The Yellow Triangle

[Nicholas Kennedy]

On many German gliders on the ASI is a yellow triangle.
This Yelloe Triangle on my ASw 20 and my LS3a is the factory recommended approach airspeed. This speed is 49 knots.
This has bothered me for a long time now, I think the factory did alot of pilots a disservice by putting this, to me anyway, very low number on there.
On both the above gliders, the max speed, normal landing flap flaps down, is 86 knots.
About 15 years ago my Gold Seal flight instructor Bob Faris, CX,indicated to me in his LS3 he planned on a much higher speed in the pattern, like 70-75 knots depending on conditions. I followed suit ever since.

[Nicholas Kennedy]

On Tuesday, February 16, 2021 at 5:56:54 PM UTC-7, Nicholas Kennedy wrote:
> On many German gliders on the ASI is a yellow triangle.
> This Yellow Triangle on my ASw 20 and my LS3a is the factory recommended approach airspeed. This speed is 49 knots.
> This has bothered me for a long time now, I think the factory did alot of pilots a disservice by putting this, to me anyway, very low number on there.
> On both the above gliders, the max speed, normal landing flap flaps down, is 86 knots.

> About 15 years ago my Gold Seal flight instructor Bob Faris, CX ,indicated to me in his LS3 he planned on a much higher speed in the pattern, like 70 to as high as 80 knots depending on conditions. I followed suit ever since.
I read in Soaring magazine frequently pilots stating that their in the pattern around 50 knots, this makes the hair on the back of my head stand on end! There is no margin for error at these low speeds. No reserve energy at all.

Stalling and spinning in the pattern has been going on forever, generally resulting in a awful crash.

I think we should all come up with a much higher speed in the pattern, and trim for and hold that speed until about 20' off the ground, this could eliminate alot of accidents IMHO.

I know all about the theory of adding half the windspeed and all of the gust factor, never the less Airspeed is everything! As a group we have to do better in this important phase of flight.
What do you Glider Gods think?
Nick
T

[Dan Marotta]

I fly around 65-70 KIAS in the pattern in my Stemme. I slow down over
the numbers.

Dan
5J

On 2/16/21 6:09 PM, Nicholas Kennedy wrote:
> On Tuesday, February 16, 2021 at 5:56:54 PM UTC-7, Nicholas Kennedy wrote:
>> On many German gliders on the ASI is a yellow triangle.
>> This Yellow Triangle on my ASw 20 and my LS3a is the factory recommended approach airspeed. This speed is 49 knots.
>> This has bothered me for a long time now, I think the factory did alot of pilots a disservice by putting this, to me anyway, very low number on there.
>> On both the above gliders, the max speed, normal landing flap flaps down, is 86 knots.
>
>> About 15 years ago my Gold Seal flight instructor Bob Faris, CX ,indicated to me in his LS3 he planned on a much higher speed in the pattern, like 70 to as high as 80 knots depending on conditions. I followed suit ever since.
> I read in Soaring magazine frequently pilots stating that their in the pattern around 50 knots, this makes the hair on the back of my head stand on end! There is no margin for error at these low speeds. No reserve energy at all.
>
> Stalling and spinning in the pattern has been going on forever, generally resulting in a awful crash.
>
> I think we should all come up with a much higher speed in the pattern, and trim for and hold that speed until about 20' off the ground, this could eliminate alot of accidents IMHO.
>
> I know all about the theory of adding half the windspeed and all of the gust factor, never the less Airspeed is everything! As a group we have to do better in this important phase of flight.
> What do you Glider Gods think?
> Nick
> T
>

[Tango Eight]

On Tuesday, February 16, 2021 at 8:09:38 PM UTC-5, wrote:
> On Tuesday, February 16, 2021 at 5:56:54 PM UTC-7, Nicholas Kennedy wrote:
> > On many German gliders on the ASI is a yellow triangle.
> > This Yellow Triangle on my ASw 20 and my LS3a is the factory recommended approach airspeed. This speed is 49 knots.
> > This has bothered me for a long time now, I think the factory did alot of pilots a disservice by putting this, to me anyway, very low number on there.
> > On both the above gliders, the max speed, normal landing flap flaps down, is 86 knots.
> > About 15 years ago my Gold Seal flight instructor Bob Faris, CX ,indicated to me in his LS3 he planned on a much higher speed in the pattern, like 70 to as high as 80 knots depending on conditions. I followed suit ever since.
> I read in Soaring magazine frequently pilots stating that their in the pattern around 50 knots, this makes the hair on the back of my head stand on end! There is no margin for error at these low speeds. No reserve energy at all.
>
> Stalling and spinning in the pattern has been going on forever, generally resulting in a awful crash.
>
> I think we should all come up with a much higher speed in the pattern, and trim for and hold that speed until about 20' off the ground, this could eliminate alot of accidents IMHO.
>
> I know all about the theory of adding half the windspeed and all of the gust factor, never the less Airspeed is everything! As a group we have to do better in this important phase of flight.
> What do you Glider Gods think?
> Nick
> T

In my experience, the minimum approach speeds given in the POH and labelled with the yellow triangle are pretty spot on where they need to be. My ASW-20B (48 kts) was perfect. My ASW-27 (54 kts) could be 50-52. Not really a big deal. These are recommended minimums. Like any other glider pilot I have had occasion to fly much faster approaches, appropriate to conditions.. Best practices are scenario dependent.

I typically fly 65 in the pattern in a glass ship (unless I need to loiter), I choose whatever speed I need to for my approach. In light conditions, it's usually right on the yellow triangle. I fly in the land of small-ish agricultural fields. It just makes sense to be good at this.

If you want to assess your margin, put the ship in landing configuration at altitude and do a slow deceleration to full stall. You might be surprised..

T8

[[email protected]]

Ditto T8.

Speed chosen for the approach should be appropriate for the conditions. If the air is smooth, there is no wind, and the pilot has good airspeed control, minimum approach speed is usually pretty spot on.

There are benefits to slowing things down, making a slightly larger pattern, with a nice stabilized approach on final. Having a bit more time to set things up usually makes for a nicer approach and landing.

Of course, this is all contingent on the conditions. There is a time and place to fly 50 knots or even 80 knots; it all depends.

All the best,
Daniel

[Hank Nixon]

On Tuesday, February 16, 2021 at 9:04:14 PM UTC-5, wrote:
> On Tuesday, February 16, 2021 at 8:09:38 PM UTC-5, wrote:
> > On Tuesday, February 16, 2021 at 5:56:54 PM UTC-7, Nicholas Kennedy wrote:
> > > On many German gliders on the ASI is a yellow triangle.
> > > This Yellow Triangle on my ASw 20 and my LS3a is the factory recommended approach airspeed. This speed is 49 knots.
> > > This has bothered me for a long time now, I think the factory did alot of pilots a disservice by putting this, to me anyway, very low number on there.
> > > On both the above gliders, the max speed, normal landing flap flaps down, is 86 knots.
> > > About 15 years ago my Gold Seal flight instructor Bob Faris, CX ,indicated to me in his LS3 he planned on a much higher speed in the pattern, like 70 to as high as 80 knots depending on conditions. I followed suit ever since.
> > I read in Soaring magazine frequently pilots stating that their in the pattern around 50 knots, this makes the hair on the back of my head stand on end! There is no margin for error at these low speeds. No reserve energy at all.
> >
> > Stalling and spinning in the pattern has been going on forever, generally resulting in a awful crash.
> >
> > I think we should all come up with a much higher speed in the pattern, and trim for and hold that speed until about 20' off the ground, this could eliminate alot of accidents IMHO.
> >
> > I know all about the theory of adding half the windspeed and all of the gust factor, never the less Airspeed is everything! As a group we have to do better in this important phase of flight.
> > What do you Glider Gods think?
> > Nick
> > T
> In my experience, the minimum approach speeds given in the POH and labelled with the yellow triangle are pretty spot on where they need to be. My ASW-20B (48 kts) was perfect. My ASW-27 (54 kts) could be 50-52. Not really a big deal. These are recommended minimums. Like any other glider pilot I have had occasion to fly much faster approaches, appropriate to conditions. Best practices are scenario dependent.
>
> I typically fly 65 in the pattern in a glass ship (unless I need to loiter), I choose whatever speed I need to for my approach. In light conditions, it's usually right on the yellow triangle. I fly in the land of small-ish agricultural fields. It just makes sense to be good at this.
>
> If you want to assess your margin, put the ship in landing configuration at altitude and do a slow deceleration to full stall. You might be surprised.
>
> T8
I'm with Evan on this.
The modern ships, with more powerful brakes than the old days, and many with flaps, allow adding some margin with the ability to shed
the excess energy fairly quickly when needed. Not so much on the older ships where speed control is important.
Going into a field, all things being equal, I will be at the yellow triangle coming over the border to the field.
It is important that this be a decision and not habit because we revert to habit in stress situations and that can lead to excess energy going into a tight space.
My observation is that more ships get broken due to too much energy in field landings than too little.
Flame suit on.
UH

[BobW]

>> On many German gliders on the ASI is a yellow triangle. This Yellow
>> Triangle on my ASw 20 and my LS3a is the factory recommended approach
>> airspeed. This speed is 49 knots. This has bothered me for a long time
>> now, I think the factory did alot of pilots a disservice by putting this,
>> to me anyway, very low number on there. On both the above gliders, the
>> max speed, normal landing flap flaps down, is 86 knots.
>
>> About 15 years ago my Gold Seal flight instructor Bob Faris, CX
>> ,indicated to me in his LS3 he planned on a much higher speed in the
>> pattern, like 70 to as high as 80 knots depending on conditions. I
>> followed suit ever since.
> I read in Soaring magazine frequently pilots stating that their in the
> pattern around 50 knots, this makes the hair on the back of my head stand
> on end! There is no margin for error at these low speeds. No reserve energy
> at all.
>
> Stalling and spinning in the pattern has been going on forever, generally
> resulting in a awful crash.
>
> I think we should all come up with a much higher speed in the pattern, and
> trim for and hold that speed until about 20' off the ground, this could
> eliminate alot of accidents IMHO.
>
> I know all about the theory of adding half the windspeed and all of the
> gust factor, never the less Airspeed is everything! As a group we have to
> do better in this important phase of flight. What do you Glider Gods
> think?

As an impeached ex-president (not PDJT) suggested - and at which other
respondents' replies hinted - it depends on what the meaning of "approach" is.
Do you mean "pattern speed" or "final approach speed"?

There's SO much in the nuances - this one and lots of others regarding "how
one flies a landing pattern".

Of the ~double-digits of late glider pilots I've known, I can recall only one
who died in the base-to-final turn...on an essentially windless, still,
day...at a camp, with lots of glider pilot witnesses (but not me). I've no
idea what her actual speed was at the time of departure from controlled
flight, but am satisfied it was almost surely "slower" than "faster"...and,
were it possible to settle the bet, would wager Real Money she was less than
perfectly coordinated during her last turn. "Slow speed" accident, or
"uncoordination-induced" accident?

Point being, there's at least one thing Joe Pilot - if interested in flying
again tomorrow - must *never* do, & that's depart from controlled flight in
the pattern. How s/he accomplishes that is the interesting bit...

As to "the yellow triangle", memory sez the only ships I semi-regularly flew
that had one were club-owned Grob variants. In benign approach conditions,
"triangle speed" seemed to me quite adequate (& comfortable) throughout the
pattern down to my "slowing down point" pre-flare. In "typical western-US
mid-boisterous day", still seriously percolating, atmospheric conditions...not
so much. Curiously, the two fastest approaches/finals I recollect were in a
G-103 (dual) and my Zuni, both "somewhere between 75-80 knots "somewhere along
upper-final-approach segment" as I recall, the Grob one due to downbursty
conditions, the other due to a howling (but remarkably steady &
lacking-in-turbulence) crosswind. I ultimately wheel-landed both, whereas in
calm conditions my default touchdown preference is the much-beloved 2-pointer.

Do what you have to do to touch down safely. Be able to do "it" consistently.
Stay away from "panacea cures" for the most part, if ever tempted to view them
as talismans in some way. Recognize any inherent paradoxes.

YMMV...

[Christoph Barniske]

The yellow triangle marks is defined in CS 22.145 as the lowest approach speed (at maximum weight without water ballast) recommended by the manufacturer. So it should be treated as a minimum value without any margins.

Christoph

[Dan Marotta]

Well, one out of three isn't bad... Fifteen gusting well over 20 is
pretty much the norm at Moriarty, and for that reason I keep my patterns
pretty close in and plenty of airspeed. It's not at all uncommon for me
to begin my base turn abeam the threshold with IAS around 65-70. My
gear door speed is 76 KIAS.

Dan
5J

On 2/17/21 6:29 AM, wrote:
> If the air is smooth, there is no wind, and the pilot has good airspeed control,

[Dan Marotta]

No flames here. The yellow triangle over the threshold, whatever that
may be, trees, fence, or a painted line, is a good thing, but you won't
find too many western pilots flying wide patterns at that low a speed.

I guess the point of contention here is the location where the YT speed
is attained.

Dan
5J

On 2/17/21 7:08 AM, Hank Nixon wrote:
> On Tuesday, February 16, 2021 at 9:04:14 PM UTC-5, wrote:
>> On Tuesday, February 16, 2021 at 8:09:38 PM UTC-5, wrote:
>>> On Tuesday, February 16, 2021 at 5:56:54 PM UTC-7, Nicholas Kennedy wrote:
>>>> On many German gliders on the ASI is a yellow triangle.
>>>> This Yellow Triangle on my ASw 20 and my LS3a is the factory recommended approach airspeed. This speed is 49 knots.
>>>> This has bothered me for a long time now, I think the factory did alot of pilots a disservice by putting this, to me anyway, very low number on there.
>>>> On both the above gliders, the max speed, normal landing flap flaps down, is 86 knots.
>>>> About 15 years ago my Gold Seal flight instructor Bob Faris, CX ,indicated to me in his LS3 he planned on a much higher speed in the pattern, like 70 to as high as 80 knots depending on conditions. I followed suit ever since.
>>> I read in Soaring magazine frequently pilots stating that their in the pattern around 50 knots, this makes the hair on the back of my head stand on end! There is no margin for error at these low speeds. No reserve energy at all.
>>>
>>> Stalling and spinning in the pattern has been going on forever, generally resulting in a awful crash.
>>>
>>> I think we should all come up with a much higher speed in the pattern, and trim for and hold that speed until about 20' off the ground, this could eliminate alot of accidents IMHO.
>>>
>>> I know all about the theory of adding half the windspeed and all of the gust factor, never the less Airspeed is everything! As a group we have to do better in this important phase of flight.
>>> What do you Glider Gods think?
>>> Nick
>>> T
>> In my experience, the minimum approach speeds given in the POH and labelled with the yellow triangle are pretty spot on where they need to be. My ASW-20B (48 kts) was perfect. My ASW-27 (54 kts) could be 50-52. Not really a big deal. These are recommended minimums. Like any other glider pilot I have had occasion to fly much faster approaches, appropriate to conditions. Best practices are scenario dependent.
>>
>> I typically fly 65 in the pattern in a glass ship (unless I need to loiter), I choose whatever speed I need to for my approach. In light conditions, it's usually right on the yellow triangle. I fly in the land of small-ish agricultural fields. It just makes sense to be good at this.
>>
>> If you want to assess your margin, put the ship in landing configuration at altitude and do a slow deceleration to full stall. You might be surprised.
>>
>> T8
> I'm with Evan on this.
> The modern ships, with more powerful brakes than the old days, and many with flaps, allow adding some margin with the ability to shed
> the excess energy fairly quickly when needed. Not so much on the older ships where speed control is important.
> Going into a field, all things being equal, I will be at the yellow triangle coming over the border to the field.
> It is important that this be a decision and not habit because we revert to habit in stress situations and that can lead to excess energy going into a tight space.
> My observation is that more ships get broken due to too much energy in field landings than too little.
> Flame suit on.
> UH
>

[Eric Greenwell[_4_]]

Hank Nixon wrote on 2/17/2021 6:08 AM:
> On Tuesday, February 16, 2021 at 9:04:14 PM UTC-5, wrote:
....
>>> T
>> In my experience, the minimum approach speeds given in the POH and labelled with the yellow triangle are pretty spot on where they need to be. My ASW-20B (48 kts) was perfect. My ASW-27 (54 kts) could be 50-52. Not really a big deal. These are recommended minimums. Like any other glider pilot I have had occasion to fly much faster approaches, appropriate to conditions. Best practices are scenario dependent.
>>
>> I typically fly 65 in the pattern in a glass ship (unless I need to loiter), I choose whatever speed I need to for my approach. In light conditions, it's usually right on the yellow triangle. I fly in the land of small-ish agricultural fields. It just makes sense to be good at this.
>>
>> If you want to assess your margin, put the ship in landing configuration at altitude and do a slow deceleration to full stall. You might be surprised.
>>
>> T8
> I'm with Evan on this.
> The modern ships, with more powerful brakes than the old days, and many with flaps, allow adding some margin with the ability to shed
> the excess energy fairly quickly when needed. Not so much on the older ships where speed control is important.
> Going into a field, all things being equal, I will be at the yellow triangle coming over the border to the field.
> It is important that this be a decision and not habit because we revert to habit in stress situations and that can lead to excess energy going into a tight space.
> My observation is that more ships get broken due to too much energy in field landings than too little.
> Flame suit on.
> UH

A long time ago, the Brits had the rule "it's better to hit the far hedge slowly than the near
hedge quickly". I wonder if they still find that rule useful.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[[email protected]]

On Wednesday, February 17, 2021 at 11:21:47 AM UTC-5, Eric Greenwell wrote:
> Hank Nixon wrote on 2/17/2021 6:08 AM:
> > On Tuesday, February 16, 2021 at 9:04:14 PM UTC-5, wrote:
> ...
> >>> T
> >> In my experience, the minimum approach speeds given in the POH and labelled with the yellow triangle are pretty spot on where they need to be. My ASW-20B (48 kts) was perfect. My ASW-27 (54 kts) could be 50-52. Not really a big deal. These are recommended minimums. Like any other glider pilot I have had occasion to fly much faster approaches, appropriate to conditions.. Best practices are scenario dependent.
> >>
> >> I typically fly 65 in the pattern in a glass ship (unless I need to loiter), I choose whatever speed I need to for my approach. In light conditions, it's usually right on the yellow triangle. I fly in the land of small-ish agricultural fields. It just makes sense to be good at this.
> >>
> >> If you want to assess your margin, put the ship in landing configuration at altitude and do a slow deceleration to full stall. You might be surprised.
> >>
> >> T8
> > I'm with Evan on this.
> > The modern ships, with more powerful brakes than the old days, and many with flaps, allow adding some margin with the ability to shed
> > the excess energy fairly quickly when needed. Not so much on the older ships where speed control is important.
> > Going into a field, all things being equal, I will be at the yellow triangle coming over the border to the field.
> > It is important that this be a decision and not habit because we revert to habit in stress situations and that can lead to excess energy going into a tight space.
> > My observation is that more ships get broken due to too much energy in field landings than too little.
> > Flame suit on.
> > UH
> A long time ago, the Brits had the rule "it's better to hit the far hedge slowly than the near
> hedge quickly". I wonder if they still find that rule useful.
>
> --
> Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
> - "A Guide to Self-Launching Sailplane Operation"
> https://sites.google.com/site/motorgliders/publications/download-the-guide-1
The problem that UH has identified is the lack of training for low energy landings. I often see pilots scorching down the runway bleeding off speed so that they can arrive at the desired point. If these same pilots were forced to make an off field landing with little or no margin for extra speed the would be in serious trouble. Practice does not make perfect, but prefect practice will result in near perfect results. There is no substitute for being prepared for what can and will at some point happen.

[6PK]

On Wednesday, February 17, 2021 at 7:32:39 AM UTC-8, Christoph Barniske wrote:
> The yellow triangle marks is defined in CS 22.145 as the lowest approach speed (at maximum weight without water ballast) recommended by the manufacturer. So it should be treated as a minimum value without any margins.
>
> Christoph

[6PK]

Spot on On Wednesday, February 17, 2021 at 3:50:01 PM UTC-8, 6PK wrote:
> On Wednesday, February 17, 2021 at 7:32:39 AM UTC-8, Christoph Barniske wrote:
> > The yellow triangle marks is defined in CS 22.145 as the lowest approach speed (at maximum weight without water ballast) recommended by the manufacturer. So it should be treated as a minimum value without any margins.
> >
> > Christoph

[6PK]

On Wednesday, February 17, 2021 at 3:51:33 PM UTC-8, 6PK wrote:
> Spot on On Wednesday, February 17, 2021 at 3:50:01 PM UTC-8, 6PK wrote:
> > On Wednesday, February 17, 2021 at 7:32:39 AM UTC-8, Christoph Barniske wrote:
> > > The yellow triangle marks is defined in CS 22.145 as the lowest approach speed (at maximum weight without water ballast) recommended by the manufacturer. So it should be treated as a minimum value without any margins.
> > >
> > > Christoph
Spot on!

[Charlie M. (UH & 002 owner/pilot)]

On Wednesday, February 17, 2021 at 12:47:03 PM UTC-5, wrote:
> On Wednesday, February 17, 2021 at 11:21:47 AM UTC-5, Eric Greenwell wrote:
> > Hank Nixon wrote on 2/17/2021 6:08 AM:
> > > On Tuesday, February 16, 2021 at 9:04:14 PM UTC-5, wrote:
> > ...
> > >>> T
> > >> In my experience, the minimum approach speeds given in the POH and labelled with the yellow triangle are pretty spot on where they need to be. My ASW-20B (48 kts) was perfect. My ASW-27 (54 kts) could be 50-52. Not really a big deal. These are recommended minimums. Like any other glider pilot I have had occasion to fly much faster approaches, appropriate to conditions. Best practices are scenario dependent.
> > >>
> > >> I typically fly 65 in the pattern in a glass ship (unless I need to loiter), I choose whatever speed I need to for my approach. In light conditions, it's usually right on the yellow triangle. I fly in the land of small-ish agricultural fields. It just makes sense to be good at this.
> > >>
> > >> If you want to assess your margin, put the ship in landing configuration at altitude and do a slow deceleration to full stall. You might be surprised.
> > >>
> > >> T8
> > > I'm with Evan on this.
> > > The modern ships, with more powerful brakes than the old days, and many with flaps, allow adding some margin with the ability to shed
> > > the excess energy fairly quickly when needed. Not so much on the older ships where speed control is important.
> > > Going into a field, all things being equal, I will be at the yellow triangle coming over the border to the field.
> > > It is important that this be a decision and not habit because we revert to habit in stress situations and that can lead to excess energy going into a tight space.
> > > My observation is that more ships get broken due to too much energy in field landings than too little.
> > > Flame suit on.
> > > UH
> > A long time ago, the Brits had the rule "it's better to hit the far hedge slowly than the near
> > hedge quickly". I wonder if they still find that rule useful.
> >
> > --
> > Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
> > - "A Guide to Self-Launching Sailplane Operation"
> > https://sites.google.com/site/motorgliders/publications/download-the-guide-1
> The problem that UH has identified is the lack of training for low energy landings. I often see pilots scorching down the runway bleeding off speed so that they can arrive at the desired point. If these same pilots were forced to make an off field landing with little or no margin for extra speed the would be in serious trouble. Practice does not make perfect, but prefect practice will result in near perfect results. There is no substitute for being prepared for what can and will at some point happen.
Part of the changes in the HHSC "Snowbird" contest over the last few decades....basically, if you do well at the contest, you have off airport skills....

[Tango Eight]

There is margin at the yellow triangle.

ASW-20B, landing flaps + full spoilers on final, spoilers held open all the way to wheel stopped.

Touch down is tail first, landing roll 200', disk brake on hard but not rubbing nose in the dirt.

https://www.youtube.com/watch?v=oBexl9GfKK0

T8

[India November[_2_]]

On Wednesday, February 17, 2021 at 10:32:39 AM UTC-5, Christoph Barniske wrote:
> The yellow triangle marks is defined in CS 22.145 as the lowest approach speed (at maximum weight without water ballast) recommended by the manufacturer. So it should be treated as a minimum value without any margins.
>
> Christoph

To be clear, my Discus 2b flight manual states the yellow triangle is the lowest approach speed at maximum mass without water ballast, with air brakes fully extended and landing gear down. For the D2B the triangle is marked at 54 kt IAS. That's over 20kts faster than the stall speed given in the flight manual for the same configuration.

In normal calm conditions where I fly near Ottawa, I try to be at, or just a few kts faster, than the bug speed on final approach but faster depending on wind and gust conditions.

Ian IN

[Eric Greenwell[_4_]]

Nicholas Kennedy wrote on 2/16/2021 4:56 PM:
> On many German gliders on the ASI is a yellow triangle.
> This Yelloe Triangle on my ASw 20 and my LS3a is the factory recommended approach airspeed. This speed is 49 knots.
> This has bothered me for a long time now, I think the factory did alot of pilots a disservice by putting this, to me anyway, very low number on there.
> On both the above gliders, the max speed, normal landing flap flaps down, is 86 knots.
> About 15 years ago my Gold Seal flight instructor Bob Faris, CX,indicated to me in his LS3 he planned on a much higher speed in the pattern, like 70-75 knots depending on conditions. I followed suit ever since.

Since so many gliders have 49 knots for the Yellow Triangle, I think this might be a regulatory
requirement, not a choice by the glider manufacturer. Perhaps gliders are required to have an
approach speed no higher than 49 knots at a certain weight with the regulation required control
authority.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[India November[_2_]]

On Thursday, February 18, 2021 at 8:11:35 AM UTC-5, Eric Greenwell wrote:
> Nicholas Kennedy wrote on 2/16/2021 4:56 PM:
> > On many German gliders on the ASI is a yellow triangle.
> > This Yelloe Triangle on my ASw 20 and my LS3a is the factory recommended approach airspeed. This speed is 49 knots.
> > This has bothered me for a long time now, I think the factory did alot of pilots a disservice by putting this, to me anyway, very low number on there.
> > On both the above gliders, the max speed, normal landing flap flaps down, is 86 knots.
> > About 15 years ago my Gold Seal flight instructor Bob Faris, CX,indicated to me in his LS3 he planned on a much higher speed in the pattern, like 70-75 knots depending on conditions. I followed suit ever since.
> Since so many gliders have 49 knots for the Yellow Triangle, I think this might be a regulatory
> requirement, not a choice by the glider manufacturer. Perhaps gliders are required to have an
> approach speed no higher than 49 knots at a certain weight with the regulation required control
> authority.
> --
> Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
> - "A Guide to Self-Launching Sailplane Operation"
> https://sites.google.com/site/motorgliders/publications/download-the-guide-1

My D2B says 54 kts. The standard does not fix a given speed.
IN

[John Galloway[_2_]]

On Thursday, 18 February 2021 at 13:11:35 UTC, Eric Greenwell wrote:
> Nicholas Kennedy wrote on 2/16/2021 4:56 PM:
> > On many German gliders on the ASI is a yellow triangle.
> > This Yelloe Triangle on my ASw 20 and my LS3a is the factory recommended approach airspeed. This speed is 49 knots.
> > This has bothered me for a long time now, I think the factory did alot of pilots a disservice by putting this, to me anyway, very low number on there.
> > On both the above gliders, the max speed, normal landing flap flaps down, is 86 knots.
> > About 15 years ago my Gold Seal flight instructor Bob Faris, CX,indicated to me in his LS3 he planned on a much higher speed in the pattern, like 70-75 knots depending on conditions. I followed suit ever since.
> Since so many gliders have 49 knots for the Yellow Triangle, I think this might be a regulatory
> requirement, not a choice by the glider manufacturer. Perhaps gliders are required to have an
> approach speed no higher than 49 knots at a certain weight with the regulation required control
> authority.
> --
> Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
> - "A Guide to Self-Launching Sailplane Operation"
> https://sites.google.com/site/motorgliders/publications/download-the-guide-1

In the EASA CS22 specification for ASI markings it includes "a yellow marking (triangle) for the lowest approach speed (at maximum weight without water ballast) *recommended by the manufacturer*". (My stars). As far as I can see there is neither a set value nor a formula for deriving it from e.g. stall speeds

[andy l]

On Thursday, 18 February 2021 at 14:48:55 UTC, wrote:
> On Thursday, 18 February 2021 at 13:11:35 UTC, Eric Greenwell wrote:
> > Nicholas Kennedy wrote on 2/16/2021 4:56 PM:
> > > On many German gliders on the ASI is a yellow triangle.
> > > This Yelloe Triangle on my ASw 20 and my LS3a is the factory recommended approach airspeed. This speed is 49 knots.
> > > This has bothered me for a long time now, I think the factory did alot of pilots a disservice by putting this, to me anyway, very low number on there.
> > > On both the above gliders, the max speed, normal landing flap flaps down, is 86 knots.
> > > About 15 years ago my Gold Seal flight instructor Bob Faris, CX,indicated to me in his LS3 he planned on a much higher speed in the pattern, like 70-75 knots depending on conditions. I followed suit ever since.
> > Since so many gliders have 49 knots for the Yellow Triangle, I think this might be a regulatory
> > requirement, not a choice by the glider manufacturer. Perhaps gliders are required to have an
> > approach speed no higher than 49 knots at a certain weight with the regulation required control
> > authority.
> > --
> > Eric Greenwell - Washington State, USA
> > https://sites.google.com/site/motorgliders/publications/download-the-guide-1
> In the EASA CS22 specification for ASI markings it includes "a yellow marking (triangle) for the lowest approach speed (at maximum weight without water ballast) *recommended by the manufacturer*". (My stars). As far as I can see there is neither a set value nor a formula for deriving it from e.g. stall speeds

There's probably some approximation or rounding

49 and 54 knots might sound like precision to us, but these are 90 and 100 km/h. 86 kts is 160 km/h

[Dan Daly[_2_]]

> My D2B says 54 kts. The standard does not fix a given speed.
> IN

My SZD-55 says 56.7 kts/105 kph.
CS 22.1545 Air-speed indicator (See AMC 22.1545)
Each air-speed indicator must show the following markings:
.... some deleted ....
(e) a yellow marking (triangle) for the lowest approach speed (at maximum weight without water ballast) recommended by the manufacturer;
2D

[Charlie M. (UH & 002 owner/pilot)]

On Thursday, February 18, 2021 at 7:48:45 AM UTC-5, wrote:
> There is margin at the yellow triangle.
>
> ASW-20B, landing flaps + full spoilers on final, spoilers held open all the way to wheel stopped.
>
> Touch down is tail first, landing roll 200', disk brake on hard but not rubbing nose in the dirt.
>
> https://www.youtube.com/watch?v=oBexl9GfKK0
>
> T8
My feeling/training others....what does it feel like, what does it look like, what does it sound like....make it all seem correct...don't break the aircraft....

[Nicholas Kennedy]

This is the problem I'm referring too:

https://www.youtube.com/watch?v=use6PnxjO7s

Nick
T

[[email protected]]

On Thursday, February 18, 2021 at 7:48:45 AM UTC-5, wrote:
> There is margin at the yellow triangle.
>
> ASW-20B, landing flaps + full spoilers on final, spoilers held open all the way to wheel stopped.
>
> Touch down is tail first, landing roll 200', disk brake on hard but not rubbing nose in the dirt.
>
> https://www.youtube.com/watch?v=oBexl9GfKK0
>
> T8
There is plenty of margin, try putting a cover over your airspeed indicator for the entire flight, actually it will be in your best interest to see exactly how your ship flies. E9 did this to me back in the day and it taught me a thing or two. Number one is that fly by the seat of your pants, number two prepare yourself for the off field landing. We did this often and it was one of the best learning experiences that I ever had.

[Tango Eight]

On Thursday, February 18, 2021 at 6:32:19 PM UTC-5, wrote:
> This is the problem I'm referring too:
>
> https://www.youtube.com/watch?v=use6PnxjO7s
>
> Nick
> T

That sort of accident has nothing to do with speed on final approach.

T8

[bgrly[_2_]]

On Thursday, February 18, 2021 at 7:12:31 PM UTC-6, wrote:
> On Thursday, February 18, 2021 at 6:32:19 PM UTC-5, wrote:
> > This is the problem I'm referring too:
> >
> > https://www.youtube.com/watch?v=use6PnxjO7s
> >
> > Nick
> > T
> That sort of accident has nothing to do with speed on final approach.
>
> T8

Discussions at my club and at a FIRC prompted my research and writing a piece on my personal website. The link is below. I suspect or expect a few comments, as undoubtedly somebody will think I"m all wet. Mention of the huey is because that's what I know.

http://www.unofficialuseonly.us/gelbe.html

Brent, 7A ,

[NG[_2_]]

I suggest avoiding blanket statements like assertions that the yellow triangle is ALWAYS based on max unballasted weight. On my 1980 Ventus A, the yellow triangle is required by the flight manual to be at 62 knots based on max allowable weight (with full ballast) at 525 kg / 1157 lbs and landing flaps down, airbrakes retracted. Design rules have undoubtedly changed since then, but the POH is always the final authority.

[India November[_2_]]

On Friday, February 19, 2021 at 11:38:02 AM UTC-5, bgrly wrote:
> On Thursday, February 18, 2021 at 7:12:31 PM UTC-6, wrote:
> > On Thursday, February 18, 2021 at 6:32:19 PM UTC-5, wrote:
> > > This is the problem I'm referring too:
> > >
> > > https://www.youtube.com/watch?v=use6PnxjO7s
> > >
> > > Nick
> > > T
> > That sort of accident has nothing to do with speed on final approach.
> >
> > T8
> Discussions at my club and at a FIRC prompted my research and writing a piece on my personal website. The link is below. I suspect or expect a few comments, as undoubtedly somebody will think I"m all wet. Mention of the huey is because that's what I know.
>
> http://www.unofficialuseonly.us/gelbe.html
>
> Brent, 7A ,

I have understood (though I can't give you a reference) the YT as marking the minimum recommended approach speed needed to complete the landing flare at max mass without water ballast in calm air. Modern gliders have a high rate of descent with airbrakes fully extended and gear down. The D2b flight manual status that the L/D in this configuration is approximately 5.8:1, which translates to a sink rate of roughly -10kts. In this condition you need enough airspeed to arrest the descent and flare. If the approach is too slow, the glider will mush and land heavily. The YT doesn't say anything per se about airspeed margin for wind gradient and gusts.

A variety of approaches can be found in the literature on the subject of approach airspeed in windy conditions, e.g.:

1.3 Vso plus the full wind speed (Soaring Association of Canada Student handbook p61) http://www.sac.ca/index.php/en/documents-en/safety-and-training/resources-for-new-pilots/447-soar-new-ed-9-aug-2011-optim/file
1.5 Vso plus half the gust speed if not otherwise specified in the POH (FAA glider manual pp 7-23 and 7-25) https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/glider_handbook/media/faa-h-8083-13a.pdf
1.5 Vso plus half the wind plus all the gust (Soaring Safety Foundation). https://www.soaringsafety.org/publications/Nov-2016-airspeed.pdf

They all give the same general advice, that is fly higher airspeed in windy and gusty conditions. I don't claim to know which exact formula which is best.

Ian IN

[India November[_2_]]

On Saturday, February 20, 2021 at 7:55:39 PM UTC-5, NG wrote:
> I suggest avoiding blanket statements like assertions that the yellow triangle is ALWAYS based on max unballasted weight. On my 1980 Ventus A, the yellow triangle is required by the flight manual to be at 62 knots based on max allowable weight (with full ballast) at 525 kg / 1157 lbs and landing flaps down, airbrakes retracted. Design rules have undoubtedly changed since then, but the POH is always the final authority.


The CS-22 standard does actually say "a yellow marking (triangle) for the lowest approach speed (at maximum weight without water ballast) recommended by the manufacturer", but you're right in case of difference the POH rules.

Ian IN

[[email protected]]

Does anyone not consider that there is a difference between pattern speed where maneuvering and other factors come into play versus on short final approach speed say starting at 1/4 mile where a stabilized approach using the manufacturers numbers by design gives you a safe, consistent, lower energy spot landing (hopefully) every time and still enough energy to use a lot of spoilers if needed whether you are heavy or light to adjust some. If you are convinced YT is wrong for your airframe, talk to others in type before you decide to move it a knot or two up or down and after you are convinced about the accuracy of your stall speed vs the POH.
Carl, 7E

[Jon May]

On Sunday, 21 February 2021 at 19:55:22 UTC, wrote:
> Does anyone not consider that there is a difference between pattern speed where maneuvering and other factors come into play versus on short final approach speed say starting at 1/4 mile where a stabilized approach using the manufacturers numbers by design gives you a safe, consistent, lower energy spot landing (hopefully) every time and still enough energy to use a lot of spoilers if needed whether you are heavy or light to adjust some. If you are convinced YT is wrong for your airframe, talk to others in type before you decide to move it a knot or two up or down and after you are convinced about the accuracy of your stall speed vs the POH.
> Carl, 7E

[Jon May]

On Tuesday, 23 February 2021 at 11:07:26 UTC, Jon May wrote:
> On Sunday, 21 February 2021 at 19:55:22 UTC, wrote:
> > Does anyone not consider that there is a difference between pattern speed where maneuvering and other factors come into play versus on short final approach speed say starting at 1/4 mile where a stabilized approach using the manufacturers numbers by design gives you a safe, consistent, lower energy spot landing (hopefully) every time and still enough energy to use a lot of spoilers if needed whether you are heavy or light to adjust some. If you are convinced YT is wrong for your airframe, talk to others in type before you decide to move it a knot or two up or down and after you are convinced about the accuracy of your stall speed vs the POH.
> > Carl, 7E



I am posting an paper on wing profiles because it helps explain the effects newer wing shapes have on the flare to land.
Jon

Effects of the step in lift-curves of wing profiles with flaps

by Karel Termaat

Introduction (third alinea modified since publication in S&G)

Some years ago, I found that when landing our new sailplane it usually made a couple of bumps onto the ground before it decided to stay there; very interesting to friends observing your flying skills with the new toy of course. Another surprising thing happened to me with this sailplane when I made a long cross-country flight, came back a little late and low and decided to divert from my shortest route to the airfield to an area where small flocks of cumulus clouds were still forming. To my surprise I could not climb there, even though the air was quite turbulent and going up on average. I tried another identical spot with the same disappointing result. I flew out of this promising area and found a weak smooth thermal which brought me home.. Quite a frustrating experience.

After a while, my son Ronald and I began to suspect the lift-curve of the wing of having an unusual part at the lower speeds, which could explain our findings that pitch control just prior to touching down had hardly any effect on sink rate and that the good climbing performance of our new sailplane degraded substantially when circling in turbulent thermals.

Because of our observations we contacted prof. ir. Loek Boermans at Delft Technical University. "We improved the not so good climbing performance of a standard class sailplane in turbulent conditions already some years ago”, he said. “A new wing profile and winglets were applied. Apart from a recent phone call of Ronald flying at the EC in Leszno, I heard no complaints about the climbing performance of the new breed of sailplanes with flaps though I was indeed worried about this. I advised Ronald to fly somewhat faster in turbulent thermals with more banking angle. I think there are ways to solve this problem also for sailplanes with flaps as we did for the standard class glider”.

Shortly after the meeting, prof. Boermans came up with a slightly redesigned profile promising a better climbing rate in turbulent conditions while maintaining optimum glide rates. I myself started to develop some software of my own to better understand the effects of the flatter part in the lift-curves of modern sailplanes with flaps and came up with a couple of practical ideas when flying in turbulent air or when landing at low speed. Slightly modifying the wing profile is of course no option to pilots having one of the current breed of high performance sailplanes.

The lift-curves:

The lift of a sailplane is controlled by the well known lift formula: L = ½.r.V2.S.CL . With this formula, together with the CL-a graph we can study the performance of a sailplane in smooth and turbulent air. Modern wings are quite thin to minimise profile drag and are normally operated at small values of a within the so called ‘laminar drag bucket’ where maximum length’s of laminar boundary layers on both the upper and under surfaces of the wing are realized. The lift formula shows that at low thermalling speeds or when landing, Cl must be as large as possible to properly carry the weight of the sailplane. Maximising CL at low speeds and small values of a is controlled by a balancing process, where with increasing a the lift in the forward section of the wing grows as expected, while the lift on the aft part of the wing breaks down at about the same rate because of earlier laminar boundary layer transitions and flow seperations at the flap hinge area. The net effect of this is that the mean lift coefficient remains about constant over quite some interval of a, especially for the larger flap settings, as shown in figure 1.

So, a linear relationship between CL and a, as common in earlier years of wing design, does not apply to modern wing profiles with flaps. Almost all have a more or less horizontal step in their lift curves. Beyond the step, CL increases again because of a retarding effect in the break down process of the lift until the airflow starts fully detaching from the wing surface and the sailplane stalls.



Figure 1: Measured CL- a curves (typical for any modern sailplane)

Landing

Now consider the case of a pilot on finals to the airfield close to touching down. No water in the sailplane (mass=450kg) and FL=20°, ie in the landing position. Assume an approach speed V = 76km/h. Then from formula (2) it folows that CL = 1.46. The FL=20° curve of figure 1 indicates that for this value of CL, a = 2.5° which is just in front of the step as indicated. In rounding off prior to touching the ground, the pilot lifts the nose of the sailplane to increase a with the idea of slowing down the descent rate of the sailplane. a increases alright, but CL doesn’t do that unless the nose of the sailplane is tilted so high that a has increased from 2.5° to more than 7.0°. Only then CL will increase to above 1.46 and the sailplane will stop its downward motion. Usually a pilot’s action in rotating the sailplane over this large angle is too cautious. The sailplane will contact the ground prematurely and will usually bounce a couple of times. Bumpy landings are quite familiar to pilots flying modern competition sailplanes and can frequently be observed. The solution to this problem is to stay more in front of the step region by flying a little faster than the minimum approach speed and use airbrakes rather than pitch to control descent rate. Once on the ground, good wheelbrakes should be applied to bring the sailplane to a halt.

Flying through turbulences

In turbulent air, serious changes in a occur because of vertical movements of air hitting the wing surface. When gliding straight on at small values of a close to 0°, which is usually the case, one can see from the CL- a graphs of figure 1, that upgusts will increase CL and downgusts will decrease CL in the same manner. On the average CL will stay constant with time and no altitude gain or loss appears in this situation where gusts have a random distribution in strength and direction. But especially at low wing loading one may expect a rough ride because of significant positive and negative accelerations due to the more or less strong variations in CL with time.

However, now consider a sailplane flying nose up with a = 4.0° and FL=15°, where CL=1.40 as indicated in figure1. So right at the beginning of the step where upgusts have no effect on CL, since an increase of some degrees in a falls right into the step where CL is constant. However, downgusts will reduce a with the same some degrees, thereby reducing CL. So CL swaps between 1.40 for positive gusts, and some lower value, say 1.20 for negative gusts. On average CL = 1.30, i.e. 7 % less than the 1.40 required to carry the weight of the sailplane. Therefore the sailplane starts a dynamic downward motion as long as the turbulences are present. Not what one really wants of course.

A computer routine

To get a better insight into the climb rate of the sailplane under turbulent conditions I wrote a small, but effective, computer routine. The first part of the routine is an outer loop, which is rather straightforward and describes vertical speed as the difference between thermal climb rate and polar descent rate in smooth thermals for 300 values of circling speeds in a practical range.

I used a theoretical thermal model with a parabolic shape and accurate speed polars as measured by Idaflieg (the German academic flight test group). I used cubic spline functions to make the ploar curves accessible to the computer routine as a function of flying speed. The straight flight sink rate data of the polar were corrected for mass and bank angle. With these formulae for the thermal model and polar descent rate, stable climbing rate as a function of flying speed can be calculated as: Vs_(th+pol) = Vs_thermal + Vs_polar ......(3) (purple curve in figure 3)

To calculate the additional effect of turbulence, some sensible dynamics equations had to be defined. In the computer routine, these equations are enclosed in an iterative inner loop having small time steps dt = 0.01s. Turbulence is described as sine waves with a time constant of 2 seconds and random amplitude. In this way the effect of turbulence, Vs_gusts, is calculated during 30 seconds for each of the 300 flying speeds considered and plotted in graphs (blue curve in figure 3). Then total climbing rate is: Vs_total = Vs_(th+pol) + Vs_gusts ......(4) (black curve in figure 3)

A key issue in the routine is the application of Newton’s second law to find the vertical speed, Vs(t), of the sailplane from the forces acting on the wing due to the airgusts present.

Results of calculations with the routine

Figure 2 shows the development of vertical speed Vs(t) of the sailplane for the first couple of turbulence gusts, Vg(t), encountered. One can say that the results are quite dramatic when looking at the blue Vs(t) curve which averages out at -0.59m/s. The sine wave shapes of the turbulence can be recognised, as well as their randomised amplitudes. More erratic forms are likely of course, but these are not expected to make a significant difference to the results of this analysis.



Figure 2: Development of sink rate Vs(t) due to sine wave shaped turbulences.

More practical results with the computer routine are given in figure 3 which shows one of the many graphs studied. The a line shows the step as expected. Considering a smooth thermal, a best climbing rate of Vs_total = Vs_(th+pol) = 1.80m/s is obtained. Circling with a speed just above something like 100km/h is OK and comfortable, but of course this is dependent on the shape of the thermal.

Considering also the turbulence in figure 2, total climbing rate is now Vs_total = Vs_(th-pol) + Vs_gusts as indicated. For speeds above, say, 120km/h, the defined turbulence has no effect because a is small (even negative) and quite less than 4.0° where the step in the CL-a curve for FL=15° begins. Then positive and negative gusts cancel out as is shown by the Vs_gusts curve. However, for speeds less than 120km/h, positive gusts raise a to into the flat part of the CL-a curve of figure 1 and give a reduced contribution and at 99km/h no contribution at all to the lift of the wing. Negative gusts however still fully reduce CL as usual. Therefore with turbulences present, the nice total climbing rate of 1.80m/s at 100km/h for the smooth thermal is dramatically reduced to 1.20m/s only, a loss of 33 per cent. The only way out of this is to circle some 10km/h faster. In this case a climbing rate of about 1.40m/s is attained at a speed of 110km/h. So, still a loss of 22 per cent due to the turbulence.

To get a beneficial effect out of random air gusts, a pilot may try to fly with an a just behind the step in the CL-a curve where positive gusts push the sailplane up and negative gusts have no effect. Figure 3 promises a nice climbing rate when circling with a speed of say 96km/h. Some pilots seem succesful in doing so, however the sailplane may be rather difficult to control and high drag may deminish this special effect substantially.

Other calculations with the routine show that having a dip in the lift curve is still worse than a flat step, but some earlier high-performance sailplanes have that. A small positive gradient in the step area, as suggested by Loek Boermans, improves climbing rate with turbulences present quite considerably, so this looks the way to go in the design process of new wing profiles. Just now, some new sailplanes having this idea are coming on the market.




Figure 3: Reduced climbing rate in a turbulent thermal

It should be mentioned that the above calculations were performed assuming that the effects of the flat part in the liftcurve occur at the same moment over the total span of the wing. This not the case however due to the random distribution of turbulences in size and in space and because of different wing profiles and Re-numbers in the spanwise direction of the wing. Therefore the results of the above calculations maybe somewhat exaggerated for the practical situation.

In conclusion

Recent optimisations of wing profiles with flaps have resulted in CL-a curves with a more or less ‘flat part’ in the lower speed range.. Because of this, landing behaviour and climbing performance in turbulent thermals are somewhat disappointing.

This article shows that pilots can partly take care of these drawbacks by flying with a (angle of attack) well in front of the ‘step’ in the CL- a curve.

The main actions to achieve that are:

a. When trying to make perfect landings, approach speeds should be a little higher than usual and speed brakes gently applied rather than pitch to finely control the descent rate in rounding off just before touching the ground.
b. When optimising total climbing rate in turbulent thermals, flying speed and bank angle should be higher than a pilot would usually prefer.
c. Thermals should be entered with redundant speed to avoid that the sudden upflow encountered pushes a into the step area.

This study shows that slightly modifying the CL-a curve with a positive gradient in the step area will improve sailplane performance substantially. A better control of the descent rate during landing will then be possible and an increase in average climbing rate of some 20 per cent may be expected when circling in turbulent thermals. I understand that only minor modifications of the wing profile are necessary to obtain these improvements without a penalty in the high speed range of the sailplane (as suggested by Prof. Ir. Loek Boermans).

Acknowledgement

I would like to thank my friends for the inspiring discussions about the step in the CL-a curve of modern sailplanes – especially my son Ronald and Loek Boermans, both of whom gave in their own specific way of practical experience and theoretical knowledge, a substantial support in the realisation of this work. Additionally the books and articles of John Anderson, Helmut Reichmann, Fred Thomas, Loek Boermans and others and many articles found on the internet were also quite inspiring.

ir. K.P. Termaat
Arnhem, NL
20 oct. 2011

[krasw]

Rounding out with high airspeed and applying airbrakes in the process? Yikes.

[Tango Eight]

On Tuesday, February 23, 2021 at 6:46:13 AM UTC-5, Jon May wrote:
> On Tuesday, 23 February 2021 at 11:07:26 UTC, Jon May wrote:
> > On Sunday, 21 February 2021 at 19:55:22 UTC, wrote:
> > > Does anyone not consider that there is a difference between pattern speed where maneuvering and other factors come into play versus on short final approach speed say starting at 1/4 mile where a stabilized approach using the manufacturers numbers by design gives you a safe, consistent, lower energy spot landing (hopefully) every time and still enough energy to use a lot of spoilers if needed whether you are heavy or light to adjust some. If you are convinced YT is wrong for your airframe, talk to others in type before you decide to move it a knot or two up or down and after you are convinced about the accuracy of your stall speed vs the POH.
> > > Carl, 7E
> I am posting an paper on wing profiles because it helps explain the effects newer wing shapes have on the flare to land.
> Jon
>
> Effects of the step in lift-curves of wing profiles with flaps
>
> by Karel Termaat
>
> Introduction (third alinea modified since publication in S&G)
>
> Some years ago, I found that when landing our new sailplane it usually made a couple of bumps onto the ground before it decided to stay there; very interesting to friends observing your flying skills with the new toy of course. Another surprising thing happened to me with this sailplane when I made a long cross-country flight, came back a little late and low and decided to divert from my shortest route to the airfield to an area where small flocks of cumulus clouds were still forming. To my surprise I could not climb there, even though the air was quite turbulent and going up on average. I tried another identical spot with the same disappointing result. I flew out of this promising area and found a weak smooth thermal which brought me home. Quite a frustrating experience.
>
> After a while, my son Ronald and I began to suspect the lift-curve of the wing of having an unusual part at the lower speeds, which could explain our findings that pitch control just prior to touching down had hardly any effect on sink rate and that the good climbing performance of our new sailplane degraded substantially when circling in turbulent thermals.
>
> Because of our observations we contacted prof. ir. Loek Boermans at Delft Technical University. "We improved the not so good climbing performance of a standard class sailplane in turbulent conditions already some years ago”, he said. “A new wing profile and winglets were applied. Apart from a recent phone call of Ronald flying at the EC in Leszno, I heard no complaints about the climbing performance of the new breed of sailplanes with flaps though I was indeed worried about this. I advised Ronald to fly somewhat faster in turbulent thermals with more banking angle. I think there are ways to solve this problem also for sailplanes with flaps as we did for the standard class glider”.
>
> Shortly after the meeting, prof. Boermans came up with a slightly redesigned profile promising a better climbing rate in turbulent conditions while maintaining optimum glide rates. I myself started to develop some software of my own to better understand the effects of the flatter part in the lift-curves of modern sailplanes with flaps and came up with a couple of practical ideas when flying in turbulent air or when landing at low speed. Slightly modifying the wing profile is of course no option to pilots having one of the current breed of high performance sailplanes.
>
> The lift-curves:
>
> The lift of a sailplane is controlled by the well known lift formula: L = ½.r.V2.S.CL . With this formula, together with the CL-a graph we can study the performance of a sailplane in smooth and turbulent air. Modern wings are quite thin to minimise profile drag and are normally operated at small values of a within the so called ‘laminar drag bucket’ where maximum length’s of laminar boundary layers on both the upper and under surfaces of the wing are realized. The lift formula shows that at low thermalling speeds or when landing, Cl must be as large as possible to properly carry the weight of the sailplane. Maximising CL at low speeds and small values of a is controlled by a balancing process, where with increasing a the lift in the forward section of the wing grows as expected, while the lift on the aft part of the wing breaks down at about the same rate because of earlier laminar boundary layer transitions and flow seperations at the flap hinge area. The net effect of this is that the mean lift coefficient remains about constant over quite some interval of a, especially for the larger flap settings, as shown in figure 1.
>
> So, a linear relationship between CL and a, as common in earlier years of wing design, does not apply to modern wing profiles with flaps. Almost all have a more or less horizontal step in their lift curves. Beyond the step, CL increases again because of a retarding effect in the break down process of the lift until the airflow starts fully detaching from the wing surface and the sailplane stalls.
>
>
>
> Figure 1: Measured CL- a curves (typical for any modern sailplane)
>
> Landing
>
> Now consider the case of a pilot on finals to the airfield close to touching down. No water in the sailplane (mass=450kg) and FL=20°, ie in the landing position. Assume an approach speed V = 76km/h. Then from formula (2) it folows that CL = 1.46. The FL=20° curve of figure 1 indicates that for this value of CL, a = 2.5° which is just in front of the step as indicated. In rounding off prior to touching the ground, the pilot lifts the nose of the sailplane to increase a with the idea of slowing down the descent rate of the sailplane. a increases alright, but CL doesn’t do that unless the nose of the sailplane is tilted so high that a has increased from 2.5° to more than 7.0°. Only then CL will increase to above 1.46 and the sailplane will stop its downward motion. Usually a pilot’s action in rotating the sailplane over this large angle is too cautious. The sailplane will contact the ground prematurely and will usually bounce a couple of times. Bumpy landings are quite familiar to pilots flying modern competition sailplanes and can frequently be observed. The solution to this problem is to stay more in front of the step region by flying a little faster than the minimum approach speed and use airbrakes rather than pitch to control descent rate. Once on the ground, good wheelbrakes should be applied to bring the sailplane to a halt.
>
> Flying through turbulences
>
> In turbulent air, serious changes in a occur because of vertical movements of air hitting the wing surface. When gliding straight on at small values of a close to 0°, which is usually the case, one can see from the CL- a graphs of figure 1, that upgusts will increase CL and downgusts will decrease CL in the same manner. On the average CL will stay constant with time and no altitude gain or loss appears in this situation where gusts have a random distribution in strength and direction. But especially at low wing loading one may expect a rough ride because of significant positive and negative accelerations due to the more or less strong variations in CL with time.
>
> However, now consider a sailplane flying nose up with a = 4.0° and FL=15°, where CL=1.40 as indicated in figure1. So right at the beginning of the step where upgusts have no effect on CL, since an increase of some degrees in a falls right into the step where CL is constant. However, downgusts will reduce a with the same some degrees, thereby reducing CL. So CL swaps between 1.40 for positive gusts, and some lower value, say 1.20 for negative gusts. On average CL = 1.30, i.e. 7 % less than the 1.40 required to carry the weight of the sailplane. Therefore the sailplane starts a dynamic downward motion as long as the turbulences are present. Not what one really wants of course.
>
> A computer routine
>
> To get a better insight into the climb rate of the sailplane under turbulent conditions I wrote a small, but effective, computer routine. The first part of the routine is an outer loop, which is rather straightforward and describes vertical speed as the difference between thermal climb rate and polar descent rate in smooth thermals for 300 values of circling speeds in a practical range.
>
> I used a theoretical thermal model with a parabolic shape and accurate speed polars as measured by Idaflieg (the German academic flight test group). I used cubic spline functions to make the ploar curves accessible to the computer routine as a function of flying speed. The straight flight sink rate data of the polar were corrected for mass and bank angle. With these formulae for the thermal model and polar descent rate, stable climbing rate as a function of flying speed can be calculated as: Vs_(th+pol) = Vs_thermal + Vs_polar ......(3) (purple curve in figure 3)
>
> To calculate the additional effect of turbulence, some sensible dynamics equations had to be defined. In the computer routine, these equations are enclosed in an iterative inner loop having small time steps dt = 0.01s. Turbulence is described as sine waves with a time constant of 2 seconds and random amplitude. In this way the effect of turbulence, Vs_gusts, is calculated during 30 seconds for each of the 300 flying speeds considered and plotted in graphs (blue curve in figure 3). Then total climbing rate is: Vs_total = Vs_(th+pol) + Vs_gusts ......(4) (black curve in figure 3)
>
> A key issue in the routine is the application of Newton’s second law to find the vertical speed, Vs(t), of the sailplane from the forces acting on the wing due to the airgusts present.
>
> Results of calculations with the routine
>
> Figure 2 shows the development of vertical speed Vs(t) of the sailplane for the first couple of turbulence gusts, Vg(t), encountered. One can say that the results are quite dramatic when looking at the blue Vs(t) curve which averages out at -0.59m/s. The sine wave shapes of the turbulence can be recognised, as well as their randomised amplitudes. More erratic forms are likely of course, but these are not expected to make a significant difference to the results of this analysis.
>
>
>
> Figure 2: Development of sink rate Vs(t) due to sine wave shaped turbulences.
>
> More practical results with the computer routine are given in figure 3 which shows one of the many graphs studied. The a line shows the step as expected. Considering a smooth thermal, a best climbing rate of Vs_total = Vs_(th+pol) = 1.80m/s is obtained. Circling with a speed just above something like 100km/h is OK and comfortable, but of course this is dependent on the shape of the thermal.
>
> Considering also the turbulence in figure 2, total climbing rate is now Vs_total = Vs_(th-pol) + Vs_gusts as indicated. For speeds above, say, 120km/h, the defined turbulence has no effect because a is small (even negative) and quite less than 4.0° where the step in the CL-a curve for FL=15° begins. Then positive and negative gusts cancel out as is shown by the Vs_gusts curve. However, for speeds less than 120km/h, positive gusts raise a to into the flat part of the CL-a curve of figure 1 and give a reduced contribution and at 99km/h no contribution at all to the lift of the wing. Negative gusts however still fully reduce CL as usual. Therefore with turbulences present, the nice total climbing rate of 1.80m/s at 100km/h for the smooth thermal is dramatically reduced to 1.20m/s only, a loss of 33 per cent. The only way out of this is to circle some 10km/h faster. In this case a climbing rate of about 1.40m/s is attained at a speed of 110km/h. So, still a loss of 22 per cent due to the turbulence.
>
> To get a beneficial effect out of random air gusts, a pilot may try to fly with an a just behind the step in the CL-a curve where positive gusts push the sailplane up and negative gusts have no effect. Figure 3 promises a nice climbing rate when circling with a speed of say 96km/h. Some pilots seem succesful in doing so, however the sailplane may be rather difficult to control and high drag may deminish this special effect substantially.
>
> Other calculations with the routine show that having a dip in the lift curve is still worse than a flat step, but some earlier high-performance sailplanes have that. A small positive gradient in the step area, as suggested by Loek Boermans, improves climbing rate with turbulences present quite considerably, so this looks the way to go in the design process of new wing profiles. Just now, some new sailplanes having this idea are coming on the market.
>
>
>
>
> Figure 3: Reduced climbing rate in a turbulent thermal
>
> It should be mentioned that the above calculations were performed assuming that the effects of the flat part in the liftcurve occur at the same moment over the total span of the wing. This not the case however due to the random distribution of turbulences in size and in space and because of different wing profiles and Re-numbers in the spanwise direction of the wing. Therefore the results of the above calculations maybe somewhat exaggerated for the practical situation.
>
> In conclusion
>
> Recent optimisations of wing profiles with flaps have resulted in CL-a curves with a more or less ‘flat part’ in the lower speed range. Because of this, landing behaviour and climbing performance in turbulent thermals are somewhat disappointing.
>
> This article shows that pilots can partly take care of these drawbacks by flying with a (angle of attack) well in front of the ‘step’ in the CL- a curve.
>
> The main actions to achieve that are:
>
> a. When trying to make perfect landings, approach speeds should be a little higher than usual and speed brakes gently applied rather than pitch to finely control the descent rate in rounding off just before touching the ground.
> b. When optimising total climbing rate in turbulent thermals, flying speed and bank angle should be higher than a pilot would usually prefer.
> c. Thermals should be entered with redundant speed to avoid that the sudden upflow encountered pushes a into the step area.
>
> This study shows that slightly modifying the CL-a curve with a positive gradient in the step area will improve sailplane performance substantially. A better control of the descent rate during landing will then be possible and an increase in average climbing rate of some 20 per cent may be expected when circling in turbulent thermals. I understand that only minor modifications of the wing profile are necessary to obtain these improvements without a penalty in the high speed range of the sailplane (as suggested by Prof. Ir. Loek Boermans).
>
> Acknowledgement
>
> I would like to thank my friends for the inspiring discussions about the step in the CL-a curve of modern sailplanes – especially my son Ronald and Loek Boermans, both of whom gave in their own specific way of practical experience and theoretical knowledge, a substantial support in the realisation of this work. Additionally the books and articles of John Anderson, Helmut Reichmann, Fred Thomas, Loek Boermans and others and many articles found on the internet were also quite inspiring.
>
> ir. K.P. Termaat
> Arnhem, NL
> 20 oct. 2011

The phenomenon is real, but it doesn't apply to the OP's examples (ASW-20, LS-3) under any circumstances and it only applies to other gliders (e.g. ASW-27) in the flare, and that's a whole 'nother topic. (spoiler: the 27 lands just fine near max CL, tail first, flaps in landing position help)

Pattern speed, speed (and CL) on approach, speed (and CL) at the beginning of the flare, speed (and CL) at the end of the flare all should be chosen by the pilot based on the conditions of the moment. It takes time and space to make smooth adjustments in speed, which is one reason that tiny, low energy patterns in challenging conditions fail.

An approach speed of 70 kts into a 600' hay field in light conditions is just as wrong, and just as likely to end in an accident as 49 kts at Truckee airport on a rough day. One cannot do this by rote.

A pattern that allows roughly 30 second stabilized approach with a rational glide slope (typically 1/2 brakes on a modern glider, more than half on many older models) on final solves many of the problems that appear to be the OP's concern.

T8

[Eric Greenwell[_4_]]

Jon May wrote on 2/23/2021 3:46 AM:
> On Tuesday, 23 February 2021 at 11:07:26 UTC, Jon May wrote:
>> On Sunday, 21 February 2021 at 19:55:22 wrote:
>>> Does anyone not consider that there is a difference between pattern speed where maneuvering and other factors come into play versus on short final approach speed say starting at 1/4 mile where a stabilized approach using the manufacturers numbers by design gives you a safe, consistent, lower energy spot landing (hopefully) every time and still enough energy to use a lot of spoilers if needed whether you are heavy or light to adjust some. If you are convinced YT is wrong for your airframe, talk to others in type before you decide to move it a knot or two up or down and after you are convinced about the accuracy of your stall speed vs the POH.
>>> Carl, 7E
>
>
> I am posting an paper on wing profiles because it helps explain the effects newer wing shapes have on the flare to land.
> Jon
>
> Effects of the step in lift-curves of wing profiles with flaps
>
> by Karel Termaat
>
> Introduction (third alinea modified since publication in S&G)
>
> Some years ago, I found that when landing our new sailplane it usually made a couple of bumps onto the ground before it decided to stay there; very interesting to friends observing your flying skills with the new toy of course. Another surprising thing happened to me with this sailplane when I made a long cross-country flight, came back a little late and low and decided to divert from my shortest route to the airfield to an area where small flocks of cumulus clouds were still forming. To my surprise I could not climb there, even though the air was quite turbulent and going up on average. I tried another identical spot with the same disappointing result. I flew out of this promising area and found a weak smooth thermal which brought me home.. Quite a frustrating experience.
>
I noticed this difficulty during landing with my ASH26E about 25 years ago, but did not know
the cause until listening to Loek Boerman lecture on the subject a few years later. He
confirmed my guess it was due to the same CL step that was affecting the thermal climb in
gliders designed in that early '90s era. Later, when I began routine use of the 40 degree
landing flap, it seemed the effect was not so pronounced. Getting the attitude correct for a
two-point (or a tail-first) touchdown seemed to be best; keeping the spoilers constant after
the flare made it easier.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[John Galloway[_2_]]

On Tuesday, 23 February 2021 at 16:10:07 UTC, Eric Greenwell wrote:
> Jon May wrote on 2/23/2021 3:46 AM:
> > On Tuesday, 23 February 2021 at 11:07:26 UTC, Jon May wrote:
> >> On Sunday, 21 February 2021 at 19:55:22 wrote:
> >>> Does anyone not consider that there is a difference between pattern speed where maneuvering and other factors come into play versus on short final approach speed say starting at 1/4 mile where a stabilized approach using the manufacturers numbers by design gives you a safe, consistent, lower energy spot landing (hopefully) every time and still enough energy to use a lot of spoilers if needed whether you are heavy or light to adjust some. If you are convinced YT is wrong for your airframe, talk to others in type before you decide to move it a knot or two up or down and after you are convinced about the accuracy of your stall speed vs the POH.
> >>> Carl, 7E
> >
> >
> > I am posting an paper on wing profiles because it helps explain the effects newer wing shapes have on the flare to land.
> > Jon
> >
> > Effects of the step in lift-curves of wing profiles with flaps
> >
> > by Karel Termaat
> >
> > Introduction (third alinea modified since publication in S&G)
> >
> > Some years ago, I found that when landing our new sailplane it usually made a couple of bumps onto the ground before it decided to stay there; very interesting to friends observing your flying skills with the new toy of course. Another surprising thing happened to me with this sailplane when I made a long cross-country flight, came back a little late and low and decided to divert from my shortest route to the airfield to an area where small flocks of cumulus clouds were still forming. To my surprise I could not climb there, even though the air was quite turbulent and going up on average. I tried another identical spot with the same disappointing result. I flew out of this promising area and found a weak smooth thermal which brought me home.. Quite a frustrating experience.
> >
> I noticed this difficulty during landing with my ASH26E about 25 years ago, but did not know
> the cause until listening to Loek Boerman lecture on the subject a few years later. He
> confirmed my guess it was due to the same CL step that was affecting the thermal climb in
> gliders designed in that early '90s era. Later, when I began routine use of the 40 degree
> landing flap, it seemed the effect was not so pronounced. Getting the attitude correct for a
> two-point (or a tail-first) touchdown seemed to be best; keeping the spoilers constant after
> the flare made it easier.
> --
> Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
> - "A Guide to Self-Launching Sailplane Operation"
> https://sites.google.com/site/motorgliders/publications/download-the-guide-1

The current generation of flapped high performance gliders (e.g. the Diana 2, ASH 30 [and no doubt the 33], the JS1 [same section as the JS3/2/5], Arcus, and the Ventus 3) have airfoils designed to avoid a step in the lift curve, as Karel Termaat pointed out in his article version here:

http://home.planet.nl/~kpt9/thermiekstoepje%20deutsch.htm

[Jonathan St. Cloud]

The chief pilot at the airport I fly from had a different explanation for my genital bounces.
He said "It is you." I always suspected...

Tuesday, February 23, 2021 at 3:46:13 AM UTC-8, Jon May wrote:
> On Tuesday, 23 February 2021 at 11:07:26 UTC, Jon May wrote:
> > On Sunday, 21 February 2021 at 19:55:22 UTC, wrote:
> > > Does anyone not consider that there is a difference between pattern speed where maneuvering and other factors come into play versus on short final approach speed say starting at 1/4 mile where a stabilized approach using the manufacturers numbers by design gives you a safe, consistent, lower energy spot landing (hopefully) every time and still enough energy to use a lot of spoilers if needed whether you are heavy or light to adjust some. If you are convinced YT is wrong for your airframe, talk to others in type before you decide to move it a knot or two up or down and after you are convinced about the accuracy of your stall speed vs the POH.
> > > Carl, 7E
> I am posting an paper on wing profiles because it helps explain the effects newer wing shapes have on the flare to land.
> Jon
>
> Effects of the step in lift-curves of wing profiles with flaps
>
> by Karel Termaat
>
> Introduction (third alinea modified since publication in S&G)
>
> Some years ago, I found that when landing our new sailplane it usually made a couple of bumps onto the ground before it decided to stay there; very interesting to friends observing your flying skills with the new toy of course. Another surprising thing happened to me with this sailplane when I made a long cross-country flight, came back a little late and low and decided to divert from my shortest route to the airfield to an area where small flocks of cumulus clouds were still forming. To my surprise I could not climb there, even though the air was quite turbulent and going up on average. I tried another identical spot with the same disappointing result. I flew out of this promising area and found a weak smooth thermal which brought me home. Quite a frustrating experience.
>
> After a while, my son Ronald and I began to suspect the lift-curve of the wing of having an unusual part at the lower speeds, which could explain our findings that pitch control just prior to touching down had hardly any effect on sink rate and that the good climbing performance of our new sailplane degraded substantially when circling in turbulent thermals.
>
> Because of our observations we contacted prof. ir. Loek Boermans at Delft Technical University. "We improved the not so good climbing performance of a standard class sailplane in turbulent conditions already some years ago”, he said. “A new wing profile and winglets were applied. Apart from a recent phone call of Ronald flying at the EC in Leszno, I heard no complaints about the climbing performance of the new breed of sailplanes with flaps though I was indeed worried about this. I advised Ronald to fly somewhat faster in turbulent thermals with more banking angle. I think there are ways to solve this problem also for sailplanes with flaps as we did for the standard class glider”.
>
> Shortly after the meeting, prof. Boermans came up with a slightly redesigned profile promising a better climbing rate in turbulent conditions while maintaining optimum glide rates. I myself started to develop some software of my own to better understand the effects of the flatter part in the lift-curves of modern sailplanes with flaps and came up with a couple of practical ideas when flying in turbulent air or when landing at low speed. Slightly modifying the wing profile is of course no option to pilots having one of the current breed of high performance sailplanes.
>
> The lift-curves:
>
> The lift of a sailplane is controlled by the well known lift formula: L = ½.r.V2.S.CL . With this formula, together with the CL-a graph we can study the performance of a sailplane in smooth and turbulent air. Modern wings are quite thin to minimise profile drag and are normally operated at small values of a within the so called ‘laminar drag bucket’ where maximum length’s of laminar boundary layers on both the upper and under surfaces of the wing are realized. The lift formula shows that at low thermalling speeds or when landing, Cl must be as large as possible to properly carry the weight of the sailplane. Maximising CL at low speeds and small values of a is controlled by a balancing process, where with increasing a the lift in the forward section of the wing grows as expected, while the lift on the aft part of the wing breaks down at about the same rate because of earlier laminar boundary layer transitions and flow seperations at the flap hinge area. The net effect of this is that the mean lift coefficient remains about constant over quite some interval of a, especially for the larger flap settings, as shown in figure 1.
>
> So, a linear relationship between CL and a, as common in earlier years of wing design, does not apply to modern wing profiles with flaps. Almost all have a more or less horizontal step in their lift curves. Beyond the step, CL increases again because of a retarding effect in the break down process of the lift until the airflow starts fully detaching from the wing surface and the sailplane stalls.
>
>
>
> Figure 1: Measured CL- a curves (typical for any modern sailplane)
>
> Landing
>
> Now consider the case of a pilot on finals to the airfield close to touching down. No water in the sailplane (mass=450kg) and FL=20°, ie in the landing position. Assume an approach speed V = 76km/h. Then from formula (2) it folows that CL = 1.46. The FL=20° curve of figure 1 indicates that for this value of CL, a = 2.5° which is just in front of the step as indicated. In rounding off prior to touching the ground, the pilot lifts the nose of the sailplane to increase a with the idea of slowing down the descent rate of the sailplane. a increases alright, but CL doesn’t do that unless the nose of the sailplane is tilted so high that a has increased from 2.5° to more than 7.0°. Only then CL will increase to above 1.46 and the sailplane will stop its downward motion. Usually a pilot’s action in rotating the sailplane over this large angle is too cautious. The sailplane will contact the ground prematurely and will usually bounce a couple of times. Bumpy landings are quite familiar to pilots flying modern competition sailplanes and can frequently be observed. The solution to this problem is to stay more in front of the step region by flying a little faster than the minimum approach speed and use airbrakes rather than pitch to control descent rate. Once on the ground, good wheelbrakes should be applied to bring the sailplane to a halt.
>
> Flying through turbulences
>
> In turbulent air, serious changes in a occur because of vertical movements of air hitting the wing surface. When gliding straight on at small values of a close to 0°, which is usually the case, one can see from the CL- a graphs of figure 1, that upgusts will increase CL and downgusts will decrease CL in the same manner. On the average CL will stay constant with time and no altitude gain or loss appears in this situation where gusts have a random distribution in strength and direction. But especially at low wing loading one may expect a rough ride because of significant positive and negative accelerations due to the more or less strong variations in CL with time.
>
> However, now consider a sailplane flying nose up with a = 4.0° and FL=15°, where CL=1.40 as indicated in figure1. So right at the beginning of the step where upgusts have no effect on CL, since an increase of some degrees in a falls right into the step where CL is constant. However, downgusts will reduce a with the same some degrees, thereby reducing CL. So CL swaps between 1.40 for positive gusts, and some lower value, say 1.20 for negative gusts. On average CL = 1.30, i.e. 7 % less than the 1.40 required to carry the weight of the sailplane. Therefore the sailplane starts a dynamic downward motion as long as the turbulences are present. Not what one really wants of course.
>
> A computer routine
>
> To get a better insight into the climb rate of the sailplane under turbulent conditions I wrote a small, but effective, computer routine. The first part of the routine is an outer loop, which is rather straightforward and describes vertical speed as the difference between thermal climb rate and polar descent rate in smooth thermals for 300 values of circling speeds in a practical range.
>
> I used a theoretical thermal model with a parabolic shape and accurate speed polars as measured by Idaflieg (the German academic flight test group). I used cubic spline functions to make the ploar curves accessible to the computer routine as a function of flying speed. The straight flight sink rate data of the polar were corrected for mass and bank angle. With these formulae for the thermal model and polar descent rate, stable climbing rate as a function of flying speed can be calculated as: Vs_(th+pol) = Vs_thermal + Vs_polar ......(3) (purple curve in figure 3)
>
> To calculate the additional effect of turbulence, some sensible dynamics equations had to be defined. In the computer routine, these equations are enclosed in an iterative inner loop having small time steps dt = 0.01s. Turbulence is described as sine waves with a time constant of 2 seconds and random amplitude. In this way the effect of turbulence, Vs_gusts, is calculated during 30 seconds for each of the 300 flying speeds considered and plotted in graphs (blue curve in figure 3). Then total climbing rate is: Vs_total = Vs_(th+pol) + Vs_gusts ......(4) (black curve in figure 3)
>
> A key issue in the routine is the application of Newton’s second law to find the vertical speed, Vs(t), of the sailplane from the forces acting on the wing due to the airgusts present.
>
> Results of calculations with the routine
>
> Figure 2 shows the development of vertical speed Vs(t) of the sailplane for the first couple of turbulence gusts, Vg(t), encountered. One can say that the results are quite dramatic when looking at the blue Vs(t) curve which averages out at -0.59m/s. The sine wave shapes of the turbulence can be recognised, as well as their randomised amplitudes. More erratic forms are likely of course, but these are not expected to make a significant difference to the results of this analysis.
>
>
>
> Figure 2: Development of sink rate Vs(t) due to sine wave shaped turbulences.
>
> More practical results with the computer routine are given in figure 3 which shows one of the many graphs studied. The a line shows the step as expected. Considering a smooth thermal, a best climbing rate of Vs_total = Vs_(th+pol) = 1.80m/s is obtained. Circling with a speed just above something like 100km/h is OK and comfortable, but of course this is dependent on the shape of the thermal.
>
> Considering also the turbulence in figure 2, total climbing rate is now Vs_total = Vs_(th-pol) + Vs_gusts as indicated. For speeds above, say, 120km/h, the defined turbulence has no effect because a is small (even negative) and quite less than 4.0° where the step in the CL-a curve for FL=15° begins. Then positive and negative gusts cancel out as is shown by the Vs_gusts curve. However, for speeds less than 120km/h, positive gusts raise a to into the flat part of the CL-a curve of figure 1 and give a reduced contribution and at 99km/h no contribution at all to the lift of the wing. Negative gusts however still fully reduce CL as usual. Therefore with turbulences present, the nice total climbing rate of 1.80m/s at 100km/h for the smooth thermal is dramatically reduced to 1.20m/s only, a loss of 33 per cent. The only way out of this is to circle some 10km/h faster. In this case a climbing rate of about 1.40m/s is attained at a speed of 110km/h. So, still a loss of 22 per cent due to the turbulence.
>
> To get a beneficial effect out of random air gusts, a pilot may try to fly with an a just behind the step in the CL-a curve where positive gusts push the sailplane up and negative gusts have no effect. Figure 3 promises a nice climbing rate when circling with a speed of say 96km/h. Some pilots seem succesful in doing so, however the sailplane may be rather difficult to control and high drag may deminish this special effect substantially.
>
> Other calculations with the routine show that having a dip in the lift curve is still worse than a flat step, but some earlier high-performance sailplanes have that. A small positive gradient in the step area, as suggested by Loek Boermans, improves climbing rate with turbulences present quite considerably, so this looks the way to go in the design process of new wing profiles. Just now, some new sailplanes having this idea are coming on the market.
>
>
>
>
> Figure 3: Reduced climbing rate in a turbulent thermal
>
> It should be mentioned that the above calculations were performed assuming that the effects of the flat part in the liftcurve occur at the same moment over the total span of the wing. This not the case however due to the random distribution of turbulences in size and in space and because of different wing profiles and Re-numbers in the spanwise direction of the wing. Therefore the results of the above calculations maybe somewhat exaggerated for the practical situation.
>
> In conclusion
>
> Recent optimisations of wing profiles with flaps have resulted in CL-a curves with a more or less ‘flat part’ in the lower speed range. Because of this, landing behaviour and climbing performance in turbulent thermals are somewhat disappointing.
>
> This article shows that pilots can partly take care of these drawbacks by flying with a (angle of attack) well in front of the ‘step’ in the CL- a curve.
>
> The main actions to achieve that are:
>
> a. When trying to make perfect landings, approach speeds should be a little higher than usual and speed brakes gently applied rather than pitch to finely control the descent rate in rounding off just before touching the ground.
> b. When optimising total climbing rate in turbulent thermals, flying speed and bank angle should be higher than a pilot would usually prefer.
> c. Thermals should be entered with redundant speed to avoid that the sudden upflow encountered pushes a into the step area.
>
> This study shows that slightly modifying the CL-a curve with a positive gradient in the step area will improve sailplane performance substantially. A better control of the descent rate during landing will then be possible and an increase in average climbing rate of some 20 per cent may be expected when circling in turbulent thermals. I understand that only minor modifications of the wing profile are necessary to obtain these improvements without a penalty in the high speed range of the sailplane (as suggested by Prof. Ir. Loek Boermans).
>
> Acknowledgement
>
> I would like to thank my friends for the inspiring discussions about the step in the CL-a curve of modern sailplanes – especially my son Ronald and Loek Boermans, both of whom gave in their own specific way of practical experience and theoretical knowledge, a substantial support in the realisation of this work. Additionally the books and articles of John Anderson, Helmut Reichmann, Fred Thomas, Loek Boermans and others and many articles found on the internet were also quite inspiring.
>
> ir. K.P. Termaat
> Arnhem, NL
> 20 oct. 2011

[Hank Nixon]

On Wednesday, February 24, 2021 at 7:05:49 PM UTC-5, wrote:
> The chief pilot at the airport I fly from had a different explanation for my genital bounces.
> He said "It is you." I always suspected...
>
Genital bounces!!!
Like banging your balls on the ground?
Funniest thing I've read in quite a while.
UH

[Jonathan St. Cloud]

Same reason I can't ride a horse. They are just too big. I really don't understand what is so funny, every damn time I stir the stick...
On Thursday, February 25, 2021 at 1:01:46 PM UTC-8, Hank Nixon wrote:
> On Wednesday, February 24, 2021 at 7:05:49 PM UTC-5, wrote:
> > The chief pilot at the airport I fly from had a different explanation for my genital bounces.
> > He said "It is you." I always suspected...
> >
> Genital bounces!!!
> Like banging your balls on the ground?
> Funniest thing I've read in quite a while.
> UH

[Nicholas Kennedy]

In my 1989 edition of "The Joy Of Soaring"
Its got alot of wonderful illustrations.
On page 125 It graphically shows the effect of wind gradient during the turn from base to final, where so many accidents occur.
The cause of these accidents? Airspeed to low.
On page 126 it shows the effects, on a gliders glide path and airspeed, of a thermal breaking off the airport and drifting with the wind in to the final approach leg.
Cause of these accidents from glider landing short, too low airspeed.
I Don't have much experience flying east of the Mississippi, some but not much.
All I know is in the West, landing anytime after noon, IMHO, This Yellow Triangle speed is a proven recipe for disaster. It's been proven many many, many times to be be WAY to low.
Ok If you HAVE TO land in a 600' long field so be it.
But 99.8% of the time I'm landing on 5-7000'+ of asphalt.
Have a look at the guy being lowered out of his ASK 21 in Germany, In a new thread.
I bet he could have used more airspeed.
I'm not trying to cause any trouble, just address a long term problem that we have had since day 1.
And don't get high on the tug either.
Fly safe in 2021
Nick
T

[Herbert kilian]

On Monday, March 1, 2021 at 10:04:02 AM UTC-6, wrote:
> In my 1989 edition of "The Joy Of Soaring"
> Its got alot of wonderful illustrations.
> On page 125 It graphically shows the effect of wind gradient during the turn from base to final, where so many accidents occur.
> The cause of these accidents? Airspeed to low.
> On page 126 it shows the effects, on a gliders glide path and airspeed, of a thermal breaking off the airport and drifting with the wind in to the final approach leg.
> Cause of these accidents from glider landing short, too low airspeed.
> I Don't have much experience flying east of the Mississippi, some but not much.
> All I know is in the West, landing anytime after noon, IMHO, This Yellow Triangle speed is a proven recipe for disaster. It's been proven many many, many times to be be WAY to low.
> Ok If you HAVE TO land in a 600' long field so be it.
> But 99.8% of the time I'm landing on 5-7000'+ of asphalt.
> Have a look at the guy being lowered out of his ASK 21 in Germany, In a new thread.
> I bet he could have used more airspeed.
> I'm not trying to cause any trouble, just address a long term problem that we have had since day 1.
> And don't get high on the tug either.
> Fly safe in 2021
> Nick
> T
Nick, it works both ways. On our about 1,800' long grass runway with a dropoff on one side and a busy road on the other, I never see landings that come short, never. However, many gliders touch down about half way down the field and stop not far from the busy road, obviously poor final speed control in that it's too high. The yellow triangle is a great min speed suggestion for short final. Maybe only Germans can appreciate it?
Herb

[Tango Eight]

On Monday, March 1, 2021 at 11:04:02 AM UTC-5, wrote:
> In my 1989 edition of "The Joy Of Soaring"
> Its got alot of wonderful illustrations.
> On page 125 It graphically shows the effect of wind gradient during the turn from base to final, where so many accidents occur.
> The cause of these accidents? Airspeed to low.
> On page 126 it shows the effects, on a gliders glide path and airspeed, of a thermal breaking off the airport and drifting with the wind in to the final approach leg.
> Cause of these accidents from glider landing short, too low airspeed.
> I Don't have much experience flying east of the Mississippi, some but not much.
> All I know is in the West, landing anytime after noon, IMHO, This Yellow Triangle speed is a proven recipe for disaster. It's been proven many many, many times to be be WAY to low.
> Ok If you HAVE TO land in a 600' long field so be it.
> But 99.8% of the time I'm landing on 5-7000'+ of asphalt.
> Have a look at the guy being lowered out of his ASK 21 in Germany, In a new thread.
> I bet he could have used more airspeed.
> I'm not trying to cause any trouble, just address a long term problem that we have had since day 1.
> And don't get high on the tug either.
> Fly safe in 2021
> Nick
> T

Aw geeze. The yellow triangle is an instrument marking with a specific purpose. It is the "proven recipe" for a minimum energy approach speed in still air. That's all.

Spinning in off base to final turn has as much to do with bad pattern planning as anything else, and the usual reason the speed is low, nose is high, turn is skidded is because the pattern sucked. Aiming for a base / final turn at 1/3 - 1/2 mile, 300 agl (or higher, as conditions warrant) solves many problems. There are plenty of conditions in which significant extra speed is warranted, but it isn't *all* conditions.

T8

[Ventus_a]

On Monday, March 1, 2021 at 11:04:02 AM UTC-5, wrote:
In my 1989 edition of "The Joy Of Soaring"
Its got alot of wonderful illustrations.
On page 125 It graphically shows the effect of wind gradient during the turn from base to final, where so many accidents occur.
The cause of these accidents? Airspeed to low.
On page 126 it shows the effects, on a gliders glide path and airspeed, of a thermal breaking off the airport and drifting with the wind in to the final approach leg.
Cause of these accidents from glider landing short, too low airspeed.
I Don't have much experience flying east of the Mississippi, some but not much.
All I know is in the West, landing anytime after noon, IMHO, This Yellow Triangle speed is a proven recipe for disaster. It's been proven many many, many times to be be WAY to low.
Ok If you HAVE TO land in a 600' long field so be it.
But 99.8% of the time I'm landing on 5-7000'+ of asphalt.
Have a look at the guy being lowered out of his ASK 21 in Germany, In a new thread.
I bet he could have used more airspeed.
I'm not trying to cause any trouble, just address a long term problem that we have had since day 1.
And don't get high on the tug either.
Fly safe in 2021
Nick
T

Aw geeze. The yellow triangle is an instrument marking with a specific purpose. It is the "proven recipe" for a minimum energy approach speed in still air. That's all.

Spinning in off base to final turn has as much to do with bad pattern planning as anything else, and the usual reason the speed is low, nose is high, turn is skidded is because the pattern sucked. Aiming for a base / final turn at 1/3 - 1/2 mile, 300 agl (or higher, as conditions warrant) solves many problems. There are plenty of conditions in which significant extra speed is warranted, but it isn't *all* conditions.

T8

+1 Evan has nailed it