Question about spoilers and pitch stability [Archive]

Evan Ludeman[_4_]

February 1st 13, 05:09 PM

On Thursday, January 31, 2013 11:42:53 PM UTC-5, Larry Suter wrote:
> In his article on avoiding PIO in Grob 103's,
>
>
>
> http://www.soaringsafety.org/pilots/ic8.htm
>
>
>
> Dean Carswell writes, "the more the airbrakes are closed, the less pitch stability the Grob will have, making a PIO more likely if otherwise mishandled"
>
>
>
> Is there a simple explanation why opening the spoilers increases the pitch stability? Does it somehow move the center of lift further aft? I believe increasing the separation between the cg and center of lift is the classic way to increase pitch stability.
>
>
>
> And if that's how it works, why does the center of lift move aft? I would guess spoilers destroy the lift downwind of their location, causing the center of lift to move forward.....
>
>
>
> Thanks,
>
> Larry

The classic problem here isn't a PIO the way we normally understand it.

The incident starts with trying to "land" on the nosewheel. The normal setup involves the pilot trying to land at 70 kts or so with spoilers closed. Landing long as in Dean's article, or perhaps getting sink in the pattern and getting a little panicky and diving for the runway with brakes closed. The nose wheel touches first and the AOA increases as the rest of the glider momentarily descends, usually resulting in some main wheel contact (usually modest on first contact). The glider, now at about 69.9 kts, with increased AOA, takes flight again. This happens *really* fast. Inertia has probably caused the control stick to move forward during this rotation, so the glider launches, noses over and hits much harder on nose / main / tail the 2nd time and it will lather rinse and repeat until the process is interrupted. If you see this happen, you will not soon forget it!

I'm familiar with a couple such incidents. In all cases I know of, it involved trying to land hot with spoilers closed. In all cases, it starts with nose wheel contact. If it ends well, it's because the pilot managed to stabilize the glider in flight, open the spoilers, achieve a better landing attitude and complete the landing normally.

Opening the spoilers increases the AOA necessary to fly at any given speed, so it decreases the chances of making first contact on the nose wheel, which is obviously to be prevented.

Evan Ludeman / T8

kirk.stant

February 1st 13, 05:57 PM

This may border on "scab picking", but I wonder how many pilots who have been involved in G-103 PIBs had initial training in 2-33s.

I know of one incident of a pilot trained in 2-33s and 1-26s, transitioned to our club G-102 (retract gear taildragger version), and proceeded to drag the nose via full forward stick after a hot touchdown.

Law of Primacy?

Just sayin'...

Kirk
66

[email protected]

February 1st 13, 06:02 PM

On Friday, February 1, 2013 12:57:09 PM UTC-5, kirk.stant wrote:
> This may border on "scab picking", but I wonder how many pilots who have been involved in G-103 PIBs had initial training in 2-33s. I know of one incident of a pilot trained in 2-33s and 1-26s, transitioned to our club G-102 (retract gear taildragger version), and proceeded to drag the nose via full forward stick after a hot touchdown. Law of Primacy? Just sayin'... Kirk 66

I teach the same principles descibed above when teaching in the 2-33. Transition to the ASK-21 is a non issue because the habits started right.
UH

Bill D

February 1st 13, 06:15 PM

On Friday, February 1, 2013 10:09:41 AM UTC-7, Evan Ludeman wrote:
> On Thursday, January 31, 2013 11:42:53 PM UTC-5, Larry Suter wrote:
>
> > In his article on avoiding PIO in Grob 103's,
>
> >
>
> >
>
> >
>
> > http://www.soaringsafety.org/pilots/ic8.htm
>
> >
>
> >
>
> >
>
> > Dean Carswell writes, "the more the airbrakes are closed, the less pitch stability the Grob will have, making a PIO more likely if otherwise mishandled"
>
> >
>
> >
>
> >
>
> > Is there a simple explanation why opening the spoilers increases the pitch stability? Does it somehow move the center of lift further aft? I believe increasing the separation between the cg and center of lift is the classic way to increase pitch stability.
>
> >
>
> >
>
> >
>
> > And if that's how it works, why does the center of lift move aft? I would guess spoilers destroy the lift downwind of their location, causing the center of lift to move forward.....
>
> >
>
> >
>
> >
>
> > Thanks,
>
> >
>
> > Larry
>
>
>
> The classic problem here isn't a PIO the way we normally understand it.
>
>
>
> The incident starts with trying to "land" on the nosewheel. The normal setup involves the pilot trying to land at 70 kts or so with spoilers closed.. Landing long as in Dean's article, or perhaps getting sink in the pattern and getting a little panicky and diving for the runway with brakes closed.. The nose wheel touches first and the AOA increases as the rest of the glider momentarily descends, usually resulting in some main wheel contact (usually modest on first contact). The glider, now at about 69.9 kts, with increased AOA, takes flight again. This happens *really* fast. Inertia has probably caused the control stick to move forward during this rotation, so the glider launches, noses over and hits much harder on nose / main / tail the 2nd time and it will lather rinse and repeat until the process is interrupted. If you see this happen, you will not soon forget it!
>
>
>
> I'm familiar with a couple such incidents. In all cases I know of, it involved trying to land hot with spoilers closed. In all cases, it starts with nose wheel contact. If it ends well, it's because the pilot managed to stabilize the glider in flight, open the spoilers, achieve a better landing attitude and complete the landing normally.
>
>
>
> Opening the spoilers increases the AOA necessary to fly at any given speed, so it decreases the chances of making first contact on the nose wheel, which is obviously to be prevented.
>
>
>
> Evan Ludeman / T8

Evan, that's an excellent explanation of the problem. I would only add that I'm unaware of any aircraft which gracefully tolerates nose-wheel-first runway contact.

I've seen a couple of these accidents and you're right, they were ugly and I won't forget them. One of them seemed to be a case of the pilot realizing too late the landing was going to be dangerously long, panicking, then trying to stop the glider by shoving an imaginary skid into the runway - likely negative transfer from his primary trainer.

BTW, manufacturers put a yellow triangle on the airspeed indicator to suggest a safe airspeed on short approach. Airplane guys call this Vref. If a Grob 103 is slowed to just above the yellow triangle on short final (not pattern) the 'arrival' is guaranteed to be on the main wheel.

BobW

February 2nd 13, 02:40 AM

On 2/1/2013 11:02 AM, wrote:
> On Friday, February 1, 2013 12:57:09 PM UTC-5, kirk.stant wrote:
>> This may border on "scab picking", but I wonder how many pilots who have
>> been involved in G-103 PIBs had initial training in 2-33s. I know of one
>> incident of a pilot trained in 2-33s and 1-26s, transitioned to our club
>> G-102 (retract gear taildragger version), and proceeded to drag the nose
>> via full forward stick after a hot touchdown. Law of Primacy? Just
>> sayin'... Kirk 66
>
> I teach the same principles descibed above when teaching in the 2-33.
> Transition to the ASK-21 is a non issue because the habits started right.
> UH
>
Apologizing for contributing to thread drift, but Kirk's mental exercise seems
a good entry for a "related safety bull session"...

The particular pitfall of "landing hot" was the only thing I ever thought my
basic training - done entirely in a 2-33 - might've been lacking, as in,
perhaps off-the-mark/incomplete. Natcherly I'd been taught to dial in "proper
approach speed/target point" and land accordionly...and doing so worked well
in both the 2-33 and subsequently in 1-26s.

But when I "advanced" to practicing short-field landings in the 2-33 on grass
- with its widely varying drag wet/dry - I soon encountered a "slightly
snaking" rollout when I used full forward stick as part of braking to a halt
as rapidly as possible (which of course was part of the exercise). My
instructor not being there that day, I sussed out the reason on my own,
concluding that - depending on the magnitude of the drag produced by the
forward-of-the-main-wheel skid - my nose-dragging 2-33 could in effect become
(briefly) a tail-dragger in ground stability terms...i.e. CG behind the
primary rotational drag point (skid), should I somehow get sideways.
Definitely food for thought for this newbie...

The next summer, probably having grown somewhat ignorantly complacent in my
1-26, I recall being distinctly surprised when the first sound I heard during
an "only slightly hot" paved runway landing, was the sound of metal scraping.
My first "thought" was I'd a flat tire, but the reality was I was fast enough
to put the skid plate below the level of the tire. (Duh!)

Those two "minor learning experiences" convinced me of the soundness of
landing as slowly as safe under existing conditions being a generally good
pattern policy...regardless of ship type/configuration. Never found any
reason(s) to change that thinking in the 30+ years since.

Different ships/configurations will react differently when landed
gracelessly/thoughtlessly fast/etc. And it's usually not good for either ship
or pilot to be surprised under such circumstances...

Bob W.

Jeremy Keeling

February 2nd 13, 02:23 PM

At 04:42 01 February 2013, Larry Suter wrote:
>In his article on avoiding PIO in Grob 103's,
>
>http://www.soaringsafety.org/pilots/ic8.htm
>
>Dean Carswell writes, "the more the airbrakes are closed, the less pitch
>st=
>ability the Grob will have, making a PIO more likely if otherwise
>mishandle=
>d"
>
>Is there a simple explanation why opening the spoilers increases the
pitch
>=
>stability? Does it somehow move the center of lift further aft? I believe
>i=
>ncreasing the separation between the cg and center of lift is the classic
>w=
>ay to increase pitch stability.=20
>
>And if that's how it works, why does the center of lift move aft? I would
>g=
>uess spoilers destroy the lift downwind of their location, causing the
>cent=
>er of lift to move forward.....
>
>Thanks,
>Larry
>

Hi Larry,

There is indeed a simple explanation as to why opening airbrakes increases
stability in pitch. It is to do with drag.

As I assume you are aware, the reason that the airspeed increases when you
'push the nose down' is because a larger component of the lift force
generated by the wings essentially points in the direction of the gliders
horizontal motion. Obviously the opposite happens when you 'raise the
nose', and again obviously the glider stops accelerating when the sum of
the forces acting on the glider is zero (drag increases as the glider
accelerates).
Clearly the total drag at a given airspeed is greater when the airbrakes
are open than when they are closed. If a glider is experiencing a
particular 'amount' of drag at a given airspeed with the brakes open, then
the same glider with the brakes closed will obviously have to fly at a
greater speed in order to experience the same amount of drag.
All of what I've just written is completely and utterly obvious, and I'm
sorry if I've insulted your intelligence.

However what I'm getting at is that, essentially, if you sit in a glider
and move the stick forward by a certain amount, the glider with the
airbrakes open is in some (rather handwavy) sense already 'closer' to being
in equilibrium (or closer to the constant speed at which it will fly given
the amount of forward stick) than the same glider with closed airbrakes.
In other (probably rather clearer) words the glider with airbrakes open
will not accelerate at the same rate, or for as long, as the glider with
closed airbrakes, and will therefore have a lower speed (remember we are
talking about moving the stick forward by a given amount).
Basically what this results in is, again as you know, the glider feeling
less responsive in pitch if the airbrakes are open - you have to push
forward more with the brakes open in order to accelerate at the same rate
as with the brakes closed, and if you want to fly at a given airspeed the
nose will be lower with airbrakes open than if they were closed. This is
why the glider is more stable in pitch, or if you like less responsive in
pitch, when the airbrakes are open.

I'm not entirely sure any of that made sense, but I hope it helps!

Best,
JK.

Martin Gregorie[_5_]

February 2nd 13, 03:12 PM

On Thu, 31 Jan 2013 20:42:53 -0800, Larry Suter wrote:

> In his article on avoiding PIO in Grob 103's,
>
> http://www.soaringsafety.org/pilots/ic8.htm
>
> Dean Carswell writes, "the more the airbrakes are closed, the less pitch
> stability the Grob will have, making a PIO more likely if otherwise
> mishandled"
>
I've not noticed that a Grob G.103a is any less stable in pitch than, say
an ASK.21 or a Puchacz regardless of airbrake setting, but all three do
have different responses to *opening* the brakes. This might cause an ab
initio or a new solo pilot some problems when moving between the types.

- the ASK.21 has almost no attitude change and not much airspeed change
when the brakes are opened

- the Puchacz has very powerful, speed-limiting brakes and a tendency to
pitch up and so will show a noticable drop in airspeed when they are
opened. In consequence you soon learn to push the stick forward as you
open the brakes.

- the Grob G.103a has the opposite reaction: when you open the airbrakes
it has a tendency to drop its nose and accelerate. I had speed control
issues when I first flew one (pre-solo). So, at altitude we trimmed it
to 60 kts hands off and, still hands off, opened the brakes fully. The
glider lowered its nose and accelerated to almost 70 kts. Closing the
brakes caused the nose to rise and the airspeed to settle back to 60
kts. After this demo I took note and found its speed control much
easier to manage.

I do wonder, though, if opening the brakes suddenly on a G.103 after a
too fast approach followed by a not fully held-off landing could tap the
front wheel on the runway. This could easily start a PIO in the hands of
an inexperienced pilot. However if this is the case I'd say the PIO was
due to the fast approach, lack of properly held-off landing and
inexperience on type for the PIO rather than any lack of pitch stability.

not forgetting that the G103 brakes do have a tendency to snatch
open, especially with the airspeed above 60 kts.

My club's fleet contains all three types. My first 20 launches were
equally split between the ASK21 and G103a. I first met the Puchacz on my
21st launch. All these were winch launches in case you're wondering.

--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Mike the Strike

February 2nd 13, 05:12 PM

I witnessed a PIO by a Grob 103 at our club a couple of weeks ago - it resulted from an attempted landing at too high a speed with first contact by the nosewheel and, as far as I remember, either no or very little applied airbrake.

One key point I learned decades ago is to first deploy airbrakes on downwind, checking visually that they are both deployed. If there is a malfunction or sudden deployment, this give you time and altitude to deal with it. Then, you keep them deployed, modulating them as necessary to control glide slope. The same instructor who taught me this also advised that I keep a bit more speed until on final. This approach seems to work for me and I haven't had a seriously bad landing in thirty years - even on land-outs.

Mike

kirk.stant

February 2nd 13, 05:26 PM

On Saturday, February 2, 2013 11:12:20 AM UTC-6, Mike the Strike wrote:
> I witnessed a PIO by a Grob 103 at our club a couple of weeks ago - it resulted from an attempted landing at too high a speed with first contact by the nosewheel and, as far as I remember, either no or very little applied airbrake.

Mike, could it have been a practice "no-spoiler" approach? I know that the instructors at our club teach this in our Grob-103 - which I personally think is dumb as a sack of hammers.

Again, a legacy of 2-33 training? I remember having to demonstrate no-spoiler, slip to a landing in 2-33s, which are really not a big deal - but in a glass ship - a real, no spoiler to touchdown approach is a serious emergency unless you have a really long runway, and not to be taken lightly. Done incorrectly, you run the risk of being too low on final with no way of making the runway, or way too fast over the threshold with no possibility of going around. Manageable if smart abort criteria are briefed, but can result in a broken glider if not done carefully.

And, it's not even necessary in the PTS, which calls for demonstrating slips, not no-spoiler landings, I believe.

Kirk
66

Tony V

February 2nd 13, 05:53 PM

On 2/2/2013 12:26 PM, kirk.stant wrote:

>..... I remember having to demonstrate no-spoiler, slip to a landing in 2-33s, which are really not a big deal - but in a glass ship - a real, no spoiler to touchdown approach is a serious emergency unless you have a really long runway, and not to be taken lightly. Done incorrectly, you run the risk of being too low on final with no way of making the runway, or way too fast over the threshold with no possibility of going around. Manageable if smart abort criteria are briefed, but can result in a broken glider if not done carefully.
>
> And, it's not even necessary in the PTS, which calls for demonstrating slips, not no-spoiler landings, I believe.

http://www.faa.gov/training_testing/testing/airmen/test_standards/pilot/media/FAA-S-8081-22.pdf

R. TASK: SLIPS TO LANDING
REFERENCES: Soaring Flight Manual, Glider Flight Manual.
Objective. To determine that the applicant:
1. Exhibits knowledge of the elements related to forward, side,
and turning slips to landing, with and without the use of drag
devices.
2. Recognizes the situation where a slip should be used to land
in a desired area.

kirk.stant

February 2nd 13, 06:32 PM

I read the FAA PTS requirement as demonstrating the use of slips to steepen the approach, not as requiring landing without using drag devices.

Kirk
66

Bill D

February 2nd 13, 07:11 PM

On Saturday, February 2, 2013 11:32:46 AM UTC-7, kirk.stant wrote:
> I read the FAA PTS requirement as demonstrating the use of slips to steepen the approach, not as requiring landing without using drag devices.
>
>
>
> Kirk
>
> 66

Yep, Most DPE's have figured out a 'slip to landing' is a 2-33 specific maneuver and it's insane to try it in a slippery glass glider. I know of a DG505 which was totaled attempting it.

Martin Gregorie[_5_]

February 2nd 13, 11:47 PM

On Sat, 02 Feb 2013 09:12:20 -0800, Mike the Strike wrote:

> I witnessed a PIO by a Grob 103 at our club a couple of weeks ago - it
> resulted from an attempted landing at too high a speed with first
> contact by the nosewheel and, as far as I remember, either no or very
> little applied airbrake.
>
Unless I misunderstood you, first contact by the nosewheel as you
described indicates a failure to round out. In which case:

(1) its pilot error pure and simple

(2) you'd expect that it would break something due to either a
PIO or mashing the nosewheel up into the fuselage.

I'd expect the same result regardless of whether the glider was a G.103
or an ASK-21.

--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Gavin Short[_2_]

February 3rd 13, 01:07 PM

waremark

February 3rd 13, 05:50 PM

In Europe standard practice is to teach fully held off landings, touching down just a fraction over stall speed in a 2 point attitude. Do the incidents referred to result from teaching a 'flown on' landing?

The fully held off approach results in far less energy to do damage if you hit a bump in an off field landing, or to generate a bounce. My flight manuals recommend it. I think there is debate about which technique involves greater risk of a PIO.

Bill D

February 3rd 13, 06:37 PM

On Sunday, February 3, 2013 10:50:16 AM UTC-7, waremark wrote:
> In Europe standard practice is to teach fully held off landings, touching down just a fraction over stall speed in a 2 point attitude. Do the incidents referred to result from teaching a 'flown on' landing?
>
>
>
> The fully held off approach results in far less energy to do damage if you hit a bump in an off field landing, or to generate a bounce. My flight manuals recommend it. I think there is debate about which technique involves greater risk of a PIO.

You are absolutely correct. A 'held off' landing with a two point contact is the way to do it. Even contact with the tail wheel a few cm lower than the main wheel won't hurt anything.

Where I've seen the too-fast landings become a habit is where a well intentioned instructor rants on endlessly about keeping a high airspeed in the pattern without defining "pattern" and without ever explaining the meaning of the yellow triangle on the ASI. Students and others may take this to mean the ideal airspeed on final approach is in Mach numbers. This has resulted in many overshoot accidents in addition to the Grob "PIO/PIB".

Final approach is where a pilot transitions from the pattern and sets up the touchdown. Short-final "over-the-fence" airspeed should be just above the yellow triangle which guarantees the correct touchdown attitude.

For those who haven't read their manuals, the yellow triangle marks the manufactures recommended minimum approach airspeed at maximum flying weight but without water ballast. That means you'll have just enough energy for a flare and hold-off.

son_of_flubber

February 3rd 13, 07:00 PM

On Sunday, February 3, 2013 12:50:16 PM UTC-5, waremark wrote:
> In Europe standard practice is to teach fully held off landings, touching down just a fraction over stall speed in a 2 point attitude.

I have a question. Assume strong turbulence such that a 70 knot speed in the pattern is needed to obtain control authority. Once the flair is correctly executed and the glider is flying level in ground effect 6-12" off the ground, does the glider become less vulnerable to turbulence because it is in ground effect, and therefore would control authority be maintained as the glider slows to stall speed?

I understand that once the pilot is flying level 6-12" above the runway at 70 knots (at that point the plane has zero vertical velocity) that he can very gradually lower the center wheel to the runway without 'bouncing". The advantage of lowering the wheel to the runway is two-fold. 1)The turbulence can no longer slam the wheel down onto the runway (it is already there). 2)The pilot can use the wheel brake and full spoilers to stop the glider as quickly as possible, thus minimizing runout and minimizing the time exposed to turbulence near the ground.

If the pilot chooses to reduce speed from 70 knots to stall speed with the wheel off the ground, he has a longer runout, the possibility of being slammed to the pavement by a downdraft, and a longer period of time vulnerable to unpredictable turbulence.

In short, I understand that there are advantages to "landing hot". I also understand that if a pilot touches down with too much vertical velocity, that he will 'bounce'. If the vertical velocity is low enough, you will not 'bounce' not matter how high the horizontal velocity.

(I understand that the 'bounce' is caused by an increase in AOA, caused by rotation around the center wheel, caused by a CG behind the center wheel, too much vertical velocity, and therefore too much momentum pressing the rear wheel down and increasing the AOA. And of course enough horizontal speed such that the increased AOA causes the wheel to leave the ground.)

I want to reinterate that I'm asking a question and just stating my very possilbly fractured understanding. This topic is of great interest to me because I'm in the process of transitioning to a glass ship and I have a lot of training time in SGS (There seems to be some vague correlation between transitioning from SGS to glass and landing related PIOs.)

Bill D

February 3rd 13, 07:54 PM

On Sunday, February 3, 2013 12:00:41 PM UTC-7, son_of_flubber wrote:
> On Sunday, February 3, 2013 12:50:16 PM UTC-5, waremark wrote:
>
> > In Europe standard practice is to teach fully held off landings, touching down just a fraction over stall speed in a 2 point attitude.
>
>
>
> I have a question. Assume strong turbulence such that a 70 knot speed in the pattern is needed to obtain control authority. Once the flair is correctly executed and the glider is flying level in ground effect 6-12" off the ground, does the glider become less vulnerable to turbulence because it is in ground effect, and therefore would control authority be maintained as the glider slows to stall speed?
>
>
>
> I understand that once the pilot is flying level 6-12" above the runway at 70 knots (at that point the plane has zero vertical velocity) that he can very gradually lower the center wheel to the runway without 'bouncing". The advantage of lowering the wheel to the runway is two-fold. 1)The turbulence can no longer slam the wheel down onto the runway (it is already there). 2)The pilot can use the wheel brake and full spoilers to stop the glider as quickly as possible, thus minimizing runout and minimizing the time exposed to turbulence near the ground.
>
>
>
> If the pilot chooses to reduce speed from 70 knots to stall speed with the wheel off the ground, he has a longer runout, the possibility of being slammed to the pavement by a downdraft, and a longer period of time vulnerable to unpredictable turbulence.
>
>
>
> In short, I understand that there are advantages to "landing hot". I also understand that if a pilot touches down with too much vertical velocity, that he will 'bounce'. If the vertical velocity is low enough, you will not 'bounce' not matter how high the horizontal velocity.
>
>
>
> (I understand that the 'bounce' is caused by an increase in AOA, caused by rotation around the center wheel, caused by a CG behind the center wheel, too much vertical velocity, and therefore too much momentum pressing the rear wheel down and increasing the AOA. And of course enough horizontal speed such that the increased AOA causes the wheel to leave the ground.)
>
>
>
> I want to reinterate that I'm asking a question and just stating my very possilbly fractured understanding. This topic is of great interest to me because I'm in the process of transitioning to a glass ship and I have a lot of training time in SGS (There seems to be some vague correlation between transitioning from SGS to glass and landing related PIOs.)

I hope our European friends jump in too. The following is based on years of experience at Boulder in wildly turbulent west wind rotors.

I don't think a "hot" landing has any value. It just means you'll float down the runway in ground effect while you are vulnerable to those gusts and with enough energy to seriously damage the glider. Better to get it down and stopped and that can't happen until the glider slows.

With a little practice, you'll find a glider can be slowed sharply with a few seconds of full airbrake while raising the nose to maintain the glide path just before entering ground effect. This can be done on short final to hit the recommended approach speed allowing the pattern itself to be flown at any speed the pilot deems safe for the gustiness.

This discussion is about nose wheel gliders where "hot" landings put the glider on the nose wheel first.

Roel Baardman

February 3rd 13, 08:12 PM

As far as I know the ASK-21 has such geometry that it will not take off with the two front wheels on the ground. This would imply that it produces no lift upwards, and that only making the
nosewheel touch the ground is very hard.

I question the existence of downdrafts during flaring... Where would the air in the downdraft go when you are already so low?

Martin Gregorie[_5_]

February 3rd 13, 08:24 PM

On Sun, 03 Feb 2013 11:00:41 -0800, son_of_flubber wrote:

> I have a question. Assume strong turbulence such that a 70 knot speed
> in the pattern is needed to obtain control authority. Once the flair is
> correctly executed and the glider is flying level in ground effect 6-12"
> off the ground, does the glider become less vulnerable to turbulence
> because it is in ground effect, and therefore would control authority be
> maintained as the glider slows to stall speed?
>
I think the topology of the field has a bearing on it: if the ground
under the base and final leg are likely to cause turbulence then you need
a higher airspeed to punch through it, but airfields generally have flat
surfaces so there shouldn't be nearly so much turbulence a couple of feet
above the runway.

I've never been taught to do anything other than a fully held-off landing.

On my home field, in the very flat Cambridgeshire countryside I don't
think I've ever used or needed more than 60 kts on finals. Its at 52
11.052N, 0 6.655W in case you're interested and the field is set up for
an easterly in the picture.

OTOH at The Mynd where, when the wind is W to SW, the approach is down
the side of a fairly steep hill with a 50-60 foot gully to land on a
slight grassy upslope, you're told to plan for an approach at at least 60
kts. When I last flew there there was enough westerly for the ridge in
front of the club house to be working nicely, so I'd guess 20 +/- 5 kts.
That day I flew finals in my Libelle at 70kts, made a fully held-off
landing and was stopped no more than 500 ft from the threshhold. The hold-
off was no more turbulent than I'd expect at home.

If you have access to Google Earth, you'll find the Mynd at 52 31.153N, 2
52.501W. That should put your pointer pretty much where the launchpoint
that day. We were touching down pretty much N of the launchpoint on a
bearing of roughly 200 degrees and stopping somewhere between launch
point and clubhouse.

You can see the north edge of the mown field about 400 ft NE of there.
The bottom of the gully is the wavery white line another 400 feet further
to the NE and the club house is 1250 feet west of the launch point. The
curving, thin white line immediately west of the clubhouse is a walking
track along the top of the ridge. You can see the winch where it was that
day at the south end of the airfield, 3750 feet away and 70 feet higher.
If you dial GE back to oldest photo (31Dec1998) the low lighting gives a
much better idea of the shape of the gully and hill under the approach we
were using.

> I understand that once the pilot is flying level 6-12" above the runway
> at 70 knots (at that point the plane has zero vertical velocity) that he
> can very gradually lower the center wheel to the runway without
> 'bouncing".
>
I believe so, though as I said, I've never been taught that sort of
landing.

> If the pilot chooses to reduce speed from 70 knots to stall speed with
> the wheel off the ground, he has a longer runout,
>
I'd dispute that. You land slower and can still use full air and wheel
brakes so the ground run has to be shorter.

> the possibility of being slammed to the pavement by a downdraft,
>
Is that significant if you're only 1 2 feet up? You're well inside ground
effect and there's almost no distance to be accelerated downward, but
what do I know? I don't normally fly where there's rotor.

However, I expect to be at the Mynd toward the end of April so I may
learn more then.

> (I understand that the 'bounce' is caused by an increase in AOA, caused
> by rotation around the center wheel, caused by a CG behind the center
> wheel, too much vertical velocity, and therefore too much momentum
> pressing the rear wheel down and increasing the AOA. And of course
> enough horizontal speed such that the increased AOA causes the wheel to
> leave the ground.)
>
That applies to most single seaters but isn't true for the G.103, ASK-21
or Puchacz, which all have the CG in front of the main wheel with one or
two pilots in them. They would not rest on the nose wheel otherwise.

> I want to reinterate that I'm asking a question and just stating my very
> possilbly fractured understanding. This topic is of great interest to
> me because I'm in the process of transitioning to a glass ship and I
> have a lot of training time in SGS (There seems to be some vague
> correlation between transitioning from SGS to glass and landing related
> PIOs.)
>
Could that be related to reactions during roll-out after landing when you
want to use the brakes? In a 2-33 or anything with a nose skid you push
the stick forward to rub the skid on the ground for braking while in
gliders without a skid, i.e. all common glass, you tend to pull the stick
hard back when using the brakes to keep the nose off the floor and the
tail wheel on the ground for better directional stability. Of course, you
have to be slow enough to know you won't take off again when you pull on
the stick!

--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

BobW

February 3rd 13, 11:59 PM

On 2/3/2013 12:00 PM, son_of_flubber wrote:
> On Sunday, February 3, 2013 12:50:16 PM UTC-5, waremark wrote:
>> In Europe standard practice is to teach fully held off landings, touching
>> down just a fraction over stall speed in a 2 point attitude.
>
> I have a question. Assume strong turbulence such that a 70 knot speed in
> the pattern is needed to obtain control authority. Once the flare is
> correctly executed and the glider is flying level in ground effect 6-12"
> off the ground, does the glider become less vulnerable to turbulence
> because it is in ground effect, and therefore would control authority be
> maintained as the glider slows to stall speed?

My experience in the intermountain west of the US - based out of Boulder
(KBDU) 3 miles from the Rocky Mountain foothills - is (for discussional
purposes, ignore crosswinds for the moment) once in "the final float" with the
main wheel within inches of the ground, turbulence ceases to be a factor.

That's not to suggest I haven't been seriously concerned about it beCOMing a
factor, but the concern has never materialized. My experience includes 3
microbursts in the pattern, one of which was a crap shoot in terms of success
vs. life-endangering-crash. (I still get the shakes thinking about that
[2-pointed] one, years later.)

See below for brief discussion of round-out thoughts given the presence of
"beer-worthy" crosswinds...
- - - - - -

> I understand that once the pilot is flying level 6-12" above the runway at
> 70 knots (at that point the plane has zero vertical velocity) that he can
> very gradually lower the center wheel to the runway without 'bouncing".
> The advantage of lowering the wheel to the runway is two-fold. 1)The
> turbulence can no longer slam the wheel down onto the runway (it is already
> there). 2)The pilot can use the wheel brake and full spoilers to stop the
> glider as quickly as possible, thus minimizing runout and minimizing the
> time exposed to turbulence near the ground.
>
> If the pilot chooses to reduce speed from 70 knots to stall speed with the
> wheel off the ground, he has a longer runout, the possibility of being
> slammed to the pavement by a downdraft, and a longer period of time
> vulnerable to unpredictable turbulence.

"What Martin Gregorie said" regarding your "longer runout" positing. In any
event, in the absence of a persistent, strong crosswind my vote favors getting
rid of the energy where the plane is generally safest, i.e. in the air.
- - - - - -

> In short, I understand that there are advantages to "landing hot". I also
> understand that if a pilot touches down with too much vertical velocity,
> that he will 'bounce'. If the vertical velocity is low enough, you will
> not 'bounce' not matter how high the horizontal velocity.
>
> (I understand that the 'bounce' is caused by an increase in AOA, caused by
> rotation around the center wheel, caused by a CG behind the center wheel,
> too much vertical velocity, and therefore too much momentum pressing the
> rear wheel down and increasing the AOA. And of course enough horizontal
> speed such that the increased AOA causes the wheel to leave the ground.)

You seem to have a decent grasp of the dynamic issues (aero and CG-positional)
judging from the preceding brief summation. Strictly by way of FYI feedback...
- - - - - -

> I want to reiterate that I'm asking a question and just stating my very
> possibly fractured understanding. This topic is of great interest to me
> because I'm in the process of transitioning to a glass ship and I have a
> lot of training time in SGS. (There seems to be some vague correlation
> between transitioning from SGS to glass and landing related PIOs.)

Knowing very little about "typical pattern conditions" at your home field, but
having most of my time in the intermountain west - which tends to be prone to
"enthusiastic pattern conditions" year 'round - maybe an observation about
landing in the presence of persistently strong crosswinds isn't unwarranted.

Next to microbursts in the pattern, howling crosswinds are my next least
favorite pattern condition to have to deal with. While they (generally) tend
to "significantly decrease" in the vertical distance descended through during
the transition from final approach attitude to hold-off attitude, they don't
always, and the thought of landing on a narrow runway with little tolerance
for being off-center, while drifting sideways due to a crosswind overpowering
the pilot's/ship's abilities to compensate for drift has always left me cold.

Under such conditions, I've tended to favor - and had my best results from so
doing - run-on landings at the lowest speed conditions permit in the absence
of "significant drift". Working from memory, my highest-ever touchdown speed
was probably around 70 knots indicated in the presence of an estimated 25-30
knot direct crosswind. I aimed for the near end of the runway, flew in ground
effect until I was essentially out of rudder, briefly used into-wind aileron
to halt developing drift, and planted the main wheel. Then - because I could -
I went to negative flap to plant the tailwheel, and used full downwind rudder
and heavy braking to minimize the into-wind arc of the main wheel. Another
pilot in an unflapped St'd Cirrus landing minutes ahead of me
independently-opted/used essentially the same technique with very similar
(also successful) results.

We both subsequently concluded held-off landing attempts would likely have
ended anywhere from "considerably less gracefully" to "genuinely ugly-ly".

Bob W.

Terence Wilson[_2_]

February 4th 13, 03:43 AM

Opening the spoilers reduces the coefficient of lift. After the first 'bounce', which results in an increase in the angle of attack, the reduced CL dampens the lift vector and amplitude of the PIO.

son_of_flubber

February 4th 13, 05:44 PM

> On Sunday, February 3, 2013 12:00:41 PM UTC-7, son_of_flubber wrote:
> In short, I understand that there are advantages to "landing hot"....<

On Sunday, February 3, 2013 2:54:33 PM UTC-5, Bill D wrote:
> With a little practice, you'll find a glider can be slowed sharply with a few seconds of full airbrake while raising the nose to maintain the glide path just before entering ground effect. This can be done on short final to hit the recommended approach speed allowing the pattern itself to be flown at any speed the pilot deems safe for the gustiness.<

Right, you want to dissipate the energy before entering ground effect.
I do this when high speed is called for in the pattern, but not needed for the final 20 feet of descent. For example, when there is the possibility of strong sink on downwind, and you need the extra energy to use if you hit the sink to avoid landing short.

On Sunday, February 3, 2013 3:24:02 PM UTC-5, Martin Gregorie wrote:
>OTOH at The Mynd ...
>> That day I flew finals in my Libelle at 70kts, made a fully held-off
> landing and was stopped no more than 500 ft from the threshold.

My home field is known for strong sink on final, strong crosswinds (one narrow runway), erratic surrounding topography (not flat), and in wave season, rotor in the pattern. (We like it because the wave and the ridges are very close to the airport!)

>The hold-off was no more turbulent than I'd expect at home.

Granted, it can be very turbulent on downwind, but relatively calm on the ground. (After reading all the great responses) I can see that even if you have plenty of runway in front of you, you should anticipate calm and slow down before you enter ground effect. Even if there turns out to be turbulence at ground, you might hit it farther down the runway after you have necessarily slowed down. Best bet is to minimize the time spent rolling on the runway when you are exposed to possible turbulence, and also minimize the energy that you carry close to the ground should something go awry.

son_of_flubber wrote:> If the pilot chooses to reduce speed from 70 knots to stall speed with the wheel off the ground, he has a longer runout,

Martin Gregorie wrote:
> I'd dispute that. You land slower and can still use full air and wheel
> brakes so the ground run has to be shorter.

But you've floated to the far end of the runway before you can apply the wheel brake and full spoiler. When I put the wheel on the ground at 70kts, I can sooner open full spoilers and sooner apply wheel brake (balanced with the elevator to maintain appropriate AOA). From a given point on the runway and given speed, the pilot who applies full spoiler and wheelbrake sooner will stop shorter. If you try to lose speed from 70kts to stall, before let your wheel touch, you will be far down the runway before you stop. Granted, I've screwed up if I have 70kts in ground effect (unless there is a hellish crosswind or wind gradient). The question is only how to best bring the glider to a stop once that mistake has been made.

son_of_flubber wrote:> There seems to be some vague correlation between transitioning from SGS to glass and landing related PIOs.

Martin Gregorie wrote:
>> Could that be related to (ed. pilot) reactions during roll-out after landing(ed. ?)...

The SGS 2-33 has relatively low control authority and high stability. That makes it "easy to fly" in easy conditions. Early in my training, I transitioned from 2-33 to Blanik L-23. At the time I found the L-23 "extremely responsive and unstable" compared to the 2-33 (novice perspective ;). After flying the L-23 for a season, I transitioned to ASK-21 and found it another (albeit smaller) step up in responsiveness. I'm glad that I transitioned to the L-23 early before my stick and rudder habits became deeply ingrained. I'd expect that some pilots who had only flown SGS, might have PIO in a glass ship in a stressful situation, should they revert to their 2-33 formed habits and overdo control inputs. That's a possibility for me too, though I've not yet instinctively pushed the stick forward to put the skid down on a glider that does not have a skid! It could happen. I love the 2-33 for early confidence building before habits are formed, but I'm glad that I moved to the Blanik sooner rather than later. The 2-33 is just too easy to fly if you ever want to fly glass.

On Sunday, February 3, 2013 6:59:17 PM UTC-5, BobW wrote:
>...(ed.ignoring) crosswinds for the moment, once in "the final float" with the
>main wheel within inches of the ground, turbulence ceases to be a factor (ed.in my experience).
> That's not to suggest I haven't been seriously concerned about it beCOMing a
> factor, but the concern has never materialized.

It's possible to have a turbulence related incident close to the ground, but not probable. And the extra speed would only help you if the nasty gust happened to hit you before you (inevitably) slow down at the far end of the runway. The high energy will hurt you if something goes wrong. I can see that in lieu of a crosswind, and once you are sure of not landing short, that it is best to lose any excess energy before entering ground effect.

>My experience includes 3 microbursts in the pattern

This might give you the willies...
http://en.wikipedia.org/wiki/File:Microburstnasa.JPG

> the thought of landing on a narrow runway with little tolerance
> for being off-center, while drifting sideways due to a crosswind overpowering
> the pilot's/ship's abilities to compensate for drift has always left me cold.

Hopefully, there are no runway lights, cliffs, trees or parked vehicles.

> Under such conditions, I've tended to favor - and had my best results from so
> doing - run-on landings at the lowest speed conditions permit ...

>...my highest-ever touchdown speed was probably around 70 knots indicated in the presence of an estimated 25-30 knot direct crosswind. I aimed for the near end of the runway, flew in ground
> effect until I was essentially out of rudder, briefly used into-wind aileron
> to halt developing drift,

So you needed the speed and consequent control authority to quickly and precisely dip and undip the upwind wing to arrest the drift, and the energy gave you the needed rudder effectiveness (before it ran out). Did the speed do anything else for you?

> and planted the main wheel. Then - because I could -
> I went to negative flap to plant the tailwheel, and used full downwind rudder
> and heavy braking to minimize the into-wind arc of the main wheel.

So you planted the wheels well above stall speed, because you were forced to level the wing (to avoid ground loop) and you used the "tracking" of the wheels to arrest the drift? The tailwheel helped prevent weathervaning into the wind (a little).

Okay, I see the (only?) justified and correct use of 70kts IAS in ground effect. In lieu of a hellish crosswind, I was incorrect to enter ground effect at 70kts. Once I had made the runway in the turbulent and strong sink conditions, I should have shed the excess energy before I entered ground effect. However, once I had made the mistake of entering ground effect at 70kts, I was justified in putting the wheel down and applying full spoiler and wheel brake because that allowed me to stop with a comfortable margin of runway still in front of me; more runway than I would have had if I had floated in ground effect at 70kts until reaching stall speed. If (at 70kts) I had touched the wheel down with too much vertical speed, I might have executed the dreaded landing PIO. At best, I would have relaunched into stable ground effect and tried again.

Back to the original thread... Shedding excess speed before entering ground effect reduces the chance of landing PIO. Reducing vertical speed at touchdown reduces the chance of landing PIO. Holding off until reaching stall speed rules out the possibility of landing PIO. Should you screw up and "bounce"(once), your first goal is to return the glider to level flight, then land by correctly "holding off" (in lieu of a devilish crosswind). If you are running out of runway at that point, you might gradually put the wheel down before you reach stall speed, then open full spoilers and wheelbrake to stop, hopefully soon enough.

Thank you for your responses. It's very helpful to think this through sitting by the fire on a stormy day in winter. Plus it's fun to re-imagine those hairy landings from last summer.

BobW

February 4th 13, 08:07 PM

On 2/4/2013 10:44 AM, son_of_flubber wrote:
<Major snip>
>
> On Sunday, February 3, 2013 6:59:17 PM UTC-5, BobW wrote:
<Snip>

>> the thought of landing on a narrow runway with little tolerance for being
>> off-center, while drifting sideways due to a crosswind overpowering the
>> pilot's/ship's abilities to compensate for drift has always left me
>> cold.

>> Under such conditions, I've tended to favor - and had my best results
>> from so doing - run-on landings at the lowest speed conditions permit
>> ...my highest-ever touchdown speed was probably around 70 knots indicated
>> in the presence of an estimated 25-30 knot direct crosswind. I aimed for
>> the near end of the runway, flew in ground effect until I was essentially
>> out of rudder, briefly used into-wind aileron to halt developing drift,
>
> So you needed the speed and consequent control authority to quickly and
> precisely dip and undip the upwind wing to arrest the drift, and the energy
> gave you the needed rudder effectiveness (before it ran out). Did the
> speed do anything else for you?
We're likely talking nuance here, but the ways I think speed assists Joe Pilot
in the presence of a strong/steady crosswind are: a) reducing vector angles (&
hence fuselage to runway angle at touchdown); b) enhancing control authority
prior to touchdown.

>> and planted the main wheel. Then - because I could - I went to negative
>> flap to plant the tailwheel, and used full downwind rudder and heavy
>> braking to minimize the into-wind arc of the main wheel.
>
> So you planted the wheels well above stall speed, because you were forced
> to level the wing (to avoid ground loop) and you used the "tracking" of the
> wheels to arrest the drift?

Yep to all. It was a 15 meter bird, and the thought of wingtip-to-ground
contact above stalling speed terrifies me. Plant the main wheel before
drift/wingtip-to-ground contact become issues, establish the best total
plane-to-ground contact(s) possible ASAP, use controls as much as you
dare/must, brake enthusiastically.

>
> Back to the original thread... Shedding excess speed before entering ground
> effect reduces the chance of landing PIO.
Yup.

Reducing vertical speed at
> touchdown reduces the chance of landing PIO.
Yup.

Holding off until reaching
> stall speed rules out the possibility of landing PIO.
Yup...insufficient energy to do anything but bounce - not "flying bounce".

Should you screw up
> and "bounce"(once), your first goal is to return the glider to level
> flight, then land by correctly "holding off" (in lieu of a devilish
> crosswind).

(Presuming a "flying bounce"...) Yup. Stay away from trying to "fine tune"
rapid stick inputs in some new-to-you way in the event of a "flying bounce"
(which likely WILL alarm you first time it happens). Lots have tried the "fine
tuned/rapid stick motions" approach...with scant success. Locking the stick in
pitch during the arc of your flying bounce and waiting for ground arrival is
pretty much guaranteed to be a better choice.

If you are running out of runway at that point, you might
> gradually put the wheel down before you reach stall speed, then open full
> spoilers and wheelbrake to stop, hopefully soon enough.

Yup...but if this intentional "fine tuned" forward stick motion is botched
(easy to do in the absence of thought beforehand), the next arrival will
generally be even harder in the vertical velocity sense. A plan beforehand is
good!

Back to botched flare/drifting touchdowns in the presence of strong/persistent
crosswinds briefly...

One of my worst arrivals happened when I ballooned a flare (too high a
speed...the X-wind made me nervous), began drifting "way beyond my comfort
level", ballooned the 2nd flare (all the while downwind drift velocity
increasing), and dropped in from about 3 feet. It was UGLY. Amazingly it (same
15 meter ship) survived without damage, thanks largely to this happening on an
empty WW-II ramp (nothing to hit...well, other than the ramp, I mean [wry
chuckle]), but both main and tail tires squealed throughout most of the
resultant ground loop into the wind. Can't remember if squealing rubber or the
downwind wingtip skid dragging was louder. The lack of ship damage had zero to
do with pilot skill. I'd well over 1,000 hours in the ship at the time. The
worst of it was I attempted the 2-point landing (rather than a run-on one,
which is easy to do in that ship) only because it was a large, empty ramp.
What WAS I thinking?!?

At least I had the sense to do it with no peanut gallery...

Never again attempted a 2-point landing in the presence of a strong X-wind.

Bob W.

Martin Gregorie[_5_]

February 4th 13, 09:16 PM

On Mon, 04 Feb 2013 09:44:08 -0800, son_of_flubber wrote:

>>> That day I flew finals in my Libelle at 70kts, made a fully held-off
>> landing and was stopped no more than 500 ft from the threshold.
>
To clarify: I meant 500ft into the field, not 500 ft from the far end.
The headwind component combined with landing uphill stopped me quite a
lot shorter than I expected considering the Libelle's famously weak
airbrakes. I don't recall using the wheelbrakes much if at all.

> Martin Gregorie wrote:
>> I'd dispute that. You land slower and can still use full air and wheel
>> brakes so the ground run has to be shorter.
>
> But you've floated to the far end of the runway before you can apply the
> wheel brake and full spoiler.
>
If I saw that I was running out of space ahead and the end of the field
looked unpleasant I'd want the wheel on the floor as a preliminary to an
intentional ground-loop - the fastest way to stop. That involves putting
the upwind tip on the ground as a pivot and using forward stick to get
the tail off the ground so the glider swings round easier. It also
minimizes the chances of snapping the tailboom due to side loads from the
tail wheel hitting grass clumps etc. I haven't had to do that yet but a
friend did, landing in a very small field with a stone wall at the far
end.

--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Martin Gregorie[_5_]

February 4th 13, 09:22 PM

On Sun, 03 Feb 2013 19:43:06 -0800, Terence Wilson wrote:

> Opening the spoilers reduces the coefficient of lift. After the first
> 'bounce', which results in an increase in the angle of attack, the
> reduced CL dampens the lift vector and amplitude of the PIO.

It also pushes the Cd up, which always makes things less twitchy because
increased drag tends to damp out pitch changes.

--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

waremark

February 4th 13, 11:47 PM

On Monday, 4 February 2013 21:22:23 UTC, Martin Gregorie wrote:
> On Sun, 03 Feb 2013 19:43:06 -0800, Terence Wilson wrote:
>
>
>
> > Opening the spoilers reduces the coefficient of lift. After the first
>
> > 'bounce', which results in an increase in the angle of attack, the
>
> > reduced CL dampens the lift vector and amplitude of the PIO.
>
>
>
> It also pushes the Cd up, which always makes things less twitchy because
>
> increased drag tends to damp out pitch changes.
>
>
>
>
>
> --
>
> martin@ | Martin Gregorie
>
> gregorie. | Essex, UK
>
> org |

Touching down much above stall speed requires a very smooth landing area, otherwise you get thrown back up into the air by a bump. It would not work on my bumpy home field. What happens in a field landing if you have been trained to land at flying speed?

[email protected]

February 4th 13, 11:57 PM

On Monday, February 4, 2013 4:22:23 PM UTC-5, Martin Gregorie wrote:
> On Sun, 03 Feb 2013 19:43:06 -0800, Terence Wilson wrote: > Opening the spoilers reduces the coefficient of lift. After the first > 'bounce', which results in an increase in the angle of attack, the > reduced CL dampens the lift vector and amplitude of the PIO. It also pushes the Cd up, which always makes things less twitchy because increased drag tends to damp out pitch changes. -- martin@ | Martin Gregorie gregorie. | Essex, UK org |

Please explain the physics behind your statement.
UH

Martin Gregorie[_5_]

February 5th 13, 01:27 AM

On Mon, 04 Feb 2013 15:57:12 -0800, unclhank wrote:

> On Monday, February 4, 2013 4:22:23 PM UTC-5, Martin Gregorie wrote:
>> On Sun, 03 Feb 2013 19:43:06 -0800, Terence Wilson wrote: > Opening the
>> spoilers reduces the coefficient of lift. After the first > 'bounce',
>> which results in an increase in the angle of attack, the > reduced CL
>> dampens the lift vector and amplitude of the PIO. It also pushes the Cd
>> up, which always makes things less twitchy because increased drag tends
>> to damp out pitch changes. -- martin@ | Martin Gregorie gregorie. |
>> Essex, UK org |
>
> Please explain the physics behind your statement.
> UH

The effect of drag in damping a phugoid is described in this PDF:
http://www.flightlab.net/Flightlab.net/
Download_Course_Notes_files/7_LongitudinalDynami%232BA157.pdf

....sorry about the URL wrapping. That's the best reference I can find,
which is annoying because I know I've seen better explanations than that
one.

The math is here - scropp down to 'Phugoid':
https://en.wikipedia.org/wiki/
Aircraft_attitude#Dynamic_stability_and_control

though its badly presented (again, apologies for the wrapped URL). This
makes the point that: "Since the lift is very much greater than the drag,
the phugoid is at best lightly damped." With the unstated implication
that reducing the L/D ratio will make the phugoid more heavily damped.

The reference I was looking for, and I'm pretty certain it was talking
about gliders, mentioned the opposite effect by pointing out that as the
L/D ratio increases, the phugoid becomes progressively less damped,
making the aircraft less dynamically stable.

Hence lowering the wheel or opening the brakes will increase dynamic
pitch stability because the deceased L/D damps the phugoid more heavily.
This is what I was getting at: I accept that my initial comment wasn't
clear on this point and plead lateness of the hour and tiredness, m'lud.

--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

kirk.stant

February 5th 13, 09:16 AM

This whole discussion is about the difference between (in taildragger terms) three point (low energy, stalled) landings and wheel (flown-on) landings. In power, both are taught; when winds are gusty or strong, a flown on wheel landing at higher airspeed (but, critically, lower groundspeed) is much more controllable and safer, compared to a held off 3-point landing. In really strong crosswinds, it's even preferable to land on one wheel, banked, then slowly lower the other main wheel, followed by the tailwheel, as speed decreases. No big deal, it's all part of good airmanship, and applies just as well to tailwheel gliders.

As far as the concern about flown-on landings on a rough field - remember this is used when there is a strong and/or gusty wind, so the groundspeed/energy is not equal to the airspeed, and the additional control lets the pilot accurately place the plane on the ground. Trying a low-energy tail down landing in strong, gusty crosswinds is asking for a slammed in landing and groundloop!

Of course, do not confuse this with "flying onto the runway" at high speed. Forcing the plane to land is ALWAYS

kirk.stant

February 5th 13, 09:30 AM

On Tuesday, February 5, 2013 10:16:51 AM UTC+1, kirk.stant wrote:
> This whole discussion is about the difference between (in taildragger terms) three point (low energy, stalled) landings and wheel (flown-on) landings. In power, both are taught; when winds are gusty or strong, a flown on wheel landing at higher airspeed (but, critically, lower groundspeed) is much more controllable and safer, compared to a held off 3-point landing. In really strong crosswinds, it's even preferable to land on one wheel, banked, then slowly lower the other main wheel, followed by the tailwheel, as speed decreases. No big deal, it's all part of good airmanship, and applies just as well to tailwheel gliders.
>
>
>
> As far as the concern about flown-on landings on a rough field - remember this is used when there is a strong and/or gusty wind, so the groundspeed/energy is not equal to the airspeed, and the additional control lets the pilot accurately place the plane on the ground. Trying a low-energy tail down landing in strong, gusty crosswinds is asking for a slammed in landing and groundloop!
>
>
>
> Of course, do not confuse this with "flying onto the runway" at high speed. Forcing the plane to land is ALWAYS

Oops, fat fingers, here is the rest:

ALWAYS a bad thing, especially with the current "nosewheel" trainers. But a student should be able to land not only with min energy (main and tail or slightly tail first) or carefully wheeled on (slightly tail low) in strong winds.

If you can get a copy of "Big Rocks & Long Props", you will see plenty of examples of taildraggers being landed in "interesting" terrain with the tail up - precisely because of the additional controllability.

Kirk
66

[email protected]

February 5th 13, 01:34 PM

On Tuesday, February 5, 2013 4:30:51 AM UTC-5, kirk.stant wrote:
> On Tuesday, February 5, 2013 10:16:51 AM UTC+1, kirk.stant wrote: > This whole discussion is about the difference between (in taildragger terms) three point (low energy, stalled) landings and wheel (flown-on) landings. In power, both are taught; when winds are gusty or strong, a flown on wheel landing at higher airspeed (but, critically, lower groundspeed) is much more controllable and safer, compared to a held off 3-point landing. In really strong crosswinds, it's even preferable to land on one wheel, banked, then slowly lower the other main wheel, followed by the tailwheel, as speed decreases. No big deal, it's all part of good airmanship, and applies just as well to tailwheel gliders. > > > > As far as the concern about flown-on landings on a rough field - remember this is used when there is a strong and/or gusty wind, so the groundspeed/energy is not equal to the airspeed, and the additional control lets the pilot accurately place the plane on the ground. Trying a low-energy tail down landing in strong, gusty crosswinds is asking for a slammed in landing and groundloop! > > > > Of course, do not confuse this with "flying onto the runway" at high speed. Forcing the plane to land is ALWAYS Oops, fat fingers, here is the rest: ALWAYS a bad thing, especially with the current "nosewheel" trainers. But a student should be able to land not only with min energy (main and tail or slightly tail first) or carefully wheeled on (slightly tail low) in strong winds. If you can get a copy of "Big Rocks & Long Props", you will see plenty of examples of taildraggers being landed in "interesting" terrain with the tail up - precisely because of the additional controllability. Kirk 66

The other reason the tail is up is so they don't knock the tailhweel off on the big rocks. The mains are much stronger than the tailwheel.
UH

Paul Corbett

February 7th 13, 03:36 PM

test
>

Paul Corbett

February 7th 13, 03:40 PM

In checking out pilots in the G-103, I refer to the nose wheel/tail
wheel oscillation as a "tire PIO" to clearly differentiate it from the
more classic PIO associated with the excessive landing speeds and ground
effect destabilization. The correction for the latter is also
classic...FREEZE THE STICK.

The correction for the tire PIO is a bit different and is the more
difficult to perform. This oscillation, as pointed out in another post,
is divergent and VIOLENT! I have seen these result in 3-4 cycles in
about a second. Avoidance is the best cure but if it happens, there is
not enough time left to think of a plan so this is best thought out in
advance.

This is my plan...since it is the ground that is going to smite me, I
get away from it. RE-FLY AND RE-LAND the aircraft. (Here is how I know
this works...once while rolling on the main gear in an ASK-21, carrying
enough energy to taxi to parking, an airplane pulled out in front of me.
I hit the wheel brake hard, the nose pitched down and when the nose
wheel contacted the runway a tire PIO ensued, despite having full
spoilers out. I stowed the spoilers briefly and froze the stick.The
glider resumed flying and I then re-landed). Don't plan on your logical,
rational mind helping much...the motion is too rapid and violent. In my
case, I was acting instinctively. Be spring-loaded to use that excessive
energy that led to the problem in the first place, to get away from the
ground.

Some may disagree with these methods but so far, they have worked for me
and the pilots that I have instructed.

Paul
ZZ

On 02/01/2013 08:18 AM, Bill D wrote:
> There are two effects in play. A wing entering ground effect will see the center of pressure move forward which tends to destabilize the glider. A pilots feels this as increased elevator "twitchiness" when near the ground. This effect is particularly noticeable in G103's.
>
> Any increase in drag will tend to damp pitch oscillations. A free flight demonstration is easy. Just set up a stick-free Phugoid oscillation then open the spoilers and watch it damp out.
>
> The only issue I have with Carswell calling the G103 issue a PIO is that term generally refers to a free flight phenomena not involving ground contact. Perhaps the G103 specific problem should be called a PIB or Pilot Induced Bounce.
>
>
> On Thursday, January 31, 2013 9:42:53 PM UTC-7, Larry Suter wrote:
>> In his article on avoiding PIO in Grob 103's,
>>
>>
>>
>> http://www.soaringsafety.org/pilots/ic8.htm
>>
>>
>>
>> Dean Carswell writes, "the more the airbrakes are closed, the less pitch stability the Grob will have, making a PIO more likely if otherwise mishandled"
>>
>>
>>
>> Is there a simple explanation why opening the spoilers increases the pitch stability? Does it somehow move the center of lift further aft? I believe increasing the separation between the cg and center of lift is the classic way to increase pitch stability.
>>
>>
>>
>> And if that's how it works, why does the center of lift move aft? I would guess spoilers destroy the lift downwind of their location, causing the center of lift to move forward.....
>>
>>
>>
>> Thanks,
>>
>> Larry
>

Andreas Maurer

February 14th 13, 06:37 PM

On Sun, 3 Feb 2013 11:54:33 -0800 (PST), Bill D >
wrote:

>I hope our European friends jump in too. The following is based on years of experience at Boulder in wildly turbulent west wind rotors.
>
>I don't think a "hot" landing has any value. It just means you'll float down the runway in ground effect while you are vulnerable to those gusts and with enough energy to seriously damage the glider. Better to get it down and stopped and that can't happen until the glider slows.
>
>With a little practice, you'll find a glider can be slowed sharply with a few seconds of full airbrake while raising the nose to maintain the glide path just before entering ground effect. This can be done on short final to hit the recommended approach speed allowing the pattern itself to be flown at any speed the pilot deems safe for the gustiness.
>
>This discussion is about nose wheel gliders where "hot" landings put the glider on the nose wheel first.

From a European (German) point of view, I can only agree.

Even when it's pretty turbulent it is not necessary to fly faster than
60 kts with a G103. Even if one is faster, bleeding off excessive
speed with full airbrakes during the flare is very simple without any
danger of a PIO.

I am absolutely sure that these PIOs on the G103 (which is one of the
most forgiving basic trainers available) are a result of lacking
training.
This kind of accident is completely unheard of in Germany where nearly
*any* student pilot is trained from the beginning that any landing
where the tail wheel does not touch down first is a bad landing.

Andreas

kirk.stant

February 14th 13, 10:18 PM

Andreas, this is the US where I've had highly experienced instructors tell me that a landing where the tailwheel touches first is dangerous and likely to damage the glider.

Being a dick I just told them they were wrong and continue to land gliders that way.

Of course, when you are teaching no-spoiler, slipped patterns to a landing in a Grob 103, minimum energy landings are the least of your problems! Have to get that main wheel down before running off the 9000' runway!

Cheers

Kirk
66

Bill D

February 15th 13, 12:13 AM

On Thursday, February 14, 2013 3:18:05 PM UTC-7, kirk.stant wrote:
> Andreas, this is the US where I've had highly experienced instructors tell me that a landing where the tailwheel touches first is dangerous and likely to damage the glider.
>
>
>
> Being a dick I just told them they were wrong and continue to land gliders that way.
>
>
>
> Of course, when you are teaching no-spoiler, slipped patterns to a landing in a Grob 103, minimum energy landings are the least of your problems! Have to get that main wheel down before running off the 9000' runway!
>
>
>
> Cheers
>
>
>
> Kirk
>
> 66

I've more than once heard "highly experienced instructors" tell their students "only crazy people go cross country".

As long as it's so easy to meet the FAA requirements for glider instructor certificates, we'll keep hearing these things from them.

Andreas Maurer

February 15th 13, 03:41 AM

Hi Kirk,

On Thu, 14 Feb 2013 14:18:05 -0800 (PST), "kirk.stant"
> wrote:

>Andreas, this is the US where I've had highly experienced instructors tell me that a landing where the tailwheel touches first is
> dangerous and likely to damage the glider.
>
>Being a dick I just told them they were wrong and continue to land gliders that way.

How dare you....!
As we would say in German: Pfui...!
;)

>Of course, when you are teaching no-spoiler, slipped patterns to a landing in a Grob 103, minimum energy landings are the least of your problems!
> Have to get that main wheel down before running off the 9000' runway!

These things are really being taught to student pilots?
Faszinating.
Do they also teach them no-elevator landings?

No-spoiler landings are my hobby, I'm doing them in the ASK-21 and the
Ka-8 at my club's annual precision landing competition all the time.
Cannot remember when I missed the mark (main-wheel touchdown) for more
than 70 ft.

But the safety margin is very, very thin and does not allow any
mistake. Not suited at all for a student pilot.
But they are fun and guarantee the admiration of the comrades if you
and your glider happen to survive. Which, unfortunately, is not always
the case...

Cheers
Andreas

Uncle Fuzzy[_2_]

February 15th 13, 01:33 PM

On Thursday, February 14, 2013 7:41:44 PM UTC-8, Andreas Maurer wrote:
> Hi Kirk, On Thu, 14 Feb 2013 14:18:05 -0800 (PST), "kirk.stant" > wrote: >Andreas, this is the US where I've had highly experienced instructors tell me that a landing where the tailwheel touches first is > dangerous and likely to damage the glider. > >Being a dick I just told them they were wrong and continue to land gliders that way. How dare you.....! As we would say in German: Pfui...! ;) >Of course, when you are teaching no-spoiler, slipped patterns to a landing in a Grob 103, minimum energy landings are the least of your problems! > Have to get that main wheel down before running off the 9000' runway! These things are really being taught to student pilots? Faszinating. Do they also teach them no-elevator landings? No-spoiler landings are my hobby, I'm doing them in the ASK-21 and the Ka-8 at my club's annual precision landing competition all the time. Cannot remember when I missed the mark (main-wheel touchdown) for more than 70 ft. But the safety margin is very, very thin and does not allow any mistake. Not suited at all for a student pilot. But they are fun and guarantee the admiration of the comrades if you and your glider happen to survive. Which, unfortunately, is not always the case... Cheers Andreas

Slips to landing are part of the Practical Test Standard in the USA, so yes, they teach students to do it.

kirk.stant

February 15th 13, 02:39 PM

On Friday, February 15, 2013 2:33:10 PM UTC+1, Uncle Fuzzy wrote:

> Slips to landing are part of the Practical Test Standard in the USA, so yes, they teach students to do it.

Slips to a landing are one thing. No spoiler landings are a different beast altogether. First of all, the slip to a landing is a legacy of draggy gliders with poor glide path control (2-22, 2-33s come to mind). Fun to do and perfectly safe. But what about a glider where slips are not recommended? My LS-6 specifically says that slips are not very effective and are not recommended? I've tried them and they are an emergency procedure if you can't use the dive brakes prior to landing or have a really long runway.

So if a student is taking his check ride in a 2-33 (like I did), then a slip to a landing is a nice, fun, interesting way to end a flight. Heck, if you hit a boomer on final, you have to slip it sometimes just to get down in time. But if you show up for your checkride in a DG-1000, and your airfield is 1500' long with trees and powerlines at both ends, it's probably not a very smart thing to do. Demonstrate a slip in the pattern to increase descent rate? Sure, good idea. But make the student carry out the whole pattern and landing without the use of drag devices? Hope you have good hull insurance!

But what do I know - I like to land tail first...

Kirk
66

Bill D

February 15th 13, 03:54 PM

On Friday, February 15, 2013 6:33:10 AM UTC-7, Uncle Fuzzy wrote:

> Slips to landing are part of the Practical Test Standard in the
> USA, so yes, they teach students to do it.

Not for long, I'll bet.

There's a realization dawning throughout the FAA that slips to a landing have a razor-thin safety margin in heavy glass 2-seaters. A DG505 was totaled at my airfield during training flight. As a result of that accident and others, memo's are flying.

The DPE's I've discussed this with are adopting a procedure of asking the applicant for a demonstration of glide path control with a slip down to about 100' AGL then using spoilers for the actual landing.

Bill D

[email protected]

February 15th 13, 04:03 PM

On Friday, February 15, 2013 10:54:02 AM UTC-5, Bill D wrote:
> On Friday, February 15, 2013 6:33:10 AM UTC-7, Uncle Fuzzy wrote: > Slips to landing are part of the Practical Test Standard in the > USA, so yes, they teach students to do it. Not for long, I'll bet. There's a realization dawning throughout the FAA that slips to a landing have a razor-thin safety margin in heavy glass 2-seaters. A DG505 was totaled at my airfield during training flight. As a result of that accident and others, memo's are flying. The DPE's I've discussed this with are adopting a procedure of asking the applicant for a demonstration of glide path control with a slip down to about 100' AGL then using spoilers for the actual landing. Bill D

What Bill describes is in accordance with FAA guidance as to the intent of this task contained within the PTS. There is no requirement to slip to land in the PTS as currently applied.
UH

Uncle Fuzzy[_2_]

February 15th 13, 04:49 PM

<snip>Not for long, I'll bet. There's a realization dawning throughout the FAA....

Perhaps they'll get rid of landing on tow as well, (though that's less dicey than a no-kidding slip to land in heavy glass)at the same time!

>....The DPE's I've discussed this with are adopting a procedure of asking the applicant for a demonstration of glide path control with a slip down to about 100' AGL then using spoilers for the actual landing. Bill D

Which is how my DPE handled it when I had my check ride a decade or so ago.

Mike the Strike

February 15th 13, 05:22 PM

What we need to practice are events that are likely to happen and, as far as I know, failure of spoilers to deploy is almost unheard of (the only one I know of is from a glider rapidly descending from a high wave flight with severe icing).

The most likely failures of spoilers are asymmetrical deployment or failure to retract. This is probably why I was taught to deploy and check spoilers close to the field and keep them at least partially deployed during landing.

I witnessed one mishap years ago where the spoiler handle on a Grob 103 broke off, leaving the pilot unable to retract them and with a glide slope that had him land short of the field. Maybe we should simulate that - have the examiner pull full spoilers and see how the pilot copes with that?

Mike

Jonathon May

February 15th 13, 11:10 PM

At 17:22 15 February 2013, Mike the Strike wrote:
>What we need to practice are events that are likely to happen and, as far
>a=
>s I know, failure of spoilers to deploy is almost unheard of (the only
one
>=
>I know of is from a glider rapidly descending from a high wave flight
with
>=
>severe icing). =20
>
>The most likely failures of spoilers are asymmetrical deployment or
>failure=
> to retract. This is probably why I was taught to deploy and check
>spoiler=
>s close to the field and keep them at least partially deployed during
>landi=
>ng.
>
>I witnessed one mishap years ago where the spoiler handle on a Grob 103
>bro=
>ke off, leaving the pilot unable to retract them and with a glide slope
>tha=
>t had him land short of the field. Maybe we should simulate that - have
>th=
>e examiner pull full spoilers and see how the pilot copes with that?
>
>Mike
>
>If you deploy brakes above max manuvering there is a good chance they
will deflect back over the wing and though there might be some control they

will not retract fully .That is a possible emergency scenario that is worth

training for ,because if you loose control ,say in cloud or extreme
turbulence
the thing you are going to do is pull full brake while you sort out which
way is
up etc,if you haven't pulled the wings off ,the next problem will be flying
with
the brakes as they are.
A modern glass ship will exceed VNE with brakes out about 45degrees nose
down ,if you get sucked into a big cu with full brake and nose well down
you
still may well have a frozen canopy and jammed brakes when you pop out.
Sleep tight little ones

Don Johnstone[_4_]

February 16th 13, 12:14 AM

At 15:12 02 February 2013, Martin Gregorie wrote:
>On Thu, 31 Jan 2013 20:42:53 -0800, Larry Suter wrote:
>
>> In his article on avoiding PIO in Grob 103's,
>>
>> http://www.soaringsafety.org/pilots/ic8.htm
>>
>> Dean Carswell writes, "the more the airbrakes are closed, the less
pitch
>> stability the Grob will have, making a PIO more likely if otherwise
>> mishandled"
>>
>I've not noticed that a Grob G.103a is any less stable in pitch than, say

>an ASK.21 or a Puchacz regardless of airbrake setting, but all three do
>have different responses to *opening* the brakes. This might cause an ab
>initio or a new solo pilot some problems when moving between the types.
>
>- the ASK.21 has almost no attitude change and not much airspeed change
> when the brakes are opened
>
>- the Puchacz has very powerful, speed-limiting brakes and a tendency to
> pitch up and so will show a noticable drop in airspeed when they are
> opened. In consequence you soon learn to push the stick forward as you
> open the brakes.
>
>- the Grob G.103a has the opposite reaction: when you open the airbrakes
> it has a tendency to drop its nose and accelerate. I had speed control
> issues when I first flew one (pre-solo). So, at altitude we trimmed it
> to 60 kts hands off and, still hands off, opened the brakes fully. The
> glider lowered its nose and accelerated to almost 70 kts. Closing the
> brakes caused the nose to rise and the airspeed to settle back to 60
> kts. After this demo I took note and found its speed control much
> easier to manage.
>
>I do wonder, though, if opening the brakes suddenly on a G.103 after a

>too fast approach followed by a not fully held-off landing could tap the
>front wheel on the runway. This could easily start a PIO in the hands of
>an inexperienced pilot. However if this is the case I'd say the PIO was
>due to the fast approach, lack of properly held-off landing and
>inexperience on type for the PIO rather than any lack of pitch stability.
>
> not forgetting that the G103 brakes do have a tendency to snatch
>open, especially with the airspeed above 60 kts.
>
>My club's fleet contains all three types. My first 20 launches were
>equally split between the ASK21 and G103a. I first met the Puchacz on my
>21st launch. All these were winch launches in case you're wondering.

Martin has this right. The problem with opening the brakes on a Grob 103 is
that doing so will cause the nose to pitch down, closing the brakes will
cause the nose to pitch up, try it. You can actually fly a Grob 103 using
the spoilers to control pitch, not very well but it works. The apparent PIO
is different to the nosewheel contact and is caused by over correction,
this is especially true if the stick and airbrake lever are moved in
different directions, magnifying the effectiveness of the elevator.

The 103 airbrakes are linear in operation as regards airspeed. At 55 knots
the airbrakes will sit at roughly half deployment with no pressure on the
lever, closing if the speed is reduced and opening if it is increased. They
sit fully open with no pressure on the lever at about 60-65 knots.
On of the requirements of an airtest following service on the military
G103s in the UK was to accelerate to VNE and also operate the airbrakes at
about 70 kts in the recovery, at 70 kts the airbrakes will suck out very
quickly and over that speed the rate of uncontrolled deployment is very
rapid indeed.
As far as I am aware the airbrakes are rated for full deployment up to VNE,
probably break your wrist though when you unlock them.