Slips in turns and landing with winglets

[[email protected]]

I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?

Sorry for the questions, but would appreciate any help in this.

[[email protected]]

On Sunday, February 7, 2016 at 5:09:42 PM UTC-5, wrote:
> I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?
>
> Sorry for the questions, but would appreciate any help in this.

For certification, winglets must withstand a 50 lb force in ant direction at the tip without permanent deformation or damage. Flight loads are a small fraction of that.
UH

[Tim Taylor]

Dick Johnson's recommendations are still correct even with winglets. Each glider is different but I have found that S-H gliders climb better with aft cg and a slight slip. Does not require much and generally the glider feels like it has found its groove.

Winglets are strong enough for slips to final and increase the safety as the glider is less likely to spin with the winglets, especially earlier generation models like the ASW-20 and Ventus.

[Martin Gregorie[_5_]]

On Sun, 07 Feb 2016 14:09:35 -0800, ucanemailmoi wrote:

> I know this is a silly question, but I was reading an article by Dick
> Johnson that talked about holding a slight slip while thermally. This
> article was written before winglets and I was wondering if the same idea
> applied to gliders with winglets. Also, as for slipping to lose
> altitude for landing in a glider with winglets, does this place a lot of
> side load on the winglets. Should you not slip in landing with
> winglets?
>
> Sorry for the questions, but would appreciate any help in this.

Doesn't this follow from the fact that the yaw string is a fair distance
in front of the wing?

Think about it: if the tipwise axis of the wing is exactly aligned with
the radius of the turn, which it should be for maximum climb efficiency,
the yaw string should also be at right angles to the turn radius but,
being a good 2m/6ft or thereabouts in front of the wing, the turn radius
it is on is angled forward of the turn radius the wing is on, which
therefore makes it point out slightly out. This looks like a slight slip
to the pilot.

Draw this situation on a piece of paper, but exaggerate the situation by
drawing the circle diameter and about twice the wingspan and you'll see
what I described.

If you redraw the diagram with correctly scaled turn radius and wingspan,
the angular difference between the wing radius line and the yaw string
radius line will be very small, but this needs to be adjusted because the
curved shape of the canopy will amplify the angle of yaw string in a
slip. Now Dick Johnson's advice makes perfect sense.

Practically, if the yaw string is central or pointing slightly out in a
thermal you're doing it right. If its pointing a long way out or to the
inside of the turn you're sideslipping while turning and this is hurting
your climb rate by generating excessive drag.


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

[BobW]

On 2/7/2016 3:09 PM, wrote:
> I know this is a silly question, but I was reading an article by Dick
> Johnson that talked about holding a slight slip while thermally. This
> article was written before winglets and I was wondering if the same idea
> applied to gliders with winglets. Also, as for slipping to lose altitude
> for landing in a glider with winglets, does this place a lot of side load
> on the winglets. Should you not slip in landing with winglets?
>
> Sorry for the questions, but would appreciate any help in this.
>

Not silly questions at all!

UH, Tim T. and Martin G. have each provided insights.

Arguably (reality always rules, of course!), because of the additional
side-surface drag, slipping in the presence of winglets should be more
effective than in their absence...which, of course, is not the same as
suggesting that slipping in a particular glider "will *always* increase the
sink rate." (See below.)

As to whether you should slip or not, if the ship has a POH, that's the best
place to start for specific input, followed by practice aloft before trying it
"near the ground." In any case, your expectation should be that nothing will
break off due to aerodynamic forces of a slip performed at pattern airspeeds.

(Kinda-sorta related, I've flown one non-spoilered,
large-deflection-landing-flap-only equipped, 15-meter glass, glider having
*reduced* sink rate when straight-ahead-slipped with full landing flap in.
That informational bit is simply by way of sharing a perhaps-non-obvious
tidbit regarding "the obvious usefulness of slipping" as a landing aid. The
devil is in the details...)

Bob W.

[Ventus_a]

Dick Johnson's recommendations are still correct even with winglets. Each glider is different but I have found that S-H gliders climb better with aft cg and a slight slip. Does not require much and generally the glider feels like it has found its groove.

snip

I fly a Ventus a and a Nimbus3D and they both climb better with the yaw string indicating some slip. In the case of the Nimbus it can be having the yaw string 30 - 40 degrees off centre at times which tallies with advice I was given by a 4 time world open class champion.

The cross flow over the Nimbus canopy exaggerates the string position in relation to the actual amount of slip but if I'm not having to hold off bank I'm pretty much in the ballpark for getting the best climb. The Ventus has Masak winglets and climbs well with the slipping but because of the trailing edge brakes doesn't often call for sideslipping for landing although it does it well enough

:-) Colin

[kirk.stant]

On Sunday, February 7, 2016 at 6:11:14 PM UTC-6, Martin Gregorie wrote:
>
> Doesn't this follow from the fact that the yaw string is a fair distance
> in front of the wing?
>
> Think about it: if the tipwise axis of the wing is exactly aligned with
> the radius of the turn, which it should be for maximum climb efficiency,
> the yaw string should also be at right angles to the turn radius but,
> being a good 2m/6ft or thereabouts in front of the wing, the turn radius
> it is on is angled forward of the turn radius the wing is on, which
> therefore makes it point out slightly out. This looks like a slight slip
> to the pilot.
>
> Draw this situation on a piece of paper, but exaggerate the situation by
> drawing the circle diameter and about twice the wingspan and you'll see
> what I described.
>
> If you redraw the diagram with correctly scaled turn radius and wingspan,
> the angular difference between the wing radius line and the yaw string
> radius line will be very small, but this needs to be adjusted because the
> curved shape of the canopy will amplify the angle of yaw string in a
> slip. Now Dick Johnson's advice makes perfect sense.
>
> Practically, if the yaw string is central or pointing slightly out in a
> thermal you're doing it right. If its pointing a long way out or to the
> inside of the turn you're sideslipping while turning and this is hurting
> your climb rate by generating excessive drag.

Martin, I believe this is only part of the answer. You are correct that a perfectly coordinated turn (slip ball in the center) should show some slip if the yaw string is forward of the CG (and conversely, a centered forward mounted yaw string indicates a skidding turn - beware in the pattern!) but there is also the overbanking effect of long wings seeing different local airflow velocity in a turn; this can be countered by holding aileron out of the turn (inefficient and draggy) or by adding a little bit of slip via top rudder. This slightly increases the angle of attack of the lower wing and decreases the angle of attack of the top wing and voila, no overbanking, ailerons are flush and efficient, and you gleefully outclimb the newb with the perfectly aligned yaw string. It's really noticeable in my LS6 (probably due to the dihedral) - it settles into a nice slipping climb with the yaw string off about 10 degrees to the outside, no aileron deflection, and just a touch of top rudder. Extra bonus - you get some lift off the fuselage! (See "String Theory").

Kirk
66

[Martin Gregorie[_5_]]

On Mon, 08 Feb 2016 09:55:16 -0800, kirk.stant wrote:

> but there is also the overbanking effect of long wings seeing
> different local airflow velocity in a turn; this can be countered by
> holding aileron out of the turn (inefficient and draggy) or by adding a
> little bit of slip via top rudder. This slightly increases the angle of
> attack of the lower wing and decreases the angle of attack of the top
> wing and voila, no overbanking, ailerons are flush and efficient, and
> you gleefully outclimb the newb with the perfectly aligned yaw string.
>
True enough, and having reasonable amounts of dihedral is essential for
this to work. I didn't mention this primarily because I wanted to talk
about the effect of having the yaw string in front of the wing.

Of course I understand how yawing a dihedralled wing causes roll: when I
were a lad I used to fly single channel RC models before I discovered
free flight models and thermals.



> Kirk 66



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

[Bob Caldwell (BC)]

On Sunday, February 7, 2016 at 3:09:42 PM UTC-7, wrote:
> I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?
>
> Sorry for the questions, but would appreciate any help in this.

There are two other considerations:

A. The side of the fuselage meeting the airflow in a slight slip produces some lifting force.
And:
B: A bit of top rudder reduces the back pressure on the stick needed to hold your turn. That reduces the induced drag at the horizontal.

So what do you think?

Bob

[[email protected]]

On Wednesday, February 10, 2016 at 8:03:15 PM UTC-5, Bob Caldwell (BC) wrote:
> On Sunday, February 7, 2016 at 3:09:42 PM UTC-7, wrote:
> > I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?
> >
> > Sorry for the questions, but would appreciate any help in this.
>
> There are two other considerations:
>
> A. The side of the fuselage meeting the airflow in a slight slip produces some lifting force.
> And:
> B: A bit of top rudder reduces the back pressure on the stick needed to hold your turn. That reduces the induced drag at the horizontal.
>
> So what do you think?
>
> Bob

I think both are a crock of stuff. Lift is better generated by a wing and top rudder only serves to make the turn rate lower than it should be for the bank angle.
UH

[Bruce Hoult]

On Thursday, February 11, 2016 at 4:09:52 AM UTC+3, wrote:
> On Wednesday, February 10, 2016 at 8:03:15 PM UTC-5, Bob Caldwell (BC) wrote:
> > On Sunday, February 7, 2016 at 3:09:42 PM UTC-7, wrote:
> > > I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?
> > >
> > > Sorry for the questions, but would appreciate any help in this.
> >
> > There are two other considerations:
> >
> > A. The side of the fuselage meeting the airflow in a slight slip produces some lifting force.
> > And:
> > B: A bit of top rudder reduces the back pressure on the stick needed to hold your turn. That reduces the induced drag at the horizontal.
> >
> > So what do you think?
> >
> > Bob
>
> I think both are a crock of stuff. Lift is better generated by a wing and top rudder only serves to make the turn rate lower than it should be for the bank angle.

The best LD ratio of the side of a fuselage is not very good, but if it's heading directly into the airflow then the lift is zero but there is still drag, which makes the LD ratio zero. Having just a little bit of yaw giving just a little bit of lift does not significantly affect the drag, so that first little bit of extra lift is for free.

[[email protected]]

On Thursday, February 11, 2016 at 4:14:58 AM UTC-5, Bruce Hoult wrote:
> On Thursday, February 11, 2016 at 4:09:52 AM UTC+3, wrote:
> > On Wednesday, February 10, 2016 at 8:03:15 PM UTC-5, Bob Caldwell (BC) wrote:
> > > On Sunday, February 7, 2016 at 3:09:42 PM UTC-7, wrote:
> > > > I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?
> > > >
> > > > Sorry for the questions, but would appreciate any help in this.
> > >
> > > There are two other considerations:
> > >
> > > A. The side of the fuselage meeting the airflow in a slight slip produces some lifting force.
> > > And:
> > > B: A bit of top rudder reduces the back pressure on the stick needed to hold your turn. That reduces the induced drag at the horizontal.
> > >
> > > So what do you think?
> > >
> > > Bob
> >
> > I think both are a crock of stuff. Lift is better generated by a wing and top rudder only serves to make the turn rate lower than it should be for the bank angle.
>
> The best LD ratio of the side of a fuselage is not very good, but if it's heading directly into the airflow then the lift is zero but there is still drag, which makes the LD ratio zero. Having just a little bit of yaw giving just a little bit of lift does not significantly affect the drag, so that first little bit of extra lift is for free.

I suspect that the gliders that seem to benefit for slight slip do so for 2 reasons.
First, as mentioned above, is that the slight slip in a glider with a good bit of dihedral will have the force opposing over banking provided without much or any control deflection. Profile drag and spanwise lift distribution benefit a bit.
Second is likely related to wing root separation issues which vary a lot from ship to ship. From my simple observation the gliders with larger well developed root fillets seem to be the ones that are not in the group described as benefiting from notable slipping.
One guys opinion.
UH

[[email protected]]

On Thursday, February 11, 2016 at 9:14:08 AM UTC-5, wrote:
> Second is likely related to wing root separation issues which vary a lot from ship to ship. From my simple observation the gliders with larger well developed root fillets seem to be the ones that are not in the group described as benefiting from notable slipping.
> One guys opinion.
> UH

That would tend to correlate with early mentions of holding a slight slip in turns from George Moffat in regards to the Standard Cirrus and Nimbus II, both of which, IIRC, suffered from poor airflow around the wing roots while thermaling.

Chip Bearden

[Tango Whisky]

I think that all of this reasoning is pure speculation.

So for me, I try to keep it simple.

Even after 3000 h in saileplanes, I can't keep the string straight.

So, rather than skidding out of the thermal, I prefer to slip towards the core.

And it works ;-)

Bert
Ventus cM "TW"

[kirk.stant]

On Thursday, February 11, 2016 at 8:14:08 AM UTC-6, wrote:
> I suspect that the gliders that seem to benefit for slight slip do so for 2 reasons.
> First, as mentioned above, is that the slight slip in a glider with a good bit of dihedral will have the force opposing over banking provided without much or any control deflection. Profile drag and spanwise lift distribution benefit a bit.
> Second is likely related to wing root separation issues which vary a lot from ship to ship. From my simple observation the gliders with larger well developed root fillets seem to be the ones that are not in the group described as benefiting from notable slipping.
> One guys opinion.
> UH

I think there is a little more to it. It would be interesting to have wind tunnel data comparing the lift and drag of a complete glider in a pure coordinated 40 degree banked turn at CL max (ball centered, forward mounted yaw string slightly to the outside due to geometry), vs the same glider with enough slip to remove the overbanking force (ailerons neutral, top rudder). My guess is that some gliders will show a small but significant amount of drag due to deflected ailerons (which may be more than the drag of the yawed fuselage and deflected rudder), while others may be less affected.

All I know, it works good on all gliders I've tried it on - from K-21s to my LS6. It may be less of an issue with more modern gliders with smaller wings and less control deflections.

But hey, if you don't believe it, just keep on thermalling with that front mounted yaw string perfectly centered.... ;^)

Kirk
66

[[email protected]]

On Thursday, February 11, 2016 at 2:44:49 PM UTC-5, kirk.stant wrote:
> On Thursday, February 11, 2016 at 8:14:08 AM UTC-6, wrote:
> > I suspect that the gliders that seem to benefit for slight slip do so for 2 reasons.
> > First, as mentioned above, is that the slight slip in a glider with a good bit of dihedral will have the force opposing over banking provided without much or any control deflection. Profile drag and spanwise lift distribution benefit a bit.
> > Second is likely related to wing root separation issues which vary a lot from ship to ship. From my simple observation the gliders with larger well developed root fillets seem to be the ones that are not in the group described as benefiting from notable slipping.
> > One guys opinion.
> > UH
>
> I think there is a little more to it. It would be interesting to have wind tunnel data comparing the lift and drag of a complete glider in a pure coordinated 40 degree banked turn at CL max (ball centered, forward mounted yaw string slightly to the outside due to geometry), vs the same glider with enough slip to remove the overbanking force (ailerons neutral, top rudder). My guess is that some gliders will show a small but significant amount of drag due to deflected ailerons (which may be more than the drag of the yawed fuselage and deflected rudder), while others may be less affected.
>
> All I know, it works good on all gliders I've tried it on - from K-21s to my LS6. It may be less of an issue with more modern gliders with smaller wings and less control deflections.
>
> But hey, if you don't believe it, just keep on thermalling with that front mounted yaw string perfectly centered.... ;^)
>
> Kirk
> 66

What more? I was agreeing.
I provided a couple reasonable explanations for why it is/may be true for some ships. I know it doesn't help my '29 and does not seem to help my '28.
UH

[kirk.stant]

Yes, I see that now (on RAS with too little sleep last night).

I wonder if winglets have any impact on it? I would think newer ships with "draglets" would prefer as little slip as possible.

Cheers,

Kirk

[[email protected]]

Yes Kirk winglets help it! Wanna race? :)

[SF]

I was training a student in a 2-33. If you don't learn to slip that thing, you are going to land it all over the place. Let's just say I was fully practiced up on slips to land. So here I am at New Castle in my LS-6, I'm high on final, spoilers and flaps are not going to get it done, that's how high I was. So I put it in a big honking slip, didn't think about it just did it. Landing and roll out went fine, but something wasn't right, can't explain it but I never did it again, and I was shaking when I got out of the cockpit.
1. Read the POH
2. Practice first at altitude
3. remember what plane you are flying

SF

[Gianni Isotope]

On Sunday, February 7, 2016 at 4:09:42 PM UTC-6, wrote:
> Should you not slip in landing with winglets?>

For the LS7 fitted with winglets (LS7-WL), the flight manual states:

WARNING: Sideslip with winglets prohibited, because during speed reduction stalling of the leading wing occurs (example: during left hand slip, stalling to the left!)

Jordan

[Eric Greenwell[_4_]]

kirk.stant wrote on 2/8/2016 9:55 AM:
> Extra bonus - you get some lift off the fuselage!

Maybe this is intended as joke, but the last thing you want is a very
low L/D "airfoil" like the fuselage involved in providing any lift.

--
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
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/Guide-to-transponders-in-sailplanes-2014A.pdf

[Bruce Hoult]

On Monday, February 15, 2016 at 6:17:13 PM UTC+3, Eric Greenwell wrote:
> kirk.stant wrote on 2/8/2016 9:55 AM:
> > Extra bonus - you get some lift off the fuselage!
>
> Maybe this is intended as joke, but the last thing you want is a very
> low L/D "airfoil" like the fuselage involved in providing any lift.

I don't agree.

As I've already said once in this thread, you're going to get a certain amount of drag from the fuselage anyway. If the presentation to the airflow for minimum drag generates zero lift then, by the calculus definition of "minimum" of a continuous function, the first little bit of lift will not add any drag. The optimum thing to do is to use it. It might be *very* little, and a very small AoA, but it's nonzero.

If the presentation to the airflow for minimum drag generates non-zero lift ... then of course you'll take it!

Yes, the fuse has a low L/D. But that's better than the 0.0 L/D if you don't take what lift you can from it...

[Tango Whisky]

Le lundi 15 février 2016 16:32:27 UTC+1, Bruce Hoult a écrit*:
>
> I don't agree.
>
> As I've already said once in this thread, you're going to get a certain amount of drag from the fuselage anyway. If the presentation to the airflow for minimum drag generates zero lift then, by the calculus definition of "minimum" of a continuous function, the first little bit of lift will not add any drag. The optimum thing to do is to use it. It might be *very* little, and a very small AoA, but it's nonzero.
>
> If the presentation to the airflow for minimum drag generates non-zero lift ... then of course you'll take it!
>
> Yes, the fuse has a low L/D. But that's better than the 0.0 L/D if you don't take what lift you can from it...

Why do you assume that the fuselage has zero lift in a perfect airflow?
Why do you assume that the increase of drag is zero for small slip angles?

If you slip, you will trade a part of the wing's lift (with a good L/D) for lift from the fuselage (with amn extremely low L/D). Your choice - it wouldn't be mine...

[BobW]

On 2/15/2016 8:16 AM, Eric Greenwell wrote:
> kirk.stant wrote on 2/8/2016 9:55 AM:
>> Extra bonus - you get some lift off the fuselage!
>
> Maybe this is intended as joke, but the last thing you want is a very low
> L/D "airfoil" like the fuselage involved in providing any lift.
>

Eric,

That's pretty dogmatic. Why do you say so? Thanks!

Bob W.

[JS]

Perhaps a good rule of thumb while landing with winglets is to land with the same number of winglets you took off with.
This didn't work for the Voyager's round-the-World flight, but most times it does.
Jim

[Dan Marotta]

On 2/15/2016 11:05 AM, JS wrote:
> Perhaps a good rule of thumb while landing with winglets is to land with the same number of winglets you took off with.
> This didn't work for the Voyager's round-the-World flight, but most times it does.
> Jim
And yet the Voyager succeeded in its flight and landed safely, to boot.
--
Dan, 5J

[howard banks]

No theory: After talking with Dick Johnson at a Hobbs contest I tried a slight sideslip in thermals in my winglet-free 20B. It seemed to help so I kept doing it. It did not help in my 27, which was designed to have winglets, so I stopped doing it.
I never flew the 27 with only one winglet and compared results turning to the winglet and away from it, so no idea if it would have made any difference or not. (PS: The only guy I saw, after landing, who had lost one winglet in flight on a 27 said he hadn't noticed any impact on his flying at all.)




On Sunday, February 7, 2016 at 3:09:42 PM UTC-7, wrote:
> I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?
>
> Sorry for the questions, but would appreciate any help in this.

[[email protected]]

Go up, fly your ship, do whatever works best for your bird after testing things out yourself. If it worked for butler or AJ or Moffat, they must have known something they discovered with their particular ship. My 1-26 doesnt care all that much but I do have the advantage of being able to work the core of the core while most others dont have the turn radius to get and stay in there unless they are flying minden or marfa thermals.

[Andreas Maurer]

On Mon, 15 Feb 2016 07:16:51 -0800, Eric Greenwell
> wrote:


>Maybe this is intended as joke, but the last thing you want is a very
>low L/D "airfoil" like the fuselage involved in providing any lift.


Depends on the type of glider.

An open class ship needs a lot of sideslip while thermalling.

I learned that a long time ago from Uli Schwenk, who told me that he
learned that from a guy named Klaus Holighaus.

Thermalling with the yawstring 30 degrees outwards makes a huge
difference in climb performance. Huge.

Explanation:
The yaw costs a lot of drag due to the "low L/D fuselage" - but it
saves even more drag because you barely need any opposite aileron
anymore, therefore you get a much better lift distribution (and
therefore much less induced drag) on the wing.


One other glider that needs a lot of yaw in a thermal: Arcus.



Best regards
Andreas

[Bruce Hoult]

On Monday, February 15, 2016 at 7:13:43 PM UTC+3, Tango Whisky wrote:
> Le lundi 15 février 2016 16:32:27 UTC+1, Bruce Hoult a écrit*:
> >
> > I don't agree.
> >
> > As I've already said once in this thread, you're going to get a certain amount of drag from the fuselage anyway. If the presentation to the airflow for minimum drag generates zero lift then, by the calculus definition of "minimum" of a continuous function, the first little bit of lift will not add any drag. The optimum thing to do is to use it. It might be *very* little, and a very small AoA, but it's nonzero.
> >
> > If the presentation to the airflow for minimum drag generates non-zero lift ... then of course you'll take it!
> >
> > Yes, the fuse has a low L/D. But that's better than the 0.0 L/D if you don't take what lift you can from it...
>
> Why do you assume that the fuselage has zero lift in a perfect airflow?

I don't. I present the argument for both cases: zero and non-zero lift at minimum drag.


> Why do you assume that the increase of drag is zero for small slip angles?

Follows directly from the definition of "minimum" for a continuous function..

The minimum is, by definition, at the point at which the function (the drag) has zero change for small changes in the input (the AoA or slip angle).

[Martin Gregorie[_5_]]

On Mon, 15 Feb 2016 14:48:52 -0800, Bruce Hoult wrote:

> On Monday, February 15, 2016 at 7:13:43 PM UTC+3, Tango Whisky wrote:
>> Le lundi 15 février 2016 16:32:27 UTC+1, Bruce Hoult a écritÂ*:
>> >
>> > I don't agree.
>> >
>> > As I've already said once in this thread, you're going to get a
>> > certain amount of drag from the fuselage anyway. If the presentation
>> > to the airflow for minimum drag generates zero lift then, by the
>> > calculus definition of "minimum" of a continuous function, the first
>> > little bit of lift will not add any drag. The optimum thing to do is
>> > to use it. It might be *very* little, and a very small AoA, but it's
>> > nonzero.
>> >
>> > If the presentation to the airflow for minimum drag generates
>> > non-zero lift ... then of course you'll take it!
>> >
>> > Yes, the fuse has a low L/D. But that's better than the 0.0 L/D if
>> > you don't take what lift you can from it...
>>
>> Why do you assume that the fuselage has zero lift in a perfect airflow?
>
> I don't. I present the argument for both cases: zero and non-zero lift
> at minimum drag.
>
>
>> Why do you assume that the increase of drag is zero for small slip
>> angles?
>
> Follows directly from the definition of "minimum" for a continuous
> function.
>
> The minimum is, by definition, at the point at which the function (the
> drag) has zero change for small changes in the input (the AoA or slip
> angle).
>
But, as soon as the fuselage generates any side-force, its drag will
increase, and by more than the energy needed to generate the side-force.
This is due to two things:

1) The energy conservation law would be violated if the energy taken from
the moving aircraft as drag is less than that needed to generate the side
force.

2) The energy consumed will be more than that used to generate the side
force because no process that consumes energy is 100% efficient.


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

[Bruce Hoult]

On Tuesday, February 16, 2016 at 3:23:18 AM UTC+3, Martin Gregorie wrote:
> On Mon, 15 Feb 2016 14:48:52 -0800, Bruce Hoult wrote:
>
> > On Monday, February 15, 2016 at 7:13:43 PM UTC+3, Tango Whisky wrote:
> >> Le lundi 15 février 2016 16:32:27 UTC+1, Bruce Hoult a écrit*:
> >> >
> >> > I don't agree.
> >> >
> >> > As I've already said once in this thread, you're going to get a
> >> > certain amount of drag from the fuselage anyway. If the presentation
> >> > to the airflow for minimum drag generates zero lift then, by the
> >> > calculus definition of "minimum" of a continuous function, the first
> >> > little bit of lift will not add any drag. The optimum thing to do is
> >> > to use it. It might be *very* little, and a very small AoA, but it's
> >> > nonzero.
> >> >
> >> > If the presentation to the airflow for minimum drag generates
> >> > non-zero lift ... then of course you'll take it!
> >> >
> >> > Yes, the fuse has a low L/D. But that's better than the 0.0 L/D if
> >> > you don't take what lift you can from it...
> >>
> >> Why do you assume that the fuselage has zero lift in a perfect airflow?
> >
> > I don't. I present the argument for both cases: zero and non-zero lift
> > at minimum drag.
> >
> >
> >> Why do you assume that the increase of drag is zero for small slip
> >> angles?
> >
> > Follows directly from the definition of "minimum" for a continuous
> > function.
> >
> > The minimum is, by definition, at the point at which the function (the
> > drag) has zero change for small changes in the input (the AoA or slip
> > angle).
> >
> But, as soon as the fuselage generates any side-force, its drag will
> increase,

Incorrect. Please study your Calculus 101 books a little more closely.

> and by more than the energy needed to generate the side-force.
> This is due to two things:
>
> 1) The energy conservation law would be violated if the energy taken from
> the moving aircraft as drag is less than that needed to generate the side
> force.

Incorrect. Force is not energy.

If you were correct then wings would not work at all.

For maximum efficiency, every part should (as much as possible) be operated at its angle of attack for best L/D, not at minimum drag.

An L/D of 5 on some part is better than an L/D of 0 on that part. Even if some other part has an L/D of 50. (provided you don't make that other part perform worse, of course)

[Tango Whisky]

> An L/D of 5 on some part is better than an L/D of 0 on that part. Even if some other part has an L/D of 50. (provided you don't make that other part perform worse, of course)

Total lift is constant. So if you shift a part of the lift to some inefficient means, your overall performance will drop.

[Bruce Hoult]

On Tuesday, February 16, 2016 at 4:20:33 PM UTC+3, Tango Whisky wrote:
> > An L/D of 5 on some part is better than an L/D of 0 on that part. Even if some other part has an L/D of 50. (provided you don't make that other part perform worse, of course)
>
> Total lift is constant. So if you shift a part of the lift to some inefficient means, your overall performance will drop.

Incorrect, because total drag is not a constant.

If you can get, say, 1% of your lift from the fuselage without increasing the drag of the fuselage, then decreasing the lift needed from the wings by that 1% will decrease the induced drag from the wings by about the same 1%.

[Tango Whisky]

Le mardi 16 février 2016 15:24:00 UTC+1, Bruce Hoult a écrit*:

> If you can get, say, 1% of your lift from the fuselage without increasing the drag of the fuselage, then decreasing the lift needed from the wings by that 1% will decrease the induced drag from the wings by about the same 1%.

Absolutely correct, but deconnected from reality. Just because a function runs through a minimum, it's not a given that slight deviations from the minimum on the x-axis yield only slight variations on the y-axis.

But again, do what you like ;-)

[Bruce Hoult]

On Tuesday, February 16, 2016 at 5:48:00 PM UTC+3, Tango Whisky wrote:
> Le mardi 16 février 2016 15:24:00 UTC+1, Bruce Hoult a écrit*:
>
> > If you can get, say, 1% of your lift from the fuselage without increasing the drag of the fuselage, then decreasing the lift needed from the wings by that 1% will decrease the induced drag from the wings by about the same 1%.
>
> Absolutely correct, but deconnected from reality. Just because a function runs through a minimum, it's not a given that slight deviations from the minimum on the x-axis yield only slight variations on the y-axis.

If it's a continuous differentiable function then exactly that is a given.

The entire concept of a polar curve relies on this property.

[Martin Gregorie[_5_]]

On Tue, 16 Feb 2016 03:26:42 -0800, Bruce Hoult wrote:

> On Tuesday, February 16, 2016 at 3:23:18 AM UTC+3, Martin Gregorie
> wrote:
>> 1) The energy conservation law would be violated if the energy taken
>> from the moving aircraft as drag is less than that needed to generate
>> the side force.
>
> Incorrect. Force is not energy.
>
If you're extracting work from the system (in this case generating a lift
force that moves the fuselage sideways) you're using energy that wouldn't
be consumed if there was no side force being generated.

This is really basic stuff: it should have been covered in School Cert
physics unless you had a really bad science teacher, and is certainly
revisited in the first year of any physics degree cause, mainly to
compensate for really bad science teachers

The only thing that aerodynamics adds to basic physics is the knowledge
that any object that generates lift also produces induced drag. There is
no induced drag if Cl = 0, but as soon is Cl is non-zero, induced drag
becomes non-zero as well and is added to the skin friction and shape-
related drag terms.



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

[[email protected]]

Wasting your time worrying about this. All you really need is the MK IV "high tech" yaw string. I hear it auto coordinates all turns while the low profile adhesive mount reduces drag.

It's the equivalent of giving your glider PED's.

[Martin Gregorie[_5_]]

On Tue, 16 Feb 2016 14:37:18 -0800, scohpilot wrote:

> Wasting your time worrying about this. All you really need is the MK IV
> "high tech" yaw string. I hear it auto coordinates all turns while the
> low profile adhesive mount reduces drag.
>
I know: got one already, plus my bum is finely attuned to understanding
what my Libelle is telling it.



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

[Eric Greenwell[_4_]]

Andreas Maurer wrote on 2/15/2016 2:17 PM:
> On Mon, 15 Feb 2016 07:16:51 -0800, Eric Greenwell
> > wrote:
>
>
>> Maybe this is intended as joke, but the last thing you want is a very
>> low L/D "airfoil" like the fuselage involved in providing any lift.
>
>
> Depends on the type of glider.
>
> An open class ship needs a lot of sideslip while thermalling.
>
> I learned that a long time ago from Uli Schwenk, who told me that he
> learned that from a guy named Klaus Holighaus.
>
> Thermalling with the yawstring 30 degrees outwards makes a huge
> difference in climb performance. Huge.
>
> Explanation:
> The yaw costs a lot of drag due to the "low L/D fuselage" - but it
> saves even more drag because you barely need any opposite aileron
> anymore, therefore you get a much better lift distribution (and
> therefore much less induced drag) on the wing.
>
>
> One other glider that needs a lot of yaw in a thermal: Arcus.

I agree there are gliders where that drag would be an acceptable
trade-off, but the pilot I was responding to thought it was a "bonus",
and it's clearly not that.

I am curious about why a 20 meter glider would need a lot of yaw to
climb well, when my 18 meter ASH 26E hardly needs any. 20 or 30 degrees
would be a poor choice for the 26E, but you say an Arcus needs that
much? Is that part of the operating manual for the glider? I would
expect the inner winglet to be stalled, and the outer winglet to be
producing outward lift.


--
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
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/Guide-to-transponders-in-sailplanes-2014A.pdf

[Eric Greenwell[_4_]]

Eric Greenwell wrote on 2/16/2016 7:58 PM:
> I would expect the inner winglet to be stalled, and the outer winglet to
> be producing outward lift.

Make that: "both winglets to be stalled"


--
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
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/Guide-to-transponders-in-sailplanes-2014A.pdf

[Andreas Maurer]

On Tue, 16 Feb 2016 19:58:42 -0800, Eric Greenwell
> wrote:



>I am curious about why a 20 meter glider would need a lot of yaw to
>climb well, when my 18 meter ASH 26E hardly needs any. 20 or 30 degrees
>would be a poor choice for the 26E, but you say an Arcus needs that
>much?


I guess it's a question of aircraft geometry and cannot be predicted
precisely.

I was surprised how nasty the Arcus was thermalling with the yaw
string centered (some of the PICs I instructed needed full aileron
input and really hard work to keep it in the turn) and how nice it
flew with significant opposite rudder.

I didn't expect that, especially since the Duo Dicus with its very
similar wing geometry doesn't show this behaviour.

Dihedral does not seem to be an important factor: The Arcus with its
huge dihedral behaves similar to the AS 22-2 that has too little
dihedral (and needs nearly full opposite aileron if you turn with the
yaw string centered) and no winglets.


>Is that part of the operating manual for the glider? I would
>expect the inner winglet to be stalled, and the outer winglet to be
>producing outward lift.

I don't think so, but I haven't read the Arcus POH that carefully I
have to admit.

I am pretty sure that the outer winglet is the one that is stalled
(camber is pointing towards the fuselage, therefore reducing stall
AOA).

[Bruce Hoult]

On Wednesday, February 17, 2016 at 1:05:18 AM UTC+3, Martin Gregorie wrote:
> On Tue, 16 Feb 2016 03:26:42 -0800, Bruce Hoult wrote:
>
> > On Tuesday, February 16, 2016 at 3:23:18 AM UTC+3, Martin Gregorie
> > wrote:
> >> 1) The energy conservation law would be violated if the energy taken
> >> from the moving aircraft as drag is less than that needed to generate
> >> the side force.
> >
> > Incorrect. Force is not energy.
> >
> If you're extracting work from the system (in this case generating a lift
> force that moves the fuselage sideways) you're using energy that wouldn't
> be consumed if there was no side force being generated.

Incorrect. Force is not energy.

If you were correct then a 20:1 glider and a 60:1 glider with the same weight and same speed would be burning through energy at the same rate because they're generating the same lift. But that's not the case .. the 60:1 glider is losing energy at 1/3 of the rate of the 20:1 glider.


> This is really basic stuff: it should have been covered in School Cert
> physics unless you had a really bad science teacher, and is certainly
> revisited in the first year of any physics degree cause, mainly to
> compensate for really bad science teachers

Thanks for your input. I got the school prizes for top physics and top math student every year, and also scholarships etc. I'm not some dummy.

[Martin Gregorie[_5_]]

On Wed, 17 Feb 2016 01:45:59 -0800, Bruce Hoult wrote:

>> > Incorrect. Force is not energy.
>> >
You forgot that the side force is affecting the flight path.

Force * distance = work. Doing work uses energy. This is the first law of
thermodynamics: https://en.wikipedia.org/wiki/First_law_of_thermodynamics

> If you were correct then a 20:1 glider and a 60:1 glider with the same
> weight and same speed would be burning through energy at the same rate
> because they're generating the same lift. But that's not the case .. the
> 60:1 glider is losing energy at 1/3 of the rate of the 20:1 glider.
>
You're forgetting drag. Two gliders with the same weight will be
generating the same lift if their descent rates are constant. However, if
their descent rates are different but flying speeds are the same, then
the one with the worst glide will have more drag.

Again, speed * drag = work. Doing work requires energy. A glider is a
device for converting potential energy (height) into kinetic energy
(airspeed), so more drag requires more work to be done if the glider is
to move at a given speed. This is why a tail chute drastically increases
the (constant) descent rate while maintaining a constant airspeed. Doing
more work consumes more energy. If it didn't, you'd have invented a
perpetual motion machine.

> Thanks for your input. I got the school prizes for top physics and top
> math student every year, and also scholarships etc. I'm not some dummy.
>
You obviously understand maths. Physics, not so much.


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

[Tango Whisky]

Le mardi 16 février 2016 23:01:09 UTC+1, Bruce Hoult a écrit*:

> If it's a continuous differentiable function then exactly that is a given..
>

Well, no. The sole fact that the slope at a minimum is zero does not give you much information on the points close to minimum. You need to know a couple of coefficients of the function, and you need to set the "small deviation" in context with "maximum deviation". With a slip angle of 5-10 degree out of a maximum of 90 degree, you can't expand a function into a series based on small values.

But on the physics side:
(1) You fly your glider at minumum sink speed straight ahead with the string centered.
(2) Next, you fly a quarter roll and then fly straight ahead at the same speed as above (that's actually possible).

Now you tell me that there exists a slip angle between (1) and (2) which gives a better minimum sink rate than (1) ? You must be kidding...

The example of Andreas may still be true: If you need full aileron against the turn in order to keep the string straight, inflicting a longer apparent wing chord to to the whole wing by a slip angle, plus the additional drag of the fuselage, might carry less drag penalty than having the inner wing running with a fully deflected aileron over 2/3 or even the whole semi-span.

Bert
Ventus cm "TW"

[Bruce Hoult]

On Wednesday, February 17, 2016 at 2:20:30 PM UTC+3, Martin Gregorie wrote:
> On Wed, 17 Feb 2016 01:45:59 -0800, Bruce Hoult wrote:
> > If you were correct then a 20:1 glider and a 60:1 glider with the same
> > weight and same speed would be burning through energy at the same rate
> > because they're generating the same lift. But that's not the case .. the
> > 60:1 glider is losing energy at 1/3 of the rate of the 20:1 glider.
> >
> You're forgetting drag.

No, I'm not forgetting drag. Drag is *precisely* the difference between the 20:1 and the 60:1 glider. The lift is the same. And both are doing zero work in a direction perpendicular to the flight path. There is a force, but no movement, hence no work and no energy. The energy is only in the drag opposing the direction of movement.


> You obviously understand maths. Physics, not so much.

I've got qualifications and experience that say otherwise.

Anyway, I'm out of here. It's obviously long ceased to be productive.

[Eric Greenwell[_4_]]

Bruce Hoult wrote on 2/17/2016 1:45 AM:
> On Wednesday, February 17, 2016 at 1:05:18 AM UTC+3, Martin Gregorie
> wrote:
>> On Tue, 16 Feb 2016 03:26:42 -0800, Bruce Hoult wrote:
>>
>>> On Tuesday, February 16, 2016 at 3:23:18 AM UTC+3, Martin
>>> Gregorie wrote:
>>>> 1) The energy conservation law would be violated if the energy
>>>> taken from the moving aircraft as drag is less than that needed
>>>> to generate the side force.
>>>
>>> Incorrect. Force is not energy.
>>>
>> If you're extracting work from the system (in this case generating
>> a lift force that moves the fuselage sideways) you're using energy
>> that wouldn't be consumed if there was no side force being
>> generated.
>
> Incorrect. Force is not energy.
>
> If you were correct then a 20:1 glider and a 60:1 glider with the
> same weight and same speed would be burning through energy at the
> same rate because they're generating the same lift. But that's not
> the case .. the 60:1 glider is losing energy at 1/3 of the rate of
> the 20:1 glider.

The gliders are not losing the same amount of energy, even though the
airspeed and weight are identical: the 60:1 glider has 1/3 the drag of
the 20:1 glider, so it is losing 1/3 energy of the 20:1 glider.

Airspeed x drag = energy loss per unit of time

The weight of the gliders is irrelevant.


--
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
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/Guide-to-transponders-in-sailplanes-2014A.pdf

[Bruce Hoult]

On Wednesday, February 17, 2016 at 11:00:36 PM UTC+3, Eric Greenwell wrote:
> Bruce Hoult wrote on 2/17/2016 1:45 AM:
> > On Wednesday, February 17, 2016 at 1:05:18 AM UTC+3, Martin Gregorie
> > wrote:
> >> On Tue, 16 Feb 2016 03:26:42 -0800, Bruce Hoult wrote:
> >>
> >>> On Tuesday, February 16, 2016 at 3:23:18 AM UTC+3, Martin
> >>> Gregorie wrote:
> >>>> 1) The energy conservation law would be violated if the energy
> >>>> taken from the moving aircraft as drag is less than that needed
> >>>> to generate the side force.
> >>>
> >>> Incorrect. Force is not energy.
> >>>
> >> If you're extracting work from the system (in this case generating
> >> a lift force that moves the fuselage sideways) you're using energy
> >> that wouldn't be consumed if there was no side force being
> >> generated.
> >
> > Incorrect. Force is not energy.
> >
> > If you were correct then a 20:1 glider and a 60:1 glider with the
> > same weight and same speed would be burning through energy at the
> > same rate because they're generating the same lift. But that's not
> > the case .. the 60:1 glider is losing energy at 1/3 of the rate of
> > the 20:1 glider.
>
> The gliders are not losing the same amount of energy, even though the
> airspeed and weight are identical: the 60:1 glider has 1/3 the drag of
> the 20:1 glider, so it is losing 1/3 energy of the 20:1 glider.
>
> Airspeed x drag = energy loss per unit of time

Exactly!

Martin Gregorie claimed that the energy loss was the same if the lift generated was the same.

[[email protected]]

Some of you boys do too much talking n not enough flying. Theory is all well and good but the proof of the pudding is in the doing. Get in your bird and fly and see the actual differences. But wait, maybe ya'll are just playing on sims lol

[Martin Gregorie[_5_]]

On Wed, 17 Feb 2016 14:44:38 -0800, Bruce Hoult wrote:

> On Wednesday, February 17, 2016 at 11:00:36 PM UTC+3, Eric Greenwell
> wrote:
>> Bruce Hoult wrote on 2/17/2016 1:45 AM:
>> > On Wednesday, February 17, 2016 at 1:05:18 AM UTC+3, Martin Gregorie
>> > wrote:
>> >> On Tue, 16 Feb 2016 03:26:42 -0800, Bruce Hoult wrote:
>> >>
>> >>> On Tuesday, February 16, 2016 at 3:23:18 AM UTC+3, Martin Gregorie
>> >>> wrote:
>> >>>> 1) The energy conservation law would be violated if the energy
>> >>>> taken from the moving aircraft as drag is less than that needed to
>> >>>> generate the side force.
>> >>>
>> >>> Incorrect. Force is not energy.
>> >>>
>> >> If you're extracting work from the system (in this case generating a
>> >> lift force that moves the fuselage sideways) you're using energy
>> >> that wouldn't be consumed if there was no side force being
>> >> generated.
>> >
>> > Incorrect. Force is not energy.
>> >
>> > If you were correct then a 20:1 glider and a 60:1 glider with the
>> > same weight and same speed would be burning through energy at the
>> > same rate because they're generating the same lift. But that's not
>> > the case .. the 60:1 glider is losing energy at 1/3 of the rate of
>> > the 20:1 glider.
>>
>> The gliders are not losing the same amount of energy, even though the
>> airspeed and weight are identical: the 60:1 glider has 1/3 the drag of
>> the 20:1 glider, so it is losing 1/3 energy of the 20:1 glider.
>>
>> Airspeed x drag = energy loss per unit of time
>
> Exactly!
>
> Martin Gregorie claimed that the energy loss was the same if the lift
> generated was the same.

Bull****. Try understanding what you read next time.


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

[[email protected]]

Martin I fully understand this discusion, having some time wheeling around an extended wing open cirrus but I am afraid Bruce there isnt quite at cloud base yet.

[C2]

I asked Dr.
Mark Maugham about how much slip angle the winglets on my discus would tolerate before they stall. This was his response.

"In our flight tests, it wasn't possible to stall the winglet at any sideslips we could generate. There is a misconception that a winglet behaves as a vertical tail, but in reality, it operates in a "bubble" of induced velocities generated by the wing. So, when the glider is at some yaw angle, the flowfield around the winglet does not "experience" nearly as much."

I hope this helps.

[2G]

On Friday, March 11, 2016 at 6:31:54 PM UTC-8, C2 wrote:
> I asked Dr.
> Mark Maugham about how much slip angle the winglets on my discus would tolerate before they stall. This was his response.
>
> "In our flight tests, it wasn't possible to stall the winglet at any sideslips we could generate. There is a misconception that a winglet behaves as a vertical tail, but in reality, it operates in a "bubble" of induced velocities generated by the wing. So, when the glider is at some yaw angle, the flowfield around the winglet does not "experience" nearly as much."
>
> I hope this helps.

Unless I missed something here, all of you misinterpreted the argument Johnson was making. Basically, he stated (I was there when he said it) that the geometry of a coordinated sailplane in a turn placed the yaw string slightly ahead of the airflow passing over the wings. Thus the yaw string would be displaced slightly to the outside of the turn (again, assuming the sailplane is in coordinated flight). At no time did he advocate flying uncoordinated in a turn.

I worked out the geometry involved and found that this displacement would be less than the width of the yaw string. I told Dick so, but he could be pretty stubborn at times :-)

Tom

[Paul B[_2_]]

This keeps coming up and seems to be generally believed, particularly when it is accompanied by a nice diagram where invariably the scale of the glider and the circle is utterly mismatched. If you do the calculations for average circle the resultent angular dis placement is about one and a half degrees. I am sure I cannot discern that in the fluttering string. Perhaps others can

Cheers

Paul

[danlj]

This discussion is exemplary of why r.a.s is not a place to learn.
There is the usual low fact:fancy ratio and high fertilizer quotient, spiced with a little unwarranted personal vindictive.
The personal experience is useful, yet the only analytic comment that is sound is the quotation of Mark Maughmer. We need less "physics" and more aerodynamics to understand this phenomenon.
In addition to aileron drag, spanwise flow is importantly improved for ships without spoilers with a little slip (the proof of the pudding is in the eating). Regardless of improved climb, there's a real safety benefit: in a steep turn, in an un-spoilered ship with long wings, if the yaw string is kept centered, the stick will be hard against the top stop. Now there is only two-axis control. Thermal turbulence will readily cause an incipient spin, as I have experienced many times -- until I learned how and why to slip steep turns. Slipping the turn enough to (approximately) center the stick:
- most importantly improves control (a good thing for the spouse & kids, and the others sharing the thermal)
- decreases span-wise flow, improving the lift distribution, and
- decreases aileron drag, which probably improves turning sink rate, but more important, helps avoid stalling the inboard wing.
Ships with spoilers have improved circulation at the outboard part of the wing, decreasing the need to slip turns.
Credits: Dr. Mark Maughmer, Tilo Holinghaus

[Karl Striedieck[_2_]]

The statement attributed to Dr Maughmer that winglets don't stall is baloney. They do stall and it is easy and interesting to observe by simply tufting them and watching as you increase yaw or slip in a turn.

I found on the Duo Discus XL the inboard winglet stalls at a yaw string angle of about 15-20 degrees (not sure how that corrolates to actual aircraft yaw angle). But the amount of slip required to cause the yarn tufts to blow backward was a little more than a pilot would naturally use. So Schempp has chosen a mounting angle that, while not optimum in perfectly straight flight, is tolerant of pilot "abuse." A stalled winglet is effectively a mini speed brake. Not welcome.

KS

[[email protected]]

On Monday, February 8, 2016 at 12:55:19 PM UTC-5, kirk.stant wrote:
> On Sunday, February 7, 2016 at 6:11:14 PM UTC-6, Martin Gregorie wrote:
> >
> > Doesn't this follow from the fact that the yaw string is a fair distance
> > in front of the wing?
> >
> > Think about it: if the tipwise axis of the wing is exactly aligned with
> > the radius of the turn, which it should be for maximum climb efficiency,
> > the yaw string should also be at right angles to the turn radius but,
> > being a good 2m/6ft or thereabouts in front of the wing, the turn radius
> > it is on is angled forward of the turn radius the wing is on, which
> > therefore makes it point out slightly out. This looks like a slight slip
> > to the pilot.
> >
> > Draw this situation on a piece of paper, but exaggerate the situation by
> > drawing the circle diameter and about twice the wingspan and you'll see
> > what I described.
> >
> > If you redraw the diagram with correctly scaled turn radius and wingspan,
> > the angular difference between the wing radius line and the yaw string
> > radius line will be very small, but this needs to be adjusted because the
> > curved shape of the canopy will amplify the angle of yaw string in a
> > slip. Now Dick Johnson's advice makes perfect sense.
> >
> > Practically, if the yaw string is central or pointing slightly out in a
> > thermal you're doing it right. If its pointing a long way out or to the
> > inside of the turn you're sideslipping while turning and this is hurting
> > your climb rate by generating excessive drag.
>
> Martin, I believe this is only part of the answer. You are correct that a perfectly coordinated turn (slip ball in the center) should show some slip if the yaw string is forward of the CG (and conversely, a centered forward mounted yaw string indicates a skidding turn - beware in the pattern!) but there is also the overbanking effect of long wings seeing different local airflow velocity in a turn; this can be countered by holding aileron out of the turn (inefficient and draggy) or by adding a little bit of slip via top rudder. This slightly increases the angle of attack of the lower wing and decreases the angle of attack of the top wing and voila, no overbanking, ailerons are flush and efficient, and you gleefully outclimb the newb with the perfectly aligned yaw string. It's really noticeable in my LS6 (probably due to the dihedral) - it settles into a nice slipping climb with the yaw string off about 10 degrees to the outside, no aileron deflection, and just a touch of top rudder. Extra bonus - you get some lift off the fuselage! (See "String Theory").
>
> Kirk
> 66

John Coutts, world champion, stated this as well. I had the pleasure of two days dual instruction with him in Minden. His demonstration of this effect convinced me of the benifit of a little top rudder, increasing the angle of attack of the inner wing, while thermaling.

[firsys]

On Sunday, February 7, 2016 at 5:09:42 PM UTC-5, wrote:
> I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?
>
> Sorry for the questions, but would appreciate any help in this.
Karl is more forthright, than I was going to be!
When I installed homebuilt winglets ( Maugner airfoil) on my 20E,
I set them at the recommended -ve angle to the horizontal axis
of 3 deg from the zero lift setting. Upon tufting these winglets,
it was quickly apparent that keeping them both unstalled was
almost impossible ( Note , the flow at the wingtip is not
along the axis) I added 2 deg extra negative AoI and the problem
became manageable, but I still now pay a lot more attention to
slip ( or not).

Slipping turns ( without winglets) are useful.
Years ago, flying a rented Libelle std I found climbing against the
Std Cirrus discouraging. After ballasting to aft CG and circling
with a fair amount of slip, I could climb with the best Cirrus
drivers. But they left me on the glide!

John firth; an old no longer bold pilot.