Slips in turns and landing with winglets

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.

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

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

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/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

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.

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.

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

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

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.

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