Slips in turns and landing with winglets

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

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

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

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

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).

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.