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In a forward slip (I'm learning to fly sailplanes, if that matters), quite a lot of back pressure is needed to keep the nose up in a forward slip. Why, aerodynamically speaking, is this the case?

I understand that, to transition from coordinated flight to a forward slip, the aircraft effectively has to rotate around its yaw axis. From the perspective of the pilot, this results in a bit of bank angle and simultaneously raising the nose. However, in a fully developed forward slip I've often had to fully pull back on the stick to prevent the nose from dropping and gaining too much speed, which is normally not necessary to slightly raise the nose.

I asked an instructor, and they told me that due to the T-tail construction of a glider, the horizontal stabilizer sees extra pressure underneath it due to the relative wind blowing at the vertical stabilizer, pushing the tail up. However, in my reasoning, the other half of the elevator should see reduced pressure as it is shielded by the vertical stabilizer. Additionally, the horizontal stabilizer is simply less effective as its projected wingspan is less in a forward slip - but surely the main wings also lift proportionally less?

Can someone shed some more light on why (almost) full stick back is needed to maintain a forward slip in a sailplane?

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    $\begingroup$ On which sailplane did you experience this? $\endgroup$
    – Darjan
    Commented Apr 9 at 13:05
  • $\begingroup$ Re "Additionally, the horizontal stabilizer is simply less effective as its projected wingspan is less in a forward slip - but surely the main wings also lift proportionally less?" - I guess that's just another way to say that it you look at the angle between the relative wind and the longitudinal axis of the aircraft, in coordinated flight 100% of that angle is "projected" as the angle-of-attack, but in slipping flight some of that angle is "projected" as the a-o-a but some of that angle is "projected" as the yaw or slip angle. $\endgroup$ Commented Apr 9 at 16:17
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    $\begingroup$ I have some recollection of noticing that this tendency is particularly pronounced in a Schweizer 1-36, which is a T-tailed glider. It's been some years ago but my recollection is that I was playing around with contrlling the glider with rudder only, hands off the stick, and if the yaw/slip angle accidentally got too severe (for example while trying to pick up a lowered wing w/ rudder only) the nose would really want to plunge down, in a way that I hadn't seen w/ 1-26, Ka6, and some others. (Not quite the same as obsrving that "almost full back stick is needed to maintn a fwd slip" though.) $\endgroup$ Commented Apr 9 at 16:22
  • $\begingroup$ I've so far slipped in an ASK21, LS4 and Duo Discus, I guess the ASK21 was most pronounced but the others required quite some back pressure too $\endgroup$
    – Sanchises
    Commented Apr 9 at 17:40
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    $\begingroup$ @quietflyer, not sure about calm day actual calmness regarding gps measurement; no clue how bad is a pitot value impacted by slip either. If it's that bad, maybe one external experimental setup using weathervaning pitot for deltaplanes and paragliders. $\endgroup$
    – user21228
    Commented Apr 9 at 20:14

2 Answers 2

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A T-tail produces a noticeable pitch-down moment change in a sideslip.

That is basically all. You go into a steady sideslip and need to retrim to keep the speed at the desired setting. This will not be needed in gliders with a low mounted horizontal tail.

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  • $\begingroup$ Thanks for the find, so this is actually a duplicate! $\endgroup$
    – Sanchises
    Commented May 11 at 16:14
  • $\begingroup$ @Sanchises Maybe, but that fact is not mentioned in the major books about flight mechanics. Few professionals know about it. $\endgroup$ Commented May 12 at 11:11
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Looking at the tail design of the Schweizer SGS 1-36 Sprite one might tend to believe the instructor and thank the designer.

Flying a forward slip involves intentionally cross controlling the aircraft in an effort to increase drag, which should steepen the glide angle at the same speed.

This does result, as the name slip implies, in a pronounced change in relative wind on the vertical stabilizer. With the T-tail mount essentially acting as an end plate, it is understandable that increased pressure on the horizontal under surface will produce a downward pitch.

A secondary reason might be a disruption of the downwash sheet the wing produces by slipping, removing downward pressure on the tail.

Since a glider flying at low speeds is only a few knots above stall speed, and most glider pilots are loathe to waste energy on higher drag configurations (except for landing), it seems the tendency to have to add back pressure on the stick is arguably better than to allow airspeed to slow unacceptably.

However, this might be a case where spoilers may be preferred to extreme forward slipping. A combination of spoilers and moderate slipping may be better. Aerodynamic performance will vary between models. Expert instruction is invaluable.

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    $\begingroup$ During my first flights in a glider, there were some construction works that we were to overfly at 100m or more. The resulting final approach was always full spoilers and slipping. Quite a memorable way to start flying. $\endgroup$
    – Sanchises
    Commented Apr 11 at 13:19

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