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This question already has an answer here:

In a glider, sometimes it's useful to do a side slip with crossed controls. However, when that happens, the pitot tube angle is out of line and the ASI goes to zero, therefore losing any indication of the current airspeed until the sideslip is interrupted.

During this maneuver, what would happen if I were not to keep proper control of the attitude, and thus airspeed, and were to stall the wings? would the glider spin, or would the unusual attitude prevent it?

Edit: The duplicate question does not necessarily responds as it seems to address powered airplanes, where other factors may be in play. My question targets specifically gliders.

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marked as duplicate by AEhere, Manu H, Koyovis, Sean, fooot Jul 23 at 14:11

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  • $\begingroup$ Bottom line here is that cross controlled flight involves greater risk and is best done at a comfortable and known margin above stall. Flaps and spoilers are also available to alter rate of descent. Every plane is different. It is imperative to safely learn the stall characteristics first. There is no general rule for all aircraft. $\endgroup$ – Robert DiGiovanni Jul 21 at 1:47
  • $\begingroup$ What do you mean by "the unusual asset"? $\endgroup$ – Timber Swett Jul 21 at 2:13
  • $\begingroup$ @TannerSwett I strongly suspect that was supposed to be "the unusual attitude", and have edited accordingly. Stefano, if I'm wrong, please feel free to Edit further. $\endgroup$ – a CVn Jul 21 at 9:41
  • $\begingroup$ @aCVn I meant asset. What I meant is the strange position. $\endgroup$ – Stefano Borini Jul 22 at 19:06
  • $\begingroup$ That is a not a normal meaning for "asset"; the edit was helpful. $\endgroup$ – quiet flyer Jul 23 at 11:00
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Yes if you were to stall the airplane while in a side slip it will probably depart, or start to depart, into a spin. How aggressively would depend on the glider's actual stall behavior, some being more benign than others, but say you were in a glider with an abrupt stall with minimal warning and a sharp nose drop. That sort of glider would spin if you didn't initiate recovery right away.

This isn't such a big problem, because the nice thing about gliders is you don't have power to confuse things when it comes to pitch attitude vs speed. For a given configuration (weight/CG etc), if attitude X gives speed Y than you can be certain that any time you pitch to attitude to X you will get speed Y, with or without an airspeed indicator.

So, if you are on final at 50 kt, and want to side slip, just make sure you lower the nose a couple of degrees during the slip and control the pitch attitude, so that you will maintain somewhere around 50kt with the additional drag and there is very little risk of stalling and spinning. Easy peasy.

You also have the other impending stall cues, like softening controls, noise dying off, and such, so if you are aware and on top of things there shouldn't be a problem and it would take some ham fisted flying to really mess up.

If you are getting trained well, there should be dedicated no-airspeed instruction and practice and once you are considered a proficient pilot you should have no problem with flying safely with no airspeed for all maneuvers (if you sent me out in a power plane on some kind of wacky challenge, and told me I could only have one operating flight instrument, my choice would be a compass).

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  • $\begingroup$ Excellent .Thank you. I was in fact concerned that doing side slip on landing would be potentially a problem if I were to fly through strong wind gradient and I were to find myself stalled while sideslipping. I don't do it very often, but one day it might be useful with airbrakes and sideslip if I really have to get into that field, so it's good training and good to know. $\endgroup$ – Stefano Borini Jul 22 at 19:09
  • $\begingroup$ Best thing is to induce side slip on landing configuration at altitude and see just what it takes. It's good to explore that behaviour for each glider you fly, because they all have variations in behaviour. Also things like spoiler deployment on pitch. Some pitch up, some pitch down. But if you're flying a glider with speed limiting dive brakes where you can come down at crazy steep angles, I can't really see a need to slip. It's more for jammed spoilers, or old gliders like 233s with not very powerful spoilers. $\endgroup$ – John K Jul 22 at 21:06
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Firstly, I would find a way to keep that airspeed info coming. Loss of airspeed info certainly could be very dangerous, especially if you are slipping into your landing approach in any kind of winds.

That being said, if you are cross controlled (ailerons in rudder away), as soon as you feel a buffet center the ailerons and rudder, and PITCH DOWN. If you haven't started yawing, you are OK. If you have, use that rudder!

Breaking/preventing the stall by pitching down is critical, as is controlling yaw with the rudder. Do not worry about your bank, you can carefully roll out after the stall is definitely broken. Only then can you safely use aileron inputs.

Provided proper inputs (especially centering ailerons) are done at stall warning, spin should not develop if the CG is set up correctly and the glider is of a stable design. I would worry a lot more about my airspeed indicator.

Remember, the best way to know the consequences of a stall is to try it with a qualified instructor at a safe altitude. This is the best way to learn the slow flight characteristics of your plane.

If your design is docile enough, simply relaxing the elevator may be enough when you feel the buffet, but practicing up high will help fine tune your technique tremendously.

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  • $\begingroup$ "Firstly, I would find a way to keep that airspeed info coming." You simply can't. slipping means the pitot receives no air in a glider. It's impressive how the ASI quickly dies to zero as soon as you have a few degrees in. $\endgroup$ – Stefano Borini Jul 22 at 19:15
  • $\begingroup$ As a researcher that would immediately get me interested in an alternative. Though the traditional pitot tube is dear to my heart, a cup anemoter mounted on a swivel with fins to hold it into the wind (not unlike a tiny windmill) might work better. At low speeds, particularly when deflected, pitot tubes are notoriously unreliable. $\endgroup$ – Robert DiGiovanni Jul 22 at 21:11
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I would suggest you take some dual spin training, and then go carefully perform some constant-heading cross-controlled stalls at altitude in all the gliders you will be flying, with the intent of exploring the flight characteristics up to the point of the stall break. (Don't intentionally take things further than the stall break unless spins are allowed in that glider, and you are competent in spin recovery.) I would suggest that in a constant-heading slip or a slipping turn with the yaw string blowing toward the outside of the turn (as opposed to a skidding turn with the yaw string blowing toward the inside of the turn), in most gliders you'll find that if you relax the aft stick pressure as soon as the nose begins to drop in the stall, the glider will not spin. (Please add a note to the end of this answer listing any glider types where you find this not to be true!)

In general, if you are slipping to lose altitude, you'll get the best results if you carry some excess airspeed-- keep that nose down. But note that a normal crosswind landing, using the wing-down method, does end up with the aircraft slowing to near stall speed in uncoordinated (slipping) flight. In most gliders this is considered to be an acceptable landing technique.

Here is a related answer: What happens in a stall during a slip?

The last paragraph of this answer also has some related content: Is this paragraph about the dihedral effect in the FAA's Glider Flying Handbook correct?

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