A question on stalling and flat spin - in gliders

Question

Very short version: Do gliders do stalls, and flat spins, as aerobatic tricks (as do say aerobatic biplanes).

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As I understand it, with an aerobatic plane, a pilot can and does, for "fun" as it were, and for training purposes, and as part of certain (?) aerobatic trick displays, deliberately stall the craft (is that the correct way to put it?) and indeed subsequently (as I understand it) put the craft in to a "flat spin" (which I guess can be looked on as either a "particularly good-looking trick from the ground" and/or a death situation). And indeed then can recover from that (by doing something tricky, I believe along the lines of waiting a breathtaking amount of time, stomping on one pedal one way and slamming another lever another way). As I further understand it, indeed for pilots in (advanced? aerobatic only?) training, deliberately stalling (and obviously recovering) is part of the training. (But maybe that's wrong, and/or only applied in the past.)

My question is simple, does the same apply with gliders?

Can/do you deliberately (ie, as part of an aerobatic trick or display) stall gliders? Or is it just not a thing with gliders?

(I suppose associated questions to clear up my misunderstanding; in general does a stall always result in a "flat spin"; I believe there's a thing where you point the plane straight up until it basically tumbles - can you do that in a glider and is that indeed the only way to "stall a plane deliberately to perform a trick" or are there other ways; beyond gliders can All aircraft be stalled deliberately and then recovered - or is that say plain impossible in a plain old Cessna (you'll die) but possible in a "Pitts special" type acrobatic plane, ie the Pitts is literally designed for that purpose, to recover from a stall? flat spin? whereas in a Cessna it's death - ?)

I guess a thrilling addendum would be, has anyone on here stalled a glider! TY

Answer 1

All aircraft, in fact anything with a wing, can stall that wing. A stall is simply the increase of angle of attack beyond a specific point.

Not all stalls result in a spin. In fact, for the purpose of flight training, the stall characteristics of a simple trainer, like a Cessna, are demonstrated and practiced repeatedly. The process for recovery is simple; at the onset of the stall - characterized by buffeting and lack of aileron control usually - the control yoke/stick is pushed forward to lower the nose (and by extension AoA) and throttle is increased to recover airspeed. You can often go beyond this and recover a fully-developed stall which is often characterized by the sounding of the stall warning horn. The recovery from this is similar.

When you fully develop a stall, and even go beyond, there is every opportunity for one wing to stall before the other - this happens if your are not in coordinated flight (ie, you're yawing). If one wing stalls before the other you may well end up with that wing dropping, which left unchecked will result in a spin. You can also force this situation by stalling the aircraft and pushing the rudder one way or the other - this is how you initiate a spin for the purpose of inducing a spin.

So why would you do that? It used to be a standard part of flight training - to recover a spin - it no longer is in most jurisdictions because (rightly or wrongly) it was deemed that the dangers posed by practicing it outweighed the benefits.

I, like many pilots, wanted to practice it in case I ever did find myself in a bad situation. So aerobatics training organisations will provide training.

The process is fairly simple (albeit vary slightly by type); In a spin your wings are stalled, so the ailerons have little or no affect. Center the ailerons and, somewhat counter-intuitively you want a bit of forward pressure and to cut the throttle to stop you over-speeding. Use rudder in the opposite direction to the spin (ie, a clockwise spin requires left rudder) to break the spin. This will then see you pointing down, but wings somewhat level. At this point you recover from the dive using elevators, but not too quickly as to over-stress the airframe. Easy, but disorienting if you've not done it before. Recovery from a flat spin has some slightly different steps, but for an experienced aerobatic pilot I'm led to believe not all that bad.

Answer 2

A spin is a quasistationary state in which the plane rapidly spins around a vertical axis, driven by lift differences between its wings. The spin axis is located close to the nose of the airplane, so its tail and wings swing around. This induces additional speeds on them, increasing the local angle of attack on the receding wing and vice versa on the progressing wing. While the receding wing shows fully separated flow, part of the progressing wing experiences attached flow.

Principle of a regular spin (picture source)

In a regular (steep) spin the angle of attack at the center is between 30° and 60°, and the control surfaces keep some of their effectiveness. A flat spin occurs with an angle of attack well above 60°, so all flight surfaces and their control surfaces show fully separated flow.

Of course you can stall gliders. However, putting them deliberately into a flat spin is rather impossible without specific modifications to them. Part of the certification tests is to enter a spin and prove that this is either impossible or easy to end. Flat spins are very hard to end, so a glider which gets easily into a flat spin will not be certified.

How a flat spin develops and what preconditions must exist for it to be possible is explained here.

When I talk of spins from here on, I mean regular steep spins.

The normal way for entering a spin is to slow down and stall, then kick the rudder hard in one direction. The airplane will yaw and roll and pretty soon enter a spin or a spiral dive.

Some gliders will not enter a spin but only a spiral dive. Due to the much lower drag in a spiral dive, speed will quickly increase and will make this maneuver very dangerous if the dive is not ended soon. For example, the popular training glider ASK-21 will not spin if flown by two people. Flown solo by a light person, it will happily spin and the spin can be ended already if that person unbuckles and leans forward. The small shift in the center of gravity will be enough to end the spin.

In glider aerobatics spins and inverted spins are performed routinely. Inverted spins are started in inverted flight, but otherwise in the same way as upright spins. It is advised, however, to pull out of the spin into upright flight instead of ending the spin inverted. Multiple negative gs are painful!