Debunking Glider Spoiler Turns Causing Spin Thinking

On Tuesday, June 2, 2015 at 2:42:53 AM UTC-5, Surge wrote:


The "no spoilers during turns" rule makes sense if you're flying the turn too slowly and not taking the increased stall speed into account.

Example: According to the POH, a 650kg (1430lbs) Twin Astir stalls at about 90km (49 knots) with spoilers fully deployed. Add a 45 degree bank and the stall speed increases to 126 km/h (68 knots).
This is usually above the normal approach speed flown during circuits so I can conceive that a pilot who is not "ahead of the glider" could possibly fly too slowly during the turn. Throw in an uncoordinated turn and things could go wrong very quickly depending on glider type.

This is just the expected % increase in stall speed due to the loading imposed by the turn, right? But remember, the turn doesn't really "impose" the extra loading. The extra loading (lift) is imposed by either an increase in airspeed (safe), or by the pilot moving the stick aft to try to maintain the old airspeed (unsafe). If the pilot doesn't move the stick aft as he increases the bank angle, it will take a bit of time (a few seconds) for the airspeed (and loading) to increase, but there will be no risk of stall. During these few seconds, the flight path will curve slightly downward because the wing is not producing the full load "required" by the bank angle. The same would happen if the pilot opened the spoilers while holding a constant (or zero) bank angle-- the wing won't stall unless the pilot increases the angle-of-attack.

I guess what I'm trying to say is-- one of the key points of Tom Knauff's books is that turns aren't unsafe as long as you don't move the stick aft. The same could be said of spoiler usage. Opening the spoiler won't produce a stall as long as the pilot doesn't move the stick aft to increase the wing's angle-of-attack.

Banking and opening spoilers aren't completely analogous though. As you increase bank angle, you must move the stick further aft just to hold the SAME angle-of-attack that you had at a shallower bank angle. To actually increase the angle-of-attack, you have to move the stick even FURTHER aft. This is a point in the pilot's favor, in terms of avoiding accidental stalls, so long as the pilot has some awareness of what he is doing with the stick. Again this comes up in Knauff's books and seminars. This has to do with the curving nature of the airflow in a turn, and the resulting change in airflow at the tail. The same is not true when you increase loading per unit wing area in other ways-- like by adding water ballast, or by "removing" part of the wing (opening spoilers).

I guess there's another factor that could be relevant. In some gliders, opening the spoilers creates a large trim change, forcing you to move the stick just to hold the same angle-of-attack that you had before opening them. If you have plenty of airspeed-- no problem. If you have less airspeed and you are going by the theory that you won't stall because you aren't doing the wrong thing with the stick, maybe you could have a problem. Even if angle-of-attack tends to stay constant as the spoilers are opened, the resulting downcurve of the flight path could tempt a pilot to haul back on the stick to arrest it. At the end of the day I guess you'd better have plenty of airspeed if you are making radical changes in spoiler setting.

S