Debunking Glider Spoiler Turns Causing Spin Thinking

I was taught that there is nothing inherently dangerous in using spoilers in the circuit, provided that the correct speeds are maintained.

With spoilers out, even just cracked open, during base and finals turns safe speed margins are reduced, i.e. inside wing flying slower, reduced lift from deployed spoilers and wind shear (again inside wing flying slower).

However, I was advised NEVER to use them on the base and finals turns (unless absolutely necessary and me paying full attention) and here is why:

Imagine that you normally use spoilers during these turns, with no problems at all, but one day you are returning from a 9 hour cross country flight, it’s been a long final glide and energy is low. Also the weather has changed and the wind is gusting from an approaching storm front.

So you are fatigued, the glider is in a low energy state and wind is gusting. The holes in the cheese line up and you crack the spoilers during a steep base turn with speed too low. The end.

I haven't read every comment in this thread from the beginning, so maybe someone has observed this previously. If so, apologies for the repetition.

There's nothing inherent about turning that will cause a stall or spin. There's nothing inherent about deploying the spoilers that will cause a stall or spin. There's noting inherent about putting out the landing flaps that will cause a stall or spin. There's noting inherent about turning low that will cause a stall or spin. There's nothing inherent about doing them all at once that will cause a stall or spin. But, the more moving parts there are in your approach maneuvers the more attention it will take to keep everything in proper order. A few examples:

Turning below a few hundred feet presents the pilot with a very different peripheral scene because the inside wing tip traces a circle against the ground that is in the direction of travel rather than the opposite direction at higher altitudes. This tends to make it feel as though you are slipping and over-ruddering the turn can become a risk. Not a problem if you fly the airplane properly.

When you deploy the spoilers two things happen. First, you spoil the lift on the portion of the wing spanned by the spoiler (that's why they're called spoilers). This means to maintain unaccelerated flight the rest of the wing needs to produce more lift, which the pilot may compensate for by increasing angle of attack. Not a problem if you fly the airplane properly, but if you are too close to the angle of attack for stall, you can end up on the wrong side of the Cl vs alpha curve.

Second, deploying spoilers increases drag so the descent angle needs to increase to keep from bleeding off airspeed. The net effect between loss of lift and increase in drag depends a bit on the glider and the airspeed and g-load, but suffice to say that airspeed and AOA will have additional influences inflicted on them. Not a problem if you fly the airplane properly, but a bunch of rates and angles are going to change when you tug on the spoiler of flap handle.

I, for one, try not to change too many things too quickly all at once in the pattern, or if I do, I tend to push the nose over a bit in case I get distracted by, you know, looking out the window. Can you yank aggressively on all the handles at once and not create a problem? Absolutely, but it helps to pay good attention if you do.

The FAA preaches stabilized approach for a reason:

https://www.faa.gov/news/safety_brie...opic_16-11.pdf

Andy Blackburn
9B

On Saturday, January 6, 2018 at 1:38:02 AM UTC-7, Andy Blackburn wrote:
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Second, deploying spoilers increases drag so the descent angle needs to increase to keep from bleeding off airspeed. The net effect between loss of lift and increase in drag depends a bit on the glider and the airspeed and g-load, but suffice to say that airspeed and AOA will have additional influences inflicted on them. Not a problem if you fly the airplane properly, but a bunch of rates and angles are going to change when you tug on the spoiler of flap handle.
....
Andy Blackburn
9B

This, I believe is the key that people are not considering. The physics of it is such that when the air brakes/spoilers are deployed, they disrupt not only the lift, but increase the drag. As such, if you kept the aircraft in the same attitude, you would increase the AOA and are more likely to stall.

However, if you maintain your airspeed, to do this and deploy the spoilers, you would need to change the attitude of the aircraft into a more nose-down position. This would decrease the AOA, thus compensating for the decreased lift and increased drag, and the net effect would be no significant change to the stall speed, whether you are turning or flying level wings. The AOA remains the same, and IAS and stall speed also remain the same.