Question about spoilers and pitch stability

On Friday, February 1, 2013 10:09:41 AM UTC-7, Evan Ludeman wrote:
On Thursday, January 31, 2013 11:42:53 PM UTC-5, Larry Suter wrote:

In his article on avoiding PIO in Grob 103's,







http://www.soaringsafety.org/pilots/ic8.htm







Dean Carswell writes, "the more the airbrakes are closed, the less pitch stability the Grob will have, making a PIO more likely if otherwise mishandled"







Is there a simple explanation why opening the spoilers increases the pitch stability? Does it somehow move the center of lift further aft? I believe increasing the separation between the cg and center of lift is the classic way to increase pitch stability.







And if that's how it works, why does the center of lift move aft? I would guess spoilers destroy the lift downwind of their location, causing the center of lift to move forward.....







Thanks,



Larry



The classic problem here isn't a PIO the way we normally understand it.



The incident starts with trying to "land" on the nosewheel. The normal setup involves the pilot trying to land at 70 kts or so with spoilers closed.. Landing long as in Dean's article, or perhaps getting sink in the pattern and getting a little panicky and diving for the runway with brakes closed.. The nose wheel touches first and the AOA increases as the rest of the glider momentarily descends, usually resulting in some main wheel contact (usually modest on first contact). The glider, now at about 69.9 kts, with increased AOA, takes flight again. This happens *really* fast. Inertia has probably caused the control stick to move forward during this rotation, so the glider launches, noses over and hits much harder on nose / main / tail the 2nd time and it will lather rinse and repeat until the process is interrupted. If you see this happen, you will not soon forget it!



I'm familiar with a couple such incidents. In all cases I know of, it involved trying to land hot with spoilers closed. In all cases, it starts with nose wheel contact. If it ends well, it's because the pilot managed to stabilize the glider in flight, open the spoilers, achieve a better landing attitude and complete the landing normally.



Opening the spoilers increases the AOA necessary to fly at any given speed, so it decreases the chances of making first contact on the nose wheel, which is obviously to be prevented.



Evan Ludeman / T8

Evan, that's an excellent explanation of the problem. I would only add that I'm unaware of any aircraft which gracefully tolerates nose-wheel-first runway contact.

I've seen a couple of these accidents and you're right, they were ugly and I won't forget them. One of them seemed to be a case of the pilot realizing too late the landing was going to be dangerously long, panicking, then trying to stop the glider by shoving an imaginary skid into the runway - likely negative transfer from his primary trainer.

BTW, manufacturers put a yellow triangle on the airspeed indicator to suggest a safe airspeed on short approach. Airplane guys call this Vref. If a Grob 103 is slowed to just above the yellow triangle on short final (not pattern) the 'arrival' is guaranteed to be on the main wheel.