On Friday, September 28, 2018 at 2:26:57 PM UTC-4, Bob Kuykendall wrote:
On Friday, September 28, 2018 at 8:42:00 AM UTC-7, wrote:
I've been mulling this over for years now...

Very interesting, good catch, JJ! I think you're off base about the direction of fuselage bending, but you might be on the right track in general.

* Up elevator tends to bend the tailboom downward.

* Bending the tailboom downward applies compression to the bottom half and tension to the top half.

* In many gliders, the elevator push-pull tube presses aft for up elevator, and pulls forward for down elevator. (Known exceptions are ASW24-29, and the later single-seat Schleicher cockpits based on them)

* In some gliders, the elevator push-pull tube runs along the bottom half of the tailboom

Given a glider with a limber tailboom, the elevator push-pull tube running along the bottom of the tailboom, and the push-pull tube pressing aft for up elevator, the system described will have some measure of positive feedback.

As the pilot applies up elevator, the increased downward force at the tail bends the tailboom downward. The bending compresses and shortens the lower half of the tailboom structure. The shortening effect reduces the distance between where the elevator push-pull tube starts and where it meets the bellcrank at the bottom of the fin. Since the push-pull tube does not shorten as well, it tends to apply additional up elevator force at the aft bellcrank and consequently the elevator.

The big question is whether the effect is pronounced enough to become divergent at any point within the glider's operational flight envelope. My suspicion is that it's not. In most gliders, the tailboom is so stiff, and the elevator push-pull tube so close to the tailboom neutral axis, that the effect will be so small as to be barely noticeable. However, if perhaps the tailboom is more limber than normal (maybe because of an engine cutout), and the elevator push-pull tube is lower than normal (maybe relocated downward to accommodate an engine installation), the effect might be significant.

However, as B-47 aficionados will recognize, there is one important additional factor: As the tailboom bends downward, the relative incidence between the wing and horizontal stabilizer decreases, reducing the downward force applied by the horizontal stabilizer. This effect will tend to negate the elevator input effect described above, and might in fact completely overpower it.

Overall, this would be a great topic for one of our friends at Akaflieg Cal Poly San Luis Obispo. Some simple FEA on typical glider shapes and structures should be enough to indicate whether further study is warranted.

--Bob K.

Interesting thoughts, Bob! One point in JJ's post that caught my eye was the fact that all these accidents happened in SH gliders with a motor. I have to claim ignorance about the way the motor is held in the stowed position but is there a possibility that the engine assembly pushes down onto the elevator control rod when subjected to high g-loads?
Uli
'AS'