Uncontrolled Loops Elevator failure

Le vendredi 28 septembre 2018 21:04:10 UTC+2, AS a écritÂ*:
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'

No. The engine bay's lower enclosure is the shell of the fuselage.

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'

No. The engine bay's lower enclosure is the shell of the fuselage.

So how are the control rods/cables routed past the engine bay? On either side of the engine bay? Between the outside shell and the engine compartment?
As I said, I am not familiar with these designs.

Uli
'AS'

You can't get by the engine compartment above or below, so anything that needs to pass goes left or right.

As a Duo owner, the Arcus and now the Duo T crash certainly have my attention. I have very carefully inspected the rudder cables everywhere that I can. Today I did some thorough checking of the elevator circuits and a few checks similar to those called for in the AD for single seaters around the elevator pushrod.

Nothing to indicate I have anything to be concerned about, but it makes you wonder.

I've certainly spent more time thinking about my response to various failure scenarios as a result of these accidents. I don't know that it will help should something go wrong in my glider, but it's better than trying to think of a response under the stress of the moment.

More importantly, my preflight bailout discussions with copilots has become more pointed and serious. Previously I was pretty lax in my bailout briefing with people. Sure, I'd explain the parachute for people that aren't used to wearing one. But it was mostly lip service that we all give the bailout. Canopy, belts, butt...

I've lost three friends this summer to bailouts that didn't happen or didn't happen soon enough. So my new briefing approach is still to cover canopy, belts, butt. But more about what I'll say "bail out, bail out, bail out" and the fact that it's very likely everyone for themselves. The chaos of a bailout situation won't afford a nice cordial, "no, you first, I insist, please do." There may only be one chance to get out.

Will I try to get my copilot out first, you bet. Can I guarantee in the moment that I'll know if they did or not, very unlikely.

I'm also very clear that if we have altitude and have any questions about controllability of the glider, we will get out. We aren't going to "see how this works out" if it seems unlikely to remain in control. Bailing out while still under semi-control seems a lot more viable than when something snaps and goes really wrong. I'd rather take my chances parachuting into inhospitable terrain than having a glider depart even semi-controlled flight.

One last thing I've decided I should start doing. Briefing the removal of the pin and the canopy latch on the right side. It's not intended for emergency use, but in the case of a right slip due to a rudder cable failure, I realize it might be very difficult to get rid of the break-away canopy if the wind is forcing it closed. A pre-briefed situation about the use of that latch might make a difference.

All things I hope to never test.

Morgan
5H