Several months ago, I wrote about a problem with 2-56 clevis's and have changed a few on my planes. I did not get around to the one I am writing about because it was flying oh so well, looked nice and I was so very close to finally getting through the pattern, (I am fairly new back to the sport and back in the early fifties when I quit, I could loop and fly upside down...usually.)
In any event, I did finally complete the first ever (for me) pattern just the last two flights before the crash. I crashed because the 2-56 clevis I was using between the bell crank and flap control horn opened. To be fair, there were a series of circumstances that led to the clevis opening and they were not all related to the clevis. So I relate this only for its informative content.
First: I had put hard "stops" in the wings to prevent hyper extension of the bell crank. I set them at about 45 degrees bell crank travel. As it turns out, the flaps travel less than the bell crank so the flaps provided the "stop" and not the stops I installed.
Second: I had originally installed a pushrod exit from the wing, made of a small diameter brass tube. Looks cool.
Third: It was necessary to bend the pushrod from the bell crank to the flap control horn to avoid binding of the pushrod. After test flying, the pushrod still "rubbed" on the brass tube so I removed the brass tube but did not straighten or replace the pushrod. As a result of not straightening the pushrod, the pushrod would tend to rotate as the bell crank was actuated.
Fourth: The flap control horn was old and I reused it. The hole that the pushrod clevis connected with was asymmetrically worn and larger that the clevis pin.
Fifth: I used a 2-56 clevis and pushrod. I believe the clevis was a Dubro. In any event, the clevis was held shut by a thin black sleeve, much like a heat shrink that you pulled over the clevis once in place. On the previous day, I had found the sleeve was not on the clevis and again slid it onto the clevis connection. I should point out here that unlike the Great Planes clevis which has barbs that prevent the sleeve from sliding off the clevis, the Dubro has not such method to prevent the sleeve from moving.
So, the flight. I was doing and outside maneuver, the last loop in the 3 consecutive loops as I recall. I was a little low so I gave and extra and firm down control to give me more room at the bottom. When I did that, everyone, myself included, heard the "pop" and watched the plane fly into the ground, albeit more level that would have otherwise been.
My belief is that when I gave the extra "tug" on the down, I reached the limits of the flaps. Since the bell crank had more travel, It pulled harder on the clevis. Since the clevis could rotate and the sleeve keeper was probably again off the clevis (it was after the crash in any event), and given the enlarged and somewhat asymmetrically worn control horn hole, the clevis probably rotated against the side of the control horn and merely opened the clevis under the pressure.
I hope his gives you some insight into potential problem areas when building.
Mike M
Topic: Clevis Failure and the Domino Effect (Read 3211 times)