elevator trim question

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planebones

New member
Greetings to all: I am ready to cover the elevators on my 14-13-2 and I notice that the trim tab "slips" in relationship to the actuating "arm"or "lever" I see no hard mechanical connection between the two????? Like all project aircraft I am the 3rd owner since it was taken down, and I can tell some "fiddling" has gone on. It seems there should be a roll-pin or something to assure no slippage between the tab shaft and the actuating arm? any help appreciated......Planebones/ Ken
 
Ken:

From your descritption, it sounds like the problem I had last year on my trim tab for my 14-13-2. The 14-13-2 is a different set-up than the later triple tails. The clevis on the bottom of the trim tab holds a brass threaded insert. The actuation rod that protrudes from the fuselage has a threaded rod on the end that spins inside the insert, driving the tab up or down depending on the rotation direction - just like the jack-screws in the wings that pull or push the gear. The inner section of the threaded rod is supported on the lower side of the elevator with a similar brass threaded insert. Here is the important detail: The outboard section of the threaded rod which drives the trim tab is reverse threaded, (Left-hand thread) and so is the trim tab brass insert. The inner rod section and brass insert that is supported on the elevator is normal thread (Right-hand). Without the reverse threads, the rod would just drive through the inserts without actuating. With the opposite threads, the two inserts work against eachother to pull or push the trim tab. It is a very elegant design in my opinion that shows Giuseppe's touch on a small detail.

If the threads on the rod and insert are good, there is zero slop and full actuation is achieved. If the threads on the insert and/or rod are worn smooth, the insert slides over the actuation rod. This was my problem and I discovered it in flight when flying straight and level. Periodically, the yoke would cycle fore and aft as the trim tab "let go" - much to my alarm...

A machinst can make these parts very simply from brass rod stock for the inserts and steel rod stock. Just bring him the rod and two inserts. You can go one better than the original with stainless steel for the rod and oilite brass for the inserts. These do get worn from dirt that gets kicked up from the tailwheel and propwash. Once you get your new parts, installation is very easy. I would recommend giving yourself more up trim (downward deflection on the tab) than down. Rarely do you need much down trim and having lots of up trim helps for those three pointers... With the new parts and some lubrication, the trim system is silky smooth with zero slop.

If you need more info or pictures, let me know off-line tbriggs3@comcast.net

Good Luck

Tim
 
Thanks Tim: Timely response...It is snowing out here in Iderho, so I was hiding in my shop this morning recovering that elevator and trim tab.

It looks like someone has reworked my trim system and the threaded brass insert and treaded rod are all tight and smooth.

BUT...the "lever" that the threaded brass insert mounts in on mine slides over the shaft of the trim tab and is held on by a screw....but there is nothing that maintains the orientation between that lever and the shaft. You put pressure on the tab and the shaft slips in the lever......what is suppossed to hold the position....surely more than friction! I am about to find the correct position and drill a hole through the lever and shaft and install a roll pin?????????????? unless you can help me figure out what is missing in the picture. Thanks you re saving my hide.....Ken
 
Ken:

I'll have to take another look. What you refer to as the "lever" is what I call the "clevis". How that clevis/lever attaches to the tab itself is your issue, correct? That may lie beneath the fabric. My plane is in for its annual, I'll take a look when I'm down there in the next few days.

Tim
 
Thanks Time : Actually the shaft of the trim tab sticks outside of the fabric, maybe you will see a roll pin put through both the "clevis" and the shaft or? Mine has a screw that keeps that part from falling off, but it does not keep the relative position....... thanks Ken
 
Ken:

I looked at my trim tab actuator and could not see how the lever stays put on the trim tab. Whatever it is, it lies under the fabric. I poked around and couldn't feel a screw or anything else. I'm guessing it is welded in place on the leading edge of the tab?? I'm out of answers for this one short of removing the fabric to take a look.... Anyone else?

Tim
 
Thanks for looking guys: I will figure something out. Yea the mounts for the tab are welded onto the elevator, but the (lever, actuator arm, crank or what ever you want to call it) slips over the shaft of that comes out of the fabric at the end of the tab & mine is secured from coming off by a screw, but that screw has no way of holding the lever from turning on the shaft. It has been too hot fo fiddle around in my shop, but I will get back to it soon. thanks Ken
 
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