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Trim tab travel

A

alpinelakespilot2000

Well Known Member
Just installed my trim tab servo...

Any ideas regarding what sort of travel I should be seeing on my trim tab in both the up and down directions once it's connected to the servo? (I have no idea whether I'm in the ball park or not because there is not a word of instructions that I can find about installing electric trim in the manual and not much more in the Ray Allen literature that comes with the servo.)

It's very easy to get a lot of travel down, but if I'm going to get more up travel I may have to elongate the slot in the elevator skin to allow the pushrod to extend further because the angle between the servo and the tab attach point gets very shallow when pushing the tab above horizontal.

Has anyone else had to elongate the slot or should I not be expecting so much travel up?

Thanks,
Steve
 
My trim

Hello Steve,
I just installed my elec. trim on my 9A last night. I initial adjusted the center of the trim servo to be mid trim tab position. I ran the servo up full range and then down full range.
I could not get full down because of binding. After investigating what was happening, I discovered that the lower skin of the tab was hitting on the aft elevator spar. I proceeded to adjust the linkage until it would no longer hit when the servo was full down.
Now I get 1 15/16 inches down and 2 15/16 up deflection.

I talk to Vans support about this and they said don't worry about it, "just keep on building". I would like to here from someone that is flying if there elec. trim is also non-symetric and is not a problem. Did you find out anything more off this list?

Kent
 
Nothing definitive Kent. Thanks for checking. I decided to worry about it later because I won't be flying for another year at best. That said, I was at Van's with my elevator to ask in May. Gus seemed to think that it was definitely asymmetric--something like 30 degrees up and 20 degrees down (or vis-versa, I can't remember). I forgot to ask to check Van's 9A.

I think part of the answer is that you want to eventually set it so that you get whatever up trim you need for final approach speed and then accept whatever nose down trim you have left, assuming it's reasonably sufficient at the top end.

I should think those who are flying -9s should be able to give us a better answer. All it would take is to put a protractor on it at full up and full down.

Steve
 
Steve

I take off with the trim at about 0. When I land it's usually about 1/4" to 3/8" down depending on CG. That's measured at the trailing edge.

Cam
 
Thanks Cam. That's more definitive than any answer I've gotten so far. Next time you have nothing better to do :D, are at the hangar, and want to measure the max up/down distance, I (and it sounds like Kent) would be very interested. Do you remember what you were thinking when you set it initially? Did you find any instructions anywhere re: the required travel? Are there any instances in which you have run out of travel in flight?
 
I have the manual trim cable not the electric so the total travel of mine probably won't relate very well. I can run mine quite a ways up and down until it binds again the elevator skin. Much further than I would ever need in flight. Just make sure you can run a little ways in both directions from zero. Until you fly it you won't know where it's going to end up. How you set your wings and flaps will effect your trim position, so not everyone's is going to be the same. That's what your phase 1 flight testing is for. If you find that you need more travel after you start flying, you can quite easily over power an out of trim elevator until you fix it.
 
alpinelakespilot2000 said:
Just installed my trim tab servo...

Any ideas regarding what sort of travel I should be seeing on my trim tab in both the up and down directions once it's connected to the servo? (I have no idea whether I'm in the ball park or not because there is not a word of instructions that I can find about installing electric trim in the manual and not much more in the Ray Allen literature that comes with the servo.)

It's very easy to get a lot of travel down, but if I'm going to get more up travel I may have to elongate the slot in the elevator skin to allow the pushrod to extend further because the angle between the servo and the tab attach point gets very shallow when pushing the tab above horizontal.

Has anyone else had to elongate the slot or should I not be expecting so much travel up?

Thanks,
Steve
Click to expand...

I'm building a 7A but had the same issue. Instead of modifying the hole, I just bent the threaded rod right after it exits the clevis on the servo side (I'd guess about 15-20 degrees but I never measured it). I marked the center travel on the servo and adjusted the trim tab to be even with the elevator at that point. With my bend, the trim tab can travel up and down to the limits of the servo with no problem. Eventually I will put Loctite on the threads on the servo clevis, in addition to the locknut, to prevent rotation.

Dennis Glaeser
7A Empennage done - waiting for wings to be delivered.
 
Try not to make a mountain out of a mole hill here.
In my RV6, the travel was limited as you have noted, but you will find that the travel you need, in both directions, is probably much less than you have available.
In flight, a little goes a long way. I wouldn't make ANY modifications until you test fly and find something is actually required.
 
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