trim tab: too much play?

[mulwyk]

Geeze! My trim mechanism was all gunked up with old grease and crap and it could barely move, so I spent hours cleaning and fitting everything until the little unit worked smooth and easy. Now I think it moves too easy. I've got almost a half inch play in the trim tab. I can move the aft end up and down 1/2 inch while holding the little pulley still!

How much play do others have on your trim tab?

[flyguy]

Re: trim tab: too much play?

I have more play in mine than I am happy about, but not as much as you are talking about. You might check by holding the pushrod in place and wiggling the tab to see how much is in the tab to pushrod connection.
As I noted before in some thread I once had a flutter problem with a tab that was made of solid plywood. The metal ones are much lighter out at the edge and not as prone to such behavior, but it is something that needs to be dealt with.
The clearances in all the joints do add up.
Darryl

[3Dreaming]

Re: trim tab: too much play?

There are like 6 places for play to come from. Just a little from each and it adds up to be to much. All but the jack screw are easily fixed. Tom

[mulwyk]

Re: trim tab: too much play?

I measured the play in each part of the mechanism using the crude setup shown in the photo. I held each section still, wiggled the trim tab, moved to the next section, held it, wiggled again...etc. then I just had to subtract to find how much each link was contributing to the total play:

trim tab hinges .025
pushrod aft .045
pushrod fore .050
rocker pivot .020
screw pushrod outbrd. .060
screw pushrod inbrd. .035
leadscrew .125

Total Play .360 measured at tab aft edge

[Hank Jarrett]

Re: trim tab: too much play?

The big risk from a loose tab is flutter. The feds REALLY worry about flutter. Looks like you really need to attack the play in the lead screw first. Someone on the list was machining replacement lead screws some time back and now I wish I had bought a few spares.
Just to give you a little more comfort, for the surface to flutter from tab play the static load on the tab would need to be very low with a forcing function displacing it from that position. Think of it this way, if the tab is in "neutral" with respect to the flow, any disturbance would cause the tab to move away from the higher pressure. That would cause the elevator to move back, changing the flow to the opposite side, and the tab would flop the other way. This can happen over and over if you happen to hit the natural frequency of the components and the amplitude can increase VERY FAST. This is how tail surfaces get torn from planes. The GOOD thing (and the reason planes aren't falling from the sky all the time) is that the tab is usually loaded in one direction or the other to take all the force off the controls. Remember that when the elevator is trimmed for no force to you, the tab is usually deflected. If there is a pre-load on the tab, taking up the slack, it doesn't flutter unless the flow goes unstable.
All the play you can get out of the system, you should. If you EVER feel a "buzz" in the controls, SLOW DOWN! That reduces the frequency and strength of the forcing function. We are lucky in that we fly planes that pretty much stay below 100 mph so flutter isn't as much of a problem, but if you feel it, DON'T MESS WITH IT! Lots of very experienced test pilots have been killed by flutter, and a loose surface is a great way to get it.
Hank

We need OLD pilots flying our OLD planes. I want everyone here to get REAL OLD!

[mulwyk]

Re: trim tab: too much play?

Hank: I know I'll never get rid of all the play in the mechanism.

I've tried to estimate what would be the minimum amount of play I could ever achieve, my base figure. Check my logic on this:

The aft end of the tab looks like maybe 10:1 ratio? (In other words, .002 play in the pushrod results in .020 movement of the tab.) Each link requires .002 clearance so it can move easily. If each of the 6 links in the system has .002 play, that's a total slop in the system of .012, which would translate to .120 movement of the tab.

If my assumptions are right, then I guess we could say a "perfect" trim tab mechanism would still have at least .120 play, or about 1/8" movement at the aft end of the tab.

My trim system has roughly 3/8" play, about 3 times what I figure a perfect system would have. So, yes I have some work to do.

[mulwyk]

Re: trim tab: too much play?

Tom:
I'd be very interested to know what you mean by "easily fixed" Do you know some easy trick to fix this SOB?

[3Dreaming]

Re: trim tab: too much play?

Other than installing a new jack screw, all the other points are replacing hardware. If holes are worn out you will have to add material and redrill, or ream and install a bushing or the next size larger fastener.

[Hank Jarrett]

Re: trim tab: too much play?

I don't know how "legal" it would be but a lot of the link ends are flat steel with holes drilled for standard bolts. If you lay the flat steel on an anvil and hammer it the hole diameter will shrink up some. Just hit (peen) it enough times for it to be snug on the bold. Also use NEW bolts. If the diameter is worn that is part of your slop.
A friend of mine had a Cessna 337 with slop in the elevator trim system and according to the Cessna manuals, using the same analysis process you used, we found you couldn't meet the minimum play requirement with all new components if more than a few parts were at the wrong end of the tolerances. New parts did not fix it. We had to "cherry pick" parts at the other end of the tolerances to get the play within spec. I suspect that is how Cessna gets them "right" at the factory.
Hank

We won't discuss any "other" methods we used to close things up. Engineers aren't to be trusted doing their own maintenance.

[mulwyk]

Re: trim tab: too much play?

Well I got my total play down to .160 measured at the trim tabs trailing edge. That's less than half of what I had originally, but still more than my calculated minimum of .120. But, I think as a practical matter, .160 is as good as I'm likely to get. Here's what I did:

Hinges: I took out the hinge pin and laying it on top of the vice tapped it with a hammer all along its aft side. This had the effect of reducing the diameter in the fore and aft direction while increasing the diameter vertically which is where the hinge bushings were worn. An unintended consequence of makeing the pin out of round is that it made the pin bow slightly and gave it a nice tension in the hinge, sorta like having a small spring in there. The result was a happy elimination of almost all hinge play.

The Pushrod links: Someone had already installed bushings in these so I only tried to improve their fit a little by tapping them all the way around with the hammer. This mighta helped a bit, but mostly I just decided to leave them alone.

Pushrod to Leadscrew inboard link:this link needs to be able to move slightly from side to side to follow the arc of the rocker. But mine had way too much side play and was letting the whole leadscrew kick sideways on its threads. I fixed it by wrapping a shim around the pushrod end and then redrilling it right through the shim. Stabilized the whole thing nicely.

Castle nut: Talk about easy pickins. While I was focused on all that mechanism inside the elevator (yes I had it on the kitchen table) the wife goes "hey, Bob, look here, this thing is moving" It was the castle nut that holds that little pulley on. I was able to tighten the nut one whole castellation while causing only minor increase in friction.

[Bravo 6]

Re: trim tab: too much play?

Anybody have a source for the hinge pin for a metal trim tab?

[Robert Lees]

Re: trim tab: too much play?

Oh my word, another old thread!

Do you mean the long steel "wire" that runs in the hinge?

I bet it's the same as modern piano hinge wire, try Aircraft Spruce or equivalent.

There's an interesting thread on the RV forum here (RV's use piano hinge to attach the cowlings together):
"The diameter of the wire used in these hinges is 0.118” Stainless piano hinge wire"

That's what I would do...measure the diameter and buy the wire from ebay or similar. Chamfer one end, bend the other to fit.

When the FAA call: "It was like that when I bought it, Guvnor".

[Mark Julicher]

Re: trim tab: too much play?

And piano wire in various diameters is available at any decent hobby shop.

[woodmw]

Re: trim tab: too much play?

When I had my elevators off to install new hinge pins and bushings, I pulled the trim tab pin and measured ~.125". I searched the net a couple of evenings, but could not find the right size piano wire. The best fit I found on hand was a piece of 1/8 brazing rod. It wasn't long enough, so I reinstalled the old rod. But it did raise a question to me. Should the pin be the softest material in the assembly? That way the pin wears, not the hinge.

Any thoughts?