Pitch trim tab - hinge play

[Nev]

Two of us here in NZ have found play in the trim tab hinges. When I assembled them, I had to remove some factory paint from inside the hinges in order for the clevis pins to fit. I recall it being difficult to remove the paint, and I suspect the remaining paint has kept them tight during the first 100 hours, and has slowly worn away leaving the play we're now seeing.

My first question is, is this normal ? Have others noticed it ? Once the paint is fully removed, does any aerodynamic chatter start and does it further wear the hinge ? Have there been any design changes in the hinge tubing size ?

[AZBearhawk272]

Nev,
What you are seeing is " normal " and undesirable, fairly common on 6 cylinder engines with large props. That said keep a close eye on it.
An incorporation of items below does keep this damped and minimized. Get from condition to condition inspection without having issues, refresh annually.
1.Tape the hinge line gap between the tab and the elevator creating a gap seal.
2. All the linkage bolts acting as hinge points at both ends of the trim tab rod should be damped and have very little slop. Have used "Bellville or Wave" washers to allow the bolts to be pulled up
and put a clamping load on the joint. Use flat nylon washers to keep everything slippery.
3. Put some RTV silicone goop in the hinge pin holes and on the clevis pins, let it cure up before flying for a couple of days. It will eventually wear away but it really stops the slop and eliminates
the metal on metal fretting.

Kevin D

[Nev]

Thanks Kev. I'll add silicon to the hinges and a gap seal. I've previously addressed point 2 - had small clevis pins originally and replaced them with AN3 bolts and snugged them up.

Much appreciated!

[Bissetg]

Nev not flying for a couple of days??? You must be joking. He doesn’t even let it cool down let alone not fly it. Good advice though and thanks for it.

BTW, I discovered it on my 4 cyl on the hundred hour too. I don’t think this is an engine specific thing.

[Mark Goldberg]

I just spoke with Bob about this. Most of the "looseness" is from the variation of the thickness of the little tubing that hinges the trim tabs to the elevators. And the #10/AN3 bolts or 3/16" clevis pins are a little under size for the ID of the hinge tubes. Just.002 or .003 difference/ looseness will make it seem loose.

Bob suggested a different approach which is not too hard to do except it does require a lathe. Bob suggested measuring the ID of the hinge tubes and machining a bolt or steel bar stock to a tighter fit. Bob thinks he:
1) cut off the head of an AN4 bolt
2) turned it down to the best OD for a tighter fit in those tubes/hinges
3) He smashed one end so it would not allow the part to move in one direction
4) Drilled a hole for a cotter pin for the other side

Bob said a cotter pin on both sides would certainly work also. He said even a mild steel bar stock turned down to fit is plenty adequate strength wise. Oil or grease the new pin so it goes in easily and stays rust free. Mark

[Bissetg]

Thanks Mark. That makes sense. My play isn’t excessive so will look into it over the next week but turning a slightly oversized pin seems like a good solution.

[svyolo]

How about using bronze pins instead of steel? Bronze makes a nice bearing.

[jaredyates]

Please run this by Bob to ensure the lower tensile strength is ok.

[svyolo]

I will but I cannot imagine they are not.

[Battson]

Yeah it's 800 MPa for an 8.8 fastener vs 125 MPa for bronze (general ballpark numbers IIRC). So like 20% as strong as steel....

[Nev]

1.Tape the hinge line gap between the tab and the elevator creating a gap seal.

Kev, what does taping the hinge line do ? Just curious.

[schu]

Probably prevents turbulent air around the leading edge of the tab which could cause it to rattle.

[schu]

One of my trim tab hinges wasn't welded on straight, so that takes all of the slack out of it, but it's not smooth through it's travel either. I took a reamer and clearanced it a little, so it's better, but if it's just going to get really loose maybe I should cut it off and fix it since I've not covered the elevator yet. The problem is that I don't want to recoat it. I wish I would have noticed this and fixed it before I coated it.

[Battson]

How much play / oversize are we talking here Nev?

[Nev]

Here it is Jon.

Kev's solution appears to have worked well (Kev always seems to have a fix ), and a small vibration that had crept in during the past 50 hours appears to have disappeared.

https://youtu.be/XtPYtMB1E3k

[corefile]

I saw a post long long time ago - of a completely different trim system. I think the guys first name was Pat?

[Nev]

I remember that, using a cable. It removed the servo action. I think Whee has a similar system.

[alaskabearhawk]

17 part series on how I did it using Pat Fagan's design as a guideline. I made a YouTube video about it and the link is in this writeup at the bottom:

http://www.mykitlog.com/users/displa...527&category=7 248&log=253564&row=11

[corefile]

17 part series on how I did it using Pat Fagan's design as a guideline. I made a YouTube video about it and the link is in this writeup at the bottom:

http://www.mykitlog.com/users/displa...527&category=7 248&log=253564&row=11

yes that is it - great YouTube video of how you did the cable trim.

[schu]

Yet another way

https://www.youtube.com/watch?v=mFQ1VcCiBJo

[Mark Dickens]

And a variation on the same theme

IMG_1258.jpg

[schu]

I highly recommend that you use multiple switches or a trim controller like the TCW or if the dynon has a trim controller.... You need to be able to deal with a sticky button on the stick causing trim run away.

In my design if you hold the button down for more than 3 seconds, it starts ignoring you until you release and press the button again. Also, I have a backup system that removes my trim controller and directly applies power to the servos with a momentary switch. Basically, a backup.

[Mark Dickens]

Yep, I'm using the Dynon autopilot panel which has the trim controller integrated. I know what a stuck trim tab is like and it's not fun. I had a G36 Bonanza elevator trim tab freeze up on me during landing. Literally it was so cold (Buffalo, MN in December), that a misinstalled metal plate jammed the trim and I had to land the thing with down trim. Almost couldn't get the nose up enough to do any kind of flare. Turned out that the plate had been incorrectly installed at the factory and since the plane had been flying in the south, temperatures hadn't gotten down to the point that the mechanism jammed. Not fun. I have a lot of respect for malfunctioning trim systems and once I'm flying, I'm going to do a full series of controllability tests with full trim up and down.

I'm also installing a manual up/down switch in case the Dynon unit fails, which it has on my RV-8. I have a VPX on the RV-8 which has it's own trim controller and I have a switch that allows me to switch between the Dynon and VPX trim controllers.

[schu]

Good deal!