March 2019 Elevator Trim Cable Tension

[jaredyates]

March 2019 Elevator Trim Cable Tension

ENGINEERING CHANGE NOTICE: Design change that requires immediate attention to affected serial numbers. This NOTICE contains the technical information to correct the unsafe condition.
Applies to all three types, Bearhawk 4-Place, Bearhawk Patrol, and Bearhawk LSA.

Check the tension in the 1/16 cable used for the trim system. Cable tension should be approximately 60 pounds. If too loose the airplane will be unstable in pitch trim.

[jaredyates]

I spoke with Bob on the phone about this today, and he provided some clarity. If the tension is too light, the trim tab will be able to move uncommanded in flight. This can create adverse handling characteristics with regard to the servo action of the tab. The minimum tension specification is intended to create sufficient friction to keep the trim wheel fixed in position when a change is not commanded by the pilot.

[Ray Strickland]

This change works!

[jaredyates]

Also, Bob says a good range is 50-70 pounds .

[davzLSA]

would a friction lock on the trim wheel or lever work?

[jaredyates]

Tightening the cable works, do you have reason to not tighten the cable?

[robcaldwell]

I think a friction lock would be in addition to tightening the cable. It seems to me that if you apply a friction lock without tightening the cable, you would have a locked wheel with a continued wobbly trim tab.

[Battson]

I have a lot of "wobble" in the trim tab because of the fit of the parts in the tail, specifically the trim pushrods where they meet the trim horns in the horizontal stabilisers.
At the tab trailing edge, they can wobble about 3/16" at most.
I have not yet experienced any negative consequences from this situation. That said, I can't physically do high speed dives with the dragwheels (bushwheels) attached.

[Mark Goldberg]

The bolt that attaches the trim pushrod to the little tab( coming out the bottom of the hor stab) has to be pretty tight but not so tight that it does not allow movement. If it is too loose, then the vibration starts to wear the hole. I had this happen and fixed the hole. After I learned the bolt needed to be tighter I have had better luck. Mark

[Bcone1381]

Responding to Marks comment......
Below is a screen shot of the Patrol Trim linkage. I think my Patrol Kit came with a 10-32 threaded end on each end of the trim tab push rod. So, I think my assembly has a Rod End Bearing on each and and allows me to fully tighten the nut at the tab under the H-Stab. Does that sound right?

Second, it would seem to me that any uncommanded movement of the elevator trim tab would be indicated (seen?) by the movement of the trim wheel/lever. So, Can I test the system for proper function be moving the elevator to full travel limits, and watching the trim lever/wheel for any movement? Maybe this could be included when doing the pre-takeoff Control Check by keeping an eye on the trim handle for movement....but I agree its not a substitute for setting the trim cable tension IAW the engineering notice.

Third, is there a way to measuring cable tension without a $200 to $800 tool?


Screen Shot 2019-04-01 at 10.43.50 AM.png

[Mark Goldberg]

If you have a rod end bearing at each end - you can tighten down the bolt/nut well and not worry. It is the forked end (as called out on the plans) that can vibrate and enlarge the hole. Mark

[Archer39J]

I recall Bob outlining a method to measure tension without the expensive tool. I think it was in a beartracks, I'll post it here if I find it.

[jaredyates]

Bob does not own a tensionometer. He calibrates his feel by hanging weights from a piece of cable. That and from having built a dozen airplanes over the last 50 years or so. Personally, I'm not there yet, so I'll stick to the measuring device!

[Battson]

The bolt that attaches the trim pushrod to the little tab( coming out the bottom of the hor stab) has to be pretty tight but not so tight that it does not allow movement. If it is too loose, then the vibration starts to wear the hole. I had this happen and fixed the hole. After I learned the bolt needed to be tighter I have had better luck. Mark

How did you fix the hole, Mark?

[Mark Goldberg]

My friend put some weld bead in the hole where it was elongated. Then re drilled. With fabric on the hor stab. Has to be done carefully and using the proper techniques needed to protect the fabric. Mark

[mbaerobat]

Just wondering if everyone has installed the bronze flat washers on the trim wheel install as shown on dwg. 22. On my trim wheel with the washers and the tightened AN4 bolt/castellated nut combined with the cable tension there seems to be more than enough resistance to keep the trim tab as set.

Dave Plans 865 QB 77 Awaiting final inspection..

[Archer39J]

I did, but I couldn’t find a reduced OD bronze washer so I ordered 100 standard ones from McMaster. The standard OD interfered with the trim wheel chain. Not having a lathe I chucked a couple up in my drill press and had at them with a file... I wouldn’t recommend that. I did manage to get two usable ones out of it fortunately. They serve as a self lubricating wear surface, I don’t think running without them is recommended.

[TrueAirSpeed]

I have read in the "Bearhawk Assistance Manual (Revised edition 18 October 2007) on page 16 that "Early kits relied on twisting the cables upon each other or a separate tension block to supply friction to the cable." I have an early kit (048) and I notice the elevator trim cables are "twisted." Does this follow what I am reading in the assistance manual or should I be concerned?

Terry

[James]

Hi Guys,

Just wondering about the bit where it says to feed the elevator trim cable through the end of the trim wheel chain - the photo in the beartracks seems to show thimbles at the forward ends of these trim cables, but I've found that the -3 thimbles are too fat to go through the chain.

Also, do people put the turnbuckles up near the wheel, or back at the trim horn?

Thanks,

James

[Nev]

I put my turnbuckles just aft of the rear bulkhead. Also got my cables back from being tested under load last week and the engineer recommended that I put small split pins through the end links on the chain to carry the load. I’ve done as he suggested. To clarify, I didn’t use thimbles, just the bare wire.

[robcaldwell]

I could not find a a thimble thin enough to go through the #25 chain, so I made my connection using bare wire. I did not install a turnbuckle in the trim system as I tightened the wire to 60lbs before crimping.

[svyolo]

I think the cable will stretch over time and the tension will relax. I think there needs to be some adjustment. You can also apply tension by pulling one wires to the side with a small pulley

[Mark Goldberg]

If you take a pair of pliars to squeeze a 1/16" cable thimble - it will fit in that #25 roller chain. Mark

[James]

Thanks for the info guys - I'll go with the thimble option Mark.
Given that it's essentially one loop of cable anchored at the rear ends, how come you can't get away with using just one turnbuckle?
The wheel position isn't indexed in use (although I see a couple of guys put a paint mark or something on the chain for TO trim position).

Svyolo - do you think I'll need more take-up than a single turnbuckle can provide as the cable tension slackens over time?

Nev - where did you get the figures for the cable proof loading? Has Bob stated it somewhere, or is it written in AC 43 or something?
I'm sure I've pressed all my cables correctly, but it'd be nice to know that they won't fail.

Ok, thanks again guys for all the help,

James

[zkelley2]

Theres a go no go gauge for nicopress. You need to have it and check them all. Most of the cables should they fail will be catastrophic.
I have one turnbuckle on the trim cable. If I run out of adjustment with that I'll replace the cable.

[James]

Thanks ZK - yep I have the approved nicopress tool and use the go-no go gauge. It sounds like Nev's been hooking them up to the back of the tractor and doing a proof load on them as well. Did you do that for your cables?

Thanks for confirming a single turnbuckle can work - I'll give that a go. Did you pre-tension your cable before swaging it?

James

[zkelley2]

I can't really see how you need to proof load them? They shouldn't be even remotely approaching their breaking point on the airplane. Follow 43.13. Use aircraft grade wire of the appropriate thickness and strands. It all comes pre-stretched.
I ran all the wire really long, then when it came time to terminate at the turnbuckle, set it so I could get like 1 thread in with my hands holding it there, marked that, cut, swaged and then you can get appropriate tension with the turnbuckle, but make sure you have enough threads engaged when you're done also.

[James]

Yep ZK - that's how I would do it as well. The tensionmeter will tell me I'm in the zone, and there should be min 3 threads engagement, I'd assume, but I'll try to get my hands on a copy of AC43.

James