Trim Turnbuckles

[Bdflies]

When you guys are speaking about 'tight' control cables, you're referring to the system on the 4 place, right? I've read those comments over and over, but can't imagine why the Patrol system would need to be more than 'snug'. Or, am I missing something?

Bill

[Mark Goldberg]

Bill, the trim cables need to be tighter than the aileron cables. MG

[Bdflies]

Thanks Mark! I think I need to take a look at that.

Bill

[Luke68]

I know it wouldn't be as easy as a turnbuckle buy why couldn't you just remove a link or use a half link in the small roller chain?

[svyolo]

After messing will all of my control cable runs, I ended up with 2 turnbuckles in the plane. One in the trim, and one in the elevator. I think I could have deleted the elevator but it would have been a PITA to adjust tension. The only other "loop" is the ailerons. All cable runs have one swaged stud with a threaded terminal to adjust the length. The ailerons included, and the tension is applied and taken off by removing the pulley next to the stick. Very easy to do. Probably 5 lbs of pressure to do it. The flaps and rudders are tensioned with springs. The threaded studs provide almost 1" of adjustment, versus 1/2 for the same turnbuckle.

I have used turnbuckles in several other applications. In those, the turnbuckles are good for adjusting length of the cable, and applying tension. The airplane turnbuckles are much less useful, and I don't understand why except they were designed 60 years ago, and "that is the way it always was". You can make an aircraft turnbuckle much more useful by having one end of the cable a swaged stud, which is longer and has a lot more threads. You can get an inch of adjustment if you use one instead of turnbuckle end on that side of the turnbuckle. I didn't, but will the next time. The studs have a hole for safety wiring.

[zkelley2]

Probably the cost difference between using turnbuckles and the machine needed to swage stud ends.

[Bcone1381]

Yes swaging machines are expensive, but you don't have to own one. Aircraft Spruce has one and there swaging fee is reasonable.

[Bdflies]

We have a local Community College "A&P School". I've known the director of that school for a long time. Anytime I need to borrow or use such specialty tools, he's happy to loan them. He knows I'll return them in good condition and I always make a donation to fund stuff like school lunches and recreation equipment. Costs me very little and makes them very happy. What could be better?

Bill

[Gr8apmech]

Local EAA could also be a resource for these tools. Cable tensiometers too.

[James]

Hi Guys - same sort of question, but different - I've finished my trim cable installation, and put 1x turnbuckle just aft of the cargo bulkhead, as helpfully suggested above. The other cable rubs on it, but I'll make up some sort of fairlead or nylon tubing, as discussed elsewhere (#36785, etc).

I have another question - when sizing up the cables and swaging them, I placed the chain over the trim wheel so that both ends were even (seemed logical). The torque arms in this position are 90 degrees to the hoz stab. Now, I notice that the forward cable end thimbles are fouling each other. Is this a problem in flight? We can't move them apart without increasing the diameter of the toothed gear on the trim wheel, or making a second fairlead path.

Or, another way to ask the question - how much time in flight is the trim system positioned so that the torque arms are 90 degree to the stab? As soon as I move the wheel a quarter turn forward or back, the thimbles move away from each other, and the cables clear.

Any suggestions?

Thanks,

James

[PaulSA]

I staggered my chain so that the fittings didn't interfere - plenty of length to get the full . I also changed my mind over the location of the turnbuckle due to interference with the second cable and put it at the back of the tailcone. Going to need a big access panel anyway for all the fittings back there.

[James]

Righto Paul - but what happens when you trim forward or back, and the thimbles are aligned and fouling again?
Can anyone tell me - on take off, and in the cruise, would you expect to have the trim in the neutral (cables even) position?

James

[Nev]

James do you have a photo you can post of the interference? I don’t have a solution, but I’ve made my own cables up too and wonder if I’m going to see the same problem. I don’t think I used thimbles in this instance. This was the only section of my cables that wasn’t tested as it wasn’t able to fit the hydraulic tensioner. But it’s also not under huge tension either.

[Nev]

Here’s mine:

411E41D7-E4B0-4F60-A044-6972C727C175.jpeg

[Nev]

Since the above photo was taken I’ve added some split pins through the hollow chain links to add strength at the recommendation of the engineer that tested my cables.

[Bcone1381]

It seems to me like I've seen a roller chain sprocket inserted between the upper and lower chains aft of the trim wheel. I never knew why though. The sprocket just sat there.....suspended by the chain. It turned along as the trim wheel was moved.

Screen Shot 2020-11-03 at 8.39.58 AM.pngScreen Shot 2020-11-03 at 8.41.48 AM.png

[James]

Righto, thanks for the responses guys

Nev - I see you've just put the cable around the rounded surface of the chain - if you do it this way, you definitely won't be seeing the interference I have. I threaded the thimbles + cable through the chain links, as suggested elsewhere on these forums. The interference stems from the width of the thimbles.

I've got a couple more questions - rather than going for a splitpin option, could you put the orginal drive pin back in? Or could you pass the cables through a link section with the solid pin? Rather than driving out the pin, I cut the outside drive plates on the very last links in half, and rounded them off with a dynafile. There's a fair amount of tension on the trim cable - 60 to 80 lbs recommended. Also, what is the minimum bend radius for this 1/16" cable? I thought that's what the thimbles allow for, minimum bend radius, and a good entry angle into the swage. Other than that, it seems like a pretty clean setup you're running there. I see that Brooks has done something similar.

Brooks - you wouldn't happen to have a part number for this gear wheel? I'll have a look, but I'd have to go for something of larger diameter that the one mounted to the trim wheel.

Thanks again for the help guys - keep the suggestions coming, especially anyone who's flying with my current setup, and hasn't reported any problems :-)

James

[Mark Goldberg]

I have only used one turnbuckle on my trim cables. Mark

[Nev]

There's a fair amount of tension on the trim cable - 60 to 80 lbs recommended. Also, what is the minimum bend radius for this 1/16" cable? I thought that's what the thimbles allow for, minimum bend radius, and a good entry angle into the swage.

James I think you are correct. I’m hoping others with more knowledge than me (which is pretty much everyone) will chime in if they see the way I’ve done it as an issue.

On the plus side, it’s a very easy part to inspect visually on a regular basis. I’ll probably be giving it a nervous glance every minute or so now

[Bcone1381]

I took the screen shot from The surplus center. Hope this helps. I like a catalog reference like this to help define variables like pitch and teeth plus nomenclature to improve Amazon, eBay, and Google searches.

https://www.surpluscenter.com/Sprockets-Roller-Chain/Roller-Chain-Sprockets/Idler-Sprockets/

[Mark Goldberg]

The sprocket used on the four place trim wheel is a #25 pitch nine tooth sprocket. The more teeth the sprocket has - the more sensitive it makes the trim. That is my understanding. Mark

[James]

Yeah, I don't think I want to go a bigger sprocket on the wheel Mark - everyone already says the trim is pretty sensitive already. The larger sprocket would be just an idling gear, to spread the two chains apart.

[James]

Thanks for the input guys - in the end, I chopped the trim system cable, and once I order some more cable, I'll offset the two thimbles, so that the lower one is about 3 inches in front of the top one, as PaulLSA suggested. My setup only has one turnbuckle, located behind the aft bulkhead. I think it's the best way to do things.

Now that I've chopped the cable and I have a chance to revisit things, is there any consensus on which way the trim wheel should turn? I've never flown a plane with a trim wheel, so just visualising it (sitting on the ground, and making plane noises), when I push the bottom of the wheel forward, I want the aircraft to pitch up. Is that the standard?

Thanks guys,

James

[Mark Goldberg]

As to which way the trim wheel should turn - that is kind of a personal preference matter. Bob has his 4 place set one way - and I did the opposite. When the Model 5 made it to Texas, I changed the cables so it would be the same as my 4 place. It didn't make sense to me to have two planes with different orientation.

I kind of visualize the two ways like this: Bob's approach "sees" the trim wheel as if it is mounted in the instrument panel like on a Cessna. With a line marking the neutral on the "aft" side.. Move the line up - trim up.

I "see" the trim wheel as if the whole wheel is my airplane. When I trim up - the wheel turns clockwise. Down trim - counter clockwise. While it might make sense for everyone to have theirs set the same way - it does not seem to be in the cards. Mark

[svyolo]

One of the reasons for the high tension on the trim wires is to increase friction. While that works, I think there might be some more efficient ways to add friction. I put a fairlead in the rear skylight connector that causes the cable to turn about 12 degrees. It seems to add a reasonable amount of friction with less cable tension. It is in an easy place to inspect the cables, and condition and fairlead and easy to change the fairlead.

Hopefully I can get by with less cable tension. Because there are two cables, if you have 60 lbs of tension, you have 120 lbs of force on the trim system, including the trim horns, torque tubes at the rear.

[James]

D'ya reckon? Good question: if I pin both ends of the trim cable run (stop both the pitch horn and the trim wheel from moving, then tension each cable at 60lbs of tension using my tensionmeter) is there really 120lbs of force total? Or 60lbs?
What happens next when I unpin both ends and let them rotate freely? Will the dial on the tensionmeter suddenly jump to 120 lbs?

This is like a Year 10 physics question - what am I missing here? :-)

When Bob recommends around 60lbs of force, is that per cable, or total, or what?

James

[svyolo]

Yeah, 60 lbs of tension, turned 180 degrees on a pulley/sprocket/whatever, causes 120 lbs of force on the pulley/sprocket/etc. It will also cause friction in the whole system, which in this case is good. It just seems like a lot force to put on the horizontal stab/ribs etc, forever.

[James]

Oh yeah, of course - there's 120lbs combined force acting on each of the anchor points, 2x 60lbs forces working in unison. But the force on each wire is just 60lbs. I remember struggling with Year 10 Physics.