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PA-18 Trim Creep

R

roeloffs

Registered User
I just had a PA-18 recovered plus a lot of minor repairs for wear etc. on the last 20 years. On the flight home I found that I was constantly adding foward trim. On inspecting farther, I find that after resetting trim, the handle creeps counter clock wise ( nose up trim) . The handle turns 1/2 to 3/4 a turn and stops. This takes about 60 seconds or so.

The party who did the recovering and replaced the trim indicator wire says he has never heard of this situation and has no idea . The crank handle does not appear to be either too tight or too loose. Is there a friction adjustment for this? He did not think so.

Just brought the plane home Thursday so have not had time to look or flight test any farther. Just hopeing someone has an easy out for me by having an answer.

The job came out beautifully and on schedule. I will be reporting in the next week or so.

Kurt
 
I flew the plane locally a few days ago with the express purpose of checking out the trim. The position does remain stable in all speeds from 35 mph to 95 mph. Up at 100mph+ and 2500 rpm the trim rapidly (3-4 seconds) creeps from level flight and trimmed to a nose up position, enough to require adjustment every few seconds. Also others who have flown the plane a lot say that the trim crank is noticably looser than before the recovering . Does anyone know if and where there is a friction adjustment or where some friction can be added?
Thanks to all who have responded to my varoius posts.
Kurt Roeloffs
 
Kurt,

On the left side of the fuelage, back just forward of the jackscrew, is a trim cable tensioner. Essentially it is a pully that applies adjustable pressure/tension on the trim cable loop, taking up slack. I'm not sure how easy access is, but it is adjusted by moving a couple of nuts on a threaded shaft.

I think the real problem is that you obviously don't have enough mods on your plane (big tires, Borer prop, belly pod, etc.). Your PA18 is overly aerodynamic and now goes too fast! :eek: :lol:

John Scott
 
Only the early 18s with single groove pulleys have the adjustment. Later models have a pulley and spring with two grooves in the pulley at the jack screw. Which sytem do you have?
 
Photos not found

thanks John and Steve,
I am not sure. I will look this weekend and post the reply

Kurt Roeloffs
 
I have the same problem. Does anyone have the answer. My system has a double groove pulley at the crank and at the screw jack. The crank has nice ball bearings for low friction. I took some loops out of the tension spring to try to tighten the system. No joy. At speeds over 100 the crank continually moves towards nose up. This is a pain, in a plane that cruises well in excess of 110 mph.

I need to insert some friction someplace.
 
try replacing the "nice" ball bearings at the crank with the original style pulley.

john
 
cubunltd said:
try replacing the "nice" ball bearings at the crank with the original style pulley.

john
Click to expand...

I was afraid that that would be the answer. Do you have any knowledge that if there were bushings instead of ball bearings that it would definitely be the answer? I never expected that with the load on the screw jack (which is supposedly irreversible) would cause it to turn as easily as it does.
 
I installed -18 trim on my -12. I had new trim crank brackets welded in since the originals were wallowed out. I installed a new pulley and new crank pin. I greased the crank pin with white grease before I assembled everything. The crank was so stiff, I could barely turn it. I took it apart and drilled out the holes in the standoff brackets, reamed them out to true the holes then pressed in oillite bushings. After reassembling everything, the crank turns like butter. I would not assemble another cub without the oillite bushings again.

Darrell Starr told me about these oillite bushings. Check his photo album. I think he has some diagrams there. Hopefully, he will comment on it.

Kevin
 
Right Kevin, I have 245 hrs now on the Oilite crank bearings and all is well. For the problem of the crank turning on its own, I would add tension on the spring loaded pulley in the back. I am assuming we are talking about the double wrap set up.
about a dozen Pipers I know of have the Oilite bearings -- everyone loves the smooth operation. The bearings are lubed for life so are maintenance free.
The Oilite bearing are part number FF520-15 available at Aircraft Spruce:
Oilite F520-15 Brgs at Aircraft Spruce.jpg
P5210006.JPG
P5220027.JPG
Trim Redesign.jpg
 

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Here is a better look at the Oilite bearing setup. The crank pulley is in blue, the frame brackets are yellow and the Oilite bearings are green as is the crank. The cross pin is red & the spacer is brown. Note that there is no spring. There are 3/8 SS AN washers between the yellow brackets and the flange of the green bearings shown as a blank space on this unfinished drawing but you can see them in the photograph in the last post.
Modified Trim - close up with text.jpg
 

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Thank you Kevin and Darrel,
Yes it is a double groove (front & back) system. I have already increased the tension on the aft idler pulley. It did no good. Do I assume correctly that the Oilite bushing will increase the friction enough to keep it from moving on it's own? Now you can move it easily with your little finger.

This is a Backcountry kit set up. Perhaps they went too far by installing the ball bearings to make everything run nice and smooth? It is interesting that when someone goes just a little extra to make something better, that it comes out too good and has to be de-engineered.

Come to think of it, I had a similar problem with my 185 when it was new. The roll pin which holds the trim wheel on it's shaft fell out. The trim spun rapidly to full nose up on it's own. That got my attention. It is good that I was not far from the airport. It took a lot of pressure on the yoke to keep the nose down. That gives me an idea. Perhaps a little detent system similar to the 185 will be possible? I'll look into that also.
 
I don't understand what you mean by aft idler pulley.

I installed a double groove PA-18 trim system. Front pulley on the crank is double groove and rear pulley on the jackscrew is double groove. The cable comes off the jackscrew, goes forward to another pulley with a spring off the longeron, then goes back around the jackscrew. I don't know how that those pulleys can be adjusted. I bought my parts new from Univair off their part numbers in the PA-18 shop manual.
 
kevin said:
I don't understand what you mean by aft idler pulley.

I installed a double groove PA-18 trim system. Front pulley on the crank is double groove and rear pulley on the jackscrew is double groove. The cable comes off the jackscrew, goes forward to another pulley with a spring off the longeron, then goes back around the jackscrew. I don't know how that those pulleys can be adjusted. I bought my parts new from Univair off their part numbers in the PA-18 shop manual.
Click to expand...

That pulley with the spring connected to the longeron is what I am calling a "idler pulley". There is another one without a spring next to the forward double groove pulley which has the crank on it.

I was just looking at mine. I think for starters that I will glue a couple of pieces of leather on the inside of the yoke which holds the forward idler pulley. This should provide some friction on that pulley. If this doesn't work I have a couple of ideas to attach a spring loaded detent arrangement to one of the pulleys similar to the Cessna 180/185 trim wheel.
 
The original system had a tapered spring that provided some friction between the trim cover plate & frame bracket - I don't use the spring but that would be one way to add some friction.
Original Trim plus Text - close up.jpg
 

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I glued a piece of leather to the yoke which holds the forward idler pulley. This created some friction on the pulley. A test flight when the wind dies down, perhaps on Monday, will tell if I'm on the right track.

I have also noticed that the cable appears to slip somewhere in the loop when trimming at higher speeds. Shortening the tension spring did not help noticeably. Perhaps I'll need to make a tensioning device which is more positive than a spring?
 
mike mcs repair said:
i think you have spring in wrong place there... or at least i have never seen it there... it's always on outside under that brown cover...
Click to expand...

That's right Mike. It's purpose is to hold that brown cover against the handle so that the pin won't fall out. The cover is pushed in to release the pin so that the handle can be removed. Also, the spring is backwards in the drawing. The small end holds the pin.
 
Mike, check out the parts book illustration, the way I see it, the spring would go the way I have drawn it but I know the ones I have seen on planes are installed to chew up the square trim cover plate so now I think the parts book is wrong.
 
I'm going to leave the spring that goes next to the cover plate out. This should save my cover plate from being torn up after a lot of years in service. I'm going to connect the trim crank handle directly to the crank pin going thru the cover, like what Darrel has done. With this and the tensioner spring connected to the longeron and the double groove system, I don't foresee any problems.
 
Darrel Starr said:
Mike, check out the parts book illustration, ..
Click to expand...

I've never owned a parts book :)

that's what Stoddards is for... I just walk in start describing and waving my hands, sometimes they point me to the parts book... but usually they know what I mean before long ;-p
 
Anybody have a rough reference tension either number or eyeball/rule of thumb for the tensioner pulley spring in the back? Just installed a new trim cable from Tony and while the spring was easily removed with the old cable, with the new one it is under pretty significant stretch, and was a bugger to get on. Is there too tight? Can put a longer bolt in there to bring the tension down, but if tight is good I’d rather not dick with it any more.
 

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mike mcs repair said:
I've never owned a parts book :)

that's what Stoddards is for... I just walk in start describing and waving my hands, sometimes they point me to the parts book... but usually they know what I mean before long ;-p
Click to expand...

Can you still go into stoddards anymore with covid?
 
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