Trimming the Stabilizer on J-3 Cub (issue)

iWantWings

iWantWings

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On the J-3 I rent I've had a number of instances where after rotating the crank handle of trim the stabilizer up or down, the trim had no effect:
- [ok] the handle can be cranked forwards or backwards without any obstructions
- [no good] the indicator on the trim markings did not move from its postion
- [no good] the pressure on the control stick did not change (angle of incidence of the stabilizor did not change) so there was no change in trim up or down.

The interesting thing is that on the ground I have never had a problem craking the handle to adjust trim up/down. The intermittent failure always happened in the air, while cruising at around 70MPH.

One time I got the trim to work in the air by slowing down to about 40MPH, then I as able to trim as I wanted. I don't know if that was a conincidence or not.

Because I'm a skinny dude with not a lot of muscle, I'm having a hard time fighting the stik to maintain s level attitude while crusiing, and this gets me tired pretty quickly. It's no fun.

Anyone have any ideas what I could look for myself? This is a rental plane.

Thank you for any ideas.

(I've asked the instructor who is also A&P and works on this Cub, but let's not consider that at the moment. I might also join one of the cub forums, but I'll try here first).
 
The cable is probably slipping on the pulley. The mechanic who works on it should be able to remedy the situation quickly. Not hard to fix. you've spoken with him and he said........what?( When you slow down, there's less pressure on the elevator trim. ) who owns the airplane? Tell THEM!
 
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What Jim said.
What little I know of the J-3/PA-12 trim system is that they can slip on the pulley for the jack screw in back near the Stab. When I fly my friends PA-12 you have trim before speed increases, or it'll slip. I think there's some kind of fix, like wrapping the cable around the pulley one more time. I think that's the way I remember it.
 
champ driver said:
What Jim said.
What little I know of the J-3/PA-12 trim system is that they can slip on the pulley for the jack screw in back near the Stab. When I fly my friends PA-12 you have trim before speed increases, or it'll slip. I think there's some kind of fix, like wrapping the cable around the pulley one more time. I think that's the way I remember it.
Click to expand...

Same with t craft, champ, whatever. They all slip for time to time. Easy fix. Strange it's not been attended to.
 
Jimmy cooper said:
Same with t craft, champ, whatever. They all slip for time to time. Easy fix. Strange it's not been attended to.
Click to expand...

Not strange at all. As a mechanic I regularly encounter stuff that hasn't been working properly for a long time. Slack cables in all sorts of systems and airplanes, frozen cable pulley bearings, sticky pivots or hinges, worn rod end bearings, frayed cables, control systems way out of rig. All of it a result of cheap annuals that really aren't cheap in the end, when stuff finally fails (sometimes dangerously) or a different mechanic discovers it and finds that the oversight has resulted in severe wear or corrosion that now requires expensive repairs that could have been avoided with proper maintenance.

That slipping trim cable might have cut way into the pulley grooves, or frayed itself, or both. Now instead of periodically retensioning it and lubricating the rotating bits, the owner might need a bunch of new stuff.

Dan
 
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All of the above. Another probable cause would be the jackscrew being gummed up, which leads to the cable slipping on the pulley. All in all not too bad of a job.
 
Arbiter419 said:
All of the above. Another probable cause would be the jackscrew being gummed up, which leads to the cable slipping on the pulley. All in all not too bad of a job.
Click to expand...
He claims he pointed it out to the instructor who is also the AP who works on the plane. The instructor then said........What?! Did not answer? Said he'd fix it? What? A J3 is as simple as it gets. Easy to work on and fix. If it's a renter it IS strange they'd let it go. What else have they ignored?
 
Single groove trim adjust pulleys slip. The solution in heavier/more powerful Supercubs was to use a double groove pulley to provide more powerful control. The described issue isn't hard for a Cub guy to fix but lots of mechanics out there have no idea what's inside a 75 year-old Cub or how to work on it.
 
Stewartb said:
Single groove trim adjust pulleys slip. The solution in heavier/more powerful Supercubs was to use a double groove pulley to provide more powerful control. The described issue isn't hard for a Cub guy to fix but lots of mechanics out there have no idea what's inside a 75 year-old Cub or how to work on it.
Click to expand...

Ahhhhmen.
 
Thank you everyone for the very helpful responses.

My instructor/A&P will be looking at it and after it gets resolved I will write another post.

Even though it's "just a trim", it is after all a flight surface control and I'm glad it will be looked at.

Thank you again.

P.S. I also found various scanned Cub Owners Manual which are a good read.
P.P.S. I will ask my instructor if I can watch/help when he checks thr trim system - I would love to see how he does things and maybe I'll learn some basic things (maybe one day I'll own a little tailwheel :D
 
My Cub trim broke the day prior to flying to OSH in 2012. Not fun. Changing a Cub trim cable is a royal PITA. The system is simple. Access is not. Typically, you have to cut some fabric.

Most Cub trim systems need attention. Try turning the crank very slowly. Sometimes this works if the condition is too far gone. Few Cubs have an acccess panel on the left side of the fuselage a few inches ahead of the stabilizer. Without it, you cannot easily access the tensioner pulley. Consequently, when the cables loosen, the cable slips. As the cable and pulleys age, they wear. Well, the pulleys wear. The cable can stretch ever so slightly as well over time. The result is loss of cable tension which causes slipping. There are some tricks which can help, but they are typically relatively short lived. I've seen bow rosin (violin/fiddle) applied lightly to front pulley under the crank. I've also seen black cloth friction tape cut into approximately 1/4" wide strip and wrapped around the pulley and laid into the groove. It can be done with cable in place, but you need 3-4 hands in a very confined space. Sometimes, just changing the front pulley will improve the performance. Depending upon the Cub, you may well have to cut fabric on outside to allow pulley shaft/pin removal.

Make sure the jack screw is well cleaned and lubed. Grease tends to collect dust and dirt making the pulley on the jack screw hard to move. Some recommend dry lube, some grease, some light oil. Whatever you lube it with, DO NOT GET IT ON THE PULLEY OR CABLE.

Another thing is ensuring the tail wires aren't too tight overall or on one side. They can cause binding on otherwise properly adjusted trim systems.

If you don't know what you are doing, don't try any of these other than possibly the rosin. It is easy to lose control of the cable allowing to jump off one or all three pulleys.

I can discuss replacement later if anyone is interested. My skinny (then) 16 year old daughter saved the day.


Jim R
Collierville, TN

N7155H--1946 Piper J-3 Cub
N3368K--1946 Globe GC-1B Swift
N4WJ--1994 Van's RV-4
 
N3368K said:
My Cub trim broke the day prior to flying to OSH in 2012. Not fun. Changing a Cub trim cable is a royal PITA. The system is simple. Access is not. Typically, you have to cut some fabric.

Most Cub trim systems need attention. Try turning the crank very slowly. Sometimes this works if the condition is too far gone. Few Cubs have an acccess panel on the left side of the fuselage a few inches ahead of the stabilizer. Without it, you cannot easily access the tensioner pulley. Consequently, when the cables loosen, the cable slips. As the cable and pulleys age, they wear. Well, the pulleys wear. The cable can stretch ever so slightly as well over time. The result is loss of cable tension which causes slipping. There are some tricks which can help, but they are typically relatively short lived. I've seen bow rosin (violin/fiddle) applied lightly to front pulley under the crank. I've also seen black cloth friction tape cut into approximately 1/4" wide strip and wrapped around the pulley and laid into the groove. It can be done with cable in place, but you need 3-4 hands in a very confined space. Sometimes, just changing the front pulley will improve the performance. Depending upon the Cub, you may well have to cut fabric on outside to allow pulley shaft/pin removal.

Make sure the jack screw is well cleaned and lubed. Grease tends to collect dust and dirt making the pulley on the jack screw hard to move. Some recommend dry lube, some grease, some light oil. Whatever you lube it with, DO NOT GET IT ON THE PULLEY OR CABLE.

Another thing is ensuring the tail wires aren't too tight overall or on one side. They can cause binding on otherwise properly adjusted trim systems.

If you don't know what you are doing, don't try any of these other than possibly the rosin. It is easy to lose control of the cable allowing to jump off one or all three pulleys.

I can discuss replacement later if anyone is interested. My skinny (then) 16 year old daughter saved the day.


Jim R
Collierville, TN

N7155H--1946 Piper J-3 Cub
N3368K--1946 Globe GC-1B Swift
N4WJ--1994 Van's RV-4
Click to expand...

Many thanks for the excellent information! I will print this (the school's A&P will work on the Cub). Thank u again
 
Slowing down to 40mph does work. As mentioned your cable is likely slipping and in need of attention. Temp fix is to slow down and adjust, then speed back up.


Sent from my iPhone using Tapatalk
 
Slowing to trim isn't much good. As fuel burns, the trim is constantly changing. Every few minutes I seem to be changing mine.


Jim R
Collierville, TN

N7155H--1946 Piper J-3 Cub
N3368K--1946 Globe GC-1B Swift
N4WJ--1994 Van's RV-4
 
This is true in a J3 with the fuel tank in the nose, your burning close to 20 pounds of fuel per hour and that weight is all forward of the center of lift. Slipping stab trim cables are common on J3's because, as noted, it's not an easy fix. If you are holding pressure on the stick to maintain your attitude the elevator is transferring that force to the horizontal stabilizer and the pinch point is the jackscrew. So slowing down or just first releasing the pressure on the stick before cranking the trim often works.
 
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