Piper J4A trim actuator
My J4A (now flying) has that infernal jackscrew/speedometer cable/overhead crank/indicator drum and tiny block and tackle for the indicator arrangement that must have been cobbled together at Piper by the night crew. It is conceptually a lousy design concept in that there is no end stop without messing up the whole thing, requiring complete disassembly (preferably only done when recovering it) of the airplane.
I have thought of placarding it with "Only the Owner is Allowed to Adjust the Trim" but the humor and my own misgivings have slipped by me.
What do other people do with this abomination? Fix the trim to ground adjustment only & hand fly the rest? Any ideas?
"Sorry" to report that the trim system in our 1940 J-4A worked flawlessly over the ten years we owned/flew it.
Originally Posted by nrpetersen
MLang - that's a relief to hear. There are a couple different trim actuator variations for J-4s. I found a different one on the Super Cub forum that seemed to have a single wire out from the stabilizer for the trim position indicator. That's not like mine.
My J-4A has a 4:1 block & tackle driven by a 1/2 inch dia drum mounted to the rear sheave shaft. That rear sheave has ~ .018 dia stainless cable wound on it and the free end is fed thru the block and tackle & then to fuselage ground. Another wire and a tension spring above the cabin shows the movement of the forward set of sheaves. My jack screw drive had been rebuilt by the former owner (I bought it as a partially finished project) and had used speedometer cable to connect the rear sheave shaft to the jackscrew in the elevator.
When I completed the J4A, it bound up after a few operations where the speedometer shaft is connected to the jackscrew (via a collet-like arrangement). Unfortunately I have since had the revelation that speedometer cable can only transmit torque in one direction without unwinding. That is because the outer steel winding is spiralled one way and the inside is only a fiber matrix of sorts. It is a huge job to replace that cable with a type that can take a bidirectional torque - if one can be found somewhere.
That's now my next winter project. It will involve removing the left elevator and stabilzer halves. removing the fabric, replacing that flex shaft, and then refinishing the fabric work, complete with an access cover that should have been part of the original aircraft design.
I notice that the jackscrew connection to the tab is presently via a much stiffer flex and looks like it could take bidirectional torque even though in this location, it is only loaded in tension-compression in the J4s. Might Piper have used the same flex-shaft material for both locations?
Any other thoughts? Was your favorable trim tab experience from a similar system to mine?
Right now I am thinking of temporarily having only a ground adjustable tab. Is that practical based on your J4 experience?
Last edited by nrpetersen; 07-14-2013 at 05:42 PM.