The Trim Woes

On my first few flights I noticed that the trim was kinda hard to adjust. I really had to put some force to it. I attributed this to the way the trim system is designed. In the Colt the Leading edge of the stabilizer moves up and down, actuated by a screw jack. The screw in turn is rotated by a cable arrangement via pulleys. I truly believed that the stiff adjustment feel may be a function of the design. Wrong! Sorry Piper engineers. Read on.

Trim screw arrangement

Trim screw adjustment mechanism

We took the aft part of the fuselage apart, Don greased all the appropriate grease points, while I was hoping that this will help with the stiff trim. Well, after all was nicely lubricated, the improvement was only marginal. The trim was still pretty hard to adjust. Don discovered another lubrication point. This one was probably last time lubricated when the plane left the factory in 1961. It was just a little hole on a top of a tube in the fuselage that acts as a bearing for the connecting tube that holds the two halves of the stabilator together. I am surprised that this lubrication point was not much more prominently presented. One mark here against the Colt designers.

As I said previously, I am not too partial to a green color. And since the empennage components were painted solid green, and I since I wanted to repaint them, they would have to be removed from the fuselage. This would present a perfect opportunity for us to also focus our attention to the trim. I attempted to pull the two halves apart, but they just wouldn’t budge. The tube connecting the two halves and the “bearing” tube in the fuselage were solidly “frozen” together. It took Don and I a better part of an hour, and the heaviest pneumatic riveter Don owns, to get the sucker out.

Frozen tubes

Frozen tubes and damaged bolts

Soon it became clear why the trim was so stiff. The mechanical advantage of the screw-jack setup was good enough to move the leading edge of the stabilizer up and down alright, but since the aft tube was not moving, this was done at the expense of the two screws holding the two halves of the stabilizer together. Not good! We noticed substantial shear wear to the screws, and the through holes in the tubing structure were no longer round. This finding would trigger another sequence of events, but that is another story to be told later.

One thought on “The Trim Woes

  1. Edward, I stumbled across you site while doing a search for PA-22 information. I noticed in the picture you have of your Colt’s Jackscrew yoke and cable assembly that it looks like the trim cable is not routing right to the idler pully. It looks like you have it crossed between the idler pully and the jackscrew spool. That might also contribute to the stab trim crank stiffness. I just recently replaced the cable on my my 52 PA-22 and routed it according to the drawing. I’ll email you a copy of the drawing.
    Good Luck, Dave

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