Citabria Is everyone's trim super sensitive and overly powerful?

[kubark42]

I've noticed on my 1998 Aurora that the trim range is about twice as wide as what seems to be useful, with full back trim leading to a situation where it takes both hands to keep the nose down enough not to stall. It could be very challenging to remove that much trim if there weren't another pilot to hold the stick. We haven't experimented with forward trim but I suspect it's equally outlandish.

From flying the pattern to cruising, I don't move the trim more than a couple inches. The problem we have is that the trim is so sensitive that it's very hard to make a tiny change, especially to fully remove stick pressure. When we flew the plane back home after buying it, we mostly settled on an airspeed within +-5mph and then adjusted power to maintain altitude.

Is this normal, and can anything be done about it? The best theory we can come up with is that it's One Trim to Rule Them All, and so the certified design is used on everything from Champs with a lightweight 65hp engine to a Super Decathlon with 180hp and a CS prop.

 

[Bartman]

The trim is very powerful, you're right.

Do you have any play between the two sticks? Hold the rear stick and see how much play there is in the front stick. I had initially given up on trying to maintain altitude because of the play in the pitch controls and I was chasing it with trim a lot of the time. Replacing the bushings and hardware in the stick sockets made a HUGE improvement, now there's little to no play and more fine control of the plane is possible. I also replaced the lower stick socket connector tube which doubles as a connection point for the lower pitch cable. The holes in that tube get worn out of round and that's another source of play. None of these parts are very expensive and it only took an hour or two to get the floorboard up and the parts swapped out. We did it as part of the annual this year.

 

[Bob Turner]

I don't even use the stick in cruise - just rudder and trim. Never even thought about exploring full trim.

 

[Big Ed]

My Super D practical trim range is about an inch. Set at R in "elevator" for takeoff, at the notch for acro, and a bit forward of the notch for fast cruise. In cruise I will tap it forwards and backwards to find level.

I think that is inherent to the design. If you wanted fine control, you would need a crank or some other mechanical device to step down the input.

 

[kubark42]

I think that is inherent to the design. If you wanted fine control, you would need a crank or some other mechanical device to step down the input.

If we're using 1-2" of a 6-9" travel, then we could reduce sensitivity by 3x or more without needing a crank. All we need is to use the full travel available. I feel like the easiest mechanical way to do this is to redrill the trim cable attachment holes so that they're 1/3 the distance from the pivot point.



Locating the holes 1/3 the distance means that the arm throw needs to be 3x larger for the same trim tab movement.

I have no idea what the easiest way to do this is once considering FAA regs. I'm still learning about what kind of things would qualify for a 337.

And there might also be a non-obvious specific reason why ACA doesn't do this at the factory.

 

[Bartman]

a 65 hp Champ with its slower airspeed probably needs the greater range of trim tab movement to be effective. changing those he locations is probably outside of what you can do with a 337, not sure why the factory would leave it unchanged all these years unless there is a good reason for it to be that way

 

[Big Ed]

I don't have any specific knowledge, but would speculate that monkeying with flight controls is significantly different than other mods. Could be that any such change would be a change to the TC and might necessitate flight testing to be legal. The impact on things like flutter might not be readily apparent.

I once knew a girl, very talented aerobatic pilot, who died due to a design mod to the trim of a Cassutt air racer. The aircraft is designed with a ground-adjustable trim. The builder, who had a reputation as an "expert" on that type, modified the design so it was adjustable in flight. She plucked the wings off on a qualifying flight at the Reno air races.

I would call Dale Gauger and Jerry Melhaf before I attempted anything like that, for sure.

 

[kubark42]

Yeah, I kinda feel the same. I'm in no rush to mess with design loads which I don't understand.

Maybe the best thing to do is to rig up a linkage which attaches to the handle knob and multiplies the trim arm movement. Since it wouldn't be permanent the FAA wouldn't care, and since it wouldn't alter any of the trim mechanics the airplane wouldn't care.

Maybe this could even be accessible from the rear seat!

 

[Hiperbiper]

I've never had a problem trimming a Citabra or Decathlon where the trim was smooth and the plane was not at the ends of the CG envelope...
Yes it has a huge tab. Yes it moves a lot. And yes, it is infinitely adjustable.
Make sure the trim isn't binding, sticky or stiff. Check the guides/pullys, tension on the adjustment lever, trim cable tension and oil the tab hinges.
Notchy trims suck.
JMPO
Chris

 

[Bob Turner]

Agree. Nothing wrong with the trim as-is if it is well maintained.

 

[kubark42]

Agreed. I don't believe anything is wrong with mine either, it's just overly sensitive in that a 1/2" movement produces a large pitch change and the 9" of range is 3x more than is useful. I find myself fussing with the trim a lot more than I would in a plane with a trim wheel, but from responses here it sounds like that's just how the plane is designed.

 

[Bartman]

I wonder if a previous owner moved the cables to make it more sensitive.

 

[Laytonl]

the trim in my ‘95 GCBC is very sensitive too. I’ve hit my head on the ceiling a couple of times when trying to add power by pushing the trim tab instead of the throttle!

lee

 

[Bob Turner]

I am still doing that - 18 year Decathlon owner. Consciously set it, then hand on the throttle.