P

Platyius

Member
I am having trouble correcting some yaw on my aircraft. Tried following the the PRODUCTION ACCEPTANCE PROCEDURES below but dont know the length of the wedge, have tried one about three inches long which is not enough correction. I can't find Fig 15-5 anywhere. Can anyone help

Tony Earle

Directional (yaw) trim:
An off-center skid ball is common for many new airplanes. Small trim adjustments should be
made so that when at cruise speed the airplane flies straight and true in a stick free mode.
To test directional trim, establish and hold level flight. Remove your feet from the rudder pedals.
If the skid ball is does not remain centered, rudder trim will be needed. Apply rudder as
necessary to center the ball and determine whether ?right or left? trim will be needed. Fig. 15-5
shows an effective and attractive method for a fixed trim tab. Unlike tabs which stick out past the
trailing edge, these do not alter the planform profile of the control surface, yet are very effective.
A temporary tab of this type can be made of wood, sawed into a wedge about 3/8'' at the thick
edge and 1 1/4 to 1 1/2'' wide. This can be temporarily taped on to the rudder trailing edge near
bottom and adjusted simply by trimming the length. Attach to the side of the rudder opposite that
of the rudder pedal effort needed to center the ball. It may take several flights to determine the
exact size. Then the temporary wedge can be replaced by a wedge made of machined aluminum,
plastic, or sealed wood, and attached with flush pop rivets.
 
Rudder Trim

Or you can do it the old fashioned way and use a piece of aluminum...bend it a little bit, tape it the other side, fly it...then bend as needed. The Avery one is fine but you need several and all you can do is make them longer (buy more) or shorter.
 
I initially added 6" of the Avery wedge and am still about 1/8 of a ball out of trim. Added another 3" and there was NO effect. If anything, the trim may have become a bit worse. Larry Geiger had close to the same effect. So, I am concluding the situation is NOT unusual.

Maybe a case of diminishing returns with the wedge.

Scott?? Comments, next step??
 
MartySantic said:
I initially added 6" of the Avery wedge and am still about 1/8 of a ball out of trim. Added another 3" and there was NO effect. If anything, the trim may have become a bit worse. Larry Geiger had close to the same effect. So, I am concluding the situation is NOT unusual.

Maybe a case of diminishing returns with the wedge.

Scott?? Comments, next step??
Click to expand...

I have never seen a case were adding more wedge didn't cause more change in trim. It is true that if you have a major out of trim condition, that adding a second 6" piece of wedge wont produce the same amount of change as the first, because the further you try and displace the rudder with a tab or wedge, the more opposing force there is trying to push the rudder back straight.

N412RV, the red RV-12 prototype has no rudder trim tab and it fly's perfectly in trim. Not all airplanes are exactly the same as far as trim goes.
It is important that the roll trim be adjusted first.

I think it was already mentioned that if you adjust roll trim to fly level when a 250 lb pilot is flying solo you may be causing a need for more rudder trim than effectively needed.
Assuming you have the roll trim adjusted properly, I would do the following...

Make sure the rudder moves very freely on it's bearings. If you disconnect the rudder cables, you should be able to lightly flip the rudder and have it swing and bounce off of the rudder stop and swing back. If it doesn't, recheck the spacer washers for the rudder bearings to make sure no side load is being applied to the bearings (it doesn't take much to add a lot of friction).

Check the rudder trailing edge shape cord wise with a straight edge. The straight edge should lay pretty flat on the skin surface all the way to the tight radius bend at the very trailing edge. If it is pillowed out it can cause the rudder to have a dead band and reduce the effectiveness of trim tabs that are mounted there.

If those two things are ok, a trim tab wedge should be able to correct for proper trim in yaw. It is possible that placement location on the rudder can change the effectiveness of the tab. I have no experience putting a tab on an RV-12, but it is possible that the canopy produces a flow disturbance down low on the rudder that would not be present further up (so it might be more effective up higher, but you guys will have to tell me).
 
MartySantic said:
I initially added 6" of the Avery wedge and am still about 1/8 of a ball out of trim.
Click to expand...

And are you sure that your ball is working perfectly? Experienced pilot should pretty easily be able to say which one is not doing ok, the plane or the meter...
 
rvbuilder2002 said:
It is important that the roll trim be adjusted first.

I think it was already mentioned that if you adjust roll trim to fly level when a 250 lb pilot is flying solo you may be causing a need for more rudder trim than effectively needed.

Assuming you have the roll trim adjusted properly, I would do the following...

Make sure the rudder moves very freely on it's bearings......

Check the rudder trailing edge shape cord wise with a straight edge.......

If those two things are ok, a trim tab wedge should be able to correct for proper trim in yaw.......
Click to expand...

Scott after reading your response I went out and did a bit of testing. At 105 knots, with 6" of the Avery wedge installed on the rudder, I am 1/8-1/4 of a ball out to the right. If I first center the ball with the pedals and then release the controls, the left wing drops slowly. This is with a 230# pilot and 15 gallons of fuel. (So I potentially have a heavy left wing).

If I reestablish level flight and then release rudder pedal pressure, the ball drifts 1/8-1/4 of a ball to the right. Then if I release the control stick, the left wing drops much faster (I'd say 3X faster). (So again, a potentially heavy left wing.)

I have read the procedure outlined in the manual and need a bit of clarification.

Your response indicated that roll trim should be adjusted first. The manual seems to infer we adjust rudder trim first.

Should I start over by removing the rudder trim wedge?

After adjusting the trailing edge of the flaperon for roll trim, how should the flight testing be conducted? Should I first center the ball and release the stick looking for the turn tendency OR should I release pedal pressure, let the ball drift right and then release the stick looking at the turn tendency? (As the two responses are quite different).

The rudder mechanically checks OK per your suggested checks.
 
Last edited:
rvbuilder2002 said:
N412RV, the red RV-12 prototype has no rudder trim tab and it fly's perfectly in trim. Not all airplanes are exactly the same as far as trim goes.
It is important that the roll trim be adjusted first.
Click to expand...



MartySantic said:
Your response indicated that roll trim should be adjusted first. The manual seems to infer we adjust rudder trim first.

Should I start over by removing the rudder trim wedge?
Click to expand...

Sorry, I was talking about rudder trim but for some reason wrote roll...you need to follow the manual and do rudder first.

As for roll trim... with only 15 gallons of fuel and a guy your size in teh left seat, I would expect it to roll left. Check roll trim with teh airplane loaded the way you would typically fly. If it will be solo most of the time, then adjust for "in trim with you and about 1/2 fuel. If you will fly with a passenger a lot of the time, then evaluate with a typical passenger and about 1/2 fuel.
 
Clock the Instrument?

Hate to admit this, but during my trim "trials" I eventually noticed that there was a fair amount of play in the mounting of the TC/Skid ball. Finally figured out that I first needed to level the airplane, then level the instrument, then go flying. Clocking the TC properly contributed about 50% to the ultimate solution.
Terry, CFI
RV-9A N323TP
 
terrykohler said:
Hate to admit this, but during my trim "trials" I eventually noticed that there was a fair amount of play in the mounting of the TC/Skid ball. Finally figured out that I first needed to level the airplane, then level the instrument, then go flying. Clocking the TC properly contributed about 50% to the ultimate solution.
Terry, CFI
RV-9A N323TP
Click to expand...

That is a very necessary check when using a traditional inclinometer. Many certificated airplanes even have sloted screw holes at teh instrument location for the Turn & Bank or Turn Coordinator.

Not a factor for the RV-12 though, because the slip/skid ball is a digital depiction on a color screen.
 
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