Level flight requires slight right hand turn input
 

Since I purchased my 1969 T337D about 4 years ago, I have had a need to put in a slight amount of right aileron deflection for straight line flight. The amount of aileron deflection needed is about 1/4-3/8 inch as measured by looking at where the aileron is w/respect to the wing tip, or about 5-10 degrees of right turn in the yoke. I have played around with using rudder trim and also the aileron trim tabs to compensate for this but there isn't enough adjustment there to center the yoke in flight. With the right turn in there from the ailerons and no rudder trim deflection the ball is pretty well centered and the plane makes book numbers for speed. It has a Robertson kit installed as well. I was wondering if anyone else has seen this behavior and if so is there a way to address it? It's not a critical problem but kind of bothers me that the yoke isn't centered and the ailerons faired with the wing tips and flaps in level flight.

Does the airplane fly straight and level? Your concern sounds like the yoke is not centered?

Hi herb

Yes once the yoke is turned slightly to the right it flies straight and level with the ball pretty much centered. I spoke to my mechanic about it and he said it is acting left wing heavy, which doesn't mean it's necessarily heavy on the left side, but probably has a slight difference in the apparent Angie of attack of the two wings. Given the amount Of Aileron input required about 1/8 of an inch or less effective difference between the two wings would probably do it. It doesnt look like there is any adjustment for this as there is on some cessnas that have a cam for adjusting the Angie of attack. Without making some careful measurements it would be hard to establish that this is the case.

Ben

You might have a mechanic check the rigging with the Robertson STOL manual. It says to rig the flaps per the cessna manual. then with the control lock in rig the left aileron to fair with the tip and the right aileron @ 1/4 inch above the tip.
Kim

Level flight requires slight right hand turn input
 

I have seen this in other Cessnas. My C172 had the problem, although this Cessna has the eccentric bushings in the trailing edge of each wings. I adjusted the left (LIGHT WING) full down and the right full up. I KNOW THIS ISN'T THE PROBLEM YOU MENTION, BUT LET ME FINISH MY EXPLANATION. After all these adjustments my IA noticed my left flap was NOT fully UP in the tract. I replaced the flap and plane flys straight! So my advice is to make certain that your flaps (on the high flying wing) is in FULL UP position.
FYI.
BILLS

I will definitely check the Robertson manual for that aileron setting. 1/4 inch of upward deflection on the right aileron would probably fix most of whats going on. I have the manual but its currently at the shop with the plane and log books. In regard to the flap suggestion I have looked pretty carefully at the flap positions and they seem to be even on both sides.

Wouldn't the simple thing be to have the rigging checked, in a methodical order, by someone that is experienced in airframe rigging? There are a number of people that have taken the CPA rigging course that could help out. It is so nice to fly a plane that is rigged correctly.

Dave

I had the rigging redone about two years ago, but it did not effect the flight characteristics. At that time the mechanic used the Robertson manual, and he's pretty careful so I need to check it to see if in fact it recommends the aileron deflection. I will be back in the states next week and will look into it.

I was able to look at the Robertson rigging procedure today when I stopped by the shop. It does indeed recommend that 1/4 inch of right upward aileron deflection be applied after the full procedure is done to allow for the airplane to fly with trim tabs zeroed. As far as I can tell this was never done either before or after I had the plane re rigged two years ago. I don't know why this is required aerodynamically, but it is part of the procedure. Based on what I have seen this should resolve at least half of the required yoke rotation. The plane is going back together during the annual and we will put this in and see how it flies. I had to do the eddy current spar inspection for 5500 hours so its taking a bit longer to get it back in the air. Its raining cats and dogs here anyway so better to be in a cozy hanger.

So as a final update I finally got to fly the plane after adding this 1/4 inch of upward right aileron adjustment on a calm day and it now flies a straight heading with no control pressure input and neutral trim tab settings. So thanks for the help and of course RTFM. The yoke is still slightly rotated to the right, in level flight, but no control pressure is required to maintain a heading, so at this point it is purely a slight yoke position adjustment which we can take on at some point in the future.

Does the plane go faster than before? Are you making book speed?

Dave

The plane has always been pretty much on book speed. I was trying to decide if it was slightly faster yesterday, possibly 3-5 mph. I will be taking it down to Mexico next week so lots of opportunity to observe cruise flight behavior. I will post if I see a distinct change.

Excellent, report your findings.

Dave