Need trimming help...
 

Just flew my new Sukhoi yesterday. Everything went well as far as maiden flights go. The next time out I want to start working on trimming the plane, but I found a conflict for CG adjustment based on the trimming charts I've looked at online.

Here are the tendencies I've noted during the first 6 flights:

- Full throttle, level flight - chopped throttle produced no change. (Up/down thrust is ok)

- Vertical up-lines require a very slight amount of right rudder. (I'll add a small amount of right thrust until I get that trimmed out)

- Vertical down-lines are perfect. (Decalage, wing incidence ok)

- Inverted flight requires quite a bit of down elevator. (* Trim chart specifies moving the CG BACK)

- Knife-Edge flight requires quite a bit of up elevator, and a slight bit of aileron. (* I can mix out the roll coupling, but the trim chart specifies moving the CG FORWARD to compensate for the tuck)


As you can see the adjustments for the last two items conflict. Does anyone have any suggestions on how I might eliminate both tendencies? According to the trim chart, I'd have to make one tendency worse in order to eliminate the other.

One final comment: I have the CG at the conservative starting point based on the manufacturer's suggestions. I'd like to move it back further to make the plane a bit more capable.

Need trimming help...
 

Try moving the CG back and see how it feels. If everything is good then mix out knife edge to your liking. Trimming is a comprimise but most importantly trimming is a verb.

Anthony

Need trimming help...
 

What sukhoi are you flying?

Need trimming help...
 

The knife edge tuck is caused by "pitch coupling" between the wing and horizontal stab. Fixing this requires that you change the height of the horizontal stab in relation of the wing, OR, use a
"P" mix on your radio to eliminate the tendency.
The large amount of up elevator while inverted suggests a nose heavy condition.
Silversurfer

Need trimming help...
 

Yes, I've added the mix with the radio and the plane feels very neutral now. Thanks.

The trick
 

Hello,
To avoid the conflict you are talking about I think it's important to do the trim tests in order. Specifically work on the inverted flight (move the CG back) characteristics before considering the knife edge pulling. It's possible that the pulling to the belly is the wing incidence also, but I think if you follow the charts in a stepwise fashion you will have better luck. Hope this helps.
Rick