[[sm=stupid.gif] Hi, I recently finished my Seagull Models PC-9 Pilatus. I have flown it three times now, (landed 6 !!!) Anyway, on each takeoff the plane rolls to the left - QUICKLY!! - I have checked the lateral balance and it seems OK (slight left wing drop..very slight) I am running the OS 46 AX and an APC 10x6 Prop. The CG is dead-on per plans. Is this NORMAL?
I have flown two other planes ( Tower Trainer 40) And the WOrld Models T-34 Mentor low wing
aerobatics trainer.(Pylon racer plane!!) Neither of these planes exhibited any of these characteristics. I like the plane except for this aspect of it I am a little nervous about taking off next time!! I always recover with hard right ailerons..but this seems like a hairy way to take off. Is there any way to tame this thing? -Ray-

www.nsrca.org has an airplane triming chart. compare the plane's performance to that chart.

My guess is you have a wing warp that hasn't been properly taken care of. what are the aileron positions when the RX and TX are on... and the plane has been trimmed for level flight? If they are left aileron down and right alieron up... you probably have a warp.

Another possibility is... you need more airspeed before lifting it off the ground. However, if that was the problem, I would expect your correction with aileron to just make things worse, resulting in the plane snap-rolling inverted and diving into the dirt. Taikng off too slow can make you think the ailerons are working backwards. (in a way they are... because adverse yaw and the tip-stall combined turns the plane sharply the opposite of the desired direction)

The last possibility I can think of is cross-correction of aileron and rudder. The rudder is more effective than aileron at low speed and would turn the plane one way... then you build speed and correct with the ailerons. So if the rudder is offset one way, and the ailerons the other... you have cross-corrected. (which could have the ailerons offset as in the first case... or in the opposite direction)

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Note that the NSRCA trim chart assumes the plane is a Pattern Competition model... and a plane with a flat bottom or semi-symetrical airfoil will not behave the same in the dive tests. (nor will a lot of older Pattern designs from the 1970's and earlier...) If it tends to pull out of a dive toward level flight... its probably OK on your model.

I too go for the warp reason or perhaps misrigged ailerons. I doubt there is any thing wrong with the configuration that would make it prone to the kind of roll you are talking about.

Does the left roll tendency continue throughout the flight?

The left roll on takeoff is typically a result of taking off too quickly, if the plane doesn't need trim in flight to counter the roll.
What happens when you let it accelerate and lift it off -gently-?
Any warp would need to be trimmed out for the whole flight, not just the takeoff.

I checked the position of the ailerons after flying this Sunday(After Church, of course...)
They were left down slightly but right was about level with the wing.
I also printed the trim chart you linked. The elevator is split and joined by a large diameter rod.
The right half is 1/32-1/16 higher than the left. I tweaked it by hand to even it up.
I don't own an incedence meter or any of that kinda stuff. I will take it back to the field for an evaluation by some more seasoned veterans to check that wing warp. However the first flight was shared by a very seasoned pilot who helped me trim it out. He did not mention the warp thing...but he may have soon forgotten my hairy takeoff. too We'll see.

I think it did to a lesser degree, but I trimmed it out to level flight (see reply to fhuber)
The left aileron was slightly lower than the right, which was level to the te of the wing.
The elevator half on the right was 1/32-1/16 (very slight) higher than the left. Would that make the right side of the plane have more lift than the left? My speed was pretty fast at takeoff. I tweaked the elevator halves to even them up. They are connected by a large diameter rod so I twisted them some to try and even them up...even thought the variance was small. Maybe at high speeds small variance adds up?

Yes at high speeds small variances will be amplified.

The combination of the misaligned elevator halves (I hadn't thought of that) and the drooped aileron could be your problem. The right elevator being up compared to left would try to roll the model right... then trim-corrected at cruise with the aileron drooped... youhave a high chance of stalling the left wing taking off early.

Sounds like a string of small problems all adding up to bite you. When corretcing the ailerons... just raise the low one. (don't want to create a bad stall characteristic by having the ailerons droop) Gain more speed for takeoff as suggested above. Retrim for level flight and check by the NSRCA chart. (again with my noted expectation that it WILL pull up from a dive on its own, and trimming that out will be a mistake.)

Raynman:
You say in your post that the roll is quick. That's probably a stall. You need to get more airspeed before you lift off and then climb gradually to allow the aircraft to gain some speed. I had the same EXACT problem with my Decathalon. A couple of times I horsed it off the ground and it would drop a wing. Luckily, I saved it, but it was a near-miss thing.

I don't see how the misalignments you described would cause such a drastic roll. I put my money on stall. Gain more speed before lifting off and correct with rudder not ailerons.

Are the aileron gaps sealed? If not, they should be. Get out the packing tape.

The misalignments alone... no, they won't cause the problem except at the low airspeed. Yes he is stalling the left wing (and lucky he hasn't spun it in yet)

Cross-correcting trims 9in this case the offset elevator halves and the one drooped aileron) invite dropping a wing in a stall. The resutls are similar to having rudder offset to the left and the aileron set to correct it to fly yawed, but level. its just not as visible that the plane is cross corrected when it flies by, because its 2 roll axis inputs countering each other, instead yaw corrected with roll. (when the plane flies sideways you can see it every time)

Thanks, for your replies. I am working this Sat 2/14 but hope to fly Sunday Afternoon. Weather report looks good for No. CA Bay Area !! I adjusted left wing aileron droop. to level with T.E. of wing (matches left Aileron now) Elevator halves are even. I noticed gaps between T.E. of wings and ailerons And am going to tape them. Do I flex the aileron to the fullest open position ; bend tape into " V " and insert narrow end of " V " into the opening?
Also, Should I re-adjust right trim (aileron) to center after the aileron adjustment? Before the next takeoff at higher speed (use all the runway this time!!!) Thanks again for all the posts!!!

i have a pilatus pc-9 and i had found out on my first few flights that mine does the same thing. i found that i have to get up to full speed before takeoff. The pc-9 doesn't like low air speed or slow landings. on take off don't give it to much elevator or it wll snap right off the bat. Just ease on the elevator until it comes to a 15 degree angle and continue to clime and build air speed. When you have done that you will find that the pc-9 will become very stable in the air until you come to a stall and move the aileron left or right it will go into a snap and then to a spin.
hope you have luck. crashed mine 3 times 4 repairs. finally got to fly it 6 times and as long as i had good air speed it is a smooth flying plane. I also found that you will need to reglue the wing servo trays and run a bead of ca on the rest of the servo trays.
hope you can tame that plane good luck.

If we didn't crash em it wouldn't be a hobby