Roll axis trim changes with airspeed change.
 

I have an L-39 arf that just will not stay in trim ( roll axis). If trimmed for any certain power setting to fly wings level it will be out of trim if flown slower or faster. For instance, I flew today and had it trimmed straight with half or 2/3 power. I then went to flull power and the airplane started a slow but noticable roll ( I forget which way). Conversely if trimmed for high speed full power and then slowed to 1/2 power or so it will be out of trim in the opposite direction. Its not a lot but it is cetainly noticeable and makes it had to fly smooth and precise and requires constant retriiming .

The truth of the matter is even when trimmed for straight and level with constant speed and power the airplane seems to wander and not stay "rock solid" level and in trim. This is less noticeable and less annnoying than the trim changes with speed and power changes but it still makes for a less than ideal situation.

Anybody have ideas why this is happening and what can be done about it ?

I have a couple of suspicions. First of all the airplane requires quite a bit of ail deflection from nuetral to be more or less in trim in roll axis, so something is obvilusly warrped or crooked some where. I wonder if the faster the airplane flies the greater the effect of the warp ??

Secondly the large nose gear door is not quite able to close 100% as it sits right now. The forward end hangs open about 3/32 to 1/8 " at the corner opposite the hinged side. I have tried in the past to fix this but so far I havnt been successful. I think I have figured out a way to fix it now. My theory is that the faster I fly the more air it catches underneath that slightly open front corner and causes it to possibly open a bit more. Im not sure why this would or could cause a roll trim isssue however.

What do you think guys ? I forgot to mention that I have already installed better more precise digital servos on ailerons and this was little or no help.

RE: Roll axis trim changes with airspeed change.
 

Brian,

Just mix some aileron with the throttle channel !!!! That should fix you right up !!! :D... I think your on the right track with the warp idea... It's either that or you have too much horsepower/torque under the hood !!!! Can't say I have ever seen such a thing in the roll axis. ... I'll be thinking on that one.

Danno

RE: Roll axis trim changes with airspeed change.
 

The problem with the warp theory is that I have many airplanes over the years that were crooked or warped but I dont recall one that ever had changing trim like this one does. Normally the proper amout of aileron to make a warped airplane fly straight is the proper amount irregardless of airspeed. The faster you fly the more effect the warp would have but it also means that the more effect the deflacted aileron has and it cancels out and flys wings level. Or so it seems to me anyway.

RE: Roll axis trim changes with airspeed change.
 

I agree, mix it out and be done with it.

RE: Roll axis trim changes with airspeed change.
 

What servos are you using on the ailerons? Check their holding power around centre. Also check to see if the surfaces centre each time ( Move the aileron to full deflection both ways and let the stick go and see if the surface returns to exactly the same spot each time)
Regards Al

RE: Roll axis trim changes with airspeed change.
 

Have you checked the lateral( left right) balance of the plane? If for whatever reason one side is heavier than the other if will need lots of trim at low speed to produce the required correction, whereas at high speed it will need very little, which sounds like the problem you have.

Rich.

RE: Roll axis trim changes with airspeed change.
 

I'm going to 2nd Ali's suggestion of checking the servo's. If they are Hitec 5645's their is your problem gauranteed

RE: Roll axis trim changes with airspeed change.
 

Servos are hi-tech 7955 titanium gear. I just got done checking and the servos have good holding power and good centering, I dont think that is the problem. I also did a quick and rather crude check of lateral balance and that doesnt seem to be it, if anything it seems to be heavy towards the left and it has left aileron trim. Even if it was out of lateral balance I dont think this would cause the problems Im having.

What I did find is that the aileron linkages are marginally sloppy. Mechanical slop is allowing the aileron to move up and down an estimated 3/32" in each direction or slightly less. That totals up to about or slightly more than 1/8" of total mechanical free play. Im inclined at this point to think that this is part if not all of the problem.

What do you guys think ?

RE: Roll axis trim changes with airspeed change.
 

Upon further investigation I find that the right aileron has quite a bit more free play (mechanical slop) than the left aileron. I have not so far been able to figure out why. I dont think it is in the servo gear trains as they seem to be quite tight.

I think Im on the right track now.

RE: Roll axis trim changes with airspeed change.
 

If your trim clicks are 1 or two for level flight, change your aileron servo to JR digital. if its more than 3 clicks then it lateral balance or use a wing GYRO lol.

http://www.youtube.com/watch?v=ZQ0dDbJvp3Y

RE: Roll axis trim changes with airspeed change.
 

What percentage is your aileron atv set too?

RE: Roll axis trim changes with airspeed change.
 

Post a pic of your aileron servo linkage / control horn set-up.

RE: Roll axis trim changes with airspeed change.
 

Ali is on the right track !!!! Been there done that.... ATV and Dual Rate settings can lead you down a path that can get very confusing if you don't understand it....


Danno

RE: Roll axis trim changes with airspeed change.
 

Aileron travel is set to 75 %. i do the majority of the flying with a dual rate of 70% with +25% expo. I think I can go in one hole on the servo arm. there is no room to go to a longer control horn on the surface itself.

RE: Roll axis trim changes with airspeed change.
 

I would try different servos as well. Try a shorter arm on the servo to see if you can get the travel up around 100% or higher. This will help your resolution which is the first thing to do. Also a JR digital servo will center far better than the Hitec. You will notice it in the trimming function as to movement of the servo based on the deadband. Change servos and try again. The key here is your statement of the model hunting in teh roll axis in level flight.

RE: Roll axis trim changes with airspeed change.
 

I think the servo centering is fine. I checked it this morning and it looked excellent. I did find that I had two (one on each end) loose jam nuts that lock up againt the clevis and were allowing a small amount of play or slop. That servos was also very slightly loose and needed mouning screws tigthened. This was also allowing a very very small amount of play. After taking care of these issues I now have an approx equal amount of slop in both ailerons. Total linkage slop movement is about 1/16" or perhaps a wee bit more than that but less than or equal to 3/32" at most. I can get a litle more slop out by supporting the aileron tourqe tube with a bearing or bushing type situatiion. I will also go in one hole on the servo arm, it looks like that is all I can do servo arm wise.

RE: Roll axis trim changes with airspeed change.
 

On your transmitter select the trim function and set the values down to 1 from the factory default of 4. I am assuming you are using a JR radio here. Once you have done that, turn on teh system and see if you see your servos move with each click of trim. I am confident you will see them move for every 3 or 4 clicks of trim. Give this a try and see what you get.

RE: Roll axis trim changes with airspeed change.
 

Will do. I will advise and I will be honest. I normally use JR digitals. i;m not opposed to changing if need be. Yes, im using a JR 12x.

RE: Roll axis trim changes with airspeed change.
 

Sounds good on you checking. Lets get to the root of the probelm and get you flying straight. I have too many friends down in the great state of Texas and don't need any of them flying crooked except for maybe old Danno.:D

RE: Roll axis trim changes with airspeed change.
 

I think simply that your plane is warped somewhere !

Trimming a warped plane is always difficult.[:'(] Trim is only effective for one speed. I can't be OK with the above theory of a trim independant of speed.

On propelled airplanes with a normal range of speed the difference of speed between normal flight and full power flight is not enough high to make the pilot feel a bad trim, and add to that that we do no so large trajectory and are nearly always acting on the commands.

On a jet the speed range is uge between slow flight (100km/h ?), normal flight (200km/h ?), and full power (300 to 350 km/h), plus we have long trajectories, so difference in trimming are encoutered.

I remember an aerobatic glider I owned for contest. It was severely warped. It was trimmed for the high speed figure, but was bad trimmed all everywhere !!! That was bul**** to fly it with judges ! During loops as speed was changing I had to correct everytimes...[:@] Not easy !

Have a look on your plane to check for warping with horizontal beam of light. Sincerely, if your plane is warped, there is nothing to do that is really effective... Sorry. But it should be electronic...

RE: Roll axis trim changes with airspeed change.
 

My Facet developed a roll only at high speed. I replaced the hitec 5645s (which had been in there since 2004) with jr 8411s and still had the roll.

I finally found one aileron servo mount had delaminated from the wing on one end alllowing the servo to move. When I tested the holding power of the servos, I had only tested in one direction (the direction that would push the servo mount against the wing). As soon as I pulled the other direction, I found the loose mount. Now the roll is gone.

The aileron with the loose mount was maybe a click or two trimmed up from neutral. I guess at high speeds the aileron was being forced to the neutral position allowing the roll.

edited to correct spelling

RE: Roll axis trim changes with airspeed change.
 

I did the trim thing. I put a 5 inch long pointer extending from the servo arm to help me see any servo movement. It appeared to move basically every other click of trim. Sometimes it would move with consecutive clicks. I still think it is as good as or better than the linkage.

RE: Roll axis trim changes with airspeed change.
 

One piece wing, or two?
If it is a one piece wing be sure that it is centered on the fuselage (i.e. you have equal amounts of wing area on both side of the fuselage).

RE: Roll axis trim changes with airspeed change.
 

Two piece plug in wings.

RE: Roll axis trim changes with airspeed change.
 

Hi,

I once turn-keyed an ARF for a customer and the left wing had a warp due to the glue bead on top of the main spar drying before they closed the top sheeting to it. You could clearly see the separation of the skin from the spar through the gear hole on the bottom sheeting, along with the long bead of glue. Anyway, the plane could never really trim out (roll-wise). As one wing was just that much fatter than the other, it was enough to change trim with speed.