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RE: Roll axis trim changes with airspeed change.
Turnburn,
I did read your email. Stop thinking about your trim issue in terms of aileron deflection. The fact is a heavy wing panel causes a constant rolling moment in one direction which will need countering with a trim induced rolling moment in the opposite direction and this will be speed dependent. You may be visually seeing a left aileron deflection but the wing system is still generating a right rolling moment which is changing with airspeed. It sounds like there may be a warp in there somewhere but for now forget that we can real with it later. Add the right wing weight retrim and tell Ta what happens. Malcolm |
RE: Roll axis trim changes with airspeed change.
After thinking about it some more I came to this realization that even thou I have left ail trim its irelevant to this disscusion. I got it now. I hope to fly it tues or wed with some weight on the right wing. The question is how much. I assume I should put only enough to bring the airplane into correct lateral balance which should be about one ounce.
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RE: Roll axis trim changes with airspeed change.
ORIGINAL: turnnburn 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.
If you are just going by the weight of a complete wing panel then you also must check to be sure that they both have the same lateral CG. That is why weight alone in not the best way to go.
If the heavy panel has its weight concentrated near the root, and the light panel has its weight concentrated near the tip, the panel that weighs more will have less leverage to cause an imbalance and could still be considered the light panel as far as lateral balance is concerned. That is why I said to put the whole jet together, get a couple of friends over to help, and check the lateral balance. One way to test to see if you have an aerodynamic issue or a weight issue is to pick a speed and trim for straight level flight. Then roll inverted at that same power setting, If it is a weight issue the weight moment will also be backwards while inverted and the aileron trim will be way off while inverted, and should be back to right on when you roll back to upright. If it remains straight and level while inverted then weight is not this problem. It is also possible to have a combination of both aerodynamic issues and weight issues. Weight is the easy one to fix so I would start there. |
RE: Roll axis trim changes with airspeed change.
Jeff is absolutely correct in his comments about lateral balance and with his test by rolling inverted. I thought about suggesting this but wasn't sure how comfortable Turnburn would be flying inverted and closely observing the trim. If I were adding weight to the right tip I would add a couple of once or so. It has to be enough to cause a change and once this has been established the correct amount can be found by trial and error.
Regarding incidence checking, I'm flying the Global Jet version of this model. I would check carefully whether both flaps line up with the moulded root fairings on the fuselage and also with the short fixed portion of trailing edge between the outboard end of the flap and the inboard end of the aileron. If there is any twist in the flap this needs to be corrected first. Then the ailerons need to be checked between the fixed portion and the moulded fairings on the tip tanks. Again any twist needs to be corrected. This is easy using a heat gun to warm the surface which can be twisted past the correct position and allowed to cool. A little trial and error is required. Only after all is straight in these areas is it worth checking incidence. Malcolm |
RE: Roll axis trim changes with airspeed change.
Good advice guys. Thanks. I have no problem flying upside down and making observations. Im hoping to go fly either tonight of first thing tomorrow. I'll will post my results once I have them. Clearly something is warped or the wing incidence is wrong or both.
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RE: Roll axis trim changes with airspeed change.
I finally got to make two flights today. For the first flight I added approx 2 ounces of lead to the right wing panel with no other changes. A noticeable improvement but still some discernable roll to the right with speed increase and roll to the left with speed decrease. For the second flight I added 3/4 ounce more to the right wing out at the tip and I also changed the trim system setting from 2 to 1 for the ailerons to get the finest, most precise triiming avail per beep or click of trim. The trim change with speed change was still better, but not quite perfect. Greatly improved for sure. I cant belive it but I completely forgot to do the inverted tests as decscribed in on of the latest posts above.
Im very gratefull for all help and suggestions. I would have never guessed that this would be the solution to my problem. Im still planning to install even better servos and probable another 1/2 to 3/4 ounce of weight. Suddenly I have a decent flying airplane. Greatly improved. Thanks again especailly to Malcom H. |
RE: Roll axis trim changes with airspeed change.
Glad to help!
Malcolm |
RE: Roll axis trim changes with airspeed change.
Suddenly I have a decent flying airplane. |
RE: Roll axis trim changes with airspeed change.
LOL. I guess so.
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