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  1. #1
    FenderBean's Avatar
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    flight trim on full stabs

    My f-16 with no trim has slight right roll at full throttle, but under 3/4 throttle it flies level. So if i trim it level at full throttle it will roll left when i back off the throttle. Is this the full stabs not being set or some kind of airfoil problem?
    Contest Director AMA # 8394

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    RE: flight trim on full stabs

    ORIGINAL: FenderBean

    My f-16 with no trim has slight right roll at full throttle, but under 3/4 throttle it flies level. So if i trim it level at full throttle it will roll left when i back off the throttle. Is this the full stabs not being set or some kind of airfoil problem?
    Fender,

    Since surface authority is airspeed related it simply means your ailerons are possibly not exactly centered, or it could be your stabs are slightly off. The effect of the off center only shows above a certain airspeed. Very similar to VNE. Below VNE the structure is stable, above it, all hell breaks loose.

    David S

  3. #3
    FenderBean's Avatar
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    RE: flight trim on full stabs

    But to have that much from such a small air speed change? Well i have no idea how to trim it when i have to use two trims in the opp direction. Like i said it rolls right until a certain speed then rolls back left. So originally i trimmed it level at full throttle to stop right roll and as soon as i throttle back it rolls left. Is both out of trim and one have more authority over the other at higher speed?
    Contest Director AMA # 8394

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    RE: flight trim on full stabs

    ORIGINAL: FenderBean

    But to have that much from such a small air speed change? Well i have no idea how to trim it when i have to use two trims in the opp direction. Like i said it rolls right until a certain speed then rolls back left. So originally i trimmed it level at full throttle to stop right roll and as soon as i throttle back it rolls left. Is both out of trim and one have more authority over the other at higher speed?
    Fender,

    This means your problem is in both the ailerons and stabs being unbalanced. For example. your aircraft might be reacting more to the offset of the ailerons at one speed, but that is then overpowered by the offset of the stabs at a higher speed, because the stabs, being larger have greater roll authority at speed, or perhaps it's the other way around. Just a theory, but aerodynamically correct, I think.

    Imagine, for example, using aileron trim to correct an offset which is actually in the stabs or vice versa. No way will you get a consistent result.

    David S

  5. #5
    Dr Honda's Avatar
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    RE: flight trim on full stabs

    You can also be lightly twisting somthing as you push the throttle higher. (from air pressure) You could simply add a mix to trim it out.
    Tony
    Owner of A&L Motorsports, LLC

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    RE: flight trim on full stabs

    Fender, just had a similar issue with my smaller F-16. I had it trimmed for medium to high speed flight, and then started noticing a tendency to bank at slower speeds, specially during final approach and flare. I was almost fighting it to the ground, which resulted on a broken nose gear strut eventually.

    It turned out one of my stabs, eventhough appeared to be centered, there was enough slop to make it asymetrical to the opposite side. To fix, I simply used one side as a reference by aux trimming until the stab was centered with the speedbrake. Next I loosened the other stab and centered it with the speedbrake and clamped it fixed, with the radio on of course. The retighten the fitting. After this then simply return the aux trim to zero or whatever the initial value was before centering with speedbrake. That took care of it.

    Just a thought.
    David
    Buying Jet Legend? Read here: http://www.rcuniverse.com/forum/m_11372496/mpage_1/key_/tm.htm

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    RE: flight trim on full stabs

    You could just build a thortte to aileron (or stab) mix to adjust the trim between 3/4 to full throttle when you get your roll.

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    RE: flight trim on full stabs

    ORIGINAL: sailing1

    You could just build a thortte to aileron (or stab) mix to adjust the trim between 3/4 to full throttle when you get your roll.
    Guys,

    Using a throttle mix to correct an airspeed issue just makes the issue worse. Example: Upwind airspeed at a certain throttle setting is higher that downwind setting at the same throttle setting. Now you have one trim for upwind and a different one for downwind. Another example: Pulling vertical, you will immediately start to bleed airspeed, even though the throttle setting is the same, the airspeed isn't, and will continue to change the longer the vertical line. Doesn't work.

    Falconwings has the right idea. You have to fix the problem at it's source, which seems to be the stabs being offset per his experience.

    David S

  9. #9
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    RE: flight trim on full stabs

    FenderBean, IMHO

    check your lateral balance first .
    your F-16 might not be balanced lateral correct.

    You have trim to counterbalance the heavy side in slow flight. Dont confuse your self by seeing your stabs in same level.All airframe can give aileron input.

    Then in high speed where CG is not that important your build in trim rolls your model.

    Balance your model and add some lead unter the edge of the wing, on the light side.witch one= work it out from the roll tendency>bear in mind the trim.


    Throtle mix wont work because our airframes dont accelerate as fast as our engines anyway.


    my 2c
    Best Regards George
    www.rcjet.gr \"props are for boats\"

  10. #10
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    RE: flight trim on full stabs

    Okay cool i will check lateral balance again, i will set my ailerons back to center and work on trim with elevators. My stabs have the typical jr 8711 play. Arrrg which is why my new jets will not be jr. Thanks for the info i figured it was both being wrong when it changed with airspeed.
    Contest Director AMA # 8394

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    RE: flight trim on full stabs

    Unfortunately a lot of the centerting issues are invisible because of the inherent slop of the linkeage. A lot of the slop goes away if you use a small wheel servo arm at the servo, and place the linkeage as far away from center on the torque rod, which is not always possible (mechanical advantage). This has been beaten to death so I'm sure you are aware by now. If you use 8711's, your elevators should not drop once you turn the radio off. If they do, it's proof of a lot of built in slop, just use a smaller servo arm or a servo wheel, it should still provide more than enough stab travel, but you greatly increase precision.
    Buying Jet Legend? Read here: http://www.rcuniverse.com/forum/m_11372496/mpage_1/key_/tm.htm

  12. #12
    DAN AVILLA's Avatar
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    RE: flight trim on full stabs

    Have you checked the incidence in the wings. I had that problem with a boomer Xl and I cured the problem by moving the wing pins. might be hard if you have tubes.Dan

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    RE: flight trim on full stabs

    So what you all are saying is taht jet engines do not have torque.  Interesting.

  14. #14
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    RE: flight trim on full stabs


    ORIGINAL: rgburrill

    So what you all are saying is taht jet engines do not have torque.Β* Interesting.
    Really!? Did they?

  15. #15
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    RE: flight trim on full stabs

    Try just a twek of left rudder. Rudder becomes more effective and less at high speed. Just a couple clicks of trim should do it.
    Tom Perry
    In Dog beers, I only had one!

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    RE: flight trim on full stabs

    How is the trim different on upwind versus downwind if the power setting is the same? Soory this keyboard is awful
    Matt

  17. #17
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    RE: flight trim on full stabs

    Yeah I will try this first, start with the easy first. I have tried the rudder before but I have not tried it at full throttle. I have found this to be true, my f-16 rudder doesnt do much unless I am about 1/2 stick, I had a 10-15mph crosswind one day and the rudder didnt do anything. I had such a bad crab in and the only thing that fixed it was coming in faster.
    ORIGINAL: tp777fo

    Try just a twek of left rudder. Rudder becomes more effective and less at high speed. Just a couple clicks of trim should do it.
    Contest Director AMA # 8394

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    RE: flight trim on full stabs


    ORIGINAL: David Searles

    ORIGINAL: sailing1

    You could just build a thortte to aileron (or stab) mix to adjust the trim between 3/4 to full throttle when you get your roll.
    Guys,

    Using a throttle mix to correct an airspeed issue just makes the issue worse. Example: Upwind airspeed at a certain throttle setting is higher that downwind setting at the same throttle setting. Now you have one trim for upwind and a different one for downwind. Another example: Pulling vertical, you will immediately start to bleed airspeed, even though the throttle setting is the same, the airspeed isn't, and will continue to change the longer the vertical line. Doesn't work.

    Falconwings has the right idea. You have to fix the problem at it's source, which seems to be the stabs being offset per his experience.

    David S
    Hehehe, did somebody say "Downwind"? I wonder how everything is affected in a downwind turn? [X(]


    Shaz

  19. #19
    rctech2k7's Avatar
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    RE: flight trim on full stabs

    This is actually not easy to figure out. David has a point but when I tried to solve it's hard to find. Maybe someone else can. The relationship between aerodynamic force and airspeed are expo but constant with its coefficient. The only thing that coefficient can change is by changing the airfoil, camber or angle. Considering flying a trimmed aircraft with no changes in surfaces, Cls are all constant and all their forces change exponentially at the same rate with airspeed therefore the difference in roll is the same throughout any speed...
    More than just sports, hobby and recreation, it’s technology…

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    RE: flight trim on full stabs


    ORIGINAL: rctech2k7

    "This is actually not easy to figure out. David has a point..."
    Big D, you getting a load of this?


    Shaz

  21. #21
    rctech2k7's Avatar
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    RE: flight trim on full stabs


    ORIGINAL: JohnMac


    ORIGINAL: rgburrill

    So what you all are saying is taht jet engines do not have torque.Β* Interesting.
    Really!? Did they?
    IMO, the torque of our jet engine is a approximately the product of mass of the rotating parts, average distance to the center and its acceleration. Not sure about the mass of rotor but radius I know is small. At constant rpm acceleration is zero therefore it's only exist as transient...
    More than just sports, hobby and recreation, it’s technology…

  22. #22
    rctech2k7's Avatar
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    RE: flight trim on full stabs


    ORIGINAL: sskianpour


    ORIGINAL: rctech2k7

    ''This is actually not easy to figure out. David has a point...''
    Big D, you getting a load of this?


    Shaz
    Just curious, one more option I'm thinking is the rudder, when center of pressure in both wings are offset...
    More than just sports, hobby and recreation, it’s technology…

  23. #23
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    RE: flight trim on full stabs

    The rudder becomes more effective as the AOA increases and thus less effective as the AOA decreases. At high speed rudder will just make the airplane yaw as you slow it will induce roll. We used to have a rudder effectiveness demo in the T38. Go about 300 kts and push rudder...jet yawed a lot. Hold rudder and pull back on stick.....then it did a nice rudder roll. If its your rudder trim the jet will roll slightly as you try to loop.
    Tom Perry
    In Dog beers, I only had one!

  24. #24
    rctech2k7's Avatar
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    RE: flight trim on full stabs

    ORIGINAL: tp777fo

    The rudder becomes more effective as the AOA increases and thus less effective as the AOA decreases. At high speed rudder will just make the airplane yaw as you slow it will induce roll. We used to have a rudder effectiveness demo in the T38. Go about 300 kts and push rudder...jet yawed a lot. Hold rudder and pull back on stick.....then it did a nice rudder roll. If its your rudder trim the jet will roll slightly as you try to loop.
    Good help as I'm also trying to find out why the trim that contributes perfect balance fail when airspeed change. I know it happen on longitudinal that change moment out of equilibrium when airspeed drops but not on ailerons. I found it that once aileron is trimmed it remains trim at any speed. I'm also thinking rapid airflow at the sweep wings, also some kind of air separation at lower speed. I can't prove it at this time due to my old 3D modelling background but I've now seen another reason why it happens when rudder is out of trim and ailerons was used to correct it.

    A very good point...
    More than just sports, hobby and recreation, it’s technology…

  25. #25
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    RE: flight trim on full stabs

    Keith, If my analysis is correct, there's an enormous down force onto the leading edges of the stabs at high airspeed, especially in a dive. If one servo is weaker than the other and start fail to hold at a certain airspeed, the 16 will pull toward that side.

    My first Jet, a Cermark F-16, everything was fine with 9411s and P-60, but this problem surfaced as soon as I had a K-80 in it. After verifying one servo being weaker then the other on the ground and replacing them with full size 7955s, problem solved.

    Perhaps something also worth look into,

    Cheers,
    Barry
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