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Jan 16, 2019, 10:50 AM
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aeronaut999's Avatar
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Trimming for a climbing turn -- a simple question for those w/ free-flight experience


Hi all --

With a CW-rotating prop, is it more common to try to trim a free-flight model for a climbing LEFT turn or a climbing RIGHT turn?

In either case, does this generally require an INTO-the-turn trim adjustment or an OUT-of-the-turn trim adjustment?

Is the required trim adjustment usually made with the rudder, or with ailerons/ wing warping/ roll trim trim tab? Or both? Or are the ailerons/ wing-warping/ roll trim tab typically adjusted to make a roll torque AGAINST the direction of the rudder trim?

Is it standard practice in free-flight flying to make an aileron (or wing-warp) trim adjustment AGAINST the direction of the intended turn, while setting the rudder INTO the desired direction of turn? Somewhere I remember reading about this. If so, does this generally work regardless of whether the desired direction of turn is INTO or AGAINST the turning tendency created by engine torque effects?

Here's what I'm trying to do in more detail--

The Hobby Zone Super Cub is a high-wing rudder-elevator plane with ample dihedral. Prop turns CW as viewed from behind, and the motor mount has some down thrust and right thrust. After trimming for a wings-level power-off glide, I don't detect a huge turning tendency in either direction when I apply power, but there may be some, probably to the right. Thrust is modest-- in wing-level light it takes about 45 seconds to gain 500' of altitude. I've modified mine by adding ailerons.

The question -- I want to set the plane up for a configuration where it will be really locked into a banked climbing turn at full power --tending to return to the climbing turn if temporarily upset into wings-level flight (which will tend to lead to a loop) or if temporarily upset to a much steeper bank angle (which will tend to lead to a spiral dive). What direction of turn, and what configuration of rudder and ailerons, is most likely to work to attain this goal?

For example, I've had some luck setting the aileron trim to centered, putting the plane into a left turn, and adding a slight amount of left rudder trim to lock the plane into the left turn. The plane seemed to stay in the climbing left turn. If I intervened to roll the plane into a right turn, and then released the controls, the bank angle tended to shallow and then the plane did some near-wings-level loops and then did finally end up in a climbing left turn. However these conclusions are only based on one flight, near the end of which I had a structural issue-- after repairs, I'd like to speed up further investigations by focussing my efforts on the configurations which are mostly likely to be fruitful-- hence this post--

Is there any reason to think I'd have better results if I used the ailerons rather than the rudder to make the into-the-turn trim input that seems to be required? Or should I set the ailerons AGAINST the desired direction of the turn, which would likely require a stronger rudder trim setting into the direction of the turn? Note that the rudder is in the propwash while the ailerons are not, and I'd like my setup to be effective over a range of thrust values, as the thrust drops off as the battery is depleted.

Stepping back a level-- and keeping in mind that I want my setup to work both when the engine is going at full power and after the power has dropped off some due to battery depletion-- given that the prop rotates CW-- is it likely that I'll have best results trying to trim for a climbing LEFT turn, or a climbing RIGHT turn? Assuming that the a/c does have some left-turning tendency under power, it seems to me that if I set it to be stable in a climbing right turn, and then the power dropped way off, the bank angle might tend to increase greatly.

I've actually set up this plane to be pretty stable in a climbing turn some years ago, but I don't remember exactly what trim inputs I used or even what the direction of turn was--

Thanks for any input--

Steve
Last edited by aeronaut999; Jan 16, 2019 at 11:22 AM.
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Jan 16, 2019, 12:09 PM
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richard hanson's Avatar
That Cub has a fairly high wing loading for use as a performance free flight
My Seniorita could be trimmed for hands off performance but it was better suited, having a lighter loading and more dihedral
I also used rudder and elevator only
Guys who flew Buzzard Bombshells , Powerhouse, or similar types will have mode salient info
Last edited by richard hanson; Jan 16, 2019 at 12:17 PM.
Jan 16, 2019, 04:00 PM
AndyKunz's Avatar
Great YT page for FF models. I'm sure he'll be happy to answer your questions, too.

https://www.youtube.com/channel/UCJ4...khuycvXmEg2caw

Andy
Jan 19, 2019, 11:43 PM
B for Bruce
BMatthews's Avatar
If the model has a fairly high wing loading as Richard suggests it may not be indeal but you should still be able to trim it for "free flight". Plus when I look at pictures of the HZSC I don't see what I consider to be "lots of dihedral" by any free flight perspective. It's barely "lots" by RC standards. I'm actually surprised if it flies well with rudder for the main turn control. So all in all it may not prove to be stable in the sort of hands off long term trim you're after.

Also CW from the rear is actually a CCW prop. Prop direction is always given by how it rotates as seen from in front of the aircraft. It's pretty much the only thing labelled that way but such is how it's done. This applies to full size too. Likely because it was always the ground crew staring into that big swinging piece of lumer just after the early mechanics proped the engine into life....

I do cross trim my own models to some degree. It aids with avoiding the repeating stall cycle where the stalls get bigger with each cycle until it crashes. And for a Cub and a CCW prop I'd suggest that the model will be happier and more stable with an open right hand turn. And to get that turn a whisper of left aileron to set up the proper twist then push against that roll trim with extra rudder will produce a reasonably stable setup.

But even with more dihedral than the HZSC has if you upset the apple cart and turn it then release the controls expect it to SLOWLY transition back to the right turn trim. And through that it'll do a few stalls or even loops if the power is that high. Lower power would help but not totally cure this trait. Free flights do this too if they get upset by turbulence found around strong lift.


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