Trim Questions

[FredN]

I have 2 questions about my current plane and would like to know what
could cause them and if it is possible to trim them out. The plane is
a fun-fly 3D type patterened after the TACO-II. It has a fat symmetrical
airfoil with the thrustline, wing and tail incidence supposedly set at 0.

1) When upright, it flys level. But, when inverted, I need to add up
elevator to keep it level.

2) In a knife edge flight, I need to add up elevator to keep it going
straight.

thanks,
Fred

[Sparky Paul]

This "up" elevator when inverted.. is it really "down", to hold the nose up?
If so, this is not unusual with a symmetrical airfoil. It must be at a positive angle of attack rightside -or- inverted. The elevator position while upright will push the nose down when the plane inverts, therefore, you muct compensate.

[FredN]

Sparky,

I did not know how to explain it. When the plane is inverted it goes up toward the
landing gear. I need to pull back on the stick (up elevator) to hold level flight.

thanks,
Fred

[Sparky Paul]

It sounds like somethings on wrong.. It must dive when flying upright then.. ???

[JMP_blackfoot]

Sparky,

I did not know how to explain it. When the plane is inverted it goes up toward the
landing gear. I need to pull back on the stick (up elevator) to hold level flight.

thanks,
Fred

It may be due to the fact that the tailplane is not aligned with the wing (is it higher than the wing ?). The deflexion from the wing hits it at a different angle when upright or upside down, and you have to pull some up to compensate. Lowering the tailplane might cure this.

[Salto]

If you have the C/G too far aft then to achieve level upright flight you will have trimmed in some down elevator trim, or else the stab incidence was set slightly "down". When inverted this down trim will cause the nose to rise as you describe. So the fix should be to move the C/G forward until you get no trim change from upright to inverted. If the design is good, at this point the trim will also be right for knife edge.

With models like you describe you can still achieve pitch stability with the C/G more aft than you expect because of the large stab area and tail moment (ie the netral point is quite a way behind the aerodynamic centre of the wing). So the above test of no trim change upright to inverted is often a better way to determine the correct C/G, as opposed to the normal dive test for required pitch stability.

The complication here is that if you want to hover your model you will probably want the C/G further aft. In this case you must suffer some reduced pitch stability and also put up with the upright to inverted trim changes you have described.

Graham.

[FredN]

Graham,

Thanks, I was curious about the CG. I have been moving the CG back to make it
easier to hover. I guess I went to far. Need to play with it more. I appreciate
the responses.

Fred