Modify position of hoz-stab

[rorywquin]

Can any of you aerodynamic fundis out there please help.

I am building an Acrowot (to replace one lost in a mid-air) which, is a low-wing (semi symetrical airfoil) sport aerobatic aeroplane .60 size with about a 58" wing span. http://www.chrisfoss.co.uk/business.htm

This is a super aircraft except for the fact that when you apply rudder, it rolls and dives in the direction of the rudder. On my previous one, I mixed elevtor and aileron to rudder to get rid of most of this coupling.

The wing is built with a slight dihedral (3/4" from center to tip) and I have read somewhere that if I lower the the position of the horozontal stab by 1" this will eliminate most of the coupling. As it is now, the horozontal stab is quite high above the wing.

Any suggestions / comments please??

[Andy W]

In my very inexperienced opinion, the dihedral is causing that. I have a very similar model - the Dynamic (http://www.ezonemag.com/~awilletts/dynamic.jpg) - and it has a very small amount of dihedral too. I notice a slight rolling tendancy when using rudder alone, but the model is so aerobatic, I find myself flying both sticks constantly anyway, so I can't be sure it's not diving with rudder also. Stab is in the same position as yours.
I'd be concerned that moving the stab would blank it out to some degree - wash over the wing affecting it.
..a

[rorywquin]

Andy

The Acrowot pitches quite badly with rudder - and flying something like knife edge (without corrective mixes) is quite difficult because of having to correct for the roll and pitch ! I would prefer to have minimal corrective mixing - ie get the plane as harmonious as possible aerodynamically. Should make for a better flyer!

[Andy W]

Can you remove the dihedral with the one you're building?

[tim hooper]

FWIW, my little D5 Turbi has the same sort of planform and exhibits the same behaviour, so I know the feeling!

I think the person to talk to about this could be Dereck; hasn't he done a couple of versions of the 4*40 with a lowered tail? I seem to recall him saying it cured the original problem.

tim

[Salto]

There are two things on this model that will cause it to roll into a yaw. The dihedral, and the side area of the fuse above the CG. Reducing either should reduce the roll.

My guess is that the dihedral is there to counter the natural slight roll instability of the low wing design, so reducing dihedral would come at a slight cost of reduced roll stability, maybe even pushing it to slight roll instability.

The only way to reduce the side area above the CG would be to raise the wing roots up into the fuse, ie. make it into a mid wing design. This would raise the CG and place it closer to the centre of the fuse side area.

I guess you're just up against the fact that a low wing design like that one can never be as sweet as a mid wing "pattern" plane in terms of lack of unwanted coupling effects.

But for a partial fix, like Andy, I'd suggest building the wing flat.

Graham.

[BMatthews]

Or just build a pattern model design in the first place.

It sounds like by the time the side area and wing position and dihedral and tail location and rudder areas are all optimized that it'll end up looking like a pattern model anyway.

I'm not TRYING to be flippant but if your flying is such that you find these sorts of atifacts in the handling are bothersome enough to do the mods to eliminate them then I think you're a prime cadidate to just build a pattern model that won't have such charactaristics in the fist place.

[rorywquin]

Guys

Thanks for the input - my feeling was that the wing could have been higher. As mentioned there are pros and cons. This aircraft is my throw in the boot (trunk in the USA) go & fly Sunday sport plane. It is a lot of fun over a wide speed range and very forgiving after a late Saturday night.