Lets talk some aero...stab location

[GremlinX]

Pardon my lack of knowledge on this subject. What makes an ideal knife edge and neutral flying aircraft? What stab/wing/thrust line's would you look for? Thanks.

[Vic3D]

Well, the way I see it, the closer the thrust, wing, and stab location are in the same line, the less coupling it will have.

But wing/stab location is not the only thing that determines coupling in KE. Wing dihedral also affects the coupling tendencies...

I'm not that big of a help, I just know that much...

Vic

[GremlinX]

Lets say there is 0 dihedral...

[Vic3D]

	Quote: Originally Posted by GremlinX
	Lets say there is 0 dihedral...

Well you got me there!

There's a few aerodynamic experts out there that could probably explain...

[GremlinX]

I keep hearing folks talk about " oh, I love that high stab, shouldnt have any coupling at all". I'd just like to hear it from some experts... I know there are some here.

[Matchless]

In a purely theoretical world, you would want everything in line. IE, thrust and all forces on the airframe would be in line and in balance.

[Shawn Berkheimer]

Say we are talking 40%. I would want the engine thrust line and stab to be on the same line and then place the wing 2.5" below that.The wing top would be flat. This is what I did on a 40% Extra 300 that I designed although it will most likley never be completed.


Shawn

[Baron]

Aside from those mentioned already (wing & stab vertical placement, dihedral), coupling also depends upon cross-sectional shape of the fuselage, fin & rudder geometries, rudder/elevator proximity, vertical and longitudinal CG location, etc.

Then if you could (remarkably) balance all these effects throught the range of sideslip angles while maintaining a CG suitable for the type of flying desired, you still have asymmetric prop forces that cause both pitch and yaw moments due to sideslip

However, generally speaking for most aerobatic aircraft - holding all other variables constant - a lower wing results in adverse roll (opposite direction of rudder application), higher wing results in proverse roll (same direction of rudder application), lower stab results in positive pitch (nose-up), and higher stab results in negative pitch (nose-down).

George Hicks wrote a very interesting piece on his research and thoughts regarding coupling, I think it's on RCU. It's a .txt file attached to a post in the aerodynamics section... very highly recommended reading on a very complex subject matter.

-Baron

[p sme]

also the size of a couterbalance can have a drastic effect on rudder to aileron coupling...

[3dubya]

	Quote: Originally Posted by GremlinX
	I keep hearing folks talk about " oh, I love that high stab, shouldnt have any coupling at all". I'd just like to hear it from some experts... I know there are some here.

High stab to the wing is a disaster for the way i trim. If the stab height is any higher than the top of the root rib of the wing if to each other its bad if you trim vertically. The QQ yaks are perfect example of what trims out good if not a shoulder wing model. I do know this, the higher the stab to the wing the more positive incidence (or down trim) to the wing it needs to dive straight down idling with the fuse apart of the test as well.

[JRO]

Wing AC to Stab AC, and Wing AC to Vert Fin AC on the same point and Wing and Stab on the Thrustline.

Jason

[MODE 1]

	Quote: Originally Posted by GremlinX
	Lets say there is 0 dihedral...

How can there be zero dihedral unless its a flat slab wing like a foamy. Even an Extra , Edge has a taper to its bottom and in some cases top so it has dihedral and anhedral built in. LOL and the spell checker doesn't recognize that last term.

[Breakstuff5050]

I've thought alot about this. I think the rudder shape is important. Im no expert by no means but can anyone confirm that the amount of surface area on each side of the thrust line should be equal on the rudder surface? I would think that this would reduce roll tendencies when rudder is applied.

[2walla]

	Quote: Originally Posted by Breakstuff5050
	I've thought alot about this. I think the rudder shape is important. Im no expert by no means but can anyone confirm that the amount of surface area on each side of the thrust line should be equal on the rudder surface? I would think that this would reduce roll tendencies when rudder is applied.

Yes that would be theoretically correct, however, the forces generated by the rudder are not equal across its area. The area out on the tip in clean air is more efficient than that by the fuse so you don't necessarily need equal areas above and below the line.

[sukhoi26mx]

	Quote: Originally Posted by GremlinX
	Lets say there is 0 dihedral...

Then there will still be effective dihedral of wing taper, stab taper, wing height, and stab height. In fact, you don't want zero dihedral, you want the correct total effective dihedral matched to the side area distribution of the airframe. This will give minimal roll coupling. Generally, a mid-low wing with slight to moderate taper of both the wing and stab, with a stab position slightly above the wing works best. This is why the Extras are so popular.

As far as pitch coupling, low wing/high stab worsens pitch to gear, whereas high wing/low stab causes slight pitch to canopy. CG plays a HUGE part in pitch coupling, but I find that again, a mid/low wing, mid/high stab and a slightly forward CG works best for most sorta scale aerobatic side area distributed models.

Scott