Drag/clamshell rudders?

[FrankW]

Anyone have any experiance with drag/clamshell rudders, like what are used on the B2 bomber? I'm building a simplified 72" Horton HO 229 from Bell Imel plans and I want to implement these on the airframe for yaw control and airbrake functions. Radio set-up is not a problem, I'm mostly just curious about dimension. What would be a good span for these rudders; 10% of the wing, 15%? I know it won't be scale, but it'll be lighter and easier to build than the scale option.

-Frank

[reylf_gnijieB]

I'm guessing that at model speeds they'll be much less effective.

[FrankW]

Why?

[AndyOne]

Frank,

I put this arrangement on an experimental fun-fly about 15 years ago. The problem I had with it was blow-back because of the servos I used. It did operate but wasn't very effective. I suggest you use the most powerful servos you can and make the rudder area as large as the geometry of the airframe will allow. I guess the reason why this works better on full size is simply down to Reynolds numbers.

Andy

[Rodney]

If you are talking only about dual rudders (one on each outboard tip) then they work quite well if you only use the rudder on the inside turn; i.e. deflect only left rudder outward for a left turn and deflect only the right rudder for a right hand turn. Otherwise the drag of the deflected rudder on the inboard side of a turn is often enough to counteract the desired effect.

[JetPlaneFlyer]

If you are talking only about dual rudders (one on each outboard tip) then they work quite well if you only use the rudder on the inside turn; i.e. deflect only left rudder outward for a left turn and deflect only the right rudder for a right hand turn. Otherwise the drag of the deflected rudder on the inboard side of a turn is often enough to counteract the desired effect.
The Ho229 has no vertical stabilisers at all, it's a pure flying wing... I think the 'rudders' in question are actually split elevons which would act like drag rudders.

I never did quite grasp how the Horten wings were stable in yaw without vertical surfaces. I think in truth many (all?) Hortens, including the 229, had very marginal directional stability which would explain why almost all other wing designs had some sort of fin, at least up to the point where computer controlled ‘fly by wire’ stability enhancement was invented.

Steve

[FrankW]

The Horton 229 used dedicated rudder slats on the top and bottom of each wing tip that would open and close. I would prefer to use drag rudders similar to what is seen on the B2 and Nothrop N9M.

Using torquey servos is no problem. I was also thinking about using digital programable servos with a gyro on each. The programable servos then would be nessesary to limit the travel to only half rotation.

The rudder need not be excessively effective, I'm not planning on doing knife-edges with this plane, but I would like to be able to land in cross-winds and use the airbrakes to steepen the langing approach.

-Frank

[Brandano]

The Horten wings had a certain directional instability, however, even if only marginally they were directionally stable, thanks to the wing sweep. If the plane was jawed to the left, the right wing would present a bigger cross section to the air than the left, and the differential drag would restore the heading. Further, if the wing had an angle relative to the air the wing outside the slip would have more lift and tend to roll the plane into the slip, therefore automatically synchronizing the turns. Training the gun on a target when the plane snakes left and right is a hard job, which made them unsuitable for the fighter role. I don't see why split elevons should be less effective at smaller sizes, other than having to operate in a different Reynold range. I can see how they would be hard to implement to work with weaker servos, though. I'd use normal elevons on the outer portions of the wing and split drag rudders aerodynamically balanced a little inboard of these.

[JetPlaneFlyer]

The Horten wings had a certain directional instability, however, even if only marginally they were directionally stable, thanks to the wing sweep. If the plane was jawed to the left, the right wing would present a bigger cross section to the air than the left, and the differential drag would restore the heading. Further, if the wing had an angle relative to the air the wing outside the slip would have more lift and tend to roll the plane into the slip, therefore automatically synchronizing the turns.

Yeah, I'd thought about the 'leading' wing in a yaw presenting more drag and I can see how that would generate some directional stability... However I'd imagine that the dihedral effect of wing sweep that you describe, coupled with the relatively weak directional stability, would tend to produce a dutch rolling/snaking motion... From what I've read (including your own comments) this is exactly how the horton wings did behave.

Steve

[raptor22]

There's a lot more to it than just adding sweep, although it is possible to fly an airplane (at least a model anyways) with nothing but sweep for direction stability.

They had a special sweep/taper/washout setup that created a "bell shaped" lift distribution instead of the more traditional elliptical one. The result was that the wingtips produced little lift, sometimes even negative lift. Additionally, at positive lift coefficients they had a forward trust vector (like winglets).

The result was that the addition of aileron actually had some proverse yaw. And...here's the cool part...the forward thrust vector would actually increase when the sweep was increased, so a trailing wing would be pushed forward to align the plane.

--Alex