"canard" rudder system (OK, dactyl head!) Help??!

[CactusJackSlade]

What are the pitfalls of using a "canard" rudder - a rudder up front instead of trailing? I guess the Wright Bro's used one didn't they.. or was that for elevator?

Anyway, some of you have checked out my bird shaped flyer and I am thinking about moving the rudder from the rear to the front... and while I'm at it making the rudder look like a pterodactyl's head.

The reason for the switch is the CG. I have too much weight up front - like 7 ounces on this fuse' length/design. I was thinking about moving the rudder up front and using a pull/pull set-up with rigid CF rods on the servo for maximum control.

I would think that the biggest challenge would be the rigidity of the whole rudder system. This will not be a high speed flyer, but a slow thermal cruiser... and of course the shape is fun!

Now if I could pull this off I'd have one cool looking bird!... er, a reptile??!!

Any suggestions? What problems will I face?

CJS

[averen]

The only thing that I can think of that you might have problems with will be the rudder authority. Since the nose of your plane is probably only about 33% or so in front of you CG you won't have as much rudder authority with the same sized rudder as you would on the back. You can counter act this by going to a larger rudder but this will induce more drag.

If it's possible you might want to consider moving the wing around to get some of the weight out of the front and keep the rudder in the rear.

Good Luck!
Averen

[mattg]

I think there will be stability problems with this setup.

Imagine what will happen as the plane starts to yaw a little. The forces acting on the "head" will cause it to yaw more, exactly the opposite of a normal fin. A snowball effect will cause the plane to enter a spin. Depending on how much area you have I wouldn't expect the plane to fly at all, it will constantly want to swap ends.

Having said that I remember a doco where some archaeologists built an RC pterodactyl and I think it had a moving head where the beak acted as a rudder.

Maybe an all-moving surface with a gyro would work.

To be honest I didn't think your wing would fly very well in the first place so you've already proven me wrong, double or nothing?

[CactusJackSlade]

mattg,

Hum, yes I see what you mean about the "snowball effect" you described.... I had not thought about that... good point.

Don't think I will take that bet!

Yes, I could see that, in straight flight it could possibly fly OK, but as soon as yaw is induced, it would undoubtedly want to keep on going just like you said... maybe I'd need a computerized fly-by-wire set-up like on the Stealth Fighter to keep it from going end over end... (ie: gyro like you suggested?)

Maybe you just saved me from a lot of experimental work for nothing??! :)


averen - Good points too - thanks

Maybe I should leave well enough alone??

Thanks!

Any other opinions???

CJS

[Ollie]

Aerovironment (Paul Mcready) built a working, flapping wings and all, pterodactile model a number of years ago. It used the head and bill as yaw control but since the arrangement was statically unstable they used a gyro for active yaw stabilization.The aerodynamic center has to be behind the CG in yaw as well as in pitch for static stability.

[Sparky Paul]

Feathers are at the -rear- of the arrow! :)

[Quacker]

A cheap gyro may get you out of enough trouble to get it working. It worked for Aerovironment.

[uscra112]

In England they fly free-flight gliders with some sort of magnetic gizmo that moves a canard rudder. Was written up in a magazine a few years ago.

[CactusJackSlade]

Interesting ideas... I guess I haven't given up the idea yet! Maybe just a gyro...

Although I could replace that 6 oz. in the nose with my nice digital camera... I did strat this project as a photo/video platform anyway! :D

Keep the ideas coming!

CJS

[Pteradactyl]

Check this site out, Jack, it might help . John


http://www.randrmodelaircraft.com/pterotec.htm

[DeuceTrinal]

What about a small control surface/head up front, and also a trailing surface - legs!

That would let you have control, with aerodynamic centering as well.

[Sparky Paul]

The only real requirement is there be more lateral surface area behind the c.g. than ahead of it.
That's how spears and finless missiles fly.
Otherwise the tendency to swap ends will be a serious problems.

[Ollie]

The stabilizing (behind the CG) or destabilizing (ahead of the CG) effect of an area is equal to the area times its distance from the CG. For over all stability the stabilizing effects of areas behind the CG has to be greater than the destabilizing effects of areas ahead of the CG.

The model has to be stable in both the vertical and the horizontal planes or it will tumble in flight.

[CactusJackSlade]

Pteradactyl,

Good site! Cool looking... animals!?

Wnder how they fly? Wishe there were more photos.

CJS

[uscra112]

But Grumman built that swept-forward-wing thingy that was dynamically unstable and controlled it with computers and servos.

[Daemon]

A forward sweep doesn't make it inherently unstable. In fact, quite the opposite. The placement of the CG relative to the center of lift and the placement of the stabilizers are what matter. Military aircraft of late have been pushing the limits of stability and relying more and more on computer control, because a less stable aircraft is a much more maneuverable one. While not necessarily for performance reasons, the F117 stealth fighter for one is so unstable, it wouldn't know which end to point into the wind if it didn't have computers between the stick and control surfaces. The F22 and others of its generation rely much more heavily on computers than the older fighters.

BTW, the Wright Flyer I is pretty much a flying wing with a forward elevator rather than a canard design as we know it today. With a canard the CG is far enough forward that the canard carries some of the weight of the glider and being smaller it flies at a higher angle of attack which is what gives it its stall resistance (canard stalls before main wing). The elevator surfaces on the Wright Flier are full flying but carry very little of the load. With the reproduction they're flying this month, they've said it takes lightning reflexes to stay on top of it. One of hardest aircraft to fly every made. My suspicion is that the most stable flight is acheived by literally shifting your body weight forward a little on the wing to shift the CG, and then having to apply more backpressure on the elevator control. It's gotta be absurdly sensitive to changes in airspeed.

Anyway, back on topic, I believe a gyro and reasonably quick servo could keep up with a forward rudder on this glider fairly easy, especially if you didn't make the rudder area overly large. The flying wing has some inherent yaw stability, so as long as you don't overwhelm that you should be ok. The main reason I wouldn't actually do it, is that you'd basically be landing on your vertical stab all the time. Consider what happens every time you drop a conventional glider on its tail. *crunch*

ian