Thread: Discussion Gyrocopter Aerodynamics
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Old Dec 29, 2006, 03:45 PM
mnowell129 is offline
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Mickey from Orlando. Really.
Joined Nov 2004
4,145 Posts
TURN the page

Ok so now we get to turning with the rudder (or not). We've already shown how a tilting spindle or swashplate can create cyclic pitch. This in turn tilts the rotor in a new direction. The tilted rotor thrust no longer goes through the CG and the model tilts in the intended direction providing pitch and roll control.
What about trying to turn with rudder.
In the first set of figures we have our happy little fixed mast gyro flying along. We have a side, top and rear view of our gyro, mostly showing our aft tilted main shaft. Now lets apply some right rudder as shown in the second set of figures. The side view looks pretty much the same but hold on. Our rear view looks pretty ugly. What happens is that our aft tilt mast, when yawed to the right, tilts the mast to the left. You can prove this to yourself by sticking a straw in a holiday cupcake at the aft mast angle and rotating your cupcake while viewing from the rear. Sure enough that aft tilt turns into side tilt when yawed. Oops. If you've followed along to this point this looks just like what it is, left cyclic pitch. I've even drawn what happens to the blade in the fore and aft position. This is a problem because this is left cyclic with right rudder.
Lets go further to the last picture. Remember asymmetric velocity. What used to be the peak advancing blade speed at A is now at A'. The rotor tries to flap up at B'. This is good because now we have a little roll component in the direction we want to turn. But wait, there's more. Because you have a fixed mast, you have to have flapping hinges to correct for asymmetric velocity. Guess what else you got. Yes, coning! So now the blades are coned and the blade with the highest angle of attack due to coning is now in the flight direction B'. But we now know that the phase lag of cyclic takes place and the maximum tilt of the rotor due to coning now takes place at C'. Double oops. Now we have left cyclic due to mast tilt, we have left coning induced roll and only a mild amount of asymmetric velocity flap to roll to the right. Which way it will roll is anybody's guess and could just as well be left as right depending on rotor speed, coning angle, flapping hinge stiffness, yada, yada, yada....
But wait. What if our rotor direction was CCW instead of CW as viewed from the top. Now our coning induced roll would peak at A' and add to the roll due to asymmetric velocity. Now we got a chance of turning. But guess what, we are going to roll better to the advancing blade not the retreating blade. This has to be confusing to the uninformed observer. The silly gyro copter will turn quite nicely into the advancing blade against all intuition and won't turn worth a hoot against the supposedly lower lift retreating blade.
Is there any wonder why those floppy hinged, rudder elevator controlled gyros didn't fly worth a D&^^ ?
So does anyone really want to try a rudder only controlled gyrocopter any more?
Here's the good news. If you do the same little mental experiment with elevator you will see that up elevator tilts the shaft back and provides up cyclic and makes the rotor tilt back, just as you desire. So at least the elevator works like we want.

So what's the simplest, highest likelyhood of success single rotor layout? At a minimum you need roll control on a tilting spindle and elevator control. Rudder control doesn't hurt because as we've seen here the gyrocopter does bad things when it skids like it would in an un-coordinated turn. So a little rudder to keep things coordinated wouldn't be a bad thing.

Next up. Is your rotor stable as well as controllable? Turns out your rotor can be too controllable.

anybody still out there or have I lost you all by now?

mickey
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