Thread: Discussion Gyrocopter Aerodynamics
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Old Dec 31, 2006, 08:10 PM
mnowell129 is offline
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Mickey from Orlando. Really.
Joined Nov 2004
4,145 Posts
Do the two step....

How about steering the two rotor gyro layout.
Shown is the typical two rotor gyro. How does it steer.
Lets look closely at one of the tips and the mast attached there. The important point is that the mast is canted inward at the "dihedral" angle of the wing or rotor mount. Note that in the level condition the mast has some right cyclic, but the right tip has left cyclic so they balance out.
Lets now yaw the aircraft with rudder. What happens is that the inward canted left tip rotates rearward and the inward cant becomes backward tilt. Note that we reduce the right cyclic some but to a lesser extent than the inward tilt becomes backward tilt. We then recognize the backward tilt for what it is, up cyclic. This makes this rotor tilt up and now being tilted up further has a greater overall angle of attack and speeds up. This produces more lift on the left tip and rolls the model to the right. Note also that the reverse happens on the right tip. It gets nose down cyclic from the inward tilt swinging forward, the rotor tilts down and slows and this tip falls.
The important points are that the turning mechanism involves using cyclic to create differential rotor speed and angle to make the turn. But speeding up and slowing down the rotor speed takes time and involves inertia. So this kind of model is slow to turn and has a tendancy to overshoot because once the yaw is removed the leading rotor still has extra speed that must be bled off. If you watch the videos of these models you see them "wallow" in the turns and the rotors changing speed with respect to each other, and so this makes sense with how it's turning. Note however that the inward tilt of a rotor becomes helpful with the model is upset and starts to slip. Now the inward tilt becomes backward tilt against the slip and automatically corrects for the disturbance. This makes this layout a good beginner setup as it tends to level itself out and fly like a three channel trainer airplane. The only downside is the "wobbly" kind of control that overshoots, but this is easily overcome by piloting. This explains why these models might fly better with lightweight low energy rotors. These will speed up and slow down faster and provide more positive control. The downside is that light rotors tend to stop easily so you need more negative pitch to keep them spinning. Hence the typical two rotor configuration of dihedraled, lightweight 4 bladed rotors, with fairly substantial negative pitch (-10 or more) seems to make perfect sense in hindsight.
Note that this dihedral is not coning, coning being the shape of a single rotor, not the difference between two rotors.
Note that if you flatten the rotors out then you have two flat rotors that when yawed, don't behave any differently that a single rotor yawed. It might roll the right way, it might not. The trick is that the inward canted rotors works differentially when yawed, thus providing the differential lift needed to roll/turn the aircraft.
You can probably think your way through how coning doesn't improve any of this in any way due to the cross coupled behavior.
Anyhow maybe this will help you understand your dual rotor a little better.
P.S. differential spindle tilt by rotating the "wings" or pivots on the ends provides the same differential cyclic as yawing the aircraft but with more differential throw available. Note that you still have to wait for the rotor to start making more lift due to being pitched up so the control response will still be slower than a single rotor with cyclic pitch directly causing the roll. But you get more control than indirect cyclic/speedup of the rudder controlled dual rotor. The differential cyclic model will likely have adverse yaw issues and need some coordinating rudder. The rudder turned dual will not have such issues since the rudder is already yawing the model the correct direction already.

regards to all.
Happy New Year.
mick
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