ferranrosello
Member
- Joined
- Nov 14, 2005
- Messages
- 398
- Location
- Madrid
- Aircraft
- Ela 07
- Total Flight Time
- FW: 600, HELOS: 3550, GYROS: 3020
Right now, and as I had promised in Understanding rotor rpm thread, and because of some disagreements between some gyro pilots and other people posting in that thread I will try to explain the mechanics of autorotation in the simplest way… Of course, this does not mean that I’m going to succeed.
Initially we are not going to enter in the complexities of the real aerodynamics acting on the blade. We are going to replace them by single forces showing the final results we can see on rotor rpm.
Instead of posting everything in a long and complicated text, I will divide the explanation in smaller steps, easier to follow. It is absolutely necessary to understand and agree every step to be able to understand and accept the new ones. Because I have not a lot of time available for posting, it will take me some days to put all the information on the thread. Of course, if you find that something is wrong, or that something is not understandable, please, tell it to me. Learning by the discussions is one of the qualities of this forum…
1st Step. The driving force.
We are going to start by the results. For a specific gyrocopter, (fixed weight and flying in a fixed air density) sometimes the rotor spins faster, and other times spins slower.
Without discussing the reason for this behavior, it is obvious that something is driving the rotor blades. And because of its rotation, the rotor has to overcome a resistance force, a drag. This drag will be bigger when the rotor is spinning faster and smaller when the rotor is spinning slower. If the rotor is stopped there is not drag to overcome.
The drag that the blade has to overcome when it is rotating is a force. And the only way to overcome a force is with another force. Then, just because we can see that a gyro rotor spins, there has to be a driving force acting on the blade, that overcome the drag force. We are going to call it “Driving Force”.
It is obvious that in the prerotation this force is produced by a mechanical device. But once the gyro starts flying this is not the case anymore. This driving force will have an exact magnitude to overcome the drag. If the driving force is bigger, then the rotor blade will accelerate its spinning speed. This will produce an increase in the drag. The rotor will reach a new balance in which driving force = drag at a faster rotor rpm.
If the driving force is reduced the rotor rpm will decay until reaching a new balanced state.
Then, the first agreement we need to reach is that it is driving force acting on the blade what keeps the rotor spinning, and that rotor rpm are totally linked to the value of this force.
Is there any disagreement with that?
Ferran
Initially we are not going to enter in the complexities of the real aerodynamics acting on the blade. We are going to replace them by single forces showing the final results we can see on rotor rpm.
Instead of posting everything in a long and complicated text, I will divide the explanation in smaller steps, easier to follow. It is absolutely necessary to understand and agree every step to be able to understand and accept the new ones. Because I have not a lot of time available for posting, it will take me some days to put all the information on the thread. Of course, if you find that something is wrong, or that something is not understandable, please, tell it to me. Learning by the discussions is one of the qualities of this forum…
1st Step. The driving force.
We are going to start by the results. For a specific gyrocopter, (fixed weight and flying in a fixed air density) sometimes the rotor spins faster, and other times spins slower.
Without discussing the reason for this behavior, it is obvious that something is driving the rotor blades. And because of its rotation, the rotor has to overcome a resistance force, a drag. This drag will be bigger when the rotor is spinning faster and smaller when the rotor is spinning slower. If the rotor is stopped there is not drag to overcome.
The drag that the blade has to overcome when it is rotating is a force. And the only way to overcome a force is with another force. Then, just because we can see that a gyro rotor spins, there has to be a driving force acting on the blade, that overcome the drag force. We are going to call it “Driving Force”.
It is obvious that in the prerotation this force is produced by a mechanical device. But once the gyro starts flying this is not the case anymore. This driving force will have an exact magnitude to overcome the drag. If the driving force is bigger, then the rotor blade will accelerate its spinning speed. This will produce an increase in the drag. The rotor will reach a new balance in which driving force = drag at a faster rotor rpm.
If the driving force is reduced the rotor rpm will decay until reaching a new balanced state.
Then, the first agreement we need to reach is that it is driving force acting on the blade what keeps the rotor spinning, and that rotor rpm are totally linked to the value of this force.
Is there any disagreement with that?
Ferran