# Flapping amplitude and variation of AoA

#### XXavier

##### Member
I know that the rotor blades flap, and that the amplitude of that flapping is a function of the rotor angular speed and of the airspeed of the gyro itself. Now –provided I've got these things right– the periodic flapping of a blade, as observed from the rotorhead axis, can be seen, when observed from the real, blade-tip path plane axis, as a periodic variation of the AoA of the same frequency. My question is: in what form are flapping amplitude and AoA variation related? In other words, if I know the amplitude of the flapping at a given moment, how can I calculate the corresponding variation of the AoA?

For two extreme cases, the variation of AoA is clearly zero for a flapping amplitude of 0º and (it seems to me, intuitively…) it may perhaps reach 90º for a flapping amplitude of 90º, but I doubt that there is a linear correlation between these two extremes that would allow me to say, for example, that for a total flapping of 10º, I would get get a total variation of AoA also of 10º…

Any comment will be welcome...

#### C. Beaty

##### Gold Supporter
The cyclic “flapping” angle viewed from the rotorhead axis is identical with the cyclic feathering angle viewed from the rotor plane. JC’s “flapping” plank on a rope illustrates this very nicely; unfortunately, i’ve lost the link to his video.

#### Uncle Willie

##### Member
For a flapping angle of 10° (2-3° Realistically) the Angle of attack would vary Plus and Minus 10° (20° total) at the 3 and 9 o'clock positions and 0° at the 12 and 6 o'clock positions and vary sinusoidally (Sin wave) between the positions.
Assuming your 10° flap example the AOA would be Sin(45) x 10 = 7.1° at the 45° positions and Sin(30) x 10 = 5° at the 30° (1,5,7,11 o'clock) positions

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#### XXavier

##### Member
Uncle Willie;n1137499 said:
For a flapping angle of 10° (2-3° Realistically) the Angle of attack would vary Plus and Minus 10° (20° total) at the 3 and 9 o'clock positions and 0° at the 12 and 6 o'clock positions and vary sinusoidally (Sin wave) between the positions.
Assuming your 10° flap example the AOA would be Sin(45) x 10 = 7.1° at the 45° positions and Sin(30) x 10 = 5° at the 30° (1,5,7,11 o'clock) positions

Thanks... It's now clear: both angles are identical. However, it's the variation of the AoA that is identical to the flapping angle, and not the absolute AoA at a given position, that depends not only on the flapping, since there are other things to consider, as the disk's angle of attack and the original pitch of the blades...

#### XXavier

##### Member
C. Beaty;n1137497 said:
The cyclic “flapping” angle viewed from the rotorhead axis is identical with the cyclic feathering angle viewed from the rotor plane. JC’s “flapping” plank on a rope illustrates this very nicely; unfortunately, i’ve lost the link to his video.
JCs video here... https://youtu.be/LNoue-Q8PjM

Try this one

#### Jean Claude

##### Junior Member
XXavier;n1137500 said:
it's the variation of the AoA that is identical to the flapping angle, and not the absolute AoA at a given position, that depends not only on the flapping, since there are other things to consider, as the disk's angle of attack and the original pitch of the blades...
You are right, Xavier.
Below, the angles of attack calculated for my project: Diameter 6.45 m at different radii and blade positions when the angle of attack of the disc is 10.2 degrees, flapping blade 2.1 degrees, aerodynamic pitch setting to 3.5 degrees, forward speed 25 m/s

Tim's video does not show the flapping angle. Just the angle of the plane of rotation with respect to the horizon.

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#### XXavier

##### Member
Very interesting... Let me suggest, JC, that you should write a book on the physics of gyros for amateur pilots, a subject that no doubt you know quite well, and include in the book the spreadsheet you have created. Many pilots in France that will be interested, since gyros are very popular in your country. So far as I know, only one book has been published covering this matter. (In Germany, where gyro flying is also very popular...). To my knowledge, nothing at all has been published in English...