down under
Newbie
- Joined
- May 8, 2009
- Messages
- 281
- Location
- Charleville Australia
- Aircraft
- gyro single seat
- Total Flight Time
- 5300
I am interested in building a four blade teetering rotor and would like to find out more about the resonant frequency of rotor blades.
It is this post from C. Beaty that has me thinking about it.
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If you take a stick of 1/8” welding rod, grasp it between thumb and forefinger ¼ of its length from one end and give it a thump, it will vibrate about as shown in the sketch. This is the in-plane free resonance that causes most of the 2/rev problems with seesaw rotors. There’s a periodic excitation from the airstream in forward flight; broadside, the rotor gets a stronger hit than endwise.
The cure, as most people know, is to make the rotor as stiff in-plane as possible and hang it on a soft mast. This way, the natural in-plane resonance is higher in frequency than the aerodynamic excitation.
A mass attached to the center will inevitably lower the in-plane resonant frequency into the range where it can be excited by the aerodynamic input.
Whether the mass attached to the center is round, square or a rod skewed at a 60º angle is irrelevant.
A rigid rotor pylon is just as bad. The magic rubber bushing on an RAF-2000 does nothing for stability but is a good solution to the 2/rev problem.
_______________________________________________________
I have tried the test as described and the resonance occurred. Next I held the welding rod in the centre and struck it in the same spot. There was a lower frequency and I could feel the vibration between finger and thumb.
I then tried holding it in the centre, but twirling it between two objects so that both ends struck at the same time. If one end struck slightly before the other there would be resonance but when both ends struck at the same time there was no resonance.
From my experience two per rev shake is worse on high coning rotors, but with fine adjustments to teeter bolt height a point can be found where there is no shake in straight and level flight. Exact teeter height does not seem critical on rotors with a low cone angle.
At the moment I have two thoughts on the issue of resonance in teetering rotors.
1. Resonance may be an issue on rotors that are flexible in plane and operating with high blade loading, but not a problem with rotors that are stiff in plane and operating at low blade loading.
2. Because the was no resonance when both ends of the welding rod were struck at the same time, and what I have found when eliminating two per rev shake, there is no problem with resonance in a teetering rotor.
It is this post from C. Beaty that has me thinking about it.
________________________________________
If you take a stick of 1/8” welding rod, grasp it between thumb and forefinger ¼ of its length from one end and give it a thump, it will vibrate about as shown in the sketch. This is the in-plane free resonance that causes most of the 2/rev problems with seesaw rotors. There’s a periodic excitation from the airstream in forward flight; broadside, the rotor gets a stronger hit than endwise.
The cure, as most people know, is to make the rotor as stiff in-plane as possible and hang it on a soft mast. This way, the natural in-plane resonance is higher in frequency than the aerodynamic excitation.
A mass attached to the center will inevitably lower the in-plane resonant frequency into the range where it can be excited by the aerodynamic input.
Whether the mass attached to the center is round, square or a rod skewed at a 60º angle is irrelevant.
A rigid rotor pylon is just as bad. The magic rubber bushing on an RAF-2000 does nothing for stability but is a good solution to the 2/rev problem.
_______________________________________________________
I have tried the test as described and the resonance occurred. Next I held the welding rod in the centre and struck it in the same spot. There was a lower frequency and I could feel the vibration between finger and thumb.
I then tried holding it in the centre, but twirling it between two objects so that both ends struck at the same time. If one end struck slightly before the other there would be resonance but when both ends struck at the same time there was no resonance.
From my experience two per rev shake is worse on high coning rotors, but with fine adjustments to teeter bolt height a point can be found where there is no shake in straight and level flight. Exact teeter height does not seem critical on rotors with a low cone angle.
At the moment I have two thoughts on the issue of resonance in teetering rotors.
1. Resonance may be an issue on rotors that are flexible in plane and operating with high blade loading, but not a problem with rotors that are stiff in plane and operating at low blade loading.
2. Because the was no resonance when both ends of the welding rod were struck at the same time, and what I have found when eliminating two per rev shake, there is no problem with resonance in a teetering rotor.