Luc De Keyser
03-22-2004, 12:07 PM
In the other forum Chuck Beaty admitted he did not know how Cartercopter would handle blade flutter in mu > 1 conditions. The Cartercopter website now features a FAQ and the latest is that issue. Too bad we have not seen Chuck on this forum yet for a second opinion.
Luc
copied from Cartercopter's website:
How do you handle the problem of blade flutter/divergence on the retreating blade?
There is an instability on the retreating rotor blade caused by reverse flow shifting its aerodynamic center from the ¼ chord to the ¾ chord. The design of our rotor, which is very torsionally stiff, has the ability to very rigidly carry torsional loads across the blade hub from one blade to the other, which means that the stability of each blade is coupled to the other blade. We developed a simple equation to predict the stability of our rotor, and have verified the equation through flight testing. We designed our rotor blade to be inherently stable up to high mu ratios (mu ~1.4) by the planform and weight distribution of the rotor blade. Near the tip of each blade, there is a leading edge extension that has a lead weight, which shifts the blade CG forward. Also at the tip, there is a trailing edge extension, which shifts the AC aft. While this combination of CG & AC shift makes the advancing blade very stable, there is a decrease in stability in the retreating blade. However because the velocity is so much lower at the tip of the retreating blade, its instability is less that the advancing blades increased stability, and since the blades are torsionally tied together, the net result is a stable rotor. At a mu of 1, the velocity at the tip of the retreating blade is zero, which results in a very small unstable pitching moment. But at some increased mu ratio the reverse flow velocity over the retreating blade will cause that blade to become more unstable than the advancing blade becomes stable and the rotor will become unstable. At this point the cyclic control will need to be very stiff and will require boosted controls. Any cyclic movement due to the rotor trying to diverge will cause one blade to increase pitch while decreasing the pitch of the other blade and this will cause the blades to go out of track. At this point the lift on the rotor is small and if the amount the blades are out of track is small due to a sufficiently rigid cyclic control, the 1 per rev vertical lift oscillation may not be noticeable.
During flight testing, we have experienced this rotor blade instability. It manifests itself in the rotor going “out of track.” One blade will be at a higher pitch than the other. As the rotor turns, the blade with the higher pitch will run higher than the other blade. The pilots will see the one blade running higher than the other, and can slowly increase speed and note how the instability increases, or slow down and note the blades going back in track. It is not a sudden, disastrous divergence.
Luc
copied from Cartercopter's website:
How do you handle the problem of blade flutter/divergence on the retreating blade?
There is an instability on the retreating rotor blade caused by reverse flow shifting its aerodynamic center from the ¼ chord to the ¾ chord. The design of our rotor, which is very torsionally stiff, has the ability to very rigidly carry torsional loads across the blade hub from one blade to the other, which means that the stability of each blade is coupled to the other blade. We developed a simple equation to predict the stability of our rotor, and have verified the equation through flight testing. We designed our rotor blade to be inherently stable up to high mu ratios (mu ~1.4) by the planform and weight distribution of the rotor blade. Near the tip of each blade, there is a leading edge extension that has a lead weight, which shifts the blade CG forward. Also at the tip, there is a trailing edge extension, which shifts the AC aft. While this combination of CG & AC shift makes the advancing blade very stable, there is a decrease in stability in the retreating blade. However because the velocity is so much lower at the tip of the retreating blade, its instability is less that the advancing blades increased stability, and since the blades are torsionally tied together, the net result is a stable rotor. At a mu of 1, the velocity at the tip of the retreating blade is zero, which results in a very small unstable pitching moment. But at some increased mu ratio the reverse flow velocity over the retreating blade will cause that blade to become more unstable than the advancing blade becomes stable and the rotor will become unstable. At this point the cyclic control will need to be very stiff and will require boosted controls. Any cyclic movement due to the rotor trying to diverge will cause one blade to increase pitch while decreasing the pitch of the other blade and this will cause the blades to go out of track. At this point the lift on the rotor is small and if the amount the blades are out of track is small due to a sufficiently rigid cyclic control, the 1 per rev vertical lift oscillation may not be noticeable.
During flight testing, we have experienced this rotor blade instability. It manifests itself in the rotor going “out of track.” One blade will be at a higher pitch than the other. As the rotor turns, the blade with the higher pitch will run higher than the other blade. The pilots will see the one blade running higher than the other, and can slowly increase speed and note how the instability increases, or slow down and note the blades going back in track. It is not a sudden, disastrous divergence.