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Question about underslung rotor systems


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To deal with Coriolis Effect I know that 2 bladed helicopters use an underslung rotor system. I'm confused about how it actually compensates however. According to RFH and another book I have, it says that one blade moves in and the other blade moves out to preserve center of mass (and the illustration presents the rotor system tilting to one side). In a no wind situation I fail to see why one blade would move out and the other would move in when both of the blades are responding to the center of mass moving inwards and therefore speeding up. In addition, the illustration adds further to the confusion by showing a tilted rotor disc. I can't see why the rotor disc would tilt one way when both blades are encountering the same forces (no wind hover).

 

It makes more sense to me to think that as the rotor disc cones up and center of mass moves inward, the underslung blades slide outward (both of them equally) to restore a balanced center of mass.

Edited by Mountaingoat
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To deal with Coriolis Effect I know that 2 bladed helicopters use an underslung rotor system. I'm confused about how it actually compensates however. According to RFH and another book I have, it says that one blade moves in and the other blade moves out to preserve center of mass (and the illustration presents the rotor system tilting to one side). In a no wind situation I fail to see why one blade would move out and the other would move in when both of the blades are responding to the center of mass moving inwards and therefore speeding up. In addition, the illustration adds further to the confusion by showing a tilted rotor disc. I can't see why the rotor disc would tilt one way when both blades are encountering the same forces (no wind hover).

 

It makes more sense to me to think that as the rotor disc cones up and center of mass moves inward, the underslung blades slide outward (both of them equally) to restore a balanced center of mass.

 

Coriolis Effect is irrelevant in a no-wind hover. It has its effect noticed in moving flight when the blades are flapping due to dyssymmetry of lift.

 

Consider the non-underslung blade. As a blade flaps up, the center of mass of that blade not only moves upward, but also moves inward toward the center of rotation. (This is because the blade's flapping movement is hinged at the hub and the tip of the blade as it moves up and down moves in an arc.)

 

In an underslung system, as the blade flaps up the center of mass of that blade only moves up because of the mechanics of the "undersling offset". This "offset" effectively moves the blade outward as it flaps up, negating the arc movement. Therefore, since the center of mass of a blade never gets any closer to the center of rotation, coriolis effect is non-existent.

 

Even with an underslung rotor system, a blade could flap up so much that the center of mass will start to move closer to the center of rotation; however, you better hope you're not in the helicopter if that happens.

 

~Jeff

Edited by Jeff
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Therefore, since the center of mass of a blade never gets any closer to the center of rotation, coriolis effect is non-existent.

 

Even with an underslung rotor system, a blade could flap up so much that the center of mass will start to move closer to the center of rotation; however, you better hope you're not in the helicopter if that happens.

 

I was under the impression that the underslung system took care of most of the coriolis effect, but even during normal operations there was some movement of the center of mass. I thought that there was always a little coriolis effect still there, and those remaining stresses were then absorbed by the blade.

 

Obviously the blades on the "semi rigid system" wouldn't deal with the amount of stress that blades of a "rigid rotor system" would, but there was some absorbtion taking place due to coriolis effect.

 

Am I off the mark with my train of thought here?

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I was under the impression that the underslung system took care of most of the coriolis effect, but even during normal operations there was some movement of the center of mass. I thought that there was always a little coriolis effect still there, and those remaining stresses were then absorbed by the blade.

 

Obviously the blades on the "semi rigid system" wouldn't deal with the amount of stress that blades of a "rigid rotor system" would, but there was some absorbtion taking place due to coriolis effect.

 

Am I off the mark with my train of thought here?

 

 

No you're right on there. There is some but the underslung gets rid of most of the need to asorb any lead/lag. Some of that force is asorbed being part of the reason that semi-rigid blades need to be structurally strong. Example: the aluminum honeycomb inside an R22 blade.

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So is there any mechanical movement of the rotor hub to compensate for coriolis effect on underslung setups in a no wind hover? Its a physical phenomenon of the rotor blades being underslung? Coriolis effect does or does not cause the upward bending blades to accelerate in a no wind hover (center of mass remains relatively the same)?

 

Thanks for the replies.

Edited by Mountaingoat
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So is there any mechanical movement of the rotor hub to compensate for coriolis effect on underslung setups in a no wind hover? Its a physical phenomenon of the rotor blades being underslung? Coriolis effect does or does not cause the upward bending blades to accelerate in a no wind hover (center of mass remains relatively the same)?

 

Thanks for the replies.

 

Coriolis effect is not an issue in a no-wind hover. The blades move upward due to coning; the angle is the same all the way around the rotation. As you lift off into a no-wind hover, coriolis effect will come into play as the blades start to cone, but the entire rotor system will tend to speed up. This increase in RRPM is handled with the throttle. This is true whether the blades are underslung or not, but obviously, the center of mass will have negligible movement in an underslung system.

 

Rotor hubs are not typically designed to handle lead-lag forces of coriolis effect. The blades absorb these forces or there is a drag hinge with a damper. I can't give you the engineer's explanation for it, but that's what you need to know as a pilot.

 

~Jeff

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