Rotor blade stresses

TJMay

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I found it very interesting as well but not quite sure what it means.

The conclusion simply satisfies his curiosity but what do the findings tell us on a practical level?

Tommy
 

500e

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I found it very interesting as well but not quite sure what it means.

The conclusion simply satisfies his curiosity but what do the findings tell us on a practical level?

Tommy
I would have thought that a stress level x 3 \ 4 would have been food for thought by both pilots & designers.:lalala:

PS. I have an old fashioned approach as well.
 

dinoa

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Jukka,


Elegantly simple and well illustrated analysis. Thanks for sharing.

I have witnessed solid aluminum hub bar fracture first hand. An audible ping in flight immediatly followed by a precautionary landing. It chilled my bones.



Dino
 
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RotoPlane

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I do not like a two-bar hub like the one shown in the "nocone" illustration because the cone bending loads are transferred out to the more flexible blades. A single bar hub with no built-in coning angle, if not overly rigid, will reduce these blade bending moments by bending a bit itself. On the other hand, a bent radius built-in coning angle on this single hub-bar has already exceeded the materials yield point in the bend area and must be compensated for with thicker material.

I prefer the Jukka/Magni head method of transferring cone bending loads from the blades to the head by allowing the blades to be aligned with the coning angle and held there by two horizontal bolts. The more common vertical bolted blades makes this arrangement a bit tricky but not impossible.

The problem with a preset coning angle is that it is only optimum at 1G of a certain weight. My question is; would a flexible material like spring steel (or even fiberglass) fair better as a hub-bar for the more common vertical bolted blades, weight ignored? I think the fatigue cracking problem would show up less on both hub-bars and extruded blades, if hub-bars were more flexible…..although the dumb in-plane vibrations may change…..
 

ckurz7000

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...The problem with a preset coning angle is that it is only optimum at 1G of a certain weight. ...
That's not correct. The coning angle for a given rotor flying in steady state conditions (i.e., turning at a constant rrpm) is independent of weight and g-loading.

If you think about it, it's pretty obvious why this has to be so. Coning is determined by the ratio of two forces: centrifugal and lift force. Both depend on the square of the rotor tip speed. If you increase the weight of the gyro you increase rotor speed, which affects both forces the same. Hence, the coning angle remains unchanged.

This, of course, doesn't hold true for helicopters, they operate at a constant rotor rpm. That's why a load increase will result in a higher coning angle.

-- Chris.
 
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RotoPlane

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What you've said is true Chris….I should have added "during initial cyclic change".
Crank and bank maneuvers will change the G loading and therefore the coning angle, without initially changing the rotor rpm enough to compensate for a consistent coning angle….causing changing rotor bending loads.
 

Mike G

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Jukka
You say
"One interesting result of the calculations is that the blade CG seems to be some centimeters lower with no built in coning angle. This apparently is a major factor in reducing 2-per rev vibrations with this type of a rotor."

I think you're saying that the designer calculated his cone angle simply (as if it was pinned at the hub end and free to rotate giving a simple triangle) and didn't account for the curvature of the blade due to the fact that it is effectively a cantilevered beam. In the attached sketch I've tried to show the cantilevered blade on the left and a simple "pinned" blade on the right.

Why do you think this reduces 2/rev vibration? Is it because a 2 bladed teeter rotor is more sensitive to 2/rev vibration when the undersling is too low (ie rotor blade C of G above the teeter bolt) than when it is too great (ie rotor blade C of G teeter below teeter bolt)?
Or is there another reason?
Mike G
 

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