Blade Cracking Discussion from RotorCraft Conference

There appears to be a pattern of RAF blades developing cracks at the trailing edges, near the root ends, usually on the top surfaces. I have them, as do many others. I started thinking about what "Mark the Balancer" and I were discussing last week while messing with the undersling. This is how I recall the conversation. Mark said that when designing a rotor system, the coning angle of the blades AND the hub-bar have to be configured and engineered so they are alike under most load conditions.

He went on to say that if this is done correctly, it is a simple matter of determining the undersling. You find the center of gravity of the 2 blades, draw a line between the centers, and this is where the...here's where I get rusty...the teeter bolt or maybe some other component of the teeter block is placed. I'm pretty sure though that it's the teeter bolt. I'm thinking that the vertical size of the teeter block would then automatically be determined. It would be long enough to maintain the correct coning angle of the bar and blades.......or something like that. You engineer types will maybe clarify this for us.

We guessed that this procedure was probably not done by RAF when initially designing the rotor system and that the coning angle of the blades and the coning angle of the hub-bar are not in sync and the coning angle of the hub-bar was most likely just a guess or a result of "copy-cat engineering."

Here's why I belive this to be true. If the coning angles are not in sync, there would be stress placed where the blades solidly connect to the straps, at the root ends. The stress would be greatest, or rather would manifest at the weakest point, which would be the trailing edges, where the blades are thinnest. This is my theory as to why the RAF blades are developing cracks at the trailing edges of the root ends.

If this mismatch was corrected, it would also be possible to reduce the 8 IPS lateral 2-pers, that are caused by improper undersling, down to a reasonable level. This can't be done with the current setup, because the 2 coning angles are incompatible and there is no way to adjust this, except by redesigning the hub-bar to get its coning angle in sync with the coning angle of the blades in flight.

I'll add one more idea. The cracks seem to be appearing on the tops of the blades more often than the bottoms, so my additional theory is that there is too much coning angle in the hub-bar, so the coning angle is less in the blades during flight, thus exerting a downward pressure on the blades at the root, where they are locked into the hub-bar, and therefore the cracks are predominant on the tops of the blades. It also could be oppositely true. I don't know if a bending motion would develop cracks on the surface being stretched or the surface being compressed by the bending motions. I'm sure someone here knows for sure.

Anyway, those are my theories as to why we're all developing cracks in our blades. Comments are as always, most welcome....as long as they agree with my thinking. ha-ha.

Ken J. - Sandy Eggo

...is explained by Martin Hollmann in an article in April 2002 Custom Planes.

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Martin outlines several principals of flight that are different in gyros vs. helicopters, however he did not address the vertical thrust line offset.

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Chuck E....R.M.E.C..