Well Sir, I finally got a few seconds to do these FEA's on your hub.
I installed some 'ball bearings' of sorts and then a shaft/stub
where the
blades would sit. I loaded this stub two ways. The first was radially
out from the center w/ a 16,000 lb load. And then on a flat face which
was 24" from center I put 325 lb's force perp to the plane of rotation.
I put symmetrical loads on either end of the hub and then used Larry's
6061-T6 Alum material properties to test it.. 32seconds later it
gave me these
I think that the area where it neck's down is really the only area
which might need a little more material.. the rest looks like it could
fly. If it was made from Alum.. that is..
Larry also gave me some material data on composites so I did these
w/ Eglass. Now the data did not provide a yield number for the material
so it doesn't show whether it's exceeded it , but hopefully these
will be
useful to you as well..
Anyway if you want me to do any more just fire away.. it takes
longer to copy and
paste the screen shots than it does to do the FEA part of the job..
Jarrett
Thanks Jarrett. You do great work.
Looks like it is over designed just about everywhere except the necked
down area. Time to save some weight :) I'll go with the e-glass results.
--
Paul Lamar ...No rotor no motor.
I think the yield strength for e-glass structures is rarely a concern -
it's so flexible that by the time you reach yield, the structure has
flexed so much that other things start going wrong. Certainly e-glass
can be used in a particular structure where the yield strength matters,
but it may be more relevant to design for stiffness..
Regards,
Matt-
I would agree in most cases. Helicopter rotor heads are unique situations
where some flexibility is required to deal with the gyroscopic forces.
You have this big gyro up there and every time you try to pitch
or yaw huge bending forces are introduced into the shaft. Also as the
blade moves forward on one side the lift is greatly increased while
the blade moving aft the lift is greatly decreased. This too causes
bending moments on the blade. Mitigating all this is the centripetal
forces on the blade due to their own weight which in effect tends
to keep them flat while still supporting the weight of the craft.
Without this effect they are no where near strong enough to support
the weight of the helicopter.
Consequently most helicopters have what amounts to a U-joint between
the blade and the shaft. This minimizes the bending loads on the shaft.
The new thing is a semi rigid rotor head made from glass composite
that provides the flexibility required while simplifying the rotor head.
Glass fiber does not fatigue.
http://www.b-domke.de/AviationImages/Rotorhead.html
Paul Lamar ...No rotor no motor.
Hi Paul;
Glass fiber DOES fatigue. Ultimate allowables are relatively low.
I have no references handy - M.Hollmann references some other works in
some of his books.
Cheers
Cary
Maybe it is relative to the way it is made.
http://infoserve.library.sandia.gov/sand_doc/1992/927005.pdf
"Fatigue of Fiberglass Wind Turbine Blade Materials"
When you get this much scatter you begin to question the quality
control and how the sample was designed and made. The study was
done in 1992 of a bunch of tech papers dating back to who knows
when. If it were made from aluminum it is guaranteed to fatigue
sooner or later regardless of the allowable. Some of these fiberglass
samples failed after 34 million cycles at 20,000 psi to 2000 psi cycle.
Never the less they are still making wind turbine blades out of composites.
It is really a question of fatigue life to required weight
ratio for a part that does the job. My money is on fiberglass.
The fatigue limit life of 4130 is about 15,000 psi. at over 10,000,000 cycles
as I recall. In other words the part will last forever if the stress is below
that level. The kicker is the density of fiber glass is about .065 pounds per
cubic inch while that of steel is .3 pounds per cubic inch. In other words the
fiberglass part is very roughly one fifth the weight for the same stress level
as a crude guess. The big difference is the modulus of elasticity (springiness)
of FRP is one third that of aluminum and one tenth that of steel so
the part will deflect greatly which is what we are looking for here
to absorb the shock. We built a race car chassis way back in the 1960's out
of FRP and crashed it at Mosport in a big way. Ripped off all four
suspension corners as I recall and broke the drivers arm. Cart wheeled
and/or end over end. We repaired it with new corners and it won the
Nurburgring the following year. Ten layers of 10 oz vac bagged and autoclaved.
The Chevy Corvette has used epoxy glass springs for many years. Many Long EZ are
flying with fiber glass landing gear. I doubt Boeing would be making the 787 out
of composites if fatigue were a problem. There are fifty year old fiberglass
boats still floating and boats take a beating.
One thing it does do is creep. In other words a steady applied load will
cause FRP to slowly deflect permanently. Not a problem in this application.
More of a problem for Long EZ landing gears and Corvettes.
Here are some pictures of carbon fiber epoxy helicopter blades.
If FRP has a problem it is probably the epoxy and not the glass.
-
Paul Lamar ...No rotor no motor.
"The big difference is the modulus of elasticity (springiness)
of FRP is one third that of aluminum and one tenth that of steel so
the part will deflect greatly which is what we are looking for here
to absorb the shock."
Paul,
Do you mean that the springiness is 3 times that of Al and 10 times that of
steel?
George ( down under)
Yes. Another way of saying it is FRP is 1/10 the stiffness of steel
and 1/3 the stiffeness of aluminum.
Paul Lamar
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