LAEV Prop

But one thing is bugging me on the LAEV prop though... Some thing is worrying me on
the carbon fiber / steel root interface area.

How are the fibers locked into the curved surfaces of the root? What retains em over
time and with strain and vibrations? Isn't the sharp (as it appears in the photo,
could be wrong here) edge at the end of the steel root section a bad area for stress
concentration? Is there a commercial product that uses this same design?
Blake

Funny you should mention that Blake. Butch was asking me the same thing.
Here is the idea we are using. Ours is better as the thermal expansion
and contraction would be less of a problem not to mention that epoxy
sticks to steel better than aluminum. Never the less there will be a glass
insulation layer between the carbon and the steel to reduce the corrosion
effect. The first half of the blade is laid up and bunge cord is wrapped
around the fibers to hold them in place while the resin cures.
Plastic film is used of course to keep the bunge cords from sticking.
The bunge cord is removed and the other half of the blade laid up
with bunge cord again. When that side is cured uni direction fibers
are wrapped around the whole works to lock it onto the steel hub.

The feather edge on the steel hub actually reduces the stress
concentrations in the carbon.
Paul Lamar


Hmmmm..... Looking at the Hartzell prop they have a definite draft angle as the root tapers.
But the LAEV has a very thin appearing outward edge that I think will bend, if not buckle
under high loading. Even minimal flexing of that lip over time will lead to delaminations in
that area and nearby, IMHO.

Interesting layup sequence on the locking wraps of the Hartzell. Also of interest is the
drilling of the "rear" of the root, the crossectional solid area, and the nearly parabolic
pocket turned into the "front" of the root section. You can almost see the areas of maximum
compressive and tensile loads in the aluminum hub and the surrounding glass fiber blade
matrix.

Blake


I don't think so Blake. Loads are ninety percent centrifugal.
This tends to stabilize those lips under compressive forces.
The FAA certification for CS prop blades does not specify bending loads.
Only centrifugal loads. It is like a helicopter. When the rotor
is rotating all the blade bending loads due to the weight of the helicopter
on the center of the rotor are canceled by the centrifugal loads and the 
rotor remains more or less flat. If this were not the case the helicopter 
blades would simply break from the bending loads. Same thing happens 
in a prop and probably to a greater degree due to much higher RPM. 

The drawing is a bit misleading as the carbon
distribution will look more like the Hartzel blade.

You are also looking at some very good stuff there. Heat treated 4130.
The fatigue limit on this material is four times higher than 
aluminum. Those hubs are nothing if not over designed. They also serve as the
inner race of the blade ball bearings. As such they need to be hard.
Think of them as heat treated 2 1/2 inch diameter 1/8 wall 4130 tubing.
As such they will endure enormous bending loads. Way more than the
carbon "pipe" or blade surrounding them.
 
In any event we have devised a torture test beyond anything I have ever
heard of for props. Tracy's test stand will be modified to nod up and down 
at a 60 degree per second rate while the prop is revving to 2800 RPM. 
This is like doing power on stalls hour after hour one right after
another.

This will simultaneously test the bending, torque pulse and centrifugal 
loads on the complete prop like no other ground test. The gyroscopic 
loads on the blades and hub will be enormous. In fact this pitch rate 
is enough to bend the flange on a Lycoming engine crankshaft while using 
a metal prop. Our prop blades of course are not metal but lighter weight 
carbon so the loads will not be that high on Tracy's PSRU flange. It will 
of course also test Tracy's PSRU for the types of loads one gets
when doing aerobatics.

BTW It is a good think Tracy lives out in the wilderness or the noise from the
test rotary engine will be loud.

Here is a chart of loads on a blade done by an independent engineer.
As you can see the centrifugal loads are in the 8,500 pound range.
The G force and weight is calculated for each segment of the blade and 
the loads are totaled up. This is higher than what I calculate so 
I am showing you these results rather than the results from my calculations. If 
anybody wishes I can send the .xls spread spreadsheet used to make these 
calculations and or the BASIC program I used to make my calculations.

BTW in case you think I am biased I am making no money what so ever
from this prop. This has been a year long ACRE NL cooperative design 
effort with Butch and Perry fronting the hardware for the prototype.

Paul Lamar
 
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