Picture above is first prototype of fiberglass bearingless hub design.  The idea of this design is to allow flapping and twisting (pitching) of the rotor, but have limited flex in lead/lag. 

This rotor hub is 24" long and 4" wide in the center.  Was made with Qty 10 - 24" x 2" strips of 6 oz plain weave fiberglass cloth.  Bonded with Fiberglast series 2000 epoxy resin with 120 minute cure time.  

After layup, I inserted pins (small finishing nails) into the holes of the mating aluminum plates.  The idea is to distribute the tensile forces across the area of the plate, and also to minimize damage to the individual glass fibers.  While they are pushed to the side slightly by the pin, severing of the fibers is minimized, thus maintaining maximum strength.

This was a messy process and in hindsight, I would do it differently.  Yes, a learning process.  I needed to get the pins through the layup before the epoxy set, but the protruding pins created a bit of a problem for the vacuum bagging process.  Many layers of duct tape later, I was finally able to get a bit of a seal on the vacuum bag.  But I know that it could have been better because I only achieved 10" Hg vacuum with a pump that is capable of 20".  Leaks abound!

From this point I intend to measure the flexibility of the hub in 3 directions (flap, pitch and lead/lag).

After that, I hope to do a tensile test up to 200% of the design operating tension to verify my 4:1 safety factor.

If all goes well in the tensile test, and I still have a functional part, I will begin cycle testing, by flexing the part in the flap and pitch axes while under design tension load.

Here are some additional photos.

This is the end plate of 1/8" thick aluminum on each side of the fiberglass layup.  There are small steel pins in each of the holes that transfer tension from the fibers to the aluminum plates.

This is the center hub which will eventually connect to the rotor main shaft.  You can see the tops of the finishing nails protruding from the holes.  There is a flat alum plate opposite these aluminum channels that will attach to the shaft hub.

This "wishbone" design was seen in a technical paper on bearingless rotors, and seemed to be the simplest to manufacture, which is why I'm trying it first.

As an asside:  While laying up this part, I calculated the resin required to get a 50% resin by weight composite.  However, I did include a bleeder cloth to distribute the vacuum pressure along the length of the part, but it ended up soaking up a LOT of the resin to the point that it entered the vacuum tubing and flowed in the tubing toward the pump.

I did not have a "drop out" chamber to protect the pump from induced resin, and I was very lucky that the resin did not make it all the way to the pump.  Especially since this pump is borrowed!!  In the future, I will use a drop-out pot between the part and pump.

Till next time.

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Comment by Rob_Lefebvre on November 26, 2015 at 5:20pm

Still interested.  Keep me posted. :)

Comment by R. D. Starwalt on November 27, 2015 at 7:20am

The pins are an interesting, but tedious, method.

Some seriously coarse sandpaper/cloth might get you similar results with less work.

Perhaps just let the resin fill the pin-less holes?

It would be an interesting study in dissimilar material s composite strength.

-=Doug

Comment by Rob_Lefebvre on November 27, 2015 at 5:30pm

Is there any chance that this could be successfully done by simply cutting out standard flat plate?

Ideally, if it was to be manufactured, I'd prefer to do a wound filament around a fixture like the full size helis.

Comment by Randy Sonnicksen on November 29, 2015 at 7:26am

R.D. - Tedious - Yes, Very!  I wanted a through connection, not just a surface bonding connection because this joint will experience bending and twisting moments as well as tension forces.  My concern is that a simple surface bond will begin to crack at the edges and propagate from there.  Your idea of just letting the resin fill the holes is a good one, but I would have to review the shear strength of the resin only. 

Rob, regarding flat plate - yes, if the layup plan was only 0 and 90 deg.  Any 45 deg layers would give torsional rigidity that I don't want.  But yeah, after all this work, not out of the question and much much simpler.  

The problem with a filament wound design for me is that I want the fibers oriented along the axis of the part, to give flexibility in the pitch axis.  I'm not sure how that would work with filament wound.  

Also, FYI, I'm not intending to go into production with this aircraft, so while any method that requires less effort is indeed an interest to me, it is not my primary goal to design with current mass production practices in mind.

Thanks for comments!

Comment by Jon Russ on November 30, 2015 at 10:48am

Randy,

You might try sewing a Kevlar or carbon fiber thread through the pin holes instead of using finishing nails.  This would allow resin to wick along the thread as vacuum is applied and give you the through connection you want.  However, you would need to ream out the holes in the aluminum plate to decrease stress on the thread.  

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