My rotor hub design continues to evolve as I learn more about the complex forces and motions of a rotor system.

One of my goals of this project was to keep it simple.  Ideally as simple mechanically as a small electric quad, but as discussed in a previous post, that is not the desired approach, so I'm designing a "collective only" variable pitch rotor head, not unlike what might be found on a conventional helo's tail rotor.  As you can see from the image, I'm trying to leverage stock structural materials such as standard architectural aluminum channel and square tube.

The design above consists of a "double wishbone" (for lack of a better term) composite strap made of approx 11 layers of 6 oz. plain weave fiberglass fabric with about 50% by weight of Series 2000 epoxy resin with 2120 slow cure hardener (FiberGlast Resin). This should give a tensile strength of about 5500 Lbs which is almost 4x the anticipated centrifugal blade tension of 1500 Lb.

My big challenge here was to come up with a way to attach mechanical structures to the composite flex-element without weakening the composite.  My approach here is to use a large number of small diameter "shear pins" which will pierce through a sandwich of aluminum plates and the composite.  The idea is to spread the load across a large area of the composite and avoid force concentrating geometries which will exceed the local strength of the fibers.

A simple test of a 1/16" diameter nail through 2 layers of this composite matrix is surprisingly strong.  

A variation of that test was to line up 4 nails in a line (spaced about 1/4" apart) along the axis of tension, and even though these nails were all aligned, the strength went up sufficiently that it was able to support my full weight (~160 Lbs).

This tells me that the composite matrix is doing a good job of spreading the load to neighboring fibers.

So, back to the rotor design....The vertical shaft is the main rotor mast.  The channel and plate at the upper end of the mast comprise a sandwich which holds the composite "wishbone" on center, and receives upward forces from the thrust generated in the blades (not shown).

The wishbone shape is a copy of a shape observed in several articles I've read.  The idea is to give flexibility for the rotor blades to flap (up and down due to dissymetry of lift), lead/lag (oscillate forward and backward in the plane of rotation) due to correolis forces generated as a result of flapping, and pitch (or feather) the blade to increase/decrease the angle of attack of the blade to control the lifting thrust of the rotor.

Based on this flat geometry, this particular design will not have a tremendous amount of flexibility in the lead/lag direction, however, I am hoping this is ok because of the measures I am taking to limit flapping.

The pitch control horn protrudes forward of the blade leading edge, and a rod extends this toward the center of rotation a bit.  The idea here is that I will connect my pitch control links to the rod at a point which is outboard of the effective flapping hinge point.  In this way, when the blade flaps up, it's angle of attack will be reduced (because the pitch control link won't flap up), and will tend to limit the amount of blade flapping.  Now I've been around control systems long enough to know that if I get too aggressive with this (by making the pitch control horn too short, or by connecting the pitch link at a point that is too far outboard) the system could become unstable and the flapping could actually be exaggerated to the point of destruction.

Another interesting observation relating quad copters to single rotor copters:

On a single rotor copter, the cyclic forces cause the rotor disc to tilt, and this is transferred to the helo airframe through the rotor hub and mast.  The airframe FOLLOWS the rotor disc.

On a quad copter it is quite the opposite.  The difference in thrust of the 4 rotors causes the airframe to tilt, which causes an effective "flapping" of the rotor discs relative to their masts new position.  Moments transferred through the mast and hub then bring the rotor disc axis into alignment with the mast.  The rotor disc(s) FOLLOWS the airframe.

This is one reason I'm so excited to try this flapping compensation technique with the forward extending pitch control horn as it promises to greatly reduce cyclic bending stresses on the mast, rotor, and blade roots.

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Comment by Gary Mortimer on September 22, 2015 at 12:37pm

I don't understand how the teeter will work smoothly with this setup?

Comment by Rob_Lefebvre on September 22, 2015 at 1:05pm

There really wouldn't be teeter as you know it.  There is still some flap, but he is attempting to limit it by employing Delta-3 geometry.  This is what full scale does.  

Randy, I like what you're working on quite a lot.  Developing a composite rotor head would go a long way to simplifying UAV helicopter construction even more than they already are.  I have been dabbling in designing UAV helicopters for a long time, and one of my primary design goals is mechanical simplicity.  Helicopters are already a "solved problem" and can be quite reliable when assembled correctly.  But it is that assembly which still scares some people off.  I have a SAB Goblin 380 which I absolutely love the design.  There are only 38 unique parts IIRC (other than screws and other standard parts) and is very easy to assemble.  The head is easily the most complicated part, and you would be simplifying that further.

I wonder if a composite head could be made at these small scales simply by cutting standard carbon fiber plate?

Comment by Randy Sonnicksen on September 22, 2015 at 1:05pm


By teeter, I assume you are referring to the "flapping" (up and down) motion of the blades. 

The composite material (which is the "Y" shaped strap) is flexible.  It can be bent to a radius of roughly 20 times it's thickness before yielding.  In the future I will try to label the components or color them to reduce ambiguity.  Did that answer your question?

Comment by Joe Renteria on September 22, 2015 at 10:17pm
This is awesome.
Are you planning to continue this method with the upper swash assembly as well?
Comment by Randy Sonnicksen on September 23, 2015 at 4:49am

Joe, I'm not that far yet (designing the arms for the upper swash assy) but it's a great idea.  

Working with composites, while within the grasp of the average hobbyist, still requires a degree of skill, and I'm just beginning.  Fiberglass' strength to weight ratio is about the same as aluminum, so composites really have to offer something that aluminum doesn't.  Thanks for your interest.

Comment by naish on September 23, 2015 at 5:46am
Comment by Randy Sonnicksen on September 23, 2015 at 12:43pm

Here is a more complete picture.  I will need to do some work on the pitch link connection.  I will probably need to add another link to couple the upper swash rotation to the rotor rotation, and let the pitch links have ball joints at both ends.

Comment by Jerry Giant on September 23, 2015 at 10:56pm

rigid rotor is popular in full size high-end helis. people fly heli models are looking for even more rigid contronl (3d) , so bearing sprint might not be a good idea

Comment by Randy Sonnicksen on September 24, 2015 at 5:51am

Jerry,  Not sure I understand your comment (bearing sprint spring?), but thanks for looking.  FYI my application is full size quad. Thanks for your interest.

Comment by gsenroc on September 25, 2015 at 8:19am

We call this compliant mechanism in the university. It has a lot of possibilities but sometimes it's tricky to have a good design.

I think is quite feasible to have such mechanism for our rotor system, we just need some nice design for it. Looking forward for your upcoming results Randy.


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