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.