I was finally able to perform a test of the new drive test stand. The previous test stand used a 4-stroke engine and a less rigid test frame. This test stand consists of a 6.5 HP, 2-stroke DuraForce engine coupled to a V-Belt centrifugal clutch with a custom made drive shaft. Couplings between the engine/drive-shaft and drive-shaft/clutch are stainless steel flex-couplings similar to the type used on the Robinson tail rotor shaft drive. I added a large flywheel to the engine to attenuate the torque pulses from the engine. The flywheel also makes starting the engine much easier.
The variable pitch mechanism has the following characteristics:
I chose to NOT include a dedicated follower link which forces the upper half of the "swashplate" to follow the speed of the rotor. Instead, the attachment of the pitch links to the control horn on the rotors has only one degree of freedom, and thus uses the pitch links to transmit torque to the upper (rotating) portion of the swash plate to make it follow the rotor. I plan to improve this design on my next rotor build, but for proof of concept, this seemed to work reasonably well. The final product will have 4 blades on each rotor, so I will just need to make sure of proper clearance between the control horn of one rotor, and the flex-element of the neighboring blade.
The lower half of the swashplate is connected to a giant scale servo at only one point. I was aware that this actuation force, being applied offset from the centerline of the rotor shaft, could induce some binding or sticktion in the variable pitch collective control. I wanted to test this approach because it so greatly simplifies the connection between the servo and the swashplate. To combat this tendency to bind, I designed the upper portion of the swashplate as a quite long "sleeve" which slides up and down on the OD of the rotor shaft.
The "stickiness" issues I experienced during this test I believe are due to the following effects and design choices:
- The upper swashplate "sleeve" material is 6061 aluminum which has a quite high coefficient of friction with steel.
- The rotor blades, in their relaxed state, have only a slight pitch angle (3 degrees) but the design pitch is around 15 degrees, so the pitch servo and linkage must overcome the torsion of the 3/8" fiberglass flex-element in order to achieve the design pitch of the rotor blades.
- Additionally, when operating at design RPM of 1410, this blade induces approx 1600 Lbs of centrifugal force, or tension, on the fiberglass rod flex-element. This force tends to make the blades want to straighten out, back to their relaxed position, and thereby increases the amount of force/torque required to achieve the design blade pitch.
To mitigate these issues, on the next hub design, I will attempt to:
- Lengthen the upper sliding section of the swashplate.
- Use lower friction brass or teflon sleeves as linear bearings.
- Build the rotors with the blades at flight operating pitch in their relaxed state.
- Keep the pitch servo attachment to the swashplate as close to the centerline of the rotor shaft as possible.
Another problem encountered during the test was the detection of false triggers from the engine RPM photo-interrupter.
The blue line is the engine RPM and the orange line is the clutch RPM. You can see all of the high spikes which go off the chart for the engine RPM. This means I was picking up false triggers on the photo-interrupter input. My circuit does include Schmitt Triggers on the tachometer signals, which seems to be working well on the other two photo-interrupters, but was getting some noise on the engine tach. I will have to hook up an oscilloscope to see what the issue is.
FYI, the Grey trace is the rotor RPM (approx 2.8 : 1 reduction from the clutch),
The cyan trace is the pitch servo signal in microseconds (inverted)
The yellow trace is the throttle servo signal in microseconds.
So while I discovered some issues (engine power, and pitch mechanism stickiness) I had basic success of proving that I could operate a home-made set of rotors, with an internal combustion engine, without excessive vibration.
I would still like to over-speed my rotors to ensure the safety factor ( I used a 4:1 safety factor over the static tensile strength of the flex-element fiberglass rod). I would like to take the rotor up to 2000 RPM, which would put the tensile force at 3200 Lbs, - double the normal operating force, but still approx 1/2 of the ultimate tensile strength of the material.
- New carburetor for engine to get the power/RPM I need.
- Tuned exhaust for engine (again, for power)
- Run rotor at 2000 RPM to verify strength / safety factor.
- Add lower friction sleeves to swashplate to reduce binding
- Tilt rotor at speed to observe gyroscoping effects on vibration.
- Attempt to control/balance test stand attitude using closed loop control from a gyro.
For those who think this is an ill-advised project, please refrain from negative comments. I readily acknowledge the difficulties that await me down the development road. I am working this project for my own satisfaction, not for a commercially viable transportation vehicle of the future. I am enjoying the process and the knowledge and experience I'm gaining at each step of the way. If you have questions or encouragement, I welcome those comments.