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Jun 23, 2020, 04:41 AM
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Variable Pitch system


Hey All,

I bought one of those small variable pitch propeller systems off your typical chinese webstore and wanted to make my own scaled up and better version for spinning some real props - justification being that you can really get to the max speed of an rc plane with variable pitch.

Just wanted to share how its going and a few thoughts on the matter. See the video linked below for the current state - pictures are more of the process.

I'm starting with brushless motors that I can feed a linkage through the centre of. One is a Tmotor that has a hollow shaft with a circlip and an m3 threaded screw to retain the bell - I removed the threaded screw and left the circlip. A 3/32" welding rod fits through the middle without binding. Another candidate motor was a sunnysky pancake which I drilled out the middle of the pressed steel prop locating pin, it also had a hollow main shaft. I had some folding T-motor props so that's what's being mounted at the moment - some questions about that at the end.

For now the design is mostly 3d printed with mainly m3 hardware. Linkages pivot on welding rod and shoulder bolts that are clamped around either side by 3d prints. The linkages are similar to the chinese ones - a stationary shaft through the centre of the motor sticks out the top, has a bearing to transfer input to the spinning components, and then the bearing housing moves up and down with the shaft. Linkages convert this up/down movement into twisting of the blades. There is a static bushing on the bottom of the motor to stop the pitch shaft from binding/touching the motor whilst its spinning.

The centrifugal load is taken by an m6 threaded rod, which also acts as the pivot/bearing for the blades which attach to nuts that screw onto this rod. There is a housing that picks up onto the features of the t-motor, but can be modified for whatever motor.

I hoped an MG90s would be sufficient to drive the approximate straight line linkage for the pitch shaft. It seems to be binding however - if you sweep the mechanism cold it works fine and then when you keep doing it the servo starts to get stuck and jerk to different positions. I'm going to work on hunting down binding issues and swap the servo out for the next size up.

Some concerns are:
- The centrifugal load is the dominating problem. Rotary moment goes up really quickly with larger diameter propellers. It also shoots up with RPM. Spinning up to max RPM would put literally tonnes of tensile load on the blades and joints with the current (~20ish) inch propeller. I think I need either a geared motor or super low KV motor to really get use out of the variable pitch. For safety i need to keep the RPM low when i test this unit properly once the servo issue is dealt with, and that means I'm never really utilising a high voltage battery and motor.
- I dont know what to use for the propeller blades in terms of geometry. There would be specific flight conditions that would prompt different geometries and I assume a neutral/symmetric blade would be the best average tradeoff. Are multirotor props like what i'm using at the moment or slow fly glider props okay?
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