V-22 Osprey Project - Prototype 1

"Two helicopters on a stick"

For my first prototype, I want to attach two of the mCPX BL helicopter frames to a central boom to test the control system in a hover. Essentially, two helicopters on a stick.  Upon examination of the stock mCPX BL frame, there is not a good way to attach the plastic frame to any cross support. I would have to design my own.

I took the measurements from the stock frame and started sketching them up in Fusion360. After a few test prints to confirm the dimensional accuracy, I created a way to attach the frames to a 15mm square dowel.

I’m using a Flip32+AIO FC board (https://www.readytoflyquads.com/flip32-aio) that I had in stock. It’s a Naze clone, and already quite old as FCs go, but it should provide enough functionality for this build.

The stock mCPX BL controller has a built-in ESC for the brushless motor, but I will have to use different ESCs since I’m not using the stock control board. I happened to have 3 12-amp ESCs in my stash that should do nicely. They are not the smallest form factor, but should do fine for this prototype. For now I’ve been testing with an Arduino Mega board, but it’s likely I’ll switch to a smaller form factor Arduino for the final craft.

The linear micro servos with the mCPX BL have a JST-SH connector on them. These are very tiny and don’t fit with the standard Dupont 2.54mm servo connections. I’ve removed the JST-SH connector and soldered on my own Dupont connections.

The real V-22 has counter-rotating proprotors with the left rotor turning clockwise and the right rotor turning counter-clockwise (as viewed from above in hover mode). The mCPX BL rotor is setup for clockwise rotation, so no changes are needed in the control setup for the left rotor. The right rotor, however, has some interesting issues:

1. The motor can be reversed simply by swapping two of the three leads. It’s using a standard brushless DC motor.
2. The rotor blades use a symmetrical airfoil so they won’t care which way they turn.
3. The rotor head cannot be flipped on the mCPX BL. The linkages from the swashplate to the blade grips have a 90-degree right-hand twist to them that won’t fit if the blade grips are reversed (upside down).

This leads to an interesting problem. On nearly all helicopters, the control inputs are done from the front of the blade 90 degrees in front of the direction of rotation. This is due to gyroscopic precession (http://www.copters.com/aero/gyro.html). Essentially, the input of a rotating mass (gyro) will have an effect 90 degrees later. Therefore, to pitch a helicopter forward additional lift is given to the blade as it passes the 3 o’clock position. The lift actually occurs behind (90 degrees later on a clockwise rotating rotor), causing the helicopter to pitch forward.

With the right rotor of my prototype turning counter-clockwise and the blade inputs coming from the rear of the blade (90 degrees behind), the controls are a bit weird! For starters, I knew that the collective inputs would be reversed (up is now down), and I thought that all other inputs would similarly be reversed from normal, but they aren’t. Because I’m controlling the blades from the rear and the blade is rotating in the opposite direction, cyclic inputs remain the same as the left rotor.