Guys, I think we are approaching end of the road here.
Please feel welcome to spot if I missed options.
The problem with this design (2 motors with prop down wash partially over the front of the wings) is it cant be stabilized properly in hover mode. In vertical flight it will likely work just lovely. In hover, the lift will only work fine for the part of prop not hitting the wing thus creating a pitch up movement. This up pitch is not canceled out by a prop on the other side of the wing, on the contrary its amplified by the other motor. In the case of a straight wing with motors at CG and at tip of wings, this down wash is canceled out. So when adding throttle the plane will rise up and eventually flip over on its back. Roll and Yaw works just fine though. If fixing the pitch by moving CG a lot forward, then the forward placement will result in strong down pitch in vertical flight. Not a workable option.
1. cut off the tip of the wings
Reducing the length of wings so the props run free will create an even lift over CG, where CG needs to be. This will likely make everything work nice in hover mode. However, the reduced wing area will decrease lift, make the stall speed higher, motors work harder and cause shorter flight time in vertical flight mode. Don't desire that. Maybe I could calculate just how much lift will be lost if cutting out that area, maybe its acceptable...
2. move the motors further out until prop downwash run free of wings.
This can be done by increasing the wing spar length by 120 mm on each side. The wing spar is now 1040 adding 240 would make total length 1280, ~ 25% longer. I think that will still work fine with adjusting yaw, roll settings but will the spar hold or need to increase diameter? How will the spar respond to motors given this increased leverage? How about vibrations?
3. add a third or forth motor
Balance pitch in hover could be done adding a third motor and place it at rear using the Mako's stock mounting place for the motor. However, this would need an additional output port on the flight controller. Since all 6 are already used and I cant see how to free up one without losing vital control, this is not an option. To try make it a quad will put even more demand on output ports. It will also need to rethink how to successfully control yaw and roll in both hover and vertical modes.
4. use another airframe with straight wings
The reason I chose current fixed wing was to have the simple Elevon controls that combines Aileron and Elevator into one control and don't use a rudder. Thus in Hover mode yaw is done by tilting rotors opposite directions but in Forward flight tilting is not desired but can be done using the Elevons only, just don't touch the yaw stick.
Elevons cant be used on a straight wing. They will need separated Elevator and Ailerons and maybe a rudder control too. I am unfamiliar with that type and not sure how to set it up to work in both Hover and Vertical flight with Libre.
5. different flight controller (HW)
As far as I understand Librepilot Rhino support at least 12 output ports so just swop the Revo board to another supported board with more than 6 ports. Then do option 3 above (add one more motor).
6. develop a vehicle type switch (SW)?
Today, there is no way to switch the current vehicle type (Multirotor/FixedWing/Heli/Custom) etc, from one to another dynamically. Maybe this limitation can be done away with?
Something like in the GCS when you import a UAV-file to a board.
Maybe one additional 'uav file' can be stored in Revo memory and with a command it will be 'imported' during flight.
This would create much more options and no need for additional output ports on the boards, they can just be used in different ways.
7. different fc (SW and HW)
Use another fc more tailored to VTOLs.
Yes can do that but outside of this scope. Purpose is to see what can be done with LP.
At the moment I am not sure how to move forward.
With the current design it can't be a V-TOL. It can at best be a Short-TOL fixed wing tilt rotor. Just accepting the pitch upward and forward motion in hover and switch to half way forward when plane is starting to lift off. In half forward there is no down wash over wings anymore. The motors should have enough thrust. I might try this.