Motor and Prop Selection (I promise this one's different)

[BayDesigns]

I'm well aware that there are plenty of threads and resources online for figuring out which motor and prop will suit your needs, but I'm really struggling here- because I'm designing a twin-motor micro/mini VTOL. For starters, the aircraft will hopefully have a 20-25 inch wingspan and be built as minimal and light as possible, with a small shell to hold the battery, FC, VTX, and camera (whether that's coroplast, 3D printed, or just cut out of foam is TBD), hotwired foam wings, and a carbon spar supporting the tail feathers. I'm shooting for a sub-350 build here but that's not a hard limit. Does anyone have any experience (or at least general advice) on selecting motors and props for this type of aircraft?

 

[quorneng]

BayDesigns
Interesting but with only 2 motors how is it to maintain stability in the hover? Cyclic pitch control like a helicopter or vanes in the air stream.
This has the makings of being a very complex project.
As an example you wont know how much static thrust you will need until the all up weight is established and the final weight will depend heavily on the power required to achieve the required thrust. A chicken and egg situation.
I am sure even with experience in high thrust set ups there would be quite a bit of trial and error required before you ended up with a workable VTOL solution.

For my EDF vertical take off job I not only determines the maximum possible EDF thrust and the weight of the battery required but also a couple of working mock ups of the intended EDF installation to check how much thrust would be lost in the ducting. All this was before I even started the final version. At least by then the problem was simply(?) keeping within a maximum weight limit.
I hope this helps.

 

[BayDesigns]

BayDesigns
Interesting but with only 2 motors how is it to maintain stability in the hover? Cyclic pitch control like a helicopter or vanes in the air stream.
This has the makings of being a very complex project.
As an example you wont know how much static thrust you will need until the all up weight is established and the final weight will depend heavily on the power required to achieve the required thrust. A chicken and egg situation.
I am sure even with experience in high thrust set ups there would be quite a bit of trial and error required before you ended up with a workable VTOL solution.

For my EDF vertical take off job I not only determines the maximum possible EDF thrust and the weight of the battery required but also a couple of working mock ups of the intended EDF installation to check how much thrust would be lost in the ducting. All this was before I even started the final version. At least by then the problem was simply(?) keeping within a maximum weight limit.
I hope this helps.

Stability in the hover is achieved the same way a bicopter flies- pitch control is achieved by simultaneously pitching the motors forward or aft, roll by altering motor RPMs individually, and yaw by pitching each motor pod in opposite directions. Depending on how Ardupilot lets me set it up, I'll hopefully be able to keep the "forward flight" control surfaces in use throughout all phases of flight to supplement the tilt servos whenever there's any airspeed.

Regarding your EDF VTOL project, was there a specific thrust-to-weight ratio that you were looking for?

Thanks!

 

[quorneng]

As it was never intended to hover but to take off vertically and gain sufficient air speed quickly enough for normal aerodynamic control to be effective so it had an initial 'burst' thrust to weight ratio in excess of 1.5 to 1. Once under control it flew and landed as a normal RC plane.
It is actually a scale model of the A4b which was a wartime V2 rocket but with wings. No rocket motor but with an EDF in the tail.

No stability control but it does have a 3 axis 'rate' gyro to tame it down a bit. More "to prove it could be done" than create a practical RC plane but it flies well enough.