Can anyone clue me in to a guide on rotor sizes and minimum/maximum RPMs needed?
35cc gas engine (3000 - 7000 RPM) --- 4-way speed increasing gearbox --- 4 x right angle speed increasing gearboxes (2 x CW& 2 x CCW) --- variable pitch props.
Weight 25 - 30 lbs
Props ~ 24 inches
I'm waiting for the tech supports at the gearbox companies to give guidance on high RPM bevel gearboxes.
What type of blades would fare best for this application?
What RPM levels would I need to maintain hover and flight with 24 inch props and 25 lbs?
Naysayers - Redundant failsafes, backups and safety features will be paramount in this design in the long run.
What is possible now.
I am on a Mac so I use Drive Calculator to give me some rough numbers. I believe most on the PC side use MotoCalc. http://www.motocalc.com/motodown.htm But that probably won't help you much for your design.
I would be curious to hear what payload requirements you have that are driving that weight. Most of those doing "Heavy Lift" multi's are carrying full sized dSLR's etc. So it's not out of the question to carry around 10lbs with the current commodity stuff. And it will scale further but I don't think most people have the need.
I would suggest running some numbers through MotoCalc with the motors they list as options. Play with numbers of lipo cells and prop size to get a feel for what's possible. As other posters have alluded to, this is all pretty new to all of us (relative to R/C in general) so there is a bunch of trial/error going on.
You just touched on the need to rewrite code yourself. You are also introducing more variables by using an air breathing engine, which will be subject to performance issues at different density altitudes. In an all electric set-ups I am subject to density altitude impacts on the rotor system but not the motor.
So, there are many reasons why those working on multi-copters have gone with direct drive electrics.