QUAD Rotor Sizes and RPMs needed

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.

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  • I have been designing a QaudCopter that will be large enough for an adult to ride. Electric brushless motors are the way to go due to their preformace. The only problem I'm having is that battery technology is not up to par yet. My design does use batteries for emergency landings. My design uses a APU (Auxiliary Power Unit ) form a small private jet to Supply power to run the electric motors. Hope this helps
  • Bill,

    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.







    Download MotoCalc
    MotoCalc electric flight performance prediction software
  • What is possible now.

    Big copter

  • William


    Have a look here. Thats a big one.

    Maybe Robero has some more precise figures on power / lift ratio.
  • Whats the efficency and gear ratio of your gearboxes?

    I guess you will use a constant RPM and collectiv pitch system. Then we would rather talk about blade size than prop size.

    350 blades would give a rotor diam of ca 710mm depending a bit on the rotorhead . @3000rpm you would get a lift of ca 5kg @7° (11") pitch @650W. Thats a very rough calculation because we do not know what blades you will use ect. Given that you should use 50% - 60% of the power available for liftoff, you could lift +- 10Kg with a motor with 4.5HP (70% efficency)

    Maybe you can post some more details and I am sure there are a few members here to add their 2 cents.



  • "normal" sized quadcopters have an RPM range of about 3000-10000, so your engine RPM is okay
  • I'm surprised at the lack of experience...or willingness to share.

    There has to be someone on board here that knows the rotors..............

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Jan 12