Gas Hybrid Quad Rotor UAV?

While trying to think of a way to increase the payload capacity, size and flight time of a quad rotor aircraft I had an idea, which may or may not be novel and was wondering if anyone else has entertained it.

Could a combustion engine powered auxiliary power unit (APU) be installed on a quad rotor aircraft that is light and powerful enough to maintain a charge of the main battery that is used to power the motors and avionics?  In other words, could a small engine powering a generator keep up with the electrical demands of the aircraft that would have to carry it and the fuel?  

Here is a link to a company that produces generators for conventional RC aircraft:

Food for thought at least.

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  • The design of a quad makes it hard the integrate a internal combustion engine of any kind, anywhere but the gas/electric idea might work well on something like a large tilt-rotor. For quads its pretty hard to beat the responsiveness and sheer acceleration of a brushless motor. In industry anything with a gas/electric setup is massive, expensive and has insane amounts of power...mining tippers, ore trains and powerplants for example...

  • I did this years ago...

    I made 1000 watts with a 23cc car motor... worked fine but was on the edge of flying..

    A pos battery was used and could lift that copter fully charged but the voltage would drop to 3.7..

    With the generator running with on a mix curve I could keep the voltage at 4.1


    • Here's a helicopter with essentially the same motor. You can buy this off the shelf for $3000 (it's a high-end machine)

    • Eddie, we're you the one that had the long build log in a blog post on RCG?

      I recall following one, but never found it again after it died down.
  • 3 kg for 500 watts... not worth it. And that's just for the engine, not the generator and fuel and power regulator and the rest of the quad. If you are talking about making a multirotor that big, you might as well start from scratch with variable pitch and individual gas powered engines.

    • And then you might as well just build a helicopter.  Far more lifting area, far less weight, less complexity, off-the-shelf parts, etc.  Guys, sorry, but I really just do not see the point of pure-mechanical VP gas powered quads.  It's an evolutionary dead end, offering not a single benefit over a helicopter platform.  Uncountable billions of dollars have been spent on full-scale VTOL development, and nobody has ever built a fuel powered quadcopter.  There's a reason for that.  The helicopter is still the king for real-world usable VTOL applications. 

      • But I still can't explain me, why the whole world uses multirotors instead of helis. Probably... because of a better video quality from multirotors rather than from helis. And imo, on small sizes quads have better payload/flighttime parameters, comparing to helicopters.

        I think, generally multirotor will beat similar sized/weighted/powered helicopter flighttime on payloads up to 5-7 kilo.

        • No.

          They don't necessarily have better video quality.  And they do not have better payload or flight time, in any size.  Small electric multirotors are good because they are very simple.  That's the main reason they are popular.

          • Rob,

            I sincerely disagree, and give for that an example. The guy here with Trex800 and DSLR says he can fly for 7 minutes. On the other hand, if we simply look at a popular DJI S900 hexa, the specs say:

            Hover Time: 18min (@12000mAh& 6.8Kg Takeoff Weight), which means about 2 Kg payload, which is actually pretty similar to the examples heli payload.

            Maybe I missing something, but I see an enorm flight time difference...

            • Just because the majority of helicopter users do not understand their machines, does not mean that their results are representative of the state of the art of the technology.

              I could double or triple the flight time of the helicopter in that video by making a very simple, $0 change.  It would not require replacing any parts.

              Magic?  No.  Slow down the rotor, by changing the batteries from 2x6S series, to parallel.  Cut the rotor speed in half.  He's running crazy high acrobatic rotor speeds that are unecessary for flying a camera.

              I have helicopters that fly for 25 minutes without trying very hard, and the current record of electric VTOL flight, is held by a helicopter, which flies for 2 1/2 hours.

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