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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I think that I have heard about people putting a brushless motor on their IC powered helicopters and have a inverter that charges their batteries in-flight.

I remember that on a picture in Model Helicopter World, it pointed out that a Jet powered helicopter had a generator mounted.

Well just go and search, I have no clue to the power needed and if it is efficient with regards to the added weight.
The simplicity of a battery and 4 motors is really the key to making a quadcopter cheap and easy.

Most of the demand is steady state. There is no coasting. You would have to build a generator that would supply primary power AND additional to charge the batteries. The battery would provide additional power when climbing.

Someone here is working on a variable pitch quad rotor driven by a single gas engine. That is a better solution than the extreme complexity of a engine and generator and batteries and motor and electronics to make it all work. Even that is a complex scheme.

It just proves there is no such thing as a free lunch.

It would still be four motors powered directly by a battery. It would just be carrying a payload consisting of the APU and it's fuel. Yes the generator would have to provide enough juice to keep up with the constant demand but would not have to produce "full throttle" demands. The battery could absorb a few minutes of that. I just wonder, as you scale up in size, would it eventually be possible and effective to trade a majority of the battery weight for an IC powered APU and fuel? I'm not trying to get a "free lunch" since all things comes at a price and it would eventually run out of fuel.

Now, to me a variable pitch IC quad sounds like that would take most of the advantages of an electric powered quad out of the equation. My initial thoughts are that the drivetrain and pitch control adds so much weight and complexity that you might as well go with a conventional helo design. It would be neat to see though.

I am curious to see if this is feasible or not.  The main step in figuring this out is finding the energy density of a battery and it's associated electronics, compared to the energy density of an engine and it's associated hardware.  There will be a point where it will be better to use an engine versus batteries (i.e. real life Aircraft) but where exactly is the point?  


There are obviously disadvantages to having an IC engine, with only a few advantages (such as higher energy density, faster re-fueling, possibility for in-air re-fueling, etc).  This is definitely not a question of whether it's possible or not, it's more a question of the size/cost/other feasibility constraints that would be required to answer.  Which means a very detailed engineering study on this.  


I am working on a quad-rotor project, (www.icarusuav.com) and one of our capabilities we plan to implement in the future is in-air refueling, so this question is of importance to me.

Air to air refuelling you say http://www.suasnews.com/2010/11/2676/ultra-slow-air-to-air-refuelin...


Fuel cells would be the way to extend range, better buy expensive ESCs to make sure they keep running under the strain.

That is ridiculous.
Given modern battery technology and the innovations in brushless motors, gas hybrids are still the elephants in this hobby.

LOL.  Maybe in another 25 years electric systems will hold a candle to IC.

Right now the energy density of batteries is around 1/100th that of gas.

We've discussed the whole hybrid multicopter idea before.  The calculations seem to show that you could *probably* get one to fly.  It would have to be around the size of a golf cart, cost several hundred thousand dollars, would fly terribly, and be horribly inefficient.

That was my recommendation a while back during a lengthy discussion on the potential for fuel cell power supply.  Gas/electric has been around a long time powering trains (diesel/electric) and hybrid cars.  I think it's the way to go for long-duration multirotor flights.  I imagine it all boils down to weight.

Efficiency and multi tortoise do not belong in the same sentence. If you want more payload and longer flight times, there is no better platform than a conventional gas heli. Multi rotors are fun and fascinating, and mechanically simple. However, this comes as a trade off, they are mechanically simply, and electrically complex, often resulting in more downed aircraft.

Nonetheless people love multis. Quite frankly the answer to endurance is by no meas a hybrid, althat is simply excess weight. The only reasonable option is a 100% gas operated multi, with collective (variable pitch) props. This reduces the weight of the electronics, bridge converter, step up step down regulators, motors, and excess wiring. In fact it may not even take a change to the flight controller, as the normal throttle (pwm signal) that would go to the esc, would instead gp to a digital servo. The PIDs would be much different of course, but should be very doable.

Going this route would result in more power, flight time, response, and payload.

In fact, I had a lengthy discus sion on rcgroups many years back where someone made a hybrid multi, and posted it in his personal blog. I can no longer find it, but he 100% accomplished it, with a much larger engine than I would have used!


Not quite that simple you need feedback to know rotor rpm. But I'd love to see a weedwacker quad powered by 4 weedeater motors. That would be a hoot! With a 2 hour flight time.

No need for any rpm feedback....

Having 4 gas engines is counter productive, with 4 failure points and slow reaction times.

The craft is simple, with one central engine powering 4 variable pitch rotors via shaft or belt. No need for rpm control, just pitch.

Of course if the rotors were properly designed for positive thrust rather than 3d it would add huge gains to efficiency as well.

I'm not willing to prove it but I'm sure you can get fast enough response from a 2-stroke to make it work if you operate in the power band of a specific motor.

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