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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Are there any videos from that device: http://imgur.com/MTP3Q5O
Because I really doubt that it works in any way better (mean peaload or flight time) than a clean electric version of this.
Hi Dave/Rob
I think overall the idea of hybridization has merit. As a avid fan of hybrid vehicles I think their main benefits are typically found in trying to overcome peak power demand, by introducing high power small electric motors that are mostly used in short bursts of acceleration. Coupled with a small battery that can deliver the peak power required to maintain flight performance, a much smaller gas generator size can be used, that in turn should result in higher overall efficiencies through the drive train.
A couple of things worth considering with combustion engines is that max fuel to rotation power - thermodynamic efficiency is typically achieved at 80-100% throttle, this means that you actually want to run the engine at relatively high throttle most of the time (which gives no extra power envelope however), which in turn means that you should use the smallest engine that can sustain the desired level of flight performance, plus a small portion for peak battery charging that leaves the battery enough of a buffer to provide peak loads. A load profile analysis for the aircraft and performance required is highly recommended to establish the base load.
A hybrid drivetrain could be used for both helicopters and multicopters, either of them have power dynamics that can be improved on, depending on the application. Even though a helicopter already has the advantage of having a bigger rotor swept area, a helicopter could still benefit from such a system, provided the correct method of "hybrid" is used. The idea is to use the hybridisation to a) reduce combustion engine size (to something that produces just over average power req.) b) decrease drvietrain weight c) resulting in reduced fuel requirement d) and increase drivetrain reliability (electric and/or gas driven) There are various combinations of serial and parallel hybrids that can be used. I think a Prius type hybrid would likely yield the best results.
This system consists of a gas motor (ICE), two electric motors and a planetary gear as a torque converter. The advantage of this type of hybrid is that torque from the gas motor can be controlled and directed on demand (generation or rotor power) and can be directly transferred to the drivetrain (in the case of a multicopter a variable pitch props would be required to maintain attitude) bypassing the need to convert all the torque generated by the ICE into electricity first. That means that power from the ICE goes straight to the rotors, and the electric motor only supplement the drivetrain for peak demand, plus this can be used to charge the peak battery and keep the avionics etc running. Because the electric motor need not power the whole aircraft on it's own, and only needs to operate to boost peak for seconds rather than minutes, this means it can be dimensioned a lot smaller as well without running into thermal issues at higher power levels. Torque command is also spontaneous so the ICE can be tuned for efficiency rather than peak performance.
This is significantly more efficient than using a APU type electric driven copter, especially in the smaller size of RC platforms. By using the Prius type hybrid the drivetrain can then be optimised to run at designed "cruise" performance from the ICE, with up to 2-3 times the power still available in peak from the small electric motor(s). On top of this the system is capable of driving the props on just electric power alone (being the more reliable power source) should the ICE fail, without the need for any clutches. The electric motor(s) obviously can also be used to start the ICE and can be configured to operate the ICE without the props moving for improved on ground safety. Autorotation should also be possible in this configuration should the battery fail, in fact it could use some of the autorotation energy to power avionics/RC control directly, and possibly even store enough energy in the drive train (motor/props/flywheel) to slow the craft down for a soft landing after a faster decent. This would work for helicopters as well as multicopters.
Although I generally only advocate reducing system complexity and weight in principle, flying "things" are very dynamic and have sometimes hard to manipulate mechanisms that can only be improved by compromising simplicity. Typically in designing such a device I'd first start to conceptualize the purpose of the intended craft and then only add mechanisms that can be modulated in such a way as to achieve the desired outcome. ie carrying a GoPro aerobatically or SLR for cruisy video camera work, or loading up 25kg of weed killer for agriculture spraying are completely different scenarios that need to be addressed individually. Also range and endurance are two completely different parameters. For example do you want 2 hours of endurance to cover an large area for surveying or just to hover on the spot like a helium balloon (no area/range required)? For range a fixed wing is better but it cannot hover or VTOL. A VTOL hybrid variable pitch quad with wings (optimised for cruise over 80kmh which makes them nice and small and slender so they don't significantly increase the booms) would of course be something I'd be very interested in.... ;)
My motto is: If the existing system does not perform as required; add an associated problem and then then simplify both into one solution.
Have you heard of monocopters with 360degree rotating camera? How about a variable pitch mono-prop (thrust vectoring) tailsitter VTOL? Only one servo and one motor with 3 axis forward flight control and hover. Analogous of a canoe driven and steered by a man with just one paddle. No quad required ;)
Put belt driven alternators (brushless motors) getting power from the main motors.
As an engineer, I think it is completely possible to make a hybrid quadcopter that flies and carries a moderate load (admittedly this is a pretty low bar). In very general terms, one way to do this is to use a large model aircraft gas engine driving a large brushless motor (which acts as the generator). Selecting off-the-shelf items, this weights less than the Li-Po batteries that big multi-copters use while providing 5-8kW. This may require directly connecting the ESC's to the output of the generator (after power conditioning) as attempting to charge an in-use battery could be problematic. There are challenges such as vibration mitigation (8x motors/prop + gas engine + spinning generator) and the complication of carrying flammable fuel, but these are not insurmountable (really pretty similar to a gas heli).
The real question is this: Is it really worth the effort and trade-offs compared to an equivalent gas heli?
Having said all of that, I fully intend to create a proof-of-concept hybrid multi-copter some day (or year... or decade... ) just for the challenge. I don't think it is the best solution to this problem, but I like trying silly stuff to see what will actually happen and seeing if I can make it work. ;-)
Interestingly, if you go variable pitch with a clutch you could then potentially autorotate on deadstick??
Thats what I was going to say ;-)
I'm not going to express to being an expert in this field - my thread detailing my flyaway on both mine and my quads maiden flight is testament to that! I am full of ideas though!
I can't help feel that there is legs in what we are talking about although nobody might have hit the nail on the head yet.
Would a system using a large centrally mounted prop powered by a fuel more efficient than lipo's, providing lift/thrust - complemented by small brushless motors for movement and direction - not give longer flight times?
If this affects the maneuverability of the craft then providing 80-90% of the thrust and the rest made up by the brushless motors would negate the effect to a degree.
Just a thought and probably a load of <insert choice of profanity here> but can't help feel this is possible in one way or form.
Hi Dave,
Read about your idea of using combustion engine powered auxiliary power unit (APU) for increasing the flight time and payload capacity. Well I would like to tell you that this will not be of any benefit as there will be energy losses while converting one form of energy to another as no system is 100% efficient, secondly additional equipment will have to be carried on-board like Fuel tank, engine, Sullivan generator, Ac to DC Converter and Li-Po Battery charger which will not help in increasing the flight time and capacity at all.
Instead I would recommend you to use only one Gasoline engine and connect all four motors with gear mechanism and use Variable pitch propeller assembly with servo to maneuver the Quad rotor.
Hi Ashutosh,
Like this Hobbyking Reaper only with gas motor.
http://www.hobbyking.com/hobbyking/store/__66936__Assault_Reaper_50...
Curtis Youngblood originated the design (Stingray 500) and also experimented with a larger gas version (but to this point has not put it into production).
http://curtisyoungblood.com/V2/products/quadcopters/stingray-500
and the (proof of concept) Manta Ray gas version right here on DIYDrones:
http://diydrones.com/profiles/blogs/curtis-youngblood-single-motor-...
Hi Dave,
Several people have broached this topic over the last couple years and a few have even tried to implement it.
Generally the results have not been encouraging.
The energy conversion ratio seems to be just not high enough.
The 75% + energy mechanical conversion loss that is inherent to all internal combustion engines plus the losses associated with even a high performance brushless generator and reconversion to mechanical energy in the motor simply result in a MG set that weighs too much for the high electric power required in order to be able to lift it.
There has been at least one multicopter with a JPU Jet Power Unit generator system that did (barely) achieve hover, but this is an awfully expensive option and specific fuel consumption was very high due to the compounded inefficiencies.
There was at least one attempt using Jet engine ducted exhaust thrust which actually achieved better results.
Both projects have not been heard from for some time now.
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