Im working on a UAV hybrid, and i have 2 engines with differing and confusing specs.

My aim is to gear down the output to around 5000 RPM to match the alternator.

The first engine, is a Kyosho GT 0.15Ci (2.5cc) rated at just under 0.5 Hp at 29,000 RPM so reduced to 5000 RPM should give me a theoretical 3 Hp or approx 2.2Kw..ish

The other is an elderly MAX OS 40, ive not found out much data on this engine, but even though from what i can find out, its 0.4Ci (about 6.2cc) but only rated at just under 0.7Hp at 12,000 RPM, geared down will give me less power.

Have i researched the wrong data, and are the facts ive found, wrong?

So from the above, which is the better engine to use?...I get the feeling it might be the Kyosho, can you please advise.


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  • The OS will run forever and the Kyosho won't. Kyosho makes good stuff but the smaller engine is really stressed at that high of rpm.

    Also, the hp of the engine doesn't change but the torque would be multiplied roughly equivalent to the ratio.


  • I wouldn't build a hybrid for its possible efficiency gains (although I do commend the research).

    I'd build it for its endurance.

  • Can we stop fighting over this?

    I am a little confused over this forums structure...!!!

  • Mr. Stew,

    I implore to research this topic a little further before you continue to badger us here. An academic discussion is taking place, not a name calling contest. 

    If you would like an example of this, please google for and find the papers regarding Boeing's current work in this field, including the fact that by using cryogenic superconductors (in a lab setting, of course) power delivery systems have been developed that are almost 100% efficient, battery to kinetic energy. Another important point in their work, however, is the use of gas turbines to generate the electricity, which are far more efficient than their reciprocating brethren. A final note in this is that efficiency gains (as in the hybrid system was shown to be more efficient than just the turbines alone) only with superconductivity, and thus cryogenic temperatures. This sort of technology for aircraft is only in its infancy, and of course comparing current consumer level hardware with this theory of operation is going to make the skeptic more sure of himself. 

    [EDIT] The most recent article I saw regarding this topic was published in Aviation Week and Space Technology magazine. I'm not sure which issue, but it was definitely within the last month. That'd be a good place to hunt for it. [/EDIT]

    As for whether or not this would work on an R/C scale is, of course, debatable. However, if a person here is interested in pursuing that question for the purpose of furthering the art for the rest of us, it would seem our best course of action to provide what insight and encouragement we can, rather than simply say it wont work. Orville and Wilbur Wright were told many times that their idea was the stuff of fantasy, yet somehow we fly heavier than air machines around the world on a daily basis today. 

  •    It is not the job of the skeptic to disprove partially developed theories.  I've challenged anyone to provide a theoretical proof of concept using real world numbers, or even the sometimes unrealistic specs provided by manufacturers.

       At this point nothing has really been accounted for except for the fact that hydrocarbons have a higher energy density than batteries.

       The rational refutation of the idea has already been explained.  You have energy losses at each conversion.

    gas->kinetic->electric->kinetic will NEVER be anywhere as efficient as gas->kinetic

       So the only real question is not weather it would be efficient, it is guaranteed by simple physics to be horribly inefficient, but weather the beast you're proposing could ever actually fly.

       I'm as interested as anyone to know the answer, and I've already put forward a quick look at currently available small gas generators.  That was my stab at answering the challenge.  I found at the very first step that the generators I looked at didn't come close to delivering the required power to weight ratio required to fly.  If it had come even close I would have gone to the next step.

       Unless someone can come up with a gas engine + alternator combination that delivers over 50W per pound there's just no reason to even discuss the idea any further.  Until that day the idea needs no further research and all the calculations in the world don't matter squat.

  • And keep in mind, the higher performance two-strokes are way less reliable than the lower performance ones.  The incidence of flame-out on the high performance ones is fairly high.  My first engine was an OS 40FP, low performance, but it just worked.  Started easy, even in freezing weather.

    On my Saito 4 strokes, I use remote glow driver, I've never once had a flame-out.  Very reliable.

    Regardless, for a multi-copter that can't autogyro, and can't glide like an airplane, you'll *HAVE* to have a battery backup to at least get down to the ground.

  • O.S is always a more reliable and lighter motor, but if your goal is only to generate then I suggest the O.S 50 which I believe idles at about 5000 rpms

  • I'd just like to point out, (siding with Brad and OP), that the hybrid concept is still a valid one for aircraft in general, and is actually under study ATM by Boeing and others for use in civil aviation. What a lot of folks are missing in this thread is a simple point: gasoline engines have a very wide range of RPMs at which they function, over which their power band varies quite a bit. For EVERY gasoline engine, there is a speed at which maximum output power occurs, and a speed at which maximum energy conversion efficiency occurs. The two are very rarely the same. 

    It's also an agreed upon fact that fossil fuels store more energy per unit weight (have higher energy density) than LiPo or any other form of battery. Ultimately, the idea is simple: run a gas engine at it's peak efficiency constantly, and extract as much energy from it as possible. Then convert the energy to usable form through electric motors. While the hardware on the hobby level may not be capable of doing this without more losses than either electric or gas hardware now, it is possible on a much larger scale. So why not do this now, do the research, and push the market towards higher efficiency products while giving yourself a huge plus on flight time?

    EDIT: Sorry, I forgot my key point. In flight, we use our motors under varying conditions: you add throttle to climb, and you take it away to descend, etc. This need for on demand power is very inconvenient in the design of efficient power delivery systems, but mostly with gas engines. What if you could take away this nuisance from the gas engine, but still garner the benefits of longer range/flight time it delivers? That's the point of hybrid.

  • I don't see any efficiencies in running a nitro engine in place of a battery for any reason.

  • Very nice analysis Brad.  I completely agree that there are very good reasons to favour a hybrid under certain circumstances.  Battery technology has a long way to go before it reaches the energy density of liquid hydrocarbon fuel, even allowing for the losses inherent in extracting it.

    I would like to add that the efficiency of a brushless motor can exceed 90%, although motors that achieve this tend to be heavier.  I would also like to say that you are right on the money with your 85% for the AC to DC conversion.  At the risk of being accused of using DIYDrones as an advertising forum, I include the following efficiency curves for my generator power management unit.  Over 90% is possible with a sensibly chosen operating area.



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