Confusion over engine sizes, can someone enlighten me?

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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Well, about autogyro gliding (autorotation)...

You undoubtedly know of "dead man's curve"?  Given the typical application of a drone for low-altitude photography, where forward speeds are low, autorotative recovery is not reliable.  For faster forward speeds and higher altitudes, automatic parachute deployment is a viable equivalent to autorotation for the electric multicopter.  The best defense against failure, of course, is redundancy.

For a battery backup, energy density is not the primary factor; it's all about current dump.  A123 has some nano-phosphate lithiums that shed charge like lightning (almost as good as capacitors without the exponential voltage collapse).

Methinks some time spent looking at newer two-strokes for line trimmers, etc. (at least that class) might be worthwhile.

Thanks, Jake.  The detracting arguments seem devoid of any tangible data or rational refutation of the points raised.  Still, when it comes to copters, most operation is very nearly at full throttle, especially hovering.  Out-of-ground-effect (OGE) hovering is the most inefficient operation mode for any aircraft.

   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.

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. 

Okay, here's a Honeywell APU brochure that confirms at a larger scale a motor-generator set can provide 40KVA (40 kilowatts into a non-reactive load, ie V*A*cos(theta)) and weighs, at most 168 pounds.  This includes an air compressor for the air-powered starters common on many larger jets (in other words, it weighs more that it has to for just the electric power).  Considering its compact turbo-shaft reduction design, it clearly could be made to be more efficient and weigh even less for an application not required to run on Jet-A.

Ahem, that's two hundred and thirty eight watts per pound.  238!!!

Can we now get past the clearly irrational "impossibility" angle?


Can we stop fighting over this?

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

Admittedly, this debate has gotten a bit "spirited".  It would seem that I was expecting any counterpoints to be made as, for example, Mr. Lefebvre has professionally asserted his; specific elements of the mathematical supposition would be questioned.  That one would issue a strident, blanket indictment of the whole concept because of a limited world-view (what's available for sale at Home Depot?) was disappointing from a community of engineers, codesmiths, and other creators of new things.

Although, I do understand a skepticism borne of lack of knowledge.  If you had come to me 15 years ago, when the electric motors with which I was familiar were the NEMA-class fractional horsepower models, and told me that a polyphase PM motor (BLDC) could produce 5 HP at the shaft, weigh 3 pounds, and fit in the palm of your hand, I would have been highly skeptical indeed. 

I put forth what I believe is a reasoned, mathematically sound, and referenced defense of the whole concept of energy conversion at a model scale.  Hopefully, to the benefit of all, I've provided some impetus for exploration into this area.  The fact that it does work on a larger scale - and works well - is beyond dispute. 

Your efforts are to be applauded, Dr. Lannigan, with the caveat that you're a pioneer and the proverbial trial and tribulation arrows are indeed coming.

P.S. Now at least, you know why the power to weight ratio, not just the power, is a highly important consideration in engine selection.

Fair enough point.  And for sure, a parachute system is infinitely more practical on a multi-rotor than it is on a helicopter.

But isn't there a "deadmans curve" to a parachute system as well?  A parachute requires some amount of altitude to deploy.  Obviously ballistic deployment helps a lot, but you still need a bit of time to let the chute open.  I think this would have the effect of shrinking the required energy storage device to something that can get you down from... ~50 feet?  Above that you'd use the chute.  

As for the gas engine, yes, a weed trimmer could be a source.  But do you think they could best a hobby engine in terms of power/weight?  You might also look at chainsaw engines.  Another application where power/weight is a big deal, and I think due to the nature of the industry, the chainsaw engines might be more advanced.

Regardless, consider this:

8351W of power, 2908g.  1304W/lb for the engine.  Just need the generator still.  Seems achieving 50W/lb for a combined system would be pretty easy?  I must be missing something.

I believe that gas powered two strokes are at a disadvantage compared to "Nitro" engine in terms of Power/weight.  But have a big advantage in terms of fuel efficiency.  Nitro engines... well they're just terrible.  And not just because gasoline has twice as much energy/weight as alcohol.

You're right though Brad, about the incredible power/weight of these BLDC motors.  I was watching a video one day of a 700 class helicopter, and they quoted 7hp from the powerplant.  I was skeptical, but I simply ran the numbers and it's true.  7hp from a motor smaller than an apple!  A 7hp AC inverter duty industrial motor weighs about 150lbs!

What a world we live in today.

Just a "napkin sketch" here but, a 1.20 size glow engine develops in the area of 3.0hp, or 2250W. Weight is around 950g.  So 1076W/lb.  Huh, the gasoline engine bests it.  I didn't expect that.  I think the 3.0hp figure is lower than the manufacturer would quote, but it probably realistic if you are looking at actual operating RPM with a propeller attached.

I'm pretty sure I remember back in the day seeing .50 size ducted fan engine rated at 7hp or something rediculous like that, and at an incredible RPM.  But this isn't at all practical for a gen-set.  The MTBF of something like this would be way too low.

   Woah!  Lets take the baseless skeptic bashing down a notch.  There's no reason to resort to personal attacks.

   If you think my "strident, blanket indictment of the whole concept" is wrong then come back with some reasoning and logic to refute my simple statement of fact.

   Obviously you're not refuting that additional energy conversions in general result in a decrease in efficiency. So what specific case is there in this instance that would counteract that general rule?  That's all I'm asking, and I gave the example of the hybrid car specifically as an example so that you would realize that I'm looking for understanding rather than just trying to be a nay sayer.

   It's also way too early to start patting yourself on the back and bashing all who doubted the concept.  A diesel locomotive does not fly, nor does an APU.  The 36-150 APU costs around $700k-1M, depending on the exact model.  The 40kVA also appears to be a "Rated Horsepower" and may not necessarily be actual power output.  40kVA = 32kW, which would potentially work out to around your same figure of 240W/lb..

   So now you have 32kW and ~125 pounds at the low low price of $700k-1M.  So if you want to answer my challenge of putting something together that could theoretically fly you still have a ways to go.  By my figuring you still need to account for 4-6 engines with enough combined power to lift the weight we're talking about, props, the heavy duty ESCs you'll need, a frame that can support all the weight, and fuel to run the APU.

   Your 32kW can potentially lift around 640 lbs..  You're already at 125, so that leaves 515 lbs. left.  That seems like a pretty doable number to me.  But sounding reasonable is not proof of concept.

   I've never said any of this was impossible, just not efficient and probably not practical.  A $1M plan to get a generator flying doesn't really do much to address the practical aspect, but it's a step in the right direction.  Once you have a plan on paper we can look at the potential efficiency. 

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

I'd build it for its endurance.

@Mr. Stew: I admire your persistence as much as this whole thread is starting to look embarrassing, so I shall make this my final comment. 

The reasoning and logic were presented in their entirety already in this thread.  That you chose not to accept either is apparently an unsolvable problem.

I never stated that there is efficiency to be gained here; that IS a ridiculous notion.  Go back and re-read my original analysis.  I estimated that a full 90% of the energy in the fuel would be lost at the first stage, followed by 25% at the second, yet the total solution would still exceed the energy density of the example LiPoly pack by 300%.  I accounted for all the other hardware, too, estimating that the ship was 40Kg and the power source was 10 Kg.

The price for the APU seems off by at least an order of magnitude, even figuring in the lofty overhead of the whole FAA TSO-compliance thing.  The turbine engines out of larger APUs are selling for just over 7 thousand dollars on the secondary market.  Here's one example on eBay.  A million dollars?  They're included in whole planes selling for 5 million.

Yes, VA does not indicate actual watts, unless it is multiplied by the cosign of the phase angle difference between the current and the voltage (theta, noted in my post).  In the 400Hz power grid of your average APU-equipped airplane, there is bound to be some inductive reactance from the distribution transformers.  There will be some ripple current in a MOSFET-switched motor environment, but a few capacitors will fix that.  Properly corrected for power-factor, VA = watts.  And yes, that's a maximum, but there will also be losses in the transmission gearing and the air compressor, as well as the added weight of both.  The APU is simply a metaphor for a commonly attainable power-to-weight ratio (and one of the few brochures that actually mentioned both weight and power).

Perhaps you believed all along that my position was that a M/G set would be more efficient than a battery.  That simply is not, and never was, the case at all.  That IS impossible; I know from experience that even the most dreadful LiPoly packs on the market yield back >90% of their input charge, depending on discharge loading.  This thread has always been about ONE thing - energy available for a given power source weight.

I have no intention of putting a "plan on paper" for a model-scale MG device beyond what I've done here.  I'll leave it to others to not be discouraged and make the attempt on their own.  Regardless, the efficiency is irrelevant; the only standard they have to exceed is the one set by the net energy density of common LiPoly packs, and that is a fairly low threshold of success.

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