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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Can anyone give me better specs on these engines?

First off, what are you using these for?  What are your project goals or requirements?  You can either design the UAV around the powerplant (what propulsion engineers like) or size the engine around the UAV (what happens most of the time in my experience).

I am confused on what you mean by gaining horsepower by gearing down the Kyosho engine.  If the peak power the engine can make at WOT on a dyno is 0.5 hp, that's all you are going to get unless you modify the engine.  I seriously doubt you could take up the hp 600% without some major work.  At that point, you would be wise to invest in an appropriately sized engine.  In the end,  you don't gain power by gearing down. Think of a car transmission, if you car has 200 hp at 6000 rpm, it will make roughly that in 1st gear or 6th gear.  However, what speed your car is going vastly changes

By gearing down, you do gain in efficiency if you are looking at a prop.  A prop give x thrust at 5000 rpm would be much more efficient than a different prop giving the same x thrust at 29000 rpm (and a lot quieter!).

Maybe I am misunderstanding something?

As Jonathan wrote, gearing down increases torque (rotational force) it will, due to frictional losses, cost you some of your horsepower, not increase it. Horsepower, like watts, is the rate of doing work. The purpose of a gearbox is to match the RPM where the engine produces the most horsepower to the required RPM of the output. 


The OS has more horsepower and does so at a lower RPM, hence have a longer life due to less wear.

I may have worded things wrong, horse power and or torque, im a newbie, but im applying basic physics to a degree.

I just want to gear down both engines, to around 5000 RPM to drive an alternator, to achieve the best output.

My question is, which engine is the best to use?

The one with the best power to weight ratio, assuming either one is powerful enough for your application.

As previously mentioned, Horsepower is a unit of energy-per-second.
It will never increase via gearing, and will in fact reduce slightly for each gear used in the gearbox.

So you will get more power out of the MAX OS 40 because it's got more power in.

To get a 5000rpm output, for the first you'll need a 5.8:1 ratio, for the second a 2.4:1 ratio.

- Both of these are reasonable ratios for a single-stage gearbox.

I would also recommend a lower RPM engine in general, because it's easier to make/find efficient gearboxes rated for lower RPMs, and the consequences of misalignment or outright failure are less.

I really missed the obvious here, and to quote Homer...DOH!!

When i posted this, i was doing things on the 'fly'

I realise now, what you are saying, i did a little homework, which to be honest, i should have done before. Can i simplify my original question, if you had to generate the maximum power in watts, electrically, which would be the best engine to use, I cannot fine enough data on the Max OS engine to be able to compare the two.


"maximum power in watts, electrically, which would be the best engine to use?"

OS MAX w/ .7 Hp (521 watts) has more power than the Kyosho w/ .5 Hp (373 watts)

You might want to rethink the hybrid UAV idea altogether.


450 watts * 60% alternator efficiency = 270 W

The maybe 450 watts you're going to get from the larger motor isn't going to be enough to fly much.  You need around 50W per pound flying weight.


You're going to end up running a motor wide open the entire flight just to barely generate enough power to stay in the air.

There's just no reason to have a hybrid in the air, it would just be squandering energy for no reason.


Where does your 60% come from Jake?

@ Jake: I'd say 60% is on the low side.  While I don't have any experience with generators at this scale, a decent BLDC motor can do 80%.  There's every reason to believe a similar efficiency in the opposite direction is achievable.  

That's why I asked Dr. John about his motor-generator weight; given an average disk loading (about 1.5 pounds/square foot) and a FM of 50, your 50 watt motor input-per-pound number is pretty close to the mark.  The reason to have a hybrid in the air is that a well-designed and tuned motor-generator set can have 3-5 times the energy density advantage of the best LiPoly cells, with the caveat being the "well-designed-and-tuned" part.

The odds of someone with little electrical or mechanical engineering expertise cobbling something together with parts from various sources are rather...long.  That said, I would expect to see such solutions come to the enthusiast market in the next 12-18 months, given the expanding market realities.

@Dr. John: Unless you're embarking on this project for your own edification, given the above, I'd wait for someone to offer a turn-key module.  It's bound to happen.

I figured the 60% as an average figure you might get with a car-type alternator.  I'm sure it's possible to get higher efficiency, but since it sounds like we're talking about cobbling something together from spare engines, homemade gearboxes, and mystery parts it's probably better to err on the conservative side.


a well-designed and tuned motor-generator set can have 3-5 times the energy density advantage of the best LiPoly cells,

   Sure... That would be the reason to use a gas engine.  But then converting the kinetic energy into electricity then re-converting it back to kinetic energy again is highly inefficient.

   You're always going to loose energy with each conversion, and each conversion step also requires components that add weight.  So the best you can hope to end up with is a heavy and highly inefficient plane that will never match the gas input you started with.

   There are a number of factors that make hybrid cars work well, but none of them are present in an airplane.  There's regenerative braking and running your motor at it's most efficient speed.  In normal cars 100% of braking energy is lost, which is essentially equal to your total power input.  So theoretically you could double your efficiency with regen braking.

   In planes you have no braking and your prop selection can be chosen to run your engine at optimal RPM.  So there's nothing to be gained with a hybrid system, only energy and weight to be wasted.

   Size also matters.  Very few regen electric bikes are made.  They typically recapture 20% of the input during regen braking.  The problem is that you have very little power input in the first place.  You don't usually go very fast, so trading your speed back in for power doesn't give you much.  Usually on a bike you either want to maintain your speed or stop quickly, so there isn't a lot of regen braking going on.

   On a plane there's no braking and you already have a good way to store kinetic energy... Just climb to gain altitude.  It's also easy to convert that altitude back into speed.

   There's just too many reasons a gas-electric hybrid UAV wouldn't work.  OTOH a regen ESC might be worth it since it doesn't add all that much extra weight.  You might use rocket assisted take off or have an emergency small solid rocket to gain altitude.  In any case you'd have a way to use your altitude to run your electronics rather than just to gain speed.


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