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.
John
Replies
You might want to rethink the hybrid UAV idea altogether.
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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.
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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.
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.
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.
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The OS has more horsepower and does so at a lower RPM, hence have a longer life due to less wear.
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?
Can anyone give me better specs on these engines?
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