Bret1

I am sure this question has been asked before, but is there a conversion chart that shows the power/weight ratio for different gas motors vs. different electric motors? Also, it would only be fair to include the weight of fuel vs. the weight of batteries in the chart...then comes the price of the different setups. Thanks!

David Cutler

This is an old, old question when applied to cars and trucks. Every few years somebody brings out a revolutionary new road vehicle that uses electric motors and batteries, and, guess what happens? It's never seen again!

It all revolves around energy density. Until batteries make a huge quantum leap in technology, they can't hope to get anywhere near the energy storage density of a bucket of oil-based fuel.

Imagine how big a battery would have to be, using current technology, to get a 10 ton truck as far as a tank of fuel.

Say, 50 gallons of diesel fuel can move those 10 tons around 500 miles, where a battery of the same weight (approx 350 lbs) would probably only move it around 10 miles if you were lucky.

Electric model aircraft have many advantages over internal combustion engines, but one of them certainly isn't efficient storage of energy!

They are so different that it's hardly worth comparing them as they don't compete in the same arena.

However, if you want to compare them, remember that 746 Watts = 1 Horsepower of anybody's money. That means , say, you would need around 20 receiver battery packs @ 4.8 volts at 1 amp to produce one horsepower, and that would only last about, say, 5 minutes.

So an OS 91 FX (2.8 horsepower) weighing approx 26 ounces with 5 ounces of fuel is equivalent to approx 50 receiver battery packs, (my guess is those would weigh about 3 times as much) and that's just for a 5 minute run.

An electric motor, to be sure, is capable of competing with an internal combustion engine in terms of power to weight ratio, but only if you could run a power lead to the plane from a wall socket, which would compromise the aerobatics a bit!

There are some approximations there, I'm sure, but the figures are approximately the right order.

. .My 2c worth!

David C.

Bret1

very well put...Thanks.

gwright

something not discussed here, however, is the huge advantage electrics have due to the ability to swing larger props instead of the little "toothpicks" a glow motor spins. I used to fly 30 to 46 size planes with piped OS32SX's, 30% nitro, and a 10X5 on most, although props were a touch different with different type planes of course. Horsepower is about 1.25 (around 900 watts in electric talk). With electric power, I fly the same size of planes, but with 14 and 15 inch props (14X7, 14X10, 15X8, etc) and i tend to test everything, trying many props, taking tach readings, current reading, thrust measurements, etc. My current setups for this type plane are right at 400 watts (just over 1/2 horsepower), but I get equal to better thrust and a few mph less speed. Propped differently I can get more speed and a touch less thrust. Up to 550 watts I can do better in both thrust and speed than the 32 equation, so you can't really compare hp and assume it's an apples to apples comparison. Another example is my funtana ("funscale" katana at 26%, roughly 80 inch span and length). It's 11.6 lbs with 30 cell nimh pack and 10 lbs with lithium pack. 2100 watts on original setup, now down to 1800 watts input power, but a larger prop (originally 20X8, now 22X12). Local funtanas with glow are using OS160 engines with pipe and 18X6 propellor. Weight is 11.5 to 12 lbs on the glow models (same to more than my electrified one). the fuel injected OS is rated at 3.7 horsepower (2760 watts in electric language), but mine outperforms the glow planes in every respect. FAR better thrust for 3D/funfly type flying which the plane is intended for, and speed is equal to the glow planes. With lithium pack, I'm even 12~15% lighter than them, and fly for 30 minutes, so flight time is even better. I haven't vibrated the tail appart in flight either , which is a great benefit of smooth electric power . The current "downfall" of electric is cost. However, in this comparison, my power system is roughly $600 whereas the injected OS is $800, so the electric is even less expensive. Batteries compared to fuel are much more expensive however, unless you rationalize the cost of how many flights you get from a pack. With nimh, it's about $160 for a pack (8 to 10 minute flight) and with the lithiums, projected cost will be about $600 for this pack (I say projected, because this particular pack is not available yet, I've been testing them for the manufacturer). You can rationalize all you want about how many flights per amount of fuel compared to battery, and electric will eventually win, but the initial cost is always going to be much greater than glow which is really electrics only downfall. Sorry for the long digression, but comparing power ratios isn't a simple task of comparing horsepower to equivalent watts, so I wanted to fully explain my thoughts on the subject .

Steve Lewin

Why power to weight ratio ? It's rather strange number to bother with. E.g. about the best power/weight available today in glow engines is the OS46LA, but what does that tell you. What are you trying to work out ?

It's worth remembering that planes designed for electrics can be built a lot lighter (the airframe at least) because they don't need all that beefing up for the vibration or that heavy fuelproofer. So the overall weight difference in flying condition is not as great as just weighing engine and fuel against motor and battery.

Also we've been assuming you mean glow rather than gas. Genuine gas engines and ancilliaries tend to be rather heavy for the power they produce so they too have low power/weight.

But as everyone says, in the real world it's the energy density and the startup cost that get you.

Steve

David Cutler

I agree with the comment about larger props, but only to the extent that this only happens because a gearbox to transmit the power at a lower speed, that is, at higher torque, isn't used in a glow engine at the moment. I don't see why though, as gearboxes are currently being used in helicopters for the same reason! Large props are more efficient (as there is less cavitation) so you are bound to get more thrust for a given horsepower.

As for the amount of raw power available when comparing a battery pack with a can of glow fuel; I must have a think, about that!

Thanks for the interesting input.

David C.

David Cutler

I mentioned power to weight ratio for a simple reason . .

In discussing anything the advantages of doing something are usually balanced against the disadvantages.

We were discussing the relative efficiency / effectiveness of electric against glow engines.

An engine's main advantage it brings to a vehicle of any type is the power (surprise!) and the biggest disadvantage is its dead weight.

To add power to a machine, you must unfortunately add weight. The more power you can add with the addition of minimum weight, the better, surely?

-DC

Steve Lewin

It's an interesting point of view but really I intended to ask Bret1 why he asked his original question about power/weight ratios.

That ratio is certainly of some interest for the complete machine e.g. plane or truck, but it's not very relevant for just the power plant. E.g. very few people would choose an OS46LA over a 46FX because it has a better power/weight ratio. Very few people would put the smallest possible batteries into a model simply to improve that ratio.

Certainly if power, torque, run time, thrust etc were all equal one would prefer the setup with the lower weight but they never are all equal.

Steve

Bret1

I am new to the hobby, and I guess the reason I asked is because I'm not sure if I want to go with a gas or electric set up. I love the idea of electric being so clean, simple, and quite. However, I very much appreciate the power of gas engines. If I can get "close" to the same output with electric, on any given plane, I'm sure I would lean towards electric, even if it costs more upfront.
I probably asked the original question wrong, since the batteries are probably the heaviest part of any electric setup and they are what increases a motors output ability. I probably should haved asked, is it possible to convert a gas plane to electric and have the same flying characteristics? And if so, how would you know what electric motor, and number of cells to use to compare to the gas engine? Thanks for all the great input,
Bret

David Cutler

I'm sure most planes can be converted to electric and there appears to be a cult of electric supporters doing that very thing as we speak!

Generally speaking though, it's done by experienced pilots / builders as the technology is so much more advanced than using a nasty smelly powerful ol' IC engine.

In fact, I reckon there appears to be a conspiracy to get people who are new to the hobby to try electric planes, even though, frankly, a simple high -wing 40 size standard trainer set up simply is probably the best way to learn. It uses such tried and tested technology that it's difficult to go wrong.

However, that's a very controversial point, so I have no doubt it will be refuted by lots of people here!

-David C.

Steve Lewin

Well David much as I love a controversy that point isn't really all that controversial even in an Electric forum.

40 size IC trainers are very successful for a couple of good reasons. First, as you say the technology is very straightforward and easy to get working and there are lots of very inexpensive ARTFs to choose from. Second and perhaps even more important there are hundreds, maybe thousands, of clubs and instructors around who are comfortable helping out with them but who don't yet like or understand electrics.

However the reason small electrics are fast taking over as first R/C models is not much to do with conspiracies but more precisely because you can buy a complete package for a lot less money than a full IC set up and you can take it out and fly without the pain and expense of getting involved with a club etc. You can (fairly) safely fly in a local park or school field. Try that with a 40-size IC trainer .

Bret1, I'd say that if you already know what type of plane you'd like then it's probably simpler to get it with the original power setup. E.g. at the moment electrics work best as small park flyer types or as multi-engine types where they are much much simpler than IC multis. It certainly is possible to convert almost anything to electric, e.g. 1/4 scale Extras, 80" span Spitfires etc (both of which I've seen flying), but it's still a bit of a black art and the bigger the plane the greater the expense.

So I'd say it depends what sort of planes you are considering. If you have a burning ambition to fly very large scale warbirds then electrics won't really do it for you (yet !).

Steve