Hybrid Powerplant

It would be VERY impractical for the reason you mention, plus one other BIG reason: weight! You now have three times the power plant weight; ehgnie, generator and motors. Not to mention that you will need some kind of smoothing device such as batteries or large capacitors. That is the reason this technique is used on locomotives but not aircraft.

Na, we use it on locomotives because it's a little hard on the clutch trying to get 18,000 tons of coal moving! :-) remove my-wife to reply :-)

A small constant speed diesel driving a small but efficient generator and a normal e-flight setup minus the batterys...

For a multi motor plane it could be an idea, but it has to be quite big to carry the weight.

Yea, that too!

I would be willint to bet that there is no diesel/generator/fuel combination that would equal the power of LiPos at the same weight. You would be MUCH better off just putting the prop on the diesel.

| I would be willint to bet that there is no diesel/generator/fuel combination | that would equal the power of LiPos at the same weight.

If *all* you care about is maximum power (and not how long it's delivered), then your statement is probably correct. But if you care about total energy delivered, you could get more by adding a certain weight of fuel than you could by adding a certain weight of LiPo cells.

| You would be MUCH better off just putting the prop on the diesel.

I don't know about MUCH better, but better in general I'll agree with. If you were making a plane designed for 1+ hour flights, and it had to have electric motors for some reason, this would be one way of doing it. But yes, you'd pay the price in extra weight over just using the engine to drive the prop directly.

Good Heavens!

For a second, I thought you were recommending a redesign of railroad locomotives. :-O

Nemmermind ...

Marty

I didn't say anything about maximum power. Total power delivered for the weight. The engine and generator are essentially dead weight that do not produce power but actually LOSE power through the conversions.

There are electrics that currently fly for well over an hour while carrying video, gps and telemetry.

With all of the combusting LiPo's, perhaps steam power will become a reality someday!

| I didn't say anything about maximum power.

You said power. Why would you not care about maximum power? Or do you just not advance your throttle past 50%? (I think there's just a breakdown of communication here.)

I think the confusion is between power and total energy.

Power is amount of energy delievered per unit time. Commonly used units are watts and horsepower. Energy is, well, energy. Some commonly used units for energy include watt-hours and joules.

Being able to produce gobs of power is what makes your plane go straight up and speck out in sixty seconds. Having lots of energy is what lets your plane speck out and come back down, over and over and over and over and over, or lets it fly for several hours without landing.

| Total power delivered for the weight.

Total power over a certain time, or total power at a given instant? The former would be called energy -- only the latter is power. `1000 watts for one hour' -- that's a measure of energy, not power.

Both electrics and gas (glow/gas/diesel/whatever) motors can provide lots of power for a small amount of weight. Given a certain amount of weight -- say one pound, I don't know which can provide more, but it doesn't really matter -- both can provide a lot. (But if I had to guess, I'd still put my money on the right 0.90 lb glow engine + 0.10 lbs of fuel instead of the best pound of motor and battery.)

But if you're looking to optimize total energy delivered for a given weight, then gas reigns supreme. Why? Because even when you remove the energy lost to an inefficient internal combustion engine, one pound of gasoline, diesel fuel, glow fuel or kerosene contains much more energy than one pound of the very best LiPo batteries.

| The engine and generator are essentially dead weight that do not | produce power but actually LOSE power through the conversions.

Yes, they're extra weight. And yes, they lose power -- that's a given. However, good motors and generators can easily be over 80% efficient and not weigh too much. It would add weight, but would add some other benefits that might (under certain conditions) be worth the weight.

| There are electrics that currently fly for well over an hour while | carrying video, gps and telemetry.

Well, duh! But it's not terribly easy to do, especially with a high performance plane. If the `1+ hour' figure bothers you, replace it with ten hours. Or 39 hours, if you wish -- approximately how long TAM-5 took to cross the Atlantic. Do that with an electric, one that does not gain lift or energy from solar cells or things like slope lift, thermals or dynamic soaring. Good luck!

If your plane needs to have electric motors for some reason, and if you need it to fly for long periods of time (let's say ten hours), then using an engine, generator and fuel tank is probably a better solution than lots of batteries. Why would you *need* to use electric motors? I don't know -- it's just a hypothetical situation. If you were making a plane that had 30 motors on it for some reason, and yet wanted it to be able to fly for several hours, this might work nicely.

I didn't say that this was a good solution for everybody, or that everybody should sell their LiPo cells and buy engines and generators

-- I said that for certain (very specific) applications, it does make sense. I was refuting (at least partially) this statement --

I would be willint to bet that there is no diesel/generator/fuel combination that would equal the power of LiPos at the same weight.

Short-term power, probably yes. Total energy, no -- absolutely not.

Did you know that people have designed laptop batteries that don't contain batteries at all, but instead have a tiny turbine (engine) that runs on alcohol and a tiny generator? If they can make the turbine and generator small enough, they can make a `battery' that will last far longer than any LiPo based battery of the same size. Same principle.

| > If *all* you care about is maximum power (and not how long it's | > delivered), then your statement is probably correct. But if you care | > about total energy delivered, you could get more by adding a certain | > weight of fuel than you could by adding a certain weight of LiPo | > cells.

Of course, what we really need is a good fuel cell -- light, but able to generate lots of power. Then we could have all the benefits of gas and electric power, at the same time. The little turbine/generator thing comes close, but I'd still rather have it all done with no moving parts :)

Am I out of touch with the rest of the world? I really LIKE the sound of RC planes - it adds to the interest and excitement - the hush when suddenly the sound stops and all look to see how the deadstick lands.

I have actually been giving some thought lately about how to make a noise maker for electrics - a small amp, oscillator, and speaker/transducer set up should do the trick.

David

C D Kilgour wrote:

Would you really need that smoothing? With the diesel/glow/petrol motor speed controlled by governer or an electronic feedback circuit operating the throttle, that should take up variations in the current draw of the electric motor, I think.

David - who used to enjoy design> It would be VERY impractical for the reason you mention, plus one other BIG

You not only have the variations in motor current draw, but variations in current output of the generator. Without some kind of speed synchronization, you would end up with times when the motor would be at max draw and the generator at minimum output. Some kind of smoothing would get you past those points.

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Wasn't that what they used in the NASA high altitude plane, a bunch of electric motors and solarcells, I could see the use for a small gas turbine spinning at optimum rpm's driving a generator supplying the motors during climb, but then you would want the turbine/generator and tank in a pod the you could drop after gaining altitude

Hi Paul

In commercial generators, the sort you use to run power tools or run your camping equipment, emergency lighting etc, the petrol/diesel motor is controlled by the load applied to the output of the generator/alternator. Draw more power, the motor responds accordingly.

It seems a relatively simple task to have a (say) glow motor throttle controlled by a servo which in turn is controlled by an electronic sensor which measures the (say) voltage drop as the load on the electric motor is increased

- eg as its speed is increased by the application of more throttle at the xmitter end of the chain.

Couldn't be bothered doing it myself tho - but could be fun to try.

David

Paul Mc> You not only have the variations in motor current draw, but variations in

Ah, but in that case the generator and motor essentially replaces a gearbox...