Has anyone has come across, or attempted to build for themselves, a hybrid
RC plane? I'm thinking of an internal combustion engine driving an
alternator which supplies power for two electric motors.
I realize this might not be very efficient, just wondering really how
practical it might be. Since the engine would run at constant RPM, it
also might be easier to tune a muffler and make the thing really quiet.
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.
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.
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
| 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.
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.
| 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
| 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.
3.6 x 10^6 joules to be exact.
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
| 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
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.
| > 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.
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 designing things like that
Paul Mc> 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.
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
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.
Paul Mc> You not only have the variations in motor current draw, but variations in