Model Forum / Radio Controlled / Air Models / May 2004

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 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.

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.

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.

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 :)