clare at snyder.on.ca wrote in message
...
On Fri, 10 Feb 2006 16:28:10 GMT, Alan Baker
wrote:

In article .com,
wrote:

But when we DO use 'horsepower' we must be careful to
never use it in isolation, always identifing the rotational speed at
which that 'horsepower' is being produced.


Absolutely and utterly wrong.

It is *torque* which must always be associated with the rotational
speed
at which it is being produced.

Read that first sentence again. He's not wrong; he just
didn't specify "torque" for those who don't know the relationship
between it and RPM and HP.
When you say "absolutely and utterly" it should be used
only where it applies. Clearly, that's not here.

But that's my point. He is absolutely and utterly wrong, when he says
that you need to know the rotational speed before you know all you need
to know when you know the horsepower.

With horsepower, you can use gearing to get any rotational speed you
want; the horsepower remains constant. Torque changes with gearing.

Yes, you CAN use gearing, at the expense of complexity.And efficiency.
Much better to design the engine to produce the power you need at the
speed you need it. However, sometimes you trade efficiency and
durability for weight - and a geared 1.2 liter 80 hp engine running
at 6000 RPM can weigh significantly less than a direct drive 2.7 liter
engine providing the same power at 2800 rpm. (well, about 40 lbs less,
anyway)

Ya' know ... there is a real problem with this entire discussion. Not just
this latest thread, but the discussion in general, and I really feel a need
to mention it before I turn in for the night--which is another ting that I
fell a need to do.

The problem, as I see it, is that there may be nearly as much difference
between different kinds of airplanes as there is between the different kinds
of ground vehicles that can be operated on public roads. That's just
counting airplanes, not helicopters, etc...

And we can probably all agree that a faster airplane can efficiently use a
smaller, and faster turning, prop for its horsepower than can a slower
airplane.

Some of us are mostly interested in airplanes that really need a redrive to
get good propeller efficiency from a 40 HP VW. Others are interested in
slippery airplanes that cruise at 150 to 200 kts. My interest is in the
faster type of airplane, and the only reason the specification isn't for
something even faster is a desire to keep the simplicity of a fixed pitch
prop. Therefore, if I want to use the old formula of 0.2G static thrust for
good takeoff performance on a 150 kt airplane, I only need to divide the
expected gross weight of the airplane by 10 to arrive at a reasonable
horsepower figure. (Since I want a static thrust of one fifth of the gross
weight, and also since each horsepower results in 2 pounds of thrust at the
150 kt speed--or would if efficiency was 100%) I really DON'T care about
efficiency, because I only intend to operate at low speed and high power for
less than a minute per flight. Propeller efficiency will always be zero, by
mathematical definition, at the beginning of the take off roll; and my
numbers work just fine with 40% efficiency during the initial climb to clear
the obstacles. On the other hand, if your plan is to cruise at 60 kts, with
a proportionately slower initial climb speed, then you probably need a
larger diameter prop than I do, even with a much lighter and less powerful
airplane.

We really need to look at what is workable, reliable, and affordable for
each specific application. I admit to being a long time advocate of
automotive conversions, and the various GM and D-C all aluminum 60 degree
V6s from 3.0 to 3.7 liters really do look promising; but I really would have
to think long and hard before I trying to adapt one to an airplane that has
already been designed around a standard airplane engine. Just making the
cooling system work reliably, with reasonable drag, would probably cause
insomnia!

Peter