Differences between automotive & airplane engines

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)

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

Peter Dohm wrote:
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



Designing a proper cooling system for a wet engine really isn't that
difficult. Air cooled engines have a much higher delta-T, but their
cooling surface area is extremely limited. Radiators have a much lower
delta-T, but thousands of square inches of surface. The problem is that
most people havn't the slightest clue or any inclination to get one as
to what the air is doing around the cooling surfaces or how to make it
better.

The secret is to make the air pass straight through the radiator vanes
so that both sides are cooled. Rotaries using air-conditioner cores as
radiators are flying very successfully. Have been for years. Only
requires 2 13'x9'x3' cores. Mine will be installed in the wing strakes.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

"Peter Dohm" wrote in message
...
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



I recommend Fred Weick's book on Propellor Design. I think you will find
that the thrust per horsepower is not a constant but rather decays
proportionately to the log of the RPM. The pounds of thrust per horsepower
gets pretty punk past about 2500 RPM of the prop. At 1000 RPM you get great
thrust out of 25 or 30 horsepower! At 2500 RPM you can get the same thrust
from 100 HP with a good prop! :-)

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )