Equal Power (not!)

wrote:
To All:

When two identical engines are turning at the same rpm it does NOT mean
they are producing the same amount of power. It is the LOAD on the
engine that determines how much power is being produced at a particular
rpm, not the rpm itself.

For an automobile engine converted for flight the handiest 'load-meter'
is a simple vacuum gauge attached to the intake manifold.

Converted car engines, from the Volkswagen to the Reo (yes, 'Reo' with
an R) do not have a set of factory specs telling you how well they
perform as an AIRCRAFT powerplant. Fortunately, an engine can't lie.
Attach a few instruments, determine your local atmosphere and let her
rip -- the engine will tell you how much power it can produce when
fitted with a given prop.

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In the same vein, be damn careful about DEFINING the prop. Most
home-builders have the idea that when it comes to props, diameter and
pitch is all you need to know. Alas, with only those two factors you
can miss by a mile. In addition to its diameter and the geometrical
pitch of its blades, in order to define the LOAD a particular prop will
impose you need to know the weight of the prop, the total area of the
blades, and how that area is distributed relative to the prop's
diameter.

Lemme give you an example: You can take a "54x42" prop that needs a
140cid engine to spin it 3200 rpm and shave that puppy down to a
tooth-pick that will turn 3200rpm on the nose of a stock (96cid) VW
engine. But it won't fly the plane, a fact you'll discover the instant
you're out of ground effect.

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It's really all about thrust, as in the Production Of. But folks tend
to hear what they WANT to hear, whereas the engine, the prop and the
airframe can only respond to the reality of physics, mechanics and
aerodynamics.

Vast Mystery, of course. Or at least, half-vast.

-R.S.Hoover

So, is there an equation or series of equations where one can calculate
thrust?

Dan, U.S. Air Force, retired