Wow! Here is a physics question for everybody. How much
     net out of the prop HP is Mark's engine generating? Assume
     the coefficient of friction between tire and pavement is one.

     Paul Lamar


Without digging out any old physics books and starting to look things up, this
is how I remember it...

Find the average velocity of the air just in front the the prop, and most of
the problem is solved. (Alternately, the air behind the prop could be used,
but things are pretty stirred up back there. No air blows behind the prop
that doesnt come from the front, right?)

I think a simplified way of explaining the problem would be to picture a
cylinder of air the diameter of the propeller and extending forward
approximately parallel to the ground, actually lining up with the thrust-line
of the aircraft. (ie A cylinder of air, lets say 6' in diameter and 100'
long). The mass of this air could then be calculated, and the kinetic energy
of this air could be calculated as it is pulled through the propeller and
thrust backwards.

E=(1/2) x M x (Velocity-Squared)

The 100' long cylinder was chosen because that is approximately how much air
would be moved by the propeller in one-second of time (100 ft/sec = 68MPH)
Actual value will vary, depending on engine and prop.

One HP is 550 Ft#/sec, so the actual HP transferred to the air could be
calculated by dividing Energy by 550.

Energy of heating would really need to be added to the Kinetic Energy of the
air too before dividing by the 550 above, and that Energy = (M) x (C) x
(Delta T) where Delta T is temperature change of air, and C is the specific
heat of air.  I seem to remember HVAC guys having a simple numeric conversion
for this, but it has been too many years for me to have a clue what that
number was.

Robin

What about a couple of scales under the mains Robin?

Paul Lamar