X-Mozilla-Keys:                                                                                 
Fred wrote:

The controversy continues on P-51 drag or thrust from the P-51 cooling
system.  Impossible to figure out without pressure drop data for the radiator
core used.  But based on my analysis of a Lancair IV with 375 HP at 25,000
feet it might be possible IF:

1) You ignore the drag of the belly scoop, and just focus on the momentum
drag of the air flow through the radiator,

2) You are going very fast (more ram pressure makes for higher likelihood of
pressure left over to reaccelerate flow  out the back once heated-- which is
why ram jets need to fly fast to work)

3) The coolant temperature is very high  to heat the air a lot, making it
less dense so it can be accelerated aft to a higher velocity.  (Making lots
of power to go fast, this seems likely)

4) The ambient air temperature is very low (winter, high altitude, say minus
60F or so) so that the air is heated a LOT in passing through the radiator,

THEN one likely could make some thrust.  On a garden variety cruise day at
more nominal conditions, I doubt it.  And I doubt it especially if you
consider the drag of the scoop.  But compared to the drag of a big radial,
well, it must feel like thrust to the airframe after dragging all those big,
finny cylinders around.

Of such "corner of the envelope" data points are modern legends made.

Fred

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