"Evan Williams" wrote in message ink.net...
A question for all of the pilots, engineers, and people who are a lot
smarter than me. Is there a simple rule of thumb for comparing the amount
of thrust or lift you can get from an engine which drives a set of propeller
or rotor blades to the amount of thrust you can get from the exhaust of a
non-turbo prop engine? For example, a T56-15 from a C-130 has 4,910 SHP and
a J85-21 from an F-5E has 5,000 lbs of thrust (These numbers are from a
twenty year old book of mine. For the sake of this discussion, lets just
assume they are correct). How much actual thrust can you get from the T56
on the C-130? I realize that there must be a lot of variables involved with
the propellers or rotors due to different airfoil shapes and the "lift" that
they provide and the pitch the blades are set at. But I was wondering if
there was a "general rule" that people went by. Keep in mind that math was
one of the main reasons that I dropped out of college and I just spend the
last twenty years as an Aviation Ordnanceman (Population Control) so any
large mathematical equations would go right over my head.
I apologize in advance for posting a question with little chance of
degrading into a political rant.

Evan Williams


Power = Speed x Thrust.

Power in Watts,
Speed in meters per second,
Thrust in Newtons (about 101 grams).

Thus a jet moving at 440mph (200m/sec) with a 2000kg (20,200N) jet
engine is equivalent to a 200 x 20,200 = 4040000 watts (4040kW,
5200hp).

Propellers are between 75% to 85% efficinet so we would have to add
about 25% to those figures to get the shaft horsepower equivalent.

Ofcrouse the equation gets a little silly (non linear) about Mach 0.66
or so as compressability reduces propeller efficiency.

Turboprops posses substantial jet thrust and inefficient engines in
terms of shaft power often perform better due to jet thrust in faster
aircraft.

The WW2 spitfire merlin possesed about 300lb (1330N) jet thrust. At
440mph (200m/s) and 75% propellor efficiency this equated to 353kw or
460hp. This is one reason NOT to use a turbo charger.