Page 228 - The Jet Engine

Performance

FV ENGINE THRUST IN FLIGHT
15. The t.h.p. is expressed as
550 . ft per sec .
where F = lb. of thrust 17. Since reference will be made to gross thrust,
momentum drag and net thrust, it will be helpful to
V = aircraft speed (ft. per sec.) define these terms:
Since one horse-power is equal to 550 ft.lb. per sec. from Part 20, gross or total thrust is the product of the
and 550 ft. per sec. is equivalent to 375 miles per mass of air passing through the engine and the jet
hour, it can be seen from the above formula that one velocity at the propelling nozzle, expressed as:
lb. of thrust equals one t.h.p. at 375 m.p.h. It is also
common to quote the speed in knots (nautical miles P ( − P0 ) A + W Jv
per hour); one knot is equal to 1.1515 m.p.h, or one g
pound of thrust is equal to one t.h.p. at 325 knots. The momentum drag is the drag due to the
momentum of the air passing into the engine relative
16. Thus if a turbo-jet engine produces 5,000 lb. of WV
net thrust at an aircraft speed of 600 m.p.h. the t.h.p. to the aircraft velocity, expressed as where
, 5 000 x 600 g
would be = , 8 000 W = Mass flow in lb. per sec.
375 V = Velocity of aircraft in feet per sec.
However, if the same thrust was being produced by g = Gravitational constant 32.2 ft. per sec. per
a turbo-propeller engine with a propeller efficiency of sec.
55 per cent at the same flight speed of 600 m.p.h., The net thrust or resultant force acting on the aircraft
then the t.h.p. would be in flight is the difference between the gross thrust
100 and the momentum drag.
, 8 000 x = 14 , 545
55
Thus at 600 m.p.h. one lb. of thrust is the equivalent 18. From the definitions and formulae stated in
para, 17; under flight conditions, the net thrust of the
of about 3 t.h.p.

Fig. 21-2 The balance of forces and expression for thrust and momentum drag.

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