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15. The conditions at the turbine inlet are the same
as the conditions at the combustion chamber outlet,
i.e. 55,417 lb.
Therefore given that the turbine--
OUTLET Area (A) = 480 sq.in.
Pressure (P) = 21 lb. per sq.in.
(gauge)
Velocity (vJ) = 888 ft. per sec.
Mass flow (W) = 153 lb. per sec.
The thrust
= 14,326 - 55,417
= -41,091
This negative value means a force acting in a
rearward direction.
Exhaust unit and jet pipe
16. The conditions at the inlet to the exhaust unit
are the same as the conditions at the turbine outlet,
i.e. 14,326 lb.
Therefore, given that the jet pipe--
OUTLET Area (A) = 651 sq.in.
Pressure (P) = 21 lb. per sq.in.
(gauge)
Velocity (vJ) = 643 ft. per sec.
Mass flow (W) = 153 lb. per sec.
Thrust distribution
210
21,235
g
(A x P) W VJ = + −
55,417
g
(A x P) W VJ = + −
21,235
32
= (580 x 93) + 153 x 309 −
55,417
32
= (480 x 21) + 153 x 888 −
The thrust
= 16,745 - 14,326
= 2,419 lb. of thrust in a forward direction.
Propelling nozzle
17. The conditions at the inlet to the propelling
nozzle are the same as the conditions at the jet pipe
outlet, i.e. 16,745 lb.
Therefore, given that the propelling nozzle--
OUTLET Area (A) = 332 sq.in.
Pressure (P) = 6 lb. per sq.in.
(gauge)
Velocity (vJ) = 1,917 ft. per sec.
Mass flow (W) = 153 lb. per sec.
The thrust
= 11,158 - 16,745
= 5,587lb. acting in a rearward direction.
It is emphasized that these are basic calculations
and such factors as the effect of air offtakes have
been ignored.
18. Based on the individual calculations, the sum of
the forward or positive loads is 57,836 lb. and the
sum of the rearward or negative loads is 46,678 lb.
Thus, the resultant (gross or total) thrust is 11,158 lb.
Engine
19. It will be of interest to calculate the thrust of the
engine by considering the engine as a whole, as the
resultant thrust should be equal to the sum of the
individual gas loads previously calculated.
20. Although the momentum change of the gas
stream produces most of the thrust developed by the
engine (momentum thrust = ), an additional
thrust is produced when the engine operates with the
propelling nozzle in a ’choked’ condition (Part 6). This
thrust results from the aerodynamic forces which are
created by the gas stream and exert a pressure
Thrust distribution
211
14,326
g
(A x P) W VJ = + − 16,745
g
(A x P) W VJ = + −
14,326
32
= (651 x 21) + 153 x 643 − 16,745
32
= (332 x 6) + 153 x 1,917 −
g
WVJ
across the exit area of the propelling nozzle
(pressure thrust). Algebraically, this force is
expressed as (P-P0) A.
Where A = Area of propelling nozzle in sq.in.
P = Pressure in lb. per sq.in.
P0 = Atmospheric pressure in lb. per sq.in.
Therefore, assuming values of mass flow, pressure
and area to be the same as in the previous calculations
i.e.
Area of propelling nozzle (A) = 332 sq.in.
Pressure (P) = 6 lb. per sq.in.
(gauge)
Atmospheric Pressure (P) = 0 lb. per sq.in.
(gauge)
Mass flow (W) = 153 lb. per sec.
Velocity (vJ) = 1,917 ft. per sec.
The thrust
= 1,992 + 9,166
= 11,158 lb., the same as previously calculated
by combining the gas loads on the individual
engine locations.
21. On engines that operate with a non-choked
nozzle, the (P-P0) A function does not apply and the
thrust results only from the gas stream momentum
change.
Inclined combustion chambers
22. In the previous example (Para. 14) the flow
through the combustion chamber is axial, however, if
the combustion chamber is inclined towards the axis
of the engine, then the axial thrust will be less than
for an axial flow chamber. This thrust can be obtained
by multiplying the sum of the outlet thrust by the
cosine of the angle (see fig. 20-2). The
cosine = and for a given angle
is obtained by consulting a table of cosines. It should
be emphasized that if the inlet and outlet are at
different angles to the engine axis, it is necessary to
multiply the inlet and outlet thrusts separately by the
cosine of their respective angles.
AFTERBURNING
23. When the engine is fitted with an afterburner
(Part 16), the gases passing through the exhaust
Thrust distribution
212
Fig. 20-2 A hypothetical combustion chamber showing values required for calculating thrust.
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