Consider a jet engine attached to an aircraft flying at M 0

Consider a jet engine attached to an aircraft flying at M = 0.8 at an altitude of 30,000 ft. Under this flight condition, the engine capture area is 9 ft² and the exhaust velocity is 2500 ft/sec. Assuming perfect expansion at the nozzle exit, determine

a)The ram drag.

b)Engine jet thrust.

c)Engine uninstalled thrust.

d)What engine inlet area would be required if we desire the Mach number at the inlet to be 0.5? What is the spillage drag in this case?

Solution

a) Thrust of the engine is F= (mdot x V )_e - (mdot V)₀ where e denotes exhaust and o denotes ambient

The second term is the ram drag.

mdot = density at 30,000ft x area of inlet x velocity at inlet = 0.04lb/ft³ x 9ft² x ( 0.8x 899.8ft) = 259.2 lb

Ram drag = mdotx Vo = 186,480 lb

c)engine uninstalled thrust is the first term of net thrust ie. (mdot x V )_e so that ram drag is zero

= 259.2x 2500ft/sec = 647500lb

engine jet thrust = mdotx ( Ve-Vo)= 461,020 lb

At 0.5 mach the area to be larger for the sqme mass flow. ie mdot = 259.2 = rhox areax velocity

hence area = 259.2/ ( 0.04 x (0.5x899.80) = 14 ft²