Jet engines burn fuel at a rate proportional to thrust delivered. The optimum cruise condition for a jet aircraft is at maximum speed for a given thrust. In steady level flight, thrust and drag are equal. Hence, optimum cruise occurs at the speed when the ratio of drag force to air speed is minimized.

A Boeing 727-200 jet transport has wing planform area Ap =1600 ft2, and aspect ratio AR=6 5. Stall speed at sea level for this aircraft with flaps up and a gross weight of 150,000 lbf is 175 mph. Below M =0 6, drag due to compressibility effects is negligible, so Eq. 9.43 may be used to estimate total drag on the aircraft. CD,0 for the aircraft is constant at 0.0182. Assume sonic speed at sea level is c =759 mph.

Evaluate the performance envelope for this aircraft at sea level by plotting drag force versus speed, between stall and M =0 6.

Use this graph to estimate optimum cruise speed for the aircraft at sea-level conditions. Comment on stall speed and optimum cruise speed for the aircraft at 30,000 ft altitude on a standard day.