# Use this graph to estimate optimum cruise speed for the aircraft at sea-level conditions.

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 A p =1600 ft 2 , 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. C D,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.