High Speed Flight: 350-760 MPH
In order to go faster, aircraft needed engines with even higher performance (power/ weight). The development of the jet engine was the first key to faster aircraft. As aircraft exceeded 500 mph, the drag increased rapidly, and a new aircraft design was needed to reduce the drag. The development of the thin, sweepback wing provided the second key to high speed flight. As before, improvements in aerodynamic knowledge and technologies such as materials contributed to the evolution of modern, high-speed aircraft.
The resulting vehicles were limited by the power of their engines and the amount of drag they made. Generally, the more swept the wings, the faster they could go. They were all sleek-looking shapes that minimized drag as much as possible. Aircraft were approaching the speed of sound, and drag was increasing dramatically. The speed of sound is about 760 mph at sea level and about 650 mph at 35,000 feet, and is called Mach I (for one times the speed of sound). At these speeds, the air has difficulty "flowing" around the airplane, with the result that shock waves form and drag increases dramatically. Also, the aircraft was difficult to control at these speeds. This largely limited aircraft to flying below the speed of sound, called subsonic flight. There are no airplanes that spend significant time flying between 700 and 800 mph because of the high drag and the control issues.
Modern airplanes that fly slower than the speed of sound (fast, but still subsonic) all peak out in speed well short of the speed of sound (below Mach <0.9). Higher speed flight would be desirable, but today the best tradeoff between speed and economy for transporting a large number of passengers or cargo is near Mach 0.8. Vehicles in this class are the commercial Boeing 700 series, the Boeing B - 7 Strato Jet, Vickers Viscount, the Lear Jet, and many military aircraft.