Page 168 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 155

High-Speed Flight 155

greater the energy loss and the greater the drag. This is The SR-71 was designated the
accomplished by making the nose and wing leading edge sharp. RS-71 until President Johnson
Blunt noses lead to energy-consuming bow shocks, which are a accidentally reversed the letters
combination of a normal shock wave on the very nose joined to in a public announcement. Rather
an oblique shock wave a little ways back. Bow shocks are than embarrass the President, the
avoided by putting sharp noses on supersonic airplanes. designation changed.
Shock waves in air have known density, pressure, and
velocity jumps. For a given shock angle, all of these properties can be
found in tables. Thus, for a given airplane geometry the shock angles
and pressures are easily determined. Supersonic flight is actually easier
to analyze than subsonic flight. In low-speed aerodynamics, engineers
must rely on complicated equations to solve for the pressures over a
vehicle. In supersonic aerodynamics an engineer can use
published tables. However, being able to compute the
Supersonic flight is actually
pressures on the vehicle more easily does not translate into
easier to analyze than subsonic
making the problem of supersonic flight easier. Now the
flight.
penalty of supersonic flight will be discussed: wave drag.

Wave Drag and Power

As discussed in Chapter 2, the faster an airplane goes the greater the
amount of air that is diverted, the smaller the vertical velocity of the
downwash, and the smaller the induced power for the same load.
Thus the induced power is small for very fast airplanes. The down
side is that the parasitic power goes as the speed cubed. Now, there is
an additional demand for power at high speeds. The demand is to
overcome wave drag.
Shock waves travel with the aircraft. Before supersonic flight over
land was banned, sonic booms (a sound like an explosion heard by
those on the ground) were a frequent occurrence around military
bases. The shock wave is a persistent phenomenon, which travels
along with the aircraft and extends miles from the airplane. This
means that the airplane is not only doing work locally to change the
airflow but it is affecting the air miles away! This results in an increase
in wave drag, and an increase in the required power. The extra power
needed to overcome wave drag is one factor that makes supersonic