AERODYNAMIC HEATING

When air flows over any aerodynamic surface certain reductions in velocity occur with cor­responding increases in temperature. The greatest reduction in velocity and increase in temperature will occur at the various stagna­tion points on the aircraft. Of course, similar changes occur at other points on the aircraft but these temperatures can be related to the ram temperature rise at the stagnation point. While subsonic flight does not produce temper­atures of any real concern, supersonic flight can produce temperatures high enough to be of major importance to the airframe and power- plant structure. The graph of figure 3-21 il – | lustrates the variation of ram temperature rise with airspeed in the standard atmosphere. The ram temperature rise is independent of altitude and is a function of true airspeed. Actual temperatures would be the sum of the temperature rise and the ambient air temper­ature. Thus, low altitude flight at high Mach numbers will produce the highest temperatures.

In addition to the effect on the crew member environment, aerodynamic heating creates special problems for the airplane structure and the powerplant. The effect of tempera­ture on the short time strength of three typical structural materials is shown in figure 3.21.

Higher temperatures produce definite reduc­tions in the strength of aluminum alloy and require the use of titanium alloys, stainless steels, etc., at very high temperatures. Con­tinued exposure at elevated temperatures effects further reductions of strength and magnifies the problems of “creep” failure and structural stiffness.

The turbojet engine is adversely affected by high compressor inlet air temperatures. Since the thrust output of the turbojet is some func­tion of the fuel flow, high compressor inlet air temperatures reduce the fuel flow that can be used within turbine operating temperature limits. The reduction in performance of the turbojet engines with high compressor inlet air temperatures requires that the inlet design produce the highest practical efficiency and minimize the temperature rise of the air delivered to the compressor face.

High flight speeds and compressible flow dictate airplane configurations which are much different from the ordinary subsonic airplane. To achieve safe and efficient operation, the pilot of the modern, high speed aircraft must under­stand and appreciate the advantages and dis­advantages of the configuration. A knowledge of high speed aerodynamics will contribute greatly to this understanding.