Aerodynamics Challanges of Supersonic Flight

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A) Rapid increase of drag coefficient at Transonic speed => Need to be minimized by streamlined shape

The force of drag is proportional to the coefficient of drag (Cd), to the square of the airspeed and to the air density. Drag rises rapidly with speed, therefore a key priority of supersonic aircraft design is to minimize this force by lowering the coefficient of drag. To minimize the drag, streemlined shapes are used. Indeed according the drag equation:


the drag force on any object is proportional to the reference area, density of the fluid and proportional to the square of the relative flow speed between the object and the fluid. Nearby Match=1 the Cd reach the highest value, Supersonic aircraft also manage drag by flying at higher altitudes than subsonic aircraft, where the air density is lower.


The  coefficient of drag (Cd) is higher at speed of sound due to the propagation property of air itselft.





B) Phenomenon of Wave Drag that increases the total drag

The wave drag is a component of the aerodynamic drag on aircraft wings due to the presence of shock waves.  Wave drag is independent of viscous effects, and tends to present itself as a sudden and dramatic increase in drag as the vehicle increases speed to the Critical Mach number. It is the sudden and dramatic rise of wave drag that leads to the concept of a sound barrier. The effect is typically seen on aircraft at transonic speeds (about Mach 0.8), but it is possible to notice the problem at any speed over that of the critical Mach of that aircraft. Supersonic aircraft must have considerably more power than subsonic aircraft require to overcome this wave drag.



C) Low Lift to Drag Ratio at supersonic speed

At supersonic speeds, airfoils generate lift in an entirely different manner than at subsonic speeds, and are invariably less efficient. For this reason, considerable research has been put into designing wing planforms for sustained supersonic cruise. At about Mach 2, a typical wing design will cut its L/D ratio in half (e.g., Concorde managed a ratio of 7.14, whereas the subsonic Boeing 747 has an L/D ratio of 17). Because an aircraft’s design must provide enough lift to overcome its own weight, a reduction of its L/D ratio at supersonic speeds requires additional thrust to maintain its airspeed and altitude.




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