Jet
age
Dr
Dewan discusses aircraft aerodynamics and elaborates on its features
and consequences while emphasising on its finer subtleties
Fluid
and the motion with which it flows play a key role in several fields.
Aerodynamics, for one incorporates the motion of air and other gaseous
fluids. Comprising of several important applications, from aviation,
road vehicle, spacecraft, missile, rocket to propulsion system and energy
conversion devices, the effect of high speed in aerodynamics results
in interesting flow behaviour. Mach number (Ma) is an important flow
parameter in aerodynamics. It is the ratio of the flow velocity to the
speed of sound of the concerned fluid. Further, the value of density
changes in such cases (termed as the compressible flow) and this adds
complexity to the flow. In contrast, a low speed flow is termed as incompressible,
because in such flow the density variation is negligible. Here we consider
some interesting aspects of aircraft aerodynamics.
Flow regimes
If the aircraft speed is less than the speed of
sound, it is termed as subsonic and compressibility effects can be neglected.
If the aircraft speed is about the same as the speed of sound, the flow
is termed as transonic (0.8 < Ma < 1.2). In such a case, the airspeed
around the aircraft may locally exceed the speed of sound resulting
in shock waves, expansion waves and flow choking. Fighter planes of
World War II were the first ones to experience transonic flow regime.
The flow is termed as supersonic if the aircraft speed is larger than
the speed of sound everywhere (Ma > 1). If the aircraft speed is
much higher than the speed of sound, special materials for aircraft
body should be employed, as the air temperature could become extremely
high. If the Mach number is greater than five, the aircraft is termed
as hypersonic. Here the temperature of the gas is so strong that the
molecular chemistry becomes important. At very high temperatures, the
molecules could break apart. A re-entry aircraft is such an example.
As the aircraft re-enters earth’s atmosphere, its Mach number is
extremely high (around 25), temperature of flow is extremely high and
thus the molecules of air are broken. Air density is usually quite low
as re-entry occurs at a height of several kilometres in the atmosphere
resulting in generation of strong shock waves on the surface. A space
shuttle is probably the only manned aircraft to experience this flow
regime.
Forces
Four aerodynamic forces act on a flying aircraft.
Drag force opposes the motion of an aircraft. Every component of the
aircraft contributes to its drag. Drag is the consequence of the relative
flow velocity Figure 1: Forces acting on a flying aircraft
....CONTD
