**3. Military aircraft**

*Military Engineering*

**2.3 Flight Mach number**

The ease with which an aircraft can be operated is judged by the handling qualities (HQ ) of an aircraft. The HQ is directly related to the aircraft stability and controllability. An aircraft is said to be statically stable if an aircraft while flying in a steady path, if unintentionally disturbed by some external forces like gust or any other reason, the aerodynamic forces and moments so created due to the disturbances bring the aircraft back to its original stable condition. This property of the aircraft is termed as stability. However, higher the stability, higher will be the demand for control power to steer the aircraft and lower will be the controllability of the aircraft. Thus, a compromise is made, and the stability requirements are specified in design regulations formulated by the airworthiness authorities. The advantages of lower stability have brought the concept of 'relaxed static stability' or 'statically unstable' aircraft. The high-performance military fighters like F-16, F-17 and F-18 are statically unstable in order to obtain dramatic increase in manoeuverability. The vehicle is kept under control by **'**fly-by-wire' (FBW) control system. In FBW, accelerometers and rate gyros are mounted in each axis which senses the aircraft position and attitude, and a FBW computer continuously monitors these data and commands the control actuators to move the control surfaces to keep the aircraft under control. In this approach, very high manoeuverability advantages can be realized without heavily taxing the pilot. Even in the transport aircraft, this leads to smaller tailplane resulting in lesser weight and drag. The light combat aircraft of India, Mirage-2000 and SU-30, belong to this category of unstable aircraft [1–5].

At low flight speeds, compressibility of air may be neglected (ρ assumed constant), but as the flight speed increases, air gets compressed, and change in ρ cannot be neglected. Above M > 0.3 (M = Mach number; defined as the ratio of aircraft speed and speed of sound, named after Austrian Physicist Ernst Mach), compressibility effect cannot be neglected. The solution of a three-dimensional flow with air

b.*Transonic flow*—the flight M No. is 0.8–1.2. In this flow, the local M No. over some parts of the airfoil is supersonic, while it is subsonic in some other parts. Transonic flight regime is associated with very high drag and instability due to formation of shock and shock oscillation. No aircraft therefore does sustain

c.*Supersonic flow*—the flight M > 1.2. In supersonic flow the normal shock formed at M = 1 becomes bow shock forming a conical region over the aircraft. The

50–80% higher than the subsonic drags. The drag arises due to the thickness effect, and a hypothetical zero-thickness flat-plate airfoil in supersonic flow will produce no drag. However, practical aircraft wings required adequate thickness for storage of fuel in wing tanks and to provide adequate structural stiffness.

d.*Hypersonic flow*—the flight M No. >7. When an airflow is brought to rest (like in the leading edges of wings, fuselages, tail planes, etc.), the kinetic energy of air is converted to thermal energy and temperature rises; this is called stagnation temperature (T s) .T s can be estimated from the equation T s = T 0[1 + 0.2M<sup>2</sup>

For flow with M = 7 and above, the temperature rise may be so high that the air

*<sup>M</sup>*. Due to formation of shock, supersonic drag is

].

viscosity terms included at higher speed thus becomes complex.

a.*Subsonic flow*—the flight M No. is 0–0.8.

flight in transonic regime.

half angle of the cone is sin−1 \_\_1

Depending on the flow M, the aerodynamic studies are classified as:

**6**

Any aircraft that is operated by a legal or insurrectionary armed force may be called military aircraft. Military uses aircraft for both combat and noncombat applications.
