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P. Mukkavilli et al.

the flier and subsequent motion of the body through air” [1]. It also deals with the

analysis of various forces generated as a result of the motion of objects through air

and the mechanism or ‘the art’ of sustaining a flier, quite often heavier-than-air, flying

object, or a machine afloat and making it move forward. These aerodynamic forces

are influenced to a great extent by the aero-thermodynamic conditions of the air viz.,

the ambient pressure and temperature which control the density of the surrounding

air, flight Mach number and the presence of aerosol. In this context, the geodetic

location or the altitude of the flying object above the ground or water surface plays a

key role. The viscous nature of air also affects the flight of the object depending on

whether the velocity of the flier results in laminar, transition or turbulent boundary

layers on the critical surfaces controlling the flight. The flight is also affected by

the Reynolds number which can be defined as the ratio of inertial forces to viscous

forces. This may be the case for all flying objects. Likewise, the Mach Number (M),

a ratio of flight velocity to the local speed of sound, affects high-speed flight. If M

is less than 0.3, the flight is in an incompressible regime as is mostly the case with

Micro Aerial Vehicles (MAVs) and the biological fliers. This is not generally true for

an aircraft due to its larger speed and high altitude flight. The biological flier or the

aircraft in these cases is supported by its wings and literally floats in the air. This is

true for the insects with low mass as well. The different forces predominant in this

kind of flight are aerodynamic forces comprising of vertical lift, forward thrust and

drag in addition to the weight due to the downward gravitational pull on the flier.

The lift forces help in overcoming the downward gravitational pull and help the flier

to take off from the ground or rise to different heights. Similarly, thrust is needed

for the flier to propel forward and to provide any necessary forward acceleration for

increasing the speed (velocity) of flight. It may be noted that the greater the forward

velocity, the greater is the thrust needed to overcome the aerodynamic drag and other

resisting forces.

The biological fliers comprising of birds, bats and insects have differences in

phylogeny, structure and physiology. However, these fliers are many times superior to

man-made flying objects like the aeroplanes and helicopters, during their low-speed

flight. The rockets and missiles operate at much higher speeds. The aeroplanes, for

example, derive their forward thrust through the propellers or the propelling jet, and

the lift is generated by the relative airflow over the asymmetrical or cambered, fixed-

wing surfaces, which are suitably contoured. The helicopter rotor by virtue of the

blade pitch angle settings is both collective and cyclic and the blade rotation develops

the necessary lift as well as the forward thrust. A tail rotor helps in producing the

balancing counter-torque to arrest the body rotation. The alteration in the lift force in

a fixed-wing aircraft is achieved by adjusting the angle of attack which controls the

relative direction of airflow with reference to the wing chord-line. The greater the

angle of attack, the higher is the lift developed. In the case of fixed-wing aircrafts,

if the angle of attack exceeds around 16°, it is counterproductive with the resulting

stalling and separation of the airflow over the wing. This leads to a total loss in the

lift, increased drag and a possible loss in height of the flier leading to an accident. It

may be mentioned that it is usually very difficult for the aircraft to recover from the

stall and restore its normal flight. It is interesting to note that in many biological fliers