Forces

In a steady-state level flight, an aircraft is in equilibrium under the applied forces (i. e., lift, weight, thrust, and drag) as shown in Figure 3.9. Lift is measured perpen­dicular to aircraft velocity (i. e., free streamflow) and drag is opposite to the direction of aircraft velocity (naturally, the wind axes, FW, are suited to analyze these param­eters). In a steady level flight, lift and weight are opposite one another; opposite forces may not be collinear. In steady level flight (equilibrium),

Force = 0;

chord line

Figure 3.10. Aerofoil section and definitions – NACA family that is, in the vertical direction, lift = weight, and in the horizontal direction, thrust = drag.

The aircraft weight is exactly balanced by the lift produced by the wing (the fuselage and other bodies could share a part of the lift – discussed later). Thrust provided by the engine is required to overcome drag.

Moments arising from various aircraft components are summed to zero to main­tain a straight flight (i. e., in steady level flight, Moment = 0).

Any force/moment imbalance would show up in the aircraft flight profile. This is how an aircraft is maneuvered – through force and/or moment imbalance – even for the simple actions of climb and descent.