content_image/543312.jpg content_image/543313.jpg
##  Forces in Flight 

The four forces that affect WSC fl ight are thrust, drag, lift, and weight. [Figure 2-12]  In level, steady WSC fl ight: 

1. The sum of all upward forces equals the sum of all downward forces. 

2. The sum of all forward forces equals the sum of all backward forces. 

3. The sum of all moments equals zero. 

Note that the lift and weight forces are much greater than the thrust and drag forces. A typical example for many WSC aircraft is that the lift/weight forces are five times the thrust/drag forces. 

<img src='content_image/543312.jpg'>

 Thrust—the forward force produced by a powerplant/propeller as it forces a mass of air to the rear (usually acts parallel to the longitudinal axis, relative wind, and fl ightpath). 

 Drag—the aerodynamic force acting on the wing and carriage in the same plane and in the same direction as the relative wind. 

 Lift—the aerodynamic force caused by air flowing over the wing that is perpendicular to the relative wind. 

 Weight—the force of gravity acting upon a body straight down and perpendicular to the Earth. 

During level flight, these forces are all horizontal and vertical. During descents or climbing, these forces must be broken down into components for analysis. 

##  Dynamic Pressure (q) 

Both lift and drag are a direct result of the dynamic pressure of the air. Dynamic pressure ( q ) is created from the velocity of the air and the air density. An increase in velocity has a dramatic effect on dynamic pressure ( q ) because it increases with the square of the velocity. Doubling the velocity means “ q ” increases by four times. Increasing the velocity by a factor of three means that the dynamic pressure ( q ) increases by a factor of nine. This is a very important concept in understanding the aerodynamics of WSC. 

Formula for dynamic pressure: q = V$^{2}$ x ρ /2 

V = velocity 

 ρ  = density factor 

##  Lift 

Lift opposes the downward force of weight and is produced by the dynamic effects of the surrounding airstream acting on the wing. Lift acts perpendicular to the fl ightpath through the wing’s center of lift. There is a mathematical relationship for lift which varies with dynamic pressure ( q ), AOA, and the size of the wing. In the lift equation, these factors correspond to the terms q , coefficient of lift (C$_{L}$), and wing surface area. The relationship is expressed in Figure 2-13 . 

<img src='content_image/543313.jpg'>

Figure 2-13 shows that for lift to increase, one or more of the factors on the other side of the equation must increase. Generally, the lift needed is about the same for most fl ight situations. A slower speed requires a higher AOA to produce the same amount of lift. A faster speed requires a lower AOA to produce the same amount of lift. 

