 Forces in Flight  Dynamic Pressure (q) The four forces that affect WSC fl ight are thrust, drag, lift,  and weight.  [Figure 2-12]  In level, steady WSC fl ight: 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.  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. 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 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 . 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.  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.  Lift Weight Lift CG Thrust Drag Relative Wind Flightpath L = C$_{L}$V 2  2 S  L = Lift (pounds)  C$_{L}$ = Coefficient of lift (This dimensionless number is the ratio of lift  pressure to dynamic pressure and area. It is  specific to a particular airfoil shape and, above  the stall, it is proportional to angle of attack.) V = Velocity  (feet per second) = Air density  (slugs per cubic foot)  S = Wing surface area  (square feet) Figure 2-12.  The four basic forces in level fl ight. Figure 2-13.  The lift equation. 2-7