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3. The sum of all moments equals zero. 

THRUST – the forward force produced by a power - plant/propeller as it forces a mass of air to the rear (usu- ally said to act parallel to the longitudinal axis). 

DRAG – the aerodynamic force acting on the airfoil lines and cart in the same plane and in the same direc- tion 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. 

## Lift 

Lift opposes the downward force of weight and is pro- duced by the dynamic effects of the surrounding air- stream acting on the wing. Lift acts perpendicular to the flight path through the wing’s center of lift. There is a mathematical relationship between lift, angle of attack, airspeed, altitude, and the size of the wing. In the lift equation, these factors correspond to the terms coefficient of lift, velocity, air density, and wing sur - face area. The relationship is expressed in Figure 2-11. 

This shows that for lift to increase, one or more of the factors on the other side of the equation must increase. Lift is proportional to the square of the velocity, or airspeed, therefore, doubling airspeed quadruples the amount of lift if everything else remains the same. Small changes in airspeed create larger changes in lift. Likewise, if other factors remain the same while the coefficient of lift increases, lift also will increase. The coefficient of lift goes up as the angle of attack is increased. As air density increases, lift increases. However, you will usually be more concerned with 

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