In straight and level flight, the wing of an aircraft must produce enough exactly enough lift to counteract the weight of that aircraft. As the lift equation is based on velocity, amongst other things, this means that less lift will be produced by the same wing at lower speeds than at higher speeds. This is why pilots can change the angle of attack of the wing in order to increase or decrease the coefficient of lift produced by the wing. The coefficient of lift is the dimensionless number that is used to compare how much lift the wing produces when it approaches the air at different angles or in different configurations.

If the aircraft is in level flight at 100 kt, then speeds up, the pilot must pitch down to decrease the wing's angle of attack to maintain the same lift force. If the aircraft instead is to slow down, the pilot will have to pitch up to increase the angle of attack. This is the most intuitive way of thinking about this relationship.

If you wish to look at the lift formula for an even more in-depth explanation of this, we will do this now.

Lift can be calculated as follows:
L= C_L × ½ rho × V² × S

Where:
• C_L is lift coefficient (without dimension)
• rho is density
• V is velocity [m/s] (velocity is TAS)
• S is surface area [m²]
• L is lift [N]

If we assume density (rho) and area (S) are constant, then that leaves us with lift being proportional to C_L x V². Therefore, any decrease in V will require an increase in C_L, and vice-versa, and C_L is directly set by the angle of attack (AoA) for a clean wing.