The Four Forces and How an Airplane Flies

The four forces

In steady, unaccelerated flight the four forces balance: lift equals weight, and thrust equals drag. Change one and the airplane accelerates, climbs or descends until they balance again.

How the wing makes lift

A wing deflects air downward and creates lower pressure above it than below. Pilots control lift mainly with angle of attack (AoA), the angle between the wing's chord line and the relative wind, and with airspeed. Lift rises with the square of airspeed: double the speed and the wing can make about four times the lift at the same angle of attack.

Drag

• Parasite drag (form, skin friction, interference) grows with speed.
• Induced drag is a byproduct of lift, highest at low speed and high AoA.
• Total drag is lowest at one speed, which is close to best glide (L/D max).

Turns and load factor

In a level turn, the wing must make extra lift to hold altitude, so you feel more G. In a level 60° bank the load factor is 2 G: the wing supports twice the airplane's weight. Higher load factor also raises the stall speed (see stalls and spins).

Left-turning tendencies (single-engine, clockwise prop as seen from the cockpit)

• Torque: the airplane tries to roll opposite the propeller's rotation.
• P-factor: at high angle of attack the descending prop blade takes a bigger bite, yawing the nose left.
• Spiraling slipstream: the prop wash hits the left side of the tail.
• Gyroscopic precession: most noticeable in tailwheel airplanes raising the tail.

That's why you need right rudder on takeoff, in climbs and in slow flight.

Ground effect

Within about one wingspan of the ground, induced drag drops, so the airplane can float on landing or fly before it can climb on a soft-field takeoff.