Lift and Drag

The Four Forces

Weight and lift

Have you ever wondered about the science behind flight? We all know that gravity is a force that pulls everything towards the Earth’s surface. This pull is called the weight force.
Planes and birds have to be able to provide enough lift force to oppose the weight force. Lift is caused by the variation in air pressure when air flows under and over an airplane's wings. It acts upwards against weight and must be greater in order for the aircraft to fly. 

Propulsion - Thrust and drag

​The power source of a bird or plane provides the thrust. Thrust is the force that moves the object forward. Thrust is provided by:

• muscles – for birds and other flying animals
• engines – for flying machines
• gravity – for gliders that actually fly by always diving at a very shallow angle (birds do this too when they glide).

The force working against thrust is called drag. It is caused by air resistance and acts in the opposite direction to the motion. The amount of drag depends on the shape of the object, thedensity of the air and the speed of the object. Thrust can overcome or counteract the force of drag.

An object in flight is constantly engaging in a tug of war between the opposing forces of lift, weight (gravity), thrust and drag. Flight depends on these forces – whether the lift force is greater than the weight force and whether thrust is greater than drag (friction) forces.

Lift and drag are considered aerodynamic forces because they exist due to the movement of an object (such as a plane) through the air. The weight pulls down on the plane opposing the lift created by air flowing over the wing. Thrust is generated by the propeller (engine) and opposes drag caused by air resistance. During take-off, thrust must counteract drag and lift must counteract the weight before the plane can become airborne.

How it works

If a plane or bird flies straight at a constant speed:

• lift force upwards = weight force downwards (so the plane/bird stays at a constant height)
• thrust force forwards = opposing force of drag (so the plane/bird stays at a constant speed)

If the forces are not equal or balanced, the object will speed up, slow down or change direction towards the greatest force.

Newton's Third Law

For example, if a plane’s engine produces more thrust, it will accelerate. The acceleration increases air speed past the wing, which increases lift so the plane gains altitude. Then, because the plane is moving faster, drag (air resistance) is increased, which slows the plane from speeding up as quickly until thrust and drag are equal again. The plane can now remain at a constant but greater height.
A plane can lose altitude by reducing thrust. Drag becomes greater than thrust and the plane slows down. This reduces lift and the plane descends.

Airplane wings are designed to take advantage of lift. They are shaped so that air has to travel farther over the top of the wing than underneath it. The reason for this is explained in Bernoulli's Principle, which states that an increase in the velocity (speed) of air or any fluid results in a decrease in pressure. When the air has to travel farther over the top of the airplane wing, it must also travel faster, which results in lower pressure. The shorter distance under the wings results in higher pressure, causing the airplane to move upward.

​You can demonstrate Bernoulli's Principle with a piece of notebook paper. Fold the paper in half the short way, so that you have a tent shape. Now, set the tent on a table and blow very carefully (slow and firm) through one of the open ends. The sides of the tent will stick together but the tent won't collapse. This occurs because the velocity of your breath is more than that of the air outside of the tent, causing lower pressure. The air outside the tent has higher pressure and pushes the sides of the tent inward.

Bernoulli's Principle

​Lift vs. Relative Wind

Ground Effect

• Reduction of induced drag during takeoffs and landings
• Caused by a reduction of wingtip vortices
• Occurs at about a wingspan above the ground
• Up-wash and Down-wash decrease
• Down-wash can hit the ground and pushes the wing from below, forming what feels like a cushion
• Causes floating if a fast approach is flown
• More noticeable in a low-wing aircraft

​
Ground Effect:

• Increases lift while decreasing drag (induced), thrust required
• The opposite is true when leaving ground effect

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