Exploring the Four Forces of Flight

The Four ForcesAn aircraft in straight and level flight is

acted upon by four forces: lift, gravity, thrust, and drag.

How the forces work togetherThe opposing forces balance each other;

lift equals gravity and thrust equals drag. Any inequality between thrust and drag, while maintaining straight and level flight, will result in acceleration or deceleration until the two forces again become balanced.

What is lift?Lift is the shape of the wings and how air

flows over and under them. The wing is designed so that the top surface is "longer" than the bottom surface in any given cross-section.

How does lift work?The air moving over the wing must travel from

A to B in the same amount of time. Therefore, the air is moving faster along the top of the wing.

This creates a difference in air pressure above and below—a phenomenon called the Bernoulli effect. The pressure pushing up is greater than the downward pressure, and lift is created.

What is gravity?Gravity is actually a force of acceleration

on an object. The Earth exerts this natural force on all objects. Being a constant force, it always acts in the same direction: downward.

How does gravity work with the other forces in order to create lift?Thrust creates lift to counteract gravity. In order for an aircraft to take off, enough lift must be created to overcome the force of gravity pushing down on the aircraft.

What is thrust?Thrust is the force which moves an aircraft

through the air. Thrust is generated by the engines of the aircraft through some kind of propulsion system.

Thrust is a mechanical force, so the propulsion system must be in physical contact with a working fluid to produce thrust. Thrust is generated most often through the reaction of accelerating a mass of gas.

How is thrust used during flight?Thrust is used to overcome the drag of an

airplane, and to overcome the weight of a rocket.

What is drag?Drag is a force exerted on an object moving

through a fluid; it is always oriented in the direction of relative fluid flow (try running against a high wind and you'll feel drag pushing you back in the direction of relative fluid flow).

Why does drag occur?Drag occurs because the fluid and the

object exchange momentum when impacting, creating a force opposing the motion of the object.

*Note: Trying to walk in a strong wind will demonstrate drag for you. A dropped weight falls faster through air than through honey largely because of drag forces.

Recap of the four forces of flight

•Lift: the shape of the wings and how air flows over and under them. •Gravity: a force of acceleration on an object.•Thrust: the force which moves an aircraft through the air.•Drag: a force exerted on an object moving through a fluid.