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What is flight? People have different ideas about what flies. Our resources explore winged flight through birds, planes, gliders, hang-gliders and kites.

This resource provides explanations of the key terms encountered when exploring flight. These are terms that should be emphasised to help give every student a good understanding as they explore winged flight through birds, planes, gliders, hang-gliders and kites.

## Aerofoil

An aerofoil is a structure designed to create lift from its motion through the air. It is usually related to wing shape and has a carefully contoured cross-section to produce lift for flight. It can be symmetrical or non-symmetrical. The leading edge of the wing is curved, and it narrows to a sharp (trailing) edge.

## Air pressure

Air pressure is the force per unit area exerted by air, whether compressed or unconfined, on any surface in contact with the air.

## Angle of attack

In aerodynamics, angle of attack is used to describe the angle at which a wing meets the airflow. If the wing is angled correctly, the air is forced downwards, resulting in an equal but opposite upwards force (lift) on the wing.

## Bernoulli principle

This states that, as the velocity of a fluid (liquid or gas) increases, the pressure exerted by that fluid decreases. In other words, the faster a liquid or gas moves, the less pressure it exerts. The principle was named after Daniel Bernoulli, a Swiss mathematician.

## Drag

Forces that oppose the relative motion of an object through a fluid (liquid or gas) are known as drag. If the fluid is air, drag is also referred to as air resistance.

## Force

Force can be described as a push or a pull in a particular direction. If an object is pushed or pulled by unbalanced forces, it will accelerate (move faster and faster) in the direction of the unbalanced force. The amount of acceleration depends on the size of the mass and the size of the force. The relationship between force (F), mass (m) and acceleration (a) can be shown in the equation: force = mass x acceleration (F = ma). The forces that are associated with this context are weight (the force of gravity pulling an object to Earth), lift, thrust and drag.

## Friction

A force that resists the relative motion between two objects in contact with each other. Smoother surfaces exhibit less friction than rougher surfaces. Friction can be helpful (such as grip for feet while walking) or a hindrance (such as cycling into a head wind).

## Lift

In aerodynamics, lift is produced as air flows over an aerofoil (wing). When the lift force is greater than the weight force, the aerofoil rises up.

## Thrust

Thrust is the reaction force that moves an object forwards, for example, the propeller on an aeroplane pushes air backwards. The reaction to this is a force (thrust) that, if big enough, pushes the aeroplane forwards.

## Weight

Weight is the force of gravity acting on a mass (the amount of matter in an object). To become airborne, the weight force of an aeroplane needs to be less than the lift force.

Find more information on flight in this introductory article.

Published 9 September 2011 Referencing Hub articles