Overview and Definitions

Overview

The primary requirements of an aircraft are as follows:

•a wing to generate a lift force;

•a fuselage to house the payload;

•tail surfaces to add stability;

•control surfaces to change the direction of flight; and

•engines to make it go forward.

The process of lift generation is fairly straightforward and easy to understand. Over the years aircraft designers, aerodynamicists and structural engineers have refined the basics and, by subtle changes of shape and configuration, have made maximum use of the current understanding of the physical properties of air to produce aircraft best suited to a particular role.

Aircraft come in different shapes and sizes, each usually designed for a specific task. All aircraft share certain features, but to obtain the performance required by the operator, the designer will configure each type of aeroplane in a specific way.

As can be seen from the illustrations on the facing page, the position of the features shared by all types of aircraft i.e. wings, fuselage, tail surfaces and engines varies from type to type.

Why are wing plan shapes different?

Why are wings mounted sometimes on top of the fuselage instead of the bottom?

Why are wings mounted in that position and at that angle?

Why is the horizontal stabilizer mounted sometimes high on top of the fin rather than on either side of the rear fuselage?

Every feature has a purpose and is never included merely for reasons of style.

Overview and Definitions 1

Definitions and Overview 1

An aeroplane, like all bodies, has mass. With the aircraft stationary on the ground it has only the force due to the acceleration of gravity acting upon it. This force, its WEIGHT, acts vertically downward at all times.

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Figure 1.2 The force of weight

Before an aeroplane can leave the ground and fly, the force of weight must be balanced by a force which acts upwards. This force is called LIFT. The lift force must be increased until it is the same as the aeroplane’s weight.

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Figure 1.3 The forces of weight & lift

To generate a lift force, the aeroplane must be propelled forward through the air by a force called THRUST, provided by the engine(s).

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Figure 1.4 The forces of weight, lift & thrust

From the very moment the aeroplane begins to move, air resists its forward motion with a force called DRAG.

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Figure 1.5 The forces of weight, lift, thrust & drag

Overview and Definitions 1

Definitions and Overview 1

When an aeroplane is moving there are four main forces acting upon it:

WEIGHT, LIFT, THRUST and DRAG.

These are all closely interrelated, i.e.:

The greater the weight - the greater the lift requirement.

The greater the lift - the greater the drag.

The greater the drag - the greater the thrust required, and so on ...

Air has properties which change with altitude. Knowledge of these variables, together with their effect on an aeroplane, is a prerequisite for a full understanding of the principles of flight.

The structural and aerodynamic design of an aeroplane is a masterpiece of compromise. An improvement in one area frequently leads to a loss of efficiency in another.

An aeroplane does not ‘grip’ the air as a car does the road. An aeroplane is often not pointing in the same direction in which it is moving.