The 4 Forces
NOTE: This document is for information only and refers to UK procedures only. Not to be taken as an authority.

Introduction

In aerodynamics, there are four main forces which act upon an aeroplane in flight. These are:

• Lift
• Weight
• Thrust
• Drag

We will study each of these in turn and then see how they act together in flight.

Weight

Weight is the force of gravity which acts upon a mass.

It always acts vertically downwards regardless of the attitude of the aircraft. It can be said to act through the Centre of Gravity (CG) of the mass.

Lift

Lift is the force acting at 90 degrees to the relative airflow as a result of the air flowing over an aerofoil. Lift can be said to act through the Centre of Pressure (CP) of the aerofoil.

In fact the lift is only part of the total reaction produced by the airflow over the aerofoil. We can draw the above diagram again showing the Total Reaction produced by the action of the airflow.

This total reaction can then be resolved into two vectors – one at 90 degrees to the airflow known as Lift and one in line with the airflow which we will see later is known as induced drag.

As the aeroplane attitude changes, so too does the direction of the total reaction and the lift.

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Angle of Attack

The angle between the chord line and the relative airflow is known as the angle of attack (AoA).

The amount of Lift produced depends on a number of variables:

• The cross-section shape of the airfoil
• The angle of attack (AoA)
• The area of the wing
• The density of the air
• The speed of flight through the air

The Coefficient of Lift is a unit-less number which depends on both the cross-sectional shape of the airfoil and on the angle of attack, and is an indication of the relative amount of lift being produced by a wing. This leads us to the formula for calculation of lift as shown:

where the Coefficient of Lift is CL, the air density ρ, the True Airspeed v and the area of the wing S.

In general, we find that as the angle of attack is increased, then so to is the co-efficient of lift, and so also the amount of lift produced. However this is only true up to a point. We cannot go on increasing the angle of attack indefinitely and expect to achieve more and more lift. Eventually, there will come an angle of attack, beyond which, lift no longer increases, and in fact decreases sharply (known as a stall).

Graph of Angle of Attack versus Lift

a wing has a maximum co-efficient of lift at the stall

This angle of attack is known as the critical angle or stalling angle of attack. Once this angle is reached, the airflow over the wing ceases to be streamlined and becomes turbulent. This is known as the stall. The wing will stall when the angle of attack reaches the critical or stalling angle. For most wings, the critical angle of attack is around 16 degrees.

Drag

Drag is the force which opposes flight and comes from a number of sources. There are two main types of drag

• Parasite Drag – this in turn divided into skin friction, form drag and interference drag
• Induced drag

Parasite Drag

As stated above, this kind of drag is sub-divided into 3 types:

• Skin friction is caused by the roughness of the surface of the object. A shiny polished surface will reduce skin friction, and accumulations of ice will increase it.
• Form Drag is caused as a result of the shape of an object

• Interference Drag is caused by turbulent airflow at the junction of various aircraft components such as where the wing meets the fuselage. It can be reduced by filleting and fairing.

Parasite drag increases as the airspeed increases

Induced Drag

We saw in the section on lift how induced drag is formed as a by-product of lift. Whenever we create lift, we also create unwanted induced drag.

So as airspeed increases, induced drag decreases

Total Drag

The total drag at any speed is just the sum of parasite and induced drag.

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16Jun06 ©T&FA