Limitations

In service an aircraft must observe certain speed limitations. These may be maximum speeds or minimum speeds, but in each case they are set to give safe operation in the prevailing conditions. The limits may be set by various considerations, the main ones being:

•strength of the aircraft structure.

•stiffness of the aircraft structure.

•adequate control of the aircraft.

Strength is the ability of the structure to withstand a load, and stiffness is the ability to withstand deformation.

Loads and Safety Factors

•Limit load: The maximum load to be expected in service.

•Ultimate load: The failing load of the structure.

•Factor of safety: The ratio of ultimate load to limit load.

For aircraft structures the factor of safety is 1.5.

The safety factor on aircraft structures is much lower than the safety factors used in other forms of engineering because of the extreme importance of minimum weight in aircraft structures. To keep the weight as low as possible, the safety factor must be kept to a minimum. Because of this it is extremely important not to exceed the limitations set on the operation of the aircraft, as the safety margin can easily be exceeded and structural damage may occur.

Loads on the Structure

The airframe structure must obviously be strong enough to take the loads acting upon it in normal level flight, that is the forces due to lift, drag, thrust and weight. However, the aircraft is also required to manoeuvre and to fly in turbulent air. Under these conditions the loads on the aircraft will be increased, so it must also be strong enough to withstand whatever manoeuvres are specified for the aircraft and the gusts which are required to be considered.

The structure should also have sufficient stiffness to ensure that phenomena such as aileron reversal, flutter and divergence do not occur within the permitted speed range of the aircraft.

Limitations 14