Definitions

Load factor is the ratio of the lift on an aircraft to the weight of the aircraft. The load factor is expressed in multiples of g where one g represents conditions in straight and level flight. In straight and level flight the lift is equal to the weight so the ratio of lift to weight is one, and the load factor is one g. Load factors greater than one, and less than one, are achieved by maneuvering of the aircraft by the pilot, and by atmospheric gusts.

In the definition of load factor, lift on an aircraft is not simply the lift generated by the wing. Lift on an aircraft is the vector sum of the lift generated by the wing and fuselage plus the lift generated by the tailplane which is almost always downwards. Lift on an aircraft is therefore almost always less than the lift generated by the wing and fuselage.

Load factor is the ratio of two forces so it is dimensionless. Flight with a load factor of one does not mean the aircraft is accelerating at 9.8 m/s2. It means the lift on the aircraft is the same as in straight and level flight.

The load factor may be positive or negative.

During straight and level flight the "right way up", the load factor is 1g. In a turn, and during some aerobatic maneuvers, the load factor is greater than 1g. Whenever the load factor is greater than zero the load factor is said to be positive.

In turning flight the load factor is greater than +1g. For example, the load factor is +2g in a turn with 60° angle of bank. In a balanced turn in which the angle of bank is $theta$ the load factor $n$ is related to the cosine of $theta$:

$n = frac \left\{1\right\}\left\{cos,theta\right\}$

If the aircraft is flown "upside-down" (inverted), or in aerobatic maneuvers in which the pilot pushes forward strongly on the elevator control, the lift on the aircraft acts in the opposite direction to normal. The load factor in these situations is negative.

Design standards

Excessive load factor must be avoided because of the possibility of exceeding the structural strength of the aircraft. The maximum load factors, both positive and negative, applicable to an aircraft are usually specified in the Pilots Operating Handbook or Flight Manual.

The maximum load factors for different classes of airplane are typically:

• In airline airplanes, from -1g to +2.5g
• In light airplanes, from -1.5g to +3.8g
• In acrobatic airplanes, from -3g to +6g

These limits are specified in airplane design standards such as Parts 23 and 25 of the US Federal Aviation Regulations.

Most helicopters are not capable of safe flight with a negative load factor so it is important for pilots of such helicopters to avoid maneuvers that will cause zero or negative load factors.

When the load factor is +1g, all occupants of the aircraft feel that their weight is normal. When the load factor is greater than +1g all occupants feel heavier than usual. For example, in a +2g maneuver all occupants feel that their weight is twice normal. When the load factor is zero, or very small, all occupants feel weightless. When the load factor is negative, all occupants feel they are upside down.

Human beings have limited ability to withstand a load factor significantly greater than 1g, both positive and negative. Unmanned aerial vehicles can be designed for much greater load factors, both positive and negative, than conventional aircraft because these vehicles can be used in maneuvers which would be incapacitating for a human pilot.

References

• Clancy, L.J. (1975), Aerodynamics, Pitman Publishing Limited, London ISBN 0 273 01120 0
• Hurt, H.H. (1960), Aerodynamics for Naval Aviators, A National Flightshop Reprint, Florida
• McCormick, Barnes W., (1979), Aerodynamics, Aeronautics and Flight Mechanics, John Wiley & Sons, New York ISBN 0-471-03032-5