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Aviation Words — Load Factor

By Ian Brown, Editor

September 2019 - This phrase has always been a bit obscure to me, but those of you with a more mathematical bent might think, "Well, it's obvious, isn't it?"

Firstly, load factor is a ratio, so there are no units to consider, even though it is often expressed as g. Secondly, it's simply the ratio of the lift provided by the lifting surfaces divided by the total weight of the aircraft.

In straight and level flight, the load factor is 1. In other words, the lift being provided by the lifting surfaces (and the aircraft's attitude) is just sufficient to keep the aircraft from descending. The load factor is an overall ratio, not a per-square-foot thing, although one assumes that aircraft designers have to understand local stresses and lifting forces at different points on the lifting surface.

Things get a bit more complicated when the aircraft is turning. For example, when on a 60-degree banking turn, the load factor is doubled, one of the reasons to avoid high-speed turns. If you decide to do that banked turn inverted, the load factor becomes -2. In other words, the loads on the lifting surfaces are in the opposite direction on the wings and empennage.

A typical load factor limit for normal category aircraft might be -1.5 to 3.8, but the Sukhoi Su-26 aerobatic family has a load factor of -10 to +12. So if you've ever wanted to test the limits of your favourite ride, try doing an inverted 60-degree banked turn and see if the wings remain attached.

Seriously though, do you know the load factors for your aircraft? I just looked it up for my RV-9A, one of the few Van's Aircraft designs that is non-aerobatic. The others are the RV-12 and the RV-10. At gross weight, my design has load factor limits of -1.5 to +3.8. Since aircraft have a weight challenge — build it too strong and it'll never get off the ground — the margins applied to aircraft design safety might be a lot tighter than say, furniture design.

Load factors, as a ratio of lift to weight, can vary with weight, so an aircraft 200 pounds under gross can expect an added safety margin over the designed load factor limits, and many pilot operator handbooks list values for load factor limits at some standard gross weight and a higher set of limits at a "utility" category at some lower gross weight.

A friend recently had a wing buckle in turbulent air, so sometimes those margins might be very marginal. Maybe we can get him to write an article about that in a future issue.

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