Wing Bending Relief
In fixed-wing aircraft, fuel is usually carried in the wings. Weight in the wings does not contribute as significantly to the bending moment in the wing as does weight in the fuselage. This is because the lift on the wings and the weight of the fuselage bend the wing tips upwards and the wing roots downwards; but the weight of the wing, including the weight of fuel in the wing, bend the wing tips downwards, providing relief to the bending effect on the wing.
When an airplane is being loaded, the capacity for extra weight in the wing is greater than the capacity for extra weight in the fuselage. Designers of airplanes can optimise the maximum takeoff weight and prevent overloading in the fuselage by specifying a MZFW. This is usually done for large airplanes.
Most small airplanes do not have a MZFW specified among their limitations. For these airplanes, the loading case that must be considered when determining the maximum takeoff weight is the airplane with zero fuel and all disposable load in the fuselage. With zero fuel in the wing the only wing bending relief is due to the weight of the wing.
Read more about this topic: Zero-fuel Weight
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—Charles Dickens (1812–1870)
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Which, like unruly children, make their sire
Stoop with oppression of their prodigal weight.
Give some supportance to the bending twigs.
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Cut off the heads of too-fast-growing sprays
That look too lofty in our commonwealth.
All must be even in our government.
You thus employed, I will go root away
The noisome weeds which without profit suck
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—William Shakespeare (1564–1616)
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—For the State of Florida, U.S. public relief program (1935-1943)