Fuselage, Wings and Stabilizing Surfaces — Page 19, Lesson 27

We’re starting the wing section now — the mainplanes. The wings support the entire weight of the aircraft in the air, so they must have sufficient strength and stiffness to do that job. And here’s the key relationship: that strength and stiffness are determined by the thickness of the wing. The thickness, and the type of construction used, both depend on the speed requirements of the aircraft. So the faster you want to fly, the more the construction has to change. There are three types of construction I want you to know: the biplane, the braced monoplane, and the cantilever monoplane. Let’s start with the biplane. Very few biplanes fly at more than 200 knots in level flight. Because the air loads are low at those speeds, a truss-type design covered in fabric is satisfactory. The wing spars, the interplane struts, and the bracing wires together form a lattice girder of great rigidity. That lattice girder is highly resistant to both bending and twisting. So the biplane gets its strength from that triangulated framework of spars, struts, and wires, not from a thick wing. Next, the braced monoplane. This type of design is also used on low-speed aircraft. So it shares that low-speed application with the biplane, but it’s a single wing, braced externally. Then we come to the cantilever monoplane. This is the important one for modern transport aircraft. The mainplanes have to absorb the stresses due to lift and drag in flight. And if the wing is of cantilever design, it also has to absorb its own weight when the aircraft is on the ground. That’s a critical point — a cantilever wing has no external bracing, so it carries everything itself. This is achieved by building the wing around one or more main load-bearing members known as spars. These spars are constructed so that they absorb the downwards bending stresses when the aircraft is on the ground. But here’s the contrast: when the aircraft is in flight, the wing not only has to have the flexibility to bend upwards, it also needs enough stiffness to resist the torsional loads which will cause twisting. So the spar has to handle downward bending on the ground, upward bending in flight, and twisting — all at once. There’s also something called bending stress relief. This is provided in a few ways: by using ‘Aileron Up-float’, by mounting the engines on the wing, and by positioning the major fuel tanks within the wing. And here’s the operational detail — during flight, the fuel in the wing tanks is the last to be used. That’s deliberate: keeping that fuel in the wing helps relieve the bending stress while you’re airborne. So to tie it together: biplane for low speed with a truss and fabric, braced monoplane also low speed, and the cantilever monoplane where the spar system takes all the loads — bending on the ground, bending and torsion in flight — with fuel and engines positioned to help relieve that bending stress.