Landing Gear — Page 100, Lesson 135

I want to walk you through the landing gear chapter, and we're starting with the fuselage-mounted undercarriage. We've already looked at wing-mounted gear, so now let's see what changes when the undercarriage is built into the fuselage itself. The requirements are basically the same as wing-mounted gear, but there are three key differences. First, with no geometric lock available, provision has to be made for locking the undercarriage up and down. Let me explain that — a geometric lock is a mechanical over-centre arrangement that holds the gear in position. On a wing-mounted unit you often get that naturally from the geometry of the folding mechanism. When the gear is buried in the fuselage, you don't have that, so you need a positive locking device for both the up and down positions. Second, depending on wheel layout, each wheel may require its own shock absorber unit, and possibly even a steering motor. So instead of one big shock absorber handling a pair of wheels, you might need individual units per wheel, and if that wheel needs to steer, it may need its own motor. Third, ease of access to the undercarriage in flight allows manual lowering of the undercarriage in emergency. Because the gear is inside the fuselage, the crew can physically get to it, so the emergency extension can be a manual operation rather than relying purely on gravity or hydraulics. Now let's talk about the loads the landing gear has to sustain. An undercarriage unit has to withstand varying loads during its life, and these loads are transmitted to the mountings in the aircraft structure, so those mountings must be very strong too. There are five distinct load types. Compressive loads, both static on the ground and on touchdown. Rearward bending. Side loads, during crosswind landings, take-offs, and taxiing. Forward loads, during push back. And torsional loads, during ground manoeuvring. Now we move to the nose undercarriage. A nose unit is usually a lighter structure than a main unit, since it carries less weight and is usually subject only to direct compression loads. But it does carry the attachment for the towing equipment, so it must withstand shear loads as well. Its design is complicated by several requirements: castoring, self-centring, steering, anti-shimmy, and withstanding shear loads. Let me define castoring, because it's the key concept here. To enable the aircraft to be manoeuvred about the airfield, the nose wheel must castor freely, though it's subjected to compression and shear loading — and that presents a problem in the bearing design. Castoring is the ability of the nose wheel to turn to either side in response to the results of differential braking or aerodynamic forces on the rudder. So think about that — when you brake one side harder than the other, or when the rudder pushes the tail around, the nose wheel is free to swivel and follow. That's what lets you manoeuvre on the ground. The challenge is that this free-swivelling wheel is also carrying compression and shear loads, so the bearing has to allow free rotation while still supporting those loads. That's the design problem. We'll pick up with self-centring and steering next.