Landing Gear — Page 94, Lesson 134

I want to walk you through the landing gear chapter now, and we're starting with the big-picture design factors. Let's look at why modern aircraft put their main undercarriage where they do. The driving force here is cost and multi-role capability. Aircraft designers today are heavily influenced by the need to keep costs down while making the aircraft do many jobs. One of those jobs is carrying both freight and passengers. That dual role pushed manufacturers toward the high-wing monoplane design, because with a high wing, the floor of the aircraft sits very close to the ground. And that's exactly what you want for freight loading — you can roll cargo straight in without heavy lifting equipment. But here's the problem. Some of these wings are as high as 20 feet off the ground. If you tried to build an undercarriage that reached from the ground all the way up to that wing, you'd need a leg of enormous length and strength. It became impossible to build an undercarriage of sufficient strength to reach that far. So the modern trend is to incorporate the main undercarriage in the fuselage instead — the body of the aircraft — rather than hanging it off the wing. Now, for aircraft that keep the standard underwing fitted undercarriage — the conventional arrangement where the gear hangs under the wing — the units comprise basically five things. Let me walk you through each one. First, a leg, pin-jointed to the aircraft structure. Pin-jointed means it's attached with a pin that allows the leg to pivot, so it can swing up and down as it retracts and extends. Second, a wheel or wheels. Third, a means of absorbing landing shocks. That's the shock absorption system that takes the impact energy when you touch down. Fourth, a means of controlling deceleration of the aircraft. That's your braking system — the way you slow the aircraft down after landing. And fifth, a means to withstand turning and braking stresses. The gear has to handle the twisting forces from steering and the heavy loads from braking without failing. There's one more fascinating point, and it's about very large aircraft like the Boeing 747. These have the ability to turn part of the main gear to assist with steering during tight turns. The reason is to reduce the turning radius — the tighter the turn, the less space you need on the ground. Here's the clever part: when the nose wheels are turned, the main wheels turn in the opposite direction. So the nose gear steers one way, and part of the main gear steers the other way, and together they let that huge aircraft pivot around a much tighter circle. Now, before we go further into the shock absorption systems, I want you to see the actual hardware. Let me bring up Figure 3.1, which shows an oleo-pneumatic strut — that's the classic shock absorber leg you'll find on most aircraft. Take a look at it now. That strut is the heart of the shock absorption system, and we'll dig into exactly how it works next.