Landing Gear — Page 94, Lesson 132

I want to walk you through the landing gear chapter now, starting with retractable landing gear. This is a big topic, so let's build it up properly. First, the fundamental idea. The majority of modern transport aircraft, and an increasing number of light aircraft, are fitted with a retractable landing gear. The whole purpose of retraction is to improve aircraft performance — once you're airborne, you don't want those wheels and struts hanging out in the airflow creating drag, so you tuck them away. Now, how does retraction actually happen? It's normally effected by a hydraulic system, but pneumatic or electrical systems are also used. So hydraulic is the standard, but you'll see the other two as alternatives. Here's an important nuance: in some instances, power is used for retraction only, and extension is effected by gravity and slipstream. That means the gear comes up under power, but to bring it down you just release the locks and let gravity and the airflow push it into place. Because this is a safety-critical system, retractable gear is also provided with three essential things. First, mechanical locks to ensure each undercarriage is locked securely in both the retracted and extended positions. Second, devices to indicate to the crew the position of each undercarriage — you need to know where that gear is. And third, means by which the landing gear can be extended in the event of failure of the power source. That's your emergency extension system. There are also two protective measures built in. Means are provided to prevent retraction with the aircraft on the ground — you don't want someone accidentally pulling the gear up while still on the runway. And there's a guard against landing with the landing gear retracted — a warning system. Finally, the undercarriage wells are normally sealed by doors for aerodynamic reasons, so when the gear is stowed, the bay is closed off to keep the airflow clean. Now let's talk about design and construction. The geometrical arrangement and physical location of undercarriage units on aircraft is by no means standard. The type, size and position are decided at the design stage, having already taken into account the many factors that must be considered. There are two main types of landing gear. Nose wheel, which are often referred to as tricycle, and tail wheel aircraft, which are also called tail draggers. Most aircraft use the tricycle layout, where the two main undercarriage units are positioned just aft of the C of G — that's the centre of gravity — and they support up to 90% of the aircraft's weight and all initial landing shocks. So the mains take the brunt of the landing impact. The nose wheel unit keeps the aircraft level, and in most cases also provides a means of steering. Now, the tricycle gear has two clear advantages over the tail dragger type. First, there is no danger of it tipping over onto its nose while taxiing in a strong tail wind. Second, there is much less danger of it ground looping — that's when the aircraft swings around uncontrollably on the ground. Finally, let's look at the factors affecting design and construction. Of the many factors taken into consideration, the main ones are: size of aircraft, weight of aircraft, role of aircraft, high or low wing, performance, and construction of aircraft and associated stowage problems. So when the designer chooses the gear, they're balancing all of those — how big and heavy the machine is, what job it does, where the wing sits, how fast it goes, and how the gear physically fits and stows within the airframe. That's the foundation of retractable landing gear. We'll build on this as we go deeper into the struts and the actual mechanisms.