All pilots botch landings. It comes with the territory. Thankfully, engineers know this and design landing gear with shock-absorbing properties to help us stay safe and save a bit of face. There are a few basic designs, including a series of bungee cords, a collection of flat pieces of steel that act as a spring, and the oleo strut.
The main structure of the oleo strut consists of a piston that’s attached to the wheel and a cylinder that’s attached to the airframe. Inside the tubes are what’s generally referred to as oil, but usually is hydraulic fluid, and compressed air or nitrogen. Between the two is a plate with a small hole, and a tapered pin that sticks through the hole.
The fluid and gas aren’t in equal proportions. When the aircraft is at rest on the ground, some amount of hydraulic fluid is above the plate to provide a small and quick amount of shock protection while taxiing.
On landing, the incompressible hydraulic fluid pushes on and compresses the air, creating a spring effect. To help dampen the spring effect, the hole and metered pin control the amount of fluid that is allowed to act on the gas. The metered pin is a simple but ingenious method of providing a large amount of spring initially, and then by gradually reducing the amount of fluid that contracts the air, reducing the spring as the weight on the wheels increases. Another small but significant design feature of the pin is a small bulb at the top. This keeps the piston from slamming out of the cylinder as it extends on takeoff.
Servicing the strut
Oleo struts require some regular maintenance, including occasional additions of air and hydraulic fluid. In most low-wing light singles, adding air is easily done through a filler port on the top of the wing. Adding fluid is a bit more complicated and involves some tubing, having the airplane on jacks, and sucking the fluid in a few times. The aircraft owner or operator can complete both of these tasks under the regulation for preventative maintenance.
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