Landing Gear Types

A critical design element of any VTOL aircraft is the landing gear. We must answer three fundamental questions to determine the optimal solution:

• What terrain type is the aircraft expected to take-off and land from?
• Is it acceptable for the landing gear to cause drag during flight?
• Does the gear need to absorb a certain amount of shock during landing?

Once we have preliminary answers, we can consider the various landing gear types available on the market today: Wheels, skids, and legs.

1- Wheeled landing gear: Aircraft that land on concrete pads, runways, or solid flat surfaces may employ wheels in their landing gear design. This type of design splits into two major groups: static and retractable. Static wheels have been used for decades starting at the dawn of aviation with the Wright Brothers aircraft. Static wheels are inherently simple devices and have made quite a comeback recently with popular air-taxi companies like Archer and Jobi. Most electrical VTOL aircraft cannot develop high cruise velocity due to propeller limitations. Hence, there is no need for the more expensive retractable wheels as drag is of no concern. On the other hand, conventional passenger airliners require retractable wheel designs. This is because their cruise velocity is much higher than propeller-powered VTOL aircraft. Furthermore, the airliners are heavy aircraft, so the wheels are also equipped with shock absorption called Oleo struts. All wheel-based landing gears offer an additional layer of safety as the aircraft can glide to a soft landing even if all engines shut-down.

2- Skid-type gear: Not all aircraft require the ability to glide into a landing (especially non-winged VTOL aircraft). Landing skids are most famously used on Helicopters which are, in fact, the first type of VTOL aircraft to have ever been designed. The skids are the simplest type of landing gear and provide the ability to land on various terrain and for this reason are also very popular for drones. The simple design of the skids allows us to easily mount all sorts of equipment. Some examples of equipment are payload racks (as used for heli-skiing) as well as pontoons (for water take-off and landing).

3- Landing legs: They are the least famous of all landing gear types. Landing legs are exclusively found in space-based VTOL applications such as the NASA Lunar Lander and SpaceX Falcon 9 Booster. Unfortunately, we have not seen much implementation of the landing legs in modern VTOL aircraft. The primary concern is that during take-off, the vehicle may drift sideways due to unexpected ground effects and one of the legs may catch on something causing the vehicle to tip over. Landings legs with oversized pads are specialized in landing on very soft terrain like send or snow as they help to distribute the weight and prevent sinking. Unlike the skid-type gear, legs are more compact and are thereby easier to fold and retract into the body of the vehicle.

Unlike conventional aircraft which require a runway, VTOL aircraft have a truly diverse set of landing gear types. From wheels to skids to legs and everything in between, there are countless ways to support the weight of an aircraft on various terrain as well as absorbing any landing shock. It will be interesting to see what sort of configurations come into play in the near future. The hope is that the designers come up with both safe and aesthetically pleasing landing structures, especially when considering passenger aircraft.