Our take on suspension
A good suspension system maximizes the contact between the tires and the road to provide steering stability and good handling. A suspension system is a protective lattice of shock-absorbing components such as springs and dampers. It helps ensure a smooth ride by absorbing the energy from various bumps and other kinetic impacts, like landings.
Suspension on ultralight aircraft is a system of tires, tire air, springs, shock absorbers and linkages that connects the craft to its wheels or skids and allows relative motion between the two. The best suspension systems must be light weight and support ground handling, and ride quality, all of which are at odds with each other.
The design of suspensions involves finding the right compromise. It is important for the suspension to keep wheels in contact with the surface as much as possible, because all the road or ground forces acting on the vehicle do so through the contact patches of the tires.
Rigid struts were the original type of landing gear. The idea was simple: weld the wheels to the airframe. The problem is the imperfect landing; a hard touchdown means that strong shock loads transfer directly and immediately into the outboard supports and airframe.
The Weedhopper is typical rigid strut gear. Large air-filled tires and airframe flex are its only suspension. There is a distinct advantage, the structure is well supported as the outboard support struts are quite a ways from the center line, in addition the outside wheel axle is used for the lower wing strut mount. This angle advantage allows for a much more ridged wing.
The Weedhopper is typical rigid strut gear. Large air-filled tires and airframe flex are its only suspension. There is a distinct advantage, the structure is well supported as the outboard support struts are quite a ways from the center line, in addition the outside wheel axle is used for the lower wing strut mount. This angle advantage allows for a much more ridged wing.
Inflatable tires Soften impact loads and are used on most all ultralights. Large diameter wheels help soften surface irregularities.
A-Arm or Wishbone shaped arms locate the wheel. Each wishbone or arm has two mounting points to the chassis and one joint for the wheel. Some sort of resistive force, bungees or a spring member are used to control suspension.
Culver Engineering ‘Backyard Flyer is a perfect example of a-arm or wishbone suspension. In this case they use a bent tube for the resistive force.
Spring struts are one of the most common landing systems on ultralights. They are effective, can be made lightweight and are strong. Flexible materials like steel, aluminum, or composites are used in their construction. Upon landing the spring flexes upward, which spreads the airframe impact out over time. However, they do not allow for any outboard support structure.
Troy Woodlands ‘Ruckus’ is a great example of spring struts.
Bungee cords are a series of elastic cords wrapped between the airframe and a flexible gear system, allowing the gear to transfer impact load to the aircraft at a rate that doesn’t hurt the plane. Rubber cushions also fall into this category.
Typical bungee system on a Sopwith Camel which the wheel axle or swing arm, picots off the lateral support member. It is allowed to move several inches between the outboard supports. The movement is controlled by the tension and amount of bungee and the stop. Many WWI craft used similar systems.
Shock struts use two telescoping hydraulic cylinders in the Oleo design, but these are not common on ultralights. Generally, on ultralight versions, the top cylinder is attached to the craft while the bottom cylinder is attached to a wheel. The bottom cylinder, typically called the piston, slides inside the upper cylinder against a resisting force, springs, bungees, etc. Wheel induced strut torsion is a huge design consideration.
The Bleriot uses a bungees on the outboard support tubes, but as one tube slides inside of another it is probably closer to a shock strut than a bungee system.
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