gear. It increases with speed. On very light, slow aircraft, the  extra weight that accompanies a retractable landing gear is  more of a detriment than the drag caused by the fixed gear.  Lightweight fairings and wheel pants can be used to keep  drag to a minimum.  Figure 1-82  shows examples of fixed  and retractable gear.  Landing gear must be strong enough to withstand the forces  of landing when the aircraft is fully loaded. In addition to  strength, a major design goal is to have the gear assembly be  as light as possible. To accomplish this, landing gear are made  from a wide range of materials including steel, aluminum,  and magnesium. Wheels and tires are designed specifically  for aviation use and have unique operating characteristics.  Main wheel assemblies usually have a braking system. To  aid with the potentially high impact of landing, most landing  gear have a means of either absorbing shock or accepting  shock and distributing it so that the structure is not damaged.  Not all aircraft landing gear are configured with wheels.  Helicopters, for example, have such high maneuverability  and low landing speeds that a set of fixed skids is common  and quite functional with lower maintenance. The same is true  for free balloons which fly slowly and land on wood skids  affixed to the floor of the gondola. Other aircraft landing gear  are equipped with pontoons or floats for operation on water.  A large amount of drag accompanies this type of gear, but an  aircraft that can land and take off on water can be very useful  in certain environments. Even skis can be found under some  aircraft for operation on snow and ice.  Figure 1-83  shows  some of these alternative landing gear, the majority of which  are the fixed gear type. Amphibious aircraft are aircraft than can land either on land  or on water. On some aircraft designed for such dual usage,  the bottom half of the fuselage acts as a hull. Usually, it is  accompanied by outriggers on the underside of the wings  near the tips to aid in water landing and taxi. Main gear that  Figure 1-82.  Landing gear can be fixed (top) or retractable (bottom).  Figure 1-83.  Aircraft landing gear without wheels. 1-36