Trunnion bearing surface Retracting mechanism Drag strut Hinge point Manual control torque tube Manual control gearbox Gearbox Retracting mechanism Universal joints Landing gear motor Drag strut Trunnion support Shock strut Drag strut 13-18 Figure 13-34.  geared electric motor landing gear retraction system. Figure 13-33.  hinged drag strut holds the trunnion and gear firm  for landing and ground operation. It folds at the hinge to allow the  gear to retract.  motor is converted to linear motion to actuate the gear. This  is possible only with the relatively lightweight gear found  on smaller aircraft. An all-electric gear retraction system is  illustrated in  Figure 13-34 .  A more common use of electricity in gear retraction systems  is that of an electric/hydraulic system found in many Cessna  and Piper aircraft. This is also known as a power pack system.  A small lightweight hydraulic power pack contains several  components required in a hydraulic system. These include the  reservoir, a reversible electric motor-driven hydraulic pump,  a filter, high-and-low pressure control valves, a thermal relief  valve, and a shuttle valve. Some power packs incorporate an  emergency hand pump. A hydraulic actuator for each gear is  driven to extend or retract the gear by fluid from the power  pack.  Figure 13-35  illustrates a power pack system while  gear is being lowered.  Figure 13-36  shows the same system  while the gear is being raised. When the flight deck gear selection handle is put in the gear- down position, a switch is made that turns on the electric  motor in the power pack. The motor turns in the direction to  rotate the hydraulic gear pump so that it pumps fluid to the  gear-down side of the actuating cylinders. Pump pressure  moves the spring-loaded shuttle valve to the left to allow  fluid to reach all three actuators. Restrictors are used in the  nose wheel actuator inlet and outlet ports to slow down the  motion of this lighter gear. While hydraulic fluid is pumped to  extend the gear, fluid from the upside of the actuators returns  to the reservoir through the gear-up check valve. When the  gear reach the down and locked position, pressure builds  in the gear-down line from the pump and the low-pressure  control valve unseats to return the fluid to the reservoir.  Electric limit switches turn off the pump when all three gears  are down and locked. As the speed of a light aircraft increases, there reaches a point  where the parasite drag created by the landing gear in the  wind is greater than the induced drag caused by the added  weight of a retractable landing gear system. Thus, many  light aircraft have retractable landing gear. There are many  unique designs. The simplest contains a lever in the flight  deck mechanically linked to the gear. Through mechanical  advantage, the pilot extends and retracts  the landing gear by  operating the lever. Use of a roller  chain, sprockets, and a  hand crank to decrease the required force is common.  Electrically operated landing gear systems are also found on  light aircraft. An all-electric system uses an electric motor  and gear reduction to move the gear. The  rotary motion of the  Small Aircraft Retraction Systems