Landing Gear Design:
Depending on an aircraft’s intended operation landing gear may be designed as either:
Fixed gear is designed to simplify design and operation
Creates constant drag, mitigated by the use of a cover called a fairing
A retractable gear is designed to streamline the airplane by allowing the landing gear to be stowed inside the structure during cruising flight The primary benefits of being able to retract the landing gear are increased climb performance and higher cruise airspeeds due to the resulting decrease in drag Retractable landing gear systems may be operated either hydraulically or electrically, or may employ a combination of the two systems Warning indicators are provided in the cockpit to show the pilot when the wheels are down and locked and when they are up and locked or if they are in intermediate positions Systems for emergency operation are also provided.
Limited to high performance aircraft
Operating Retractable Landing Gear:
The landing gear, if retractable, may function with either electrical or hydraulic power.
An electrical landing gear retraction system utilizes an electrically driven motor for gear operation When a switch in the cockpit is moved to the UP position, the electric motor operates Through a system of shafts, gears, adapters, an actuator screw, and a torque tube, a force is transmitted to the drag strut linkages.
The gear retracts and locks The struts that open and close the gear doors are also activated If the switch is moved to the DOWN position, the motor reverses and the gear moves down and locks Once activated the gear motor will continue to operate until an up or down limit switch on the motor’s gearbox is tripped.
A hydraulic landing gear retraction system utilizes pressurized hydraulic fluid to actuate linkages to raise and lower the gear When a switch in the cockpit is moved to the UP position, hydraulic fluid is directed into the gear up line The fluid flows through sequenced valves and down-locks to the gear actuating cylinders.
A similar process occurs during gear extension The pump which pressurizes the fluid in the system can be either engine driven or electrically powered If an electrically powered pump is used to pressurize the fluid, the system is referred to as an electro-hydraulic system The system also incorporates a hydraulic reservoir to contain excess fluid, and to provide a means of determining system fluid level Regardless of its power source, the hydraulic pump is designed to operate within a specific range When a sensor detects excessive pressure, a relief valve within the pump opens, and hydraulic pressure is routed back to the reservoir Another type of relief valve prevents excessive pressure that may result from thermal expansion Hydraulic pressure is also regulated by limit switches Each gear has two limit switches-one dedicated to extension and one dedicated to retraction These switches de-energize the hydraulic pump after the landing gear has completed its gear cycle In the event of limit switch failure, a backup pressure relief valve activates to relieve excess system pressure.