Complex & Robust Flight International F-35 Special 2014
"Mark Ayton explains the highly complex landing gear systems used on the F-35...
......Patented by Carpenter Steel, Aermet 100 has very high strength and slow crack propagation properties, so if a crack develops in the material, the crack will spread slowly with further load applications. By contrast 300M or 4340M grade steel has the same strength quality, but poor crack propagation. This gives more opportunities to discover cracks in the structure before a catastrophic failure occurs.
Each type of F-35 landing gear has a Goodrich-proprietary system integrated within the aircraft’s maintenance system to help the maintainer assess the level of the gas and oil in each shock strut during servicing....
...Cats and Traps
Landing gears for the F-35C CV variant have to be able to withstand extreme high energy landings typical of naval aircraft operating from an aircraft carrier as well as the nose tow launch. Both the F-35C nose and main gears are made primarily of Aermet 100 steel.
The nose gear of the CV variant is a dual stage gas over oil cantilever strut with a staged air curve that provides a source of high energy, which helps the aircraft to achieve adequate angle of attack when released from the catapult during take-off from the aircraft carrier. The CV nose gear carries a complex mechanism which positions the launch bar in readiness for various stages of operation during the launch of the aircraft off the carrier. The mechanism is driven by a power unit comprising a number of powerful springs and a small internal actuator.
There are two reasons for having a staged shock strut for the nose gear on the F-35C CV variant. One is to provide a stable platform for loading and unloading weapons and for engaging the catapult equipment. The second is to store energy gained from the compression of the strut under the high pressure effect of the catapult. When the catapult lets go of the launch bar, the energy is released, providing a rotation that helps achieve the angle of attack necessary to get off the deck.
Similarly when the aircraft hits the deck on landing the strut is compressed and energy is stored to help rotate the aeroplane and get it back off the deck if the arrestor cables are missed and a ‘go-around’ or ‘bolter’ is required. Bolter is the term used when the aircraft’s tail hook misses the arrestor cables on the carrier deck forcing the pilot to go around for another landing.
The CV nose gear also has a locking drag brace and a launch bar that acts to transmit the high launch load from the catapult equipment to the airframe. A separate retract actuator provides the force to retract the gear into the wheel well. One end of the retract actuator is attached to the landing gear structure and the upper end to the airframe structure. Fitted to the aft of the strut is a power unit housing an actuator that hydraulically lowers the launch bar to the deck to engage the catapult. When the launch bar hits the deck a second set of springs inside the power unit provide lighter power so that the launch bar can move up and down to engage the shuttle, without jamming or binding, or badly wearing the deck or the launch bar. Large powerful springs are able to pull the launch bar back up to an intermediate position when the hydraulic power is released.
The power unit also has a linkage that operates off the motion of the drag brace during retraction to position the launch bar in a stowed position (virtually parallel to the strut) when the gear is retracted. During the retraction process the launch bar moves upwards but also rotates around the strut to reduce the actual footprint within the stowage bay. The torque arms that typically maintain alignment between the strut piston and the steering unit are on the aft of the strut as well, and have a fitting at the apex that engages the repeatable release holdback bar (RRHB) of the ship. This bar holds the aircraft back during engine runs and while the load builds during the start of a catapult sequence. Once the load reaches an adequate level, the RRHB releases the torque arm fitting, allowing the aircraft to be catapulted to flight. In comparison to the F-35A CTOL and the F-35B STOVL, the nose gear of the F-35C CV has a dual wheel/tyre arrangement to straddle the catapult equipment and to adequately react to the loads. Nose wheels are the same as those used on the other variants but the tyre was developed specifically for the F-35C.
Like the CTOL and STOVL variants, the CV main gear is a dual stage gas over oil cantilever strut with staged air curves that provide a stable platform for loading and unloading weapons and hold stored energy to assist in getting airborne in the case of a ‘bolter’ during carrier operations.
The main gears have a retract actuator between the strut and the airframe, providing the force to retract the gear into the wheel well. Each also has a drag brace with locking linkage and locking actuator with backup springs to react fore and aft ground loads. The F-35C’s drag braces attach to a collar on the strut and a pivot pin in the aircraft that roll around the strut centreline during retraction to minimize the amount of space in the bay when retracted.
Featuring a long main strut the F-35C’s main gear has a shrink mechanism to shorten the strut prior to retraction so it will fit within the available space. The Goodrich-proprietary shrink mechanism utilizes a novel transfer cylinder to convert high pressure and low flow aircraft hydraulics into a low pressure and high flow shock shrink hydraulics. Unlike the nose gear, the CV main gear system utilizes the same main wheel and brake as the F-35A CTOL. All tyres used on the F-35C CV variant are significantly more robust than the CTOL and STOVL variants, because of the high energy landings on top of arrestor cables."