Damper stops wheel wobble

Cary, NC-Runway surface irregularities, tire non-uniformity, and worn landing gear components induce oscillatory motion in aircraft nose wheels during takeoff, landing, and taxiing. This oscillation, called nose-wheel shimmy, is similar to a caster wheel wobbling on a shopping cart. Instability results if the oscillation grows, so aircraft nose gear typically uses hydraulic dampers to lessen oscillatory effects by reducing the amplitude or preventing shimmy onset.

Hydraulic shimmy dampers consist of a hollow tube filled with oil. A rod and piston move through the fluid to generate velocity-dependent, viscous-damping forces. Such designs require frequent maintenance, and temperature increases reduce the damping efficiency because as hydraulic fluid expands it leaks past the seals. Just 20 drops of leakage can almost cut damper performance in half, and can occur after only 100 hours of operation.

Thanks to advances in elastomer and lubricant materials, Lord Corp. has resolved two shimmy damper failure modes, leakage and bent shafts, by adapting a mature industrial technology to the aerospace industry. The result is a line of nose-gear shimmy dampers with a thicker shaft, containing no hydraulic fluid. Instead, these oil-less dampers use surface-effect technology that depends on resistance between a rubber and non-rubber element sliding against each other in concert with a lubricant to effect energy dissipation. The new dampers eliminate maintenance, cost less, perform consistently over a wider temperature range, and have longer life than the hydraulic dampers they replace.

Lord engineers got the idea after a visit to a local fixed base operator (FBO) at the Raleigh-Durham International Airport. An FBO is comparable to a full-service gas station for aircraft-offering everything from tie downs and maintenance, to taxiing and re-fueling. "We were looking for applications for our wetted surface-effect technology in general aviation aircraft," explains Geoffrey Nicholson, Lord's marketing specialist for aerospace products. "Our first thought was to replace the large struts in the landing gear, but then we noticed a smaller damper on the Cessna nose gear. We asked the technician what it was. He told us it was the shimmy damper. It fails quite often, requires a lot of service, and that we would have a good product if we could come up with a better one."

Nicholson brought a failed hydraulic shimmy damper back to establish design criteria. In the process, Lord engineers determined two reasons why hydraulic damper performance deteriorates after losing a few drops of hydraulic fluid. First, when fluid weeps from a cylinder, air replaces it, which offers less resistance than oil when flowing through an orifice.

The other reason is that when the air bubble isn't flowing through the orifice, it's entrained in the fluid. Air's compressibility makes it act like a tiny spring that stores energy when the piston moves. "So, for example, if you have a number of air bubbles at the orifice," Nicholson explains, "there's little resistance as the piston initially begins moving. Then, each air bubble entrained in the fluid stores energy instead of dissipating it. Surface effect technology gets away from all that because it can't leak."

Eliminating hydraulic fluid in the damper also made for a lighter design. But instead of taking a weight savings, Lord engineers decided to go from a 3/8- to 1/2-inch shaft diameter to increase durability and structural integrity over the hydraulic design. "That addresses another big failure mode: scrapped shimmy dampers due to bent shafts," Nicholson notes. "The heftier shaft is expected to be more durable than the original equipment shaft."

The biggest challenge pervading the design process was that surface effect response differs from viscous damper response in that it's not velocity dependent. "Viscous dampers are very velocity dependent," explains Nicholson. "When you push the damper shaft slowly, there is almost no resistance, and the greater the velocity the greater the resistance. In contrast, surface-effect dampers offer substantial resistance at low velocities, and at higher velocities resistance increases, but not nearly as fast as in a hydraulic damper."

Many forms of damping, including viscous, coulomb, and hysteretic, are evident in the performance of surface-effect devices, according to Andrew Meyers, senior engineer at Lord. "So analytical testing is virtually impossible." The design proceeded empirically, evaluating many configurations until steering resistance, shimmy resistance, and initial motion brake-away forces were fine-tuned for the application.