November 2004Features

Revere the gear

Good landings are more than ego-boosters; they're kind to your airplane's landing gear

Next time you misjudge the flare and really drop one in -- don't worry, we all do that from time to time -- spare a thought for your airplane's landing gear. Of all the jobs that you could get on an airplane, being the landing gear may be the most difficult, and it's almost certainly the least glamorous. (OK, maybe being reincarnated as the engine's drain plug gasket would be worse.)

Think about the engineering challenges. The landing gear must be strong enough to survive brutally hard landings, in straight-down-the-runway form as well as with slewing, tire-smoking side loads. The gear must absorb this shock without damaging the airframe. And it must be as light as possible, because during the main point of the exercise -- flying -- it's just stealing useful load and creating aerodynamic drag.

Over the years, airplane manufacturers have come up with lots of ways to do landing gear -- and here we're talking about fixed landing gear. (When you get to retractable designs, the engineering permutations are many.) Let's look at some of the common designs to understand how they work and what to look for during a preflight inspection.

Bending steel, the Cessna way

Cessna, because it has made so many airplanes in so many categories, was once an influential engineering juggernaut. Its en- gineers could balance weight, rigidity, and cost effectiveness to startlingly good effect. You see this in Cessna's fixed-gear designs.

The problem, of course, is that the wing is up there and the ground is a long way away. Too long for the landing gear to attach to the wings, in fact, which didn't deter Cessna at all. Bolting the landing gear directly to the fuselage is one of those time-tested technologies that just happen to be based in good engineering.

Obviously, the landing gear has to carry the weight of the airplane, and it's best if the loads carried by the gear can go someplace that's very strong. Like the fuselage. It's possible to transmit gear loads into a beefy fuselage with comparatively little weight gain. That's because all the loads can be concentrated where the structure is naturally stronger, and if it needs to be stronger yet to carry the landing-gear loads, it won't take a lot of extra metal to do the job. Consider the alternative: Place the landing-gear loads in the wing -- as the Piper Cherokee does, for example -- and those loads must be carried through the wing spar to the fuselage, which is the heaviest part of the airplane. All the components between the wheels and the fuselage have to be stronger than if the wing carried no gear loads.

So it was a natural for Cessna to poke the gear out of the lower corners of the fuselage-even more logical than you think, because our legacy airplanes are based on taildraggers, whose main-gear legs entered the fuselage very near the inboard end of the wing strut. Cessna could make this part of the airplane extremely beefy to carry the loads while the rest of the airframe remained extraordinarily lightweight.

But even Cessna couldn't resist toying with its designs -- listening to the marketplace -- and soon the tricycle models arrived, using many of the same design ethics. If you've flown an early 150 or a 172, you've seen Cessna's flat spring-steel gear legs. They are what they sound like -- long, flat flanks of steel that taper down to the wheels. They're bolted to the airframe just inside the fuselage and transmit landing loads through a built-up saddle. Steel? Isn't that anathema to low weight? Yes and no. Turns out steel is an excellent material for landing gear because it's very strong and resistant to fatigue. That Cessna's steel gear legs were flat helps aerodynamics.

On the preflight: Cessna spring-steel gear is extremely strong but can be broken by abuse or lack of maintenance. Look at the airplane for symmetry -- it should not be resting one wing low. Check the area where the gear leg penetrates the fuselage for loose rivets or skin wrinkles. Sometimes there will be a fiberglass or plastic fairing over this junction, so damage can be hard to spot. Naturally, look for any leaking fluids. Brake fluid is red. As you're taxiing, listen for clunks or rattles as the airplane traverses pavement joints; they could indicate a gear leg coming loose.

Cessna's other tack

Spring-steel gear is great for a lot of things, but it has a few drawbacks, including the weight. It can be -- how shall we say this? -- lively. It wants to bounce and wobble a bit on landing. Also, you may have noticed, it takes a noticeable downward set in flight. This means that the tires will have to scrub sideways during the first part of the landing, which wears them out quickly. Cessna's response was to switch from flat-steel gear legs to tubular steel. You'll notice this in all the new Cessnas and the old ones from about the mid-1960s.

The tubular-steel gear rides better and doesn't have the big camber (angle) issue. It also allowed Cessna to lower the fuselage of the various models, which makes them more stable while taxiing in high winds.

On the preflight: You're looking for the same things as on the flat-steel gear plus a close inspection of the fairings. The structure itself is a relatively small tube hidden inside an aerodynamic fairing.

And now there's fiberglass

Most of the new-think airplanes -- Cirrus, Diamond, Lancair -- use fiberglass legs in place of steel, but stick with the same basic idea as Cessna: install the gear either directly into the fuselage, where the airplane is strongest, or very near to it. Fiberglass -- and its close cousin, carbon fiber -- have some compelling advantages here. First, composites don't corrode. Second, they're surprisingly fatigue resistant. What's more, fiberglass can be designed to flex one way at the fuselage junction and another way at the wheel. It's very adaptable, and one design can be modified easily to handle more weight. In this way, fiberglass can be made to control the ride of the gear and its geometry quite easily.

On the preflight: It's just like the spring-steel and tubular-steel systems with one addition: look for delamination of the gear. It will look like deep cracks in the surface, perhaps even sections pulling away, like string cheese. It's unlikely you'll see such a thing, however, as fiberglass gear is known to be quite rugged.

Just add oil

As you know, the landing gear must be able to bend and flex to absorb not only the small jolts from taxiing and the occasional greaser landing, but also the big dumps from botched approaches and severe drop-ins. If the gear weren't allowed to bend and dissipate the energy of the, er, "arrival," it would damage the airframe. That's the point of poking a piece of steel or fiberglass out of the fuselage and letting it bend at will.

Not all manufacturers use this kind of gear, of course, and the dominant alternative is called the oleo strut. Think of a shock absorber with a spring inside, or a motorcycle fork. Only in most cases there's not actually a spring, but a column of compressed air. Air is a good springing medium because it's obviously a lot lighter than a piece of wound steel, but it also is extremely progressive. That is, when the strut is fully extended, the spring rate -- or the resistance to movement -- is comparatively light. But as the strut compresses, the spring rate increases dramatically. It's just what you want: a smooth ride when the load is light, as during taxi, but a measure of bottoming resistance for clunker landings.

There's a maintenance downside to the oleo strut, and that comes from its greater complexity compared to the flexible gear leg. There are seals, compressed air -- nitrogen is preferred because it is dry and will not promote corrosion inside the strut -- and hydraulic fluid inside the strut that need to be serviced periodically. Also, because the movable part of the strut is free to rotate around the leg that attaches it to the wing, there must be some kind of linkage to keep the wheel pointed straight. It all adds up to more moving parts and things to break.

On the preflight: The big thing is to look for leaks. Both the strut's hydraulic fluid and the brake fluid are red, so be really curious if you see any fluid on the strut or wheel. Look carefully at the connecting linkages for missing nuts or bolts, and check the routing of the brake lines to make sure they're not rubbing on any moving parts of the strut or axle assembly. Look at the shiny part of the exposed strut for nicks or corrosion. A small amount of oil on the shiny part of the leg is normal, but a gusher is not.

Rubber wonders

Inside the aircraft tire, there's....more than air

So much in aviation is different from the world of cars, bikes, and golf carts that it shouldn't be a surprise to learn that general aviation tires are not like the others. Some salient points:

  • The vast majority of GA tires have tubes. Cars changed to tubeless tires decades ago because they're more durable and easier to replace. The tube itself can rub on the inside of the tire and eventually wear away. A tube inside a tire also creates more heat than would be the case with a tubeless.

  • Aircraft tires aren't designed primarily for traction. Indeed, they don't really need much as they don't carry much in the way of lateral (side) loads and don't carry acceleration loads. They do need to disperse water well and have good braking action. Auto tires are a compromise to handle a much more dynamic environment, but airplane tires can be tailored to the task-braking action and longevity.

  • A ply rating doesn't indicate the number of plies. Tires carry a ply rating that tells you how much load the tire can carry-the higher the ply rating the more weight-but doesn't indicate the number of actual plies.

  • Holes in the wall. Tire manufacturers intentionally put holes in the tires' sidewalls. On tube-type tires, the pin-size holes go all the way through and are marked by white paint or ink. On tubeless tires, the holes go only partway through, into the carcass, and are highlighted green. The reason: to let air escape from between the inner carcass and the tube or between plies inside a tubeless tire.

  • Red marks the (light) spot. On new tires, you may see a red dot or triangle. This is the measured light part of the tire and is supposed to be aligned with the valve stem, presumably the heaviest part of the wheel.

When inspecting tires, look for proper inflation pressure-if the sidewalls bulge significantly, have the pressure checked with a gauge and refill as needed. Always check tire pressures cold, or wait three hours after a flight. The pressure goes up a lot when the tires get hot. Also check for bald spots on the tread and visible cords, as well as chunking or tearing of the tread. When you roll the airplane out of its parking space, watch the tires to see if they're out of round or flat-spotted. (Airplanes left to sit for long periods will have some sag in the carcass that will remedy itself.) Superficial cracking of the sidewalls is common and not a reason for tire replacement, as are cracks in the tread area that do not expose cord or run out of the groove to the tread crown.

On the nose

Nosewheel designs differ, but the majority uses a variation on the oleo-strut theme because it packages well and provides a neat, controllable suspension action. Normally, the differences in design point back to how the nosewheel is steered. Our legacy airplanes all use nosewheel steering through the rudder pedals. (Have you ever wondered why Cessnas feel squishy and Pipers don't? It's in the design. Cessna uses a thing called a bungee -- basically a set of caged springs -- between the rudder-pedal linkage and the nosewheel steering mechanism. This allows the wheel to remain pointing more or less straight ahead during a crosswind landing. The Pipers use a direct linkage, so you need to be careful to center the nosewheel just before it touches down else the airplane will take an ungainly swerve.)

The trend now -- for reasons of simplicity and low weight -- is toward free-castering nosewheel steering. That is, the leading wheel is free to turn on its own, like a shopping-cart wheel. You control ground track with a combination of differential braking -- push one brake pedal to swing the nose in the desired direction -- and rudder. American Yankee/Grumman trainers have had this system for decades. The castering nosewheel can be implemented like the Cessna-style main gear -- at the end of a steel pole (Cirrus, Diamond, Grumman) -- or beneath an otherwise conventional oleo strut (Lancair).

In either design, engineers strive to eliminate shimmy, a wobbling vibration felt in the rudder pedals or the airframe. This is the nosewheel moving rapidly left and right, usually at the point of maximum load or highest speed. It is influenced by a variety of factors including wheel balance, tire profile, and the strength or stiffness of the landing gear itself. One solution is a shimmy damper, a small shock absorber mounted on the steering gear that prevents the nosewheel from twisting rapidly yet allows the normal slow movement needed for taxiing.

On the preflight: Check the oleo strut for leakage and make sure the hardware is all there and tight. If the airplane has a shimmy damper, check that it's not leaking and loose. On the designs without the oleo strut, inspect the area where the nosegear tube fits into the fuselage or onto the firewall; there should not be wrinkles or other evidence of structural trauma.

There's more...

There are other variations in gear design beyond what we've discussed relative to the mainstream aircraft. Mooneys use rubber doughnuts between the free-to-swing gear-itself made of welded-steel tubes -- and the wheels. They're strong and durable but offer a bouncy ride. Some airplanes have what's called trailing-link landing gear -- the EADS/Socata retractables are an example -- that is famously smooth and kind to the fragile pilot ego. They combine a massive swinging arm with an oleo strut to absorb the shocks.

But they all do the same thing: balance the needs of being strong and light, waiting patiently while you fly to make you look good on the landing. So next time you drop it in, please don't blame the gear.

Marc E. Cook has logged 3,000 hours in 16 years of flying a variety of light aircraft. A former senior editor for AOPA Pilot magazine, he now writes about aircraft, automobiles, and motorcycles. He is based in Long Beach, California.

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Illustrations by David Diamond
Photography by Mike Fizer