Fixed Gear Is the New Retractable

How come the fastest airplanes have fixed gear?

Technicalities Cessna TTx

Technicalities Cessna TTx

Cessna's TTx is powerful, sleek and high-flying.
But wait — they forgot to retract the gear!

"Landing gears are made retractable in all modern high-speed airplanes. The drag of fixed 'under-carriages' used in old-type, and still in small and slow airplanes, is avoided in this manner."

Thus wrote Sighard Hoerner, the great compiler of drag data, in 1951.

One of the oddities of the present moment is that of the three swiftest single-engine piston airplanes manufactured in this country only one has retractable landing gear. A few decades ago this would have seemed an unimaginable situation. Without exception, retractable airplanes outperformed ones with fixed gear. It was simply understood, without dispute, that an airplane with fixed landing gear was aerodynamically handicapped.

Perhaps the barrier separating retractables from nonretractables began to crack in the late 1960s when several manufacturers, beginning with Piper, brought out retractable versions of traditionally fixed-gear models, I suppose to colonize a perceived niche halfway between fast airplanes and slow ones and to capitalize upon the perception that real pilots fly retractables.

Retractable models often came with slightly larger engines and constant-speed props that made it impossible to assess how much difference retractable gear really made. Results, such as they were, were mixed. If POH numbers could be believed — and manufacturers had every reason to exaggerate the gains — the Piper Arrow was 11 knots faster than its stiff-legged counterpart; the Piper Lance 9; the Beech Sierra 15; the Cessna Cardinal RG 8; the Skylane RG 14; the Cutlass more than 20. (I have adjusted some of these figures when the retractable version had more horsepower than the other; the effect of a constant-speed rather than a fixed-pitch prop is more difficult to guess, and I didn't try. Very large unexpected disparities, for instance between the speed gains for the Cardinal RG and the Cutlass, may be due to wheel pants being taken into account in one case and not the other.)

The prestige of retractable gear may also have taken a hit from homebuilts. Horsepower is seldom a fair criterion when comparing homebuilts with factory products because homebuilts are almost always much smaller, but it is impossible not to notice that many fixed-gear homebuilts are much faster than many factory retractables of similar or greater power.

Perhaps the stage was set far earlier, when builder and race pilot Steve Wittman decided that a steel leaf spring, similar to those in the rear suspensions of cars, would make a satisfactory landing gear. Even without a fairing the thin steel leg, though heavy, produces relatively little drag — a faired tubular one even less. That leaves the exposed tire. A good pant, fully enclosing the brakes and making a clean intersection with a thin gear leg, does away with at least half of a wheel and tire's drag, provided that most of the tire is inside it. Manufacturers leave more of the tire in the airstream than homebuilders, who are willing to patch their pants if they taxi over a rock.

Landing gears have two tasks to perform. First, they cushion the shock of touchdown — 3 G is a typical design load — by smearing it out over time and distance. Most landings are pretty good, and spring gear is fine for any pretty good landing. It's prone to bounce, though, and therefore worse for really bad landings.

The second job of a landing gear is to dissipate energy. If landing gear springs were made of some almost perfectly elastic material like Silly Putty, an airplane stalling a few feet above the ground would bounce back almost to the height from which it fell. Pure spring gears — solid steel or composite legs, Mooney-style rubber donuts, coil springs a la Beech Staggerwing or bungees like those on a J-3 Cub — dissipate some energy by the friction of the tires scrubbing the runway if the gear splays outward under load and some by the internal damping of the spring material itself. Ideally, however, a landing gear should absorb the shock of a rude arrival and not bounce back at all. Pure spring gears fall short of that ideal.

The virtue of the oleo strut, in which compressed gas is the spring, is that it absorbs the energy that would otherwise become a bounce. It does so mostly by forcing oil through a small orifice. A well-designed strut is squishy on first impact, re-extends slowly, and is capable of absorbing several successive impacts despite the heating and foaming of the damping oil. The damping rate is often regulated by a tapered needle that increases the resistance to oil flow as the gear compresses. When there are multiple orifices, a poppet valve can be used to close some of them on the spring-back, so that the piston encounters more resistance when extending than when compressing.

We tend to think of the loads on landing gear as vertical ones, but there can be a large horizontal component as well. Part of it, which becomes more significant with larger wheels and tires, is the so-called spin-up load that occurs when contact with the ground starts the wheels spinning — an event heralded, when any large airplane lands, by a puff of smoke. Fore-and-aft loads also occur whenever the gear encounters a bump or dip. These are particularly severe for nosewheels, because any backward drag on the gear pitches the airplane nose-down, adding a vertical load on the nose strut. Hence the preference for tailwheel gear at unimproved strips: A tailwheel is lightly loaded and jumps away from a bump.

A simple oleo strut, in which the strut itself contains the air spring and oil damper, does nothing to soften fore-and-aft shocks. A different type of design, called a trailing link or trailing arm strut, does a little more. A trailing link leg resembles a human thigh and calf with an oleopneumatic shock absorber added between them. The reputation of trailing link gears for making all landings good ones is due in part to the way they smooth out fore-and-aft shocks — though some airplanes, such as the Ryan PT-22, have put the wheel ahead of the hinge, sacrificing that benefit.

Oleo struts are comparatively difficult to streamline. The outer cylinder can be no more costly, aerodynamically, than a steel spring, because although it is thicker it is much shorter. But 6 or 8 inches of exposed piston are a different story. One of the more striking illustrations in the aforementioned Hoerner's compendium of drag data compares a streamlined fairing with a cylinder: In terms of frontal area, the cylinder's drag is nine times that of the airfoil-shaped fairing. I would venture a guess that the exposed oleo piston on the nose gear of a Cessna TTx produces more drag than the much larger, but better-streamlined, wheel pant does.

As airplanes like the TTx demonstrate, however, it is simpler to add power than to subtract drag. Turbocharging helps too, at least on paper; the higher you fly, the smaller the portion of the total drag that parasite items, like exposed landing gear, represent, because a larger and larger fraction of the total drag is of the induced variety.

I have enough grasp of arithmetic to see that the cost of manufacturing, maintaining and insuring retractable landing gear is comparable to, and can be greater than, that of the extra power to make the fixed gear go fast.

Nevertheless, I cannot reconcile myself to fixed gear. No doubt this is just a stupid prejudice that became ingrained in me at the time when Hoerner could state flatly that all high-performance aircraft had retractable landing gear. But looks have to count for something, and fixed gear just looks as bad to me as dangling legs would on a bird. I admit, however, that it looks worse when appearing from behind and receding into the distance ahead.

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