what's in a landing gear?

Sep 24, 1971 - He spotted a "Kingfisher" which had capsized while trying to effect a rescue. Burns landed and picked up the two pilots and a crewman, then ...
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KINGFISHER . . .

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He spotted a "Kingfisher" which had capsized while trying to effect a rescue. Burns landed and picked up the two pilots and a crewman, then taxied to a rendezvous with the submarine Tang and delivered the three men. Bums took off again and saved a pilot from a life raft, taxied two hours and picked up the crew of a downed torpedo bomber. Taxiing back toward the Tang, he rescued three more downed fliers. With his floatplane now loaded with seven extra men, Burns taxied three hours before the submarine rescued the entire "crew." So badly was it damaged by the long beating of the waves, the "Kingfisher" had to be sunk by the sub's guns. "Kingfishers" also distinguished themselves in antisubmarine warfare. Two planes from VS-9 based at Cherry Point, North Carolina helped sink a German submarine, the V-576, off the east coast. Another OS2U based at Key West helped sink the U-176 off the Cuban coast. OS2U's could carry 350 lb. bombs under each wing in addition to having a fixed gun firing through the propeller and one or two swivel mounted machine guns in the rear cockpit. While spotting gunfire from the cruiser Pensacola off Iwo Jima, Lt.Jg. D. W. Gandy flying an OS2U was jumped by a Japanese "Zeke" fighter, far superior in all performance except turning radius. Gandy was able to turn inside the attacking "Zeke" in the ensuing dogfight and shot it down. Although small for the job, "Kingfishers" even served as ambulance planes during the war. Wounded men on stretchers were loaded into the rear cockpits and flown to areas where hospitalization was available. Because they were launched from battleships and cruisers by being fired from catapults, "Kingfishers" were among the most ruggedly built planes in the world. The firing charge in the catapult was that of a five-inch artillery shell, and the plane reached a flying speed of 60 mph at the end of its short catapult run. Battleships usually carried three "Kingfishers", two being positioned on the two catapults on the stern, and a third tied down between them. After a mission, the "Kingfishers" would land on a slick made by the ship's turning into the wind. After taxiing up to a rope-net sled being towed by the ship, the "Kingfisher" pilot would engage a hook on his main float to the sled. The ship would then attach a cable and hoist the plane aboard with a crane and place it on the catapult for another launch. )

VOLKSPLANE SAFETY BULLETIN NO. 1 Ref. Aileron Horn bend radius (VP-C-7), page 24 of plans.

It has come to our attention that an Australian builder has had trouble forming this part about the 1/8-in. bend radius specified without developing cracks during the forming operation. Therefore, it is requested that all builders carefully inspect this item for cracks in the bend. It is a matter of record that no cracks have developed during fabrication or since on the prototype horns. FIX

Use a 8/16-in. radius rather than the 1/8-in. specified, or make the part of the same thickness in 1020 steel which will form much easier than the 4130 material. It is requested that all plan holders mark this correction on the face of the drawing. In all cases, BEND ACROSS THE GRAIN, not parallel to it. Evans Aircraft Box 744

La Jolla, California 92037 24

SEPTEMBER 1971

£)

What's In A Landing Gear? By Harry Gorjanicyn (Courtesy ULAA Australian AirsportJ

IS OUTSIDE THE scope of this article to outline the of designing a spring-leaf landing gear system, as a IfairTmeans bit of work (and usually some testing) is required to produce a good and efficient design. However, some general notes on the subject may be of use and of general interest. Before discussing a spring-leaf system let us consider the fundamental features of landing gears in general. We are here mainly concerned with the system of absorbing the landing impact and transferring the resultant loads into the airframe. The question of wheel, tire and brake design is a separate problem. So, the main purposes of the landing gear are to absorb the landing impact, and to provide an elastic suspension during taxiing and ground maneuvering over irregularities in the field surface. Once the landing gear is designed to achieve the first of the above requirements, the second is almost always met automatically. It is necessary, of course, to ensure comfortable and stable taxiing characteristics. An exception to this is the nose wheel unit which almost invariably is designed for post-landing conditions when main wheel brakes are applied and the aircraft pitches nose down, deflecting the nose wheel strut. This "nose slam" case develops the highest loads and energy absorption requirements for the nose gear. Let's now consider what happens in a landing. The aircraft changes its direction of motion from a downward glide to one almost parallel to the ground at the same time descending until the wheels touch the ground. During this period, the aircraft possesses two separate types of energies: "Potential" energy, which is a function of its weight and distance above ground, and "kinetic" energy which is a function of the weight and the vertical descent velocity. The aircraft is considered being "airborne" during landing. This means that the potential energy is carried by wing lift, and the vertical descent is at steady and constant descent velocity (in simple terms, you don't just drop 'er with a thump!). So, the object is to absorb this constant descent velocity — kinetic energy.

The energy is absorbed by deflection of the landing gear leg. As the leg deflects, a load is generated which is

Lo*t* *•

greater than the static load on the wheel due to the aircraft

weight. In fact, the wheel load times the deflection equals

the energy absorbed for an ideal system. As the wheel touches the ground and the leg deflects from zero to its maximum deflection, the load on the wheal- increase* frorn, zero to its maximum value as shown in Fig. I. The form in which the leg deflects (or the shape of the load deflection diagram of Fig. 1) dictates the energy that is being absorbed. In fact, the energy equals the area underneath this load deflection diagram. And this is the essential feature which distinguishes one type of landing gear from another. For an ideal leg, the load on the wheel would immediately be the maximum value and our diagram would have a rectangular shape as in Fig. 2(a). This shape would give the maximum possible area for the smallest deflection, and hence the best energy absorption characteristic for the smallest deflection, and the system would be 100 percent efficient. But, alas, like anything else nothing is perfect or 100 percent efficient! The nearest we can get to the ideal is the oleo-pneumatic strut which has an efficiency of around 85 percent. Its load deflection diagram looks something like that of Fig. 2(b). A simple steel spring has an efficiency of 50 percent and the appropriate load deflection is as shown in Fig. 2(c). It is seen that the area under this diagram is only one half that of the ideal 100 percent efficient unit. This means that to absorb the same energy (and this is dictated by the descent velocity requirements) the simple spring would either develop double the load or would have to deflect twice as much! Our general rule can now be written as: The wheel load x deflection x efficiency must equal the energy to be absorbed. The energy to be absorbed by each main wheel unit is equal to:

Fig. 2

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D e.FLJgCTio*j

This is one of the main features of the leaf spring — large deflections! In actual fact, a well designed leaf-spring system can be around 65 percent efficient. The same applies to "rubber" struts or rubber bungee systems where efficiencies of about 60 percent can be achieved. A tire on its own (as in some aircraft) has an efficiency of approximately 50 percent and this is why very large wheels and tire thicknesses have been used to achieve the necessary deflections together with high wheel loads because of practical tire size, and hence deflection limitations. This is also why the landing in this case is rather "sudden" and the

system is often referred to a "rigid" landing gear.

2g

where "W" is the weight of the aircraft, the descent velocity "v" is equal to 6-8 fps, and "g" is the gravitational acceleration of 32 fps. Therefore, say for 8 fps descent velocity, the energy to be absorbed is: (i.e. energy equals aircraft weight) Because of structural and weight reasons the maximum load is usually limited to about twice the weight (i.e. wheel load reaction of 2g). Higher values are used on military aircraft but 2g is about the limit for passenger comfort. So we now see that the required deflection will be: Deflection (in feet) = 2 x e fe ncy (oretfgg in inches) (This is obtained as follows; Deflection x load x efficiency = energy, and for Energy = W and load - 2W we have Deflection x 2W x efficiency = W). The result of the preceding is that for an oleo strut we would need 35 = 7 in. of deflection, and for a spring gear g|$ = 12 in.

Fig. 3(a)

Mor/art Fig. 3(b)

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This rather lengthy, but important, description covers only one aspect of landing impact. A spring, tire, or rubber

system will absorb the energy, store it at maximum deflection and then immediately release it again during rebound, with the aircraft perpetually bouncing up and down. This naturally does not occur in practice because we

vs a T/CM.

Fig. 1

have some system of damping. The aircraft itself has some damping characteristics because of aerodynamic damping and friction of the air. But this alone is nowhere sufficient and the energy must be dissipated by other additional means. This second problem of energy dissipation, or damping, is equally an important one. Because of ground stability and comfort the landing impact energy must be damped out quickly, ideally in about 1M> strokes of the landing gear leg. That is during deflection under landing and extension to the static position. This, in all systems, is achieved by converting the energy into heat. In an (Continued on Next Page) SPORT A V I A T I O N

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LANDING GEAR . . . (Continued from Preceding Page)

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oleo-pneumatic strut the air acts as a spring to absorb the energy while the incompressible oil flows through a small orifice and dissipates the energy into heat. Rubber struts (rubber discs closely packed inside an outer tube) absorb the energy by compressing the rubber as a spring, and dissipate energy by friction of the rubber against the cylinder walls. The leaf-spring landing gear dissipates the energy by friction between the tire and the ground as the wheel moves sideways across the surface during the vertical deflection (see Fig. 3). This is another reason for having large deflections of the leaf-spring gear to obtain large lateral movement and hence adequate damping. It is because of this method of "tire scrubbing" damping that spring-leaf gears are rather severe on the tire wear, particularly when operated from concrete runways. This is also why one "bounces" more on wet grass than, say, dry concrete — coefficient of friction between the tire and the ground is vastly different in the two conditions. It is seen from the above that the behavior and characteristics of the spring leaf are quite different from other systems. Appreciating this, quite a different approach is called for in designing a "spring" landing gear. Let us now have a look at some of the other features of spring-leaf gears. First, a spring requires quite different attachment techniques and, in fact, has to transmit much heavier loads into the airframe than an oleo or rubber strut. This necessitates a completely different type of back-up structure layout, if structure weight is to be kept within efficient bounds. This is why adapting a spring-leaf system to an existing aircraft can present tricky problems. Fig. 4 shows how the loads reacting into the airframe are higher. In the strut type, except for a small offset of the wheel from the leg axis, the wheel load passes straight up the leg. With the leaf, the wheel load is magnified something like three times because of the "lever" action. Furthermore, the leaf deflects as a beam in bending. This produces large bending stresses at its top end. Because of this, and fatigue considerations, one cannot just drill a couple of bolt holes and bolt the leaf to the structure. The leaf at its outboard attachment has to be "clamped" by some arrangement as shown in Fig. 5. The design of the "clamp" has to be thought out carefully because of a

Fig. 4

P, -'ooovg Pa. =

CLOMP

Fig. 6 26

SEPTEMBER 1971

number of problems. Because of leg contour (when a spring leaf is bent to shape it also bends across the width of the leg as in Fig. 6) clamps have to accommodate this shape. Forward and rearward loads on the wheel tend to imbed one corner of the leg as shown in Fig. 7, and this can cause surface cracking or damage, if not accounted for as these loads are also fairly high. Because of the high loads in general and the nature of loads (oscillating loads in taxiing), the clamp arrangement must provide tight, secure, and chatter free holding characteristics. The inboard end of the leg is often secured by a single bolt through a hole in the leaf. As the leg deflects and extends it is very difficult to prevent this bolt from flexing by working in bending, and it is very prone to fatigue because of this. Any owners of aircraft with such an arrangement where no special bolts are used (necked and shaped bolts) would be well advised securing the bolt every 200-400 hours or so, unless otherwise advised by the designers of the aircraft. Because the leaf spring has to be quite flexible to achieve the required deflections, it is invariably quite thin ('/2 in. or less). It is also thin because of aerodynamic reasons (one advantage of springs being—-low drag), and it is also thin because it has to be fairly wide (around 4-6 in.) in order to give as large a "spread" for reacting drag loads as shown in Fig. 7; the narrower the spring the higher the load "P"! As the deflection is proportional to thickness cubed, very small variations in thickness can make a significant difference in its characteristics. Therefore, the thickness has to be selected and made fairly accurately. Also, as the stresses in the leg are proportional to thickness squared and the leg is quite thin, the stresses are large. This requires both good detail design and high strength heat treatment. Fortunately, commercial materials and manufacturers can be utilized (with some relatively inexpensive material testing involved) and the unit can be made relatively inexpensively. And this is the main advantage of a leaf spring. It is inexpensive, very much so compared with an oleo-pneumatic strut. It is also simple, and when well designed, pretty well trouble and maintenance free. The biggest disadvantage is that leaf springs are heavy. To a designer of an ultra-light aircraft, really too darn heavy, but inexpensive. A leaf spring could, for a twoseater, be expected to weigh around 15 lbs. each, almost twice as heavy as a good oleo! And this means some two gallons of fuel and half an hour of flying less per aircraft. However, this is the penalty one has to pay for simplicity and initial cost economy. Incidentally, it is almost impossible to break a spring under overload as it will keep deflecting usually until it hits the wing — on a low-wing aircraft, anyway — before failure. There are variations to the leaf spring that are mainly attempts to reduce the weight. There is the spring rod, tube, or even a fiberglas leaf spring is used on at least one aircraft. To us, the amateur builders, fiberglas for primary structure is difficult because of quality control, manufacturing control, etc. Spring rod has some possibilities but it also has attachment complications and I doubt if it would turn out any lighter for the same deflection. Tube would be the lightest arrangement, but spring steel tube is not easy to come by. Using chrom-moly tube results in smaller deflections, hence higher loads. One cannot taper a tube simply to save weight as is possible with a leaf, and aerodynamic drag is certainly higher. To conclude, leaf-spring landing gears have their problems, mainly in the design stage. They are, unfortunately, relatively heavy but simple and inexpensive to manufacture. Also, the large deflections associated with a spring leaf gives good soft ride characteristics. Their design is not for eye-balling, but if properly developed there is no reason why they cannot result in a simple, inexpensive and reliable landing gear system as indeed already proven by Cessnas, Victa "Airtourers" and others, and so many amateur-built aircraft. Q