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Articles by Triplane Builder - William (Bill) Woodall

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Aircraft Tire History

It is one of the least understood pieces of equipment on any airplane.
(You can still generate a heated discussion by asking how the load that
the tire is carrying, gets from the axle to the earth.)
Its dimensions are fixed rather early in the planning of an airplane,
usually bound by the dimensions of a wheel well.
If its use involves a braked wheel, its bead diameter will also be frozen
since the capacity of the brake stack must be sufficient to provide the energy
required for an "RTO,*" or some other severe stop, and rest assured,
only the aircraft is allowed to "grow" in weight and performance.
The tire's option is to have its inflation pressure raised, and its ply rating
and thickness increased to accommodate the new conditions.

It was not always this way. The Wright brothers, with their catapult launching system,
found that skids could handle their grass field landings. Those pioneers who did opt
for tires, learned that their load carrying capacities and speed requirements
were easily met by tires made for existing bicycles; motorcycles; or automobiles.
Cotton cloth, woven as for clothing, provided the reinforcement for most early tires.
Since braking was provided by the tailskid, the bead diameters were large
and the axle diameters were quite small. Wheel weight could be kept down
by using spokes, and tire aspect ratios were in the range of 100%.
Although some purpose built tires had become available, it remained possible
to utilize tires sized for cars and trucks for most applications,
even as airplanes became larger and heavier. As was the case with other tires
of the time, early airplane tires employed the "clincher bead" concept.
A fabric reinforced, hard rubber cored, grooved, triangular shaped terminus
for the lower sidewalls was engaged by a special inward hook developed
by the top of the rim flange. When this "paradigm shifted," it was one of 180 degrees.
The lower sidewall plies became anchored to coils of high tensile steel wires,
and instead of the inflation pressure resulting in a tensile load on the rim,
it now developed a compressive one. The new tires were once referred to as
"straight sided" to differentiate from the clincher, but the clincher
eventually was phased out. (Clinchers had always been subject to
accidental dismounts and blowouts from sudden deflections, and side loads).

Another invention was the "bias" tire principle. This tended to eliminate the
short life span of cloth tires, which literally "sawed" themselves to pieces,
and were in the process of "failing" from their first loaded revolution.
The bias construction involved distinct layers of rubberized, parallel cords
lying at an angle to the centerline. Alternate layers were disposed at opposite angles.
It is of some interest that this development first emerged in aircraft tires,
and then spread to surface vehicle tires. Tail wheel tires sometimes were built
as "single tube" tires. This involved vulcanizing the upper portion of an innertube
to the interior of the tire structure. The exterior of the tube [that faced the rim]
was provided with a raised rubber pattern that engaged matching depressions
in the outer surface of the rim. This was to reduce rim slippage and damage
to the tube valve. Prior to the discovery that carbon black added strength
and durability to rubber, aircraft tires, like their ground bound relatives
might be white or in varying shades of gray. With a fixed landing gear hanging
in the breeze, parasitic drag was an ever-present consideration, so the spoked wheel
was "streamlined" by covering it with cloth. Tires were made with hooks in their
lower sidewalls to provide anchors for the wheel covers. The first change
in tire shape was to a streamlined tire profile. These tires were widest
at the rim and tapered from there to the tread, and became identified as
S.C. (smooth contour) tires. The outside diameter, in inches,
was the only size marking. Early retractable landing gear systems often
left a portion of the tire exposed to the slipstream.
The S.C. profile, or any other streamlining effort helped in this regard.

Ever heavier aircraft weights made greater demands on the reinforcing
cord material. Finding a replacement for cotton became a driving force.
Rayon was the first man-made fiber that entered the field. Made from
wood fiber, it was stronger than cotton; adhered to rubber well;
and was dimensionally stable. It could absorb water if punctured
however, and its strength could degrade suddenly.

A "Standards Body," The Tire and Rim Association, began organizing data
that would assure the proper "fit" of tires on rims. Tires were categorized
as either "landing wheel tires" or "tail wheel tires," and in the
1939 yearbook, separate pages were devoted to "Streamline Airplane Tires,"
(intended for tail-wheel service which operated at 40% deflection)
and "Smooth Contour Airplane Tires" (which operated at 35% deflection).
The maximum pressure for either was 65 psi.

Another page listed "High Pressure Airplane Tires." The highest pressure
shown was 85 psi. These tires used the same sizing system as some early
truck tires. That is, they stated an O.D. and a section width. (The tire's
maximum dimensions were somewhat different from the numbers on the sidewalls).
E.g., a 32X8 had a max. OD of 30.84" and a max section of 8.96."
Ply rating was not used, just the simple statement "plies." A "max.
Static load" was listed, and "Air press." but a notation was made that
these were approved as EXPERIMENTAL PRACTICE; other data: "information only."

By 1943, another category of tire had appeared. This was called
"Low Pressure Tires." Sizes were designated by section width and rim diameter.
Here, the minimum tire section width might agree with the number
on the sidewall, (e.g., 17.00-16) but not necessarily.

The 1945 Yearbook sought to categorize aircraft tires by "TYPES."
TYPE I referred to the "S.C."(smooth contour).
(E.g., 56" [B-17; Flying Fortress]; [B-24 Liberator]).
Aspect ratios were in the low 70% area; - pressures ranged as high as 95 p.s.i..
TYPE II replaced what had been the former "high pressure" (e.g., 30X7; SBD [Dauntless]).
Aspect ratios were about 85%; pressures were less than 90 p.s.i.; deflection - 27.5%
TYPE III replaced "low pressure" type ( e.g., 17.00-16; DC-3 (Skytrain [Dakota, Gooneybird]
Aspect ratios - 84%; pressures up to 75 p.s.i.; deflection - 25%
TYPE IV were Extra Low Pressure tires (Airwheel); tail wheel sizes: e.g., 12X5-3/ 18X3-3
Aspect ratios - 90%
TYPE V catalogued the "Streamline Tires," e.g., 15.50; 18.00
Aspect ratios - 70%; deflection - 40% for mains, 35% for tailwheels; pressures below 45 p.s.i..
TYPE VI low profile nose wheel tires for jet aircraft. eg. (22X7.25-11.50; P-80 [Shooting Star].
Aspect ratio - 65% to 72%; deflection - 25%.
TYPE VII was added by 1948. (High pressure versions of Type II's). A decimal point and
number added to the section width, repeating the section width definer. (E.g., 30X7.7
[Republic P84]. Aspect ratio 80 to 87%; 32% deflection
TYPE VIII ended the TYPE categories. These were 3 part name tires in as the 41X15-18.
Aspect ratio 79%; 32% deflection.
TYPE VIII later were called "New Design", and they eventually became the basis for the
practice of listing the outside diameter; section width; and rim diameter (all maximum
dimensions) as the method of naming the tire.

Other "types" of tire, "Helicopter"; "British," and "Beaching Gear" held or hold places
in U.S. documents. "British"-- for imported aircraft;
"Beaching Gear" for pulling flying boats ashore.

Early versions of nylon had entered the scene in the late 40's /early 50's.
It provided a major leap in tensile strength.
Cord surface treatments that would provide good rubber-to-cord adhesion,
and better applications of the heat and tension needed to enhance
uniformity and limit the stretch of the cord were new challenges.
Learning how to dimension tire molds for the new phenomenon of "tire growth" entered the scene. One other facet of nylon became apparent-- "flat-spotting." It seemed that the footprint area of a warm tire, allowed to cool under load, would shrink and preserve an "out of round" condition. The next use of the tire provided a "bumpy" ride until internal "warming" cured its "morning sickness," and concentricity was restored.
Nylon was water resistant, but if the cord were overheated, it would melt, whereas rayon and cotton, at a higher temperature, would char. Since nylon was so much stronger however, the tires built of it were thinner and thus ran cooler under the same conditions. Inflation pressures began to exceed 100 psi.
It became the practice to list a "Ply Rating" instead of the earlier "plies". A lesser amount of nylon layups could more than equal the tensile strength of the earlier cotton or rayon.
The bead seats of airplane wheels have always had design points that differed from surface vehicle tires. This results in molded tire bead diameters that are smaller than those of surface vehicles of the same nominal diameter.
In spite of this, there are continuing efforts to use aircraft tires in non-aircraft applications. In the tire scarce days of the second world war, many substandard 32X8 aircraft tires somehow ran on autos as "6.00X16's".
Since aircraft tires are subjected to sudden high deflection events like landing, new tires were tested to confirm adequate margins of safety with regard to bursting strength. A multiple of the maximum inflation pressure has been used, (usually "4" but sometimes "3"). This test has designed the amount of bead wire required, as well as the number of body plies, and seems to have served the industry well.
Early tires had been devoid of any surface designs. The grass fields and relatively slow speeds interacted well with smooth rubber surfaces.

As more and more airports came to feature paved runways (for all-weather operations), the tailwheel replaced the tailskid. Braked wheels became the norm, and the need to improve tire traction on wet pavement took the form of "button" type tread designs.
These worked on either paved or unpaved runways. As time passed, more sophisticated tread designs appeared.
Some strange attempts have been made to enhance tire performance. In one case, short coil springs were vulcanized into the circumferential tread ribs with the intent of providing ice traction. These could suddenly depart and bombard the adjacent structure with "sound, fury", and some damage. (not to mention an "F.O.D.**" potential for subsequent users of the runway). [Apparently little research had been done about the lack of success in producing sled runners from rubber].
With increases in take off speed, the detrimental effect of using 100% aspect ratios reared its head.
The phenomena of the "traction wave" appeared, and was studied. As the revolutions per second of a loaded tire increased, repeat images of the footprint recurred around the circumference, and sidewalls. If these had not damped before a new footprint developed, the tire was about to self-destruct.
Although multiple wheeled landing gear had appeared on bombers in the first world war, the tendency continued toward large tire, single wheel applications throughout the second world war.
[That trend was resisted by the Douglas C-54 ,the Lockheed C-69, the Boeing B-29 "Super Fortress," and a few others.]
(The transports would become better known as the DC-4, and the Constellation).
Single tire applications soldiered on in some rather high risk designs such as the Douglas B-19, and the prototype Convair B-36 (which used 110 inch diameter main wheel tires).
In spite of some pioneers having used "tricycle" landing wheel setups, almost from the dawn of flight, it wasn't until the late 1930's that it began to see concerted use. Douglas, Martin, Bell, North American, Lockheed, Consolidated-Vultee and Boeing, produced successful military aircraft that employed the concept. Germany and England also found it necessary for some advanced designs.
Landing gear vibration has been a long-standing problem for designers. Tire companies offered changes in aspect ratio and tread design, hoping to reduce un-commanded oscillatory excursions from straightforward rolling.
The channel tread tire, with its depressed center, provided a "dual tire" effect by concentrating load onto the shoulder ribs. These saw widespread use in tailwheel and some main and nose wheel applications, but treated symptoms rather than the causes.
Following the second world war, the "conventional" landing gear was all but abandoned in favor of the tricycle configuration, and the nose wheel tire became a genre' all its own.
Propeller driven airplanes could, in many cases use reverse propeller pitch to provide aerodynamic braking and overcome the loss in drag that the "3 point" attitude, (afforded by the tail-wheel), had provided. This was not so readily available from the jet engine. Wheel braking energies became significantly higher, and the tire environment became more hostile.
The need to prevent tire explosions from overheated brakes fostered the tubeless tire for main wheel applications. (It was essential that the inflation medium be able to be vented should a critical temperature be reached, and inner-tubes made this all but impossible).

Even though the operating conditions in which tires were now functioning had changed drastically, the design process whereby they were selected had not. Limited space, (not required for something else), were left for the tire or tires.
When size was inadequate, higher pressure was presumed to provide the solution. Aspect ratios tended to remain "high," even though greater and greater take off speeds might have mandated something else.
It might well have been better for user and tire manufacturer had several sizes never existed at all. They helped condemn their aircraft to a short life span; plus many redesigns for the tire maker.
Some of these unhappy sizes were selected from either the Mil Spec. or the T&RA "catalogues" as "existing items," and forced into yet another unhappy incarnation. Some of these choices took place because a qualified wheel already existed, and the certification process could be shortened and made less expensive.
Examples of the process: what had been the main wheel tire size for the 1920's Ford Trimotor, was selected as the nose tire for the last 4 engined- piston engined transport to be offered by one major manufacturer; the main wheel tire for the first U.S., 4 engined jet transport, used the same main tire dimensions as the C-54 from several decades before.
Certain military and "X" plane applications did foster unique approaches to unique needs, but it wasn't until the 1970's that some of the advances in tire technology, pioneered in racing and other high performance areas, began to make their way into the mainstream aircraft tire industry.
Tire testing had begun with variable mass dynamometers that allowed a finite amount of energy to be transferred through a simulated landing roll out. These machines were incapable of producing the rotational speeds that tires would routinely see on take off from most of the turbine engined aircraft.
Enter the programmable dynamometer. These could accelerate a loaded tire through a simulation of the actual take off variables that a tire might experience. In deference to the past, simulated landings continued to be performed, along with some taxi testing which had proven to be a more rigorous part of the duty cycle than was originally envisioned.
One of the "facts of life" that was realized early in the game, was that proven, long wearing synthetic rubber compounds that were so successful in surface vehicle tires, could not get past the aircraft tire "qualification" test on the dyno. "Heat failures," (localized reversions of these synthetics), ensured that only treads made largely of natural rubber would ever see the light of day on an airplane. Rightly or wrongly, the dynamometer pretty much designed the tire. Even though 99% of the common dynamometer degradation was never seen in the "real world". So whether the problem was real, or maybe just a facet of the concave footprint engendered by the curved surface of the test wheel, could not be explored. (A real "catch 22").
The advent of the rear positioning of jet engines revealed another area where help was requested from the aircraft tire designer. Nose wheel tires (like any other tire) when forced to roll through standing water, displace substantial amounts of liquid. The resulting plumes are elevated, sprayed aside and aft. The overtaking engine air intakes are frequently targets for this water. The quantities can produce "flameouts". By adding strategically placed bands of rubber (protruding from the outboard sidewalls), the trajectories of the water can be steered away from the engines.

Tires have been described as a "combination of totally dissimilar materials operating in loose cooperation in a hostile environment in an unknown manner."

Those who design them, have been accused of practicing a "black art". This because the mathematics needed to characterize them has been so difficult to develop.
The power of the computer, and finite element analysis, has seemed to unlock some of the mysteries, but the large number of "degrees of freedom" needed for the components, make it necessary that the analyzer introduce "characterizations" that can skew results. This is particularly true of bias tires.

"Ply rating" (followed by a number) persists in the aircraft tire arena, even though it has little direct relationship to the number of plies used in the construction of a tire. However, the number, along with a pressure relates directly to the maximum load that the tire can be expected to support.
Aircraft tire users, accustomed to the "ply rating" system have been reluctant to move to a "load range" or other description that may be operative in other tire areas.

The number of participants in the manufacture of aircraft tires has steadily declined.
A company has to want to manufacture them to continue to do so. They constitute a small fraction of the tire company's total production, and traditionally have a small profit margin.
They are the most expensive tire to qualify, and are subject to high insurance charges to protect against a catastrophe. They require the highest degree of quality control during manufacture; a dedicated field service organization to assist in their use; and diligent care by the user.
More and more specialized tests are being required to guard against unique operational situations, and to develop data points on theoreticians wish lists. Even when all hurdles have been passed, there is often no assurance that there will be a market for the product. What had been a plethora of would-be producers has declined to a precious few.

Retreading began as a military experiment to save rubber, and was gradually taken up by independent entrepreneurs and helped the airlines bottom lines. The major tire companies reluctantly entered the field as a defensive measure, but ultimately came to better understand their product's shortcomings, when too many new tires failed the NDI to become R1's.
The current success of retreading is verified by the high percentage of tires so treated, that permeate the daily rolling stock of most airlines. The increase in the successive number of times that tires return for an additional tread life is a silent testament to improved new tires, and especially the skills of the retreader. The retreader has learned to evaluate his basic raw material (the worn tire), specify the correct rebuild materials, and perform those tasks necessary to produce and inspect his final product.

The TSO process, mandated by the F.A.A. has, through its several iterations, resulted in more realistic testing conditions. The replacement of landing simulations with demonstrations of take off; multiple taxi cycles; and overload taxi; and an overload take- off, has refined many of the constructions.
Changes in tire contours, and aspect ratios have resulted in families of tires, especially those for commercial transport aircraft, that are much better suited for their roles than were their immediate predecessors.

The radial tire entered as an aircraft type, long after it had come to dominate most other tire use areas. Its development has been slower than anticipated. Unlike the bias tire, the radial has a minimal body structure made up of cords that take almost the shortest path from bead to bead. The tread portion is stabilized by a nearly circumferential reinforcement of cable-like materials made of either textile or metal.
The large number of wheel diameters, and the need for costly specialized building equipment associated with each size has negatively impacted the speed of changeover.
The fact that the unique demands of aircraft service--high speed--high deflection--high circumferential torque---high side loadings--and the expectation that multiple retreading will be realized, have lead to multiple construction and material changes.

Radials, as new tires, have demonstrated superior overload capability, and longer tread life.
The different load paths utilized by the fewer cords in radials have engendered the need to increase adhesion in different areas of the tire profile from those needed in bias constructions.
These differences have also highlighted the need for wheel manufacturers to provide different design philosophies for radial wheels. This results in wheels either intended for bias or radial use, but rarely, both.
As with the earlier bias experience, the problems uncovered in retreading operations will eventually pinpoint the solutions needed to finalize the construction details of new tires.

Aircraft tires have come a long way since their inception.
The success of the worlds: airlines; air forces; and civil aviation would have been impossible without them.
We still may not be able to "prove" how they support a load,but we don't understand much about gravity either.

Bill Woodall







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