Aircraft Tire Preservation - Inexpensive and Simple

  • You have watched airliners land and heard a screech of tires and seen puffs of tire smoke arise when the tires first touch the runway.

  • This is because each tire and wheel is very large and heavy, where it has a lot of what science calls Rotational Inertia, and it is not spinning. The contact with the runway must nearly instantly cause the massive wheels and tires to start to spin relatively fast, in order to roll on the runway.

  • This results in a brief instant where the tires are all SLIDING on the runway rather than rolling! This necessarily causes immense stresses to occur in the small area of tire tread which first contacts the tarmac, which the tire cannot endure very well. Science explains that TORQUE has to be applied to each tire-wheel assembly in order to accelerate its rotational movement and increase its rotational speed to that required to stop sliding and start rolling on the tarmac. The result is that the tire tread in that one spot on each tire is severely worn off nearly instantly, in a process often called flat-spotting.

  • The enormous weight of the airliner and the good traction surface of the tarmac attempt to up-spin the tires instantly, but the massive weight and size of the wheel-tire assemblies (Rotational Inertia) makes this impossible to actually occur. The result is that each tire is flat-spotted where the tread in one spot is worn off nearly immediately, resulting in the puffs of smoke and the sound of tires screeching. Automobiles can flat-spot tires when a driver slams on the brakes at high speed and maintains the brakes where the tires cannot roll at all and so only one spot on each tire receives all the frictional wear with the road, also with screeching and clouds of tire smoke. When that severe braking is done to an automobile, the entire set of tires generally have to be immediately replaced, as the wheels then "thump" as the car goes down a road. (I know this for a fact as I intentionally did this to a very old car I had that was about to be replaced, where I applied the brakes hard at 80 mph on a desolate road, and held the brake pedal down until the car stopped. The car then made thumping sounds after that!) If the automobile tires are severely flat-spotted, the vehicle then needs a full set of new tires as it otherwise bounces and thumps each time the tires rotate past the non-round spot! One small advantage of airliner tires (with a good pilot) is that they do not remain fixed but they quickly start spinning, which makes this flat-spotting a rather brief but extremely severe wear on the tires. The fact that airliners are hundreds of times heavier than a car and the fact that many airliners touch down at around 130 mph speed makes their situation worse. The fact that each wheel-tire on a large airliner weighs around a ton, makes it harder for that assembly to be rotationally accelerated.

  • Those giant aircraft tires and wheels are very expensive to buy and to replace. Where we expect automobile tires to last for 40,000 miles on the highway, the tires on airliners commonly HAVE TO BE REPLACED after each hundred landings, that is, after about just 100 miles of travel on the tarmac! This incredibly short tire lifetime is entirely due to this effect of flat-spotting of the tires at the instant of first touching the tarmac on every landing.

  • A very simple and inexpensive improvement to airliners, essentially special hubcaps on the wheels, would save several major Airlines more than a million dollars every day!

  • In February 1988, I invented and fully Designed and Engineered such a device, which is simply a unique hubcap for airliners. Textured ribs on the surface of these unique hubcaps have a vague resemblance to the toy pinwheels that children sometimes play with, which catch the wind and therefore upspin the wheels (entirely due to air motion, no motors) as soon as the wheels were lowered our of their retracted positions, before landing.

  • Instead of the wheels and tires having to get up to 130 mph tread spinning rate in a fraction of a second, they generally each have around 30 seconds to GRADUALLY upspin before contact with the runway.

  • I designed my textured hubcaps to have many Z-shaped ribs that are each only 13 mm (or half an inch) thick, such that the many ribs each 'catch the wind' (in one direction) where they create a Torque of around 14 kg-m which accelerates the rotation of the wheel and therefore the tire. During the 30 seconds that this action occurs the angular velocity of the wheel increases from zero up to approximately the needed final rotational speed.

  • The result of this is that the tires are already spinning at approximately the correct rotational speed at the moment the treads first contact the tarmac. This greatly reduces the Torque to get each tire to spinning the exact speed to roll down the runway, and therefore the extreme wear of the tire treads is eliminated!

  • The result would be an elimination of the screech and the puffs of smoke from the current non-spinning tires when they first touch the runway.

  • Instead of tire lifetimes which are now around 100 miles, airliner tires might last the 40,000 miles we expect of automobile tires.

  • MASSIVELY less labor would be involved, the aircraft would not be out of service while worn tires are replaced, and the huge cost of buying and replacing such huge tires would nearly be eliminated, saving some Airlines close to half a billion dollars every year!

  • I was astounded in 1988, and many times since, that no Airline and not even any Airline Official, has ever shown the slightest interest in this simple concept!

  • I was often told that the Regulations regarding modifications to aircraft are so immensely complex that it was foolish to even consider such an idea! I wonder if Detroit carmakers had to spend billions of dollars and years of bureaucracy to get approvals, just when they added hubcaps to cars! But all the Airlines that were willing to waste their time in talking to me, and all government Regulators, made it absolutely clear that they PREFERRED the status quo, of wasting at least three billion dollars every year (by Airlines in the US), rather than considering adding $5 hubcaps to each wheel! In fact, several Airline Executives told me to mind my own business, as THEY had the smartest Engineers working for them and IF there was ever a good idea, their Engineers would have thought of it themselves! And then a couple of those Execs hung up the phone on me!

  • Amazing behavior by those Executives and Engineers when the ONLY reason I had even called them was to try to help THEIR COMPANY to save half a billion dollars every year. I had NOT called them to even try to sell them $100 of hubcaps!

  • To upgrade an entire Airliner, only around $100 cost and around one hour of labor should be required (around $200 for a 747)

  • After significant effort to arrange meetings with many dozens of Airline Executives regarding this in 1988, 1989 and 1990, I came to believe that no one really cared about their companies wasting as much as half a billion dollars every year! That seems to be the case because airlines are heavily subsidized by the government, and there is no real incentive for their Executives to even care about whether their company makes a profit or not! Seems very peculiar to me, where America seems to inspire EVERYONE to try to make enormous profits in any business!

  • I first presented this information on the Internet in May 1998. Of the many people who have visited this site and read this information, one man had asked many questions around late 2000. Later news reports indicated that HE then applied for a US Patent on MY invention, and it was GRANTED to him (in 2001)! (That was supposedly impossible, for one person to Apply for a Patent of an invention of a different person, and even receive the Patent!) He was a soldier, a member of the US Military, so he was not allowed to have ownership of that Patent, but our Government apparently now owns that Patent (on my invention)! News reports of the time indicated that the soldier was given $300 for my invention!

  • Oh, even though he had asked many e-mail questions of me, he clearly did not really understand the Physics or Mechanical Engineering behind my invention, and so his Application for Patent on my invention contained a number of design errors in it.

  • I had assumed that the US Patent Office was supposed to be thorough in their research, but clearly they did not realize that I was actually the one to have invented it many years earlier, or that the man had even gotten his information from my web-page and from e-mails from me. It seems inconceivable that anyone would have thought that HE should have been able to obtain a Patent for my invention!

  • It appears to be immensely difficult to get the Patent Office to recognize or to correct such errors on their part! So that Patent is still considered valid, even though it certainly is not!

  • I later learned that every major airport has to shut down each runway on a regular basis so that work crews can grind off massive accumulations of tire treads which can become several inches thick on the tarmac, at the single location on the runway where airliners first touch down. I calculated the amount of tread that accumulates based on the number of airliners that land on a specific runway and the maintenance schedule of removing those accumulations, and the numbers which result regarding the quantity of tread lost in a single landing are in reasonable agreement with the statement of airliner tires needing to be replaced after around 100 landings. This is a confirmation of the scale of this problem.

  • (Comment added late in 2011) So the result is that larger Airlines have managed to each spend around an additional $12 billion to replace tires since my contact with them in 1988. And they clearly have no intention to ever change that fact! Maybe they own Stock in the tire manufacturers???

This concept was invented and Engineered by February 1988. This presentation was first placed on the Internet in May 1998. Late in 2000, a man asked many e-mail questions about my invention, and I later learned that the US Patent Office LATER gave HIM a Patent for my much earlier invention! That ain't supposed to happen, as I had intentionally caused this concept to go into the Public Domain which cannot be Patented by anyone!

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A simple and inexpensive modification of aircraft wheels could greatly increase the durability of airline aircraft tires. This would substantially reduce the cost of airline operations and it would also reduce the maintenance time necessary to regularly change the tires.

Whenever you have seen a large airplane land, there is always a screech and a puff of smoke when the tires first contact the runway. You may have noticed another symptom of this when taxiing on the runway while preparing to take off. There are a LOT of tire skid marks right where the large airliners first touch down on the runway. In fact, a regularly scheduled chore of all major airports is to shut down each runway to remove those accumulations, as once the tread accumulations have gotten several inches thick, a new danger to the aircraft remaining under control exists!

If you do much flying, you might try to look at how extensive the skid marks are on any particular runway, as that is an indication of where in that maintenance schedule that runway happens to be. It would probably be really interesting to see a time-lapse movie (over several months) of that spot on a runway where airliners first touch down. In addition to all the puffs of smoke and the screeches, the accumulating deposits of tire tread on the runway should be frighteningly obvious!

Recently, I heard in a TV commercial that American Airlines has 3,900 flights every day, which means 3,900 landings every day. They do not seem to want to admit how long tires last, but comments from Airport and Airline Technicians seem to indicate that 100 landings seems believable. That means that, every day, just that single airline needs to replace 39 full sets of airliner tires! They also seem not to want to divulge the total cost of removing a huge wheel from an airliner, dismounting the tire, installing a new tire, balancing it, testing it and re-mounting the wheel on the airliner, including highly paid technicians, but $5,000 per tire seems likely reasonable. That suggests that American Airlines must spend around two million dollars every day on replacing tires! Imagine if the tires lasted a hundred times or four hundred times as long! That one company would save nearly two million dollars every day!

On your car, you buy a set of tires that last for 40,000 miles or more. Airplanes spend much of their lives in the air (or parked). Their tires are only moving on the ground during the one-mile runway run and another mile to the terminal. Their VERY advanced, very expensive tires have a tread life on the scale of one hundred miles!

OK! When could enormous wear occur? During low speed taxiing? No, there is extremely little tire wear under those conditions. During a takeoff? A moderate amount of wear can occur then, due to the fairly high speed attained just before liftoff. But that cannot possibly account for a tire tread lifetime of just a hundred miles or so.

There are two situations during landing that can (and do) wear the tires extremely rapidly. Prior to large airplanes having ABS brake systems, it was possible for a pilot to apply the brakes so aggressively in slowing the plane down that he could "lock up" one or more of the wheels. If that wheel remained in that position, a LOT of wear could occur on one side of that tire, resulting in something called "flat-spotting". That is virtually unheard of today because of the automatic action of ABS brakes.

That leaves only one remaining situation that can result in rapid wear in the tires, at that instant of touchdown, when the very heavy non-spinning wheel and tire assemblies must suddenly be sped up to 130 mph (110 knots) by friction with the pavement of the runway. This fraction of a second, mentioned above, with its puff of smoke from wearing of tire tread and its skid marks that are additional parts of the tire that came off, is therefore the cause of virtually all of the wear on aircraft tires. In Physics terms, the contact with the runway must apply an enormous moment (torque) to give the tire/wheel an angular acceleration to get the rotation up to the necessary speed to avoid further skidding.

Many years ago, I was curious about the effect of almost instantaneous wear on the tires of large aircraft. The plane is often traveling about 130 mph when the tires first touch the runway. Therefore, the massive tires/wheels have to suddenly be accelerated to a rotation rate appropriate for 130 mph movement, by the contact with the ground. This necessarily occurs in a small fraction of a second. This sudden rotational acceleration due to contact with the ground is responsible for the screech and the puffs of smoke, because, at first, the tires must skid down the runway, until they start spinning.

In Engineering terms, it is fairly simple to estimate the necessary Torque necessary to accomplish such a spin-up, given just the time interval involved and the rotational inertia of the tire and wheel assembly. The time interval and the necessary final rotational speed ω determine α the rotational acceleration. (ω = α * time) The necessary torque is defined as equal to the product of α and (I) the rotational inertia. (torque = α * I) And the necessary frictional force between the tire tread and the runway is (Force = torque / radius) By knowing the radius of the tread, and these other well known values, it is then easy to determine the necessary frictional force at the tire/pavement interface. These are all very simple and traditional Engineering calculations.

Here are some interesting numbers!

For a general aviation (light aircraft), which weighs 2200 pounds at landing, which lands at 70 mph and which has 6.00x16 tires and wheels:

2200 pounds weight is equal to 69 slugs of mass (don't ask!). 70 mph is equal to about 103 ft/second. Therefore, the kinetic energy of the airplane just before touchdown is 1/2 * m * v2 or 0.5 * 69 * 1032 or about 364,000 ft-lb of energy.

In order to stop, all that kinetic energy has to be removed. Newton told us that energy cannot be created or destroyed, so it all must be converted into some other sorts of energy. Let's see how much is involved to spin up one wheel/tire.

That size tire is around 26" in diameter, and the tire and rim weighs around 25 pounds. There is a quantity called the Rotational Inertia for spinning things that is sort of like the Mass for s something going straight. The Rotational Inertia is usually called I, and it is calculated by m * r2. M is the total mass of the wheel/tire, or 25/32 slugs. R is an effective radius, which is sort of hard to explain simply. In this case, it is around 8" or 2/3 foot. So I is equal to 25/32 * (2/3)2 or 0.35 slug-ft2.

We also need to know how fast the tire will be spinning when it is fully at speed. We commonly describe this as in RPM, but in Physics, it is more useful to describe it in radians/second. In this example, once the tire stops skidding, it will be spinning at the aircraft speed/tire circumference times per second (or 103/6.8) revolutions per second and we just multiply that by 2 * PI to get radians/second, or 95 rad/sec.

The kinetic energy of rotation that the wheel/tire will eventually have is given by 1/2 * I * ω2. In our example, it is 0.5 * 0.35 * 952 or 1580 ft-lb. The aircraft has two main tires/wheels, so double this to 3,160 ft-lb.

So, if the aircraft has 364,000 ft-lb of kinetic energy the moment before touchdown, it will have around 361,000 ft-lb left after fully spinning the wheels/tires up! An almost irrelevant effect as regarding stopping the aircraft. This is mentioned here because a number of critics have informed me that it is "long known" that the landing distance would be 20% longer if the tires were pre-spun. Apparently, no one has ever done the math, because that is clearly not the case! In the case of this aircraft, if it normally took 500 feet before stopping, if the wheels were pre-spun, it would take 503.3 feet, slightly more than one yard longer! NOT 20% longer. Not even 1% longer!

Yes, the aircraft had that 364,000 ft-lb of kinetic energy to dissipate in order to stop. But up-spinning the tires absorbs an extremely small amount of it. Actually, if the aircraft began the landing at 69.75 mph instead of 70 mph, there would be a much greater effect of shortening the landing!

For a 747 airliner, which weighs around 400,000 pounds at landing, and which lands at about 130 mph, the numbers are all bigger but the effect is very similar:

400,000 pounds weight is equal to 12,500 slugs of mass. 130 mph is equal to about 191 ft/second. Therefore, the kinetic energy of the airplane just before touchdown is 1/2 * m * v2 or 0.5 * 12500 * 1912 or about 227 million ft-lb of energy.

That size tire is around 8 feet in diameter, and the tire and rim probably weighs around 1000 pounds. The Rotational Inertia is equal to 1000/32 * 2.52 or 195 slug-ft2.

Once the tire stops skidding, it will be spinning at 191/25 revolutions per second and so ω is 48 rad/sec. (The giant wheels actually spin more slowly than the small aircraft tires do!)

The kinetic energy of rotation that the wheel/tire will eventually have is then 0.5 * 195 * 482 or 225,000 ft-lb. The aircraft has sixteen main tires/wheels, so this total is 3.6 million ft-lb if kinetic energy needed to up-spin all the main landing gear tires/wheels.

So, if the aircraft has 227 million ft-lb of kinetic energy the moment before touchdown, it will have around 223 million ft-lb left after fully spinning the wheels/tires up! Again, an almost irrelevant effect as regarding stopping the aircraft.

Say the 747 normally takes 5,000 feet of runway to completely stop. We can easily calculate the deceleration that occurs. Another Physics formula is 2*a*d = v2. We know everything but a, 2 * a * 5000 = 189.062. Solve for a and get 3.5744 ft/second, a gentle deceleration of around 1/10 G.

Let's see how far that exact same aircraft would have taken to stop if it had pre-spun the wheels/tires, and applying the same deceleration! Same equation:
2 * 3.5744 * d = 1912. This gives 5103 feet as the needed landing distance, roughly one hundred feet longer, half the length of the aircraft. That also is certainly not any "20% longer landing distance"!

That necessary tangential force for upspinning the wheels/tires on impact must entirely be provided by friction with the ground. This gives a value that indicates how much heating and wear is likely to occur to a tire under those circumstances.

In a very small fraction of a second, the heavy wheel and tire assemblies must be spun up to the 130 mph (191 feet per second) tread speed. From the lengths of runway skid marks (seemingly under 20 feet), this seems to occur in well under 1/10 second. Simple calculations show the extreme frictional forces present at the tire-runway surface. This is certainly the cause of extremely rapid tread wear on the tires, which is apparently virtually the only wear that occurs during their lives. (Additional math is not presented here because you have been punished enough!)

I have a feeling that tire life is dependent on the specific pilot. Some pilots seem to impact the runway hard, which gives even less time for the upspinning of the tires, and certainly more wear on the tires. In general, it seems that tires on airliners last a few dozen trips, although the airlines don't seem to want to publicize such things. These tires are very high-speed, high-load-capacity tires, and clearly represent a major expense when they are replaced.

An Improvement

There would seem to be a simple modification that could greatly reduce the wear on these tires. It is a narrow disk or ring (possibly FRP), resembling a automobile wheel trim ring (or hub cap), the size of the aircraft wheel that is bolted to one side of each wheel. This disk would have curved radial ribs (or scoops) on the outer surface of it. This device is actually a simplified version of a Savonius rotor (that is pretty simple to start with!) This whole concept is also similar to a child's pinwheel.

For practical circumstances of large airplanes, calculations suggest that it is likely that these wind-catching fins can be less than 1/2 inch in width. For weight reasons, it probably could be constructed of FRP or straight fiberglass.

As soon as the airplane lowers its landing gear (generally more than fifteen seconds before touchdown), these fins would act like a child's pinwheel (or even a windspeed anemometer rotor) that would slowly start the wheel rotating. It would not represent any significant additional drag on the aircraft, so it wouldn't affect handling or safety. But, over the next fifteen seconds, each tire would spin up to a speed more suited to the plane's landing speed. Then, when the tires contacted the ground, there would be no screech and no puffs of smoke and no massive wear of the tires. A set of tires might then last a hundred times as long. Great economy would result!

Airliner tires are necessarily extremely expensive. They must be capable of supporting around 25,000 pounds on each tire, the tire must be capable of safely spinning at around 150 mph, and they MUST NOT fail under any conditions. Besides the tire being extremely expensive as a result of these requirements, mounting, balancing and testing the tires is also complicated, time consuming and expensive. Finally, the large, massive wheel/tire assembly must be mounted on the aircraft, which involves additional time and money.

Again, airlines don't seem to want to divulge the details of such things, so I am estimating that each tire on a large airliner costs $5,000 total to replace and install. For a mid-sized airliner, with 10 wheels, that's $50,000 each time the tires need to be replaced. If the tires last an average of 100 flights (landings, actually!), that's an expense to the airline of $500 for every landing.

If, by spinning up the wheels prior to touchdown, the wear could be reduced to 1/100 the currently accepted value, that would reduce that tire expense to $5 per landing, a savings of $495 for the airline for every single flight. This is a SIGNIFICANT saving! For this reason alone, I think there is great value in further exploring this new invention. Increasing the tire lifetime by a factor of one hundred may at first seem preposterous, but consider that, even then, the total tread lifetime would still only be about 20,000 miles, most of which was at very low speed during taxiing. Automobile tires last longer than that! This suggests that the benefits might even exceed these amazing estimates!

I would think that additional safety might occur as well, since there would not be the effect of that large instantaneous frictional drag force at the moment of contact trying to pull the nose downward when the main gear contacted the runway, and induce great stresses in the landing gear struts, as well as far less chance of a tire failure during landing that could be catastrophic.

By trying different rib curvatures at the outer edge of the disk, and different rib/scoop width, different rotational torques could be caused. By appropriate choice, it should be possible to ensure that the tires are rotating at approximately the correct rotational speed for the specific airplane. (If too tangential an angle was used, or too large a scoop size, it would actually be possible that the tires could be rotating too rapidly, and there would be a screech as the tires slowed down!)

All large commercial and military airplanes would benefit from this improvement. General aviation airplanes have lower landing speeds, and far less expensive tires, and the benefits may or may not be practical for those light aircraft.

Airlines and aircraft manufacturers have showed no interest in my letters to them. A small general aircraft company in Valparaiso, Indiana showed moderate interest for a while, but eventually lost interest when they found that the value to small aircraft was apparently minimal. Given the airlines present financial problems, I expect one or more of them to eventually show interest in my simple device that can save them many millions of dollars every year!

As to general aviation aircraft: Flying is such an expensive activity that private pilots seldom seem to worry about the cost of tire replacement. I recently heard from a pilot who got 11 years of use out of his tires, which he estimates to involve around 1650 landings. After eleven years of use, the cost of two $200 tires seems irrelevant to him! Probably true! But look! 1650 landings, at 500 feet length of actual landings, is around 160 miles of travel at any speeds greater than very slow taxiing speeds. Taxiing causes extremely minimal wear on tires, due to the very low speeds and stresses. So it could be argued that he got 160 miles of use out of a set of tires! Tires very similar to automobile tires, which we all expect to get at least 40,000 miles of high speed driving out of! Hmmmm!

So even though light aircraft do not create impressive puffs of smoke and loud screeches on landing, apparently there is still great amounts of wear due to the tire up-spinning. In practical terms, there is therefore very little economic benefit of upspinning the tires on a general aviation light aircraft. But in a more theoretical sense, doesn't it seem wrong to only get 160 miles of use out of tires?

I first invented this device in February 1988. This presentation was first placed on the Internet in May 1998. In 2003, I was informed that someone invented a similar device in 1942! It appears that the military's (very negative) reaction at that time initiated the old wives' tales about very long landing distances! And such folklore continues, more than 60 years later!

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago