Everything you wanted to know about tires, and were afraid to ask...

From:         rdd@rascal.ics.utexas.edu (Robert Dorsett)
Date:         06 Jan 93 01:06:29 PST
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The following is from _Landing Gear Design Handbook_, by Norman S.
Currey, January 1982, published by Lockheed-Georgia, Marietta,
Georgia 30063.  It's a highly readable book, covering every facet of
landing gear design I can think of, at least (no great task :-)).

No restriction on redistribution or duplication of the material
is asserted in the cover sheet: quite the opposite, in fact, so
here it is...


---------------------------

Pp. 6-25-6-35:

TIRE PRE-ROTATION  
-----------------

At first glance, it would seem logical to spin up the tires prior to
touchdown to alleviate tire wear and spin-up loads.  Several methods 
have been devised to do this, and some have been tested with various 
degrees of success.  One methods uses an electric motor and antother
uses fan-like blades on the wheel or tire.  However, the
cost/weight/maintainability  penalty must be assessed and traded off
aginst the advantages gained.  First, tire wear at spin-up is minor. 
Most tire wear is caused by braking and turning.  Secondly, spin-up
loads do not usually design a great deal of the gear--usually parts
of the torque links and piston. Experience has indicated that tire
pre-rotation devices are just not worthwhile. For further reading on
this subject, reference should be made to "Prerotation of Landing
Gear Wheels," by H.F. Schippel, SAE Journal, Volume 52, No. 10,
October 1944.


TIRE ECONOMICS 
--------------

Tires represent a significant cost element in aircraft operation.
Their initial cost is relatively low, but their life is short, and
even with retreading the operating cost is of some consequence. Any
attempts to increase tire life usually involve additional weight
and/or initial cost.

It is impossible to recommend any specific number of retreads that
may be applied.  It depends on the tire scrappage rate due to cuts
and other abnormalities, and these depend to some extend upon the
operating environment. A typical airline, however, could be expected
to retread four or five times.   [Refers to figures, showing a cost
per airplane landing, in 1963 dollars, indexed by tire pressure: a
707-323C, with 180 PSIG of pressure, is listed at 25 dollars per
landing).  From SAE Journal, December 1963, "The Cost of Landing," by
J.E. Davis and R.C. Curry.  A more modern, separate illustration,
which we can assume is near the date of publication of the book,
lists a "new" cost of $346, $70 per retread.  --rdd] 


HYDROPLANING 
------------

When a tire is traversing a wet runway, there is a certain
relationship of forward speed and inflation pressure at which the
tire is essentially lifted above the water film. When this occurs,
the tire is said to be hydroplaning.  [...]

As the leading edge of the tire encounters the water film, a
hydrodynamic wedge is formed, lifting the leading edge and producing
an inclined surface at the contact area. The upthrust on the tire is
equal to the change in momentum of the water squeezed out beneath the
tire, and the momentum change is dependent upon water depth, tread
configuration, and tire forward speed. An approximate speed that has
been used for many years to determine the minimum hydroplaning speed
is: Vp = 9.0 * SQRT(P) where vp = minimum hydroplaning speed (mph), P
= tire inflation pressure (psi).

Of all the variants involved, tread configuration is the only one
that we can do anything about--although it must be admitted that
water depth is being reduced in some cases by runway grooving.  It
should be noted that some tests have indicated hydroplaning speeds
1.5 times greater than that predicted by the formula.

To improve hydroplaning characteristics, tire treads have been
modified to remove water from under the contact area. The approach
taken on automobile tires has been described well in several papers
[...], but the tread fragmentation used on those tires is not
applicable to aircraft tires since the latter have far higher
inflation pressures, and under such conditions the tread would
distort badly and have more wear. Also, high aircraft braking loads
would tend to tear the automobile tread patterns.  Extensive research
is being undertaken to improve aircraft tire treads, an example being
Dunlop's Aquagrip in which the entire tread surface is covered with
small holes.  These act as reservoirs, and as such they collect the
surface water as the contact area interfaces the runway, and then
release it as the tire rolls forward.  Thus, the water is not
squeezed sideways, the hydroplaning wedge is minimized, and under
tests it has proved to be remarkably effective.  For instance, an
aircraft with conventional tires stopped in 6350 feet on a wet
surface. Using Aquagrip tires it was stopped in 4700 feet.

All of the above refers to Dynamic Hydroplaning, where the water
depth is more than the tire tread depth; i.e., more than about 0.40
inch.  There is, however, viscuous hydroplaning and reverted rubber
skidding, both of which are discussed in [Horne, Walter B., NASA
Langley Research Center, "Skidding Accidents on Runways and Highways
Can be Reduced," Astronautics & Aeronautics, August 1967].

Viscuous hydroplaning (due to a thin film of water acting as a
lubricant) can occur even when the pavement is covered with a heavy
dew, and is generally only a problem on very smooth runways.  Tests
have shown that a textured runway surfaces satisfactorily alleviates
this condition, and the noted reference includes data to show the
effects of moisture on the surface, both smooth and textured. It also
lists numerous other sources of data on this subject.  In general,
there is really very little that can be done to the tire to alleviate
viscuous hydroplaning.  The solutions to the problem are to groove
the surfaces, and to use the more sophisticated skid control systems
which constantly monitor the available friction coefficient and
thereby minimize the possibility of skid.

The latter device is also the best protection against reverted rubber
skidding. During a prolonged skid, the heat generated by the braking
tire turns surface water into steam.  Indications are that this steam
is hot enough to melt the surface rubber.  In any event, the tire
effectively planes across the surface on a cushion of steam, leaving
distinctive white streaks on the runway.  The melted rubber fills the
pores in the runway surface, making it extremely slick and therefore
further compounding the problem.

 [ The author concludes the section by discussing the #1 preventive
maintenance technique, namely correct pressure, and lists signs of
tire damage--overall, a fascinating book!]




--
Robert Dorsett
Internet: rdd@rascal.ics.utexas.edu
UUCP: ...cs.utexas.edu!rascal.ics.utexas.edu!rdd