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Background
The technical challenge of
ensuring satisfactory braking characteristics for
aerospace vehicles, especially on wet or icy runways,
has been a key element in a research program at
the Langley Research Center. The contributions
of this program have probably had more impact on
the daily safety of the flying and nonflying public
than many other Langley programs. Langley’s
leadership in this field is based on years of studies
ranging from the fundamental tendencies of precipitation
to affect hydroplaning and road/runway friction
to the analysis and development of improved tire
and landing gear designs. Recognized as international
experts, Langley researchers are frequently requested
by the civil aviation industry to provide analysis
during new aircraft development programs, by international
organizations to participate in cooperative studies
to develop advanced instrumentation and friction
measurement normalization, and by government agencies
to assist in accident investigations for both aviation
and ground vehicle mishaps. Thanks to the efforts
of Langley researchers, their technical peers,
and interorganizational cooperation, this critical
technology has been transferred into products and
services that have significantly enhanced the quality
of life and safety of the U.S. public.
Langley Research and Development
Activities
Langley’s approach
to accumulating its vast expertise and operational
experiences in this area has included extensive
hands-on research involving all aspects of surface
friction characterization, precipitation runoff
characteristics, the development of advanced instrumentation
for friction measurements in all-weather conditions,
landing gear dynamics, tire cornering characteristics,
and tire wear. Throughout the program, extensive
use has been made of the unique Langley Aircraft
Landing Dynamics Facility (ALDF), which permits
detailed studies of tire-runway interactions under
actual and simulated environmental conditions.
Perhaps the most important aspect of the program,
however, has been the extremely active participation
of the Langley staff during actual runway braking
and friction-measuring studies at worldwide sites
using actual aircraft operations and representative
weather conditions.
Langley Aircraft Landing Dynamics
Facility
The ALDF is a test track
primarily used for landing gear research activities.
The ALDF uses a high-pressure water-jet system
to propel the test carriage along the 2,800-ft
track. The propulsion system consists of an L-shaped
vessel that holds 28,000 gallons of water pressurized
up to 3,150 lb/in2 by an air supply system. A timed
quick-opening shutter valve is mounted on the end
of the L-shaped vessel and releases a high-energy
water jet, which catapults the carriage to the
desired speed. The propulsion system produces a
thrust in excess of 2,000,000 lb, which is capable
of accelerating the 54-ton test carriage to 220
knots within 400 ft. This thrust creates a peak
acceleration of approximately 20g. The carriage
coasts through the 1,800-ft test section and decelerates
to a velocity of 175 knots or less before it intercepts
the five arresting cables that span the track at
the end of the test section. The arresting system
brings the test carriage to a stop in 600 ft or
less. Essentially, any landing gear can be mounted
on the test carriage, including those exhibiting
new or novel concepts, and virtually any runway
surface and weather condition can be duplicated
on the track.
Runway Grooving
The process of bringing a
relatively fast-moving aircraft to a stop following
a landing on a wet runway involves the interactions
of factors contributed by the environment, runway,
aircraft, and the human pilot. For example, the
water depth on the runway is determined by interactions
between the rainfall rate, wind, and the slope
and texture of the runway surface. In addition,
the ability of a tire or pavement to drain water
is affected by interactions of the tire ground
speed, inflation pressure, and tread condition
together with the microtexture and macrotexture
of the runway surface. The resulting water depth
and drainage capability in turn determine the tire-pavement
friction coefficient available for stopping the
aircraft. The efficiency of the friction coefficient
is in turn affected by aircraft characteristics,
such as aerodynamics, engine thrust, the brake
system, and landing gear characteristics. Finally,
piloting techniques and control inputs for braking
and steering can also impact aircraft wet runway
performance.
Launch of test sled at
Langley Aircraft Landing Dynamics Facility.
Thomas Yager (left) and
Walter Horne inspect grooved concrete surface at
NASA Wallops Flight
Facility in front of Convair 990 test aircraft.
In 1968, these experiments evaluated
effectiveness of grooved runway surfaces for safer
wet pavement landings.
Langley researchers Walter
B. Horne and Thomas J. Yager led Langley’s
program to improve the safety of aircraft landing
operations on wet runways, beginning in the late
1950s. Their efforts started with the very fundamental
consideration of the mechanics of water buildup
on runways and the impact of runway surface configurations.
The phenomenon of tire hydroplaning was identified
as the key factor causing poor braking on wet runways.
Hydroplaning occurs when water penetrates between
a rolling tire and the pavement. This penetration
results in the formation of water pressure, which
raises a portion of the tire off the pavement.
The pressure increases as the speed of the aircraft
increases supporting more and more of the tire
until, at the critical speed termed the “hydroplaning
speed,” the tire is supported only by water
and loses all frictional contact with the pavement.
Major factors that aggravate hydroplaning include
the depth of water, a medium to high aircraft speed,
poor pavement texture, and worn tire tread. In
hydroplaning conditions, longer aircraft stopping
distances are required, a potential loss of directional
control may occur, the aircraft experiences a greater
sensitivity to crosswinds, and the pilot must use
a greater reliance on reverse thrust for stopping.
In 1962, Horne and Yager
developed a method of cutting thin grooves across
concrete runways; thus, channels were created through
which excess water could be forced and the risk
of tire hydroplaning was reduced. Grooving studies
at the ALDF evaluated tire friction and wear performance
for many different test pavement specimens. By
1966, track testing had been completed to identify
the optimum groove arrangement, and an industry
conference was held to review the test results
and define a potential flight test program for
actual aircraft evaluations. On the basis of these
test track results at Langley, an optimum transverse
groove arrangement was selected for installation
on a specially constructed runway at the NASA Wallops
Flight Facility in 1967. The grooves cut into the
research runway surface at Wallops were 0.25-in.
wide and 0.25-in. deep on 1-in. centers.
In 1968, with the assistance
and support of the U.S. Air Force and the FAA,
testing was initiated at Wallops to evaluate the
effect of pavement grooving on aircraft braking
performance for an F-4 fighter, a Convair 990 jet
transport, and a Beech Queen Air twin-propeller
aircraft. Impressive results from these tests were
presented at a Langley Conference on Pavement Grooving
and Traction Studies. By the time of the conference,
pavement grooving had been installed on a few British
and U.S. runways, as well as on some selected highway
curves known for relatively high wet-pavement skidding
accident rates. Results from these early evaluations
were very positive.
The civil and military aviation
sectors, however, desired additional full-scaled
aircraft data to substantiate these initial promising
studies. Therefore, new joint programs were conducted
with the U.S. Air Force and the FAA from 1969 to
1972. A C-141, a Boeing 727, and a Douglas DC-9
were all instrumented by Langley to conduct braking
performance tests for over 50 different grooved
and ungrooved runway surfaces. Results from these
tests at Wallops clearly indicated the benefits
of grooving that improved wet tire traction. As
a result of this additional testing, the application
of grooving to both airport runways and public
highways was accepted as a promising means for
minimizing wet pavement accidents. By 1991, runways
at 646 U.S. airports (including several Air Force
bases) were transversely grooved, and every state
in the United States has now grooved some of its
main highways. The greatest use of highway grooving
has occurred in California, where frequency of
accidents on wet pavements has been reduced 98
percent on some highway sections. In addition,
the grooved highways required less maintenance
costs and last about 5 to 10 years longer than
ungrooved highways. Other grooving applications
to nontransportation scenarios have reduced injuries
caused by wet surfaces on swimming pool decks,
playgrounds, and work areas in refineries.
Langley’s pioneering
efforts on runway grooving have been extended
to national highways for enhanced safety in inclement
weather.
This relatively small NASA
investment (about $1.4 million from 1962 to 1973)
to develop and evaluate pavement grooving leveraged
action by other organizations to improve the Nation’s
airport runways and public highways; this has significantly
minimized wet pavement skidding accidents and improved
the safety of aircraft and ground vehicle operations
during wet weather. In recognition of this important
safety spin-off from NASA’s aeronautical
program, in 1990 the U.S. Space Foundation inducted
this Langley technology into the Space Technology
Hall of Fame in Colorado Springs, Colorado.
Winter Runway Friction Measurement
Program
Despite the advances in technology
and operational procedures, safe winter operations
remain a challenge for airport operators, air traffic
controllers, airlines, and pilots who must coordinate
their efforts under rapidly changing weather conditions.
Complicating the winter weather picture is the
fact that criteria for safe operations on a given
runway snow condition differ from airport to airport
because of differences in grooving, temperatures,
and pavements. Runway water, ice, or snow was identified
as a contributing factor in over 100 aircraft accidents
between 1958 and 1993; many of these accidents
involved fatalities. Inaccurate, incomplete, or
confusing runway surface information has been a
contributing factor in a number of accidents where
airliners have slid off the end of the runway upon
takeoff or landing or have been dangerously slow
in reaching liftoff speed because of the slowing
effect of snow, ice, or rain. To help reduce this
type of accident, Langley partnered with Transport
Canada and the FAA in a runway friction measurement
program called the Joint Winter Runway Friction
Measurement Program. Also participating were organizations
and equipment manufacturers from Europe and several
Scandinavian countries. The research effort included
instrumented aircraft, friction-measuring ground
vehicles, and an international test group. Langley
researcher Thomas J. Yager served as the lead NASA
participant in the program.
Langley’s Boeing
737 research aircraft during winter runway friction
studies.
Langley’s Boeing 757
aircraft during test run at joint runway friction
testing
at Kenneth Ingle Sawyer Air Base at Gwinn, Michigan,
February 1999.
The research program was
designed to meet several objectives. First, the
researchers coordinated readings from different
ground vehicle friction measurements to develop
a consistent friction scale for similar potentially
hazardous runway conditions. Second, the objective
was to establish reliable correlation between ground
vehicle friction measurements and the braking performance
of aircraft. These two objectives provide airport
operators a better way to evaluate runway friction
and maintain acceptable operating conditions. Results
also enhance safety for all ground operations and
provide information to help relieve airport congestion
during bad weather. Results also help industry
develop improved tire designs, better chemical
treatments for snow and ice control, more reliable
ground vehicle friction-measuring systems, and
runway surfaces that minimize bad weather effects.
In a related precursor study
in 1994, about 80 engineers representing 43 organizations
from 10 countries participated in controlled tire-runway
friction studies that were conducted at the NASA
Wallops Flight Facility. A better understanding
of the factors that influence tire-runway friction
performance was obtained from over 800 friction
test runs and over 400 runway surface-texture measurements.
Thirteen friction-measuring devices and 7 pavement-texture
data collection techniques were operated on 11
different pavement surfaces.
In 1996, actual aircraft
testing began in the Joint Winter Runway Friction
Measurement Program with braking tests for instrumented
aircraft and ground vehicles in the U.S. and Canada.
Langley’s Boeing 737 research aircraft and
Canada’s National Research Council (NRC)
Falcon 20 aircraft completed a weeklong series
of landing tests on ice-, snow-, and slush-covered
runways at the Jack Garland Airport in North Bay,
Ontario, Canada, about 200 miles north of Toronto.
Surface conditions were artificially varied to
expand the range of data collected. Many different
runway friction-measuring ground vehicles—vans,
trailers, and modified cars—took readings
with continuous and fixed slip devices under similar
runway conditions for comparison with each other
and with the braking performance of the two instrumented
aircraft. Subsequent winter test seasons have involved
9 aircraft and 18 different ground vehicles. Test
aircraft include the Langley Boeing 737 and Boeing
757, the FAA Boeing 727, the NRC Falcon 20, a de
Havilland Dash 8, a Dornier 328, Airbus A319 and
A320 aircraft, and a Boeing 737-400.
Data collected in this program
from 1996 to 1999 included nearly 400 instrument
aircraft test runs and more than 10,000 ground
vehicle runs. These tests were performed on a variety
of runway conditions, from dry to various combinations
of ice, snow, and slush. Researchers also took
manual measurements to monitor conditions including
ambient temperature, temperatures of pavement surface
and snow, depth of cover material, and the density
of cover material for snow or slush.
Langley Instrumented Tire
Test Vehicle during friction measurements.
Using this substantial database,
the Langley researchers and their peers have developed
an International Runway Friction Index (IRFI) that
standardizes friction reporting and minimizes piloting
difficulties in making critical takeoff and landing
decisions. The IRFI is anticipated to become a
standard criterion used by airport operators to
assess a runway under winter conditions. Safe takeoff
and landing decisions will then be facilitated
by use of the index. The index—probably in
the form of a simple chart—will help pilots
with “go/no-go” runway decisions based
on readings taken by a ground friction-measuring
vehicle on the same runway. The index will help
airport operators determine if their runways are
suitable for aircraft operations and when maintenance
is required. A methodology standard that defines
procedure and accuracy requirements is under review
for approval by The American Society for Testing
and Materials.
The research will also help
industry develop improved tire designs, better
chemical treatments for snow and ice, and runway
surfaces that minimize bad weather effects. In
nonaerospace applications, much of the equipment
being used to monitor runways is being used to
measure highway pavement friction performance.
In areas with high accident rates, pavement textures
can be modified, on the basis of friction measurements,
to improve the safety of automobile travel.
Effects of Deicer Fluid on
Aircraft Tire Friction
Cooperative NASA and FAA
tests were conducted by Thomas J. Yager, Sandy
M. Stubbs, Granville L. Webb, and William E. Howell
at the Aircraft Landing Dynamics Facility in 1993
to determine the effects of deicer fluid on tire
friction. A conventional transport aircraft main-gear
tire was tested at speeds up to 160 knots on a
nongrooved concrete test surface. Surface test
conditions included dry, wet (water only), Type
II deicer chemical-water mixture, and 100 percent
Type II chemical. Test tire operational modes included
antiskid controlled braking at zero yaw angle and
yawed rolling at a yaw angle fixed at 6∞.
The results indicated that for the 3-to-1 mixture,
the friction values were similar to the water-wet
condition. The friction coefficient for 100 percent
deicer was about 30 percent lower than the value
for the water-wet condition. The 3-to-1 mixture
is probably more representative than the 100-percent
mixture of what might be found in normal aircraft
operations. Therefore, the results suggested that,
in practice, the deicer effects on friction will
be similar to those of water. The information assisted
in establishing a national database on the effects
of aircraft Type II chemical deicer depositions
on aircraft-tire–pavement friction performance.
These data also help improve the safety of aircraft
ground operations during winter runway conditions.
Tire and Landing Gear Studies
In addition to its extensive
research on runway friction, Langley has conducted
in-depth research on tires and landing gear. The
scope of research includes tire cornering and durability
characteristics, tire tread design, and landing
gear dynamic behavior.
A major test program to measure
the comparative dynamic response characteristics
of radial-belted and bias-ply aircraft tires using
radial-belted aircraft tires from an F-4 fighter
was conducted by Pamela A. Davis. Both tire designs
were tested in a free-vibration environment under
combined vertical and lateral loads and under combined
vertical and fore-and-aft loads. The free-vibration
test data showed that the radial-belted tire was
less stiff than the bias-ply tire under both loading
conditions. The increase in elasticity of the radial-belted
tire could adversely affect the operation of an
antiskid braking system that was designed for the
bias-ply tire. Damping of the bias-ply tire was
greater than for the radial-belted tire under both
test conditions, which suggested that this radial-belted
tire should be a cooler-operating tire and should,
thus, tend to have less wear during normal cornering
and braking operations. Stiffness characteristics
revealed by the tests have resulted in the reassessment
by the manufacturer of the design of this radial-belted
tire. Data from these tests helped to establish
a national database for radial-belted aircraft
tires that will be used to compare the response
characteristics with those of bias-ply tires.
In another radial tire study,
Langley joined with the Michelin Aircraft Tire
Corporation in 1995 to define durability characteristics
expected on new commercial aircraft tires. Various
tire properties, including relaxation length and
spring characteristics, are of great concern to
the aerospace industry as the tire is the conduit
between ground forces and the airplane structure.
The project included testing at the Langley Aircraft
Landing Dynamics Facility with the research test
carriage in a quasi-static mode. Langley’s
Robert H. Daugherty led the joint investigation.
The extremely high tire loads tested in the program
required that concrete weights be placed on the
drop carriage portion of the test carriage. A 6-month
effort was required to design and fabricate a special
test loading setup for the project. A 15-ft-long
built-up runway was fabricated in the carriage
hangar capable of withstanding 60 tons of vertical
load combined with 40 tons of side load. A “frictionless
platform,” which allowed side loads to be
induced into the test tire, was modified and installed
as an integral part of the built-up runway. The
breakaway tests define not only the friction values
achieved by the tire, but also the spring characteristics
of the tire as well as the movement of the center
of pressure in the tire footprint. Eight breakaway
and 8 unyawed relaxation tests were completed,
as well as 35 yawed relaxation length tests. The
test matrix contained vertical load and yaw angle
combinations that encompassed the full range of
tire conditions expected on the newest commercial
transport aircraft. These data were used by Michelin
in the design and certification of advanced radial
tires for the Boeing 777 and will be used for other
future commercial transports.
Michelin tire subjected
to large side loads in joint testing at Langley
in 1995.
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