The NASA Landing Gear Test Airplane

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NASA Technical Memorandum 4703
The NASA Landing Gear Test
John F. Carter and Christopher J. Nagy
June 1995

The NASA Landing Gear Test

John F. Carter
NASA Dryden Flight Research Center
Edwards, CA

Christopher J. Nagy
PRC Inc.
Edwards, CA

Technical Memorandum 4703
June 1995

A tire and landing gear test facility has been developed and incorporated into a Convair 990 aircraft.
The system can simulate tire vertical load profiles to 250,000 lb, sideslip angles to 15 degrees, and wheel
braking on actual runways. Onboard computers control the preprogrammed test profiles through a feed-
back loop and also record three axis loads, tire slip angle, and tire condition. The aircraft to date has pro-
vided tire force and wear data for the Shuttle Orbiter tire on three different runways and at east and west
coast landing sites.
This report discusses the role of this facility in complementing existing ground tire and landing gear
test facilities, and how this facility can simultaneously simulate the vertical load, tire slip, velocity, and
surface for an entire aircraft landing. A description is given of the aircraft as well as the test system. An
example of a typical test sequence is presented. Data collection and reduction from this facility are dis-
cussed, as well as accuracies of calculated parameters. Validation of the facility through ground and
flight test is presented.
Tests to date have shown that this facility can operate at remote sites and gather complete data sets of
load, slip, and velocity on actual runway surfaces. The ground and flight tests have led to a successful
validation of this test facility.
deg degrees
Dryden Flight Research Center, Edwards, California
knots ground speed
knots indicated airspeed
John F. Kennedy Space Center, Florida
Landing Systems Research Aircraft
n mi
nautical miles
pounds per square inch
Space Transportation System
Tire and landing gear development and testing for aircraft are usually done by ground test facilities
due to the expense and hazards associated with testing on aircraft.
Tire dynamometer and sled tire track are the two facilities used mainly for dynamic tire testing. Exist-
ing facilities have limitations in simulating the landing surface, time varying vertical loads, and tire slip

Tire dynamometer facilities roll aircraft tires against a metal drum at any combination of velocity,
vertical load, and slip angle. These facilities have the advantage of long run times, very good load and
speed control, and good control of the slip angle of the tire. However, dynamometers have disadvantages
for dynamic tire testing, such as
1) the dynamometer rotary drum surface does not accurately simulate a runway surface,
2) the curvature of the contact area of the drum causes incorrect radial tire deflection during the test,
3) heat build up of the drum causes the temperature of the test tire to be abnormally high.
Because of these problems, dynamometer data are used primarily to measure the strength and endurance
of tire carcass material, not the tire surface forces or wear. Appendix A shows a tire dynamometer at
Wright Patterson Air Force Base in Dayton, Ohio. An example of data obtained from this type of dyna-
mometer is given in reference 1.
Tire sled-type facilities mount the tire on a carriage and move the carriage down a straight path.2 A
test surface can be constructed which simulates an aircraft runway, but the process can be time consum-
ing and may not accurately represent the surface. Existing facilities also have problems due to their limit-
ed run times, limited capability for time varying vertical load, speed, and tire slip angle control. Because
of limited track length, simulations of complete aircraft landings typically are completed in segments,
with a single landing test requiring as many as five test runs. In addition to the inconvenience of multiple
runs, cooling of the test tire between runs can cause inaccurate results.
The unique design of the Space Shuttle Orbiter landing gear with its highly loaded tires, hazards asso-
ciated with tire failure, as well as limited opportunities for landing test data from the vehicle resulted in a
strong reliance on tire test facilities. Because of high landing speeds, high vertical loads, long roll out dis-
tances, and unusually rough runway surfaces, existing tire test facilities have had difficulties in accurately
simulating the tire wear and forces of an entire shuttle landing.
The Landing Systems Research Aircraft (LSRA) is a unique addition to complement existing aircraft
dynamic tire testing facilities. Its capabilities are compatible with the Space Shuttle Orbiter requirements.
The design goal of the LSRA is to conduct dynamic tire testing on an actual surface while simulating ver-
tical loading, tire slip angle, and speed of an entire aircraft landing simultaneously. Computer control of a
tire test fixture allows for precise control of vertical load and slip angle of the test tire. The computer con-
trol software also provides a speed advisory to the pilot. These capabilities make it possible for the LSRA
to recreate a realistic combination of run distance, runway surface, vertical load, tire slip angle, and
ground velocity for aircraft landings.
The LSRA is the result of a cooperative effort of the Dryden Flight Research Center (DFRC), Lyndon
B. Johnson Space Center (JSC), John F. Kennedy Space Center (KSC), Langley Research Center
(LaRC), Ames Research Center (ARC), and many industry and military organizations. Flight test has
been conducted on runways at Edwards Air Force Base and KSC.
This paper describes the systems and capabilities of the LSRA vehicle. In addition, this paper discuss-
es ground calibration and flight tests used to validate the LSRA as a test facility.

The NASA Convair 990 (SN 10-29, tail No. 810) is a high-speed, medium range, low-swept-wing jet
transport (fig. 1). This aircraft is equipped with four wing-pylon mounted General Electric® CJ805-23
aft fan turbojet engines and a fully retractable tricycle landing gear (the main gear can no longer be re-
tracted with the LSRA modification). The aircraft is controlled by dual wheel and columns located in the
cockpit. The control surfaces are moved using a combination of mechanically driven flight tabs and hy-
draulics. The basic control system is augmented with a yaw damper which drives the rudder.
The LSRA underwent significant structural modification to provide space for the test gear and also to
react the test gear loads into the aircraft. Normal aircraft structural factors of safety were maintained for
all the original structural design conditions plus the additional loading conditions for landing gear testing
as defined in this report.
The primary components of the landing gear test system added to the LSRA are shown in figures 2
and 3. Figure 2 identifies the test pallet system elements within the aircraft. The hydraulic power of the
gear test pallet is provided by accumulators which use compressed nitrogen gas. Onboard hydraulic
pumps are used to pressurize the accumulators. The test pallet system is controlled by a test conductor
console which contains hardware switching capability and system monitoring capability. Included in the
system is a computer which controls the motion of the test gear pallet. In addition to the vertical load, the
test pallet system can apply braking to the test tire. Aircraft performance specifications before and after
the LSRA modification are presented in table 1.
Figure 3 shows the pallet which is the interface point between the test fixture and the landing gear test
system. The pallet is attached to the aircraft through a pair of parallelogram swing links which restrain
the test gear in pitch, roll, and yaw. The top of the test pallet is attached to two hydraulic actuators which
provide the vertical reaction load. The vertical loads are reacted into the airframe through a truss system
located inside the cabin.
®The CJ805-23 engine is a registered trademark of General Electric, Lynn, MA.
Table 1. Aircraft operational limits before and after LSRA modifications.
CV990 Aircraft
Max. taxi weight, lb
Max. takeoff weight, lb
Max. landing weight, lb
Max. landing speed, kgs
Max. range, n mi
Max. ceiling, ft
Max. velocity, KIAS
Empty weight, lb
NOTE: data taken from operations manuals of the CV990/LSRA.

Many landing gear test fixtures can be attached to the LSRA test pallet. Currently two attachments
have been designed. One is a modified shuttle main landing gear strut with dual tires, the other is a single
tire fixture that contains a rotary actuator which can be turned for desired slip angle. Table 2 presents the
maximum load and steering capability of the LSRA for these two fixtures.
Figure 4 shows a model of the single tire fixture. The fixture frame is attached to the test pallet. This
frame houses the rotary actuator which turns the test tire axle assembly. Tire braking is applied through
the braking assembly. This test fixture was used exclusively for all the testing described in this report.
A test computer controls the test tire vertical load and slip angle using feedback loops, and sends a
discrete signal to activate the wheel brake. Table 3 shows the capabilities of the control system. During a
test, the computer also displays to the pilot the difference between the current measured ground speed and
the desired speed profile for the test. The test tire vertical load feedback is provided directly from vertical
Table 2. Structural load capabilities of the LSRA.
Main gear
Variable yaw fixture
dual tire
single tire
Vertical load, lb
250,000, –50,000
150,000, –25,000
Drag load at tire contact point, lb
± 100,000
± 50,000
Side load at tire contact point, lb
± 40,000
± 40,000
+ 800,000
Brake torque, in–lb
– 250,000
Steering torque, in–lb
Table 3. Performance of the LSRA test system with the single rotational tire fixture.
Load control system max rate, unloaded
15 in/sec
Load control system max rate, max load
7 in/sec
Steering control system max rate
35 deg/sec
Maximum steering angle
± 20 deg
Load control system bandwidth
2 Hz
Steering control system bandwidth
3 Hz
Maximum error from commanded profile, load
± 3000 lb
Maximum error from commanded profile, slip
.25 deg
Typical error from commanded profile, speed
± 10 kts

load cell measurements while the slip angle is computed, as seen in figure 5, from a combination of an
angular displacement sensor on the steering fixture and two optical ground velocity sensors which pro-
vide aircraft slip angle across the runway.
The test pallet system includes a fail-safe feature which retracts the test pallet to its stowed position.
Pallet retraction can be caused by fault detection in hardware or software. The gear control system fault
detection software performs comparisons between redundant input signals, compares input signals to
minimum and maximum output values, and compares steering and extension values to simulated predic-
tions. Hardware fault switches detect over extension, ground contact, and over rotation of the test pallet.
If the test pallet cannot be retracted due to a mechanical failure, the hydraulic actuators can be separated
from the test pallet by explosive bolts. The tire retraction and the actuator separation can be performed
manually by either the test conductor or the pilot.
In addition to the test pallet retraction system, there are two fire suppression systems associated with
the test pallet system. A water deluge system was installed which can spray water directly on the CV990
main landing gear tires, brakes, and the test tire. A halon fire suppression system was placed in the cargo
bay near the hydraulic pumps to extinguish any fires in that area.
Flight planning and data analysis are performed with the aid of a six-degree-of-freedom simulation
resident on a desktop workstation. This simulation was programmed using the FORTRAN® computer
language and executes at 100 Hz, with a 400-Hz execution for landing gear dynamics. Aerodynamic data
used in the simulation were obtained from wind tunnel models, and then refined using data obtained dur-
ing early NASA flight test of the CV990. The workstation is interfaced with a gear control computer
which is a duplicate of the aircraft gear control computer. This configuration allows for production and
hardware-in-the-loop simulation testing of new time history profiles, as well as verification and valida-
tion of flight software revisions. The workstation and duplicate aircraft gear control computer configura-
tion were designed to be portable so that simulation, analysis, verification, and validation functions
would be retained at remote testing sites.
The time history profiles of load, slip, and speed are produced using output from this simulation, and
then converted to a binary format which is loaded onto a data diskette. After testing the profiles on this
disk using the hardware-in-the-loop configuration, this data diskette is then used to load the profiles onto
the aircraft gear control computer. A new time history profile can be developed in approximately one
hour. A new gear control software version can be qualified for flight in approximately three hours.
Figure 6 shows a typical landing test sequence. The CV990 aircraft makes a final approach. After
touchdown and derotation, the pilot calls for test initiation and uses spoilers, thrust reversers, and brakes
to follow the pilot speed advisory. The test gear is extended and controlled to match the preprogrammed
test profiles of vertical load, slip angle, and braking on the test tire. Upon completion of the test, the test
gear is automatically retracted. If a problem occurs during the test, either the computer or the hardware
fault detection system will command a retraction of the test gear. If a retraction does not occur, the test
®FORTRAN is a registered trademark of Information Processing Techniques Corp., Palo Alto, CA.

conductor or pilot can unload the test fixture by exploding the bolts connecting the test gear assembly to
the hydraulic actuators, thus relieving the vertical load to the test gear assembly.
The LSRA has performed approximately 100 test operations at Edwards AFB and KSC. During these
operations, all flight test profile preparation, data reduction, and analysis were performed at the test site.
The LSRA can collect onboard data or telemetered data. The data rates for the parameters range from
25 to 200 Hz. The test pallet has been instrumented with load cells in three axes. Appendix B presents the
equations for calculations and corrections for vertical, side, and drag loads. Accuracies of the measured
loads for the Shuttle Orbiter tire tests are ± 3000 lb vertical load, ± 500 lb side load, and ± 300 lb drag
In addition to the onboard and telemetered data, the LSRA has video cameras which can provide five
different views of the tire fixture. These cameras allow for real-time monitoring of tests, as well as post
flight analysis using video tape which is synchronized with the other data. High-speed film of tests is also
Calibration of the LSRA load cells was performed at DFRC. This was done by attaching static test
equipment to the test pallet and loading it to known values of vertical, side, and drag loads. This calibra-
tion effort provided information to validate the LSRA gear control software calibrations, provided infor-
mation on elastic deformation of the test fixture, and verified post flight data measurements.
The LSRA has performed two validation landing simulations; one was performed at the Edwards
AFB concrete runway, the other at KSC. The purpose of these tests was to validate the LSRA as a tire
testing facility by simulating an actual Space Shuttle Orbiter landing and comparing the test tire
wear from the LSRA to the tire wear of the Space Shuttle Orbiter. While both tests were successful, only
the KSC test will be discussed in detail to illustrate the process. The Space Shuttle Orbiter landing chosen
for the comparison was the STS 51-D landing. On this landing, the Space Shuttle Orbiter landed on
Runway 33 with approximately 8 knots of crosswind from the right-hand side of the vehicle. The weight
of the Space Shuttle Orbiter was approximately 200,000 lb. Inertial platform data as well as strain
gage data recorded from this landing were used to derive the load, slip, and speed profiles for the left
inboard main gear tire of the Space Shuttle Orbiter. The local tilt angle of this tire during the Orbiter
landing was simulated by raising the right-hand strut of the LSRA until the test tire tilt angle was approx-
imately –1.6 deg (left wing down).
The LSRA performed the profile shown in figure 7 on Runway 33. A load “spike” was placed at the
beginning of the load time history profile to create the initial load of 70,000 lb to simulate initial tire
touchdown. After the initial load, the average load control for the time history stayed within ± 3000 lb of
the target value. The slip controller held the slip angle to within approximately .40 deg until the speed fell
below 50 knots, at which point the resolution of the optical sensors caused some steering oscillations. The
steering system exhibited an oscillation of approximately .2 deg at 2 Hz. Subsequent slip controller im-
provements have eliminated these two anomalies. Figure 8 shows a time history of the achieved slip an-
gle plotted against the commanded slip angle after the improvements were made. The steering system

currently holds the slip angle to within .25 deg of commanded slip angle throughout the speed range. The
roll out of this test was estimated to be 10,300 ft, and the roll out of the Space Shuttle Orbiter test was
10,000 ft. The outside air temperatures were approximately the same between the LSRA test and
STS 51-D landing. Figure 9 compares the LSRA test tire with the STS 51-D Space Shuttle Orbiter tire.
For both tires, three cords were exposed on the left-most rib, and most of the other ribs were worn off.
This near identical tire wear was a significant factor in the validation of the LSRA as an Space Shuttle
Orbiter tire test bed.
The LSRA effort has provided significant data to the Space Shuttle program. Tests on the Edwards
Air Force Base dry lakebed runways were used to redefine the tire drag model used in Space Shuttle Or-
biter simulations and flight planning. A 20-knot crosswind capability was demonstrated for Space Shuttle
Orbiter landings on the Edwards Air Force Base concrete runway, and LSRA testing helped define the ef-
fects of ply steer and wheel tilt on the Space Shuttle Orbiter tire force model. The most significant contri-
bution of the LSRA to the Space Shuttle Orbiter program is the tire wear data that contributed to defining
the need for the KSC shuttle landing facility runway resurfacing.
The Landing Systems Research Aircraft (LSRA) provides a unique test bed for landing gear testing
which can reproduce vertical load, speed, slip angle, and actual runway surface.
Validation of the LSRA concept was achieved by recreating tire wear from actual Space Shuttle Or-
biter landings based on profiles from Space Shuttle Orbiter data. Static load calibration tests have verified
the flight measurements of the LSRA. Flight testing has shown the LSRA to be an efficient test facility at
remote sites.
The LSRA has had a significant impact on the Space Shuttle Orbiter program. Tire force and wear
data from the LSRA were instrumental in upgrading tire force and wear models used by the Space Shuttle
Orbiter program. LSRA data helped to define the resurfacing requirements for the smoothing of the KSC
runway surface.
The testing on the LSRA is complementary to the existing national dynamometer and test track facil-
ities. By comparing and cross checking tire force and wear data under actual landing conditions, this fa-
cility can validate results from other tire testing facilities.
Flight test of the system showed that the vertical load time history profile can be tracked within
± 3000 lb, and the tire slip profile can be tracked within ± .25 deg. This performance was considered ac-
ceptable for this application.
Features such as a generic test pallet that can have many different test fixtures attached to it, and the
ability to change commanded time history profiles of the load, slip, and speed have ensured that the
LSRA is a useful tool as a generic test bed.

1. Beall, Leman G., Dynamometer Evaluation of Continuous Tape Wound Type III Aircraft tires, Tech-
nical Report AFFDL-TR-69-102, Dec. 1969.
2. Davis, Pamela A., Sandy M. Stubbs, and John A. Tanner, Aircraft Landing Dynamics Facility, A
Unique Facility With New Capabilities. SAE Tech Paper Ser. 851938, Oct. 1985.
Beall, Leman G.: Dynamometer Evaluation of Continuous Tape Wound Type III Aircraft tires, Technical
Report AFFDL-TR-69-102, Dec. 1969.
Davis, Pamela A., Sandy M. Stubbs, and John A. Tanner, Aircraft Landing Dynamics Facility, A Unique
Facility With New Capabilities,
SAE Tech Paper Ser. 851938, Oct. 1985.
Daugherty, Robert F., Sandy M. Stubbs, and Martha P. Robinson, Cornering Characteristics of the
Main-Gear Tire of the Space Shuttle Orbiter,
NASA TP-2790, 1988.
Leland, Trafford J. W., Thomas J. Yager, and Upshur T. Joyner, Effects of Pavement texture on Wet-Run-
way Braking Performance,
NASA TN D-4323, 1968.
Tanner, John A., Sandy M. Stubbs, and John L. McCarty, Static and Yawed-Rolling Mechanical Proper-
ties of two types VIII Aircraft Tires,
NASA TP-1863, 1981.
Vogler, Wiliam A., and John A. Tanner, Cornering Characteristics of the Nose-Gear Tire of the Space
Shuttle Orbiter,
NASA TP-1917, 1981.

Document Outline

  • Cover Page
  • Title page
  • Abstract
  • Nomenclature
  • Introduction
  • Aircraft Description
  • System Capabilities
  • Safety Systems
  • Test Operations
  • Data Reduction
  • Test Validation / Flight test Results
  • Concluding Remarks
  • References
  • Bibliography
  • Appendix A
    • Test Machine
      • Applications
      • Data Collection
      • Accessories
  • Appendix B
    • LSRA Parameters and Plot Descriptions
    • RDP Page