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THE NEW HEAVY GAS TESTING CAPABILITY IN THE NASA LANGLEY TRANSONIC DYNAMICS TUNNEL Stanley R. Cole Jose A. Rivera, Jr.
NASA Langley Research Center Hampton, VA 23681-0001 USA Abstract To stagnation temperature, R V velocity, ft/sec X purity (fraction of heavy gas) The NASA Langley Transonic Dynamics Tunnel ratio of specific heats "t (TDT) has provided a unique capability for aeroelastic fluid density, lb. sec2/in 4 testing for over thirty-five years. The facility has a rich P history of significant contributions to the design of O) frequency, rad/sec many United States commercial transports and military aircraft. The facility has many features which contribute Sobscripts to its uniqueness for aeroelasticity testing; however, perhaps the most important facility capability is the use a air of a heavy gas test medium to achieve higher test h heavy gas densities. Higher test medium densities substantially wind-tunnel model W improve model building requirements and therefore full-scale vehicle V simplify the fabrication process for building 1 local condition aeroelastically scaled wind-tunnel models. The heavy X gas mixture gas also provides other testing benefits, including reduction in the power requirements to operate the facility during testing. Unfortunately, the use of the Introduction original heavy gas has been curtailed due to environmental concerns. A new gas, referred to as Historical perspective on aer0elasticity R-134a, has been identified as a suitable replacement for the former TDT heavy gas. The TDT is currently Although this paper is about the NASA Langley undergoing a facility upgrade to allow testing in R-134a Research Center's Transonic Dynamics Tunnel (TDT), heavy gas. This replacement gas will result in an to a very large extent the TDT is about aeroelasticity.
operational test envelope, model scaling advantages, and To this end, an historical perspective on aeroelasticity is general testing capabilities similar to those available offered here as a method of introducing the TDT and to with the former TDT heavy gas. As such, the TDT is shed a great deal of light on the past importance and expected to remain a viable facility for aeroelasticity potential future contributions of the TDT.
research and aircraft dynamic clearance testing well into Aeroelasticity is a field of aeronautics that deals with the the 21st century. This paper describes the anticipated interaction of vehicle structural components, in terms of advantages and facility calibration plans for the new elastic and inertial characteristics, and aerodynamic loads heavy gas and briefly reviews several past test programs that develop over the vehicle in flight. Aeroelasticity that exemplify the possible benefits of heavy gas encompasses dynamic phenomena such as buffet and testing.
flutter and static phenomena such as aileron reversal and wing divergence. Dynamic phenomena are highly undesirable and can result in catastrophic instability if Nomenclature not eliminated during the design and development process. Aeroelasticity is predominantly thought of in a speed of sound, ft/sec terms of detrimental dynamics. However, static b reference length, ft phenomena such as the deformation of an elastic wing Btu under steady aerodynamic loads are also important constant-pressure heat capacity coefficient, -- Co lb.R considerations in vehicle design. Such deformations Btu may or may not be catastrophic. Even if the Cv constant-volume heat capacity coefficient, -- lb. R deformations are not catastrophic, they can degrade desired lift and drag properties. The field of m generalized mass, slugs molecular weight aeroelasticity also deals with methods to prevent M e instabilities, such as through aeroelastic tailoring or static pressure, Ib/ft 2 P stagnation pressure, Ib/ft 2 through active control methodologies. For the reader Po fi2 with an interest in learning more about aeroelasticity, R references 1-3 are three classic textbooks on the subject.
gas constant, sec2. R Aeroelastic behavior has been important to many universal gas constant technological advancements for a very long time.
T temperature, R Reference 4 briefly describes someearly,unusual 1) that the facility be as large as feasible to enable accurate simulation of model details, such as control
encounters withaeroelasticity. Twoexamples of these
early aeroelastic effects areproblems in windmills that surfaces; 2) that the facility be capable of operating were empirically solved four centuries agoin Holland over a wide range of density in order to simulate various altitude conditions, because flutter characteristics often
andsome19thcentury bridges thatweretorsionally
weak and collapsed fromaeroelastic effects. Manyother change with altitude; 3) that the facility use Freon gas examples exist of aeroelastic problemsin civil as the test medium which, based on previous experience, enables the use of heavier, less expensive models,
engineering; however, the widest attention hasbeen
results in higher Reynolds number, and allows more givento aeroelasticity in the field of aeronautics.
Virtually fromthebeginning of flightaeroelasticity has efficient power usage; and 4) that the facility be capable played aroleinthedesign or flightreadiness process of of operating at Mach numbers up to !.2.
new vehicles. One of the earliestexamples of The NACA's answer to Regier's request for a new conscientious and beneficial use ofaeroelasticity was the facility was the conversion of the Langley 19-ft Pressure Tunnel to the Transonic Dynamics Tunnel (TDT). The
Wright Brothers' application of wingwarping to take
new wind tunnel would have all the features proposed by
advantage of wingflexibilityforthepurpose of lateral
control oftheiraircraft. 5
Regier: a 16-by-16 ft test section that could operate at Mach numbers up to 1.2 with variable pressure
Asflightcapabilities progressed rapidly in theearly
conditions in either air or a heavy gas with the chemical
20th century,aeroelasticity continued to play an
name dichlorodifluoromethane and hereinafter referred to
important partin aircraft design. Aeroelasticity was
as R-12. The design and conversion process began in
generally looked upon asa problem andaeroelasticians
1954 and the TDT became operational in early 1960. 6
were usually consulted tofixthese problems rather than
An early description of the initial heavy gas processing
beinginvitedto join the designteamearlyin the
system for the TDT can be found in Ref. 7. Figure 1
process to anticipate andmakebeneficial use of
shows an aerial view of the TDT. The TDT represented
aeroelastic characteristics. Thisledto manyexpensive
a significant advancement in aeroelastic testing
vehicle redesigns, aswellasthelossof flightvehicles
capabilities, primarily because of its large size, heavy
andhuman livesalongthe way. While theoretical
gas test medium and transonic speed capabilities.
developments progressed sothat there wasa continually
improving understanding of aeroelasticity, thedriveto
achieve faster flight forced vehicles in thedirection of
ever lighter structures and thinner, more flexible lifting
surfaces. This trend continued tomake aeroelasticity an
importanttechnical field for flight. As vehicles
approached and exceeded transonic speeds, theneed for
experimental assessment of aeroelastic behavior grew
substantially because of the pronounced effectof
transonic aerodynamics onphenomena likewingflutter.
At thetimethatthetransonic flightregime wasbeing
conquered, the ability totheoretically determine unsteady
aerodynamics forusein theprediction of flutterdidnot
exist. This inabilityto handle transonic aeroelastic
effects wasoneof themajorconsiderations thatledto
Fig. 1- Aerial photograph of the TDT.
the ideaof the NASA Langley Transonic Dynamics
Tunnel.
The TDT had a significant success within months of coming on-line. In late 1959 and early 1960 the History of the TDT Lockheed Electra aircraft experienced two catastrophic crashes. Evidence from these crashes pointed in the As the flight capabilities of aircraft advanced, wind direction of violent wing flutter. In an attempt to tunnel testing capabilities were also advancing to satisfy rapidly solve the Electra problem, a one-eighth scale the need. By the early 1950's transonic wind tunnels aeroelastic model was assembled for testing in the TDT.
were available. Aeroelastic experiments could then be A photograph of this first-ever flutter clearance model conducted at transonic conditions, which tended to be the tested in the TDT is shown in Fig. 2. By the time the critical flight regime for many aeroelastic issues. A TDT test occurred, a Lockheed engineer had identified significant early effort to specifically address this need the possibility that the Electra was experiencing a was the conversion of a 4-fl heavy gas tunnel at the coupling between the wing structure, engine gyroscopic National Advisory Committee for Aeronautics (NACA) torques, and aerodynamic forces in a phenomena referred Langley Memorial Aeronautical Laboratory to a 2-ft to a propeller-whirl flutter. The TDT wind tunnel tests continuous flow transonic tunnel for the purpose of showed that reduced stiffness engine supports on the flutter testing: However, the lack of a particularly outboard engines would cause the Electra to experience suitable facility in which to determine the aeroelastic propeller-whirl flutter. Based on these findings, the behavior of new high-speed aircraft designs led engine mounts were stiffened on the flight vehicles and A. A. Regier in 1951 to propose that the NACA design the aircraft never experienced a catastrophic flutter and build a large-scale, transonic facility dedicated to incident again. An unsubstantiated story has circulated aeroelastic testing. Reference 4 lists the following over the years that the money saved by the aircraft requirements that were originally stated by Regier: industry in quickly solving the Electra propeller-whirl flutter initselfmore than equaled thefacilityconversion were considered, including sulfur hexafluoride (SF6) which has been used in some recent test facilities on an costs inconstructing theTDT.
experimental basis. However, the gas of choice for replacing R-12 in the TDT was decided to be 1,1,1,2-Tetrafluoroethane (CH2FCF_), also identified as R-134a.
R-134a is a relatively inert gas with properties fairly similar to R-12. Like R-12, it is an odorless, tasteless, invisible gas. It has been determined to be incombustible within the temperature and pressure ranges which it will experience at the TDT, both for pure R-134a and for gas-air mixtures. Some of the principle properties of R- 134a, R- 12, and air are shown in Table 1. The data in this table shows that the properties of R-134a are relatively close to R-12. It is this similarity to R-12 that was considered critical to the continued viability of the TDT because of the great advantages that are realized in scaling and testing aeroelastic models (discussed in more details in the following section).
Fig.2- Lockheed Electra model mounted intheTDT.
Table 1 : A comparison of some properties of R- 134a,
Over the decades, the TDT has served as a
R-12, and air.
workhorse for experimental aeroelastic research and
Test medium
vehicle clearance testing. Testing hasincluded such
R-134a R-12 Air Property
varied aeroelasticity concerns asbuffet, divergence, gusts
102.03 121.00 28.97 M_
loads, flutter,anddynamic response. In addition to
1.13 i.14 1.40 T
testing forthese phenomena, manypassive andactive
540 5O5 a, ft/sec
control studies have been carried out in the TDT to
demonstrate methodsof overcomingaeroelastic
At the time of the writing of this paper, the project
obstacles to flight. References 8-10 provide overviews
to convert the TDT from an R-12 testing capability to of testing thathasoccurred in theTDTovertheyears.
an R-134a testing capability is ongoing. The
Most military fightersand commercial transports
conversion is planned to be completed in July 1997, at
developed in theUnited States have been tested in the
which time a calibration effort will begin (discussed TDTatsome timeintheirdevelopment history.
below) for the new operating test medium. Following
Althoughthe TDT enjoyedsignificantearly
the calibrations, the TDT will return to operational
success, the continued progression of aircraft flight
status with both air and R-134a heavy gas operational
performance eventually began to pushthe realmof
capabilities available. Even during this shutdown
suitabilityof usingtheTDT for aeroelastic clearance
period, the demand for the TDT clearly remains high as
studies. In theearly1980's, vehicle configurations had
exhibited by a full test schedule for approximately 13
advanced to thepointthatit wasbecoming evermore
months after the facility returns to normal operations
difficult to scaleaeroelastic modelsto matchthe
and only a few weeks of currently undefined test time
lightweight, relatively flexiblemodern aircraft.In an
during the first 24 months of operations.
attempt to reduce the challenge of scaling transonic
aeroelastic models, anupgrade to theTDTfacilitywas
accomplished which increased the drive motor
TDT Characteristics
horsepower to a levelwhichresulted in a 50 percent
increase in the dynamic pressure capability. This
The TDT is a large wind-tunnel built for the
upgrade was completed in 1985,thus easingthe
purposes of conducting aeroelastic research and of
difficulty ofdesigning and building aeroelastically scaled
models for taskssuchas flutterclearance of flight clearing vehicles of aeroelastic phenomena such as flutter. The TDT is capable of achieving Mach numbers vehicles.
above the speed of sound, reaching M = 1.2 in air and an estimated M = 1.1 in the new heavy gas, R-134a. The TDT has a variable pressure capability from near Heavy Gas Conversion vacuum to about one atmosphere. The 16 x 16 ft test section allows the testing of reasonably large models.
The need to eventually discontinue the use of the And, finally, the high density available by using the R-12 heavy gas in the TDT was identified at the end of heavy gas capability (compared to air) provides a great the 1980's. Environmental constraints on the use of advantage in the scaling of aeroelastic models. It is this R-12 were being accelerated such that its future combination of large scale, high speed, high density, availability for wind-tunnel testing was at risk and its and variable pressure that makes the TDT ideally suited cost was rising rapidly. An effort was initiated at for testing aeroelastically scaled models. In addition to NASA Langley to identify a new candidate heavy gas for these facility operating characteristics, there are a use in the TDT in place of R-12. A number of gases
y
Adjustable Vanes Fan Blades Nacelle Fixed 2 Row Vertical Tube Cooler-, 30,000 HP Electric Equipment Motor Building Test Chamber Butterfly Gate Settling I I Valve !
Chamber Room T_oml Pressure Probe Equipment Building Laboratory Building
I I I t t !
0 50 Scale, ft Fig. 3- Plan view of TDT facility.
Special Facility Features number of other facility features that help make the TDT particularly suitable for aeroelasticity testing.
Figures 3 and 4 show a plan view and a test section area Bypass Valves- A unique safety feature of the TDT cross-sectional view of the TDT. These drawings show is a group of four bypass valves connecting the test a number of the special facility features that will be chamber (plenum) of the tunnel to the return leg of the discussed in more detail in the following sections.
wind-tunnel circuit (see Fig. 3). In the event of a model instability, such as wing flutter, these quick-actuating Manifold bypass valves can be opened. This opening causes a rapid reduction in the test section Mach number ard dynamic pressure; hopefully resulting in saving the wind-tunnel model from a catastrophic failure. The bypass valve system results in approximately a 25 percent reduction in operating Mach number and up to a 38 percent reduction in dynamic pressure in the transonic operating range, with significantly smaller reductions in dynamic pressures at low Mach number conditions. Half of these reductions occur in about three seconds.
Air_tream Oscillator System- Another special capability available in the TDT is a set of four oscillating vanes, referred to as the airstream oscillator system. The vanes are located upstream of the test section and can be driven sinusoidally to simulate atmospheric turbulence or gusts. These vanes have been used in a number of tests for the purpose of gust loads studies and active gust load alleviation demonstrations.
Reference 11 contains a good description of the TDT I L f | I I airstream oscillator system, and references 12 and 13 0 25 Scale, ft describe two different experimental studies conducted using the airstream oscillator system.
Fig. 4- Cutaway view of test section area of TDT.
Te_ting Abilities Control Room- Another convenient feature of the TDT is that the control room, from which the tunnel is operated and from which the wind-tunnel test is directed, As previously stated, the TDT has been used to is physically situated directly adjacent to the test section conduct many types of tests on many types of vehicles for almost four decades. Tests have included flutter, within the pressure shell of the test chamber plenum (see Figs. 3 and 4). The control room has a large divergence, buffet, gust loads, rotorcraft aeroelasticity matrix of observation windows so that direct visual and loads, unsteady pressure measurements, ground wind observation of the wind-tunnel model is possible. This loads, dynamic response, atmospheric reentry loads and feature has proven to be very valuable because of the dynamics, propeller-whirl flutter, stall flutter, aileron reversal, control surface buzz, flight stability, stores dynamic nature of aeroelastic testing and because constant visual monitoring is essential to the success of flutter, fuel-slosh dynamics, active structural mode control, maneuver load control, active buffet alleviation, testing. Also, the close proximity of the facility active and passive flutter suppression, and many others.
operators and the test engineers allows immediate, clear and concise communication in the event that model Vehicles tested have included general aviation airplanes, instabilities must be overcome by tunnel operations. commercial transports, military fighters, rotorcraft, tiltrotor vehicles, launch vehicles, space shuttle Test Section Isolation- The test section and test concepts, planetary landers, high-speed civil transports, unmanned high-altitude vehicles and others. In the chamber plenum area of the TDT can be isolated from interest of aeroelastic research many tests have been the remainder of the tunnel circuit by a butterfly valve conducted on non-vehicle-specific models. In general, and a gate valve (see Fig. 3). This isolation allows access to the wind-tunnel model with the convenience of the special scaling relationships provided with the heavy gas testing capability have driven the need to conduct leaving the R-134a heavy gas in the remainder of the wind-tunnel circuit, even under low pressure. This these types of tests in the TDT.
The ability to support and test models in many feature significantly reduces gas processing time and, different configurations has also added to the value of the therefore, greatly increases the test efficiency of the TDT. Model support systems include sting-supported facility. With the isolation valves closed, only about models, semispan models mounted on a sidewall 25 percent of the test medium in the entire tunnel circuit turntable, "free-flying" cable-mounted models, and floor- has to be processed to allow access to the wind-tunnel mounted models. Figure 5 shows several examples of model.
the different model mounting systems that will be discussed in the remainder of this sub-section.
Fan-Protection Screen- Although this feature does Semispan models are often tested on the sidewall not directly result in any benefit to conducting turntable with half-body fuselage sections which provide aeroelasticity studies, there is a model debris catch appropriate wing root aerodynamics and also remove the screen located at the wind-tunnel turning vanes just wing root area from the boundary layer along the wind- upstream of the drive motor fan blades. The provision tunnel test section wall. The sidewall turntable of this catch screen recognizes the fact that aeroelastic mechanism allows models to be tested at various angles model testing is very high risk and that the probability of attack which are remotely controlled during testing.
of a model failure that could damage the facility fan Semispan models can also be tested mounted against blades is fairly high. This catch screen has protected the splitter plates for the purpose of ensuring that the flow fan blades from model debris in the past and is over the model is not contaminated by the wind-tunnel considered a very valuable facility feature that wall boundary layer. A special semispan model-mount contributes to the suitability of the TDT for system that has been widely used for conducting aeroelasticity testing.
aeroelastic research tests in the TDT is referred to as the Pitch and Plunge Apparatus (PAPA). The PAPA Cooling Coils- A final feature of the TDT that provides for testing rigid aerodynamic surfaces that are contributes to the ability to complete successful mounted to a flexible support system to allow pitch and aeroelastic studies is the ability to regulate the airstream plunge motions and even classical two-degree-of-freedom temperature in the TDT. A set of cooling coils is flutter. This apparatus assists in the difficult task of located internal to the test circuit at the turning vanes determining and separating aerodynamic and structural immediately downstream of the drive motor for the effects by providing a dynamic support system with a purpose of maintaining a reasonably constant operating rigid aerodynamic surface. References 14 - 17 describe temperature in the facility during testing. The cooling some results that have been obtained using this PAPA system is not actively controlled so temperature is not mount system.
precisely held; however, typical testing in the TDT A recent addition to the semispan model test occurs with temperatures in the vicinity of 105 ° F.
capability at the TDT is a retractable sidewall turntable.
Operating temperatures rise to an extreme of about This turntable is located approximately 2.5 ft 140 ° F at the highest operating dynamic pressures, downstream of the primary TDT sidewall turntable and which require the most drive motor power to achieve.
is able to move in and out with respect to the sidewall The ability to control operating temperatures is to potentially allow ease of access to certain areas of the important because the material stiffnesses of the types wind-tunnel model systems, as well as to allow for the of materials that must be used in order to build installation of systems such as the PAPA mount aeroelastically scaled models are sometimes quite system or force-and-moment balances beyond the plane sensitive to temperatures.
of the test section sidewall.
Sidewall Turntable Sting Two-Cable System Rotor Testbed Floor Turntable Pitch and Plunge Apparatus Fig. 5- Model mounting systems for the TDT Two primary floor mount systems are frequently R-134a Properties used in the TDT. A large, removable floor turntable can be used in the TDT. This turntable is generally used for One of the difficulties in dealing with R-134a as a the testing of ground wind loads models, most often of test medium in the TDT is the necessity to account for launch vehicles on the launch pad, to provide a gas properties under the recognition that a small fraction mechanism for changing the wind azimuth angle of the of the test medium will always be air. Equations are freestream flow over the model. The second primary being developed that estimate flow properties for the floor mount system is the Aeroelastic Rotor appropriate mixture of R-134a and air. In order to Experimental System (ARES) that is used for rotorcrafl accomplish this, thermodynamic properties of the gas aeroelasticity and loads testing of rotor and rotor-hub must be calculated as a function of static pressure, static systems.
temperature, and gas purity. It is not the intention of In addition to floor mounts, full-span models can be this paper to give a thorough explanation of the required tested in the TDT on a test-section centerline sting mixture equations; however, in order to give the reader support apparatus. This apparatus provides for vertical some feel for what goes into calculating R-134a flow translation of models in the test section, which is properties, the following discussion will address gas generally more important during model setup or properties for pure R-134a. The molecular weight of configuration changes than during actual testing. More R-134a, as shown in Table 1, is importantly, the sting apparatus also allows for the remote positioning of model angles of attack, within Mg= 102.03 kg/kmol .
approximately a +23 ° range of motion. This range can also be extended by the use of 5 °- and 10°-offset sting Therefore, the gas constant for R-134a is sections that are readily available at the TDT facility.
The final primary support technique used in the kJ TDT is a cable-mount model support system. Fig. 5 E 8.314-- kJ _ kmol. K m = 0.0815-- shows an F/A-18 E/F aeroelastically scaled model kg-K Mg 102.03 kg mounted on a two-cable support system typical of kmol current test set-ups. For configurations in which the ft 2 interaction of fuselage flexibilities, flight stability = 487.3 modes, and aeroelastic modes are important, the cable- sec 2- R mount system enables this interaction to be simulated in the tunnel.
Under ideal gasassumptions, theconstant-pressure heat designed and built primarily for the purpose of full-scale Reynolds number simulation through the combination capacity coefficient isgiven by: of testing at high pressures and at cryogenic temperatures. In a similar fashion, the Langley TDT Co = 0.3437 + (3.988 x 10-_)T -(2.113 x 10-7)T was designed and built to allow for the proper simulation of parameters that are important in the field +(7.295 × 10-")T 3 Btu lb. R of aeroelasticity.
The following discussion will detail the essential and the constant-volume heat capacity coefficient is scaling parameters associated with an aeroelastic model, given by: such as a flutter clearance model, in an attempt to explain the advantages that the TDT offers for aeroelastic testing with regard to scaling. This Cv = 0.7540 - (2.199 x 10 -3)T + (3.211 x 10-')T 2 discussion is not an attempt to thoroughly explain the various implications of all aspects of scaling parameters -(1.539× 10-")T 3 - 2.341 × 10 4 Btu T 2 lb. R and their impact on the usefulness of aeroelastic measurements. Reference 18 contains a good description of scaling considerations in designing For nearly pure R-134a at T=530 R (-70 ° F), this leads to a ratio of specific heats of dynamically scaled wind-tunnel models.
One of the first scaling parameters that must be Cp considered, and perhaps the easiest to handle, is a y =- = 1.13.
geometric length scale. This is primarily driven by the Cv facility size. From the standpoint of building an aeroelastic model that properly simulates structural In comparison, the ratio of specific heats for air is elasticity, it is generally advantageous to build as large a approximately _, =1.4 and for the previous heavy gas, model as possible. However, this maximum size is R-12, ], =1.14. Under the assumptions of nearly pure constrained by wall interference and shock reflection R-134a and that R-134a is an ideal gas, the speed of considerations and, in a few cases, by the streamwise sound in R-134a is region of good flow within the test section. In general, a wind-tunnel model designed for testing in the TDT is a = .f-yRT = 540 ft/sec.
limited to a maximum span of approximately nine feet, whether a semispan or a full-span model. Ref. 19 From this point, all flow parameters for pure R-134a contains a table of model dimension ratios suitable for can be calculated using compressible flow equations.
use as sizing guides in building models for testing in Table ! shows a comparison of several properties of transonic tunnels such as the TDT.
the operating gases discussed herein. As can be seen, The other scaling relationships that are of primary compared to the properties of air, R- 134a has properties importance for aeroelastic scaling are Mach number, relatively close to the previous operating test medium of frequency, and mass. It is imperative that Mach number R-12. The fairly large change in gas properties between be matched in order to properly simulate transonic the heavy gases and air results in a number of aerodynamic conditions. The ability to match Mach advantages in terms of facility operations and number requires a facility that can obtain the desired capabilities. In the heavy gas, the TDT can achieve Mach number. In the case of flutter and other higher densities and, therefore, higher dynamic pressures aeroelastic phenomena, the Mach numbers of concern for the same Mach number. In R-134a at a fixed Mach are often in the transonic range. The TDT offers the number, approximately a 100 percent increase in ability to test in the transonic range. The required test dynamic pressure can be achieved as compared to air velocity to match Mach number is significantly redtr.ed operations. Other parameters are likewise affected. For (see speed of sound in Table 1) when using a heavy gas instance, significantly higher Reynolds numbers can be test medium. These reduced velocities require lower achieved due to the changes in gas density, velocity facility drive motor speeds (approximately one-half) to (speed of sound), and kinematic viscosity. In addition to achieve a given Mach number condition. This results in these improved operating capabilities, the heavy gas ieduced tunnel power consumption for a given Mach allows for many advantages in aeroelastic model scaling.
number-dynamic pressure condition compared to air operations.
Scaling Relationships A slight limitation of the new heavy gas in the TDT is that it is anticipated that the maximum In order to properly interpret wind-tunnel results operating Mach number of the facility will not be quite with regard to a flight vehicle, it is necessary to account as high as it was with the previous heavy gas. This for scale effects. The effects of different phenomena limitation is generally viewed as acceptable since the generally require different scaling considerations. For facility will still be able to test beyond the speed of instance it is well known that Reynolds number sound so that the most critical flutter condition can be simulation is often of prime importance in simulating measured for most models. Figure 6 shows the the proper aerodynamic flow field. Much effort has been estimated operating boundary in R-134a compared to the directed toward proper representation of full-scale previous operating boundary in R-12 and the operating Reynolds number. One example being the National envelope in air.
Transonic Facility at NASA Langley which was 600 -- Other parameters need to be considered when designing models, such as Froude number and Reynolds /_ -_ ,_/---R - 12 number. However, some compromise is generally 500- R-134a -_/_, _\ required in order to build and test aeroelastically scaled 40O models. As previously discussed, testing in the heavy \ gas at the TDT does provide increased Reynolds Dynamic numbers, although the Reynolds numbers will not pressure, 300 psf generally match flight conditions. Another benefit of -- Air ._/ \l using the heavy gas is that in addition to matching the -- // L_ _ I primary aeroelastic scaling parameters, Froude number can also be matched for a model that is approximately 1oo - /// "_I one-quarter geometric scale. Froude number scaling can z I I i Ill be important because it will ensure proper scaling of static deflections. If subsonic Mach numbers are 0 .4 .8 1.2 MachNumber considered the critical regime for a particular model, then Mach number scaling can be sacrificed and Froude Operating boundaries in the TDT for three test Fig. 6- number scaling is considered a more critical parameter.
mediums: air, R-12, and R-134a (R-134a boundary estimated).
Calibration Plans The real advantages of the heavy gas properties for A series of calibration tests have been planned for an aeroelastic model begin to manifest themselves when the TDT to quantify flow properties and flow quality one begins to consider scaling the dynamic properties of after the completion of the facility conversion to allow the vehicle to properly match frequencies. Frequencies operation in R-134a. The current focus of the are properly scaled when the following relationship is calibration efforts will be to determine a suitable set of satisfied: instruments and measurement techniques to ensure that accurate test section flow conditions are being measured.
Beyond this, there are also plans to make Mach number distribution measurements in the TDT test section area along the test section walls, along the centerline of the test section, and at a matrix of locations across the test Since the length has been previously constrained, section cross section for several streamwise locations.
primarily by facility size, the ratio of model structural Additionally, boundary layer thickness will be measured dynamic frequencies to vehicle frequencies is further around the test section and at several streamwise influenced only by the test medium velocity, which is locations in the test section. Turbulence and flow directly related to the speed of sound at a given Mach angularity measurements will be made at several number. As shown in Table 1, this means that model streamwise locations using a sting-mounted flow survey frequencies in heavy gas (R-134a) will only be about rake. Finally, an attempt will be made to directly half the values they would have had to have been in an measure the speed of sound of the test medium to air test medium. This frequency reduction makes the construction of a model less difficult since it can be less potentially improve flow property measurement accuracy for mixtures of heavy gas and air. In addition stiff than an appropriately scaled model that would be to these efforts to determine the flow properties in the designed for testing in air at the same flow conditions.
TDT for both air and R-134a test mediums, another goal Also, with regard to the safety aspects of testing such of the calibration effort is to determine the maximum models, it is easier to observe, and potentially save, a operating capabilities of the TDT in terms of dynamic wind-tunnel model if destructive phenomena such as pressure and Mach number. The following subsections flutter occur at lower frequencies. Lower frequencies affect visual observation abilities as well as data will discuss in more detail the objectives and plans for the calibrations efforts.
acquisition and monitoring equipment requirements.
The final parameter that is essential for proper Primary Tunnel Parametgrs scaling of an aeroelastic model is mass: The most important aspect of the planned calibration efforts will be to determine proper instrumentation locations to ensure accurate flow property measurements, particularly with the new For this parameter, the increased density of R-134a R-134a heavy gas operating capability. The determination of the primary flow parameters relative to air at a given temperature and pressure combination allows a properly scaled wind-tunnel model fundamentally requires the measurement of only four properties: stagnation pressure, static pressure, to be built that is approximately four times heavier than stagnation temperature, and R- 134a purity.
would result for air testing. This weight increase eases Historically at the TDT, stagnation pressure has the difficulty of building aeroelastically scaled models been measured in the settling chamber (see Fig. 3) of because it is very difficult to match stiffness and meet strength requirements on a lightweight model. the TDT by a total pressure probe mounted two feet
away fromthewest wall of the settling chamber at a
R-134a heavy gas testing, test medium purity. Under
position slightlybelowthe vertical centerline of the
the assumptions that the ratio of specific heats is settling chamber. Duringthe newcalibration effort, available based on the test section purity (using real gas
stagnation pressure probes will be mounted at nine
mixture equations)and that ideal gas flow equations are
different locations several feetdownstream of thetunnel
sufficiently accurate, the local Mach number will then
turningvanes (located just upstream of the settling be calculated based on the equation
chamber). Measurements will be made at these new
probelocations, as well as the previous location,
primarily to determine if theoriginal probe provides a
sufficiently representative measurement of stagnation
pressure or if a newmeasurement location or technique,
suchas averaging several probes, maybe needed in
future testing.
Mach number distribution measurements will be made
The primarystatic pressure measurement has
in three general categories summarized in the following
historically been made via a tubelocated between the
subsections. The ultimate goals of these local static west wallofthe plenum chamber and thecontrol room, pressure measurements (and calculated local Mach again near thevertical centerline of thetunnel circuit.
numbers) are to determine if there are significant
This appears to be a reasonable locationunder the
variations in Mach number through the test section and
assumption thatthe test medium in the plenumis
to determine if corrections to the measurement of test
relatively stilland atnominally uniform pressure except
section Mach number are required.
in theimmediate vicinityof thesidewall slotsin the
testsection.In order to check on theaccuracy of the
Sidewall Pressure Measurements- The term
existing static pressure measurement, anumber of tubes
sidewall pressure measurements is being used here to
will be located at various positionsin the plenum
describe any measurement of local static pressure along
duringthe calibrations to assess static pressure
any of the primary wall, ceiling, or floor surfaces of the measurement asafunction of location in theplenum.
TDT test section. Four primary streamwise rows of
In the past, stagnation temperature has been
static pressure ports will be located in the test section,
measured with thermocouples in the TDT. This
one row on each of the primary test section surfaces.
measurement wasmade just a few feetdownstream of
There will be approximately 28 static pressure ports
thecooling coilsin thetunnel circuit(see Fig.3). As
along each of these rows. Fig. 7 shows a conceptual
withstagnation pressure and static pressure, anumber of
drawing of the placement of these static pressure port
thermocouples will beused during thecalibration effort
rows in the TDT test section. The static ports will be
to determine the most appropriate location for
spaced more densely in the vicinity of the sidewall
measuring the facility stagnation temperature. It is
turntable, where sting-, cable-, and sidewall-mounted
anticipated thata measurement in thesettling chamber
models are tested.
may become the primarystagnation temperature
measurement locationin the future. The previous
arrangement may haveplaced the thermocouple too
closeto the facility coolingcoils, not allowing
sufficientmixing beforereaching the measuring
instrument.
Thefinal parameter needed tocalculate all pertinent
flow properties is thepurityof theR-134a gaswith
respect toaircontamination. It is possible toeliminate
theneed to directly measure gaspurityif the speed of
Static pressure o f c s
sound ofthetest medium mixture can bemeasured under
theassumption thatthemixtureis of two thermally
perfect gases forwhich theindividual gasproperties are
Fig. 7- Conceptual drawing showing approximate
known. This possibilitywill be discussed in more
locations of sidewall pressure orifices.
detail ina latersection.Previously, flowproperties in
the TDTwere calculated forthe R-12 heavy gas medium
Centerline Tube Measurements- To enable based onpuritymeasurements made withgasanalyzers.
the measurement of static pressures along the centerline
Thistechnique will still betheprimary technique, but
of the TDT test section, a new centerline tube apparatus
will employ a newsystem of modern gasanalyzers for
is being fabricated. A conceptual drawing of the thenew heavy gas.
placement of the centerline tube is shown in Fig 8.
This apparatus will attach to the TDT sting support and Mach Number Distributions extend forward through the test section into the aft region of the settling chamber. Positioning the nose of An important aspect of calibrating the TDT will be this centerline tube in the lower flow speeds of the to measure static pressure variations as a function of settling chamber will minimize wake disturbances that position in the test section. In general, these test could cause erroneous static pressure measurements section static pressures will then be converted to local downstream along the centerline tube. The tube will Mach numbers based on the settling chamber stagnation have static pressure ports at approximately 127 pressure and stagnation temperature and, in the case of streamwise positions. Thecenterline tubewill also regarding the streamwise variation of Mach number in have anorificeformeasuring stagnation pressure at the the test region around the primary model mount nose of the tubein thesettling chamber. This will positions for sting, cable and sidewall mounts and are serve asanadditional stagnation pressure measurement very useful particularly since swept-wing models will to assist in determining thebest technique with which actually cover a range of test section stations.
tomeasure theprimary flowstagnation pressure.
Boundary Layer Measurements Side Vie_,,f
,,e.um :: ! IIII
To assess the possible influence of proximity of a model to a test section sidewall, a series of boundary
.,,as.va,v;>., IIII
I I /-Center,.e t,.be IIII layer rakes are being fabricated for use at up to six positions around the test section perimeter
Tes, se iog IIII
I I Chamber III I simultaneously. These rakes will extend from the
IIII
sidewall surface approximately one foot into the flow.
The boundary layer rakes will have numerous stagnation
3..__1111
pressure tubes along their span to give the variation in stagnation pressure from the wind-tunnel wall out into Fig. 8- Sketch of centerline tube in TDT test section.
the freestream flow. Figure 10 shows an approximate layout of the stagnation pressure tubes along the Survey Rake Measurements- The final boundary layer rake. In addition to the positioning of apparatus that will serve as a measurement of local these probes around the inside perimeter of the test Mach number in the test section will be a flow survey section, the six probes (or subsets thereof) can also be rake that will be sting mounted in the test section.
moved to other streamwise positions to allow the Figure 9 is a conceptual drawing illustrating the measurement of streamwise variation in boundary layer positioning of this survey rake. This rake will be a characteristics.
single horizontal blade with eleven probes on the leading edge. Several probe devices will be available for use on this flow survey rake. The pertinent devices with regard to the measurement of local Mach number will be static and stagnation pressure probes. By mounting these probes at the various positions across the rake, the Mach number distribution of the central m span of the test section can be determined. The probes m w on the rake will span approximately five feet on either m side of the test section centerline. In addition to this m m spanwise measurement of local Mach number, the rake m m will be able to traverse on the sting to determine the n m vertical distribution of Mach number in the test section.
m m m m Top Views.
m Test-section m m m m 2 stati°ns 1 J m m !
Settling Fig. 10- Boundary layer rake drawing.
Chamber ( Control Room ) Turbulence Measurements Turbulence levels in the TDT will also be measured Fig. 9- Drawing representing survey rake device.
during this calibration program. Turbulence Three positions for locating the survey rake are information is considered very important, particularly also shown by dashed lines and numbers.
when dynamic response is of prime interest and may be highly influenced by background turbulence levels. To The survey rake will have the capability of being make an assessment of turbulence in the TDT, a number installed at three different streamwise positions in the of hot-wire probes will be available for testing on the test section (alternative positions indicated in Fig. 9).
sting-mounted survey rake.
Position I will be at the test section station position corresponding to the center of the hub location for the Speed Of Sound Measurements ARES rotorcraft testbed that is often used in the TDT to conduct rotor aeroelastic and loads research. Position 2 An attempt is currently being made to assemble a wilt be at the streamwise location of the pitch axis of system of acoustic transmitters and receivers that could sidewall semispan models, 10 ft downstream of potentially measure the speed of sound of the test position 1. Position 3 will be 3 ft further downstream medium. Figure 11 is a conceptual drawing showing a where the retractable turntable is currently positioned.
general idea of how the transmitters and receivers could Position 3 measurements will provide information
belocated inthetestsection or thetestsection plenum
downstream of the test section. A part of the calibration
area.Fromthespeed of sound, theproportion of the
efforts will be to make an assessment of the optimum
gasconstituents canbe determined giventhe known
re-entry flap settings to be used at different Mach properties ofpure R-134a and air,andfromthese purity number and dynamic pressure combinations to provide
proportions andothermeasured tunnelconditions, all
the best Mach number distribution through the test other flow properties can be calculated. The proportion section. Recommendations for re-entry flap settings of gases is calculated based on the following equation, may very well be dependent on the test medium, so that the change over to R-134a as the operating gas may result in changes in the use of the re-entry flaps.
Another facility variable is the position of the pre- k?h rotation vanes that align the flow prior to entering the drive system fan blades for improved facility operating
f[
efficiency. The pre-rotation vanes may have an effect on
[x( o
the flow turbulence level and the flow angularity, so measurements will be made for variations in the pre- as given in reference 20. Such a speed of sound rotation vane settings. However, the effect of the pre- measurement system has the potential for improving the rotation vanes setting is probably more of an issue in accuracy of R-134a property calculations, although it determining maximum operating conditions of the remains to be determined if the most accurate process facility, particularly with the new test medium, than in will be the direct measurement of test medium purity determining effects on the flow properties in the test section.
via new gas analyzers or through the direct measurement Another tunnel variable that will be considered for of the speed of sound.
its effect on flow properties will be sidewall slots on the Bypass Valves wall of the test section where semispan models are Top View typically tested. For most tests conducted in the TDT, the effect of the proximity of the test article to the test section sidewall slots has been considered minimal =^_\l Test Section _,v. _ because most aeroelastic models are tested at nearly zero- lift conditions. Also, most often the important lift Settling loads for aeroelastic testing are dynamic in nature and therefore the proximity of the sidewall slots may not be as important as they would be for large, steady aerodynamic loads. However, the possible influence of Fig. 11- Conceptual drawing of possible arrangements the sidewall slots has led to a decision to conduct of equipment for measuring speed of sound.
facility calibrations, particularly for the new heavy gas R-134a, with these sidewall slots opened and closed.
Flow Angularity Measurements Another important flow characteristic to understand Typical TDT Tests for proper testing in a wind-tunnel facility is flow angularity. A fourth set of probes will be available for After the completion of the calibrations, the TDT will mounting on the survey rake that will allow the return to operational status with the capability of testing measurement of flow angles with respect to both the in either air or the new R-134a heavy gas test medium.
horizontal and vertical planes of the test section. The The new R-134a test medium capability will be probes that will be used for the purpose of flow beneficial to the aeroelastic testing community, angularity measurement are generally referred to in the supporting aircraft vehicle programs and aeroelastic literature as five-hole probes.
research developments for many years to come. In an attempt to emphasize the potential future impact of Tunnel Configurations TDT testing capabilities, the following sub-sections summarize several key recent TDT wind-tunnel test In addition to the above types of calibration programs completed prior to the conversion to the measurements that will be made after the completion of R-134a testing capability. These four programs cover a broad range of aeroelastic vehicle-development and the heavy gas upgrade of the TDT, another important research objectives and represent the wide range of aspect of calibrating the facility will be to account for vehicle types that are typically studied in the TDT.
configuration variables of the facility itself. Aside from drive motor speed, the primary TDT facility variable is FIA-18 E/F the position of the re-entry flaps located on the ceiling and the floor at the downstream end of the test section.
These flaps provide for efficient operation of the facility A series of five wind-tunnel test entries was completed during transonic testing. The re-entry flaps essentially in the TDT for the purpose of flutter clearance of the re-capture facility flow that has escaped, or expanded, new F/A-18 E/F fighter. The wind-tunnel model is through the test section sidewall slots. The flow shown in the cable-mount configuration in Fig. 12. In impinges upon the re-entry flaps and is drawn back into many ways this test series represents a typical flutter the tunnel circuit through the tunnel expansion cone clearance program that might be conducted in the TDT fora militaryaircraft.Thetests consisted of multiple controls and composite tailoring of the wing structure entries thatbuilt upon one another. Model flying has been shown to improve propeller whirl flutter stability was first verified on the cable-mount system instability margins. A recent summary of the active with a "rigid" version of the model. The second goal of vibratory loads reduction as well as a description of the WRATS testbed is available in reference 22.
the test series was to verify the aeroelastic characteristics of the individual surface components on a sting mount to minimize the risk of catastrophic loss of the whole model. Following the components testing, the entire flexible vehicle was flutter cleared on the cable-mount system: first in a clean-wing configuration followed by stores-clearance testing of many store configurations.
Fig. 13- Model with aerodynamic shell sections removed to show piezoelectric actuators.
Fig. 12- F/A-18 E/F model cable-mounted in the TDT.
Flutter Suppression Using Piezoelectric Ac_uator$ Experimental aeroelasticity research programs, often not directly associated with any specific flight vehicle, are frequently carried out in the TDT. One such base research program involved the design and fabrication of a model that was fitted with many piezoelectric elements that could be used to induce strain in the structure of the wing model. Through the use of active control, these piezoelectric elements were employed to suppress flutter Fig. 14- WRATS tiltrotor model mounted against a and reduce loads. Figure 13 is a photograph of the wing splitter plate in the TDT.
with the aerodynamic-geometry shells removed to expose the piezoelectric elements. Control laws tested Launch Vehicle.s.
in this program resulted in as much as 12.5 percent increases in the flutter dynamic pressure. This testing A number of launch vehicle tests have also been proved that piezoelectric control of dynamic instabilities conducted in the TDT in recent years. These tests is possible. Reference 21 summarizes some of the include a ground wind loads test of the Atlas-Centaur II results from this test program.
vehicle and three tests primarily concerned with the buffet response over hammerhead payload configurations TiltrQtor Research in the transonic flight regime, one test for each of the following launch vehicles: the Atlas-Centaur I Large As previously indicated in this paper, the TDT is often Payload Fairing, the Delta II Composite Payload used for conducting rotorcraft tests. In recent years, a Fairing, and the Delta III. Reference 23 summarizes the number of research studies have been completed TDT test results for the Atlas-I wind-tunnel model. A associated with a tiltrotor model testbed. Figure 14 photograph of the Delta IH launch vehicle model is shows a photograph of this testbed, called the Wing and shown in figure 15. These tests proved to be significant Rotor Aeroelastic Testing System (WRATS). Through risk mitigation steps in verifying the flight readiness of the WRATS test program the following objectives have the vehicle designs with regard to many dynamic been successfully demonstrated: tiltrotor vibratory loads aeroelastic concerns.
have been reduced using active swashplate/flaperon .
Dowell, Earl H.; Crawley, Edward F.; Curtiss, Howard C., Jr.; Peters, David A.; Scanlan, Robert H.; and Sisto, Fernando. A Modern Course in Aeroelasticity. Kluwer Academic Publishers, 1995.
4.
Garrick, I. E.; and Reed, Wiimer H., III: Historical Development of Aircraft Flutter. AIAA 81-0491 R, Journal of Aircraft, Vol. 18, No. 11, November 1981.
.
Garrick, I. E.: Aeroelasticity -- Frontiers and Beyond. Journal of Aircraft, Vol. 13, No. 9, September 1976.
6. Baals, Donald D.; and Corliss, William R.: Wind Tunnels of NASA. NASA SP-440, 1981.
Fig.15- Photograph ofDelta IHlaunch vehicle model
sting-mounted in theTDT.
.
Moxey, R. L.: Transonic Dynamics Wind Tunnel.
Compressed Air Magazine, Vol. 68, Number 10, October 1963.
Concluding Remarks .
Reed, WilmerH.: AeroelasticityMatters: Some The NASA Langley Transonic Dynamics Tunnel Reflections on Two Decades of Testing in the (TDT) was designed and built for the specific purpose of NASA Langley Transonic Dynamics Tunnel.
aeroelasticity research. Over the years it has been NASA TM-83210, 1981.
maintained and modified to allow for continued relevant contributions to the advancement of the fundamental .
Doggett, Robert V., Jr.; and Cazier, F. W., Jr.: understanding of aeroelastic phenomena. Most major Aircraft Aeroelasticity and Structural Dynamics United States commercial transports and military aircraft Research at the NASA Langley Research Center- which are capable of flight at transonic speeds have been Some Illustrative Results. Presented at the 16th tested at the TDT at some point in their design or Congress of the International Council of the development phases. The TDT provides unique Aeronautical Sciences (ICAS), Jerusalem, Israel, capabilities through the combination of large scale, high September 1988.
speed, high density, and variable pressure that make the facility ideally suited for testing aeroelastically scaled 10.
Noll, T.; and Perry, B., III: Activities in clearance models. The high density capability is Aeroelasticity at NASA Langley Research Center.
perhaps the most significant feature of the TDT that Paper to be presented at the ASME 4th makes the facility very suitable for aeroelastic testing.
International Symposium on Fluid-Structure This capability was historically provided through the Interactions, Aeroelasticity, and Flow-Induced use of the heavy gas R-12 as the test medium.
Vibrations and Noise, Dallas, Texas, November However, recent environmental concerns have led to 1997.
discontinuing the use of the former heavy gas. To retain its unique capabilities, the TDT is currently being 11.
Gilman, Jean, Jr.; and Bennett, Robert M.: A modified to use a new heavy gas, known as R-134a.
Wind-Tunnel Technique for Measuring Frequency- With R-134a, the TDT will continue to be a viable test Response Functions for Gust Load Analyses.
facility for the purpose of leading edge aeroelasticity Journal of Aircraft, Vol. 3, No. 6, Nov.-Dec.
research and dynamic vehicle clearance testing well into 1966.
the 21st century. The benefits of R-134a as a test medium have been discussed in this paper, calibration 12.
Stewart, Eric C.; and Doggett, Robert V., Jr.: plans have been summarized, and several past test Dynamic Wind-Tunnel Tests of an programs have been reviewed that show the potential Aeromechanical Gust-Alleviation System Using benefits of R- 134a heavy gas testing.
Several Different Combinations of Control Surfaces. NASA TM 78638, March 1978.
References 13.
Redd, L. Tracy; Hanson, Perry W.; and Wynne, Eleanor C.: Evaluation of a Wind-Tunnel Gust I.
Bisplinghoff, Raymond L.; Ashley, Holt; and Response Technique Including Correlations With Halfman, Robert L.: Aeroelasticity. Addison- Analytical and Flight Test Results. NASA TP Wesley Publishing Company, 1955.
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. Scanlan, Robert H.; and Rosenbaum, Robert: 14.
Rivera, Jos6 A., Jr.; Dansberry, Bryan, E.; Aircraft Vibration and Flutter. Dover Publications, Bennett, Robert M.; Durham, Michael H.; and Inc., 1968.
Silva, Walter A.: NACA 0012 Benchmark Model 19. Ruhlin,
Charles L.;Destuynder, Roger M.;and
Experimental Flutter Results WithUnsteady
Pressure Distributions. NASA TM 107581,
Gregory, Richard A.: Some Tunnel-Wall Effects
March 1992. onTransonic Flutter.Journal ofAircraft, Vol.
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Rivera, Jos6 A.,Jr.;Dansberry, Bryan, E.; 20. Pozniak,
Durham, Michael H.;Bennett, Robert M.;and O.M.: Investigation Intothe Use of
Freon 12 as a Working Medium in a High-Speed
Silva, Walter A.:Pressure Measurements ona
Wind-Tunnel. CoA Note No. 72, The College of
Rectangular WingWithaNACA0012 Airfoil
Aeronautics, Cranfield, United Kingdom,
During Conventional Flutter.NASA TM
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16. McGowan, Anna-Maria R.; Heeg, Jennifer; and Dansberry, Bryan, E.; Durham, Michael H.; Bennett, Robert M.; Rivera, Jos6 A., Jr.; Silva, Lake, Renee C.: Results of Wind-Tunnel Testing From the Piezoelectric Aeroelastic Response Walter A.; Wieseman, Carol D.; and Turnock, Tailoring Investigation. AIAA-96-1511.
David L.: Experimental Unsteady Pressures at Presented at the 37th Structures, Structural Flutter on the Supercritical Wing Benchmark Dynamics, and Materials Conference, Salt Lake Model. AIAA Paper No. 93-1592. Presented at City, UT, April 15-17, 1996.
the 34th Structures, Structural Dynamics, and Materials Conference, LaJolla, CA, April 19-22, 22.
1993. Nixon, M. W.; Kvaternik, R. G.; and Settle, T. B.: Tiltrotor Vibration Reduction 17. Through Higher Harmonic Control. Presented at Dansberry, Bryan, E.; Durham, Michael H.; Bennett, Robert M.; Turnock, David L.; Silva, the American Helicopter Society 53rd Annual Walter A.; and Rivera, Jos6 A., Jr.: Physical Forum, Virginia Beach, VA, April 29-May 1, 1997.
Properties of the Benchmark Models Program Supercritical Wing. NASA TM 4457, September 23.
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