The move to the Full-Scale Tunnel was a gigantic step in the development of the wind tun-nel free-flight technique and the construction methods used for the free-flight models. This large tunnel became the centerpiece of free-flight studies at Langley because of its unique design features. The tunnel is closed-circuit and open-throat, characterized by an open quasi-elliptical test section with dimensions of 60 feet across by 30 feet high and a length of 56 feet. The air is drawn through the test section at speeds up to about 80 mph by two 4-bladed, 35.5-foot-diame-ter propellers powered by two 4,000-horsepower electric motors. After the airstream passes through the test section, it returns to it in a bifur-cated manner through wall passageways within the building enclosing the test section.
The huge Langley Full-Scale Tunnel has been the focal point of NASA’s free-flight model studies. The large test section and open-throat design permit flight-testing of relatively large sophisticated models of aerospace vehicles.
Cross-sectional views of the Langley Full-Scale Tunnel show the unique open-throat test section and large dimensions of the facility. The arrangement permitted pilots and crews to be outside the airstream during free-flight operations. With a continual air-speed control from about 20 mph to about 80 mph and the ability to withdraw the model from the test section in the event of loss of control, the tunnel provided an ideal test site for flying models.
For free-flight investigations, pilots fly the remotely controlled, powered model with minimal restraint in the 30- by 60-foot open-throat test section of the tunnel for various test conditions and airspeeds. Flying the model involves a carefully coordinated effort by the test team, whose members are at two sites within the wind tunnel building. One group of researchers is in a bal-cony at one side of the open-throat test section, while a pilot who controls the rolling and yaw-ing motions of the model is in an enclosure at the rear of the test section within the structure of the tunnel exit-flow collector. Com-pressed air powers the model, and a thrust pilot in the balcony controls the level of thrust. Seated next to the thrust pilot is the pitch
pilot, who controls the longitudinal motions of the model and conducts assessments of dynamic longitudinal sta-bility and control during flight tests.
Test setup for free-flight studies in the Langley Full-Scale Tunnel. The pitch pilot and other operators concerned with the longitudinal motions of the model and its horizontal and vertical positions in the tunnel test section are in a balcony at the side of the test section, out of the airstream. The pilot who controls the rolling and yawing motions of the model is seated in an enclosure at the rear of the tunnel, behind the model. A digital flight control computer is used to simulate control laws.
The requirement for multiple pilots because of the impact of dynamic scaling of models has already been discussed. In addition to the model’s high angular rates, the pilot is limited by flying the model remotely with only visual cues, and he cannot sense the cues provided by the accelerations of the model—in contrast to a pilot onboard the full-scale airplane.
The lack of acceleration cues can result in delayed pilot inputs and pilot-induced oscillations, which can be critical in the relatively constrained area of the tunnel test section. Other key members of the test crew include the test conductor and the tunnel airspeed operator.
Free-flight evaluation of the dynamic stability and control of a powered model of the YF-22 fighter prototype at high angles of attack in the Langley Full-Scale Tunnel. The model was pow-ered with compressed air ejectors and included thrust-vector-ing engine nozzles and a nose boom equipped with angle-of-attack and sideslip vanes and sensors for implementation of critical flight control system elements of the full-scale air-craft. The outstanding behavior of the model at high angles of attack was subsequently verified by the airplane.
A light, flexible cable attached to the model serves two purposes. The first is to supply the model with compressed air, electric power for control actuators, and transmission of feedback signals for the con-trols and sensors carried within the model. The sec-ond purpose involves safety. A portion of the cable is made up of steel that passes through a pulley above the test section. This part of the flight cable is used to snub the model when the test is terminated or when an uncontrollable motion occurs. The entire flight cable is kept slack during the flight tests by a safety-cable operator in the balcony, who accom-plishes this job with a high-speed winch.
The introduction of larger models and the emer-gence of advanced flight control systems for full-scale aircraft posed significant challenges for the design, fabrication, and instrumentation of free-flight models and the Full-Scale Tunnel. Model dimen-sions greatly increased as model wingspans approached 6 feet or greater, scaled weights for the
CHAPTER 2: HISTORICAL DEVELOPMENT 27
28 MODELING FLIGHT
models approached 100 pounds, and new types of propulsion units such as miniature turbofans and high thrust/weight propeller propulsion units required development. Fabrication of models changed from the simple balsa free-flight construction used in the 12-Foot Free-Flight Tunnel to high-strength, lightweight composite materials. The development and implementation of new model structures compatible with the rigors of flight-testing and loads imposed during severe instabilities were made even more complicated by the requirements of maintaining accurate scaling relationships for weight and mass distribution of the models.
In recent years, the control systems used by the free-flight models have been upgraded to permit simulation of the complex feedback and stabilization logic involved in flight control systems for contemporary aircraft. The control signals from the pilot stations are transmitted to a digital computer in the balcony, and a special software program computes the control surface deflections required in response to pilot inputs, sensor feedbacks, and other control system inputs. Typical sensor packages include control-position indicators, linear accelerometers, and angular-rate gyros. Many models employ nose-boom-mounted vanes for feedback of angle of attack and angle of sideslip, similar to systems used on full-scale aircraft. Data obtained from the flights include optical and digital recordings of model motions, as well as pilot comments and analysis of the model’s response characteristics.
The free-flight technique is used to obtain critical information on the behavior of aerospace configurations in 1 g level flight from low-speed stall conditions at high angles of attack to high-speed subsonic conditions at low angles of attack. Specific objectives are to evaluate the configuration’s dynamic stability and response to control inputs for flight at several angles of attack, up to and including conditions at which loss of control occurs at extreme angles of attack. In addition, an assessment is made of the effectiveness of the flight control system feedbacks and augmentation systems as well as airframe geometric modifications.
A typical test sequence begins with the model ballasted for a specific loading. The model initially hangs unpowered from the safety cable and is lowered into the test section. Tunnel airspeed is then increased to the condition of interest, at which the model is trimmed with thrust and control inputs from the pilots and flies without restraint for the specified flight conditions.
The technique has proven to be excellent for obtaining qualitative evaluations of the flying characteristics of aircraft configurations, especially unconventional and radical designs for which experience and data are lacking or nonexistent. Because the tests are conducted indoors, the test schedule is not subject to weather, and a relatively large number of tests can be conducted in a short time. Configuration modifications are quickly evaluated, and the models used in the flight tests are also used in conventional wind tunnel force and moment tests, which provide aerodynamic data for use in the analysis of the observed flight motions and the development of mathematical models of the configuration for use in piloted simulators.
For over 50 years, the Langley Full-Scale Tunnel has been the site for a continual stream of free-flight investigations of aircraft and spacecraft configurations. As will be discussed in later sections, configurations
studied have included high-performance fighters, vertical/short take-off and landing (V/STOL) aircraft, parawing vehicles, advanced supersonic transport configurations, general aviation designs, new military aircraft concepts, and lifting bodies. Now under the operation of Old Dominion University (ODU) of Norfolk, VA, the Full-Scale Tunnel continues to support aeronautics research and development to this day.