The efforts of the NACA and NASA in developing and applying dynamically scaled free-flight model testing techniques have matured impressively. Although the scaling relationships have remained constant since the inception of free-flight testing, the facilities and test attributes have become dramatically more sophisticated.
The size and construction of models have changed from unpowered balsa models weighing a few ounces with wingspans of less than 2 feet to large powered composite models weighing over 1,000 pounds. Control systems have changed from simple solenoid bang-bang controls operated by a pilot with visual cues provided by model motions to hydraulic systems with digital flight controls and full feedbacks from an array of sensors and adaptive control systems. The level of sophistication integrated into the model testing techniques has given rise to a class of free-flight models that are considered to be integrated concept demonstrators rather than specific technology tools. Thus, the line between free-flight models and more complex remotely piloted vehicles has become blurred, with a noticeable degree of refinement in the concept demonstrators.
Research activities at the NASA Dryden Flight Research Center illustrate how far free-flight testing has come. Since the 1970s, Dryden has conducted a broad program of demonstrator applications with emphasis on integrations of advanced technology. In addition to the previously discussed X-48B program, two other Dryden projects, the Highly Maneuverable Aircraft Technology (HiMAT) program and the X-36 program, are related to the main theme of this document on research for dynamic stability and control of advanced vehicles.
The author observes low-speed tow stability tests of a free-flight model of the Ames M2-F1 lifting body in the Full-Scale Tunnel in 1962.
87 After the success of the exploratory remotely
piloted vehicle project using the three-eighths-scale unpowered F-15 Spin Research Vehicle, Dryden accelerated its efforts in the development of remotely piloted research vehicle (RPRV) tech-nologies in the cooperative HiMAT program with the Air Force Flight Dynamics Laboratory in 1979.90 The objective was to demonstrate integrated advanced fighter technologies such as advanced composites and aeroelastic tailoring, close-cou-pled canards, and winglets. Launched from a B-52 mother ship at altitudes of about 45,000 feet, the HiMAT remotely piloted research vehicle was about half the size of an F-16 fighter and was pow-ered by a J85 jet engine. Designed by the team of
Ames, Dryden, and Rockwell International, the configuration demonstrated twice the turn rate capability of the F-16 at transonic speeds. An onboard digital computer provided fly-by-wire control capabilities, includ-ing propulsive control. The vehicle was designed with relaxed static stability (from 10- to 30-percent unsta-ble) and direct force control, and active control was a major area of emphasis in the research flights. Two HiMAT vehicles conducted 26 flight tests during the program.
The HiMAT remotely piloted research vehicle flies over Edwards Air Force Base. In its role as an advanced technology demonstrator, the vehicle incorporated a highly sophisticated flight control system, a canard, winglets, aeroelastic tailoring, and relaxed static stability.
The HiMAT program produced extensive infor-mation regarding the flight dynamics characteris-tics of a radically new configuration for highly maneuverable aircraft. In addition to results on aeroelastic tailoring, automated flight maneuvers for extraction of aerodynamic data, and remote-piloting technologies for supersonic aircraft, the study inspired aircraft design features. For exam-ple, designers of the Grumman X-29 forward-swept wing research aircraft employed much of the technology from the HiMAT experiments, including a close-coupled canard, relaxed static
stability, and lightweight composite materials.91 NASA research pilot Bill Dana flies the HiMAT vehicle from a fixed-base ground cockpit.
In 1997, another milestone was achieved at Dryden in remotely piloted research vehicle technology when a NASA–Boeing X-36 vehicle demonstrated the feasibility of using advanced technologies to ensure satis-factory flying qualities for radical tailless fighter designs. The X-36 was designed as a joint effort between the NASA Ames Research Center and Boeing Phantom Works (previously McDonnell-Douglas) as a 0.28-scale powered free-flight model of an advanced fighter without vertical or horizontal tails to enhance surviv-ability. Powered by a F112 turbofan engine and weighing about 1,200 pounds, the 18-foot-long configura-tion used a canard, split aileron surfaces, wing leading- and trailing-edge flaps, and a thrust-vectoring nozzle for control. A single-channel digital fly-by-wire system provided artificial stability for the configuration,
CHAPTER 4: DYNAMIC STABILITY AND CONTROL
The X-36 tailless advanced fighter flight demonstrator in flight at NASA Dryden. The radical configuration conducted a successful flight investigation in one of the more remarkable remotely piloted vehicle programs at Dryden.
which was inherently unstable about the pitch and yaw axes. For the first time, a microphone was carried onboard a powered free-flight model in the cockpit area of the X-36. The aural cues provided alerts to the pilot for engine stalls or other propulsion-related problems. The ground-based pilot used a fighter-type cock-pit outfitted with a standard head-up display and a moving-map representation of the location of the X-36 in a successful series of 31 subsonic research flights, during which the vehicle reached an altitude of about 20,000 feet and a maximum angle of attack of 40 degrees. The pilot’s crew station was in a ground control station trailer, where distractions were minimized.92
Summary
This overview of over 80 years of applications of free-flight dynamically scaled models for assessments of the dynamic stability and control characteristics of full-scale vehicles has discussed the remarkable advances that the NACA and NASA have made in conducting and correlating results of the studies. The impact of the model results for advanced military and civil aircraft, spacecraft, and utility vehicles has influ-enced the development of new aircraft, provided an early identification of problems and potential solutions, and reduced the risk of subsequent full-scale flight tests.
88 MODELING FLIGHT
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