On October 5, 1955, R.J. Woods, an Airplane Design Consultant at Bell Aircraft Corporation, submitted 90 copies of a draft outline of a new “Proposed Supersonic Research Test Aircraft” to John W. Crowley, Associate Director for Research at NACA. Woods had presented this proposal at a conference at NACA Headquarters the previous spring. At the time, NACA was interested to some degree, but not wildly enthusiastic. Woods sensed that he shouldn’t push the proposal more until NACA’s planned Advanced Research Aircraft was definitely committed for procurement. As this was the case, Woods wanted to present his proposal at the next meetings of the Committee on Aircraft Construction, Aerodynamics and Power Plant. If NACA was opposed to this, Woods would withdraw his suggestion of placing his proposal on the agenda.
In his proposal, Woods noted the development of a series of new military aircraft that had been placed with industry. Most of these new aircraft types were required to have sustained supersonic flight speed capabilities in specified combat missions. Woods felt that these aircraft include as much research and development know-how as the state-of-the-art permitted when they came into existence. This applied not only to the airframe but also to all the aircraft and weapons system components.
Woods noted that any piloted supersonic aircraft was a compromise of subsonic and supersonic utility. He felt that the US aerospace industry needed as much supersonic full scale flight test experience as possible to rationalize the existing model test and calculation data and develop the prospective concept needed to apply these data to new aircraft design projects.
The Proposed Research Test Aircraft was described in an outline form as a rough draft to be critically edited and serve as a basis for discussion rather than a specific proposal. He thought that there was an urgent need for such a “workhorse,” a research test aircraft to back up the programmed military aircraft. Woods hoped that it would be considered and discussed by the NACA Committees on Aerodynamics, Aircraft Construction, and Power Plant.
Intended Use: The proposed aircraft was intended to be designed, constructed, and equipped in the concept of a laboratory research tool rather than that of a military weapon for use by NACA and the armed services engineering and research organizations. The aircraft was to have generated greater amounts of higher quality supersonic flight research data on a faster basis than could be obtained from military aircraft diverted for this purpose. Just as the military aircraft is a better weapon because it is designed, constructed, and equipped with that concept in view, so the research test aircraft should be a better and more effective research and development tool when its design, construction and equipment is so specialized.
The proposed research test aircraft was to have assessed the problems and established the criteria and parameters for satisfactory flight and function of large, multi-engine, military aircraft with sustained supersonic flight endurance. More specifically, this work in basic research was to have included flight test operation at supersonic speed in such areas as:
- Propulsive System Research
- Structural Integrity Research
- Dynamic Stability Research
In applied research and development, the aircraft was intended to be used as a full capability, flight test platform for the assessment, test, and evaluation of components, equipment, personnel, and systems intended for use in military aircraft of supersonic flight capability.
At the time, there was urgent need for as much information and data in the areas of basic and applied research and development to ensure the successful and scheduled development of the advanced military aircraft and weapons systems then programmed. Woods argued that construction a specialized research test aircraft to do this work was more desirable than waiting until the advanced military equipment came out and then having to backtrack to determine the mistakes made. The large number of known problems and the galaxy of suspected or potential problems associated with the development of effective supersonic military aircraft appeared to justify the time and cost to construct the specialized aircraft needed and do the flight test work intended for it.
A great amount of data for the design and development of supersonic military aircraft and components was available from studies, simulators, wind tunnels, models, etc., and was constantly being supplemented by the work-in-progress at NACA and the laboratories and engineering stations of the Armed Services. The work intended for this research test aircraft was to complement and coordinate this fund of information and data by providing the full-scale, free flight test data needed to properly rationalize it. The available flight test data, limited as it was, did not appear to justify the construction of new advanced and larger supersonic military aircraft types then were then flying without obtaining considerably more basic full-scale supersonic flight information and experience.
It was proposed to design the research test aircraft on the premise that it would attain maximum flight speeds of approximately Mach 2.5 at altitudes of 50,000 to 60,000 ft. with a sustained endurance at such speeds of 20 to 30 minutes after climb and acceleration. (Wood suggested that a diving start could be used to shorten the acceleration period to save fuel.) It was recognized that engines suitable and capable of providing such performance did not yet exist. Further, it was doubted that the programmed engines then under development would offer the needed thrust at altitude. Wood felt that the unavailability of the power plants needed to develop the maximum intended flight speed of the research test aircraft did not preclude the desirability for the rapid initiation of this aircraft project, as it might in a military type, because the research test aircraft was intended to do its work at speeds from Mach 1.0 on up. This meant then, upon completion of the airframe, the best available engines could have been installed and flight test work intended for the aircraft undertaken. The engines available at the time (with afterburners) could have provided flight speeds of Mach 1.4 to 1.5 with adequate power for takeoff and climb. If the proposed research engine program materialized, such an engine, optimized for the research test aircraft, could have offered a performance potential up to the maximum contemplated and possibly to speeds representing thermal limits for the airframe and turbojet engines.
The airframe configuration proposed for the research test aircraft was a mid-wing monoplane with a thin, essentially unswept, moderately low aspect ratio wing. Twin turbo jet engines in outboard nacelles were designed to accommodate experimental installations of current and proposed turbojet engines of the 10,000 – 20,000 lb thrust class. The fuselage had a high fineness ratio of circular cross section which contained the crew and their associated equipment, all fuel tankage, and all elements of the tricycle landing gear. A conventional swept leading edge horizontal and vertical tail were proposed.
It was proposed that if the design of the research test aircraft was initiated, a careful assessment of the airframe configuration details would have been made by calculating and tests to direct the design toward a configuration and arrangement that would have:
- Generated a minimum of aerodynamic complications unknowns in the intended flight operating range.
- Applied and offered facilities for assessment of the most advanced state-of-the-art in aeronautical research, such as area rule, etc.
- Offer the best potential as a vehicle for stability and control research as regards size, shape, location, and details of the tail planes and control surfaces.
The scope and context of the preliminary design work and associated wind tunnel and model test work was to have been discussed, studied, and agreed upon between the interested agencies.
The arrangement and environmental installation for the crew, the research equipment installation, facilities, and special arrangements to be provided for in connection with the intended use of the aircraft was to have also been discussed, studied, and agreed upon between the interested agencies.
A realistic time schedule was to have been established and maintained by the relevant agencies. Wood felt it was important to stay within existing know-how limits, using as much developed components and equipment as possible in the interests of quick and economical design and construction of rapid orientation into the intended flight test work.
Construction: Representative conventional construction design and materials under consideration for use for military aircraft intended for operation in the speed range of Mach 1.0 to 2.5 was proposed for the research test aircraft. This was done so that structural integrity test data could be applied with minimum extrapolation to the military aircraft program. Information on the general structural design and materiel characteristics of military aircraft to have been incorporated into the research test aircraft was to have been furnished by the Armed Services.
1. Structural Assessment Facilities – it was proposed that in preliminary design conferences with agencies interested in the structural integrity research work intended for the research test aircraft that a reasonable number of areas and points in the structure be designated as desirable for critical observation or inspection. It was proposed to fasten the structure in these areas with bolts, rather than rivets or other semi-permanent attachments so that periodic removal for laboratory inspection or replacement could be made.
2. Design Criteria – it was tentatively proposed that conventional fighter load factors be used in the design of the research test aircraft modified by current practice methods for flight at speeds of Mach 2.5 at altitudes of 50,000 to 60,000 ft. The final design criteria were to be agreed on in detail by conferences between the interested agencies with the objective of providing criteria that would be flexible enough to concentrate the structural integrity data obtained from flight tests in areas where it would be applicable to military aircraft structural design problems with minimum extrapolation.
3. Design Data Records – It was proposed to more completely document the design, construction and operation of the research test aircraft than was conventional practice in the interest of the structural integrity research programs. Records of all design calculations made, both those used and those discarded, were to have been retained in data records. Aerodynamic and structural analysis calculations and test data were to have been made in somewhat more detail than was conventional practice, and also be retained in the data records. The same was to have been done for study work done vibration, flutter, fatigue, aerodynamic heating, etc. During construction, detailed records of structural changes, repairs, unusual fabrication situations and other factors or circumstance having a potential for effect on the aircraft’s structural integrity history were to have also been kept in the data records. It was not the thought that complicated reports or extensive duplicate copies were to have been made, but rather that all original paperwork be collected, suitably identified, and supplied with any need explanatory notes and organized into an indexed record and transferred with the aircraft to the using agency. Wood did not think this procedure would have appreciably increased the engineering costs or time, and it was thought the complete design and construction record would have been of considerable help in structural integrity research work.
It was proposed to provide accommodations in a pressurized cabin for a pilot and two flight technicians. Environmental equipment appropriate for flight at altitudes up to 75,000 ft. altitude and flight speeds of Mach 2.5 for a duration of 30 minutes at 50,000 ft. altitude was proposed. Pending further design study, a weight allowance of 2,500 lbs. was made for the installation.
In consideration of the desirability of continuous and complete case histories of each aircraft and to parallel records on similar aircraft so far as possible, it was proposed that if several research test aircraft were placed in operation on diversified research work, such as propulsion, dynamic stability, structural integrity and aero-medical research, armament and equipment test platforms operation, etc., that certain amount of structural integrity flight history data would be collected on all the aircraft for all operations. This meant that all research test aircraft would have carried a prescribed minimum of indicating and recording test equipment. A tentative weight allowance of 1,000 lbs. was made for this equipment.
Specialized test and recording equipment was to have been provided for to conduct flight tests in specialized research areas. An allowance of 1,500 lbs. was made for such equipment.
Pending more information on the intended research test aircraft use, the following general instrumentation was proposed:
- Pressure distribution on wings, fuselage, nacelles and tail.
- Strain gauges (where feasible) in critical structural areas.
- Thermocouples – extensive installation in wings to access thermal conditions due to frictional heating as required for other thermal problems.
- Micro grids – on structure in critical areas.
- Optical alignment – major structural alignment during flight; more complete alignment check for ground operation.
- Flight path and control action recording equipment.
- Conventional flight, navigation, engine, and equipment indication.
It was proposed that all research test aircraft be equipped with a reasonable minimum of safety installations to warn the crew if flight conditions approach limiting values of potential trouble. In the case of the research test aircraft, the safety devices would have indicated the approach to dangerous or potential danger areas, but permit controlled approach to such areas so that the maximum value data could be obtained with controlled risk. In later phases of flight testing, assessment of problems and development and test of suitable safety devices and indicators for general supersonic flight use was important work was to have been programmed, as continued use of subsonic concepts appeared inadequate and uncontrolled loading of supersonic aircraft with new safety “gadgets” would have been prohibitively restrictive.
Discussion of Proposed Research Program
In the area of propulsion research, it had become generally recognized that for supersonic speed flight with air breathing engines, the engine environmental installation could exert powerful controlling influence and limitations on engine functions. The air intake, diffuser, duct system, exhaust nozzle and tail pipe could be as critical to engine operation as the internal components of the engine itself. The external airflow conditions around the aircraft could have had a great influence on engine operation and function, and conversely the engine internal flow could induce changes in the external airflow on the airframe which could produce changes in trim, stability and control.
Woods believed that there was an urgent need for full scale propulsion system research flight testing at speeds from Mach 1.0 up to the engine thermal limits at altitudes from 40,000 ft. up. It was also vital that for such tests the engine internal flow conditions and environment equipment be as free as possible from limiting influences of the airframe to permit isolated assessment and evaluation of varying installation conditions. Sufficient flight tests were needed to rationalize the corresponding wind tunnel and test chamber work on propulsion systems because reliable interpolation of ground based test work for full scale installation design was impossible without it. Woods felt use of early production models of new military types for this work was not satisfactory as the process was slow, conditions were restrictive and limiting to flight research test scope and technique, and it came too late to prevent serious basic installation mistakes.
Woods asserted that when a critical appraisal was made of the many millions of dollars worth of ground based propulsion research and development facilities in existence, the meager quantity and inadequate capabilities of the available propulsion research aircraft and the magnitude and quantity of critical problems that require full scale flight test coverage, it was obvious that there was an urgent need for a supersonic research test aircraft adapted for propulsion system flight research. It appeared unrealistic to hope that the programmed development of engine capabilities would be realized in future aircraft installations without adequate full-scale flight research and development tests being made.
It was proposed that the engine installation conditions in the research test aircraft be as flexible as possible. Location of the engines in nacelles outboard and independent of the fuselage was proposed to provide engine air intake and exhaust conditions as free as possible of the fuselage influence. It was also proposed to provide, in addition to provisions for using alternate engines, for a reasonable amount of flexibility for experimental change of the engine nacelle size arrangement and location. Underslung nacelles faired into the wing with the engine center-of-gravity at approximately mid-chord on the wing were proposed, but structural attachment provisions suitable to permit underslung pod type engine nacelles to be substituted were included. It was also proposed to provide a wing panel joint at the engine nacelle location arranged so that the outer panels could be removed and experimental wing-tip nacelle installations made. Use of inboard panel mounted lateral controls and high lift devices appeared to permit satisfactory flight operations with or without the outer wing panels installed.
Ballast compartment space in forward and aft ends of the fuselage was included to provide aircraft c.g. control for variation of engine location in experimental nacelles from the basic design location of plus or minus .5 M.A.C. length (approximately 16 ft. total) to permit assessment of air intake and airflow interference variations and other external airflow conditions as these could be found critical.
Woods believed that the provision for one or two flight technicians in the aircraft in addition to the pilot to supervise and observe the power plant function and test procedure should not only considerably improve both the quantity and quality of the flight test information obtained, but also considerably increase the safety of tests near critical limits and prevent flight accidents caused by the inability of a single pilot to continuously assess and supervise the power plant operating conditions during flight.
Structural Integrity Research
In the area of structural integrity research, it had become increasingly obvious to Woods that a careful reconsideration of aircraft structural design criteria was necessary to ensure safe flight operation where sustained supersonic speeds were programmed. In general it appeared that, in addition to increased loadings caused by the increased speed, changes in load distribution, aerodynamic heating, aeroelasticity, fatigue and secondary structural efforts, to name a few, seem to present problems which had to be assessed and evaluated to ensure acceptable structural integrity at supersonic speeds. The investigation of these and other effects had been the subject of a large amount of study, calculation, and wind tunnel test work. Full scale flight tests were, however, required to establish quantitative values and extrapolation factors to make the available data properly accurate for full scale design purposes. Woods argued that failure to do the needed basic full-scale flight research in this area would leave the door open for a rash of structural failures in supersonic flight without the proper background being established to assess trouble or take steps to guard against its reoccurrence. A major complication in the structural integrity problem for supersonic flight speeds was that adequate static structural strength was not necessarily a guarantee of flight integrity, as had been the case generally for subsonic flight speeds, and “new rules,” some of which were not yet clearly defined, appeared to apply in many areas. Woods claimed that a well-planned structural integrity full-scale flight research program based on using several of the research test aircraft proposed could contribute tremendously to the available data in this area.
A brief outline of some of the points proposed for consideration for structural integrity research in the research test aircraft read as follows:
a. Complete design and construction data records.
b. Somewhat more complete and detailed than usual aerodynamic, structural, flutter, vibration, aerodynamic heating and fatigue calculations and tests.
c. Complete and continuous flight records on all available aircraft.
d. Construction and materials representative of programmed supersonic military aircraft.
e. Facilities for detail inspection and test of structural components during useful life of aircraft.
Dynamic Stability Research
In the area of dynamic stability research it was proposed to endeavor to provide in the research test aircraft, by careful design of a simple airframe configuration and special control and damping considerations, the ability to fly level, stabilized, constant speed, supersonic flight tracks into which controlled displacing impulses could be introduced. Dynamic stability assessment and record equipment was to have also been included. Adjustable damping equipment in simulator form, air adjustable split trailing edges on all control surfaces, and provisions for controlling the longitudinal position of the aircraft c.g. by means of ballast and fuel transfer were proposed to assist in dynamic stability research.
Woods felt that assessment of the aerodynamic problem and establishment of dynamic stability parameters for flight speeds from Mach 1.0 on up was urgently needed design information for the development of successful military aircraft and the research test aircraft proposed should provide exceptional facilities for doing this work rapidly, effectively, and economically.
A brief outline of some points proposed for consideration for dynamic stability research in the research test aircraft read as follows:
a. Slab type horizontal tail.
b. An air adjustable vertical tail (with a 10° leading edge) with rudder that could be locked and unlocked in flight.
c. Adjustable split trailing edges on all control surfaces.
d. Inboard ailerons (which also could be used as flaps or split open for air brakes) – an air adjustable trailing edge flap on outboard panels.
e. Power controls – with adjustable boost and damping on all three axes (perhaps some arrangement like an electronic simulator that could be pre-flight-rigged or set up to duplicate system functions such as the Lockheed F-104 Starfighter system, etc.)
f. Extra attention to aerodynamic accuracy in airframe construction.
J. W. Crowley, Associate Director for Research at NACA, informed Woods on October 11, 1955 that he had no objection to bringing his proposal for a supersonic research test aircraft up for discussion at the next meetings of the Committees on Aircraft Construction, Aerodynamics, and Power Plants. However, Crowley emphasized that the NACA staff was unable to support his proposal. It was their feeling that the objectives of direct interest to NACA could be accomplished in large measure more economically and just as quickly by proper use of new military aircraft. In the field of dynamic stability and automatic systems, for example, NACA was proposing to use a new supersonic military aircraft with special equipment that would permit great flexibility in research. However, Crowley had no objection to Woods still distributing copies of his proposal at the next meeting of the Aerodynamics Committee.
Woods responded on October 25. He still wanted to distribute his Supersonic Research Test Aircraft proposal to the Committee on Aerodynamics at their meeting on November 4. However, in view of the lack of NACA support, he withdrew his previous suggestion to have it distributed to the Committee on Aircraft Construction and Power Plant. No further correspondence regarding the proposal has been found in NACA files; one can reasonably assume that it was rejected by the Committee on Aerodynamics and subsequently shelved.
Drawings ©2011 Jared A. Zichek.