Category Airplane Stability and Control, Second Edition

Tactical Airplane Maneuverability

Tactical airplanes have always had special stability and control problems because of the extreme maneuvers required of them. The rapid aileron roll, the sharp pullup, and the rapid turn entry all present special problems. Some examples are the level of rolling velocity actually required, overcontrol in pullups, and badly coordinated turn entries. Finally, controlled flight at angles of attack beyond the stall is a new field of required maneuvers for tactical airplanes.

10.1 How Fast Should Fighter Airplanes Roll?

Fighter roll capability became a crucial question during the early days of World War II. Many allied fighter airplanes carried gun cameras into combat. Gun cameras are movie cameras pointed in the direction of the ship’s fixed-wing guns. Movies are taken as long as the firing trigger was pressed, witnessing hits (or misses) on enemy airplanes or missiles. Gun cameras carried on Curtiss P-40s and North American P-51 s witnessed interesting moments in dogfights and bore out pilots’ accounts of loss in combat advantages due to relatively low rates of roll on the U. S. aircraft.

Some Axis aircraft, particularly the Mitsubishi Zero at low airspeeds, would feint a roll in one direction and then roll rapidly in the other direction. The horizon or cloud background in the gun camera pictures would show the Allied airplane following the feint, a bit slower perhaps, then be left behind as the Zero did a rapid roll in the opposite direction and disappeared from the gun camera’s view.

Clearly, high-rolling velocity performance was needed at dogfight airspeeds in order not to lose firing opportunities when in the favorable trailing position. At the low end of the fighter airspeed range the Gilruth/NACA criterion pb/2V = 0.07 was a reasonable guide, although higher levels, up to 0.10, were considered. Higher pb/2V levels could be attained with extra-large ailerons. But in early World War II days, before hydraulically powered controls, the wide-chord, long-span ailerons that provided high pb/2V values meant high stick forces, restricting rolling velocities at high airspeeds.

In other words, an airplane could be designed for fast rolling performance at either low airspeeds, say below 200 knots, or high airspeeds, but not both. The Curtiss P-40 was typical in that its maximum rolling velocity of 95 degrees per second occurred at an airspeed of 270 miles per hour. At 400 miles per hour (not shown in Fig. 10.1) maximum available rolling velocity dropped to 65 degrees per second, limited by a nominal 30-pound stick force.

Restricted maneuverability due to high stick forces started an intense research program on both sides of the Atlantic. The British seemed to have had the innovative edge, coming up with two significant stick force reduction schemes: the spring tab, ultimately used on the Hawker Tempest, and the beveled-edge control surface. The history of these devices is given in Chapter 5, “Managing Control Forces.” Beveled-edge ailerons worked quite well for the P-51 Mustang, almost doubling the available rate of roll.

Tactical Airplane Maneuverability

Figure 10.1 Rolling velocities obtainable with 50 pounds of stick force for a number of World War II fighter airplanes, all at an altitude of 10,000 feet. These data were heavily classified during the war. (From Toll, NACA Rept. 868, 1947)

Hydraulic power assistance came into the picture for fighter-type airplanes only at the very end of World War II, on the ailerons of the late version Lockheed P-38J Lightning. However, once power controls became common, in about 1950, stick force limitations to rate of roll were overcome. Now the only limits were hydraulic system capacity, control system and wing strength, wing torsional stiffness, and the inertial coupling phenomenon discussed in Chapter 8. The military specification version of that period reflected these new capabilities. Fighter roll rates up to 360 degrees per second were required. A limiting factor in fighter roll maneuverability at high airspeeds and low altitudes is wing twist, treated in Chapter 19, “The Elastic Airplane.”

Concluding Remarks

While the special carrier-approach problems for swept-wing jet airplanes have been recognized for over 30 years, there seems to be no clear-cut method for predicting the severity of such problems in preliminary design, much less for adopting solutions at an early stage. The detail specification for one of the U. S. Navy’s recent jet airplanes, the McDonnell

Douglas F/A-18, makes that point, listing no fewer than six possible determinants for that airplane’s approach speed.

The closed-loop systems analysis approach to the carrier-landing problem would seem to offer the best chance of answering difficult questions, such as whether a new design will need direct lift control and what the upper limit might be for thrust lag following throttle motions. However, the closed-loop systems analysis approach apparently requires additional development before it is ready to be used in this design sense. Systems analysis study reports typically close with a “Need for Further Research” section.

Stability and Control Issues with Variable Sweep

Variable wing sweepback is an attempt to combine the best performance, stability, and control characteristics of straight and swept wings. Straight wings have benign low – speed stability and control characteristics, good low-speed maximum lift, and low cruise drag, while sweptback wings have low transonic and supersonic drag and good high-Mach – number stability and control. In a variable-sweep airplane, wings are spread fully, orunswept, at low speeds and are swept back at high Mach numbers.

16.1 The First Variable-Sweep Wings – Rotation and Translation

The designers of the first variable-sweep airplanes, the Messerschmitt P1101, the Bell X-5, and the Grumman XF10F-1, found it necessary to move the wing inboard ends forward on the fuselage as the wing tips were moved aft. This was to keep the wing’s mean chord in about the same fore-and-aft position along the fuselage. This kept the distance from the airplane’s cg to the wing aerodynamic center about the same as the wing was swept back.

It can be imagined that the complication of a wing-fuselage attachment that translated as well as rotated was a powerful deterrent to aircraft designers. In fact, while this was the only available variable-sweep method, the concept turned up only in research aircraft.

Suborbital Flight Mechanics

The effects of the earth’s curvature are quite negligible on the airplane modes of interest to the stability and control engineer under ordinary flight conditions. However, some significant effects are expected for the suborbital case. A number of investigators have extended the flat-earth equations to spherical or oblate models in order to examine these effects.

Linearized airplane motions have been examined in perturbations from great-circle and minor-circle trajectories about a spherical earth (Myers, Klyde, McRuer, and Larson, 1993). In principle, this is the same procedure followed by Bairstow (1914) in his extension of the Bryan equations of motion to perturbations from steady turning flight. An extra longitudinal mode of motion is found, in addition to the usual short-period and phugoid modes. This is a first-order density mode, also referred to as an altitude mode. Aside from this extra complexity, with a typical hypersonic configuration at Mach numbers from 3 to 20 the density mode occasionally couples with real phugoid poles.

There is also an extra lateral-directional real mode, in addition to the usual Dutch roll, spiral, and roll modes. This is called a kinematic mode, generally of very long time constant. At some high Mach numbers, the kinematic mode couples into the spiral mode, producing a very low-frequency stable oscillation.

Propulsion-Controlled Aircraft

Multiengine airplanes that rely on hydraulically powered controls can be controlled in an emergency by differential applications of thrust. This is for the emergency situation in which all control surfaces are either fixed or freely floating, but are no longer under the control of the flight crew. NASA calls airplanes controlled by differential operation of thrust propulsion-controlled aircraft, or PCA.

While differential thrust might in principle provide sufficient control moments to guide an airplane to a safe emergency landing, it should be clear that lags in engine thrust re­sponse to throttle movements would make successful control an almost impossible task. The emergency noted in Sec. 5.23, “Safety Issues in Fly-by-Wire Control Systems,” where a Lockheed L-1011 with a jammed elevator was controlled with differential thrust by a highly skilled pilot, did not involve full loss of flight control operation. A rather less successful outcome of differential thrust control occurred with a DC-10 that had lost all flight control operation (Tucker, 1999).

The difficulty of throttle-only control for emergency control of airplanes with failed con­trol systems led NASA to authorize a research program for propulsion-controlled aircraft
that would lead to workable systems. The key concept was the use of stability-augmentation techniques that would overcome the thrust lag problem, without requiring unusual pilot skills. The research was authorized by the then director of NASA’s Dryden Flight Research Center, Kenneth J. Szalai, and took place starting in 1990 (Burken and Burcham, 1997).

Propulsion-Controlled AircraftThe NASA PCA program was carried far enough to prove in flight testing that in the absence of surface hardovers or large mistrim conditions, a three-engine commercial jet, the MD-11, could be returned to an airport and landed without the aid of aerodynamic surfaces. The tail engine was used in the tests, although the PCA system is designed primarily for airplanes with two wing engines. The MD-11 engines were modified for PCA operation with full-authority digital controls and special idle settings to avoid large time lags in response to thrust change commands. The longitudinal and lateral PCA control laws are illustrated in Figure 20.7. In conclusion, the NASA PCA effort has provided a viable option, with

Propulsion-Controlled Aircraft

Left engine gain

moderate hardware and software costs, for transport designers to consider in the quest for safety.

Short Biographies of Some Stability and Control Figures

Abzug, Malcolm J. 1920-, b. New York, NY. B. S. (1941) Mass. Inst. of Tech., M. S. (1959), PhD. Engr. (1962), U. of Calif, at Los Angeles. After government laboratory work, he joined Douglas Aircraft, where he was stability and control lead engineer for the A2D-1 and A4D-1. His later industrial experience was at Sperry Gyroscope, TRW Systems, and Northrop on the A-9A, YF-17, and B-2 programs.

A’Harrah, Ralph 1931—, b. Warren, PA. B. S. Aero. (1955), Penn. State U. A’Harrah’s career is balanced between North American Aviation, the U. S. Department of Defense, and NASA. He used ground-based fight simulation as a tool in solving flight dynamics problems associated with hazardous flight. On the AGARD Flight Mechanics Panel, he developed V/STOL flying qualities criteria.

Anderson, Seth B. 1918-, b. Los Altos Hills, CA. B. S. (1941), M. S. (1942), Purdue U. Anderson’s long career at NACA and NASA dealt with handling quality requirements for conventional and VTOL airplanes. He is the principal author of AGARD Report 577 on V/STOL handling criteria.

Ashkenas, Irving L. 1916—, b. New York, NY. B. S. (1937), M. S.M. E. (1938), Ae. E. (1939), Calif. Inst. of Tech. His stability and control career started in industry, first at North American Aviation, then with the Northrop P-61 spoiler ailerons and design requirements forthe XB-35 power controls and artificial-feel systems. He is best known for applying man – in-the-loop theory for flying qualities prediction and as a co-author of Aircraft Dynamics and Automatic Control.

Bairstow, Leonard 1880-1963, b. Halifax, Yorkshire, U. K. Royal College of Science, London. Bairstow’s major stability and control contributions were the extension of the Bryan equations of motion to the nonsymmetric steady-flight case and development of efficient methods for root extraction, both done in 1914. The 1939 (second) edition of his Applied Aerodynamics was a useful stability and control reference for years.

Barnes, Arthur G. 1929-, b. Wigan, U. K. B. S. (1950), Manchester U. RAF and RauxAF pilot. His career in the United Kingdom industry from 1954 to 1990 included research and development for flight controls, flying qualities, and flight simulation. Barnes proposed the original numerical rating scale for pilot opinion on flying qualities. He is a consultant to the Kungl Tekniske Hogskola (KTH) and SAAB in Sweden.

Bihrle, William, Jr. 1925-, b. New York, NY. B. Ae S. (1945), Rensselaer Poly. Inst. Bihrle contributed to the stability and control designs of the Republic F-105 and XF-103 airplanes. He invented the widely used control anticipation parameter for pullups and plays a leading part in developing advanced spin tunnel rotary balance techniques and methods for improving high angle of attack stability and control.

Bowman, James S., Jr. 1924-, b. Burlington, NC. B. S. (1951), N. C. State Coll. As a leading NASA expert on spinning, Bowman consulted with military and commercial designers on spin problems for many years. He is the author or co-author of more than 40 reports on spinning, including NASA TP 2939 on pressure distribution at spinning attitudes.

Bratt, Robert W. 1918—, b. Palisade, MN. B. S. (1941), M. S. (1942), U. of Michigan. Bratt was a stability and control engineer at the El Segundo Division of Douglas. He pioneered in the application of digital computers to maneuvering flight. He solved drop vehicle instability problems involving aeroelasticity and inertial coupling. He later became Chief of Preliminary Design at Northrop.

Breuhaus, Waldemar O. 1918—, b. Lowell, OH. B. S.Ae. (1940), Carnegie Inst. ofTech., M. S. (1961), State U. of New York at Buffalo. Breuhaus was in charge of stability and control at Vought-Sikorsky during World War II. At Cornell Aero. Lab., later Calspan, he was responsible for the development of the B-26 and T-33 variable-stability airplanes, and he used these machines in flying qualities requirement research.

Bryan, George Hartley 1864-1928, b. Cambridge, U. K. Cambridge U. Bryan’s monu­mental contribution to the field was the equations of aircraft motion, developed in 1911 in essentially modern form from a preliminary study (with W. S. Williams) in 1904. He later made contributions to compressible flow theory.

Cantrell, Coy R. 1924-, b. Muskogee, OK. B. S. (1953), M. S. (1954), Calif. Inst. ofTech. Cantrell’s long career at Lockheed’s Advanced Development Company (Skunk Works) started in 1954. He shared stability and control responsibility for the SR-71, the Have Blue prototype, and the F-117A, whose air data measurement system he designed. He was also involved in the YF-22A Advanced Tactical Fighter prototype.

Cook, Michael V. 1942-, b. Colchester, U. K. B. Sc. (1965), U. of Southampton, M. Sc. (1967) Coll. of Aeronautics, Cranfield. At Elliott Flight Automation, Ltd., Cook was involved with flight control system research and design on the Hovermarine HM2 hovercraft, the Westland Lynx helicopter, the Panavia Tornado, and the Jaguar fly-by-wire. He teaches at Cranfield College and is the author of Flight Dynamics Principles (1997).

Cook, William H. 1915-, b. Plainview, TX. B. S.M. E. (1934), Rensselaer Poly. Inst., M. S. (1938), Mass. Inst. ofTech. Cook was a designer of the Boeing High-Speed Wind Tunnel and was involved with the stability and control development of many Boeing designs, including the B-29, XB-47, and 707. He was co-inventor of the B-47 electronic yaw damper, one of the first of its kind.

Cooper, George E. 1916-, b. Burley, ID. B. S. (1940), U. of Calif. Cooper combined NACA/NASA engineering and research test pilot careers to become an important stability and control contributor. He is the Cooper of the Cooper-Harper handling qualities rating system and the author of a NASA Technical Note that is a text for test pilot training schools.

Czinczenheim, Joseph 1919-1994, b. Hungary. La Sorbonne, Centre Superieur de Mecanique, Paris. He worked on stability and control problems of the STOL Breguet 941, the transonic Breguet Taon, and the BAC-Breguet-Dassault Jaguar. Later, he was involved with certification of the Dassault Civil Transport /C and with stability and control of several Israeli prototypes. He was a member of the AGARD Flight Mechanics Panel.

Doetsch, Karl-H. 1910-, b. Kaldenhusen, Germany. Dipl.-Ing. (1934), TH Aachen, Dr. – Ing. (1943), TU Berlin. Professor Doetsch is an aeronautical scientist as well as a 3,000- hour test pilot. His contributions are fly-by-wire control (Avro 707C, Do 27, Pembroke), flight simulation, flight recording, and advanced aircraft flight controls. He chaired the

AGARD Flight Mechanics Panel and has made special efforts to broaden international cooperation in education and research.

Duncan, William Jolly 1894-1960, b. Hillhead, Glasgow. D. Sc. (1930), U. of London. Duncan was co-author of the important textbook Elementary Matrices and author of the 1952 book Control and Stability of Aircraft. His other contributions were in the theories of aileron reversal, tail buffeting, aerodynamic derivatives, and flap hinge moments.

Dunn, Orville R. 1916-1997, b. Wayne, PA. B. S. (1939), Mass. Inst. of Tech. Dunn was chief of stability and control at the Douglas Aircraft Santa Monica Division during the designs of the DC-4, C-74, DC-6, DC-7, and DC-8 transports. He produced a useful synthesis of methods for control force reduction by various tab systems. As Director of Aerodynamics he saw the DC-10 through certification.

Efremov, Alexander V. 1944-, b. Gorky Cty, U. S.S. R. Ph. D. (1973), D. Sc. (1996), Moscow Aviation Inst. As an expert in flight dynamics and control and in pilot-in-the-loop problems, Dr. Efremov participated in the flight control system designs for the aerospace vehicle Buran, the airship ALA-40, and the TU-204 and IL-96 airplanes. He is a member of the SAE control and guidance systems committee.

Etkin, Bernard 1918-, b. Toronto, Canada. B. A.Sc. (1941), M. A.Sc. (1947), U. of Toronto, D. Eng. (Hon) (1971), Carleton U. Dr. Etkin had a long career at the University of Toronto, becoming University Professor in 1982. He wrote three standard stability and control texts, which have German, Russian, and Chinese editions. Etkin made many con­tributions to the theory of flight dynamics, including flight in turbulence and dynamic longitudinal stability at high altitude.

Gates, Sidney B. 1893-1973, b. Watton, England. Gates was a brilliant theorist who did remarkable work on analyzing spins and predicting spin recovery with minimal facilities. Gates is responsible for the important flying qualities parameters of static and maneuver margins and stick force per g. With A. V Stephens, he established the effect of air density on spins. The scope of his stability and control work is truly wide. Gates was the British counterpart of R. R. Gilruth in flying qualities research.

Gee, Brian 1933-, b. Manchester, U. K. B. Sc. (1954), Manchester U. Gee was head of the Flight Control Systems Design Group at British Aerospace, Warton, involved with the Jaguar and Fly-by-Wire Jaguar, the Toronado, EAP, Eurofighter, and the RAE VAAC Harrier. His main contributions were in the areas of component requirements, digital flight control specifications, and system clearance for flight control/structural mode interactions.

Gera, Joseph 1937-, b. Szentes, Hungary. B. Ae. (1961), Auburn U., M. Appl. Mech. (1965), U. of Virginia. At NASA Langley and Dryden Flight Research Facility Gera con­tributed to understanding the effects of wind gradients on pitch stability. He led efforts at Dryden to integrate simulators into flight research and to measure stability margins “on-line” for such aircraft as the X-29A.

Gibson, John C. 1929-, b. Swatow, China. M. Sc. (1958), Cranfield, Ph. D. (1999), Delft U. of Technology. At English Electric/British Aerospace, 1952-1992, he worked on the flight control systems of the Lightning, TSR-2, and Jaguar and developed new fly-by-wire handling design methods and criteria for the Tornado, BAe FBW Jaguar, the Experimental Aircraft Programme (EAP), the Eurofighter, and the VAAC (vectored-thrust) Harrier. He is responsible for the phase-gain, dropback, and other criteria used to prevent pilot-induced oscillations by design.

Gilruth, Robert R. 1913-2000, b. Nashwauk, MN. M. S., U. of Minnesota. He joined NACA in 1937. His major stability and control contributions were design methods for static longitudinal stability and roll performance and an early complete set of flying qualities requirements. He later was Director of the NASA Manned Spacecraft Center. He retired in 1973 and was a consultant to NASA from 1974 to 1983.

Glauert, Hermann 1892-1934, b. Sheffield, U. K. B. S. (1915), Trinity Coll., Cambridge. Glauert’s notable work was in unsteady lift, airfoil theory, control surface effectiveness, and propeller theory. He originated the lag in downwash theory that explained damping discrep­ancies in the longitudinal short-period mode. He made the first nondimensionalization of the equations of airplane motion.

Goett, Harry J. 1910-, b. New York, NY. B. S. (1931), Holy Cross, B. S. (1933), New York U. Goett’s important contribution to stability and control came at NACA, on methods of predicting flying qualities from wind-tunnel tests. In charge of large NASA Ames wind tunnels, he directed high-lift and stability research on swept wings. He later became the Director of NASA’s Goddard Research Center.

Goto, Norohiro 1943-, b. Sasebo, Nagasaki, Japan. B. Eng. (1966), D. Eng. (1972), U. of Tokyo. Dr. Goto developed methods to identify pilot-control behavior in practical multi-input and multi-output aircraft control systems. At Kyushu University he is devel­oping an autonomous flight control system for an unmanned observation blimp. He had been an NRC research associate at NASA Ames Research Center and a Fulbright Scholar at M. I.T.

Graham, F. Dunstan 1922-1992, b. Princeton, NJ. B. S.E. (1943), M. S.E. (1947), Prince­ton U. As an aerodynamicist at Boeing in 1947 and 1948, Graham made an early analysis of inertial coupling on a pilotless aircraft. At Lear, Inc., he was in charge of automatic controls development for the KC-135 and other jet aircraft. However, he is best known as the co-author with McRuer and Ashkenas of Aircraft Dynamics and Automatic Control and the co-author with McRuer of Analysis of Nonlinear Control Systems.

Hamel, Peter G. 1936-, b. Hamburg, Germany. Dipl.-Ing. (1963), Dr. – Ing. (1968), TechU. Braunschweig (TUBS), S. M. (1965), Mass. Inst. of Technology. Dr. Hamelhadalong career as the director of the Institute of Flight Research of the German Aerospace Research Center (DLR) and as a professor at TUBS. He is recognized internationally for the development and use of in-flight simulators. He is a leader in European vehicle system identification and in handling qualities research.

Harper, Robert P., Jr. 1926-, b. Gallipolis, OH. S. B. Ae. (1952), S. M.Ae. (1953), Mass. Inst. of Tech. Harper was a Calspan engineer and test pilot who is noted for his part in developing the Cooper-Harper flying qualities rating. He was project engineer on the F-94 and NT-33A variable-stability airplanes during simulation of reentry vehicles and the X-15, as well as during basic flying qualities research.

Harris, Thomas Aubrey 1903-1987, b. Whites, VA. B. S. (1929), William and Mary. Harris designed the NASA Langley Atmospheric and 7 by 10-foot wind tunnels during a long career at Langley He was an expert on flaps and tabs, and he contributed to numerous wind-tunnel studies of control surfaces.

Haus, Frederic Charles 1896-1993, b. St. Gilles, Belgium. Brussels U. (1922). In a long, productive career, Professor Haus headed the famous aeronautical laboratory of Rhode-St.-Genese, published a 1930 book (in French) on airplane stability and control, served as professor at both Ghent and Liege Universities, and was a member of AGARD panels on flight mechanics, guidance, and control.

Heald, Ervin R. 1917—, b. Sultan, WA. B. S.A. E. (1940), U. of Michigan. Heald headed stability and control at the El Segundo division of Douglas Aircraft during the years when that division produced new airplanes on the average of one every two years. He took part in the stability and control work on the U. S. Navy’s XSB2D-1, XBT2D-1, AD-1, XA2D-1, D-558-1, D-558-2, F3D-1, F4D-1, XF5D-1, A3D-1, and A4D-1. Later, Heald was Chief Engineer for the U. S. Air Force’s C-17 transport.

Heppe, R. Richard 1923-, b. Kansas City, MO. B. A. (1944), M. S. (1945), Stanford U., A. E. (1946), Calif. Inst. of Tech. At Lockheed Aircraft, Heppe made significant contributions to understanding the inertial coupling problems of the F-104 and other USAF fighters, and helped find corrections for those problems. He contributed in the unlimited angle-of-attack maneuvering areas of the YF-22A prototypes. He became president of the Lockheed-California Company.

Hodgkinson, John 1943-, b. Ilseworth, U. K. B. Sc. (1965), U. of Southampton, M. S. (1971), St. Louis U. After training at British Aerospace, Warton, he joined McDonnell and then led controls R&D at Northrop. He later was at Eidetics and McDonnell Douglas (Boeing). Hodgkinson’s stability and control contributions are in equivalent systems, agility, and safety. He is the author of Aircraft Handling Qualities.

Hunsaker, Jerome C. 1886-1969, b. Creston, IA. B. S. (1908), Annapolis, M. S. (1912), Mass. Inst. of Tech., D. Sc. (1914), Williams Coll. Dr. Hunsaker was the author of NACA Technical Report No. 1 on inherent dynamic stability, 1915. He taught airplane stability and control at MIT, starting in 1914, and headed the Department of Aeronautical Engineering at MIT for many years.

Jex, Henry R. 1929-, b. Baltimore, MD. S. B. (1951), Mass. Inst. of Tech., M. S. (1958), Calif. Inst. of Tech. Jex developed analytical models of operator-vehicle control and applied them to handling qualities, landing displays, and workload studies. He is the principal devel­oper of the critical-instability tracking task, used for detecting impaired pilots and drivers. Jex designed the control system for the first autostabilized-while-flapping ornithopter, the Q-N pterodactyl replica.

Johnston, Donald E. 1924-1995, b. Huron, SD. B. S. Eng. (1952)., U. of California, Los Angeles. Johnston’s contributions have been in the fields of man/machine control analysis, synthesis, simulation, and full-scale flight test. He was a vice president of Systems Technol­ogy, Inc., where he was assigned to the most critical investigations. He conducted studies into control problems of the F-4, F-111, F-14, F-16, and F-18 airplanes and designed control laws for the McDonnell Douglas C-17 cargo airplane.

Jones, Bennett Melvill 1887-1975, b. Birkenhead, England. B. S. (1909), Emmanuel Coll., Cambridge. He joined the National Physical Laboratory in 1910. He contributed the “Dynamics of the Airplane” division in W. F. Durand’s Aerodynamic Theory, published in 1934. This is a key reference, the first complete derivation of aircraft equations of mo­tion, in modern form. His research at Cambridge was on stalling. Jones was a pilot and a decorated gunner in World War I.

Jones, Robert T. 1910-1999, b. Macon, MO. U. of Missouri, 1928, Catholic U. of America, 1933. After working as an airplane designer for the Nicholas Beasley Company, Jones joined NACA in 1934. His long career there produced notable stability and control contributions in lateral control, in the theory of two-control flight, in all-movable controls, and in a very early (1936) application of operator theory to the solution of the equations of aircraft motion.

Kalviste, Juri 1935-1996, b. Tartu, Estonia. B. S. (1957), M. S.E. E. (1960), U. of Washington. He worked on the flight control designs of the Boeing X-20 and Northrop YF-17 airplanes and on the ATF proposal. Kalviste made innovative formulations of the large-amplitude equations of airplane motion to develop departure parameters and methods of combining rotary balance and oscillatory aerodynamic data.

Katayanagi, Ryoji 1946-, b. Gumma Prefecture, Japan. B. S.M. E. Waseda U., M. S. (1972), Ph. D. (2000), U. of Tokyo. At Mitsubishi Heavy Industries, Katayanagi analyzed flying qualities and flight controls of the T-2 trainer. He designed flight control laws for the T-2CCV research airplane, the QF-104 drone, and the F-2 fighter. His research interests are multiloop flight controls and PIOs. He leads the engineering team for the NAL scaled supersonic research airplane.

Koppen, Otto C. 1901-1991, B. S. (1924), Mass. Inst. ofTech. Koppen’s career went back to the design of the Ford “Flying Flivver,” a contemporary of the Ford Model A. He joined MIT in 1929 and taught airplane stability and control and airplane design courses there until his retirement in 1965. Koppen did early work on the effects of closing loops on stability. He designed one of the first two-control airplanes, the Skyfarer, as well as the famous STOL Helioplane. Koppen test-flew the Helioplane prototype in 1949 and continued to fly at a ripe age, getting an FAA instrument rating at age 80.

Larrabee, E. Eugene 1920-, b. Marlboro, MA. B. S.Me. (1942), Worcester Poly. Inst., M. S.Ae. (1948), Mass. Inst. ofTech. Larrabee did stability and control design work on the Curtiss C-46, XF15C-1, and XP-87 airplanes. He developed stability derivative extraction methods using time vector analysis. He taught airplane stability and control at MIT and Northrop University for many years. He is a recognized expert on propeller design.

Lecomte, Pierre 1925-, b. France. Ecole Polytechnique, ENSAE. Lecomte was Professor of Flight Mechanics at ENSAE and author of the book Mecanique du Vol. He initiated a new handling qualities approach based on normal and peripheral flight envelopes and a theoretical explanation of wing drop. He was a test pilot in the French Flight Test Center, a Concorde evaluator at Aerospatiale, and chairman of the AGARD Flight Mechanics Panel.

McDonnell, John D. 1937-, b. Hollywood, CA. B. S. (1960), M. S. (1965), U. of Calif, at Los Angeles. At Systems Technology, Inc., he contributed to the analysis and evaluation of flying qualities. At McDonnell Douglas he contributed to the design and evaluation of avionics and control systems for the DC-10, MD-80, T-45, C-17, MD-11, and the space shut­tle (HUD). He was the chief avionics engineer and chief avionics FAA DER at McDonnell Douglas, Long Beach.

McRuer, Duane T. 1925-, b. Bakersfield, CA. B. S. (1945), M. S.E. E. (1948), Calif. Inst. of Tech. McRuer is perhaps best known to stability and control engineers as the senior author of “The Green Book,” whose real title is Aircraft Dynamics and Automatic Control. His enormous personal contributions to the field include mathematical models for human control and information processing in closed-loop systems and a well-tested theory of vehicle handling qualities.

McWha, James 1939-, b. Millisle, N. Ireland. B. S., Queens U., Belfast. McWha was chief engineer of flight systems at Boeing Commercial Group throughout the development of the fly-by-wire 777 transport. Prior to a 30-year employment at Boeing, he worked at Shorts Brothers, N. Ireland. He is vice chairman of an SAE control and guidance subcommittee and a member of a NASA Flight Controls and Guidance Panel.

Milliken, William F., Jr. 1911-, b. Old Town, ME. B. S. Ae. and Math. (1934), Mass. Inst. of Tech. At Cornell Aeronautical Laboratory, later Calspan, he was a leader in the application of servomechanism techniques to airplane stability and control, including the determination of airplane stability derivatives and transfer functions from flight-test frequency-response measurements.

Mueller, Robert K. 1909-1994, b. Waterbury, CT B. S. (1932), M. S. (1934), ScD. (1936), Mass. Inst. of Tech. Mueller produced one of the first electronic analog computers while doing his Sc. D. thesis. He also developed time vector analysis of airplane dynamics at that time. He invented the Microsyn transducer, used in servomechanism systems.

Mulder, Jan A. (Bob) 1943-, b. The Hague, The Netherlands. MSc. Aero. (1968), Ph. D. (1986), Delft U., a student of Professor Otto Gerlach. Dr. Mulder is head of the division of Control and Simulation, Aerospace Engineering, Delft U. of Technology and an active captain on the Boeing B-757. His current research interests are in intelligent flight control and dynamic flight-test techniques.

Neumark, Stefan 1897-1967, b. Lodz, Poland. Dipl. Ing. and Sc. D., Tech. U. of Warsaw. Dr. Neumark was a most versatile engineer. In stability and control, he was noted for the atmospheric density change effect on the phugoid mode and for the theory of airplane stability under constraints. He also contributed to the theories of dynamic stability with rudder free and of gust effects on automatic control.

Nguyen, Luat T. 1947-, b. Vietnam. B. S. (1968), M. S. (1970), E. A.A. (1970), Mass. Inst. of Tech. Nguyen is a NASA expert on aircraft flight dynamics at high angles of attack. He has contributed to control system design for enhanced maneuverability and departure resistance.

Osder, Stephen 1925-, b. New York, NY. B. E.E. (1946), City Coll. of New York, M. S. (1951), Johns Hopkins U. He pioneered in the design of digital flight control, fly-by-wire systems, and redundancy management. He was Director of R and D at Sperry and Chief Scientist, Flight Controlsand Avionicsat McDonnell DouglasHelicopters. Hisflight control design experience included the QF-104 drone, MD-80, DC-10, NASA CV-990, AH-64 helicopter, various reentry bodies, NASA STOLAND and Space Shuttle Autoland, and recently the Boeing Canard Rotor Wing aircraft.

Perkins, Courtland D. 1912-, b. Philadelphia, PA. B. S. (1935), Swarthmore Coll., M. S. (1941), Mass. Inst. of Tech. Perkins helped launch the stability and control function at Wright Field in World War II. He wrote the stability and control portion of the important text Airplane Performance, Stability and Control. He taught the subject at Princeton University and later became Chief Scientist of the U. S. Air Force.

Phillips, W. Hewitt 1919-, b. Port Sunlight, Merseyside. S. B. (1939), S. M. (1940), Mass. Inst. of Tech. Phillips was a well-known model aircraft builder before joining NACA in 1940. His achievements in stability and control are many, but they are perhaps topped by his discovery of the roll or inertia-coupling phenonemon in 1947. Phillips also made important theoretical contributions to the design of spring tabs, the landing approach problem, gust alleviation, and pilot-airplane interactions that cause instability.

Pinsker, Werner J. G. 1918—, b. Mannheim, Germany. BEUTH (1939), Berlin College. Pinsker was the preeminent expert in inertial coupling at the British Royal Aircraft Establishment. He also contributed to the theories of landing large airplanes and of nose – slice departures. As a consultant, he helped solve lateral-directional problems of the multi­national Tornado airplane.

Poisson-Quinton, Phillipe 1919—, b. Loches, France. La Sorbonne, ENSAE (1945). He was a professor/lecturer on aerodynamics, flying qualities, and control systems and a visit­ing professor at Princeton U. in 1975. At ONERA, he initiated transonic research on flying qualities and on optimized shapes for aircraft from V/STOL to hypersonic types. He de­livered the 1967 AIAA Wright Brothers Lecture and was a member of the AGARD Flight Mechanics Panel.

Reid, Lloyd D. 1942-, b. North Bay, Canada. B. A.Sc. (1964), M. A.Sc. (1965), Ph. D. (1969), U. of Toronto. He is Associate Director of the University of Toronto Institute for Aerospace Studies. He is co-author of two standard stability and control textbooks. Dr. Reid’s research contributions include the study of aircraft response to the planetary boundary layer and the development and operation of a facility for flight simulation of the effects on stability and control of pilot-aircraft interactions.

Relf, Ernest Frederick 1888-1970, b. Beckenham, Kent. A. R.C. Sc. Relf combined mastery of aerodynamic theory with extraordinary talents as an experimentalist. He devised methods for the testing of autorating wings, yawed propellers, and apparent mass. In 1922, he built a small, powerful electric motor for use in powered wind-tunnel models.

Ribner, Herbert S. 1913-, b. Seattle, WA. B. S. (1935), Calif. Inst. of Tech., M. S. and Ph. D. (1937, 1939), Washington U. Dr. Ribner’s contributions to stability and control are in propeller and slipstream theory and in gust response. While at NACA, he solved the problem of the forces on yawed propellers, an important factor in static stability.

Rodden, William P. 1927-, b. San Francisco, CA. B. S. (1947), M. S. (1948), U. of Calif., Ph. D. (1958), U. of Calif, at Los Angeles. Dr. Rodden made important stability and control contributions as a co-developer of the Doublet Lattice method for oscillating lifting surfaces and of the first correct equations of motion for quasi-steady aircraft utilizing restrained aeroelastic derivatives. He is a co-author of the MSC/NASTRAN Aeroelastic Analysis User’s Guide.

Root, L. Eugene 1911-1992, b. Lewiston, ID. B. S., U. of the Pacific, M. S., Cal. Inst. of Tech. As chief of aerodynamics at the El Segundo plant of the Douglas Aircraft Company, Root led the team that developed in a systematic way excellent flying qualities for the U. S. Navy SBD Dauntless and AD Skyraider aircraft. Root went on to become one of the founders of the RAND Corporation and later president of the Lockheed Missile and Space Company.

Roskam, Jan 1930-, b. The Hague, The Netherlands. M. S.A. E. (1964), Delft U. of Tech­nology, Ph. D. (1965), U. of Washington. Dr. Roskam worked for Cessna (1957-1959) and Boeing (1959-1967) on a variety of airplane projects. He is a major stability and control influence through his teaching at the University of Kansas, his consulting work, and his papers and textbooks.

Ross, A. Jean 1931-, b. Sussex, U. K. B. Sc. (1953), Ph. D. (1956), U. of Southampton. At the RAE and DERA, Farnborough, Ross specialized in modeling and analysis of aircraft responses to nonlinear dynamics and aerodynamic effects. She contributed to wing rock theory and to model testing of spin prevention and maneuver limitation systems. She partic­ipated in experimental wind-tunnel and free-flight work, culminating in the active control of forebody vortices.

Schairer, George S. 1913—, b. Pittsburgh, PA. B. S. (1934), Swarthmore Coll., M. S. (1935), Mass. Inst. of Tech. During Schairer’s long career at the Boeing Company, he was responsible for the stability and control designs of many of their airplanes. He redesigned the Stratoliner vertical tail to include one of the first dorsal fins and was responsible for sweeping the B-47’s wing. He became Boeing’s Corporate Vice President for Research.

Shaw, David E. 1932-, b. Bradford, U. K. B. Sc. Aeronautics (1954), Queen Mary Coll., London. He worked in all areas of aerodynamics at AV Roe Weapons Division and at BAe Warton. Shaw’s career highlights were clearance of the Lightning for rapid rolling, Tornado wing design, and Aerodynamics Team Leader for the Experimental Aircraft Programme (EAP), from design to full-flight clearance.

Smith, Terry D. 1947-, b. Norwich, U. K. B. Sc. Eng. (1968), Imperial Coll., London. As a flight-test engineer specializing in flight controls, he was deeply involved in stability and control testing and flight control system development on the Jaguar, Tornado, and Eurofighter Typhoon. He led the flight-test teams for both the fly-by-wire Jaguar and the Experimental Aircraft Programme (EAP) digital flight control systems.

Soule, Hartley A. 1904-, b. New York, NY. B. S.Ae. (1927), New York U. Soule started at NACA in 1927. He pioneered in spin research and made the first comprehensive measure­ments of airplane flying qualities. He was a co-inventor of the NACA Stability Wind Tunnel. Soule wrote a set of flying qualities requirements that eventually led to civil standards and military specifications.

Stengel, Robert F. 1938-, b. East Orange, NJ. S. B. (1960), Mass. Inst. of Tech., M. S.E. (1965), M. A. (1966), Ph. D. (1968), Princeton U. At the Draper Laboratory, Stengel followed airplane flying qualities principles in designing the manual attitude-control system for the Project Apollo Lunar Module. At Princeton, he converted the Navion variable-stability research airplane to digital control and conducted flying qualities and control system re­search.

Szalai, Kenneth J. 1942-, b. Milwaukee, WI. B. S.E. E. (1964), U. of Wisconsin, M. S.M. E. (1970), U. of Southern California. Mr. Szalai was principal investigator for the NASA Dryden F-8 Digital Fly-by-Wire program, the first of its type. He led the development of the U. S.-Russian Tu-144 supersonic flying laboratory. As director of the NASA Dryden Flight Research Center, he supervised research in thrust vectoring, high-angle-of-attack aerodynamics, and advanced flight controls. The X-29, X-31, X-36, and X-38 experimental programs were under his direction.

Thomas, H. H. B. M. (Beaumont) 1917-2000, b. Llanelli, U. K. B. Sc. (1939), D. Sc. (1980), U. ofWales, OBE 1979. In the Aerodynamics Department of the RAE, Farnborough, his expertise was in stability and control at the edges of the flight envelope. He contributed to control surface aerodynamics during World War II and to the dynamic stability of slender aircraft during the basic research that led to the Concorde. He also contributed to spin entry and recovery testing and analysis.

Toll, Thomas A. 1914—, b. Bridgewater, SD. B. S. (1941), U. of Calif. Toll made a wide range of stability and control contributions, including control surface aerodynamic balance, swept wings, and variable geometry. He is perhaps best known for two valuable summary reports, on lateral control research and on the supersonic transport.

Tonon, Aldo 1957—, b. Caracas, Venezuela. Politecnico ofTurin (1982). At Alenia ofTurin (formerly Aeritalia), Tonon’s major activity was in combat aircraft controls development. He was on the AMX program and then on the Eurofighter 2000 control law design, starting with the EAP technology demonstrator.

Wanner, Jean-Claude L. 1930—, b. Brest, France. Ing. (1950), Ecole Polytechnique, Ing. (1955), ENSAE. Dr. Wanner’s career in airplane stability and control includes serving as a military pilot, flight test engineer, and as professor in a number of institutions, including the ENSAE. He is author of the French text Mecanique du Vol. He pioneered in using computer methods in the teaching of stability and control.

Washizu, Kyuichiro 1921-1981, b. Ichinomiya, Aichi, Japan. B. Eng. (1942), Imperial U. of Tokyo, Dr. Eng. (1957), U. of Tokyo. Dr. Washizu’s important contribution to airplane stability and control was to train a generation of Japanese engineers in the field, having spent time in the United States to study the educational system. He did research on human controllability limits and finite-element methods and is the principal author of the stability and control section in Japan’s Handbook of Aerospace Engineering (1974).

Weick, Fred E. 1899-1993, b. Chicago, IL. B. S. (1922), U. of Illinois. Weick was at NACA’s Langley Aeronautical Laboratory from 1925 until 1936, contributing to lateral control research. He developed the W-1 pusher airplane, incorporating important stability and control innovations. The W-1 was a two-control airplane that had limited up-elevator travel and a tricycle landing gear. He later became known as the designer of the Ercoupe, the first agricultural airplane, the Ag-1, and a series of Piper aircraft.

Westbrook, Charles B. 1918-2001, b. Port Jervis, NY. M. S. (1946), Mass. Inst. of Tech. Westbrook joined the USAF Flight Dynamics Laboratory in 1945 as head of stability and control. He oversaw the development of post-war flying qualities specifications and the USAF Stability and Control Handbook. Westbrook managed for the Air Force much flying qualities research, including work on variable-stability airplanes.

White, Roland J. 1910-2001, b. Missoula, MT. B. S. (1933), U. of Calif., M. S.M. E. (1934), M. S.A. E. (1935), Calif. Inst. of Tech. His long stability and control career started at Curtiss-Wright, St. Louis, where he incorporated a springy or “vee” tab to the C-46 Commando, adding to its allowable aft cg travel. White designed a mechanical yaw damper for the Boeing B-52 and made one of the first servo analyses of electronic yaw dampers, for the B-47.

Wykes, JohnH. 1925-1988. B. S. (1949), Mass. Inst. ofTech., M. S.,U. of So. Calif. Wykes was a leading stability, control, and aeroelastics engineer at the Rockwell International Aircraft Division from 1949 to 1986, where he contributed to the designs of the F-86, F-100, F-107, B-70, andB-1 airplanes. He joined Northrop in 1987 to work on their YF-23A airplane. In addition to innovative work on stability augmentation, he also was responsible for the design of the B-1 gust alleviation system.

Zimmerman, Charles H. 1907-1995, b. Olathe, KS. B. S. (1929), U. of Kansas, M. S.Ae. (1954), U. of Virginia. Zimmerman started at NACA in 1929. He produced the classical NACA dynamic longitudinal and lateral stability analyses in 1935 and 1937, complete with stability boundary design charts. This was a considerable accomplishment for those times and the main design source for dynamic stability for years afterwards. He was instrumental in developing the Langley 20-foot spin and free-flight tunnels.

The V/STOL Case

Vertical or short takeoff and landing (V/STOL) airplane flying qualities require­ments present special problems because V/STOL airplane technology covers a large range of possibilities. So far, we have seen tilt rotor, lift fan, vectored thrust, blown flaps, and con­vertible rotor wing versions. Although the military services have taken up the challenge and in 1970 issued a V/STOL flying qualities specification, MIL-F-83300, there is a danger that the requirements are specific to individual designs, those available for testing at the time.

MIL-F-83300 recognizes three airspeed regimes, from hover to 35 knots, from 35 knots to an airspeed Vcon where conventional flying qualities requirements apply, and airspeeds above Vcon. Requirements are either for small perturbations about some fixed operating point or for accelerated or transitional flight. The V/STOL small-perturbation longitudinal

The V/STOL Case

Figure 3.13 The Air Force Wright Laboratory’s VISTA, or multiaxis thrust-vectoring airplane, a variable-stability machine based on the General Dynamics F-16D. Thrust is vectored up to 17 degrees in pitch and yaw, primarily for high-angle-of-attack research. (From Aerospace America, Dec. 1993)

dynamics requirements take the familiar MIL-8785 form of acceptable and unsatisfac­tory boundaries in terms of real and imaginary parts of the system roots. So do the lateral-directional requirements resemble those for conventional airplanes, as requirements on the shape of the bank angle versus time curve for rolls and on permitted adverse yaw.

A complication when applying the familiar period and damping requirements to the roots of V/STOL motions is convergence of the ordinary modes of motion at very low airspeeds. For a powered-lift STOL configuration the longitudinal short-period and phugoid modes merge at an equilibrium weight coefficient, equivalent to the lift coefficient, of 3.5 (Figure 3.14). A similar trend shows up in the lateral case, where an (unstable) spiral mode approaches in time constant the usually much shorter rolling mode at a large value of the equilibrium weight coefficient.

The problem of establishing V/STOL flying qualities requirements that are not tied to specific configurations was taken up again after MIL-F-83300, for the most part with the help of ground simulations and variable-stability airplanes. In 1973, Samuel J. Craig and Robert K. Heffley used analysis and ground simulation to explore the role of thrust vector inclination during STOL landing approaches (Craig and Heffley, 1973).

Still later, in papers delivered in 1982 and 1983, Roger H. Hoh, David G. Mitchell, and M. B. Tischler looked for flying qualities generalizations in VTOL transitions and STOL path control for landings. Precise pitch attitude control at high bandwidth appears to be critical in transitions because of the sensitivity of vertical rate to pitch attitude. However,

The V/STOL Case

Figure 3.14 A complicating factor in specifying STOL flying qualities. The long – or phugoid and short-period longitudinal modes converge at low airspeeds, where part of the airplane’s weight is supported by thrust. (From Etkin, Dynamics of Atmospheric Flight, 1972)

the writers found a number of possible requirements for the landing flare control maneuver of STOL airplanes. That series closed out with a major effort to extend the MIL Prime Standard and Handbook (Anon., 1987) concept to STOL Landings (Hoh, 1987).

An area that seems to require more attention is the lift loss in the vortex ring state (Glauert, 1934; Coyle, 1996) during increases in descent rate. Vortex rings recirculate the rotor downflow back into the rotor, instead allowing it to descend and produce lift. This is a performance problem for single-rotor helicopters. However, for the tilt-rotor V-22 Osprey, a vortex ring on one of two laterally located rotors is believed to have produced an unrecoverable roll.

The considerable experience gained by DERA and BAE systems in V/STOL projects, leading to the Harrier and the VAAC (Vectored thrust Aircraft Advanced flight Control) Harrier, is summarized by Shanks (1996) and Fielding (2000). One key finding was that eliminating conscious mode changing provides a large reduction in pilot work load. The V/STOL becomes a “conventional aircraft that can hover.” Another finding was the need to use closed-loop analysis to specify propulsion system characteristics in terms of bandwidth and response linearity.

Pilot-in-the-loop technology (Chapter 21) has made significant contributions to under­standing the special flying qualities requirements of STOL and VTOL airplanes. This ap­proach is especially valuable because it is not closely tied to the design details of specific machines.

All-Movable Controls

All-movable tail surfaces became interesting to stability and control designers when high Mach number theory and transonic wind-tunnel tests disclosed poor performance of ordinary flap-type controls. Effectiveness was down, and hinge moments were up. More consistent longitudinal and directional control over the entire speed range seemed possible with all-moving surfaces. However, application of all-moving or slab tail surfaces had to await reliable power controls.

One of the first all-moving tail applications was the North American F-100 Super Sabre. According to William E. Cook, a slab horizontal tail was considered for the B-52 and rejected only because of the unreliability of hydraulics at the time. In modern times, there is the Lockheed 1011 transport, with three independent hydraulic systems actuating its all-moving horizontal tail. Of course, modern fighter airplanes, starting with the F-4 in the United States; the Lightning, Scimitar, and Hawk in Britain; and the MiG-21 in Russia, have all-moving horizontal tails.

An interesting application is the all-moving tail on a long series of Piper airplanes, beginning with the Comanche PA-24 and continuing with the Cherokee and Arrow series. A geared tab is rigged in the anti-balance sense. The geared tab adds to both control force and surface effectiveness. Fred Weick credits John Thorp with this innovation, inspired by a 1943 report by Robert T. Jones.

The F4D Skyray Experience

At the Douglas Aircraft Company’s El Segundo plant the F4D (later F-6) Skyray was being delivered in service test quantities at the height of the inertial coupling excitement. While it was becoming clear that the coupling problem could be eased, if not avoided, by artificial pitch and yaw damping, these devices were still heavy and unreliable. In any case, properly applied, they are placed in series into a control circuit. That is, artificial damping devices are not a parallel installation, as are many automatic pilots, but are an integral part of the control run from stick to control surface. A series-type artificial damping device cannot be easily retrofitted to a production airplane.

The F4D Skyray Experience

Figure 8.7 The relative importance of aerodynamic, inertial coupling, and engine gyroscopic terms in yawing acceleration during a rapid roll in the North American F-100. In this example, aerodynamic moment is the major factor initially; then inertial coupling overpowers it. (From Wykes, WADC Conf. 56WCLC-1041, 1956)

High levels of static longitudinal and directional stability were likewise known to be helpful, increasing the roll rates that would cause coupling to occur. In the case of the tailless F4D, high levels of static longitudinal and directional stability could hardly have been designed into the basic airframe, even if the need had been known when the ship was laid out.

The only remaining option for the F4D at that stage was to determine safe maneuver limits, the points at which inertial coupling effects did not cause problems. The airplane could then be restricted by placard, with its pilots depended upon to operate within these limits. The problem of determining F4D safe maneuver operating limits fell into the hands of Robert W. Bratt, who was able to collect the large body of numerical data needed on Douglas Accounting Department digital computers.

The analog computers belonging to the Douglas Flight Controls Department could have in principle run the problem, except for two factors. First, the inertial coupling equations of motion include a large number of mathematical nonlinearities, such as trigonometric functions, needed to resolve gravity along airplane axes, and multiplications, needed to solve the Euler, or moment, equations. But representing nonlinearities on the analog computers of those days was a severe problem. The best analog trigonometric function generators and multipliers were small electromechanical servomechanism devices that were costly and somewhat slow. A few systems boasted “quarter-square” multipliers, which had no moving parts but were inaccurate.

The second drawback to having used analog computers for the F4D inertial coupling problem was that accuracy and repeatability were poor. Bratt’s problem was to cover systematically in small increments all combinations of lateral and longitudinal stick throws, including phasing or sequence between the two, trimmer settings, and airspeeds and alti­tudes, in order to establish the points of coupling onset. Vacuum-tube analog computers were not only subject to frequent breakdowns, but also to drifts and gain changes that could make a hash out of a systematic study. These considerations led to using the digital computer, an early use for these machines in flight dynamics.

The First Dive Pullout Problems

The Lockheed P-38 Lightning, a powerful and effective fighter airplane of World War II, is believed to have been the first airplane to have experienced adverse compressibility

The First Dive Pullout Problems

Figure 11.1 Pressure distributions on the NACA 4412 airfoil at an angle of attack of 1.9 degrees. From top to bottom, the Mach numbers are 0.60, 0.66, and 0.74. Separation, or the compressibility burble, starts at a Mach number of 0.66 on the upper surface areas between the arrows. (From Stack, Lindsey, and Littell, NACA Rept. 646, 1938)


effects on stability and control. A May 1942 Lockheed Company report by C. L. (Kelly) Johnson reported P-38 problems in dives and dive recoveries. These problems were typical of those found later in high-speed dives of other airplanes.

An important effect of compressibility on the P-38 was a great increase in the static longitudinal stability contribution of the horizontal tail. This was due to loss in lift curve slope of the thick center portion of the wing due to the compressibility burble. The P-38’s wing center section was the 15-percent-thick NACA 23015; the tip section was the 12-percent-thick NACA 4412. The reduced lift curve slope of the wing center section also re­duced the rate of change of downwash at the tail with angle of attack, accounting for the static longitudinal stability increase. Equally serious was a large nose-down trim change at the positive lift coefficients needed for pullouts from dives. Figure 11.2 shows both the increased stability and the trim change measured on a P-38 model in the NACA Ames 16-foot wind tunnel (Ericson, 1942).

Ericson and the NACA staff looked at fixes for these problems in a later series of 16-foot wind-tunnel tests that same year. The rather stubby P-38 fuselage aggravated the

The First Dive Pullout Problems

Figure 11.2 Variation of pitching moment coefficient with lift coefficient for two models tested in the NACA Ames 16-foot high-speed wind tunnel. Static longitudinal stability increases greatly at the higher Mach numbers and the trim lift coefficient (at zero pitching moment) is reduced. The upper data are for the Lockheed P-38, the lower for the Douglas DC-4 airplane. (From Hood and Allen, NACA Rept. 767, 1943)

The First Dive Pullout Problems

Figure 11.3 Dive-recovery flaps fitted to the Lockheed P-38 in the NACA 16-foot high-speed wind tunnel and later used on the P-38J-LO. (From Erickson, NACA WR-A-66, 1943).

problem by inducing high velocities over the already too thick wing center section, and some improvement was made when the model’s fuselage lines were straightened, by lengthening it. However, the most significant gain was made with lower surface auxiliary split-wing flaps, located 1/3 of the wing chord behind the leading edge (Figure 11.3). The auxiliary flaps, later to be called dive recovery flaps, increased the P-38’s trim lift coefficient by 0.55 at a Mach number of 0.725 (Ericson, 1943).

Best of all, auxiliary flap effectiveness decreases as the Mach number decreases. The importance of decreased effectiveness is that overcontrol is avoided during the dive recovery. Overcontrol in a dive recovery was a serious problem without dive recovery flaps, when pilots had only elevator control power to rely on to produce positive or nose-up pitching moments. In contrast to dive recovery flaps, pitching moment coefficient per degree of up- elevator travel actually increases as the Mach number decreases. Thus, if the pilot exerts a heavy pull force and nose-up trim through trim tabs to eke out a dive recovery against strong longitudinal stability and a nose-down trim change, when the Mach number drops and the latter two factors go away the airplane will respond to the heavy pull force and nose-up trim tab setting by making an excessively rapid pullout. If the normal acceleration or pullout g is high enough, the pilot blacks out and the wings fold upward.

The -25 block of the P-38J-LO series was fitted with dive recovery flaps. This same block of airplanes got power-boosted ailerons, further improving high-Mach-number con­trollability. Dive recovery flaps also were installed on P-47, A-26, P-59, and P-80 airplanes.