Category Airplane Stability and Control, Second Edition

Wright Controls

In the Wright brothers’ 1902 glider and their 1903 Flyer the pilot had a vertical lever for the left hand that was pulled back to increase foreplane incidence. The pilot lay on a cradle that shifted sideways on tracks to cause wing warp. To roll to the left the pilot decreased the incidence of the outer left wings and increased the incidence of the outer right wings. The rudder motion was mechanically connected to the wing warp mechanism to turn the nose left when the pilot wished to lower the left wing, and vice versa for lowering the right wing, thereby overcoming the adverse yaw due to wing warp.

When they began to fly sitting up in 1905, the Wrights retained the left-hand vertical lever for foreplane incidence but added a right-hand vertical lever for wing warp and rudder. They moved the new right-hand lever to the left for left wing down and forward for nose – left yaw. The right-hand lever was moved to the right for right wing down and aft for nose-right yaw. Turn coordination required the pilot to phase control motions, leading with yaw inputs. These unnatural control motions had to be learned and practiced on dual control machines or simple simulators. Bicyclists to the last, they never used their feet for control. They retained this scheme until 1909. Since wing warping involved considerable elastic deformation of the wing structure, they later changed the fore-and-aft motion of the right – hand lever to wing warp and mounted a new, short lever on its top for side-to-side movement to control the rudder. When the Wrights abandoned the all-moving foreplane array for an all-moving rear horizontal tail in 1911, the left-hand lever still controlled its incidence, but now reversed.

The Wrights’ patent was for mechanically linked roll and yaw controls. Other airplane builders, notably Curtiss, built airplanes with ailerons, rudders, and elevators, providing independent three-axis control. Curtiss and others asserted that the Wright machine now had independent three-axis control, but U. S. courts upheld the Wright patent against them. The courts maintained that the coupling of roll and yaw controls in the Curtiss machines existed in the mind of the aviator and was essential to the art of flying. Therefore, the Curtiss independent three-axis control infringed on the Wright patent!

Modern Light Twin Airplanes

The situation is different again in the case of the modern light twin airplanes. The first of these planes was the five-to-seven place Aero Commander 520, introduced by the Aero Design and Engineering Corporation of Culver City, California, in 1950. A year or so later Beech introduced its Model 50 Twin Bonanza, Piper its Model PA-23 Twin Stinson (later called Apache), and Cessna its Model 310 twin. These aircraft and their successors have a great deal of appeal to aviators who regularly fly on instrument flight plans into bad weather and those who want the extra safety of a second engine.

Yet by the early 1980s the safety records compiled by the modern light twins did not bear out this expectation. Writing in the AOPA Pilot of January 1983, Barry Schiff pointed out that the fatality rate following engine failure in light twins was four times that for engine failures in single-engine airplanes. It seems that relatively low-time private pilots were being trapped by the yaw and roll caused by the failure of one engine at low speeds and altitudes.

The Beech Model 95 Travelair and its higher power military derivative, the U. S. Army’s T-42A, are good examples of what could happen. After several fatal stall-spin accidents following power loss on one engine a courageous Army pilot made a series of T-42A stall tests, with symmetric and antisymmetric power. His report told of moderate wing drop in symmetric stalls, but of vicious behavior in stalls with one engine idling. The airplane would roll nearly inverted, clearly headed for a spin.

The response of the Federal Aviation Administration (FAA) to this generic light twin hazard was not to require design changes, but to warn pilots and to stress recognition and compensation for single-engine failure during training and flight tests for multiengine pilot ratings. Pilots are drilled to instantly recognize the failed engine by the mantra “Dead foot, dead engine.” Since accidents occur during the incessant single-engine drills in training, there is a special minimum airspeed for “intentionally rendering one engine inoperative in flight for pilot training.”

This is Vsse, the fourth of the special airspeeds the poor pilot has to memorize in order to legally operate multiengine airplanes. The others are Vmc or Vmca, the minimum airspeed for control with the critical engine’s propeller windmilling or feathered, the other delivering takeoff power; and Vxse and Vyse, the best angle of climb and rate of climb airspeeds with one engine inoperative. Vyse has its own marking on the airspeed dial, a blue line usually used as the landing approach airspeed under normal conditions. Evidently, if an engine fails on landing approach, one wants the airplane to be already at its best airspeed to climb away or to lose as little altitude as possible. The four special airspeeds for multiengine airplane operation are added to nine other special airspeeds (six if the airplane has no wing flaps or retractable landing gear) to be remembered.

In spite of the FAA’s apparent disinterest in obliging light twin builders to design safe single-engine behavior into their airplanes, there have been some attempts made in this direction. There is an FAA-approved design retrofit of vortex generators for the upper wing surfaces of some light twins. The installation reduces Vmca, the minimum airspeed for control with an engine out (Figure 4.3). Vortex generators are tiny (about 2 inches square)

Modern Light Twin Airplanes

Figure 4.3 Vortex generators fitted to the upper wing surface of a Piper PA-31-3 50 Chieftan light twin-engine airplane, to reduce minimum single-engine control speed Vmca. This installation of 43 generators on each wing was designed by Boundary Layer Research, Inc., of Everett, WA.

low-aspect ratio wings that stick out of a surface. The tip vortices from a spanwise row of generators set at angles of attack energize the surface’s boundary layer by mixing in with it high-energy air from the surrounding flow. The energized boundary layer tends to remain attached, avoiding separation or stall.

According to John G. Lee (1984), vortex generators were invented by “an introspective and rather unapproachable loner” named Hendrik Bruynes, who used eight vortex generators to correct separation from the walls of the diffuser in a new 18-foot United Aircraft Research Department wind tunnel. While Bruynes was named in the vortex generator patent, Lee credits Henry H. Hoadley with the key idea of reversing the angles of alternate generators. The Forty-Second Annual Report of the NACA, dated 1956, flatly credits H. D. Taylor of United Aircraft as the developer of vortex generators; no mention is made of either Bruynes or Hoadley.

Stability and Control at the Design Stage

In the preliminary layout of a new airplane, the stability and control engineer is generally guided by some well-known principles related to balance and tail sizing, for example. Once a preliminary design is laid out, its main stability and control characteristics can be predicted entirely from drawings. This includes the neutral point (center of gravity for zero-static longitudinal stability), static directional (weathercock) and lateral (dihedral effect) stability, and assurance that the airplane can be trimmed to zero-pitching moment over its lift coefficient and center of gravity ranges.

In the best of circumstances, the new design has a family resemblance to an earlier design. Then the estimations resemble extrapolations from known, measured characteristics. All airplane manufacturers seem to maintain proprietary aerodynamic handbook collections and correlations of stability and control data from previous designs. This is a great help if the extrapolation route is indicated. Aside from these private collections, there is a large body of theory and correlations from generalized wind-tunnel data that can be called upon for prediction or estimation.

A closely related subject to the prediction of stability and control characteristics entirely from drawings is the problem posed at the next stage in an airplane’s development, when wind-tunnel test data have been obtained. In former times, one was often asked to prepare a complete set of predicted flying qualities using the wind-tunnel data and any flight control details that may have been available at the time. Instead, current practice is to plug wind- tunnel test and control system data into a flight simulator, for pilot flying qualities evaluation. Radio-controlled flying scale models are an alternate stability and control source for projects that cannot afford wind-tunnel tests.

The three design-stage topics – layout principles, estimation from drawings, and estima­tion from wind-tunnel data – are treated in this chapter.

The Role of Rotary Derivatives in Spins

The rotary derivatives are the force and moment coefficient derivatives with respect to dimensionless angular velocity The rotary derivatives appear in the airplane equations of motion for normal unstalled flight, as well as for spinning flight. However, at the relatively low airspeeds and high angular velocities for spinning flight, the rotary derivatives are much more important than they are for unstalled flight. Physically, under spinning conditions there will be large differences in local flow angles of attack at different parts of the airplane, and possibly local separated flows.

Stated otherwise, the rotary derivatives are generally of secondary importance to flight simulation and flight control design for normal unstalled flight. If the airplane has stability augmentation systems that drive the control surfaces to provide artificial damping, this is even more true; artificial damping swamps out the rotary derivatives that supply natural aerodynamic damping. Thus it is that, at least in modern times, the drive to refine analytic and measurement techniques for the rotary derivatives has come from spinning studies.

The early 1950s saw a rush of 5- and 6-degree-of-freedom inertial coupling computer simulations, as told in Chapter 8, “The Discovery of Inertial Coupling.” It is interesting that some of the same investigators, such as Cecil V Carter, John H. Wykes, and Leo Celniker, who helped crack inertial coupling with their simulations moved on to spin simulation using analog or digital computers. The motivation was there, because the same airplane loading characteristics that lead to inertial coupling also lead to post-stall gyrations and departures, motions not easily studied in free-spinning wind tunnels.

The problem was that this period coincided with a shutdown of rotary balance testing at NACA. The NACA rotary balance was updated in the late 1950s, but it was not used for analytical studies until several years had passed. Thus, the spin computer analysis results reported at the 1957 Wright Air Development Center Airplane Spin Symposium (Westbrook and Doetsch, 1957), made without the benefit of current rotary balance data, came under criticism for using inadequate rotary derivatives by knowledgeable people such as Dr. Irving C. Statler and Ronald F. Sohn.

Supersonic Directional Instability

A rather simple static directional instability problem first appeared in a test flight of the North American F-100 Super Sabre. It is simple because the problem has one well-known cause, the loss in lifting surface effectiveness as Mach number increases be­yond 1. The instability of bodies of revolution, on the other hand, remains essentially invariant with Mach number. Static directional stability is to a first order the balance be­tween the unstable fuselage and the stabilizing vertical tail. The vertical tail is supposed to

Supersonic Directional Instability

Figure 11.15 A North American XB-70 airplane in flight. The wing tips are deflected downward for increased directional stability at supersonic speeds. (From Bilstein, Orders of Magnitude, 1989)

dominate, but as its effectiveness, or lift curve slope, drops off neutral stability is eventually reached.

The point of neutral directional stability on any supersonic airplane evidently should be beyond the attainable flight envelope. However, supersonic directional instability actually occurred in a dive on an early F-100 before an enlarged vertical tail was adopted, leading to a tragic accident. On the F-100 vertical tail, bending contributed to the loss in effectiveness. Modern stability augmentation techniques can provide artificial directional stability at su­personic speeds, if it is impractical or economically undesirable to have a large enough vertical tail.

The North American XB-70 bomber used a configuration change to return directional stability to acceptable levels at high supersonic Mach numbers. The wing outer panels folded down 65 degrees for flight at a Mach number of 2 and a larger angle above (Figure 11.15). Unfortunately, this made the dihedral effect negative, resulting in poor flying qualities. This was corrected on the second XB-70 prototype by a triangular wedge welded between the fuselage and wing, producing 5 degrees of geometric dihedral.

There was concern that if the XB-70’s wing tips ever stuck down in the folded position, the airplane could not be landed because of lack of ground clearance. Fortunately, this never happened. An additional benefit of the folded-down wing tips was reduction in excess static longitudinal stability at supersonic speeds, due to the change in planform. Also, compression lift was generated at supersonic speeds by shock waves from the folded tips producing positive pressures on the bottom of the wing and fuselage.

The British Aircraft Corporation’s TSR-2, designed for a Mach number of 2.0, had neutral directional stability at a Mach number of 1.7. The vertical fin was made small to

reduce tail loads in high-speed flight at low altitudes. The airplane was canceled for other reasons before a directional stability augmenter could be installed for flight faster than a Mach number of 1.7.

Early Safe Personal Airplane Designs

Aeromarine-Klemm As imported from Germany, it had unsafe spin character­istics. The wing was modified to have less taper and thicker tip sections. Control movement was restricted, and the center of gravity range was moved forward. All of these modifications, apparently arrived at empirically, were in a direc­tion to improve spin resistance, and this airplane became one of the very first to be called incapable of spinning. Actually, a spin could be forced, but the airplane had to be held into the spin; and with free controls it would recover. Aeromarine-Klemm models were produced with several different engines from the late 1920s to 1932.

Stout Sky Car Designed in 1931, the Sky Car was one of the first two-control airplanes. It had floating wing tip ailerons that were weight overbalanced, mak­ing them float symmetrically with slight negative lift. When deflected for a roll, proverse yaw, or yaw in the direction of the roll, resulted. No rudder control was needed to coordinate the roll. The Sky Car had a tricycle landing gear and limited up-elevator travel. It was a stubby, odd-looking machine, a biplane with a small vertical tail.

Weick W-1A In 1935 and 1936, this airplane was a test bed for several safety innovations. It had full-span flaps that could be deflected to 80 degrees to make steep descents into small fields. Slot lip spoilers provided lateral control (Figure 15.2). The not-yet-famous Robert T Jones studied two-control oper­ation and told Weick that the W-1A’s spoiler ailerons would be ideal for the purpose, as they turned out to be. As in the Stout Sky Car, elevator control was limited to prevent stall.

Stearman-Hammond Model Y and the Gwinn Aircar Both of these airplanes were designed with features of the Weick W-1A. The Model Y won a safe airplane competition sponsored by the Department of Commerce. The Aircar had no rudder at all. Its interior looked like an Oldsmobile, with Oldsmobile steering wheel and instruments.

ERCO Model 310 and the Ercoupe Fred Weick’s Ercoupe was the only one of the early safe airplanes to make it into production, which started in 1940 (Figure 15.3). The Ercoupe has the two-control, restricted elevator control and tricycle landing gear features ofthe W-1 A. The U. S. Civil Aeronautics Authority certified the Ercoupe as “characteristically incapable of spinning” and cut the dual time required to solo from 8 to 5 hours and the time for private pilot cer­tification from 35 to 25 hours.

With the yoke hard back, rapid full aileron control deflections from side to side produce nothing more exciting than falling-leaf motions. Cross-wind touchdowns are made with the airplane headed into the relative wind. When the pilot releases the controls the Ercoupe straightens out for its ground roll.

Early Safe Personal Airplane Designs

Figure 15.2 The 1935 Weick W-1A airplane, photographed in front of an NACA Langley Field hangar. This innovative airplane had full-span flaps and spoiler ailerons, limited up-elevator travel, and two-control operation. (From Weick, From the Ground Up, 1988)

Stability Boundaries

Until the advent of electronic analog and digital computers, numerical solutions of the equations of airplane motion were essentially limited to finding stability boundaries, the combinations of airplane stability derivatives and other parameters that divide stability from instability. Stability boundaries are found by Routh’s Criterion, developed by the Briton E. J. Routh in the early 1900s.

Airplane stability boundaries were first calculated in Britain (Bryant, Jones, and Pawsey, 1932). This was in a study of dynamic stability beyond the stall. Bryant and his co-authors found stability derivatives for a number of airplanes up to an angle of attack of 40 degrees. With these data, they produced stability boundaries as functions of static directional and lateral stability derivatives, both nondimensionalized by Glauert’s airplane relative density parameter /г.

There was an earlier British paper by S. B. Gates that presented contours of constant damping ratio and natural frequency for the longitudinal phugoid, as functions of tail volume and center-of-gravity position (Gates, 1927). While not strictly a stability boundary analysis, the Gates work certainly laid the groundwork for Bryant’s boundaries.

Two NACA reports by Charles H. Zimmerman (1935 and 1937) carried on Gates’ and Bryant’s pioneering stability boundary work. Zimmerman’s ambitious goal was to produce charts for the rapid estimation of the dynamics of any airplane. The Zimmerman reports have charts for both longitudinal and lateral motions, 40 of the former and 22 of the latter (Figure 18.6). As in Bryant’s work, the results are normalized using Glauert’s airplane density parameter г. The Zimmerman charts include period and damping estimates for the phugoid and Dutch roll motions.

Stability Augmentation

Stability augmentation is the artificial improvement, generally by electromechani­cal feedback systems, of airplane stability and control while the airplane remains under the control of the human pilot. Stability augmentation generally changes the airplane’s stability derivatives and modes of motion.

We make the important distinction between stability augmentation, artificial feel systems, and airplane automatic pilots. While artificial feel systems, discussed in Chapter 5, may alter stick-free stability for the better, their main function is providing manageable control forces. Automatic pilots replace the human pilot when they are in use.

20.1 The Essence of Stability Augmentation

To be a true stability augmenter, the device must change the airplane’s flight characteristics without the pilot’s perception. This means that augmenter outputs must add to those of the pilot in a series fashion. Augmenter outputs put into the primary control circuit between the cockpit and the control surfaces must move only the control surfaces, and not the cockpit controls. The requirement to not move the pilot’s controls is sidestepped if the augmenter is not inserted into the primary control circuit but moves a separate, or dedicated, control surface. Still another way around the need for augmenters not to move the pilot’s controls is the integrated control surface actuator (Chapter 5), used in fly-by­wire control systems. Integrated servo actuators accept and add electrical signals from both cockpit controls and stability augmenters.

In fly-by-cable control systems, isolation of primary-control-circuit stability augmenter outputs from the cockpit controls is a surprisingly difficult mechanical design problem. Control valve friction in control surface actuators acts to hold the surfaces fixed for small stability augmenter signals. When this happens, the augmenter in effect backs up and moves the cockpit controls instead. The result is an unaugmented airplane for small disturbances and limit cycle oscillations, such as yaw snaking. One cure for excessive valve friction can be as bad as the small signal backup problem. This is to center the cockpit controls with husky spring detents, which have to be overcome by the pilot in normal control use.

The degree of authority of stability augmentation systems is another important design consideration. Since augmenters operate ideally without moving the pilot’s controls, the pilot will be unaware of abrupt failures to the limit of augmenter authority until the airplane reacts. Then, there should be enough pilot control authority left to add to and cancel the failed augmenter inputs, with something to spare. This was the design philosophy until the advent of redundant, self-correcting augmentation systems, which make feasible augmentation at full authority or control surface travel.

Automatic pilots, which replace the human pilot when they are in use, are expected to move the cockpit controls. Abrupt full autopilot failures are instantly apparent to an attentive flight crew. Larger control authority than for stability augmenters is feasible, even for systems without the redundant, self-correcting feature.

Challenge of Stealth Aerodynamics

The invention of aircraft that are almost invisible to ground or surface-to-air – missile radars promises to be an effective defensive measure for reconnaissance and attack airplanes. This development has taken six paths so far, the first three of which are a distinct challenge to stability and control designers:

Faceted airframes replace the smooth aerodynamic shapes that produce at­tached flows and linear aerodynamics. Radar returns from faceted shapes, such as the Lockheed F-117A, are absent except for the instants when a facet faces the radar transmitter.

Parallel-line planforms have the same sweep angle on wing leading and trailing edges and on surface tips and sharp edges. Parallel-line planforms concentrate radar returns into narrow zones that are easily missed by search radars. This is the Northrop B-2 stealth method, augmented by special materials and buried engines.

Suppressed vertical tails are either shielded from radar by wing structure or eliminated altogether. The Lockheed F-22 has shielded vertical tails, the B-2 none at all.

Blended aerodynamics eliminate internal corners such as wing-fuselage inter­sections. Internal corners can act as radar corner reflectors. The Rockwell B-1 uses this technique to reduce its radar signature.

Buried engines and exhausts hide compressor fan blades and hot exhaust pipes from radar and infrared seekers.

Radar-absorbent materials are used, generally nonmetallic. This is a highly classified subject.

The challenges of faceted airframes, parallel-line planforms, and suppressed vertical tails to stability and control engineers are illustrated by current stealth airplanes.

Flying Qualities Become a Science

The stability and controllability of airplanes as they appear to a pilot are called flying or handling qualities. It was many years after airplanes first flew that individual flying qualities were identified and ranked as either desirable or unsatisfactory. Even more time passed before engineers had design methods connected with specific flying qualities. A detailed and fascinating account of the early work in this area is given in Chapter 3 of Stanford University Professor Walter G. Vincenti’s scholarly book What Engineers Know and How They Know It. We pick up the story in 1919, with the first important step in the process that made a science out of airplane flying qualities.

3.1 Warner, Norton, and Allen

Vincenti found that the first quantitative stability and control flight tests in the United States occurred in the summer of 1919. MIT Professor Edward P Warner (Figure 3.1), working part time at the NACA Langley Laboratory, together with two NACA employees, Frederick H. Norton and Edmund T Allen, made these tests using Curtiss JN-4H “Jennies” and a de Havilland DH-4. They made the most fundamental of all stability and control measurements: elevator angle (with respect to the fixed part of the tail, or stabilizer) and stick force required for equilibrium flight as a function of airspeed.

Warner and Norton made the key finding that the gradient of equilibrium elevator angle with respect to airspeed was in fact an index of static longitudinal stability, the tendency of an airplane to return to equilibrium angle of attack and airspeed when disturbed. The eleva­tor angle-airspeed gradient thus could be correlated with the 1915-1916 MIT wind-tunnel measurements by Dr. Jerome C. Hunsaker of pitching moment versus angle of attack on the Curtiss JN-2, an airplane similar to the JN-4H. In the words of Warner and Norton (1920):

If an airplane which is flying with the control locked at a speed corresponding to the negatively sloped portion of the elevator position curve is struck by a gust which decreases its angle of attack, the angle will continue to decrease without limit. If the speed is low enough to lie on the positively sloping portion of the curve, the airplane will return to its original speed and angle of trim as soon as the effect of the gust has passed. A positive slope [of the elevator angle-airspeed gradient] therefore makes for longitudinal stability. (Italics added)

A strange aspect of the Warner and Norton JN-4H test results was the effect of airspeed on static longitudinal stability. The JN-4H was stable at airspeeds below about 55 miles per hour and unstable above that speed (Figure 3.2). One would be tempted to look for an aeroelastic cause for this, except that wind-tunnel tests of a presumably rigid model showed the same trend. The cause remains a mystery. The 1915-1916 MIT wind-tunnel tests were supplemented in 1918 by the U. S. Air Service at McCook Field with JN-2 wind-tunnel tests, in which the model had an adjustable elevator angle.

The McCook Field group was active in stability and control flight tests at the same period. As part of an armed service procurement activity, McCook’s primary interest was in airplane

Flying Qualities Become a Science

Figure 3.1 Edward Pearson Warner (1894-1958). His DC-4E flying qualities requirements launched a new science. (From National Air and Space Museum)

suitability for military use, rather than in aeronautical research. Thus, it is understandable that there were no measurements at the level of sophistication of the Norton and Allen tests at the NACA. Captain R. W. (Shorty) Schroeder was one of the Air Service’s top test pilots. His 1918 (classified Secret) report on the Packard-Le Pere LUSAC-11 fighter airplane’s handling qualities was completely qualitative.

In the course of the pioneering stability and control flight tests at the NACA Langley Laboratory, instrumentation engineers including Henry J. E. Reid, a future Engineer-in­Charge at Langley, came up with specialized devices that could record airplane motions automatically, freeing pilots from having to jot down data while running stability and control flight tests. Langley Laboratory individual recording instruments developed in the 1920s measure control positions, linear accelerations, airspeed, and angular velocities.

Flying Qualities Become a Science

Figure 3.2 Warner and Norton’s measurements of elevator angles required to trim as a function of airspeed and power for the Curtiss JN4H (Jenny) airplane. They correctly interpreted the data to show static longitudinal instability at airspeeds above the peaks of the curves. (From NACA Rept. 70, 1920)

In each recording instrument, a galvanometer-type mirror on a torsion member reflects light onto a photographic film on a drum. A synchronizing device keys together the record­ings of individual instruments, putting timing marks on each drum. Frederick Norton said in later years that the work at Langley in which he took the most pride was the development of these specialized flight recording instruments (Hansen, 1987).

The instrument developments put NACA far in front of other groups in the United States who were working on airplane stability and control. The photorecorder was typical technology at other groups running stability and control tests, such as the U. S. Army Air Corps Aircraft Laboratory at Wright Field. In the photorecorder, stability measurement transducers, ordinary flight instruments, and a stopwatch are mounted in a bulky closed box and photographed by a movie camera. Data are then plotted point by point by unfortunate technicians or engineers reading the film.

As another indication of NACA’s advanced flying qualities measurement technology, one of this book’s authors (Abzug) who served in the U. S. Navy during World War II remembers having to borrow a stick force measuring grip from NACA to run an aileron roll test on a North American SNJ trainer.

NACA flying qualities research in the 1920s and early 1930s also trained a group of test pilots, including Melvin N. Gough, William H. McAvoy, Edmund Allen, and Thomas

Carroll, in stability and control research techniques, including the ability to reach and hold equilibrium flight conditions with accuracy As with all good research test pilots, the NACA group worked closely with flight test engineers and in fact took part in discussing NACA’s flying qualities work with outsiders. All of this helped lay the groundwork for the comprehensive flying qualities research that followed.