THE CONTROLWING AIRCRAFT
By George G. Spratt EAA 17426 P.O. Box 351 Media, Pa. 19063
lesy of the Author)
PHOTO NO. 1
1908 Glider. Wing rocks fore and aft, lateral contro by aileron. Single surface circular arc airfoil.
INTRODUCTION Rudder, aileron, elevator . . . the principal components of a "conventional" system for controlling an aircraft.
But not the ONLY way. Stalls, spins . . . dreaded killers or great fun,
depending on whether you are an N.T.SJ3. accident inspector or an aerobatic pilot. But not an inevitable handmaiden of flight . . . there are aircraft incapable of stalling or spinning. What do you want of an aircraft? Are you satisfied with the machine in its present form? What would you change? It is doubtful that many existing pilots give any of these questions a great deal of thought. Finding the time and money to fly at all is challenge enough for most of us. Still, these are legitimate questions and probing for the answers brings us face to face with one of the more perplexing problems of aviation - to wit, why do so few
people fly? The population of the United States is, according to the Census people, inching up around the 210 million mark, yet the F AA tells us that considerably less than one million living Americans have at one time in their lives held a pilot's license. All the obvious 48 JUNE 1974
reasons why the 219 million don't pilot aircraft have been well aired - they are too young, too old, too afraid, too poor, too lazy ... or, and this may be the greatest deterrent of them all, their wives won't
let them. There are other factors - many centering around the aircraft itself. Some persons can't tolerate turbulence, some detest the cabin noise and some simply find the conventional three control system too difficult for them to master ... or, at least to ever feel comfortable with when they fly only 30 or 40 hours per year.
Are there alternatives that might make the aircraft itself more acceptable to a significant number of people who, for their own varied reasons, have chosen not to fly? Is there a better choice for the pilot who wants to fly occasionally but can't spare the time to stay current in existing, "conventional" aircraft? George Spratt (EAA 17426), an aeronautical engineer, has spent much of his lifetime - as did his late father before him - attempting to answer the foregoing questions in the affirmative. This month we begin a two part story of the Spratt Controlwing, a story that could point the way to acceptance of private flying for many more people than enjoy it today.
Jack Cox, Editor-in-Chief
J. HE "FATHER OF AVIATION", Octave Chanute, once told my father that two young men he was helping in their attempt to fly would probably succeed but he was afraid it would not be in the best way. Chanute felt that their system, using a controlled horizontal vane ahead of the lifting airfoil for longitudinal control was not desirable. In fact, he felt so strongly
about it that he designed a "rocking wing" aircraft and had
Mr. C. H. Lamson in California build it and send it to the
Wright brothers at Kitty Hawk. Chanute wanted them to
fly and compare this "rocking wing" type control with the "elevator" they were using. Unfortunately, the brothers made no attempt to fly this machine but abandoned it to destruction by the elements when they left their camp at Kitty Hawk. Thus two opposing theories for controlling dynamic
flight were taking shape even before powered flight was achieved. On the one side a rigid, stable aircraft controlled in its flight path by movable vanes. These vanes located far enough from the CG to supply moments power-
ful enough to overcome any stability built into the aircraft and to correct the adverse effect of other components of the aircraft and of turbulence.
PHOTO NO. 2
1912 land plane powered by Curtiss air cooled V8. Wing rocks fore and aft and laterally.
On the other hand, aircraft with movable wings so hinged that they would yield to the forces of turbulent air without transmitting these loads to the structure. At the same time these wings would be so controlled that it would not be necessary to overcome the stability of the aircraft in changing the flight path. Nor would it be possible to fall into an uncontrolled flight condition such as a
stall or spin. In other words, the object was an aircraft inherently stable under any and all conditions.
HOW THE RUDDER-ELEVATOR-AILERON DEVELOPED
Due primarily to the energy, persistence and singlemi ndedness of the Wright brothers the rigid concept was
the first to make practical powered flights. Once this was accomplished the vast majority of experimenters built on this success. How the Wright brothers succeeded in doing in three years what others had unsuccessfully spent time, fortunes and even their lives on is a fascinating story.
In THE STORY OF FLIGHT by Wilbur Wright published several years after his death in Aeronautics, April
PHOTO NO. 3
Water glider wing rocks fore and aft as well as laterally about an axis sloping forward and down. Built 1929.
1915, Wilbur said: "My brother and I became seriously interested in the problem of flight in 1899. We accordingly decided to write to the Smithsonian Institution and inquire for the best books relating to the subject. We had heard that the Smithsonian was interested in matters relating to human flight. In response to our inquiry we received a reply recommending Langley's "Experiments in Aerodynamics", Chanute's "Progress in Flying Machines"
and the "Aeronautical Annual" of 1895, 1896 and 1897.
These last were yearly publications, edited by James Means, giving from year to year reports of efforts being made to solve the flying problem. The Smithsonian also sent a few pamphlets extracted from their annual reports containing reprints of Mouillard's "Empire of the Air", Langley's "Story of Experiments in Mechanical Flight" and a couple of papers by Lilienthal relating to experiments in soaring."
After carefully studying this literature the brothers
saw three fundamental weaknesses in the work that had
been done. These were the problems that required first
attention; 1) While Lilienthal and Pilcher had contributed much to the art, their system of stabilizing and controlling the aircraft by moving the CG was not practical. Control must be by "utilizing dynamic reactions of the air instead of shifting the weight." 2) Particularly if an engine was ever to be used, a more practical structure must be found. This was confirmed by
PHOTO NO. 4
1934 land plane. Used primarily to test various directional control axes. Lilienthal and Pilcher — both loosing their lives through structural failure of their gliders. 3) Some way must be found that would let the operator gain more experience in a shorter time before venturing into free flight. At the time of his fatal accident SPORT AVIATION 49
THE CONTROLWING . . .
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Lilienthal had been flying for five years and had spent
approximately five hours in the air. This average of one hour per year consisted of many glides, even the longest being measured in seconds. Wilbur remarked that it would be unsafe even for a person to ride a bicycle in traffic with this training. In his book Mouillard described a flight of 138 feet he made in 1865 using an aircraft with the wings so hinged, "that the angle of the wings with each other could be varied at pleasure." Perhaps this was the dynamic control they were looking for. In the Aeronautical Annual for 1897 Chanute described tests with his bridge trussed double decker. Here was a structure that could be made light yet as strong and rigid as desired. Interestingly, this same article mentions that the erect position of the pilot, "produces a body resistance due to about 5 square feet of surface, while it would be that due to only about 1 square foot if the man were placed horizontally, as in the body of a bird." Also, he described the advantages of using starting rails. In this same Annual there was also an article written by A. M. Herring mentioning his "Equilibrium Paradox", a model made of two triangular pieces of bristol board. The large triangle is the wing with the smaller triangular piece glued erect in the center. Weighted thumb tacks were placed in this keel near the apex and some distance from the wing surface. Herring explains the paradox by saying: "If not weighted too heavily, it will always fly with the fin side up, even if dropped with the weight and fin side undermost." To the perceptive eyes of the brothers here were all the ingredients of a practical flying machine: The rugged strength of Chanute's trussed biplane. The "paradox" which showed that the aircraft would be stable with the engine and prone body of the pilot above the lifting surface. The upper surface of the biplane would supply the drag of the vertical fin and the apex of the triangle could be made adjustable to control the angle of attack and the longitudinal flight path. For lateral control Mouillard's differential wing incidence system looked good. There was one problem, however. To differentially rotate the right and left wings about some transverse axis would destroy the continuity of the bridge-trussed biplane that was the heart of the design. As has happened in aviation many times since, a compromise was made. The tips of the wings only would be rotated, thus twisting or warping the wings. After working out a mechanism for flexing the forward horizontal rudder and twisting the wings, a glider was built and taken to Kitty Hawk in the summer of 1900. Here steady winds of 20 to 30 miles per hour are common and it was planned to attach the glider to a short horizontal rope letting it float a few feet from the ground. This way they could practice manipulating the front rudder and wing warping allowing the operator ample time to
creased angle would go down and to the rear. Actually what they found was that the pair with increased angle instead of going up and forward would go down and to the rear. Usually landing with this wing dragging in the sand. At the time Wilbur said, "When we left Kitty Hawk at the end of 1901, we doubted that we would ever resume our experiments. At this time I made the prediction that man would sometime fly, but that it would not be within our lifetime." After returning home and discussing the problem with Chanute and others, they realized what was happening. The wings with increased angle not only produced more lift but more drag as well. This drag decreased the speed of this wing which more than wiped out the increase in lift. In solving this problem Wilbur said, "We reasoned that if the speed of the right and left wings could be controlled, the advantage of the increased angle of incidence of one wing and the decreased angle of the other could be utilized as we had originally intended. Two ways of controlling the relative speed of the wing tips were open to us — one consisted in providing variable resistance to the wing tips at the will of the operator so that the wing that tends to forge ahead could be retarded; the other consisted in providing a surface at the rear with which a torque about the vertical axis could be created to counterbalance that produced by the difference in resistance of the wing tips. We decided to use the surface at the rear on account of its greater dynamic efficiency since every pound of push in the propeller while the surface at the rear exposed almost edgewise eight or ten pounds of turning power could be obtained at an expenditure of one pound backward resistance or of one pound of propeller thrust."
acquaint himself with the new feel. Unfortunately, lift
was much less than expected and they found it would be necessary to resort to gliding to get enough velocity. Such a glider was built and taken to Kitty Hawk in 1901. At first the wing warping control was made rigid and only the forward horizontal rudder used. Since flying altitude was only a foot or two the aircraft could be quickly landed at the onset of a roll or yaw. Of the next move Wilbur said: "After we had acquired some skill handling the horizontal front rudder, we loosened the warping wires and attempted to control the lateral balance also, but when we did this we found ourselves completely nonplussed." It was expected that the pair of wings having increased angle would go up and forward while the pair with de-
50 JUNE 1974
PHOTO NO. 5 The 1937 Bendix ship, the first Controlwing to use the
thick, stable N.A.C.A. 23112 airfoil. This aircraft had the backing of Vincent Bendix and the Bendix Corporation for a time, however, the company's board decided against further development on the grounds that as a major supplier of aircraft components, they should not be building planes in competition with their customers. The craft was eventually sold in the South Bend area for $50!
In the fall of 1902 they returned to Kitty Hawk with a machine fitted with a fixed vertical vane at the rear. Wilbur continued, "When we tried the apparatus, we found that under favorable conditions the appartus performed as expected, so that we could control lateral balance or steer
to the right or left by the manipulation of the wing tips.
But as we proceeded with our experiments, we found that the expected results were not always attained. Sometimes the machine would turn up sidewise and come sliding to the ground in spite of all the warp that could be imparted to the wing tips. This seemed very strange. The apparatus
would sometimes perform perfectly and at other times, without any apparent reason, would not perform at all. Every now and then it would come tumbling to the ground and make such a rough landing that we often considered
ourselves lucky to escape unhurt."
After much observation of the nature of this action Wilbur reasoned as follows: "If the tilt happened to be a little worse than usual, or if the operator was a little slow in getting the balance corrected, the machine slid sideways so fast that the sidewise movement of the machine caused the vertical vane to strike the wind on the side towards
the low wing, instead of the side toward the high wing, as
it should have done. In this state of affairs, the vertical
vane instead of counteracting the turning of the machine about a vertical axis, as a result of the difference of resistance of the warped wings on the right and left sides, on the
contrary assisted in its turning movement and the result
was worse than when the vertical vane was absent. We
PHOTO NO. 5A
Another shot of the 1937 Bendix ship — with the fabric on. Powered by a two cylinder Aeronca engine driving a ducted, pusher prop via a belt drive and drive shaft, plus a tricycle gear makes this little aircraft look like a current project rather that one 37 years old!
felt that if this were a true explanation, it would be necessary to make the vertical vane movable."
After some experimenting with movable vanes Wilbur
said, "With this apparatus we made nearly 70 glides in the
two or three weeks following. We flew it in calm and we
flew it in winds as high as thirty-five miles an hour. We
steered it to the right, or left, and performed all the evolutions necessary for flight." The basic rudder-elevator-aileron had now emerged. At
first the rudder was connected to the wing warping but
was later made a separate control. Even later the elevator was moved to the rear and the machine made "headless." By 1911 ailerons had replaced the cumbersome wing
warping. It is to the eternal credit of the Wright brothers that in three years of part time work, (during which, incidentally,
they at no time allowed this hobby to seriously interfere with their business of building bicycles), they had answered the age old problem of mechanical flight. They had developed a controllable aircraft. Or was this aircraft really controllable? Sometimes despite all the operator could do, the machine would suddenly drop its nose and plunge to the earth, or, possibly,
spin its way down with equally disasterous results. These are occurrences that even today, according to safety rec-
ords, account for some 7(X? of aircraft fatalities in private aviation. When questioned about this, Wilbur replied that time would be better spent training the pilots to avoid this condition than to try to design out this defect. Or that
"the remedy for the difficulty lies in more skillful operation of the aeroplane." This decision has probably cost more lives than any
other ever made in aircraft history.
Now that we have briefly reviewed the development of the fixed wing aircraft, let us now study in more
detail the development of the controlwing aircraft. To the best of my knowledge Octave Chanute was the first to build movable wing aircraft that successfully flew. Chanute had for many years studied the physical principles of flight and carefully followed the work of others. In 1894 he published the book "Progress in the Flying Machine", a complete and detailed report of all known work done in aviation up until that time. Chanutebuilthis first man-carrying, movable wing aircraft in 1896 and on June 22 of that year took it and a Lilienthal machine for testing to the sand hills on the south
PHOTO NO. 6
Flying boat with sloped directional axis. Powered
by Lycoming 65 in bow.
shore of Lake Michigan, just north of the station of Miller, Indiana. The first flights were made with the Lilienthal machine but he found it dangerous. The following quotation is directly from his report:
"Having discarded the Lilienthal machine, we next turned our attention to the apparatus after my own design. This was based upon just the reverse of the principle involved in the Lilienthal apparatus. Instead of the man moving about to bring the center of gravity under the cen-
ter of pressure, it was intended that the wings should
move automatically so as to bring the movable center of pressure back over the center of gravity, which latter
should remain fixed. That is to say that the wings should move instead of the man. To establish priority of inven-
tion, a patent has been applied for." The apparatus consisted of 12 wings each 6 feet long
by 3 wide, each pivoted at its root to a central frame so
that it could move fore and aft. The main frame was so
constructed that the wings could be grouped in various ways. "After considerable testing 5 of the pairs of wings had accumulated at the front, and the operator was directly under them, while the sixth pair of wings formed a tail at the rear, and being mounted so as to flex upward behind in flight, preserved the fore and aft balance. It was at once demonstrated that this apparatus was steady, safe and manageable in winds up to 20 miles per hour. With it about 100 glides were made." SPORT AVIATION 51
THE CONTROLWING . . .
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After returning to Chicago July 4th, Chanute wrote:
"It may safely be asserted that more was learned concerning the practical requirements of flight during the two weeks occupied by these experiments than I had gathered during many previous years of study of the principles
involved, and of experiments with models. The latter are instructive, it is true, but they do not reveal all the causes for the vicissitudes which occur in the wind. They do not explain why models seldom pursue exactly the same course, why they swerve to the right or left, why they oscillate, or why they upset. When a man is riding on a machine, however, and his safety depends upon the observance of all the conditions, he keenly heeds what is happening to him, and he gets entirely new and more accurate conceptions of the character of the element which he is seeking to master." On August 21 of the same year the party of five left Chicago by sailing vessel for the sand hills. With them were three aircraft; first, the rebuilt movable wing machine, now equipped with ball bearings in the pivots; the
second, a rather large machine designed by Mr. William Butusov, who had been present at the preliminary trials in June and the third, a machine containing an automatic stability device designed by Mr. Herring. "Having on the previous occasion found the vicinity of Miller too accessible to the public, we went this time, five miles further down the beach, where the hills were higher, the solitude greater, and the path more obscure to the railroad, which it reached at a sand pit station consisting of a single house, called Dune Park. The distance from our camp was about two miles, through a series of swamps, woods, and hills, so that intending visitors not infrequently got lost." Besides the improved bearings, the spacing between wings had been increased bringing the top of the machine to 10.5 feet above ground. This proved difficult to handle so the top pair of wings was removed. In this shape it was steady and manageable and made flights twice as long with the same fall as had the original machine in June. The flight path could be controlled up or down by moving both wings forward or backward. It could be controlled to the right by moving the left wing forward or to the left by moving the right wing forward. Despite this apparent success there is a note of discouragement when Chanute said of these tests: "It must be confessed that the results of this apparatus were rather disappointing, and yet the principle is believed to be sound." Seven years later the Wrights were flying with their rudder-elevator-aileron system and very few researchers of the time continued to buck the trend. One who did was my father, Dr. George A. Spratt, who started a development that I have carried on since his death in 1934.
Over the years many people have designed, written about or made models of controlwing airplanes. Only a few have actually built and flown full scale powered aircraft. Still fewer have achieved anything like practical flight characteristics. Exceptions are Professor A. A. Merrill,
moves rapidly rearward until at 0° it may be off the trailing edge of the airfoil. This means that the airfoil is stable from high angles down to perhaps 15°, but unstable below this angle. This instability was more than could be reasonably corrected with a horizontal rudder and lead to many early mishaps. This is the reason my father recommended to the Wrights that they change from the arc
to an airfoil having more stable characteristics, even if some lift was lost. Although my father could see no other immediate solution, he did not consider the suggested airfoil a final
answer and realized much more knowledge of airflow would be needed to build an inherently stable aircraft.
He gave to the Wright brothers the design of his wind
tunnel which they adopted and used. Continuing his own approach, he soon realized that while his pressure testing device showed where the line of force passed through the surface it did not show the direction of this force. If he
were to assume the skin friction on the circularly arched surface was zero, this force vector could only be normal to the point where it passed through the surface. If this were so, all flight vectors would pass through a single point, the center of radius of the airfoil. With the center of gravity of the aircraft at this point, wind turbulence might change the angle which the air struck the wing but could not tend to nose the aircraft up or down. Wind tunnel tests and then scale models confirmed this theory.
Photo No. 1 is a glider of this type built in 1908.
The wings were a typical Chanute bridge-trussed biplane having a span of 21 feet and chord of 4 feet. A pivot was located midway between the wings in the center of the span and chord. This allowed the wings to be rocked fore and aft by the lever extending downward. As the machine weighed but 40 lbs. the weight of the pilot sitting on the triangular framework of the landing skids lowered the center of gravity to coincide with the radius of the two
concentric airfoils. Ailerons were used to keep the craft level for the only object of this machine was to prove the automatic longitudinal equilibrium of this configuration. The tow car was furnished by Mr. Rupert Bonsall, the local Studebaker dealer. Both the longitudinal control and stability came
entirely up to expectations. It was now time to put the power in the aircraft and also to develop a better lateral and directional control. Incidentally, the small assistant in the foreground is your author. Photo No. 2, taken in 1912, was the first powered machine. Longitudinal control was the same as in the glider but the wings also rocked about a longitudinal horizontal axis for lateral control. The engine, now in the Smithsonian Air and Space Museum, was a Curtiss eight
cylinder air cooled job that developed about 40 h.p. Longitudinal control and stability were good but the lateral wing
rocking left much to be desired. We were again being told that all controls must be dynamic.
Three other powered craft were built and flown between this and the water glider shown in Photo No. 3,
formerly of the California Institute of Technology, and Mr. Louis P. de Monge of France.
taken in 1929. Towing this craft behind a boat taught us
to the study of mechanical flight about 1895. He became
desired, flights could be made only a foot or two above
My father, a medical doctor forced to discontinue practice because of a heart ailment, started devoting full time
a close friend to Octave Chanute and the Wrights. During his early study of air flow over airfoils in a wind tunnel he was the first to discover the reversal of center of pressure travel on a circular arc airfoil. When an airfoil made of the segment of an arc is held at 90° angle of attack, the center of pressure is at 50% of
the chord, as the angle is decreased the center of pressure moves forward to some position determined by the curvature used. Then with further decrease in angle it 52 JUNE 1974
many things, probably the most important was the advantage of testing over the water. There was always an unobstructed landing field below, no end of the runway and, if
the surface. This experience proved valuable for our
later water work.
The first powered aircraft to do any real flying was a monoplane built in 1934 and shown in Photo No. 4. While we were studying the geometry of the controlwing in actual flight, this model went through many modifications. It was simple and light, weighing but 180 lbs. without pilot. It was powered by an outboard motor, the original liquid cooled cylinders having been replaced by finned cylinders
for air cooling. This engine is now in the EAA Museum at Hales Corners. In theory, rocking the wings from side to side about a longitudinal horizontal axis should slope the lift vector to one side or the other of the low center of gravity and roll the aircraft. It did reasonably well at low speed but
at high speed the response was far from positive. Recalling Chanute's movable wing glider, a vertical
axis was tried. This way one wing would go ahead and
the other to the rear. With proper dihedral and vertical
fin area aft of the fuselage to overcome adverse yaw, control at high speed was quite good, although very deficient at low speed. Although control forces were light, still they were unstable at high speed. It was realized that longitudinal stability in the aircraft was not enough,
the control forces must also be stable. With the help of my good friend Burke Wilford and the
design guidance of Elliot Daland, this aircraft was redesigned and the next model, shown in Photos No. 5 and
No. 5A, built by Bendix at South Bend. A complete study of airfoils then available showed that there was one, NACA 23112, having just as sharp focal point in the flight range as the circular arc but much higher. The vector diagram in Figure 1 shows it is actually above the chord. This would give two advantages, not only the stable control force needed but allow the wing to be much closer
to the center of gravity, making a lower and more practical machine. A multiple V belt drove the propeller within a Venturi at the rear. This not only increased the thrust
at low speed but assures directional stability of the fuselage, a requirement of this design.
The 1939 flying boat, Photo No. 6, had a Lycoming engine mounted in the bow ahead of the passengers. A long shaft drove the propeller located over the transom
between the extended hull sides. This was by far the most
practical aircraft up to now. It flew for many years, being
used for some test work as recently as 1960. If the identification number looks strange it is because it is licensed as a boat, not an aircraft. CONTINUED NEXT MONTH -
'O' '2' '4
The present day Spratt Controlwing flying boat. Land versions are also under development.
SPORT AVIATION 53
George G. Spratt, aeronautical engineer, has spent a good part of his life in the development of a totally different kind of aircraft . . . one originally conceived by his late father, Dr. George A. Spratt, around the turn of the century. Last month Mr. Spratt related the early history of the Controlwing and his father's association with aviation pioneer Octave Chanute and the Wright brothers. Various experimental machines built before World War II were pictured and described. This month the story is taken from the immediate postwar period to the present.
THE CONTROLWING AIRCRAFT By George G. Spratt (EAA 17426) P.O. Box 351 Media, Pa. 19063 PART TWO — POSTWAR DEVELOPMENT
N THE EARLY 1940's an article written by Wayne IMorris came to the attention of Bill Stout who quickly saw
the potential of the Controlwing as a roadable aircraft. In 1944 the project was moved to the Stout Research Division of Consolidated Vultee (later to become Convair) at Dearborn. Designers came from all directions; they mathematically redesigned all the components— the wing was "improved" from 80 pounds to 250 pounds, a ratio that also held for most of the other parts. As heavy as it was, it actually flew as you can see in Photo No. 7, much to the credit of Bob Townsend who flew it for many hours and wrote a very good report despite the poor weight-to-power ratio. The next summer the Stout Research Division was moved to Nashville. Now with fewer engineers and Tony La Nave in the shop, we cut the aircraft in two at the pilot's seat. The front part was reworked, the aft part discarded and an entirely new structure built including the wing attachment. Now nearly 200 pounds lighter, performance was much better and the aircraft, after considerable flying at Nashville was taken to the home plant at San Diego where tests continued without incident. (Photo No.
8.) After completion of the roadable, in 1947 I went back to my shop in Connecticut to concentrate on the flying boat. Two models were built, the first showed clearly what not to do. It was an all aluminum hull made from very thin metal in an effort to keep the weight down. On the first high speed water run the bottom was punctured, the engine drowned and the entire boat sank to the bottom of the Connecticut river where it still rests. The second (Photo No. 9) had a much longer life, flying for over 12 years. It was made from a steel framework (Photo No. 10) with riveted plywood skin. A strong light structure but, we later found, subject to rusting between the steel and plywood surfaces. At first the steering wheel was so connected that turning it rotated the wing about the forward and downward sloping axis. Moving it back and forth tilted the wing about an axis parallel with the spar. In turbulence the wing flies at a constant angle of attack but the angle of
incidence varies with the turbulence, so the roughness is felt in the control wheel. To overcome this the pitch control was made a separate lever allowing the wheel to be fixed as in a car. This was an improvement but the feel was still not right, the inertia of the wing about the longitudinal axis was high and gave an uncomfortable feel to the control while responding to turbulence in roll. While running at high speed over rough water there was also considerable feed back into the wheel. This was because the hull often rolls rapidly while the wing tends to be steady because of inertia and aerodynamic damping. Even at anchor in a chop there was a constant slatting in the control system because of wing inertia. This single rigid, straight through wing was simple to
build and required but three fittings. On the other hand with the larger boat it became heavy for one person to handle for trailering and storage. These were some of the facts considered in the design of the present boat.
PHOTO NO. 7
First flight of the Convair readable Controlwing at Elizabeth City, N.C. in 1945. THE CONTROLWING FLYING BOAT
In 1962 my friend Elliot Daland joined with me to build the present boat. (Photo No. 11). This craft (Fig. II) has a hull not unlike a typical racing boat, the engine is mounted low just behind the passenger compartment. The shaft, however, goes up and rearward to an air propeller just over the transom, rather than downward to a water propeller under the transom as does a conventional boat. The hull sides extend outward at about a 45° angle on either side of the propeller to prevent spray being drawn through the propeller disc and to provide weather
cocking ability. Two wings, a right and left, are mounted above the center of gravity high enough to clear the water surface when banking steeply in a turn and to adequately clear a small boat or docking float when coming along side. Each wing is independently hinged about an axis parallel with the span so that it is free to rock fore and aft. In other words the angle of incidence is not fixed. This hinge line is located about one quarter of the way back from the leading edge of the wing and just under the lower surface. Each wing is
supported by struts at the center of lift to minimize the load at the center span attachment. The mechanism required for this control is quite simple as shown in Figure III. The steering wheel is connected by cable to the water rudder and a quadrant pivoted concentrically with an arm having a "T" member pivoted at the far end. A link (Continued on Next Page) SPORT AVIATION 25
CONTROLWING AIRCRAFT . . .
(Continued from Preceding Page)
connects the bottom of the 'T" and the quadrant. The left side of the top of the "T" connects to the left wing and the right side to the right wing. The speed control lever applies a torque equally to each wing without restricting the wings travel. The wings are thus allowed to move freely in pitch collectively while being controlled differentially by the wheel.
ever, in the interest of simplicity we have tried a two position lever and found it adequate. One position for land and take off and one for cruise. In order to change the speed the wing hinge must be moved to the desired flight vector, since as pointed out the flight vector must always pass through the hinge. There are three ways to do this: 1) Move the hinge in relation to the wing; 2) Deflect a trim tab on the trailing edge of the wing; 3) Apply a torque about the hinge with a spring. Although method 3 is used in this flying boat, the following explanation will use the first method, that of actually moving the hinge. This is because it is the easiest to understand and possibly the best aerodynamically. The only disadvantage is that it is a little more complicated mechanically. The vector diagram, Figure 1, shows how sharply all vectors in the flight range focus above the wing and how symmetrically they spread out at the hinge line. This is plotted as a curve on Figure V with speed in miles per hour shown for reference. Let us take some examples and see what all this has to do with longitudinal control and stability. First, suppose the hinge is on the 18° lift vector, the aircraft will be flying at 40 mph. The lift curve has so flattened at this point that little more speed reduction is possible, perhaps only 2 mph at 22°. Now suppose while flying at 18° a gust should increase the angle to 22°. There would be essentially no change in *^L but see what has happened to the center of pressure. At 18° it was 13 inches from the leading edge, now it is 14 inches. If the aircraft weighs 1000 pounds there is a 1000 inch pound moment tending to reduce the wing angle and prevent a stall. Beyond this point the curve is so steep that one additional degree gives an added moment of nearly 2000 inch pounds. Polar moments of the light wing are insignificant about this axis so recovery is almost instantaneous. Now, let's look at the other end of the range and put the hinge on the 2° vector 10.5 inches from the leading edge, giving a speed of 104 mph. The curve is now sloping upward with increasing steepness so that a down gust or increase in speed will quickly be corrected and the aircraft continue to fly level with little speed change. Between these extremes the slope is nearly constant but sufficiently steep to overcome bearing friction and inertia so as to hold the speed within close limits. DIRECTIONAL CONTROL
In normal flight the resultant aerodyanamic force of the wing must pass through the hinge, thus holding the wing at the correct angle of attack. Any tendency for the wing to increase its angle is met with a rearward movement of this force vector and conversely a decrease in angle causes a forward movement of the vector. Regardless of
any disturbance, the wing always tends to maintain the desired angle of attack. This action can be better understood by a careful look at the vector diagram, Figure 1, and airfoil characteristics, Figure IV. This is a constant speed aircraft that can fly only at the speed for which it is set. Additional thrust cannot push it faster. The added thrust will instead make it climb. If less thrust is supplied by the propeller than required for level flight, the aircraft descends, taking only enough potential energy to maintain the set speed. In other words longitudinal or up and down control is the throttle.
Few people would want an airplane that takes off, flies and lands all at the same speed, so some provision must
be made for changing this speed. This could be made
infinitely variable over the flight range if desired. How26 JULY 1974
This aircraft is steered directionally by a control wheel, much like a car or boat. Moving this wheel tilts the wings differentially and moves a small water rudder. The ratios are such that the control feel on the water or in the air is almost the same. Because the wings are free to float this differential motion does not necessarily make the angle of one wing increase and the other decrease. If this were so it would be possible to stall one wing, as sometimes happens in the conventional system when both wings are flying at maximum lift. For example, with the aircraft flying at minimum speed, which is maximum angle of attack, if the control wheel is turned to the right the
left wing does not increase its angle but the right wing takes full travel, decreasing its angle. Conversely, at high speed the exact opposite may occur, now in response to control the wing being given positive pitch will travel at a speed between these extremes, the tilting may be evenly
divided between the wings. This is not a mechanical proportioning but an aerodynamic proportioning so the way the motion is divided between the wings depends on air
flow at that particular instant. Another look at the center
of pressure curve should make this clear.
Adverse yaw that so troubled the early experimenters
with tilt wings is no longer a problem; with this aircraft
it is possible to limit the angle of attack, and therefore the wing drag, to any desired value.
PHOTO NO. 9
1947 boat with Continental 65 aft of passengers.
Looking at the drag curve you will see the lift line at
18° begins to bend over and then descend. There is little to be gained by going beyond this 18° point for cut off. The drag curve is relatively low up to this point so by
locating the hinge at 13 inches from the leading edge the lift is practically maximum and the drag limited to 0.16. With this sharp limit and a positive knowledge of what the
maximum drag will be it is easy to design for it. These curves show dramatically how, if it were not for this positive cut off, an increase in angle of one wing
would not noticeably increase the lift but the drag could increase many times over. It should be noted that the cen-
ter of pressure shown on this curve is not the conventional which is on the wing chord. Instead it is shown 2 1/2
inches below the chord on the plane of the hinge. The reason for this departure from the conventional is to show directly the restoring force available at any trim angle. An often overlooked fact about adverse yaw is that if there were no resistance to roll there would be no adverse yaw. In other words, adverse yaw is a function of roll resistance. There are two principal sources of this resistance in an
aircraft. Aerodynamic damping of the wings and other
surfaces about the roll axis and inertia about the roll axis. In the conventional aircraft the first is usually the greatest while in the controlwing roll control does not have to overcome this resistance because the incidence of the entire wing is varied. In an aircraft having the engine in the fuselage, most of the inertia is due to wing weight because the wings represent a mass the farthest from the roll axis. The controlwing has some roll inertia but it is far less than the conventional because of the much lighter wing construction.
PHOTO NO. 8
Convair readable after rebuilding. Photo taken at San Diego in 1946. SAFETY OF FLIGHT
The advantages of an aircraft that will not stall or spin
are too obvious to dwell upon. That published figures showing almost 70% of all private aviation fatalities are associated with stalling should be evidence enough of its
importance. Apparently, this aircraft is inherently stable, both statically and dynamically. When left to fly by itself, it will not go into the tightening spiral that makes blind flying dangerous. When the steering wheel is centered, it flies
straight and level, except, of course, for wandering and the
buffeting of turbulent air. Longitudinal dynamic stability is most interesting. While flying straight and level, we have intentionally introduced quite violent disturbances by suddenly tilting (Continued on Next Page) SPORT AVIATION 27
CONTROLWING AIRCRAFT . . . (Continued from Preceding Page)
the wing up or down, in this way giving the aircraft a violent surge up or down. The surprising thing is that this ascent or descent lasts only as long as the wing is held in this abnormal angle. As soon as the wing is released, the aircraft again flies straight and level, there is no oscillation or phugoid. Disturbances in the hull have little or no effect because the wing is entirely independent. The center of gravity position is not critical to flight for the same reason. In an aircraft controlled by vanes the control effectiveness varies with the speed squared. Thus, an aircraft having a speed ratio of three with adequate control at take off will have nine times too much control at top speed. Conversely if it is designed to have proper control at top speed, it will have only one ninth enough at minimum speed. The controlwing has constant control sensitivity throughout the speed range. The fact that control sensitivity does not increase at high speed prevents stunting, another potent cause of fatalities. So far no one has found a way to loop, roll or dive this aircraft. Occasionally, a fixed wing aircraft is lost because of structural damage from severe turbulence. According to NASA Report CR-1523, the effect of a sharp edge gust on the controlwing is only about one fourth that of the conventional wing. The same report also points out that lateral control is considerably more effective than ailerons, adding much to the safety of low level flying as well as landing or take off. EASE AND COMFORT OF FLIGHT
Simplicity of control means much, particularly to the non-professional flier. In this aircraft no coordination of controls is required as there is only one directional and one up and down control. The transition from water to air and air to water is made with almost no change in control feel. In fact, when the water is smooth, it is often hard to tell whether the boat is on the water or in the air. Several inventors have tried putting springs between the aircraft and its wings, finding it tends to soften the ride a little. The problem is that while this system works with car wheels, bumps in the road are finite in height and the springs can be designed to cope with them. On the other hand gusts in the air are all but infinite and a spring can at best only soften the shock when the gust strikes. Floating wings can tilt as required to spill the gusts regardless of their duration. Sometimes a pilot flying this craft for the first time is disturbed by the apparent erratic fluttering of the wings while flying through turbulent air. To anyone who has driven a car with exposed wheels over a rough road and watched these wheels, it is understandable for the action is very similar. Occasionally, a gust strikes principally one wing tending to overturn the conventional aircraft. The only way to correct this is to attempt to push the wing back down with the aileron, a small vane to overcome the gust force on the entire wing. Should this same thing happen to the controlwing the attack of the entire wing is reduced so no additional lift is felt and the wing requires no pushing down against its will.
Hinging and therefore isolating the wing from the hull allows complete freedom of both hydronamic and aerodynamic design. For instance, the step may be designed to give optimum water performance, it is not necessary to rock the hull on the step to adjust wing angle for take off. Stability on the water is adequate without wing tip floats because the engine and passengers are low in the boat. High speed water operation is improved by wing control being connected to the water rudder. The entire craft is stabilized by the wing and maneuverability is improved. There is no tendency of the craft to fly at the mooring because the wings may be locked at a slight negative incidence. Take off is as simple as opening the throttle, allowing the boat to come up on the step and then fly off. The only effort needed is to maintain the correct heading with the steering wheel. Landing is equally simple: The boat is lowered with the throttle until a few feet above the water. The throttle is then completely closed allowing the bow to come up gently, providing much additional lift in ground effect. Thus the hull bottom plays an important part in a soft landing at a speed even lower than minimum flying speed. RECENT CONTROLWING DEVELOPMENTS
While we were flight testing N-910Z on the Chesapeake and in Florida, many observers became interested in this unusual little craft and some of these people started building their own versions. On first thought this was fine because the more people working with a new concept the sooner it will become practical. It was only after looking over some of these attempts that I had second thoughts. True, This aircraft is easy and simple to build and fly. However, the apparent ease of design is deceptive. There is little help from the literature and few flyers or aerodynamicists fully understand the principle. The would-be builder is on his own with an aircraft that must be built to exact specifications in the area of the all-important wing pivot/ control system. It must be understood that the basic design philosophies behind the conventional aircraft and the Controlwing are at opposite poles—and that this enters into the construction of each. It has been said (in the case of the conventional aircraft) that the sole purpose of the rudder is to cover up the mistakes of the designer — this could also be extended to cover the ailerons and elevator. This was intended to imply that the conventional airplane was designed purposely to be partially unstable—with the re-
LIGHT AND SIMPLE STRUCTURE
For the same capacity this aircraft may be more compact because no tail is required and no minimum span is needed for adequate aileron control. Absence of gust loads allow a lighter and more simple structure. The weight of passengers and engine are either side of a fire wall directly under the wing permitting a most direct support structure. The control mechanism is simple, compact and centrally located so can be built ruggedly for very little weight. 28 JULY 1974
PHOTO NO. 10
Showing steel framework used to support the plywood skin in construction of N-3915A.
SPRATT 'CONTROLWING" FLYING BOAT nt
;• > O'T
mainder of control left up to the pilot. In other words,
the designer did not complete his job. The design of the Controlwing is complete, leaving little for the pilot to do other than pilot the aircraft. To do this he needs only one directional and one up and
down control. Obviously, the design must be precise, the pilot has no means to cover up the designer's mistakes.
After much serious thought about what to do before someone got themselves into trouble, I decided to make an individual license available under these patents to the homebuilder for a reasonable fee. The builder would then have drawings of our latest prototype for free and our
aerodynamic data would be available to him. His creative
thought would not be stifled but, conversely, he would be given sound data upon which to build his own creation. This approach appears to be working: Randall Mathues who flew the more difficult portion of the testing of N-910Z was the first licensee. His emphasis is on long
range with good cruising speed. Dr. J. M. Fanucci (EAA 84024) in Westville, Natal is building his hull of wood because he likes working with this material. Victor Lenhart (EAA 82699) in Anchorage wants to cope with more rugged conditions so is designing for greater wing (Continued on Next Page)
PHOTO NO. 11 Latest two place all plastic flying boat powered by Mercury 800.
Spratt Controlwing Flying Boat schematic of c o n t r o l system details of wing strut and root fittings
SPORT AVIATION 29
congestion and restrictions of large airports are building exact copies of N-2236.
DESIGNEE CORNER . . .
The conventional aeroplane has had many millions of flight hours. The total experience of the Controlwing is measured in hundreds of hours. It is my hope that as more of us build and fly this
(Continued from Preceding Page)
span and more power, a Mercury 135 h.p. Lynwood Smith (EAA 76160). a professional marine biologist in Bothell, Washington, is building a closed cabin version for comfort and protection from the weather in his area. In West
new concept that we will be able to work together in solving the many problems that are bound to appear. Only this way will a practical aircraft evolve to match the needs of the non-professional flyer.
Chester, Pennsylvania Joe March (EAA 70091) is attempting to use the ubiquitous VW for power. Many others
who want only the joy of boat flying, away from the
FIGURE V 30 JULY 1974
M . P. H . ° T ITL E °
A T T I TU P E
C L O N G I T U DI NAD