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AUTOGIRO—Into the Future
It is certainly strange, perhaps uncanny, that although many inventors, designers and space-minded dreamers dreamed of and/or built various versions of fixed-wing, flapping-wing and powered rotating wing aircraft. Only one, Sr. Juan de la Cierva worked with the idea of un-powered rotating wings.
During the three decades, plus, that unpow-ered "autorotating wing" craft flew, the payload ranged from a few pounds to about 800 pounds.
They were used for lowering light cargo as unpiloted, u npowered craft. They were used for carrying payloads from the tops of buildings to the surface and vice-versa. They dispersed fungicides and other agricultural materials. They carried from one to five people on their un-powered rotating wings.
Early craft required a short run to become airborne if the flight was made with no wind. They suffered from control limitations because some minimal airspeed was needed to make their airplane-type controls function.
Early in the second decade, exact control of the craft at low or even zero airspeed was available. Vertical takeoffs to limited altitudes of 10 or 20 feet with unpowered rotors were routine. Through this no appreciable payload increase was offered.
The designers of helicopters, with powered rotors, quickly adopted the control systems used by these autorotating wing machines called AUTOGIROS. The helicopter, at this time gave great promise of flight completely independent of a prepared surface. In fact, in many cases, could perform their flight function of lifting and delivering their payload without taking off or landing on any surface. The transfer could be done while hovering above the lift or delivery point.
As helicopter brought utility, so came complexity. Powered rotors need full-time powertrains from the engine to the rotors. Single-rotor types induce torque from the rotor to the airframe and the fuselage wants to turn in the opposite direction from the rotor. To offset the torque an antitorque rotor at the tail is needed. When two helicopter rotors turn in opposite directions either laterally, in tandem or on a common axis the torque of one offsets the torque of the other adding even more complexity.
Autogiros, in forward flight climbed with added power, or descended with no power. The angle of incidence of their blades was permanently set at about 4 degrees. Regardless of what angle of attack the aircraft assumed, the blades remained at this angle and the blades auioroiated.
To make helicopters ascend and to fly point-to-point, the rotor has to assume an 8 to 10 degree angle of incidence.
As long as the helicopter has power available it maintains the 8 to 10 degrees. If power is lost, the angle must be reduced to 4 or so degrees or the rotor will slow down and control is lost. With a reduced angle the rotor will continue to turn in autorotation. The craft must, of course descend, but it will be under control. It can land in an area not much larger than its rotor diameter.
Autogiros, except for the last few models, flew from takeoff to landing with a minimum angle of incidence built in the rotor.
Later models incorporated temporary incidence or pitch change for vertical takeoff and landing.
The helicopter can be flown at low speed that the fixed-wing craft cannot enjoy. The helicopter can hover, or descend at zero airspeed without risk of aerodynamieally stalling the rotor.
The autogiro can be flown in each of these phases without rotor stall. Early autogiros, however, depended on fixed wing controls to guide them to a desired spot. When the airspeed dropped below 30 mph, control was nil.
The rotor was, however, inherently stable and could descend vertically at low airspeeds or zero airspeed without diverging from a steep glide or vertical approach.
To sum up these statements; the autogiro, at the end of its development could do all that the helicopter could do with an edge of safety, but could not hover.
To hover the helicopter must have its rotor driven constantly by some kind of powerplant. This requires a heavy and complex reduction gear system and in the case of the single-rotor craft, an antitorque rotor at the tail.
What would the future of the autogiro be, if it employed already-tried techniques? It could take off vertically, fly slowly, as low as zero mph and land vertically. Adding simple also-tried devices, could perform limited hovering with rotor tip drive jets. These do not require a gear reduction system. Driving the rotor at the tip does not impart a torque to the airframe as shaft-driven systems do.
There is no reason to believe that autogiros could not be built to carry pavloads equal to the loads carried by the largest helicopters.
There are no reasons to believe that the rotor systems, single, dual lateral, tandem rotors could not be used on autogiros without heavy gear reduction systems.
Many ultra-light autogiros have been constructed as home-built projects and are being flown for pleasure or for limited business purposes.
New, lighter power plants, new airframe construction techniques; new better and lighter rotor systems, temporary drive systems such as torque converters; rotor tip drive with air driven by an engine driven compressor. In the thirties, engines ran small compressors which were used to start the engine. Autogiros could be designed with short wings for several purposes; part of the landing gear structure, to store fuel or light cargo and to accept some of the totai lift in forward flight to relieve the rotor.
There is no apparent limit to the possibilities of the autogiro when some of the mentioned already-tried features are reapplied and some of the now-available techniques applied with modern concepts.
The Theory and a Discussion of the Autogiro
(Illustrations by Author.)
The AUTOGIRO literally flashed across the sky in the early twenties with Sr. Juan de la Cierva's craft making a successful flight in Spain on January 9, 1923. The rotor was turned by a phenomenon called "autorotation."
In about a twenty year flurry of activity it seemed as though the autogiro was sent to make the helicopter, with which many inventors had labored for years and, years, a success. The helicopter interests had been trying since the latter part of the nineteenth century. The problems had been many, but two outstanding ones were power for flight and control in flight. With the arrival of the gasoline engine, the problem of power dissolved. Adequate control for such a machine that was intended to lift straight up and come straight down and fly with a great range of speed was not so easily solved.
By the early thirties the autogiro had a control system that used the rotating blades for control for vertical flight and at very low speeds.
About this time the clouds of World War II could be seen and the United States Military released a request for bids from aircraft manufacturers to design and build a helicopter.
Contracts were given to Sikorsky Aircraft in Bridgeport, Connecticut and Piatt-LePage in the Philadelphia area.
In 1940, Sikorsky had a helicopter that could fly but the control system was so complicated that it was an impractical heiicopter system to market.
Col. Frank Gregory, who was in charge of rotary wing design and procurement for the U.S. Air Corps, urged Sikorsky to enter into an agreement with the Autogiro Company of America as a licensee and thus have the use of all Autogiro Company's patents and designs. Sikorsky did this and with the autogiro rotor system added, had a very successful and relatively simple helicopter. Sikorsky began delivering helicopters to the United States Military in the early forties and with these deliveries, the autogiro activity effectively ceased. Without the work that the licensees of Cierva's principals in Europe and the work of licensees in the United States, success would not have come to the helicopter so quickly.
Autorotation was not invented by the helicopter engineers as a way to lower their caffs safely to the ground when their poweipiant fails. Nor was it invented by Juan de la Cierva.
The force that makes autorotation possible was known to aeronautical inventors at least as far back as 1909. Nature has produced millions of tiny autorotating "craft" that deliver the maple seed to earth each year.
From a book titled Practical Aeronautics by Charles Hayward, copyright 1912; "—constants used by Lielienthal show the arched surface (of an airfoil) still possess supporting powers when the angle of incidence becomes negative, i.e. below the horizontal. The air pressure becomes a propelling force at angles exceeding 3 degrees up to 30 degrees." So it can be seen that the scientists of those days knew of a forward propelling force.
A quote of an unknown author from the same book "—by this construction, the air was thrust upward on the outer surface while the air rushed in to fill the partial vacuum thus formed, exerting a powerful lift at the same time was pushed forward, thus tending to dimmish head resistance."
Later in the book, while discussing some soaring experiments by Octave Canute as early as 1909: "—at certain angles, the total air pressure acting on the plane (wing) cease to act in a line normal to the plane (wing) or its chord, instead, the line of action of this force takes a position well in front. The pressure thus materially acting in the dual role of supporting and propelling force."
A "force diagram" from Practical Aeronautics (fig.l) shows the action of an airfoil in flight
Force Diagram for a Soaring Plane
showing one inventor's understanding of the forces: one vertical arrow, showing "lift" and another arrow pointing forward to illustrate the propelling force.
Messrs. Jackman and Russel, in the same book, referred to experiments with stuffed birds used as "flying models." "—Thus we have a bird weighing 4.25 pounds, not only thoroughly supported, but propelled forward by a force of 0.359 pounds at 17 miles per hour." Other experiments discovered similar actions.
Figures 2, 3, and 4 try to explain the "rules" for lift, drag (resistance) and the forward propelling force.
Figure 2 shows the "rules":
1. Lift acts at 90 degrees from the relative wind.
2. Drag acts parallel to the relative wind.
Angle of Attack Relative^^p —■—
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