Rotary wing aircraft can be either powered or unpowered. The unpowered version saw substantial development first. Its practical development came at the hands of Juan de la Cierva working in France shortly after the Wright Brothers. Mr. Pitcaren, an American, purchased a French gyroplane and continued to advance the state of the art. The first production models included a horizontally disposed unpowered rotor having a hub mounted upon the top of a rigid mast that was in turn rigidly affixed atop a small airplane. The airplane portion of the autogyro also employed an engine and tractor propeller for propulsion. Vertical and horizontal stabilizers as well as the wings were necessary for attitude and direction control.
Takeoff was accomplished by the Pitcaren machine by taxiing the craft at an angle of attack to allow air to pass upwardly through the rotor thus causing rotation. As the aircraft continued to move forward, rotation increased until enough lift was produced for takeoff. In reality, the distance required for takeoff was excessive. As is shown in ELEMENTS OF PRACTICAL AERODYNAMICS by Bradley Jones, M.S., John Wiley and Sons, Inc., N.Y., 1936, the rotor blade was brought up to speed prior to ground roll by a power takeoff shaft that was clutched to the propulsion engine. As rotor lift neared vehicle weight, the body of the craft would begin to rotate in response to rotor torque. At this point the power takeoff clutch was disengaged, brakes released and thrust applied to produce a reasonably short takeoff run.
A jump takeoff was accomplished by the Pitcaren machine by adding collective pitch control to the rotor blades, permitting the rotors to come up to full speed with zero lift. Since a gyroplane has no means for counteracting powered rotor torque, it was still necessary to disengage the power takeoff clutch before leaving the ground. With the rotor spinning at top speed, the collective pitch control was moved from zero to maximum lift to accelerate the craft vertically into the air using the stored energy of the rotor, while forward propulsion accelerated the craft into horizontal flight with normal autorotation ensuing.
Pitcaren also discovered that pitch and roll control could be accomplished by supporting the aircraft below the rotor like a pendulum, thus eliminating the necessity for wings and horizontal stabilizer. This brought attractive simplicity and the gyroplane became an autogyro.
As is described in the aforementioned reference, ELEMENTS OF PRACTICAL AERODYNAMICS, the gimbaled and hinged rotor with powered spin-up was the state of the art in 1936 pertaining to autogyros. Cyclic pitch was apparently added shortly thereafter. During the 1940's the Armed Forces became interested in an aircraft that could hover and even fly backward. At that time Igor Sikorsky was flying a powered rotor machine or helicopter but was blocked in development efforts because Pitcaren owned the dominating patents on rotary wing aircraft. The government subsequently ordered Pitcaren to allow Mr. Sikorsky the use of his patents in order to bring helicopter technology to the level of government objectives. As a consequence helicopters and not autogyros came into common use and remain so to this date. The fact that a helicopter can hover and an autogyro cannot hover contributed to this trend.
Now a helicopter is a powered rotary wing aircraft employing collective and cyclic pitch for stability and control and a tail rotor for antitorque and directional control. As in the autogyro, hinged rotor blades are also employed to reduce or eliminate cyclic stresses developed during forward flight. The antitorque rotor represents a sizeable power loss and developers over the years have sought to eliminate it. This can be accomplished by employing propulsion means on the rotor tips or by using counter-rotating rotors.
With respect to propulsion means on the rotor tips, a European company built an aircraft called to Rotodyne during the 1950's. It was a gyroplane that employed ramjet engines on the rotor tips for powered takeoff, hovering and descent if desired. Since ramjets cannot start themselves, propulsion engine bleed air was ducted up the mast and out the blades to activate the ramjets. During the 1960's Hughes Aircraft Co. was building a hot cycle research helicopter according to "Aviation Week and Space Technology", June 22, 1964, page (cover). Also, Hiller Aircraft Co. was testing a tip-mounted turbojet according to "Aviation Week and Space Technology," Aug. 10, 1964, page 10.
With respect to counter-rotating rotors the following U.S. Patents are representative of the prior art: 1,403,909, 1/17/1922, G. E. Moir; 1,849,943, 3/15/1932, R. J. McLaughlin; 3,395,876, 8/6/1968, J. B. Green. The U.S. Navy flew a counter-rotating coaxial shaft drone helicopter as a torpedo delivery system before rocket launched torpedoes came into use. The only practical counter-rotating helicopters utilize separate shafts for each rotor disc.
The idea of combining counter-rotating rotors and tip propulsion has also been explored in U.S. Pat. No. 4,589,611, 5/20/1986, Ramme.
The above described methods for eliminating the tail rotor of a helicopter involve considerable complexity and expense and as yet have not in commercial practice replaced the standard single rotor helicopter having a tail rotor.
The present invention seeks to achieve the simplicity of the autogyro and the performance capabilities of a helicopter. Earlier workers appreciated how difficult this was to do. If a counter-rotating rotor assembly were gimbal mounted atop an autogyro then power could be applied to or deleted from the rotor without any torque problems. Assymetric lift would cease to be a problem. The rotor blades would still have to be hinged to reduce or eliminate cyclic stresses at the hub. The blades would not be as long as those of a single rotor assembly, but would still be long enough to require a substantial vertical separation between the counter-rotating blade discs to keep the tips from running into each other. The U.S. Navy counter-rotating helicopter drone demonstrated this reality very clearly with substantial vertical separation between the rotor discs. Combining such a tall rotor assembly with a gimbal mounted fuselage gives rise to many questions concerning control forces and to my knowledge has not been attempted. I reasoned that if the vertical separation could be reduced, or for all practical purposes eliminated, and a round disc supported structure provided, then an attractive hovering autogyro efficient in forward flight could be envisioned. Elimination of the vertical separation between rotor discs would require stiff rotor blades fixedly attached to a hub. A stiff rotor blade tends to look like the wing of an aircraft in order to resist root bending moments which are cyclic. Consequently long stiff blades are not desirable.
This invention provides an autogyro incorporating a pair of closely spaced rigid counter-rotating circular planform wings or disc structures integral with closely spaced sets of peripherally distributed rotor blades with means for applying power to the rotors for vertical takeoff, landing and hovering.