The term rotorcraft is used to designate an aircraft in which all or some of its lift is provided by one or more propellers of substantially vertical axis and large diameter, referred to as rotors or a rotary wing.
There are several distinct types in category of rotorcraft.
Firstly there is the helicopter in which at least one main rotor driven by a suitable power plant provides both lift and propulsion. A helicopter is capable of hovering flight, remaining at a fixed point in three dimensions, and it can take off and land vertically, and it can move in any direction (forwards, rearwards, sideways, upwards, downwards).
Then there is the autogyro (first made by La Cierva) which is a rotorcraft in which the rotor does not receive power, but rotates in autorotation under the effect of the speed of the rotorcraft. Propulsion is provided by a turbine engine or by a propeller having an axis that is substantially horizontal in forward flight and that is driven by a conventional engine. That configuration is not capable of vertical flight unless the rotor is initially set into rotation by an auxiliary device enabling the rotor to be caused to rotate faster: an autogyro is therefore not capable of performing hovering flight but only of moving upwards or downwards on flightpaths having very steep slopes. It is, so to speak, an airplane with a wide range of flying speeds that is not liable to stalling, and that can use short runways.
A gyrodyne is a rotorcraft intermediate between the helicopter and the autogyro in which the rotor provides lift only. The rotor is normally driven by an engine installation during stages of takeoff, hovering or vertical flight, and landing, like a helicopter. A gyrodyne also has an additional propulsion system that is essentially different from the rotor assembly. In forward flight, the rotor continues to provide lift, but only in autorotation mode, i.e. without power being transmitted to said rotor. The Fairey Jet Gyrodyne is an embodiment of this concept.
Several other novel formulae have been studied to a greater or lesser extent, and some have given rise to practical embodiments.
In this respect, mention can be made of the compound rotorcraft that takes off and lands like a helicopter and that cruises like an autogyro: its rotor, driven in autorotation motion by the forward speed of the rotorcraft provides some of the lift, the remainder being provided by an auxiliary wing; a tractor propeller of substantially horizontal axis generates the force needed to move in translation. As example, mention can be made of the experimental Farfadet SO 1310 compound rotorcraft having its rotor propelled by reaction in the takeoff/landing configuration and rotating under autorotation in the cruising configuration, propulsion then being provided by a propeller. The vehicle is provided with two separate turbines for actuating the rotor and the propeller.
The convertible rotorcraft constitutes another particular rotorcraft formula. This term covers all rotorcraft that change of configuration in flight: takeoff and landing in a helicopter configuration; and cruising flight in an airplane configuration, e.g. with two rotors being tilted through about 90° to act as propellers. The tilting rotor concept has been implemented in the Bell Boeing V22 Osprey, for example.
Of those various forms of rotorcraft, the helicopter is the simplest, and as a result it has been successful in spite of having a maximum horizontal speed in translation of about 300 kilometers per hour (km/h) which is small and less than that which can be envisaged by compound rotorcraft and convertible type formulae that are technically more complex and more expensive.
Under such conditions, improvements to the above formulae have been proposed for increasing rotorcraft performance, but without that leading to solutions that are complicated, difficult to manufacture and to operate, and consequently expensive.
Thus, patent GB-895 590 discloses a vertical takeoff and landing aircraft comprising a fuselage and two half-wings (one on either side of the fuselage), a horizontal stabilizer and rudder control, four interconnected drive units, a main rotor, two reversible pitch propellers that are variable relative to each other, and means under pilot control for transmitting drive power continuously or from time to time to the rotor and to the propellers.
Under such circumstances, the main rotor is rotated by the power units during takeoff and landing, during vertical flight, and for horizontal flight at low speed. At high speed, the rotor turns freely without power being transmitted thereto, like an autogyro, the rotor shaft being fitted with decoupling means.
U.S. Pat. No. 3,385,537 discloses a helicopter comprising in conventional manner a fuselage, a main rotor, and a tail rotor. The main rotor is rotated by a first power unit. That vehicle is also fitted with two other engines, each engine being disposed at the outermost end of two half-wings disposed on either side of said fuselage. The patent relates to automatically varying the pitch of the blades as a function of the acceleration exerted on the vehicle while maneuvering or during gusts of wind, for example, so as to maintain a proper distribution of lift between the rotor and the half-wings. As a result, the corresponding device contributes to increasing the horizontal speed of the rotorcraft by reducing the risks of the blades stalling, constituting sources of variation and damage to the mechanical assemblies and structures.
U.S. Pat. No. 6,669,137 describes an aircraft fitted with a rotary wing for operating at very low speed. At high speeds, the rotary wing is slowed down and then stopped, with lift then being produced by a scissors wing. At maximum speeds, the rotary wing and the scissors wing are put into a determined configuration so as to form a kind of swept-back wing.
The rotorcraft according to U.S. Pat. No. 7,137,591 has a rotor rotated by a power unit, in particular for takeoff, landing, and vertical flight. A thrust propeller is used in cruising flight, with lift being generated by autorotation of the rotor, possibly with assistance from an auxiliary wing. Furthermore, the rotor mast can be tilted forwards and rearwards a little so as to eliminate the effects due to changes in the attitude of the fuselage that might harm the performance of the rotorcraft by increasing its aerodynamic drag.
U.S. Pat. No. 6,467,726 discloses a rotorcraft comprising a fuselage, two high wings, four propulsion propellers, two main rotors without cyclic pitch control, each connected to one of the two wings, two engines, and the associated means for transmitting power to the rotors and to the propellers, and a collective pitch control system for each propeller and for each rotor.
In cruising flight, lift is developed by the two wings, so that the lift due to the rotor is eliminated either by decoupling the rotor via a clutch provided for this purpose, or by appropriately setting the collective pitch of the rotor blades.
U.S. Pat. No. 6,513,752 relates to a rotorcraft comprising a fuselage and a wing, two variable-pitch propellers, a rotor with “end” masses, a power source driving the two propellers and the rotor, control means for adjusting the pitch of the propellers so that in forward flight the thrust from the propellers is exerted towards the front of the rotorcraft, and such that in hovering flight, the antitorque function is provided by one propeller providing thrust towards the front and the other propeller towards the rear of the rotorcraft, with the rotor being driven by the power source.
The power source comprises an engine and a clutch that, by disconnecting the rotor from the engine, enables the rotor to turn faster than an outlet from said engine, because of the above-mentioned masses.
That patent specifies that the clutch permits autogyro mode in forward flight. In addition, a power transmission gearbox disposed between the power source and the propellers enables said propellers to operate at a plurality of speeds of rotation relative to the speed of an outlet from said power source.
Patent application US-2006/0269414 A1 deals with the particular problem of improving the performance of a helicopter both during vertical flight and during cruising flight. A high speed of rotation for the rotor is then desired during vertical flight in order to increase lift, whereas in cruising flight, said speed of rotation can be reduced while increasing the forward speed of the helicopter.
That patent describes a main gearbox associated with a second power gearbox driven by the engine installation. The second gearbox includes a clutch device which, when engaged, entrains the main gearbox at a first speed of rotation, with disengagement communicating a second speed of rotation thereto that is lower than the first speed of rotation. Naturally, the main gearbox drives the rotor(s).
Nevertheless, from the above considerations, it can be seen that technical solutions that tend to improve the performance of a rotorcraft are based essentially on the following proposals:                operating the rotor at two distinct speeds of rotation relating firstly to vertical flight and secondly to cruising flight, by means of a drive system with variable speed ratios between the engine installation, the rotor, the propeller(s), and the various component elements of the drive system.        Operating the rotor in autogyro mode during cruising flight: the rotor rotates without driving power being delivered, and then provides some or all of the lift of the rotorcraft, but also leads to drag that nevertheless leads to a loss of power because of a low lift/drag ratio, while in contrast the rotor of a helicopter propels the rotorcraft in the direction desired by the pilot.        
In particular, the operation of a rotor in autorotation like an autogyro during cruising flight makes it necessary in principle to disconnect the shaft for driving rotation of the rotor from the entire power transmission system.
Consequently, this separation is obtained by means such as a clutch having the sole function of preventing the rotor being rotated by the power source(s), and to do so only during the transition from vertical flight to cruising flight.
A device of that type therefore implies additional weight and cost, and constitutes a penalty in terms of safety.                Stopping the rotor and reconfiguring it, e.g. a three-blade rotor stopped in a particular configuration serves as a swept-back wing for flight at a high forward speed, or indeed, after stopping, it is possible to envisage folding the rotor over the fuselage during a rotorcraft-to-airplane transition stage.        
It can be understood that those solutions complicate the technical implementation and contribute to increasing weight, and thus to increasing installed power and consequently expense, but without that achieving an optimized vehicle.