WO1996025328A1

WO1996025328A1 - Combined cycle compressed air tip jet driven helicopter

Info

Publication number: WO1996025328A1
Application number: PCT/CA1995/000080
Authority: WO
Inventors: Michel Milot
Original assignee: Michel Milot
Priority date: 1995-02-16
Filing date: 1995-02-16
Publication date: 1996-08-22
Also published as: AU1702995A

Abstract

Disclosed herein is a helicopter drive system (106) comprising: a combustion engine (108) having a hot gas exhaust outlet (140); an air compressor (110) drivingly connected to the combustion engine (108); a rotor blade assembly including a hollow rotor hub (114) and at least two hollow rotor blades (116, 118) each having a proximate end and a distal end; the hollow rotor hub (114) being connected to each hollow rotor blade at the proximate end thereof. The helicopter drive system also includes conduits (146) for conveying compressed air from the compressed air output of the compressor (110) to the hollow rotor hub (114), the conveyed compressed air being discharged through an air outlet (120, 122) at the distal end of each hollow rotor blade. Finally, a heat exchanger (112) is provide for transferring heat from the hot gas exhaust outlet (140) of the combustion engine (108) to the compressed air, to thereby increase the temperature of the compressed air conveyed to the hollow rotor hub through the conduits.

Description

TITLE OF THE INVENTION

COMBINED CYCLE COMPRESSED AIR TIP JET DRIVEN HELICOPTER

FIELD OF THE INVENTION

The present invention relates to helicopters. More specifically, the present invention relates to helicopters having a rotor driven by high pressure compressed air exiting at high velocity through air outlets located at the tip of the rotor blades.

BACKGROUND OF THE INVENTION

The ability to accomplish long stationary flights combined with the possibility of safely realising low altitude manoeuvres, have given the helicopter great flexibility. The helicopter is a highly adaptable vehicle since it provides easy access to virtually any location without the need for ground installations.

Helicopters use at least one rotating main rotor comprising two or more rotor blades to create a force, perpendicular to the plane created by the rotating rotor blades. This force has a component, called the lift force, which is in opposition with the gravitational force present between the earth and the helicopter. The lift force causes the helicopter to rise if it is greater than the gravitational force. On the other hand, if the lift force is smaller than the gravitation force, the helicopter will descend at a speed depending of the difference between these two forces. Evidently, if the two forces are equal, the altitude of the helicopter is stationary with respect to the ground.

To rotate the main rotor, it is necessary to exert a force thereon. To exert this force, conventional helicopters utilize a piston engine or a turboshaft engine having a rotating shaft mechanically linked to the rotor (these types of helicopters will hereinafter be referred to as mechanical helicopters) . These helicopters include pieces of equipment of great complexity as a number of mechanical components, such as reduction boxes, driving shafts, gears, freewheels, etc., are required to efficiently transfer the required amount of mechanical power from the rotating shaft of the engine to the main rotor.

In mechanical helicopters, the power is transmitted from the engine to the main rotor as a driving torque. This driving torque induces a fuselage torque which has the same amplitude as the driving torque, but is of opposite direction. This fuselage torque tends to impart rotational motion to the fuselage in the direction opposite to the direction of rotation of the main rotor. To compensate for the induced fuselage torque, and therefore to prevent the rotation of the fuselage, a compensation torque is applied to the fuselage of conventional mechanical helicopters. This compensation torque is created by a second rotor having a rotation axis perpendicular to the rotation axis of the main rotor. This second rotor is mounted on the tail of the helicopter. The compensation torque produced by this tail rotor is a function of the speed of rotation of the tail rotor, of its pitch and of the distance between the shafts of the main and tail rotors.

Another type of known helicopter (hereinafter referred to as a tip jet driven helicopter) uses the power of gases discharged through the tip of the blades of the rotor to impart rotating motion to the rotor. No torque is transferred from the fuselage to the rotor. One commercial construction of tip jet driven helicopters is the SO- 1221 "DJINN" which will be discussed hereinafter.

Many advantages arise from the utilization of gases discharging through the tip of rotor blades to impart motion to the rotor. For example, a helicopter drive system using gas discharge has a considerably simpler construction than a helicopter drive system transferring torque from an engine to a rotor. Therefore, the resulting helicopter is lighter. Furthermore, since there is no torque involved in the transfer of power from the fuselage to the rotor, the previously discussed induced fuselage torque does not exist and a tail rotor is not necessary, nor desirable. Indeed, the tail rotor of mechanical helicopters is a major cause of accidents and it adds complexity to the helicopter drive system.

S.N.C.A.S.O. (Societe Nationale de

Construction Aeronautique du Sud Ouest) has constructed, on a commercial scale, between 1956 and 1958, a two-seater tip jet driven helicopter, the S0- 1221 "DJINN" . Less than 200 of these small tip jet driven helicopters have been constructed during this period.

The SO-1221 "DJINN" has a rotor with two hollow blades. A modified turboshaft engine having an oversize air compressor which can supply the combustion chamber of the engine and the rotor is used in this tip jet driven helicopter. A major portion of the compressed air exiting the air compressor is ducked to a hollow oscillating rotor hub, then passes through longitudinal passages of the hollow rotor blades. Finally, the compressed air is discharged to the atmosphere by nozzles located at the tip of the blades .

One major disadvantage of the SO-1221

"DJINN" is the low efficiency of its drive system. Indeed, the power transmission efficiency to the rotor of the SO-1221 "DJINN" is approximately 33%, which is low compared to the power transmission efficiency of more than 80% usually found in mechanical helicopters. It is to be noted, however, that since a tip jet driven helicopter is considerably lighter than a mechanical helicopter of equivalent size, a power transmission efficiency to the rotor of a tip jet driven helicopter in the order of 65% would presumably be equivalent to the usual power transmission efficiency of a mechanical helicopter. Essentially, a tip jet helicopter and a mechanical helicopter having substantially equal dimensions wherein the tip jet helicopter would have a power transmission efficiency to the rotor in the order of 65% and wherein the mechanical helicopter would have a power transmission efficiency to the rotor in the order of 85%, would use the same amount of fuel to carry the same useful load. Furthermore, their speed performances would be comparable.

OBJECTS OF THE INVENTION

An object of the present invention is to provide an improved compressed air helicopter drive system.

Another object of the invention is to provide a compressed air helicopter drive system having a power transmission efficiency to the rotor comparable to the power transmission efficiency of a mechanical helicopter on a useful load basis.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a helicopter drive system comprising: a combustion engine having a hot gas exhaust outlet; air compressor means drivingly connected to the combustion engine, the air compressor means including compressed air output means; a rotor blade assembly including a hollow rotor hub and at least two hollow rotor blades each having a proximate end and a distal end, each hollow rotor blade including a longitudinal air passage extending from the proximate end to the distal end, the longitudinal air passage having a predetermined cross-sectional area; the hollow rotor hub being connected to each hollow rotor blade at the proximate end thereof; the distal end having an air outlet; conduit means for conveying compressed air from the compressed air output means of the compressor means to the hollow rotor hub, the conveyed compressed air passing through the longitudinal air passage of each hollow rotor blade and being discharged through the air outlet at the distal end of each hollow rotor blade; and heat exchanger means for transferring heat from the hot gas exhaust outlet of the combustion engine to the compressed air, to thereby increase the temperature of the compressed air conveyed to the hollow rotor hub through the conduit means.

Another aspect of the invention is to provide a nozzle for installation at a distal end of a hollow rotor blade of a helicopter, the hollow rotor blade including a longitudinal air passage having a predetermined cross-sectional area; the nozzle comprises: a converging portion for gradually restricting the cross-sectional area of the longitudinal air passage; a throat located downstream from the converging portion, the throat having a cross-sectional area smaller than the predetermined cross-sectional area of the longitudinal air passage of the hollow rotor blade; and a diverging portion located downstream from the throat, the diverging portion having a gradually expanding cross-sectional area.

Objects, advantages and other features of the present invention will become more apparent upon reading of the following non restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings:

Figure 1, which is labelled "PRIOR ART", illustrates the principle of operation of a tip jet driven helicopter using compressed air;

Figure 2, which is labelled "PRIOR ART" is an elevational view taken along line 2-2 of Figure 1;

Figure 3 illustrates a tip jet driven helicopter having a drive system according to the present invention; Figure 4 is a line flow sheet diagram of the drive system of the present invention; and

Figure 5 is a schematic diagram of a helicopter converging-diverging nozzle according to the present invention.

DESCRIPTION OF THE PRIOR ART

The principle of operation of a conventional tip jet driven helicopter drive system will first be described with reference to Figures 1 and 2 of the appended drawings. This description is concise since the principle of operation of a tip jet driven helicopter is believed to be well known to the ones of ordinary skill in the art.

As illustrated in Figure 1, the drive system 10 comprises: an air compressor 12, a turboshaft engine 14, an air duct 16, a swivel joint assembly 18, a hollow oscillating hub 20, two hollow rotor blades 22, 24, and two nozzles 26,28. It is also possible to design a tip jet driven helicopter drive system having more than two rotor blades .

Air compressor 12 is mechanically connected to the turboshaft engine 14 so that the rotation of a shaft (not shown) of the engine 14 supplies mechanical power to the compressor 12. Using this mechanical power, the compressor 12 compresses air 30 supplied thereto by an air inlet 32. The air compressed by the compressor 12 is supplied to the air duct 16 by an air outlet 34 of the compressor 12. The air duct 16 is connected to the hollow rotor hub 20 through the swivel joint assembly 18, thereby allowing rotation of rotor hub 20 with respect to the air duct 16. The compressed air enters the hollow rotor hub 20 where it is directed to the two hollow rotor blades 22 and 24. Finally, the compressed air is discharged through nozzles 26 and 28 located at the tip of hollow rotor blades 22 and 24, respectively.

The turboshaft engine 14 is supplied with liquid fuel (not shown) and the hot gases forming the combustion exhaust are discharged through a hot gas outlet 42.

Figure 2 is an elevational view illustrating nozzle 28 installed at the extremity of rotor blade 24. As illustrated, the compressed air is discharged in a direction 36 which is substantially perpendicular to the longitudinal axis 38 of the hollow rotor blade 24. This discharge of compressed air causes a force which, in turn, causes the rotation of the hollow rotor blade 24 in a direction 40, which is opposite to the direction 36 of the discharged air.

As will be apparent to one skilled in the art, a system similar to the system for changing the angle of attack (known also as the pitch) of the blades according to their angular position is required. This system may be similar to systems used on conventional mechanical helicopters. DESCRIPTION OF THE PREFERRED EMBODIMENT

In a tip jet driven helicopter, the power transmitted to the rotor is proportional to (a) the angular speed of the rotor blades, (b) the distance between the nozzles and the center of the rotor hub, and (c) the thrust delivered by the nozzles.

Thus, by increasing any of the preceding three parameters, the power transmitted to the rotor will increase. Furthermore, if the power transmitted to the rotor increases without increasing the fuel consumption, the efficiency of the power transmission efficiency to the rotor increases.

The angular speed of the rotor blades has a limit which is set by the geometry of the blades. It is not within the scope of the present disclosure to go into these design choices and constraints, but it is generally recognized that to attain greater angular speeds, the blades must be thin to optimize the aerodynamic performances. In a tip jet driven helicopter, if thin rotor blades are used, the conduits conveying the compressed air to the nozzles must be small . The use of small conduits means that the pressure of the compressed air must be higher than when larger conduits are used if the same mass of air is to go through by unit of time. Thus, to increase the angular velocity of the rotor blades, a high pressure cycle must be used. The distance between the nozzles and the rotor hub may be increased by increasing the length of the rotor blades. However, physical and practical limits exist in the determination of the length of the rotor blades since the speed of the tip of the rotor blades is a limiting factor.

The thrust delivered by a nozzle is proportional to the mass of gas passing through the nozzle multiplied by the maximum speed attained by the gas in the nozzle.

As will be apparent to the ones skilled in the art, the thrust delivered by the nozzle is also proportional to the relative remaining gas pressure (with respect to the atmospheric pressure) at the exit of the nozzle multiplied by the cross-sectional area of the outlet end of the nozzle. Since the designs of nozzles usually include the restriction that the discharged gas must be at atmospheric pressure when it exits the nozzle, the result of the multiplication equals or is near to zero.

A limit to the maximum speed of gases flowing through a throat, which is the portion of a nozzle having the smaller cross-sectional area, exists. This limit is the speed of sound.

For converging nozzles, e.g. nozzles having an internal wall converging to a throat, there is a fixed pressure ratio between the pressure upstream from the throat and the pressure downstream from the throat of the nozzle to attain the speed of sound. Once this pressure ratio is reached, any additional pressure upstream from the throat will only increase the mass of air passing through the nozzle.

Since the thrust of a nozzle is proportional to the mass of gas multiplied by the maximum speed of the gas, the pressure ratio involved in a tip jet driven helicopter is usually greater than the fixed pressure ratio needed to attain the speed of sound. Indeed, if the pressure is increased, the mass of gas passing through the nozzle is increased and the thrust is also increased. Therefore, there is a decrease in efficiency since a large portion of the pressure is only used to increase the mass of air, and not the speed of the air, discharged by the nozzle.

To reduce this loss of efficiency, it is proposed therein to increase the temperature of the discharged air. Indeed, it is well known that the speed of sound is not constant but increases with temperature. The following equation is used to evaluate the speed of sound at different temperatures:

V = 331 1 +

N 273

where V is the speed of sound in meters per second (m/s) and T' is the temperature of air in Celsius (°C) .

For example, the air discharged by the nozzles of conventional tip jet driven helicopter, such as the SO-1221 "DJINN", is at a temperature of approximately 200°C at the throat, the speed of sound at the throat of the nozzle is therefore approximately 436 m/s. If the air were to be heated to 400°C, the speed of sound at the throat of the nozzle would become approximately 519 m/s. Since, as explained earlier, the thrust delivered by the nozzle is proportional to the maximum speed of the air in the nozzle, the thrust delivered by the nozzle, while keeping a constant mass of air flow through the nozzle, is increased simply by increasing the temperature of the air discharged by 200°C. Indeed, the maximum speed attainable by the air discharged is increased by locally increasing the speed of sound.

Referring to Figures 3 and 4 of the appended drawings, the proposed approach for increasing the temperature of the air in a compressed air helicopter drive system will now be described.

Schematically illustrated in Figure 3, a tip jet driven helicopter 100 comprises a fuselage 102, a tail 104 and a drive system 106.

Drive system 106 comprises a turboshaft engine 108, an air compressor 110, a heat exchanger 112, a hollow rotor hub 114, two hollow rotor blades 116 and 118, and two nozzles 120 and 122.

, Turboshaft engine 108 is supplied with liquid fuel (not shown) and includes a rotating shaft 126 (Figure 4) supplying mechanical power to the air compressor 110. Air (see arrow 130) enters compressor 110 through an inlet 128. The air is then compressed by compressor 110, exits through a compressed air outlet 132 and enters a conduit 146 leading to a fresh air inlet 134 (see Figure 4) of heat exchanger 112. The compressed air entering heat exchanger 112 is heated by heat exchanger 112 as explained hereinafter. The compressed and heated air then exits heat exchanger 112 through a fresh air outlet 136 (see Figure 4) . It enters the hollow rotor hub 114 through a swivel joint assembly 138. The compressed and heated air is then divided between the two hollow rotor blades 116 and 118 to finally be discharged through nozzles 120 and 122 located at the tip of hollow rotor blades 116 and 118, respectively. This discharge of compressed and heated air through nozzles 120 and 122 creates a force which causes the rotation of rotor blades 116 and 118 in a direction opposite to the direction of the discharge.

The heating of the compressed air entering heat exchanger 112 by the fresh inlet 134 will now be explained with reference to Figure 4.

Heat exchanger 112 is mounted so that heat is transferred from the hot gases forming the combustion exhaust of turboshaft engine 108 to the compressed air exiting the air compressor 110.

More specifically, as illustrated in Figure 4, an exhaust outlet 140 of turboshaft engine

108 is connected to a hot gas inlet 142 of heat exchanger 112 through a conduit 148. Heat exchanger

112 also includes a hot gas outlet 144 to which a conduit 150 is connected. The hot gases forming the combustion exhaust of turboshaft engine 108 enter the heat exchanger 112 to thereby increase the temperature of the compressed air entering the heat exchanger 112 through fresh air input 134.

Since heat exchangers are well known in the art, heat exchanger 112 will not be further detailed.

As will be easily understood by one of ordinary skills in the art, heat exchanger 112 is preferably a gas-to-gas heat exchanger since both mediums are gaseous. However, other types of heat exchangers could also be used.

Conduit 150 pipes the hot gases exiting heat exchanger 112 to a control rudder 124 mounted on the tail 104 of the helicopter 100. By changing the orientation of control rudder 124, it will be possible to change the position of the helicopter 100 with respect to the ground.

Figure 4 also schematically illustrates the various portions of turboshaft engine 108. An air compressor 152 supplies air to a combustion chamber 154. This combustion chamber 154 increases the energy content of the gas fed to a power turbine 156 which, in turn, causes the rotation of shaft 126.

It is to be noted that the air compressor 110 is illustrated, in Figures 3 and 4, as being separated from turboshaft engine 108, but, air compressor 110 could be integrated to the turboshaft engine 108. If fact air compressor 110 and air compressor 152 could be integrated into a single air compressor (not shown) having a first compressed air outlet supplying compressed air to combustion chamber 154 and a second compressed air outlet supplying compressed air to heat exchanger 112. This approach was selected in the case of the SO-1221 "DJINN" previously discussed.

By using a heat exchanger, the temperature of the discharged air is increased by transferring heat from the hot combustion gases exiting the turboshaft engine to the compressed air exiting the air compressor. energy is recuperated instead of being lost in the atmosphere; thus, the efficiency of the power transmission to the rotor is increased.

As stated above : - the thrust delivered by a nozzle is proportional to the mass of gas passing through the nozzle multiplied by the maximum speed attained by the gas in the nozzle; and

- the maximum speed of gases flowing through a throat, which is the portion of a nozzle having the smaller cross-sectional area, is the speed of sound.

From these facts, another aspect of the present invention is that a nozzle having a diverging portion extending downstream from a throat allows the compressed air, having the previously discussed maximum throat speed, to dilate and thus attain, downstream from the throat, speeds greater than the maximum speed at the throat while still being within the nozzle. This greater speed is the speed producing the thrust delivered by the nozzle. Therefore, the thrust delivered by the nozzle increases for a given mass of gas passing through the nozzle, and for a given pressure of gas upstream from the throat .

To increase the efficiency of tip jet driven helicopters, it is therefore proposed to use a converging-diverging nozzle as illustrated in Figure 5.

Figure 5 illustrates a converging- diverging nozzle 200 comprising an inlet 202, a converging portion 204, a throat 206, a diverging portion 208 and an outlet 210.

The compressed air to be discharged enters through inlet 202. The internal cross-sectional area of converging portion 204 gradually decrease to thereby increase the speed of the compressed air since the flow is constant in the entire length of the nozzle 200. As previously discussed, the speed at the throat 206 may not exceed the speed of sound. The internal cross-sectional area of the throat 206 is the smallest internal cross-sectional area of the nozzle 200. In the diverging portion 208, downstream from the throat 206, the internal cross-sectional area gradually increases, thereby allowing the compressed air to dilate. This dilatation of the compressed air rapidly decreases the pressure towards the atmospheric pressure. During the rapid dilatation of the compressed air into the diverging portion 208 of nozzle 200 the speed of the air attains speeds greater than the speed of sound.

It is to be noted that to simplify the description, a nozzle having a circular cross-section is disclosed referring to Figure 5. However, other shapes of converging-diverging nozzles could be used while still being within the scope of the present invention. Essentially, it is preferable that the shape of the nozzle be consistent with the shape of the rotor blade so as to optimize the aerodynamics of the rotor and therefore attain higher rotor blade angular speeds .

As will be understood by experts in the art, to accelerate to supersonic speed in the diverging portion 208 of nozzle 200, the pressure of the compressed air, upstream from the nozzle 200, must be higher than the pressure necessary to attain the pressure ratio required to attain the speed of sound at the throat 206 of nozzle 200.

Therefore, by using a converging-diverging nozzle, the thrust delivered by the nozzle for a given mass of air flow and for a given air temperature is increased. Indeed, the diverging portion of the nozzle allows rapid dilatation of the compressed air leading to the acceleration of the compressed air to supersonic speeds. As described hereinabove, two techniques have been used to increase the thrust of a nozzle mounted at a distal end area of a rotor blade of a tip jet driven helicopter. By combining and optimizing these two techniques, it is believed possible to attain a power transmission efficiency to the rotor which is comparable to the power transmission efficiency to the rotor of a conventional mechanical helicopter.

Although the present invention has been described hereinabove by way of specific forms, it is noted that, it can be modified, without departing from the spirit and nature of the subject invention as defined in the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A helicopter drive system comprising: a combustion engine having a hot gas exhaust outlet; air compressor means drivingly connected to said combustion engine, said air compressor means including compressed air output means; a rotor blade assembly including a hollow rotor hub and at least two hollow rotor blades each having a proximate end and a distal end, each said hollow rotor blade including a longitudinal air passage extending from said proximate end to said distal end, said longitudinal air passage having a predetermined cross-sectional area; said hollow rotor hub being connected to each said hollow rotor blade at the proximate end thereof; said distal end having an air outlet; conduit means for conveying compressed air from said compressed air output means of said compressor means to said hollow rotor hub, said conveyed compressed air passing through said longitudinal air passage of each said hollow rotor blade and being discharged through said air outlet at said distal end of each said hollow rotor blade; and heat exchanger means for transferring heat from said hot gas exhaust outlet of said combustion engine to said compressed air, to thereby increase the temperature of the compressed air conveyed to said hollow rotor hub through said conduit means.

2. A helicopter drive system as defined in claim 1, wherein said combustion engine is a turboshaft engine.

3. A helicopter drive system as defined in claim 1, wherein said heat exchanger means is an gas-to-gas heat exchanger.

4. A helicopter drive system as defined in claim 1, wherein said air outlet of said distal end of each said hollow rotor blade includes nozzle means.

5. A helicopter drive system as defined in claim 4, wherein said nozzle means comprise a converging portion leading to a throat restricting the area of said air outlet to thereby increase velocity of the compressed and heated air discharged through said air outlet.

6. A helicopter drive system as defined in claim 5, wherein said nozzle means further comprise a diverging portion located downstream from said throat to dilate said compressed air, thereby increasing the velocity of said compressed and heated air in said diverging portion of said nozzle.

7. A helicopter drive system as defined in claim 1, further comprising a control rudder mounted on a tail of the helicopter and conduit means for conveying hot gases from said hot gas exhaust outlet to said control rudder, thus enabling steering of said tail of said helicopter.

8. A nozzle for installation at a distal end of a hollow rotor blade of a helicopter, said hollow rotor blade including a longitudinal air passage having a predetermined cross-sectional area, said nozzle comprising: a converging portion for gradually restricting said cross-sectional area of said longitudinal air passage; a throat located downstream from said converging portion, said throat having a cross- sectional area smaller than said predetermined cross- sectional area of said longitudinal air passage of said hollow rotor blade; and a diverging portion located downstream from said throat, said diverging portion having a gradually expanding cross-sectional area.

9. A nozzle for installation at a distal end of a hollow rotor blade of a helicopter, said hollow rotor blade including a longitudinal air passage having a predetermined internal cross- sectional area, said nozzle comprising: a converging portion having an inner wall, said converging portion including a first end and a second end; said first end having an internal cross- sectional area substantially equal to said air passage internal cross-sectional area; said second end having an internal cross-sectional area smaller than said internal cross-sectional area of said first end; said inner wall of said converging portion converging toward said second end; a throat located downstream from said converging portion, said throat having an internal cross-sectional area substantially equal to said cross-sectional area of said second end of said converging portion; and a diverging portion located downstream from said throat, said diverging portion having an inner wall, said diverging portion including a first end and a second end; said first end having an internal cross-sectional area substantially equal to said internal throat cross-sectional area; said second end having an internal cross-sectional area larger than said internal cross-sectional area of said first end; said inner wall of said diverging portion diverging toward said second end.

10. In a helicopter drive system including: a combustion engine having a hot gas exhaust outlet; air compressor means drivingly connected to said combustion engine, said air compressor means including compressed air output means; a rotor blade assembly including a hollow rotor hub and at least two hollow rotor blades each having a proximate end and a distal end, each said hollow rotor blade including a longitudinal air passage extending from said proximate end to said distal end, said longitudinal air passage having a predetermined cross-sectional area,- said hollow rotor hub being connected to each said hollow rotor blade at the proximate end thereof; said distal end having an air outlet; conduit means for conveying air from said compressed air output of said compressor means to said hollow rotor hub for discharge through the air outlet of said distal end of each said hollow rotor blade: the improvement consisting in a nozzle means at said air outlet of said distal end of each said hollow rotor blade, said nozzle means comprising: a converging portion leading to a throat having a cross-sectional area smaller than said predetermined cross-sectional area of said longitudinal air passage, thereby increasing the velocity of the compressed air discharged; and a diverging portion located downstream from said throat to dilate said compressed air thereby increasing the velocity of said compressed and heated air in said diverging portion of said nozzle.

11. A helicopter drive system as defined in claim 10, wherein said combustion engine is a turboshaft engine.

12. In a helicopter drive system including: a combustion engine having a hot gas exhaust outlet; air compressor means drivingly connected to said combustion engine, said air compressor means including compressed air output means; a rotor blade assembly including a hollow rotor hub and at least two hollow rotor blades each having a proximate end and a distal end, each said hollow rotor blade including a longitudinal air passage extending from said proximate end to said distal end, said longitudinal air passage having a predetermined cross-sectional area; said hollow rotor hub being connected to each said hollow rotor blade at the proximate end thereof; said distal end having an air outlet; and conduit means for conveying compressed air from said compressed air output means of said compressor means to said hollow rotor hub, said conveyed compressed air passing through said longitudinal air passage of each said hollow rotor blade and being discharged through said air outlet at said distal end of each said hollow rotor blade: the improvement consisting of heat exchanger means for transferring heat from said hot gas exhaust outlet of said combustion engine to said compressed air, to thereby increase the temperature of the compressed air conveyed to said hollow rotor hub through said conduit means .

13. A helicopter drive system as defined in claim 12, wherein said combustion engine is a turboshaft engine.

14. A helicopter drive system as defined in claim 12 wherein said heat exchanger means is an gas-to-gas heat exchanger.