GB2535231A

GB2535231A - Propeller for an aircraft for vertical take-off and landing

Info

Publication number: GB2535231A
Application number: GB1502495.3A
Authority: GB
Inventors: Reiter Johannes
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-02-13
Filing date: 2015-02-13
Publication date: 2016-08-17
Also published as: GB201502495D0

Abstract

An aircraft 100 for vertical take-off and landing comprises a wing arrangement 110 coupled to a fuselage 101 such that the wing arrangement 110 is tiltable around a longitudinal wing axis. The wing arrangement 110 is adapted in such a way that, in a fixed-wing flight mode, the wing arrangement 110 does not rotate around the fuselage 101. The wing arrangement 110 is further adapted in such a way that, in a hover flight mode, the wing arrangement 110 is tilted around the longitudinal wing axis with respect to its orientations in the fixed-wing flight mode and that the wing arrangement 110 rotates around the fuselage 101. An engine 120 comprises a drive shaft 121 with an engine rotary axis 122 and a propeller unit 130 which is mounted to a wing. The propeller comprises blades that are hollow with spars and may be filled with a foam material.

Description

Propeller for an aircraft for vertical take-off and landing

FIELD OF THE INVENTION

The present invention relates to a propeller for an aircraft for vertical take-off and landing comprising an engine and a propeller unit and to a method for operating an aircraft for vertical take-off and landing comprising an engine and a propeller unit.

BACKGROUND OF THE INVENTION

It is an aim to have aircraft that are able to start and land without a runaway for example. Hence, in the past several developments for so called Vertical Take-Off and Landing aircraft (VTOL) have been done. Conventional VTOL-Aircraft need a vertical thrust for generating the vertical lift. Extreme thrust for vertical take-off may be produced by big propellers or jet engines. Propellers may have the disadvantage in travel flight of an aircraft due to a high drag.

An efficient solution for a hover flight capable aircraft is performed by helicopters, using e.g. a big wing area. In a known system, an aircraft comprises an engine for vertical lifting the aircraft (e.g. a propeller) and e.g. a further engine for generating the acceleration of the aircraft in a travel mode up to a desired travelling speed.

In the hover flight mode, the rotating wings or blades of an aircraft (e.g. a helicopter) generate the vertical lift. The rotating wings comprise a chord line, -2 -wherein an angle between the chord line and the streaming direction of the air may be called angle of attack. A higher angle of attack generates a higher lift and a lower angle of attack generates a lower lift but also less drag. In order to achieve a higher efficiency of the rotating wings it may be helpful to adjust the angle of attack. Thus, the wings may be tilted around its longitudinal axis.

In the hover flight mode where the wing arrangement rotates around the fuselage, so that the wing arrangement generates lifting forces for lifting the aircraft. Different loads and load changes act onto the wings during the rotation of the wings. Furthermore, propulsion units are mounted to the wing arrangement for driving the wing arrangement around the fuselage. The propulsion unit comprises rotating masses e.g. the propeller or the driving shaft of the propulsion unit. The rotating masses which comprise a rotating axis that directs during rotation of the wing arrangement along a circumferential direction around the fuselage generate, amongst others, a precession force. The faster the wing arrangement and the propulsion unit, respectively, rotates around the fuselage, the higher is the precession force which can negatively affect the structural stability of the wing arrangement.

OBJECT AND SUMMARY OF THE INVENTION

It may be an object of the present invention to provide an aircraft for vertical take-off and landing comprising a wing arrangement onto which a reduced precession force act darting a rotation of the wing arrangement around the fuselage.

This object may be solved by an aircraft for vertical take-off and landing and by a method for operating an aircraft for vertical take-off and landing according to the independent claims. -3 -

According to a first aspect of the present invention, an aircraft for a vertical take-off and landing is presented. The aircraft comprises a fuselage and a wing arrangement. The wing arrangement is coupled to the fuselage such that the wing arrangement is tiltable around a longitudinal wing axis of the wing arrangement and such that the wing arrangement is rotatable around the fuselage. The wing arrangement is adapted in such a way that in a fixed wing flight mode, the wing arrangement does not rotate around the fuselage. The wing arrangement is further adapted in such a way that in a hover flight mode, the wing arrangement is tiltable around the longitudinal wing axis with respect to its orientation in the fixed wing flight mode and that the wing arrangement rotates around the fuselage.

The aircraft further comprises an engine comprising a drive shaft with any direction of the rotational axis of the propeller engine rotary axis. In particular, the engine rotary axis is parallel to the rotary axis of the wing arrangement around the fuselage.

Furthermore, the aircraft comprises a propeller unit which is mounted to the wing arrangement. The propeller unit comprises a propeller shaft which is coupled to the drive shaft such that the engine drives the propeller unit.

According to a further aspect of the present invention, a method for operating the above-described aircraft for vertical take-off and landing is presented. According to the method, the aircraft is converted in the fixed wing flight mode by arranging a wing arrangement such that a fixed wing flight is enabled. Furthermore, the aircraft is converted in a hover flight mode by tilting the wing arrangement around the longitudinal wing axis and by rotating the wing arrangement around the fuselage of the aircraft. Furthermore, the aircraft is driven by an engine comprising a drive shaft with the engine rotary axis. -4 -

The above described aircraft provides the hover flight mode and the fixed wing flight mode. In a hover flight mode, the wing arrangement is rotating around a rotary axis (e.g. a fuselage axis or an axis which comprises an angle to the fuselage axis) around the fuselage, so that due to the rotation of the wing through the air a lift is generated even without a relative movement of the aircraft (i.e. the fuselage) through the air. The fuselage may be rotatable together with the wing arrangement around the rotary axis. Alternatively, the wing arrangement may be rotatable with respect to the fuselage, so that only the wing arrangement rotates in the hover flight mode for generating lift.

Moreover, if the wing arrangement rotates in the hover flight mode, a stabilizing moment (e.g. a gyroscopic moment, i.e. a conservation of angular momentum) for stabilizing the aircraft is generated. In a fixed-wing flight mode, the wing arrangement is fixed to the fuselage without having a relative motion between the wing arrangement and the fuselage, so that by a forward motion of the aircraft through the air lift is generated by the wing arrangement by a forward movement of the wing arrangement through the air.

The aircraft according to the present invention may be a manned aircraft or an unmanned aircraft vehicle (UAV). The aircraft may be e.g. a drone that comprises for example a wing span of approximately 1 m to 4 m (meter) with a weight of approximately 4 kg to 200 kg (kilograms).

The wing arrangement comprises a longitudinal wing axis, wherein the longitudinal wing axis is the axis around which the wing arrangement is tiltable with respect to the fuselage. The longitudinal wing axis may be defined by the run of a main wing spar or by a bolt (e.g. one of the below described fixing elements) that connects for example a wing root of the wing arrangement with the fuselage. The wing arrangement is mounted with its wing root to the fuselage, wherein at an opposite end of the wing arrangement with respect to the wing root the wing tip is defined, which is a free end of the wing -5 -arrangement. The longitudinal wing axis may be parallel e.g. with a leading edge or a trailing edge of the wing arrangement. Moreover, the longitudinal wing axis may be an axis that is approximately perpendicular to the fuselage axis and/or the rotary axis.

The wing arrangement may comprise a first wing, a second wing or a plurality of wings. Each wing may comprise an aerodynamical wing profile comprising a respective leading edge, where the air impinges and a respective trailing edge from which the air streams away from the wing. A chord line of the wing arrangement and the wings, respectively, refers to an imaginary straight line connecting the leading edge and the trailing edge within a cross-section of an airfoil. The chord length is the distance between the trailing edge and the leading edge.

The fuselage describes a main body of the aircraft, wherein in general the centre of gravity of the aircraft is located inside the area of the fuselage. The fuselage may be in one exemplary embodiment of the present invention a small body to which the wing arrangement is rotatably mounted, so that the aircraft may be defined as a so-called flying wing aircraft. In particular, the fuselage may be a section of the wing and the fuselage may comprise a length equal to the chord line (e.g. a width) of the wing. Alternatively, the fuselage comprises a length that is longer than e.g. the chord line (e.g. the width) of the wing that connects the leading edge and the trailing edge. The fuselage comprises a nose and a tail section.

The wing arrangement rotates through the air and the air has a defined streaming direction with respect to the wing arrangement. A so-called angle of attack defines the alignment of the wing arrangement with respect to the streaming direction of the air, through which the wing arrangement moves (i.e. in the hover flight mode and the fixed wing flight mode, respectively).

The angle of attack is defined by an angle between the cord line of the wing -6 -arrangement and the streaming direction of the air which attacks and impinges at the leading edge of the wing arrangement. If the angle of attack is increased, the coefficient of lift c is increased till a critical angle of attack is reached, where generally stall occurs.

Hence, in order to control the device adequately it is necessary to adjust a predefined lift of the aircraft. The lift of the aircraft may be defined for example by the rotational speed of the wing arrangement around the rotary axis and by adjusting the angle of attack. The term "lift" denotes a force which forces the device to move along a defined direction, e.g. horizontally or vertically.

Furthermore, in the hover flight mode, the rotating wings generate lifting forces for lifting the aircraft. Different loads and load changes act onto the wings during the rotation of the wings. Between the load changes flapping hits act onto the wings within each rotation. Furthermore, wind gusts and side winds act onto the rotating wings which cause further flapping hits. Hence, high bending cycle loads acts onto the rotating wings.

The engine comprises an electrical engine, a hybrid engine, a piston engine, a turbojet engine, a turbofan engine, a turboprop engine, a prop fan engine and/or a propeller engine. Generally, the engine comprises a movable and rotatable mass. The rotatable mass is in particular the drive shaft of the engine. Furthermore, the engine may comprise a movable piston which conducts a piston stroke along a certain piston stroke direction. The engine generates a driving torque which is transmitted by the drive shaft. The drive shaft extends along the engine rotary axis and to the propeller. According to the present invention, the propeller is arranged such that the moment of inertia in the propeller is minimized. Specifically, if the momentum of inertia on the propeller is near or almost a value of zero, the adverse effect of the precession force caused by the rotation of the propeller blades is minimized to -7 -an acceptable level for the operation. An acceptable level for the operation might mean that vibration caused by the propeller in rotation around the drive shaft and / or propeller axis and further in rotation around the fuselage axis is lower so the adverse effect on structure due to vibration is lower or even disappears.

The propeller unit comprises propellers (propeller blades) which rotate through the air such that thrust along a driving direction is generated. In particular, the propeller unit comprises a propeller shaft which is coupled to the drive shaft for transmitting the driving torque from the drive shaft to the propeller shaft. The propeller shaft and the propeller rotary axis, respectively, might even be non-parallel with respect to the engine rotary axis of the drive shaft in the hover flight mode. For example, a gear, such as a bevel gear, may be coupled between the propeller shaft and the drive shaft for transmitting the drive torque. This invention is not limited to any sort of propeller to engine coupling.

The engine and the propeller unit are mounted to the wing arrangement and thus rotate in the hover flight mode around the fuselage.

The rotation of the drive shaft around the engine rotary axis and the rotation of the propellers of the propeller unit and the propeller shaft define rotating masses. The wing arrangement and thus the engine and the propeller unit run during rotation around the fuselage along a circumferential path around the rotary axis. In particular, the mass of the propeller and the mass of the propeller shaft try to run along a linear and tangential direction with respect to the circumferential path. Due to the rotation of the wing arrangement around the rotary axis, the propeller unit and the engine are forced to rotate around the rotary axis around the fuselage along the circumferential path, so that a constraint force, which is directed radially to the rotary axis, forces the propeller unit and the engine unit to leave its desired longitudinal and -8 -tangential direction and hence forces the propeller unit and the engine unit to move along the circumferential path around the rotary axis around the fuselage. The constraint force acts on the rotating mass, such as the propeller shaft which rotates around the propeller rotary axis and causes a precession force. The precession force acts along a direction which is approximately perpendicular (90°) shifted with respect to the constraint force along the tangential direction of the rotating mass around the propeller rotary axis (i.e. a rotary axis which directs along the tangential direction with respect to the circumferential path).

The precession force is dependent on the rotational speed of the rotating mass and in particular on the alignment of the respective mass rotary axis of the rotating mass. Also, it is dependent on the moment of inertia of the propeller. More in detail, it is dependent on the moment of inertia of each of the propeller blades. Specifically only rotary masses which comprise a mass rotary axis that is tangentially with respect to the circumferential path generate a part of the precession force.

Hence, by the approach of the present invention, because the propeller and in particular the propeller blade is made of material and design methods that result in an extra low momentum of inertia in the propeller blades, and a center of gravity of the propeller blades more near to the propeller center point, so the propeller does not generate or reduce only a small amount of precession force. In other words, the propeller is a "low momentum of inertia"-propeller.

Hence, by the approach of the present invention, because the propeller is designed as a low momentum of inertia propeller, the mass of the propeller generates none or a marginal precession force. Hence, by this arrangement, the precession force is reduced such that a reduced amount of disturbing loads acts on the wing arrangement during the hover flight mode. Hence, a more -9 -stable aircraft is provided and adverse vibration effects from the propeller can be lowered.

As described above, according to an exemplary embodiment of the present invention, the propeller blades comprise a shells to introduce the forces, but the propeller blades inside are hollow. Hence, the shell may be for example a titan, a carbon, or other high strong material, respectively. Hence, in the hover flight mode, the propellers are affected by precession force the most. By this means the momentum of inertia of propellers is lowered and so the precession force decreases in comparison to full material propellers.

Additionally it is to say that the precession force is also dependent on the weight, the rotational speed of the wing arrangement around the rotary axis around the fuselage and the center of gravity of the rotating mass.

According to a further exemplary embodiment, the hollow propeller blade has a spar or other design means inside that increases the stiffness of the propeller blade structure while not increasing the weight so much.

According to a further exemplary embodiment, the propeller may consist of a propeller governor pitch adjustment mechanism. This mechanism can adjust angle of attack of the propeller blades and therewith makes the required rounds per minute to the lowest necessary in dependence to the angle of attack of the propeller blades to its surrounding air. Thus may minimize the rounds per minute to a required minimum for propulsion and therewith minimizes precession force, which is dependent on the rounds per minute value of the propeller.

The engine may be mounted in several ways to the support structure, for example such that the wing arrangement is tiltable around the longitudinal wing axis independently with respect to the engine.

-10 -In the following an exemplary low inertia momentum propeller buildup is described. The shells surround the propeller spar. The rest is empty or filled by filling material like foam.

Furthermore, the wing root might be designed in a way not losing too much diameter, like it is done in ordinary propellers, so the propeller can take on the left precession force and/ or momentums.

If the size of the propeller is too small to make it hollow, just stiff material should be used (carbon or titan propeller, or carbon propeller with titan spar for example).

In an exemplary embodiment, the propulsion unit may be adapted for generating a thrust of 3 kg to 5 kg (kilograms). In the hover flight mode, approximately 25 kg are liftable. The aircraft for vertical take-off and landing may thus have a thrust-to-weight ratio of approximately 0,2 to 0,4, preferably 0,3.

It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application.

BRIEF DESCRIPTION OF THE DRAWINGS

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

Fig. 1 shows a schematical view of an exemplary embodiment of an aircraft for vertical take-off and landing according to the present invention.

Fig. 2A to Fig. 2G show a propeller unit with propeller blades and respective cross sections according to exemplary embodiments of the present invention;

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The illustration in the drawing is schematically. It is noted that in different figures, similar or identical elements are provided with the same reference signs.

Fig. 1 shows an aircraft 100 for vertical take-off and landing. The aircraft 100 comprises a fuselage 101 and a wing arrangement 110. The wing arrangement 110 is coupled to the fuselage 101 such that the wing arrangement 110 is tiltable around a longitudinal wing axis of the wing arrangement 110 and such that the wing arrangement 110 is rotatable around the fuselage 101. The wing arrangement 110 is adapted in such a way that, in a fixed wing flight mode, the wing arrangement does not rotate around the fuselage. In the fixed wing flight mode, the aircraft 100 flies through the air, wherein the wing arrangement 110 generates lift due to the speed of the aircraft 100 through the air.

-12 -Fig. 1 shows the aircraft 100 in a hover flight mode. The wing arrangement 110 is adapted in such a way that in the hover flight mode, the wing arrangement 110 is tilted around the longitudinal wing axis with respect to its orientation in the fixed wing flight mode and that the wing arrangement 110 rotates around the fuselage 101 (see arrow in Fig. 1).

The wing arrangement 110 comprises two wings, namely a first wing 111 and a second wing 112. The longitudinal wing axis is split in a first longitudinal wing axis 113 and a second longitudinal wing axis 114. The first wing 111 extends along the first longitudinal wing axis 113 from the fuselage 101 and the second wing 112 extends along the second longitudinal wing axis 114 from the fuselage 101. The first wing 111 is tiltable with a first rotary direction around the first longitudinal wing axis 113 and the second wing 112 is tiltable with a second rotational direction around the second longitudinal wing axis.

Each wing 111, 112 comprise a respective leading edge 115 and trailing edge 116.

A sleeve 103 is rotatably mounted to the fuselage 101. The wing arrangement is mounted to the sleeve 103 such that in the hover flight mode the 20 sleeve 103 rotates together with the wing arrangement 110 around the fuselage 101.

Furthermore, a respective engine 120 is coupled to each of the first wing 111 and the second wing 112. The engine 120 comprises a drive shaft 121 with an engine rotary axis 122. The engine 120 might be coupled to the respective wings 111, 112 in such a way that in the hover flight mode the engine rotary axis 122 comprises at least one component which is parallel to a rotary axis 102 of the wing arrangement 110 around the fuselage 101.

Furthermore, respective propeller units 130 are mounted to respective wings 111, 112. Each propeller unit 130 comprises a propeller shaft 131 which is -13 -coupled to the respective drive shaft 121 such that the engine 120 drives the propeller unit 130.

In the hover flight mode as shown in Fig. 1, the propeller unit 130 rotates along a circumferential path around the rotary axis 102.

The rotation of the drive shaft 121 around the engine rotary axis 122 and the rotation of the propeller shaft 131 around the propeller rotary axis 132 define rotating masses. The wing arrangement 110 and thus the engine 120 and the propeller unit 130 run during rotation around the fuselage along a circumferential path 117 around the rotary axis 102. In particular, the mass of the propeller shaft 131 and its propellers try to run along a linear and tangential direction with respect to the circumferential path 117. Due to the rotation of the wing arrangement 110 around the rotary axis 102, the propeller unit 130 and the engine 120 are forced to rotate around the rotary axis 102 around the fuselage 101, so that a constraint force Fc, which is directed radially to the rotary axis 102, forces the propeller unit 130 and the engine 120 to leave its desired longitudinal and tangential direction and hence forces the propeller unit 130 and the engine 120 to move along the circumferential path 117 around the rotary axis 102. The constraint force Fc acts on the rotating mass, such as the propeller shaft 131 which rotates around the propeller rotary axis 132 and causes a precession force Fp. The precession force Fp acts along a direction which is approximately perpendicular (90°) shifted with respect to the constraint force Fc along the tangential direction of the rotating mass around the propeller rotary axis 132 (i.e. a rotary axis which directs along the tangential direction with respect to the circumferential path 117).

The precession force Fc is dependent on the rotational speed of the rotating mass and in particular on the alignment of the respective mass rotary axis of the rotating mass. It can have a vibration affect due to the unsteady cross -14 -section of a propeller. Specifically, only rotary masses which comprise a mass rotary axis that is tangentially with respect to the circumferential path 117 generate a part of the precession force.

The propeller unit 130, which propellers need a propeller shaft 131 and a respective propeller rotary axis 132 along a tangential with respect to the circumferential path 117, generate a precession force.

Hence, due to the rotating masses of the propeller unit 130, a precession force is generated.

Due to the specific propeller setup, the precession force origin in the propeller (center of gravity of propeller blades set more into direction of propeller center, total weight and therewith momentum of inertia of propeller blades set to minimum) is lowered by design.

In a fixed wing flight mode (not shown), the first wing 111 is tiltable with a first rotary direction around the first longitudinal wing axis and the second wing 112 is tiltable with a second rotational direction around the second longitudinal wing axis 114. The first rotational direction differs to the second rotational direction. Hence, both propulsion units 130 of the respective wings 111, 112 generate thrust in order to drive the aircraft 100 through the air. By the speed through the air of the aircraft 100, lift is generated by the respective wings 111, 112. In the fixed wing flight mode, the angle 13 may be around +/-20°, in particular 0°, depending on an angle of attack a between the respective wings 111, 112 and the direction of air 301 (see Fig. 3).

Furthermore, in the tail section of the fuselage 101, a further propulsion unit 104, such as a jet engine, may be installed. Furthermore, at the tail section of 30 the fuselage 101, tail wings 105 may be attached which provide for example control surfaces for stabilizing the aircraft 100 during the flight.

-15 -Fig. 2A to Fig.2G shows a propeller unit 130 with propeller blades and respective cross sections according to exemplary embodiments of the present invention.

Fig. 2A shows a propeller unit 130 comprising three propeller blades, which are mounted to a propeller root405. The propeller blades rotate around the propeller rotary axis 132.

Fig. 2B shows propeller unit 130 comprising one propeller blade, which is mounted to a propeller root405. The propeller blade rotates around the propeller rotary axis 132.

Fig. 2C shows a view of cut B-B of the propeller blade shown in Fig. 2B.

Fig. 2D shows a view of cut A-A of the propeller blade shown in Fig. 2A.

Fig. 2E shows a schematic view of a propeller shell 402 of the propeller blade. The propeller shell 402 encloses an inner volume, in particular a propeller hollow room 404, in order to provide a light weight and a structurally stiff design of the propeller blade.

Fig. 2F shows a schematic view of a propeller shell 402 of the propeller blade. The inner volume of the propeller shell 402 is filled with foam. Also other matrix material can be used instead of foam.

Fig. 2G shows a schematic view of a propeller shell 402 of the propeller blade. The propeller shell 402 encloses the inner volume. In particular, a propeller spar 403 may be arranged within the inner volume in order to raise the structural stiffness of the propeller unit 130. Additionally, foam or other light -16 -weight filling material may be filled within the inner volume. Furthermore, two or more propeller spars 403 may be arranged within the inner volume.

It should be noted that the term "comprising" does not exclude other elements or steps and "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

-17 -Reference Signs: aircraft 101 fuselage 102 fuselage axis, rotary axis 103 sleeve 104 further propulsion unit tail wing 106 gear wing arrangement 111 first wing 112 second wing 113 first longitudinal wing axis 114 second longitudinal wing axis leading edge 116 trailing edge 117 circumferential path 122 engine rotary axis 123 piston 124 piston stroke direction propeller unit 131 propeller shaft 132 propeller rotary axis 301 direction of air 302 coupling section 303 chord line 402 propeller shell 403 propeller spar 404 propeller hollow room or foam 405 propeller root a angle of attack R angle between propeller rotary axis and engine rotary axis Fc constraint force Fp precession force

Claims

-18 -Claims: 1. Aircraft (100) for vertical take-off and landing, the aircraft comprising a fuselage (101), a wing arrangement (110), wherein the wing arrangement (110) is coupled to the fuselage (101) such that the wing arrangement (110) is tiltable around a longitudinal wing axis of the wing arrangement (110) and such that the wing arrangement (110) is rotatable around the fuselage (101), wherein the wing arrangement (110) is adapted in such a way that, in a fixed-wing flight mode, the wing arrangement (110) does not rotate around the fuselage (101), and wherein the wing arrangement (110) is further adapted in such a way that, in a hover flight mode, the wing arrangement (110) is tilted around the longitudinal wing axis with respect to its orientations in the fixed-wing flight mode and that the wing arrangement (110) rotates around the fuselage (101), an engine (120) comprising a drive shaft (121) with an engine rotary axis (122), and a propeller unit (130) which is mounted to the wing arrangement (110), wherein the propeller unit (130) comprises a propeller shaft (131) 25 which is coupled to the drive shaft (121) such that the engine (120) drives the propeller unit (130), wherein a propeller blade of the propeller unit (130) is formed such that an extra low weight is achieved and a momentum of inertia of the -19 -propeller is reduced in order to have lower precession and vibration effects in the hover flight mode.
2. Propeller according to claim 1, wherein the propeller blade is built in a hollow way.
3. Propeller according to claim 1 or 2, wherein the hollow propeller blade consisting of a shell.
4. Propeller according to claim 3, wherein the shell consist of a stiff material, in particular carbon or titan which withstand effective forces in operation.
5. Propeller according to one of the claims 1 to 4, wherein the propeller blade is built in a way comprising a propeller spars inside.
6. Propeller according to claim 5, wherein the propeller blade spar consist of a stiff material, in particular carbon or titan to withstand effective forces in operation.
7. Propeller according to one of the claims 1 to 6,wherein the shell material is less stiff than the spar so that a shell material with less weight is usable.
8. Propeller according to one of the claims 1 to 7, wherein a propeller blade root is designed in a more stiff way, in particular by using a bigger diameter and/or a continuing spar.

-20 -
9. Propeller according to one of the claims 1 to 8 wherein the propeller blade is adjustable during operation by a governor mechanism.
10. Propeller according to claim 2, wherein the hollow blade is filled with light weight filling material, in particular foam.