US20110277447A1

US20110277447A1 - Engine, Particularly CROR Engine, for an Aircraft

Info

Publication number: US20110277447A1
Application number: US13/190,990
Authority: US
Inventors: Arne Stürmer
Original assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Current assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Priority date: 2009-01-31
Filing date: 2011-07-26
Publication date: 2011-11-17
Also published as: DE102009007013A1; EP2391536B1; WO2010086338A2; EP2391536A2; WO2010086338A3

Abstract

An engine for an aircraft flying in a cruise flight direction comprises at least one rotor having a plurality of rotor blades. The at least one rotor is driven about a rotor axis at a rotational frequency. The rotor axis is oriented in the cruise flight direction of the aircraft, and at least one geometric parameter of the rotor blades that influences the propulsion characteristics of the rotor is variable. The engine further comprises a controller which periodically varies the geometric parameter of the rotor blades at at least one frequency that is at least as high as the rotational frequency of the rotor.

Description

REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of International Application No. PCT/EP2010/050950, filed Jan. 27, 2010, that claims priority to German Patent Application No. 10 2009 007 013.3, filed Jan. 31,2009.

FIELD

The invention relates to an engine for an aircraft flying in a cruise flight direction. Particularly, the invention relates to an engine for an aircraft flying in a cruise flight direction, which comprises at least one rotor driven about a rotor axis oriented in the cruise flight direction and having a plurality of rotor blades, wherein at least one geometric parameter of the rotor blades that influences the propulsion characteristics of the rotor is variable. Even more particularly, the invention relates to such an engine for an aircraft which is designated as a CROR (Contra Rotating Open Rotor) drive or engine, and in which two open rotors are driven in opposite directions about a common rotor axis oriented in the cruise flight direction of the aircraft.
The formulation “rotor axis oriented in the cruise flight direction” used here is not intended to mean that the rotor axis has to be exactly parallel to the cruise flight direction of the aircraft. Instead there may be a pitch angle or pitch between the rotor axis and the cruise flight direction. Further, there will typically be a variation of the pitch of the rotor axis over different flight configurations of the aircraft. In the present invention, however, the pitch of the rotor axis remains comparatively small and is always smaller than 45° . As a rule, it remains below 15° . In the present invention, it is also possible, but by no means mandatory, to track the cruise flight direction with the rotor axis of the engine. Usually, the rotor axis will be fixed with regard to the structure of the aircraft to which the engine is mounted.

BACKGROUND

Aircraft with engines comprising at least one rotor driven about a rotor axis oriented in the cruise flight direction of the aircraft, which are generally designated as propeller-driven aircraft, display basic advantages in short-haul flight operation as compared to jet-driven aircraft. This particularly applies to aircraft with CROR engines. Propeller-driven aircraft, however, are also connected with general disadvantages. The excitation of vibrations in the structure of a propeller-driven aircraft, i.e. of structure-borne noise, and of air-borne noise at a frequency which is a product of the number of blades of the rotors and their rotational frequency as well as higher harmonics thereof belongs to these disadvantages.
In CROR engines so-called interaction tones provide a further source of noise. Interaction tones are generated by the interaction of different flows of both rotors, like for example of blade tip vortices and blade wakes. The frequencies of the interaction tones results from arbitrary sums of the products of the blade numbers and the rotational frequency of the rotors and of higher harmonics thereof.
Further, blades of propellers and open rotors are directly subject to any variations of the incident flow resulting from pitch or stall effects, which may result in azimuthally variable blade loads and thus also in generation of noise.
In a known engine for an aircraft, which comprises a rotor driven about a rotor axis oriented in the cruise flight direction of the aircraft, a variability of a geometric parameter which influences the propulsion of the rotor serves for having a further variable for adjusting the thrust of the engine besides the rotational frequency of the rotor. Here, the geometric parameter influencing the propulsion of the rotor is simultaneously adjusted for all rotor blades of the rotor, and this is done at a very low frequency as compared to the rotational frequency of the rotor. As a rule, the geometric parameter influencing the propulsion of the rotor is the pitch angle of the profile of the rotor blades with regard to their plane of rotation about the rotor axis.
In CROR engines it is known that some design parameters have a beneficial effect on the interaction noise generated in that the interaction of the back rotor with the blade tip vortices and the blade wakes of the front rotor is reduced. The distance between the two rotors, the load, particularly on the front rotor, a reduced diameter of the back rotor, and the rotational frequencies of the rotors or the resulting blade tip velocities belong to these design parameters. These measures for noise reduction, however, often result in a reduction of the performance and/or the efficiency of the engine. Thus, up to now, always a compromise between performance and noise emission had to be found.
A periodic variation of the pitch angle of the individual rotor blades of a rotor is known from helicopters and is a precondition for a lateral movement of helicopters having a rotor axis which is oriented essentially vertically.
US 2006/0097103 A1 discloses an integrated propulsion and guiding system for an aircraft in which the blade pitch variation principle of a helicopter is used for an engine having a rotor axis which is not oriented vertically but which is strongly inclined with regard to the cruise flight direction of the aircraft.
A dynamic pitch control for wind power plants is known from DE 202 20 134 U1. By using the signal of sensors integrated into the rotor blades of a wind power plant the energy take-up of the individual rotor blades is determined. On the basis of these data, the pitch angle, i.e. the angle of incidence, of the individual rotor blades is adjusted for homogenizing the energy take-up of all rotor blades. In this way, the torque transmitted to the drive train of the wind power plant is more uniformly distributed over the entire flow area covered by the rotor blades.
There still is a need for a engine for an aircraft, which comprises at least one rotor driven about a rotor axis oriented in the cruise flight direction of the aircraft, and in which a generation of noise which increases with the performance or efficiency of the engine or which is due to stall effects is reduced or even avoided.

SUMMARY

The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later
The present invention relates to a engine for an aircraft flying in a cruise flight direction. The engine comprises at least one rotor having a plurality of rotor blades. The at least one rotor is driven about a rotor axis at a rotational frequency; the rotor axis is oriented in the cruise flight direction of the aircraft; and at least one geometric parameter of the rotor blades that influences the propulsion characteristics of the rotor is variable. Further, the engine of the present invention comprises a controller which periodically varies the geometric parameter of the rotor blades at at least one frequency. This frequency is at least as high as the rotational frequency of the rotor.
Further, the present invention relates to a Contra Rotating Open Rotor (CROR) engine for an aircraft flying in a cruise flight direction. The CROR engine comprises a first open rotor having a first number of rotor blades, and a second open rotor having a second number of rotor blades. The first number and the second number of rotor blades are equal; the first and the second rotors are driven about a common rotor axis in opposite rotational directions at same rotational frequencies; the rotor axis is oriented in the cruise flight direction of the aircraft; the first rotor is arranged in front of the second rotor with regard to the cruise flight direction; and at least one geometric parameter of the rotor blades of the second rotor that influences the blade loading of the second rotor is variable. The CROR engine of the present invention further comprises a controller which periodically varies the geometric parameter of all blades of the second rotor at the same time and at at least one frequency depending on a relative angle position of the first rotor and the second rotor. This frequency is at least 2 n times as high as the rotational frequencies of the first and second rotors, n being the equal number of blades of the first and the second rotors.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and the detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. In the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a perspective view on a CROR engine with two rotors each having eight rotor blades.

FIG. 2 is a side view on the CROR engine according to FIG. 1; and

FIG. 3 shows a front view on a CROR according to FIGS. 1 and 2, and a plot of the unsteady blade loads over the rotation angle of the two rotors of the CROR engine.

DETAILED DESCRIPTION

In the engine of the present invention the geometric parameter of the rotor blades, which is variable, and which influences the propulsive force of the rotor is periodically varied by a controller at at least one frequency which is at least as high as the rotational frequency of the rotor. This means that, in the engine of the present invention, the variation of the geometric parameter of the rotor blades takes place much quicker as it would be necessary and useful for adjusting the rotor to different operation conditions of the engine, like for example to optimize the engine for providing different thrusts in different flight conditions of the aircraft. In the engine of the present invention, the geometric parameter of each rotor blade, which influences the propulsion characteristics of the rotor, is at least once changed and returned to its starting value over each rotation of the rotor blade about the rotor axis. The general intention of the present invention is to avoid unsteady loads on the rotor blades by which vibrations, particularly noise, may be excited. In principle, this is achieved in that different incident flows on each individual rotor blade over its various rotational positions about the rotor axis are compensated for by varying its geometric parameter in such a way that the load on the rotor blade is kept constant to an as far extent as possible.
The frequency at which the controller has to vary the geometric parameter for keeping the load constant depends on the particularities of the engine. As a rule, it is as high as the rotational frequency of the rotor or it is a plurality thereof.
Particularly, the controller may vary the geometric parameter of the individual rotor blades depending on their angle position with regard to an engine mount of the engine. The location of the engine mount by which the engine is mounted to the structure of its aircraft influences the flow incident on the rotor and particularly the distribution of different incident flows over the flow area covered by the rotor. These influences are due to the engine mount itself or due to a resulting location of the engine with regard to, for example, a wing of the aircraft. A known phenomenon in this regard is the wing box humming in propeller-driven aircraft, which is caused by the individual rotor blades passing through the flow areas in front of the high pressure side and in front of the low pressure side of the wing. By means of the present invention, this wing box humming may be reduced if not avoided completely.
It has already been mentioned that the controller preferably varies the geometric parameter in such a way that any unsteady forces to which the rotor blades are subjected are minimized. For this purpose, force sensors may be provided at the rotor blades whose signals are supplied to the controller as a value to be kept constant. In this case, the controller is a closed loop controller for keeping the force signals constant.
The controller may also be designed in such a way that it minimizes a secondary effect of the instationary incident flow on the rotor blades in that it varies the geometric parameter in such a way that excitations of air-borne or solid-borne noise by the rotor are minimized. In this case, corresponding vibration sensors for monitoring the respective noise are to be provided whose signals are supplied to the controller. Here, the controller is a closed loop controller which minimizes the supplied vibration signals.
The present invention is particularly intended for use with engines comprising open rotors. Even more particularly, the engine according to the present invention is a CROR engine having two open rotors driven in opposite directions about a common rotor axis. In this case, the controller at least varies the geometric parameter of the rotor blades of the back rotor, i.e. of the rotor located in a more downstream position with regard to the incident flow. Additionally, the controller may also vary the same or another geometric parameter of the blades of the front rotor.
Particularly advantageous conditions for applying the present invention in a CROR engine are present, if the blade numbers of both rotors are the same. In this case, the controller may vary the geometric parameter with all rotor blades of the back rotor always at the same time to compensate for a varying incident flow on the rotor blades of the back rotor.
In a CROR engine, the controller may generally vary the geometric parameter of the rotor blades of the back rotor depending on the relative angle position of both rotors, as this relative angle position determines the variation of the flow incident on the rotor blades of the back rotor by the rotor blades of the front rotor. The frequency at which the controller varies the geometric parameter influencing the propulsion characteristics in a CROR engine is preferably at least 2 n times as high as the rotational frequency of its two rotors. Here, n is the blade number of the front rotor. The absolute frequency at which the controller varies the geometric parameter of the Blades is in a typical range from 10 to 250 Hz.
The geometric parameter influencing the propulsion characteristics of the rotor may particularly be the angle of incidence of the respective rotor blade which is also designated as the pitch of the rotor blade. It may, however, also be a torsion of the respective rotor blade or the airfoil shape distribution of the respective rotor blade should this deometry of the rotor blades be dynamically variable.
In a particular embodiment of the present invention, the CROR engine is designed to be mounted in a pusher configuration to the aircraft. Here, variations of the flow incident on the rotor blades of both rotors due to, for example, a pylon to which the engine is attached may also be compensated for by the variation of the geometric parameter influencing the propulsion characteristics of the rotor blades according to the present invention to such an extent that no vibration, particularly no noise is induced by them.
Referring now in greater detail to the drawings, FIGS. 1 and 2 depict an engine 1 which is a so-called CROR engine 2. A CROR engine has two rotors 3 and 4 one arranged behind the other which are rotating in opposite directions about a common rotor axis 5. Both rotors 3 and 4 are open rotors in which the incident flow is not limited in radial direction with regard to the rotor axis 5. Here, both rotors 3 and 4 comprise a same number of eight rotor blades 6 and 7, respectively. The pitch 12 of the individual rotor blades 6 and 7 with regard to the rotational planes of the rotors 3 and 4 is variable by means of a controller 13 as indicated with rotor blade 7′. Particularly, the pitch of each rotor blade 6 and 7 may be varied for a plurality of times over each rotation of the rotors 3 and 4 about the rotor axis 5 depending on their absolute rotation angle about the rotor axis 5 and on the relative rotation angle of the two rotors 3 and 4. In this way, unsteady loads on the rotor blades 6 and 7 and resulting excitations of air-borne and solid-borne noise are reduced or even completely removed.
The CROR engine 2 depicted in FIGS. 1 and 2 is intended to be mounted to an aircraft in a pusher configuration, and is attached to the structure of the aircraft via an upstream located pylon not depicted here. By this pylon or by wakes starting from the pylon, unsteady loads on the rotor blades 6 and 7 occur at a frequency which is a product of the rotational frequency and the number of blades 6 and 7 of the rotors 3 and 4. Such unsteady loads are also known from engines having one rotor only. Due to the interaction of the flow variations generated by the two rotors 3 and 4 there are additional unsteady loads at a frequency which is 2 n-times the rotational speed of the rotors 3 and 4. These additional unsteady loads particularly occur at the rotor blades 7 of the back rotor 4 but also at the rotor blades 6 of the front rotor 3.
In FIG. 3 these unsteady loads are plotted for one rotor blade of each rotor over the rotation angle 4) about the rotation axis 5. The curve 8 indicates the load on one rotor blade 6 of the front rotor 3, whereas the curve 9 indicates the load on one rotor blade 7 of the back rotor 4. The curves 10 and 11 show the development of the average values of the curves 8 and 9. The variation of the curves 8 and 9 indicate that the unsteady loads on the blades of the back rotor 4 are higher but generally of the same frequency as the unsteady loads on the blades of the front rotor 3 which is attributable to the same number of blades of the rotors 3 and 4 here. Superimposed on the unsteady loads of this higher frequency is a variation of the loads on the blades 6 and 7 which is shown by the curves 10 and 11, and which, for example, results from a pylon to which the engine 1 is mounted or from an incident angle of the rotation axis with regard to the cruise flight direction of the entire aircraft. All unsteady parts of the loads on the blades 6 and 7 may be minimized in that a parameter influencing the propulsion characteristics of the individual rotor blades 6 and 7, respectively, such as the pitch 12 of the rotor blades according to FIG. 1, is varied by the controller 13 according to FIGS. 1 and 2 at a frequency of the unsteady components of the loads such that in case of a higher than average load on the respective blade the parameter is varied towards a smaller thrust production. This, for example means, that the pitch of the respective blade is reduced. Vice versa, in case of a lower than average load on the respective blade the parameter is varied towards a higher thrust production. This, for example means, that the pitch of the respective blade is increased.
The realization of the present invention may be based on an adjustment of the incident angle (pitch) of the rotor blades which is anyway provided with most engines. For the realization of the present invention it is, however, at least necessary that the adjustment of the pitch may be varied at a higher frequency than usual. It may also be necessary that the adjustment is possible for the individual rotor blades at different points in time. Particular in case of a CROR engine, however, there are also embodiments of the present invention in which the geometric parameter of all rotor blades of a rotor may be synchronously changed (see above). Then, the individual rotor blades do not have to be controllable with regard to their parameter varying the thrust individually.
Many variations and modifications may be made to the preferred embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention, as defined by the following claims.

Claims

1. An engine for an aircraft flying in a cruise flight direction, the engine comprising at least one rotor having a plurality of rotor blades,

the at least one rotor being driven about a rotor axis at a rotational frequency,

the rotor axis being oriented in the cruise flight direction of the aircraft,

at least one geometric parameter of the rotor blades that influences the propulsion characteristics of the rotor being variable, and

a controller which periodically varies the geometric parameter of the rotor blades at at least one frequency,

the at least one frequency being at least as high as the rotational frequency of the rotor.

2. The engine of claim 1, wherein the at least one frequency is a multiple of the rotational frequency of the rotor.

3. The engine of claim 1, wherein the controller varies the geometric parameter of the individual rotor blades depending on an angle position of the individual rotor blades relative to an engine mount of the engine.

4. The engine of claim 1, wherein the controller varies the geometric parameter in such a way that unsteady forces to which the rotor blades are subjected are minimized.

5. The engine of claim 1, wherein the controller varies the geometric parameter in such a way that excitations of air-borne noise or solid-borne noise by the rotor are minimized.

6. The engine of claim 1, wherein the at least one rotor is an open rotor.

7. The engine of claim 1, and comprising a further rotor having a plurality of rotor blades, which is arranged downstream of the at least one rotor with regard to the cruise flight direction, and which is driven in an opposite direction to the at least one rotor about the rotor axis.

8. The engine of claim 7, wherein a numbers of the blades of the further rotor and a number of the blades of the at least one rotor are equal.

9. The engine of claim 8, wherein the controller varies the geometric parameter of all blades of the at least one rotor simultaneously.

10. The engine of claim 8, wherein the controller varies the geometric parameter of all blades of the at least one rotor depending on a relative angle position of the further rotor and the at least one rotor.

11. The engine of claim 7, wherein a rotational frequency of the further rotor is equal to the rotational frequency of the at least one rotor.

12. The engine of claim 11, wherein the frequency at which the controller varies the geometric parameter of all blades of the at least one rotor is at least 2 n times as high as the rotational frequency of the at least one rotor, n being the number of blades of the front rotor.

13. The engine of claim 12, wherein the frequency at which the controller varies the geometric parameter of all blades of the at least one rotor is in a range from 10 to 250 Hz.

14. The engine of claim 7, wherein both the further rotor and the at least one rotor are open rotors.

15. The engine of claim 7, wherein the geometric parameter is selected from a group of geometric parameters including a pitch angle of the rotor blades, a torsion of the rotor blades, and a airfoil shape distribution of the rotor blades.

16. A Contra Rotating Open Rotor (CROR) engine for an aircraft flying in a cruise flight direction, the CROR engine comprising

a first open rotor having a first number of rotor blades,

a second open rotor having a second number of rotor blades,

the first number and the second number of rotor blades being equal,

the first and the second rotors being driven about a common rotor axis in opposite rotational directions at same rotational frequencies,

the rotor axis being oriented in the cruise flight direction of the aircraft,

the first rotor being arranged in front of the second rotor with regard to the cruise flight direction,

at least one geometric parameter of the rotor blades of the second rotor that influences the blade loading of the second rotor being variable, and

a controller which periodically varies the geometric parameter of all blades of the second rotor at the same time and at at least one frequency depending on a relative angle position of the first rotor and the second rotor,

the at least one frequency being at least 2 n times as high as the rotational frequencies of the first and second rotors, n being the equal number of blades of the first and the second rotors.

17. The engine of claim 16, wherein the geometric parameter is selected from a group of geometric parameters including a pitch angle of the rotor blades, a torsion of the rotor blades, and a airfoil shape distribution of the rotor blades.

18. The engine of claim 16, mounted in a pusher configuration.