AIRCRAFT HAVING A DRAG COMPENSATION DEVICE BASED ON A BOUNDARY LAYER INGESTING FAN

Abstract

An aircraft includes a fuselage having a tapered rear shape, a landing gear for moving the aircraft on a runway, a wing attached to the fuselage, at least a main engine for providing a main thrust and a rear fan, wherein the rear fan is attached to a tail section of the fuselage, wherein the aircraft is designed for conducting a take-off rotation around the landing gear during take-off from the runway, such that the tail section of the fuselage approaches the runway, wherein the rear fan is an open fan having fan blades extending in a radial direction to a longitudinal axis of the fuselage, wherein the fan blades are dimensioned to equal at least a boundary layer thickness of the flow along the fuselage and to be smaller than the gap between the runway and the tail section of the fuselage during the take-off rotation.

Claims

1. An aircraft comprising: a fuselage having a tapered rear shape; a wing attached to the fuselage; and a main landing gear for moving the aircraft on a runway, at least one main engine for providing a main thrust and a rear fan, wherein the rear fan is attached to a tail section of the fuselage, wherein the aircraft is configured for conducting a take-off rotation around the main landing gear during take-off from the runway, such that the tail section of the fuselage approaches the runway, and wherein the rear fan is an open fan having fan blades extending in a radial direction to a longitudinal axis of the fuselage, wherein the fan blades are dimensioned to equal at least the thickness of a boundary layer of the flow along the fuselage and to be smaller than a gap between the runway and the tail section of the fuselage during the take-off rotation.

2. The aircraft of claim 1, wherein the fan blades comprise a plurality of local blade incidence angles, which evolve from a radial inward section of the fan blades in a radial outward direction to adapt to the velocity profile of the boundary layer of the fuselage.

3. The aircraft of claim 2, wherein the local blade incidence angles at an outer diameter of the rear fan are lower than at an inner diameter of the rear fan.

4. The aircraft of claim 1, wherein the rear fan is connected to a drive unit adapted to rotate the rear fan at least with a first rotational speed.

5. The aircraft of claim 4, wherein the first rotational speed is chosen such that the rear fan produces a thrust force compensating the viscous drag of the fuselage at a maximum.

6. The aircraft of claim 4, wherein the first rotational speed is configured for a first cruise velocity at a first cruise altitude.

7. The aircraft of claim 4, wherein the drive unit is additionally adapted to rotate the rear fan at least a second rotational speed for at least a second flight state.

8. The aircraft of claim 1, further comprising a first control unit coupled with the drive unit, wherein the first control unit is adapted for influencing the rotational speed of the drive unit at an output coupled with the rear fan.

9. The aircraft of claim 1, wherein the drive unit is a torque transfer means between the at least one main engine and the rear fan.

10. The aircraft of claim 1, wherein the rear fan comprises variable pitch fan blades rotatable around a longitudinal blade axis so as to adjust the general pitch of the respective fan blade.

11. The aircraft of claim 10, further comprising a second control unit coupled with an actuator for adjusting the pitch angle of the variable pitch fan blades, wherein the second control unit is adapted for adjusting the pitch angle according to the flight state of the aircraft.

12. The aircraft of claim 10, wherein the rear fan is adapted to create negative thrust by providing a negative fan blade pitch angle

13. The aircraft of claim 10, wherein the fan blade pitch angle is configured such that the rear fan produces a thrust force compensating the viscous drag of the fuselage at a maximum.

14. A method for compensating the viscous drag of a fuselage of an aircraft, at least during cruise flight conditions, the method comprising: providing an open air fan attached to a tail section of the fuselage, wherein the aircraft has a tapered rear shape, a main landing gear for moving the aircraft on a runway, a wing attached to the fuselage, at least a main engine for providing a main thrust and a rear fan, wherein the aircraft is configured for conducting a take-off rotation around the landing gear during take-off from the runway, such that the tail section of the fuselage approaches the runway, and wherein the rear fan comprises fan blades of the open air fan, the fan blades extending in a radial direction to a longitudinal axis of the fuselage, wherein the fan blades are dimensioned to equal at least a boundary layer thickness of the flow along the fuselage and to be smaller than the gap between the runway and the tail section of the fuselage during the take-off rotation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Other characteristics, advantages and potential applications of the present invention result from the following description of the exemplary embodiments illustrated in the figures. In this respect, all described and/or graphically illustrated characteristics also form the object of the invention individually and in arbitrary combination regardless of their composition in the individual claims or their references to other claims. Furthermore, identical or similar objects are identified by the same reference symbols in the figures.

[0027] FIG. 1 shows an exemplary embodiment of the aircraft in a lateral view.

[0028] FIGS. 2 and 3 show basic illustrations of rear fans in lateral views.

[0029] FIGS. 4a and 4b show examples of general principles of coupling a rear fan to a source of energy in schematic views.

DETAILED DESCRIPTION

[0030] FIG. 1 shows an aircraft 2 having a fuselage 4, a wing 6, a main landing gear 8, a nose landing gear 10, a tail section 12 and a rear fan 14. The fuselage 4 may consist of one or more central portions having constant cross sections as well as a tapered section at a nose and a tail. The fuselage 4 may be designed as a cylindrical (tubular) fuselage with a main or longitudinal extension axis 16.

[0031] The term cylindrical does not necessarily require a circular cross-section, but merely expresses an elongate shape along the longitudinal axis 16 with a cross section that remains unchanged along a major section of the fuselage 4. Commercial aircraft are known having a circular cross-section, while others may tend to be more oval, i.e. having a slightly unequal relationship between height and width of the fuselage. Other commercial aircraft are known having a clearly oval cross-section at least in a certain region. These are not ruled out by the term cylindrical.

[0032] The wing 6 is arranged on a substantially central section of the fuselage 4 and carries exemplarily two main engines 18. Exemplarily underneath the wing 6, the main landing gear 8 is arranged, which is designed for absorbing a landing shock of the aircraft 2 through a dedicated shock absorber (not shown). The landing gear 8 may also be arranged at other installation position close to a center of gravity of the aircraft at the wing 6, the fuselage 4 or devices therebetween.

[0033] During take-off and landing, the aircraft 2 conducts a rotation around the main landing gear 8, before lifting off or after touchdown. During such a rotation, the tail section 12 approaches the runway 20. The latter is conducted in order to increase the aerodynamic lift during take-off and landing, such that the aircraft attitude is rotated to point nose-up. In this condition the rear fuselage is closest to the runway but not touching it.

[0034] The tail section 12 comprises a sweeping region 22, at which an underside 24 of the fuselage 4 makes a transition between a horizontal orientation (in front of the sweeping region 22) and an almost angular upward oriented region 26 (behind the sweeping region 22). During a take-off rotation, the sweeping region 22 assumes a minimum distance to the runway 20, wherein the distance of the angular oriented region 26 exemplarily increases in a rearward direction. This is indicated by a dashed contact line 28, which resembles a rotated fuselage 4 on contact with the runway 20. It is apparent that a rear end 30 of the tail section 12 may comprise a certain free space, into which the rear fan 14 is integratable.

[0035] The rear fan 14 may be connected to a drive unit 32, which is in this example placed forward of the rear fan 14. The drive unit 32 is exemplarily realized as an electric motor (not shown), which is connected to at least one electrical system of the aircraft 2.

[0036] The fuselage 4 depicted in FIG. 1 may comprise a length of approximately 40 m, which leads to an estimated thickness of a boundary layer 34 of approximately 40 cm over a constant fuselage cross-section during cruise flight conditions and more over a tapered rear end. However, this may be a rather rough estimation and is not to be considered an accurate measure.

[0037] FIG. 2 shows a very basic illustration of the rear fan 14 in a lateral view. The rear fan 14 in general is designed for ingesting the boundary layer 34 that passes the tail section 12 at the rear end 30. Hence, blades 36 of the rear fan 14 comprise a length 1 that is at least equal to the thickness of the boundary layer 34. In this case, the length 1 of the fan blades 36 may exemplarily be 40 cm or more. For the sake of providing a certain safety factor, the length 1 of the fan blades 36 may be double the size of the thickness of the boundary layer 34.

[0038] Depending on the size of the rear fan 14, including the number of fan blades 36 and the design of the rear end 30 of the aircraft 2 the rear fan 14 may comprise an end cap 38 arranged on or being an integral part of a hub for holding the fan blades 36.

[0039] FIG. 3 shows another example of a rear fan 40, which comprises a variable pitch. For this purpose, fan blades 42 are supported so as to be rotatable around their longitudinal axes 44. To maintain a smooth operation, the individual pitch angles of all fan blades 44 are coupled. The rotation of the fan blades 42 is conducted through an actuator, which is not shown in this Figure. The actuator may be integrated into the hub of the rear fan 40 or may be arranged in the fuselage 4 coupled with the fan blades 42 through a linkage (not shown).

[0040] FIG. 4a shows a first example of a general principle of coupling a rear fan 14 with a source of energy in the aircraft 2. In this example, the rear fan 14 has constant pitch fan blades 36. The rear fan 14 is mechanically connected to the drive unit 32, which in turn is coupled with an electrical network 46 illustrated as a box for simplification. The electrical network 46 is fed with electrical energy through generators 47 integrated into the main engines 18. Of course, the generators 47 are merely an example and further sources of electrical power are conceivable, such as fuel cells, batteries and other elements. The drive unit 32 is connected to the electrical network 46 through an electrical line 48, which extends to the rear end 30 of the fuselage 4.

[0041] The drive unit 32 may comprise a power electronics unit 52, which processes the voltage and the current supplied by the electrical network 46 into a required form suitable for operating the drive unit 32. The power electronics unit 52 may be controlled through a first control unit 50 in order to adjust the drive unit 32 to a desired rotational speed. The first control unit 50 may be coupled with an integrated flight control unit of the aircraft 2, a flight management system or a user input device accessible by an aircraft operator in the cockpit of the aircraft 2. Hence, the rear fan 14 may be set into operation once a certain flight state is reached with a certain rotational speed to compensate the viscous drag of the fuselage 4. During the operation of the rear fan 14 the rotational speed may be adjusted to the actual flight state, i.e. it may be increased or decreased inter alia depending on the flight velocity and altitude.

[0042] FIG. 4b shows a second example of a general principle of coupling a rear fan 40 with a source of energy in the aircraft 2. Here, a rear fan 40 with a variable pitch is used. The primary source of energy in this case is a main engine 18, which is coupled with the rear fan 40 through a torque transfer means 54. This may be a set of shafts and links, which extend from the respective main engine 18 to the rear fan 40. A second control unit 56 may be connected to an actuator (not shown) of the rear fan 40 in order to adapt the pitch angle of the fan blades 42 according to the actual flight state of the aircraft 2 and under consideration of a substantially constant rotational speed of the rear fan 40.

[0043] The second control unit 56 may be adapted to influence the pitch angle of the fan blades 42 such that after landing a reverse thrust may be created to support the brakes of the aircraft 2.

[0044] In addition, it should be pointed out that comprising does not exclude other elements or steps, and a or an does not exclude a plural number. Furthermore, it should be pointed out that characteristics or steps which have been described with reference to one of the above exemplary embodiments may also be used in combination with other characteristics or steps of other exemplary embodiments described above. Reference characters in the claims are not to be interpreted as limitations.

[0045] While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms comprise or comprising do not exclude other elements or steps, the terms a or one do not exclude a plural number, and the term or means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.