Blade for a fan of a ducted engine

Abstract

The invention relates to a blade (10) for a fan (20) of a ducted engine (30) of an aircraft (100), the fan (20) being located in an engine duct (34) between an engine inlet section (32) and an engine outlet section (36), wherein the blade (10) comprises a blade body (12) having an aerodynamic shape to generate a propelling force and extending from a blade root (14) to a blade tip (16), and wherein the blade body (12) comprises an aerodynamic profile (AP) comprising a leading edge (LE) and a trailing edge (TE) defining a chord line (CHL) and further comprising an upper profile (UP) and a lower profile (LP) defining a camber line (CAL), and wherein the tangent (T) at the leading edge (LE) to the camber line (CAL) defines a leading edge direction (LED) for the leading edge (LE), the leading edge (LE) having an incidence angle (IA) to a flow direction (FD) of the ducted engine (30), characterised in that the blade body (12) comprises a tip section (17) at the blade tip (16) having the aerodynamic profile (AP) with the tip incidence angle (TIA) being smaller than an optimum incidence angle (OIA) of the aerodynamic profile (AP).

Description

[0001] The present invention is related to a blade for a fan of a ducted engine, a ducted engine for an aircraft having a fan with such blades as well as an aircraft with such ducted engines.

[0002] It is common knowledge that aircraft use engines, in particular ducted engines, to provide a propelling force for the aircraft. Those ducted engines usually are provided with one or more fans having at least two blades. By rotating those blades an airflow is propelled to exit the ducted engine at a rear end and thereby a propelling force for the aircraft moving the aircraft forward is generated. It is also common knowledge that beside the wings of the aircraft also the blades are optimized from an aerodynamic perspective. The aerodynamic optimization leads to aerodynamic profiles or aerodynamic shapes of the blades which are optimized in particular to reduce aerodynamic pressure loss. One of the key variables driving the pressure loss is the so-called incidence angle at the leading edge of the blade. A blade that is rotating inside of a ducted engine will be in contact with an airflow flowing through an engine inlet of the ducted engine. The engine inlet section is configured to ingest air, rotate the fan blades to generate an active airflow and exhaust airflow out of the engine outlet section to generate a forward movement of the aircraft. The air flows from the engine inlet section through the engine duct to the engine outlet section. Based on the desired operational conditions for the system, flow velocity triangles and blade geometries are optimized, the incidence angle , which is the angle between the relative air flow and the blade camber line at the leading edge, being one of the key design parameters.

[0003] The common approach is to optimize the blade geometry to minimize aerodynamic losses and consequently, minimize the energy consumed to perform a certain mission. However, optimizing for minimum pressure loss can lead to a reduced margin with respect to fan stall. This might be critical in largely three-dimensional intake geometries in which significant inlet flow distortion might be expected, thus generating large variations in the incidence angle experienced by one blade along each rotation. Inlet distortion can become specially large at the blade spanwise region close to the shroud surface, namely the region close to the blade tip. If part of the blade is brought into stall condition, unsteady and abrupt variations in the aerodynamic forces can be expected, potentially leading to aerodynamic and structural unstable behaviour.

[0004] It is one object of the present invention to overcome aforesaid disadvantages at least partly. In particular, it is an object of the present invention to provide a blade for a fan of a ducted engine having an increased stall margin as well as an efficient propelling functionality.

[0005] Aforesaid object is achieved by a blade with the features according to independent claim 1, a ducted engine with the features according to independent claim 9 as well as an aircraft with the features according to independent claim 14. Further features of the sub claims can be combined freely with the features of the main claims if of technical sense.

[0006] According to the present invention a blade for a fan of a ducted engine of an aircraft is provided. That fan shall be located in an engine duct between an engine inlet section and an engine outlet section. That blade comprises a blade body having an aerodynamic shape to generate a propelling force. Furthermore, that blade body extends from a blade root to a blade tip. The blade body additionally comprises an aerodynamic profile comprising a leading edge and a trailing edge defining a chord line. Further, the aerodynamic profile comprises an upper profile and a lower profile defining a camber line. Additionally, the tangent of the camber line at the leading edge defines a blade leading edge direction. The incidence angle is defined as the angle between the relative flow direction of the ducted engine and the blade leading edge direction. An inventive blade is characterised in that the blade body comprises a tip section at the blade tip having an aerodynamic profile with a tip incidence angle being smaller than an optimum incidence of the aerodynamic profile.

[0007] In general, an inventive blade is based on the aerodynamic principles known from the state of the art. To fulfil and provide the propelling force it is provided with an aerodynamic profile. The definitions of the geometry and the aerodynamic profile also follow the principles of aerodynamics. Such an aerodynamic profile can have a leading edge and a trailing edge which are connected with a straight line defining the chord line. Furthermore, the aerodynamic profile has curvature to provide the propelling force to the airflow.

[0008] According to the present invention that incidence angle at the leading edge is now configured in a new and different way compared to the commonly known solutions. In commonly known blades the incidence angle would always be configured and aligned as close as possible to an optimum incidence angle. The optimum incidence angle relates to an angle which creates as little aerodynamic losses as possible when the airflow is contacting the leading edge along the flow direction and thereby flowing along the upper profile and the lower profile generating the propelling force of the airflow. Having the defined flow direction of the engine duct and having a defined construction and position of the leading edge with a defined optimum leading edge direction the commonly known blades achieve an optimum incidence angle resulting in a minimum of aerodynamic losses as well as a maximum of efficiency for propelling.

[0009] The present invention now differs from that commonly known optimization idea and does deliberately accept a lower efficiency in exchange of a higher robustness against inlet flow distortion. To achieve this, the tip incidence angle at least at the tip section of the blade tip, in particular only at the tip section of the blade tip, differs from the optimum incidence angle by being smaller than the optimum incidence angle. In other words, the geometric configuration and / or positioning in the engine duct is "de-optimized" by having an incidence angle different from the optimum incidence angle and thereby accepting an increase in aerodynamic losses compared to a design aimed at minimum losses by selecting optimum incidence angles.

[0010] A first solution is that from a configuration perspective the aerodynamic profile is kept identical from the root section all over the blade body to the tip section. However, by tilting the aerodynamic profile at the tip section, the tangent of the camber line at the leading edge is also tilted coming closer to the flow direction of the airflow being defined by the geometric boundaries of the ducted engine. This creates a reduction of the incidence angle leaving the optimum incidence angle and resulting in a smaller tip incidence angle compared with this optimum and thus a larger margin to profile aerodynamic stall.

[0011] A preferred geometric configuration is to change the aerodynamic profile by modifying the camber line. By reducing the angle between the tangent at the leading edge and the chord line, the leading edge direction is moved closer to the flow direction of the airflow. By reducing the incidence angle compared to an optimum incidence angle, a larger stall margin is provided and therefore, the fan robustness to inlet distortion is increased.

[0012] Of course, also both possibilities can be combined meaning a rotation or tilting of the profile can be combined with the change and the amendment of the lower profile and the upper profile.

[0013] Independently from how the reduction of the incidence angle is achieved, several advantages can surprisingly result from this. Although, by actively differing from an optimum incidence angle and thereby actively accepting higher aerodynamic losses in regard to linear flow conditions and a decreased efficiency of the blade, a so-called stall margin can be increased significantly. A stall margin can be defined according to the present invention as the robustness of the blade against a stall situation. By having a tip incidence angle being smaller than an optimum incident angle, the inlet condition variation along a blade rotation is less prone to lead to a stall scenario. This means, if due to circumstances like atmospheric wind, flow detachment on the intake walls of the ducted engines or the like, highly distorted flow can potentially be encountered at the tip section of the blade, which might change the flow direction at the tip section of the blade. By that external change of the flow direction also the tip incidence angle is changed explicitly from the design tip incidence angle to the operational incident angle. In particular, having a smaller tip incidence angle compared to an optimum incident angle enables the blade to operate with a larger stall margin to the profile stall limit at a high incidence angle. The blade can be expected to have about 0 to 3 degrees more margin to the high incidence stall limit.

[0014] In other words, if due to outer circumstances the flow direction within the engine duct changes, and inventive blade responds more robustly. Commonly known blades would respond with a stall situation losing its aerodynamic effect completely or at least to a significant portion.

[0015] It can be summarized that according to the present invention the difference between the optimum incidence angle and the smaller tip incidence angle can be defined as a margin angle range against stall situations. Those stall situations may or may not apply due to external circumstances like flow distortion which is brought or generated into or inside the engine duct. By having a decreased aerodynamic efficiency and accepting increased aerodynamic losses for regular flight circumstances, the stability and robustness in running those fans with such blades is significantly increased for irregular circumstances having a change of the flow direction within the engine duct.

[0016] It can be an advantage if according to the present invention the blade is characterised in that the tip incidence angle is at least 2°, preferably 3° smaller than the optimum incidence angle. This is surprisingly a value range having an increased stability by accepting a minimum add on aerodynamic loss.

[0017] It is further an advantage if according to the present invention the blade is characterised in that the tip incidence angle is at a maximum 15°, preferably to 11° smaller than the optimum incidence angle. This is the maximum level of that range meaning that this is an upper threshold for a non-optimum but inventive tip incidence angle. Having a tip incidence angle being between 2° and 15°, preferably between 3° and 11° relative to the optimum incidence angles leads to a significantly increased stall margin by an acceptable increase of aerodynamic losses for example from a range of 10° to a range of 20° before the blade stalls. In particular this is combined and for all blades the same. Furthermore, in particular the reduction from the optimum incidence angle is only present at the tip section so that the incidence angle of the rest of the blade body is at an optimum or at least optimum value.

[0018] It is further an advantage if according to the present invention the blade is characterised in that the tip section extends over at least 15%, preferably at least 20% and / or maximum 45% preferably maximum 40% of the blade body from the blade tip. In other words, the tip section only creates a part of the blade body. Having a reduced extension of the tip section on an extension between 15% and 45% leaves the rest of the blade body in particular the root section free of this efficiency reduction. In other words, the main part of the blade body can have an aerodynamic profile with an incidence angle being close or almost identical to an optimum incidence angle. The reduction of efficiency is only applied to a tip incidence angle smaller than the optimum tip angle to a small part of the blade being the tip section having an extension less than 50% of the overall extension of the blade body from the root section to the tip section. This has surprisingly been found being an optimum between an increased stability and a still acceptable efficiency of the blade of an aerodynamic perspective.

[0019] It is further an advantage if according to the present invention the blade is characterised in that the tip section has a smaller aerodynamic surface in axial direction of the blade body than the root section of the blade body. This can also be defined as having a shorter chord line and in particular also a shorter camber line compared to the rest of the aerodynamic surface area of the blade body. In other words, the blade is getting smaller in axial direction at the tip section coming from the root to the tip of the blade body. Due to the higher rotational speed at the tip compared to the root of the blade body a reduced thickness or extension in axial direction can be accepted and is also increasing stability and efficiency of the blade.

[0020] It is further an advantage if according to the present invention the blade is characterised in that the camber line tangent at the trailing edge defines a trailing edge direction, the trailing edge having a deviation angle to a blade outlet flow direction. The deviation angle is different from an optimum deviation angle compensating at least partly the deviation angle with respect to the tip incidence angle. In other words, by changing the incidence angle from an optimum incidence angle to a tip incidence angle this also results in a change of a deviation direction at the trailing edge. By having a non-optimum correlation to the flow direction at the leading edge a change is also to be expected when this airflow leaves the trailing edge along the deviation direction which has the defined deviation angle. While in a regular aerodynamic profile the chord line and the camber line are constructed in a geometric way to have an optimum deviation angle it is also defined to have an optimum deviation direction since there a further, in particular static, components downstream of the blade. Those static components additionally have an optimum angle at which the flow direction downstream of the blade needs to address those static components. To have and keep that optimum deviation direction the present invention changes the deviation angle. While in commonly known blades the deviation direction as well as the deviation angle can freely be constructed in a way that an optimum deviation direction is combined with the optimum deviation angle, an inventive blade needs to have at least one of those optimum values changed. According to the present invention the more helpful choice is to keep the optimum deviation direction and change the deviation angle to be different from the optimum deviation angle. In other words, by allowing the deviation angle to be different from the optimum deviation angle the deviation direction can be kept at or at least closely to the optimum or desired deviation direction all despite the fact that the tip incidence angle has already been defined to be smaller than the optimum incidence angle. In other words, the change of the deviation angle at the trailing edge leads to an aerodynamic compensation of the deviation direction due to the change that happens at the trailing edge resulting from the change of the tip incidence angle at the leading edge.

[0021] It is further an advantage if according to the present invention the blade is characterised in that the blade body is twisted from the blade root to the blade tip. That blade twist means that the leading edge direction changes moving along from the root section to the tip section. In particular also the tip incidence angle might change from the root section to the tip section moving from and changing from an optimum incidence angle at the root section of the blade body to the tip incidence angle being smaller than the optimum incidence angle at the tip section.

[0022] It is further an advantage if according to the present invention the blade is characterised in that the transition from the root section to the tip section is continuous or in general continuous. As already discussed further above the rest of the blade body beside the tip section can have a different incidence angle, in particular having the optimum or at least the optimum incidence angle. By having different incidence angles all along the blade body there must be a transition between the different sections. In particular having a continuous or general continuous transition resulting in a transition without or at least without any stepwise transition resulting in further aerodynamic optimization of the blade.

[0023] It is a further object of the present invention to provide a ducted engine for an aircraft, in particular for a vertical take-off and landing aircraft. Such a ducted engine comprises an engine duct, an engine inlet section and an engine outlet section. At least one fan is positioned between the engine inlet section and the engine outlet section defining a flow direction for airflow from the engine inlet section to the engine outlet section to generate a propelling force. The fan comprises at least two blades having the features according to an inventive blade. Thereby, an inventive ducted engine comes along with the same advantages as discussed in detail with respect to the inventive blade.

[0024] It is a further of advantage if according to the present invention the ducted engine is characterised in that an inlet geometry at the inlet section is configured as a non-circular geometry. According to a preferred embodiment, a plurality of ducted engines are aligned next to each other forming one array of engines. An aerodynamic advantage can be achieved by having an increased opening square section of those inlet geometries being of non-circular geometry.

[0025] The configuration of a non-circular cross-section of an engine intake is advantageous, particularly when a large number of engines are distributed along the wing span next to each other. While such a configuration has demonstrated advantages in terms of space utilization and external aerodynamics, it has also been noted that it introduces irregular flows in the inlet area, leading to an inefficient flow to the blades.

[0026] The utilization of non-circular cross-sections in engine ducts presents a trade-off between space efficiency and aerodynamic performance. The distribution of numerous engines along the wing span necessitates unique geometric configurations to optimize space utilization. However, this approach introduces irregular flows in the inlet area, creating challenges for maintaining efficient airflow to the blades. The irregular flow patterns can lead to increased drag, reduced fuel efficiency, and potential performance degradation, especially during non-optimal conditions.

[0027] It could be found out, that the inventive blade is designed to compensate for the disadvantages introduced by irregular flows. This inventive approach focuses on making the blades less prone to stall for distorted airflow conditions in the inlet section of the duct, i.e. the blade propelling performance is ensured even in the presence of irregular flows associated with non-circular cross-sections by adjusting the stall characteristics of the blades compared to blade designs optimized for linear airflow conditions.

[0028] The implementation of blades with modified stall characteristics serves as a compensatory mechanism to address the inefficiencies introduced by irregular flows in the inlet area. Traditional fixed-blade (i.e. the pitch of the blade is constant and not variably controlled) designs may struggle to maintain optimal performance when faced with non-uniform airflow patterns.

[0029] It is further an advantage if according to the present invention the inlet geometry is different from a duct geometry at the position of the fan. This allows that the geometry changes from the inlet geometry to the duct geometry and thereby an optimization of both geometries can be provided free and separate from each other. Due to the fact that an inventive blade provides an increased stall margin due to the inventive correlation of the tip incidence angle being smaller than an optimum incidence angle an increased turbulence can be accepted at the outer end at the tip section of the blades. This leads to an increased freedom of geometric construction of the inlet geometry as well as the duct geometry.

[0030] It is further of advantage if according to the present invention the ducted engine is characterised in that the inlet geometry is rectangular or in general rectangular geometry, and / or the duct geometry is a circular or in general circular geometry. By having those two different geometries a separated optimization is achieved. Having a rectangular geometry for the inlet geometry a maximized inlet of airflow is achieved. By having a circular or in general circular geometry for the duct geometry a maximum correlation with the circular fan and the blades is achieved. The combination of those two geometries is only possibly by having the increased stall margin according to the present invention and being provided by the blades of the fans.

[0031] A further advantage can be achieved if according to the present invention the ducted engine is characterised in that degree of difference between the inlet geometry and the duct geometry correlates with the degree of difference between the tip incidence angle of the blades and the optimum incidence angle of the blades. In other words, the freedom of separation of the two different geometries at the engine inlet section and the engine duct depends on the difference and the increased stall margin at the tip incidence angle. This can be in particular a relative measurement having 0 degree of freedom at 0° of difference between the tip incidence angle and the optimum incidence angle and increasing that geometric freedom by increasing the difference.

[0032] It is a further object of the present invention to provide an aircraft, in particular vertical take-off and landing aircraft comprising at least two propulsion units with two ducted engines according to the present invention. Thereby, an inventive aircraft comes along with the advantages as discussed in detail with respect to the ducted engines as well as with respect to the blades. It has to be noted that in particular not only two of those ducted engines but multiple ducted engines are applied on multiple wings of the aircraft. The present invention is further described in relation to their accompanying drawings. Those show schematically: Fig. 1a picture of a fan with inventive blades, Fig. 2a picture of an aerodynamic profile, Fig. 3a tilted aerodynamic profile, Fig. 4a further aerodynamic profile, Fig. 5a shorter aerodynamic profile, Fig. 6the shorter profile of figure 5 in a tilted position, Fig. 7a square section of a ducted engine, Fig. 8a geometry change from an inlet geometry to an outlet geometry and Fig. 9an inventive aircraft, Fig. 10a diagram showing different incidence angles.

[0033] Figure 1 shows schematically the definitions at a fan 20. For example, this fan 20 is provided with two blades 10 being attached at a central axis defining an axial direction AD with their blade roots 14. The blade bodies 12 extend from the blade root 14 along a root section 15 to the tip section 17 at the tip 16 of the blade body 12. Those parts and body definitions of the blade 10 will be used later on in particular in definition of the aerodynamic profiles AP. Figure 1 shows on the right blade 10 a cut showing a schematical version of the aerodynamic profile AP of the blade 10 which is discussed in more detail with respect to the following figures.

[0034] Figure 2 shows a square section of the blade 10 at the blade body 12. It has an aerodynamic profile AP with an upper profile UP and a lower profile LP. The middle line between the upper profile UP and the lower profile LP is defined to be the camber line CAL. The aerodynamic profile AP further extends from a leading edge LE to a trailing edge TE. The direct line connection between the leading edge LE and the trailing edge TE is defined to be the chord line CHL.

[0035] By having a tangent T applied to the camber line CAL and fixed at the leading edge LE the leading edge direction LED can be defined. By having the blade 12 of a fan 20 positioned inside of an engine duct 34, as for example depicted in figure 7, the boundaries of the engine duct 34 also define a flow direction FD of an airflow through the engine duct 34. That flow direction FD is thereby defined from the outer boundaries and the attack angle at the leading edge LE which is defined to be the incidence angle IA is dependent from the flow direction FD and thereby from the boundaries on one hand and on the geometry as well as the orientation of the blade 10 in particular the leading edge direction LED on the other hand.

[0036] Figure 3 shows the situation where the tip incidence angle TIA has been changed from the optimum incidence angle OIA. Where for example figure 2 shows a situation for example at the root section 15 of the blade body 12 this can be an incidence angle IA being at or at least at an optimum incidence angle OIA. At the tip section 17 now the same aerodynamic profile AP is used for the blade body 12. However, the aerodynamic profile AP has been tilted counter clockwise compared to figure 2. This counter clockwise tilt leads to a change of the leading edge direction LED since the camber line CAL has also been tilted together with the aerodynamic profile AP. Due to the fact that the flow direction FD is still defined by the unchanged outer boundaries of the engine duct 34 it remains unchanged. By changing the leading edge direction LED coming closer to the non-changed flow direction FD the optimal flow direction OFD is no longer aligned with the real flow direction FD. This results in a situation where the exact tip incidence angle TIA is smaller than the also depicted optimum incidence angle OIA, which would for example be present at the root section in figure 2.

[0037] The situation in figure 3 leads to a situation where the flow direction FD now attacks the leading edge LE at a tip incidence angle TIA being smaller than the optimum incidence angle OIA. Although this results in an increased aerodynamic loss of the airflow along the aerodynamic profile AP an increased stall margin is achieved. In particular now if turbulences occur at the tip section 17 the flow direction FD might change due to that turbulence. The margin between the real flow direction FD and the optimum flow direction OFD can be defined as a direction margin allowing a change of flow direction FD without any negative effect or in particular without any stall releasing flow from contacting the lower profile LP of the aerodynamic profile AP.

[0038] In figure 4 a further advantage of the present invention is depicted. Here the focus is on the trailing edge TE. At the trailing edge TE of the aerodynamic profile AP a tangent T at the trailing edge TE to the camber line CAL defines a trailing edge direction TED. That trailing edge direction TED has an optimum deviation angle ODA since the airflow being released from the aerodynamic profile is deviated from that tangent by a defined angle. Figure 4 shows a situation with an optimum deviation angle ODA resulting in a desired deviation direction DDD which is the desired direction of the airflow. In figure 5 the aerodynamic profile AP has been shrunken to a shorter extension. To do so the camber line CAL has a significantly reduced radius at the trailing edge TE and thereby the desired deviation direction DDD has been moved closer to the trailing edge direction TED resulting in a reduced deviation angle DA being different from the optimum deviation angle ODA. In the next step and shown in figure 6 during construction the situation according to figure 5 is tilted to have a tip incidence angle TIA being smaller than the optimum incidence angle OIA. By that tilt now the reduced deviation angle DA is remained but the real deviation direction DD is now brought and aligned again with the desired deviation direction DDD as it can be seen in figure 1. By now accepting a non-optimum deviation angle ODA a part compensation can be achieved and thereby having the deviation direction DD realigned with the desired deviation direction DDD.

[0039] Figure 7 shows in general a side view of a square section of the ducted engine 30. Air is sucked into an engine inlet section 32 and flows along a flow direction FD through the engine duct 34. While arriving from an inlet geometry IG a further bigger freedom of the inlet geometry IG is achieved so that in this case a more or less rectangular shape of the inlet geometry IG is combined with a more or less circular shape with the duct geometry DG.

[0040] In figure 9 an example of an inventive aircraft 100 is shown having four wings 110. Each of these wings 110 comprises multiple ducted engines 30 where in each of ducted engines 30 comprises a fan 20 with inventive blades 10.

[0041] Figure 10 shows a diagram for the effect of different tip incidence angles TIA. In particular, the x-axis displays the tip incidence angle TIA and the y-axis the corresponding pressure loss. As it can be seen, there is a flat plateau between the stall situations at low and high incidence angles. Within that plateau an optimum can be found for minimum total pressure loss. The present invention now differs the chosen tip incidence angle TIA from such an optimum incidence angle OIA. While this results in a minor increase in total pressure loss, the margin to stall in this example is increased. This can be derived from the diagram directly since the distance between the selected tip incidence angle TIP to the stall at high incidence is increased for example by about additional 10° compared to the optimum incidence angle OIA. Moving away from the optimum pressure loss results in increased distance to stall points in this diagram and thus result in higher stability.

[0042] Aforesaid description of the examples describes the present invention only by way of examples and do not limit its scope.Reference signs

[0043] 10blade 12blade body 14blade root 15root section 16blade tip 17tip section 20fan 30ducted engine 32engine inlet section 34engine duct 36engine outlet section 100aircraft 110wing APaerodynamic profile UPupper profile LPlower profile LEleading edge TEtrailing edge CHLchord line CALcamber line Ttangent LEDleading edge direction TEDtrailing edge direction IAincidence angle TIAtip incidence angle OIAoptimum incidence angle DAdeviation angle ODAoptimum deviation angle ADaxial direction IGinlet geometry DGduct geometry FDflow direction OFDoptimum flow direction DDdeviation direction DDDdesired deviation direction

Claims

1. Blade (10) for a fan (20) of a ducted engine (30) of an aircraft (100), the fan (20) being located in an engine duct (34) between an engine inlet section (32) and an engine outlet section (36), wherein the blade (10) comprises a blade body (12) having an aerodynamic shape to generate a propelling force and extending from a blade root (14) to a blade tip (16), and wherein the blade body (12) comprises an aerodynamic profile (AP) comprising a leading edge (LE) and a trailing edge (TE) defining a chord line (CHL) and further comprising an upper profile (UP) and a lower profile (LP) defining a camber line (CAL), and wherein the tangent (T) at the leading edge (LE) to the camber line (CAL) defines a leading edge direction (LED) for the leading edge (LE), the leading edge (LE) having an incidence angle (IA) to a flow direction (FD) of the ducted engine (30), characterised in that the blade body (12) comprises a tip section (17) at the blade tip (16) having the aerodynamic profile (AP) with the tip incidence angle (TIA) being smaller than an optimum incidence angle (OIA) of the aerodynamic profile (AP).

2. Blade (10) according to claim 1, characterised in that the tip incidence angle (TIA) is at least 2°, preferably 3° smaller than the optimum incidence angle (OIA).

3. Blade (10) according to any of the preceding claims, characterised in that the tip incidence angle (TIA) is at a maximum 15°, preferably 11° smaller than the optimum incidence angle (OIA).

4. Blade (10) according to any of the preceding claims, characterised in that the tip section (17) extends over at least 15%, preferably at least 20%, and / or maximum 45%, preferably maximum 40% of the blade body (12) from the blade tip (16).

5. Blade (10) according to any of the preceding claims, characterised in that the tip section (17) has a smaller aerodynamic surface in axial direction (AD) of the blade body (12) than a root section (15) of the blade body (12).

6. Blade (10) according to any of the preceding claims, characterised in that the tangent (T) at the trailing edge (TE) defines a trailing edge direction (TED), the trailing edge (TE) having a deviation angle (DA) to a deviation direction (DD) of the ducted engine (30), wherein the deviation angle (DA) is different from an optimum deviation angle (ODA) compensating at least partly the deviation direction (DA) with respect to the tip incidence angle (TIP).

7. Blade (10) according to any of the preceding claims, characterised in that the blade body (12) is twisted from the blade root (14) to the blade tip (16).

8. Blade (10) according to any of the preceding claims, characterised in that the transition from the root section (15) to tip section (17) is continuous or in general continuous.

9. Ducted engine (30) for an aircraft, in particular a vertical take-off and landing aircraft (100), comprising an engine duct (34), an engine inlet section (32) and an engine outlet section (36), wherein at least one fan (20) is positioned between the engine inlet section (32) and the engine outlet section (36) defining a flow direction (FD) for airflow from the engine inlet section (32) to the engine outlet section (36) to generate a propelling force, and wherein the fan (20) comprises at least two blades (10) with the features of any of claims 1 to 8.

10. Ducted engine (30) according to claim 9, characterised in that an inlet geometry (IG) at the engine inlet section (32) is configured as a non-circular geometry.

11. Ducted engine (30) according to claim 10, characterised in that the inlet geometry (IG) is different from a duct geometry (DG) at the position of the fan (20).

12. Ducted engine (30) according to claims 10 or 11, characterised in that the inlet geometry (IG) is a rectangular or in general rectangular geometry, and / or the duct geometry (DG) is a circular or in general circular geometry.

13. Ducted engine (30) according to any of claims 9 to 12, characterised in that the degree of difference between the inlet geometry (IG) and the duct geometry (DG) corelates with the degree of difference between the tip incidence angle (TIP) of the blades (10) and the optimum incidence angle (OIA) of the blades (10).

14. Aircraft (100), in particular a vertical take-off and landing aircraft (100), comprising at least two ducted engines (30) with the features of any of claims 9 to 13.

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