VTOL aircraft using rotors to simulate rigid wing aerodynamics

Abstract

A vertical takeoff and landing aircraft uses fixed rotors for both VTOL and forward flight operations. The rotors form a synthetic wing and are positioned to achieve high wing span efficiency. The rotors are positioned to shed lift evenly across the synthetic wing span. The synthetic wing can also have narrow forward and aft airfoils that can provide structural support as well as provide lift during forward flight. The wing rotors are tilted forward and provide some forward propulsion during horizontal flight.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 62 / 416,168, filed November 2, 2017, by Mikic et al., which is incorporated herein by reference in its entirety. Technical Field

[0003] This invention relates to aircraft vehicles, specifically an aircraft vehicle that uses rotors to simulate the aerodynamics of a rigid wing. Attached Figure Description

[0004] Figures 1A-1D This is a shadow rendering of an aircraft with a composite wing using stacked counter-rotating thrusters, according to some embodiments of the present invention.

[0005] Figures 2A-2D This is a drawing of an aircraft with a composite wing using a stacked counter-rotating propulsion system, according to some embodiments of the present invention.

[0006] Figure 3 This is a solution diagram of an aircraft flying forward according to an embodiment of the present invention.

[0007] Figures 4A-4B This is a shadow rendering of an aircraft with synthetic wings according to some embodiments of the present invention.

[0008] Figures 5A-5C These are drawings of aircraft with synthetic wings according to some embodiments of the present invention.

[0009] Figures 6A-6G These are lift and drag curves for various parameter values.

[0010] Figure 7 This is a view of an aircraft having a composite wing using longitudinally staggered rotors, according to some embodiments of the present invention.

[0011] Figure 8 This is a view of an aircraft having a composite wing using vertically interleaved rotors, according to some embodiments of the present invention.

[0012] Figures 9A-9E This is a schematic diagram illustrating the uniformity of lift for different vehicle configurations.

[0013] Figures 10A-10B This is a view of a rotor according to some embodiments of the present invention.

[0014] Figures 11A-11C This is a view of an aircraft according to some embodiments of the present invention. Summary of the Invention

[0015] A vertical takeoff and landing (VTOL) aircraft uses a fixed rotor for both VTOL and forward flight operations. The rotor forms a composite wing and is positioned to achieve high span efficiency. The rotor is positioned to uniformly reduce lift across the span of the composite wing. The composite wing may also have narrow front and rear airfoils, which provide structural support and lift during forward flight. The wing rotor is tilted forward and provides some forward thrust during level flight. Detailed Implementation

[0016] In some embodiments of the invention, an aircraft has an array of rotors configured to function as a composite wing or pseudo-wing. The rotors are positioned relative to the aircraft body and relative to each other, such that the lift of the rotor array is relatively uniform along the wingspan of the pseudo-wing. The composite wing produces (relatively) uniform lift and relatively uniform tail vertical velocity (downslip). Shed vorticity converges at the wingtip, similar to a conventional airfoil.

[0017] In some embodiments, the rotors are fixedly mounted to a structure coupled to the body of the aircraft. The rotors may be staggered along the longitudinal axis of the aircraft, such that differential thrusting along the longitudinal axis allows for forward pitching of the vehicle to provide a forward thrust component from the rotors. Pitch control may also be achieved using other means, such as elevator control. In some embodiments, the rotors may have several descriptive aspects.

[0018] In some embodiments, the rotor array constituting the composite wing may have a short-chord forward wing in front of the rotor. Since the aircraft uses the rotor for vertical thrust during takeoff, it does not require significant lift from a conventional wing during takeoff operations. During higher airspeed operations, the short-chord forward wing allows for lift during forward flight. The aircraft may also have a short rearward wing behind the rotor. A rearward wing with a short chord can provide lift during higher-speed forward flight operations. The rearward wing can also further even out the downswing of the pseudowing. Both the forward and rearward wings can serve as structural elements supporting struts that support the rotor. The forward and rearward wings can be joined at the outer ends, allowing for increased strength and stability. Additionally, the forward and rearward wings, located around the outer (horizontal) perimeter of the rotor array, provide elements of safety for users and ground personnel as a protective enclosure around the rotor blades.

[0019] In some embodiments, only a tail wing may exist. In some embodiments, only a leading-edge wing may exist. In some embodiments, both a leading-edge wing and a tail wing may exist.

[0020] In some embodiments, one or more rotors are used to provide forward thrust. In some aspects, a single propeller rotor may be used at the rear of the aircraft. In some embodiments, the axis of rotation of the wing rotor may be tilted relative to vertical. In some embodiments, the wing rotor may be tilted forward. The forward-tilted wing rotor contributes to the forward thrust of the aircraft during forward flight. In some embodiments, the total forward thrust during forward flight comes from a combination of the forward-tilted wing rotor and one or more conventional rotors (whose axes of rotation are parallel to the roll axis of the aircraft).

[0021] An aircraft can be constructed such that, during forward flight, both the wings and the wing rotor provide lift, and forward propulsion is provided by a tail thruster and a forward-tilted wing rotor.

[0022] While balancing other aircraft parameters, the efficiency (lift / drag ratio) of the aircraft during forward flight can be sought to be as high as reasonably possible. For example, the ratio of the tip velocity of the rotating wing rotor to the velocity of the aircraft during forward flight is an important ratio to consider when designing for efficiency. Furthermore, the percentage of lift provided by the wing rotor relative to the total lift (wing plus wing rotor) is an important ratio to consider when designing for efficiency. Additionally, the ratio of the power distribution between the wing rotors to the total power delivered to both the wing rotor and the conventional forward thrust propulsion unit (e.g., the tail rotor) is an important ratio to consider when designing for efficiency. The interactions between these ratios can be complex and not necessarily conducive to obvious optimization. A potential design parameter is that the wing rotor provides sufficient vertical thrust during vertical takeoff and landing to enable the aircraft to take off and land safely.

[0023] The aircraft according to embodiments of the invention is suitable for vertical takeoff and landing using wing rotors. Regarding takeoff, the wing rotor rotates to a speed within its hovering rpm range. In embodiments where the wing rotor tilts its axis of rotation forward relative to the horizontal axis of the aircraft, the aircraft will tilt upwards during takeoff and landing, so that the plane of the wing rotor is horizontal. After takeoff, the aircraft can cause a forward-pushing (or pulling) propeller (e.g., a tail rotor) to rotate, while the wing rotor remains underpowered. As the aircraft gains forward speed, the front and tail wings generate lift, and the proportion of lift provided by the wing rotor decreases. As the aircraft approaches its cruising speed, the wing rotor typically causes its rpm to decrease, but it remains underpowered and provides a portion of the aircraft's total lift.

[0024] An aircraft according to embodiments of the invention will be capable of hovering, takeoff, landing, and forward flight, including all required maneuvers and attitude adjustments by manipulating the speed of individual rotors without requiring any additional control surfaces. In some embodiments, the aircraft will have no ailerons, elevators, or any other controllable control surfaces. In some embodiments, the wing rotors for each wing are arranged along a linear straight line or along a curved line, wherein each rotor is further away from the aircraft body. The wing rotors may also be in a forward-swept or backward-swept configuration, such that the rotors are located at different stations along the aircraft's roll axis. By using differential stations along the roll axis, the wing rotors are able to provide control about the pitch axis. By being spaced at a distance further away from the aircraft body, the wing rotors are able to provide control about the roll axis.

[0025] In some embodiments, such as Figures 1A-1D Shadow rendering and Figures 2A-2D As can be seen in the line drawing, the aircraft 100 has a body 101 coupled to the right composite wing 102 and the left composite wing 103 using stacked counter-rotating propulsion units. In this embodiment, multiple rotor assemblies 104 are positioned along the wingspan via a short-chord forewing wing 107 and a short-chord tailwing wing 108. The rotor assemblies 104 are mounted on a wingspan support 109, which is coupled to the forewing wing 107 and the tail wing 108. A wingtip section 110 joins the forewing wing 107 to the tail wing 108.

[0026] The rotor assembly 104 may have a first thruster 105 and a second thruster 106. In some respects, the first thruster 105 and the second thruster 106 rotate in opposite directions. In this configuration, the leading edge of one thruster travels forward on the outer side of the rotor's axis of rotation, while the leading edge of the other thruster travels forward on the inner side of the thruster's axis of rotation. In forward flight mode, the thruster blades will have a higher thrust than they would have when moving forward into the dominant airspeed wind relative to the prevailing airspeed wind. By counter-rotating the coaxial thrusters, the downward thrust on the rotor shaft, both inside and outside, is evenly distributed.

[0027] In an exemplary embodiment, the aircraft can accommodate one passenger and has a takeoff mass of 315 kg. The wingspan can be 6 m, and the length of the vehicle can be 3.5 m with a height of 1.5 m. The pseudo-wing can consist of four coaxial rotor assemblies, each with a pair of counter-rotating thrusters. Each thruster has three blades, with the blade chord at 75% of its radius of 0.05 m. The aircraft has a short-chord canard with a chord of 0.5 m and a short-chord aft wing with a chord of 0.75 m. The ideal cruise speed is 100 km / h at a maximum cruise speed of 150 km / h. The power consumption during ideal cruise is 20 kW, and the power consumption in hover mode is 80 kW. The battery mass is 50 kg, and the range of the vehicle is 50 miles.

[0028] One aspect of the composite wing is that the rotors are configured to provide relatively uniform thrust along the wingspan of the composite wing. One method, as described above, involves placing counter-rotating thrusters on the rotor assembly adjacent to each other along the wingspan of the pseudo-wing. Another method, without using coaxial counter-rotating thrusters, involves having thrusters with overlapping backward and forward blade regions. For example, a rotor with forward-moving blades on the outboard side and backward-moving blades on the inboard side could be placed. Exactly within the rotor could be another rotor with its forward-moving blades on the outboard side. The rotors can be vertically spaced to allow this overlap. In some respects, the rotors can be spaced along the longitudinal axis to allow this overlap.

[0029] Figure 3 An aircraft with three counter-rotating rotor assemblies on each side of the vehicle body is used as an exemplary embodiment to illustrate the functional aspects of a composite wing. The left side of the figure shows the vertical velocity behind the wing, and the right side of the figure shows the magnitude of vorticity. The composite wing is designed to address the variation in lift reduction along the wingspan of the composite wing. By modeling the vertical velocity behind the wing, the variation in lift along the wingspan can become apparent. The left side of the figure shows the relatively stable amount of vertical velocity along the wingspan, demonstrating that the composite wing using three counter-rotating rotor assemblies operates aerodynamically as if it were a conventional wing constructed with a solid airfoil shape.

[0030] The right side of the attached diagram shows the magnitudes of vorticity along the wingspan and at the wingtip. As mentioned above, the uniformity of the vertical velocity along the wingspan, as a first approximation, should indicate the symmetrical uniformity of the vorticity magnitude. When using a rotor arrayed into a composite wing, there is a real possibility of points introducing high vorticity along the wingspan. The rotor position is crucial for minimizing the magnitude of the outflow vorticity along the wingspan.

[0031] Although Figure 3The results shown represent the output of the modeled synthetic wing, but it should be understood that there are variations in the output that are related to the temporal changes in blade position.

[0032] In some embodiments, such as Figures 4A-4B In shadow rendering and Figures 5A-5C As seen in the online diagram, the aircraft 200 has a body 201 using wing-mounted rotors coupled to a right composite wing 202 and a left composite wing 203. In this exemplary embodiment, multiple wing-rotor assemblies 204 are positioned along the wingspan via a short-chord forewing wing 207 and a short-chord tailwing wing 208. The wing-rotor assemblies 204 are mounted on a wingspan support 209, which is coupled to the forewing wing 207 and the tail wing 208. A wingtip section 210 joins the forewing wing 207 to the tail wing 208. The aircraft 200 does not use stacked rotor assemblies rotating in opposite directions. While stacked assemblies can have unique advantages with respect to load distribution, in some embodiments where the percentage of lift delivered by the rotor during level flight is lower, efficient flight can be achieved with a single rotor (e.g., by the aircraft 200). Furthermore, a single rotor allows for reduced cost and complexity.

[0033] In some aspects, the wing-rotor assembly 204 is not perpendicular to the constant-altitude cruise plane of the aircraft, but is tilted forward at an angle 212. In some aspects, the rotor is tilted forward in the range of 5-20 degrees. In some aspects, the rotor is tilted forward in the range of 5-15 degrees. In some aspects, the rotor is tilted forward in the range of 8-20 degrees. In some aspects, the rotor is tilted forward in the range of 8-12 degrees. In an exemplary embodiment, the rotor is tilted at 10 degrees. The tilt angle can be defined as the angle between the rotor axis and the mean aerodynamic chord of the maximum wing (which in some embodiments may be the tail wing).

[0034] Aircraft 200 has a horizontally driven rear rotor assembly 211 adapted to provide horizontal thrust during normal flight. Details of the size and configuration of an exemplary embodiment of aircraft 200 are shown in Table 1. Figures AB show a rotor 204 having its central hub 221 and blades 220; the central hub 221 may include an electric motor.

[0035] In some aspects, the wing-rotor assembly 204 has a rotary propulsion unit with an electric motor. In some aspects, the horizontally driven rear rotor assembly 211 has an electric motor. In some aspects, the motor is powered by a power source such as one or more batteries.

[0036]

[0037]

[0038] Table 1

[0039] The use of a forward-tilted wing rotor assembly allows forward propulsion to be provided partly by the wing rotor assembly and partly by a conventional horizontally driven thruster. A factor to consider when sharing this forward propulsion is the fraction of the total power delivered to the wing rotor during forward cruise relative to the total power delivered to both the wing rotor and the conventional thruster.

[0040] Another factor to consider is the fraction of lift carried by the wing rotor during forward flight relative to the total lift provided by both the wing and the wing rotor. In embodiments where all attitude controls, including roll control, are induced or maintained by manipulating the speed of the wing rotor motors, the fraction of lift provided by the wing rotor must be high enough to allow wing rotor manipulation to be effective in controlling the aircraft's attitude. In some aspects, the fraction of lift provided by the wing rotor at cruise speed is greater than 0.2. In some aspects, the fraction of lift provided by the wing rotor is greater than 0.25. In some aspects, the fraction of lift provided by the wing rotor is greater than 0.3.

[0041] Another factor to consider is the ratio of the wing rotor tip speed to the aircraft speed during forward flight. This factor can come into play as one or more wing rotors reduce their speed to decrease lift on one of the wings used to make rolls effective. In such a maneuver, the wing rotors on the other wings can rotate to increase lift. When operating at nominally low multiples of the aircraft speed, the risk of stall arises on the retreating rotor blades. In some respects, the nominal cruise wing rotor tip speed is greater than 2.0 times the aircraft cruise speed. In other respects, the nominal cruise wing rotor tip speed is greater than 2.5 times the aircraft cruise speed.

[0042] The factors discussed above have been modeled to determine the lift-to-drag ratio L / De (effective drag) for vehicle 200 as a function of the wingtip velocity relative to the aircraft's forward velocity against the angle of attack, and this data has been represented graphically. The lift-to-drag ratio can be viewed as a color-coded representation of the lift-to-drag ratio. The lift-to-drag ratio (effective drag) is the combined shaft power of the aircraft divided by the aircraft's forward velocity. Overlaid on this color code are outlines representing the other factors mentioned above: the fraction of total power transmitted to the wing rotor, the fraction of lift carried by the wing rotor, and also the aircraft velocity. The lift-to-drag ratio is modeled at different wing rotor tilt angles and also at different wing rotor blade pitches. By adding parentheses to portions of the graph within the cruise speed range (50-75 m / s) of vehicle 200, and by reviewing the area with a wing rotor tip velocity-to-aircraft velocity ratio greater than 2.0, it has been found that the less obvious range of parameters gives the highest lift-to-drag ratio to the operating mode. The preferred result is a lift-to-drag ratio greater than 8 within the constraints discussed above, where a higher ratio of 9 or 10 is even more desirable.

[0043] Figures 6A-6G The results are CFD results that weight the parameters discussed above to differentiate between different versions of aircraft. Figure 6A The results regarding the aircraft vehicle 200 discussed above are shown. Profile 601 forms the boundary around the region defined by cruise speeds of 50 m / s and 75 m / s and above the rotor tip speed-to-aircraft speed ratio of greater than 2.0. It is evident that there are significant regions with L / De ratios greater than 8, including those greater than 9, and some greater than 10. This indicates that the vehicle, with its 10-degree forward-tilted wing-mounted rotor, can achieve these L / De ratios in various operating conditions.

[0044] Figure 6B A similar vehicle with a rotor tilted forward at 0 degrees is shown. It can be seen that there is almost no region with an L / De as high as 9 above the 2.0 speed ratio line.

[0045] Figure 6C A similar vehicle with a rotor tilted at 20 degrees is shown. It is evident that there is no area with an L / De ratio higher than 8 available for operation.

[0046] Figure 6D A similar vehicle is shown with a rotor tilted forward at 0 degrees, but in which the rotor blades have a larger diameter than those of a conventional vehicle. Figure 6B The situation is a tilt of more than 5 degrees (pitch).

[0047] Figure 6EA similar vehicle is shown with a rotor tilted forward at 10 degrees, but in which the rotor blades have a larger diameter than those of a conventional vehicle. Figure 6A The case involves a tilt of more than 5 degrees. It can be seen that there is almost no region with an L / De as high as 8 above the 2.0 speed ratio line, but this appears to be optimized relative to other cases where the rotor blades are tilted by more than 5 degrees.

[0048] Figure 6F A similar vehicle is shown with a rotor tilted forward at 20 degrees, but in which the rotor blades have a larger diameter than those of a conventional rotor. Figure 6C The situation is tilted by more than 5 degrees. It can be seen that there are almost no regions with L / De as high as 9 above the 2.0 speed ratio line, especially in the level flight region (0 degrees AOA).

[0049] In some embodiments of the present invention, such as Figure 7 As can be seen, the aircraft 200, with a left composite wing 202 (the right composite wing is not shown), has rotors staggered along its longitudinal axis. The inboard rotor 207 has a forward-facing side and a return side 209 of its thruster 208. The return side 209 of the inboard rotor 207 significantly overlaps with the forward-facing side of the next rotor 204 along the wingspan direction of the pseudo-wing 202. This situation repeats with respect to successive rotors 205 and 206. Because the forward flight speed enhances the lift on the forward-moving portion of the thruster rotation, this significant overlap between the forward-facing and return sides of adjacent rotors evenly distributes the lift provided during forward flight.

[0050] In some embodiments of the present invention, such as Figure 8 As can be seen, the aircraft 300, having a left composite wing 301 and a right composite wing 302, has rotors that are staggered vertically along the wingspan of the composite wing. The inboard rotor 303 has a forward-direction side and a return side 308 for its thruster 307. The return side 308 of the inboard rotor 303 significantly overlaps with the forward-direction side of the next rotor 304 along the wingspan of the composite wing 301. This situation repeats with respect to successive rotors. Because the forward flight speed enhances the lift on the forward-moving portion of the thruster rotation, this significant overlap between the forward-direction sides and return sides of adjacent rotors evenly distributes the lift provided during forward flight.

[0051] Figures 9A-9E The spanwise lift distribution of conventional and synthetic wings is shown. Figure 9A This illustrates a hypothetical uniform span lift distribution, as would be seen in a conventional wing. Figures 6B-6ELift distribution curves for various types of composite wings are shown, with each curve compared against an elliptical ideal lift distribution (visible in the dashed lines). A rotor array configuration with a more uniform spanwise lift distribution will give a lower ratio of induced drag relative to an ideally loaded solid airfoil. The advantage of using a larger number of smaller rotors, compared to fewer larger rotors, is that there is a smaller total blade area and less drag for a given wingspan. Generally, for a given total lift and tip velocity, the mean blade chord does not change with the number of rotors; therefore, smaller rotors have a more suitable aspect ratio (radius to mean chord ratio).

[0052] Figure 9B A 4-rotor system is shown (2 rotors per composite wing). The load variation is 20% due to the rotor configuration. The induced drag of this configuration is 1.22 times that of an ideally loaded wing. Figure 9C An 8-rotor system is shown (4 rotors per composite wing). The load variation is 20% due to the rotor configuration. The induced drag of this configuration is 1.43 times that of an ideally loaded wing.

[0053] Figure 9D A 4-rotor system is shown (2 rotors per composite wing). The load variation is 40% due to the rotor configuration. The induced drag of this configuration is 1.88 times that of an ideally loaded wing. Figure 9E An 8-rotor system is shown (4 rotors per composite wing). The load variation is 40% due to the rotor configuration. The induced drag of this configuration is 2.71 times that of an ideally loaded wing.

[0054] Figure 10 illustrates a rotor according to some embodiments of the invention. It can be seen that there is a significant radius (as sized in Tables 1 and 2) at the inboard end of the rotor blades. In this configuration, the inboard end of the rotor blades is less prone to stalling of the retracting blades compared to a configuration where the blades are further oriented towards the rotation axis. This central region of the rotor allows for the placement of a single motor (or a pair of motors in a counter-rotating blade configuration). Therefore, the rotor blades can begin radially (relative to the rotation axis) outside the motor itself.

[0055] In some embodiments, such as Figures 11A-11C As can be seen, the aircraft vehicle 300 may have a dual assembly of forward-tilted rotors forming a right pseudo-wing 302 and a left pseudo-wing 303. The leading-edge wing and the tail wing also provide lift during forward flight. The aircraft body 301 supports the forward-propelling tail rotor 311. Details and dimensions of the exemplary embodiment can be seen in Table 2.

[0056]

[0057]

[0058]

[0059] Table 2

[0060] In some embodiments with dual-row wing rotors, the rows of wing rotors may be separated and include one or more wing elements between the rows. In some embodiments, there may be more than two rows of wing rotors.

[0061] As will be apparent from the foregoing description, various embodiments can be configured from the description given herein, and additional advantages and modifications will readily arise for those skilled in the art. Therefore, the invention is not, in its broader aspects, limited to the specific details and illustrative examples shown and described. Thus, deviations from these details may be made without departing from the spirit or scope of the applicant's overall invention.

Claims

1. A flight method for a vertical takeoff and landing aircraft having a fixed forward-tilted rotor, the method comprising the following steps: Power is supplied to multiple right-side wing rotor assemblies and multiple left-side wing rotor assemblies, wherein the rotation axis of each of the right-side wing rotor assemblies is tilted forward at a fixed angle relative to the normal to the horizontal flight line of the aircraft in nominal forward flight, the rotation axis of each of the left-side wing rotor assemblies is tilted forward at a fixed angle relative to the normal to the horizontal flight line of the aircraft in nominal forward flight, and the rotation axis of each of the right-side wing rotor assemblies and the left-side wing rotor assemblies is tilted forward at a fixed angle in the range of 5-20 degrees relative to the normal to the horizontal flight line of the aircraft in nominal forward flight; The aircraft is raised to an upward-tilted vertical takeoff and landing configuration, thereby generating the main vertical thrust from the plurality of right-side rotor assemblies and the plurality of left-side rotor assemblies; Height is obtained using the vertical thrust; The aircraft is tilted forward so as to obtain a forward thrust component from the multiple right-side wing rotor assemblies and multiple left-side wing rotor assemblies that are tilted forward; Increasing the power of the horizontally driven rotor increases the forward speed of the aircraft and the lift provided by the aircraft's wings; and By reducing the power of the plurality of right-side wing rotor assemblies and the plurality of left-side wing rotor assemblies to fly in a forward-flying mode, the ratio of lift provided by the plurality of forward-tilted right-side wing rotor assemblies and the plurality of left-side wing rotor assemblies to the lift of the wings is greater than 0.

2.

2. The method according to claim 1, wherein, The ratio of lift provided by the forward-tilted right wing rotor assemblies and the forward-tilted left wing rotor assemblies to the lift of the wing is greater than 0.

25.

3. The method of claim 1, further comprising performing the maneuver of the aircraft by distinguishing the thrust provided by different wing rotor assemblies.

4. The method of claim 2, further comprising performing aircraft maneuvering by distinguishing between thrust and torque provided by different wing rotor assemblies.

5. The method of claim 1, further comprising maneuvering the aircraft solely by distinguishing between thrust and torque provided by different wing rotor assemblies.

6. The method of claim 2, further comprising performing the maneuver of the aircraft solely by distinguishing between thrust and torque provided by different wing rotor assemblies.

7. The method according to claim 1, wherein, The rotation axis of each of the right wing rotor assembly and the left wing rotor assembly is tilted forward at a fixed angle in the range of 5-15 degrees relative to the normal to the horizontal flight line of the aircraft in nominal forward flight.

8. The method according to claim 2, wherein, The rotation axis of each of the right wing rotor assembly and the left wing rotor assembly is tilted forward at a fixed angle in the range of 5-15 degrees relative to the normal to the horizontal flight line of the aircraft in nominal forward flight.

9. The method according to claim 1, wherein, The rotation axis of each of the right wing rotor assembly and the left wing rotor assembly is tilted forward at a fixed angle in the range of 8-12 degrees relative to the normal to the horizontal flight line of the aircraft in nominal forward flight.

10. The method according to claim 2, wherein, The rotation axis of each of the right wing rotor assembly and the left wing rotor assembly is tilted forward at a fixed angle in the range of 8-12 degrees relative to the normal to the horizontal flight line of the aircraft in nominal forward flight.

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