[0001] This invention relates primarily to VTOL vehicle design and, more specifically, to ducted- fan configurations for VTOL vehicles.

BACKGROUND OF THE INVENTION

[0002] A typical Vertical Take Off and Landing (VTOL) vehicle may have one or more ducted fan units along or parallel to a longitudinal axis of the vehicle. Ducted fans have several advantages over free rotors (i.e., rotors not enclosed within ducts), the most prominent of which is the ducted fan's thrust augmentation that is attained when the incoming air accelerates over a suitably designed duct inlet lip, causing low pressure that acts upon the upper lip surface, creating up to 25-30% of additional thrust as compared to a free rotor.

[0003] Unfortunately, such augmentation comes at a price, mainly in the form of considerable rolling and pitching moments that are produced as a result of sideslip (i.e, sideward translational motion). While the following discussion applies equally to the pitch and roll axes of the vehicle, it will be dedicated to the description in roll, keeping in mind that a similar discussion is fully applicable also to the pitch axis.

[0004] VTOL vehicles that rely on ducted fans for lift and that need to operate in gusty wind conditions, must combat rolling moments that can interfere with the vehicle's operation and stability. One method that is advocated by Yoeli (US 6,464,166 Bl) is to use either a plurality of parallel, spaced, control vanes pivotally mounted to, and across, the inlet end of the duct, or a combination of two rows or groups of rotatable control vanes, one row at the duct inlet and one row at the duct exit or outlet. These rows or groups of control vanes, each located at a distance from the center of gravity of the duct that is approximately half the depth of the duct (i.e., the vertical distance between the duct inlet and outlet), are, when rotated, able to produce rolling moments in a direction that is parallel to the vanes' axes of rotation, thereby opposing the adverse rolling moments caused by the lateral motion of the vehicle or alternatively, by a side-blowing wind when the vehicle is in hover. As this sideward motion increases, the vanes need to be rotated an increased amount until they reach the limit of the force that they can produce, with a consequent limit on the lateral wind or vehicle motion velocities. A method is therefore desired for increasing the effectiveness of the vane control system advocated by Yoeli to enhance the ducted fan's ability to move sideways, perpendicular to the vanes' longitudinal axes.

BRIEF DESCRIPTION OF THE INVENTION

[0005] One way of increasing the effectiveness of the control vanes is by increasing the distance between the upper (or inlet) row or group of control vanes and the vehicle's center of gravity and/or the lower (or outlet) row or group of control vanes, if installed. However, as will be explained below, unless some special measures are taken, the vertical position for mounting the upper row or group of vanes is highly constrained by the height of the duct.

[0006] Accordingly, in one exemplary but non-limiting example, there is provided a ducted fan for a VTOL vehicle comprising a substantially cylindrical duct having an inlet at an upper end and an outlet at a lower end and an air mover unit; and, at least one row of adjustable control vanes, for at least partially controlling sideways and rolling motion of the vehicle in flight, located above said inlet a distance equal to at least one-quarter vane chord length.

[0007] In another exemplary but nonlimiting example, there is provided a ducted fan for a VTOL vehicle comprising a substantially cylindrical duct having an inlet at an upper end and an outlet at a lower end and an air mover unit, the inlet defined by a peripheral surface of the duct tapering radially outwardly in a direction toward the outlet; and at least one group of control vanes located at least partially above the inlet.

[0008] The invention also relates to a method of increasing effectiveness of vanes mounted at an inlet end of a ducted fan unit comprising an open-ended duct having an inlet and an outlet and an air mover unit, the method comprising: (a) shaping the inlet of the duct to have a diameter equal to or less than a diameter of the duct proximate the outlet; and (b) locating a first group of adjustable inlet vanes a distance above the inlet as a function of duct inlet diameter.

[0009] In another exemplary but nonlimiting example, the vanes mentioned hereinabove may be statically fixed relative to the duct at one end and movable at their other end, yielding various control forces when moved and cambered. [0010] Accordingly, the invention also relates to duct having an inlet and an outlet and an air mover unit supported in the duct, between the inlet and the outlet; a first plurality of flexible vanes located at least about the inlet, one end of each of the vanes fixed against rotation relative to the duct, and an opposite end of each vane fixed to a movable actuator and thereby movable through flexure of the vane.

[0011] The invention also relates to a method of creating side forces on a VTOL vehicle in flight, the VTOL vehicle having at least one ducted fan, the ducted fan having an inlet and an outlet and a propulsion unit supported in the duct, between the inlet and the outlet; a first plurality of flexible vanes located at least about the inlet of the duct, the vanes extending substantially parallel to a longitudinal axis of the VTOL vehicle, the method comprising:

[0012] (a) fixing one end of one or more of the first plurality of flexible vanes against rotation relative to the duct; and

[0013] (b) fixing an opposite end of the one or more flexible vanes to a linear actuator movable to flex the opposite end in directions substantially perpendicular to the longitudinal axis.

[0014] Exemplary embodiments of the invention will now be described in greater detail in connection with the drawings identified below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIGURE 1 is a schematic cross section through a conventional VTOL vehicle ducted fan unit;

[0016] FIGURES 2a-2c illustrate how fan duct geometry affects the shape of the flow field in the vicinity of the duct inlet;

[0017] FIGURES 3a-3c are similar to Figures 2a-2c but with the addition of inlet and outlet control vanes at various locations relative to the duct inlet, duct outlet and to the vehicle center of gravity; and

[0018] FIGURES 4a-4c illustrate vanes movable at one end to a cambered shape yielding control forces. DETAILED DESCRIPTION OF THE INVENTION

[0019] Fig. 1 shows schematically a cross section through a ducted fan 10 of a VTOL vehicle V (shown in phantom) comprising an open-ended generally cylindrical duct 12, inside of which is mounted an air mover unit 14 which may be driven by either a powerplant or a gearbox (not shown, and located inside or outside the duct). The air mover unit 14 may include a rotor (or propeller or fan) 16 which draws air into the duct via an inlet 18, the air exiting the duct at an outlet 20.

[0020] Fig. 1 also shows the typical flow pattern that exists around a ducted fan when the VTOL vehicle V is in hover. As can be seen by the streamlines 22, at the duct outlet 20 and to a fair distance away from the duct, the flow is predominantly straight. Achieving a greater moment arm by mounting exit or outlet vanes at a distance below the duct outlet is useful and has been done on several vehicles, such as the Honeywell MAV and others. On the inlet side of the duct, however, air entering the duct normally forms a wide funnel shown as 24, partly as a result of the wide, radially outwardly extending inlet lip 26 that typically provides additional thrust as compared to a free rotor. The flow pattern shown in Fig. 1 is typical of ducted fans having inlet lip radii that are 10-12% of the duct diameter. Because the mass flow into the rotor at any point in time must be constant for any of the (imaginary) curved cross sections substantially perpendicular to the flow streamlines, the average flow velocity through the imaginary curved surfaces (shown at 28, 30) diminishes quickly, whereas the velocity at curve 32 is relatively low and increases towards curve 30 and further increases towards curve 28. This phenomenon of lower velocity afar from the inlet lip of the duct does not encourage the mounting of the vanes at any significant distance above the duct inlet 18. It should be mentioned that for ducts that have a relatively large inlet lip radius (typically above 12% of the duct diameter), the optimal location for the vanes may even lie below the upper end of the duct (or duct inlet), where the average air velocity is already close to that of the cross section at the plane of the rotor. All this is because the side forces that each vane produces are proportional to the local air velocity squared, whereas the loss or gain in moment arm with the decrease or increase of distance between the upper and lower rows or groups of vanes and the vehicle center of gravity is linear, as will be further discussed below. [0021] Figs. 2b and 2c show how the duct 12 of Fig. 1 (also shown in Fig. 2a) can be modified to affect, through the design of the duct inlet, the shape of the flow field in the vicinity of the duct inlet 18 so that the flow into the duct 12 can be made more uniform. With this added uniformity of flow, the control vanes, when mounted outside the duct, would be able to benefit from having a larger moment arm but would still be exposed to relatively high inflow velocities (a desired effect), in spite of being either completely or partially outside the boundaries of the duct.

[0022] Fig. 2b shows a duct 34 of generally cylindrical shape, with no out-turned inlet lip, and with a substantially uniform diameter between the inlet and the outlet. This variation shows how, with a modification of the shape of the inlet 36 relative to Fig. 2a (which shows the duct 10 with a conventional inlet lip 26), the incoming flow that lies upstream of the duct inlet can be modified so that the high velocity region that characterizes the flow at the inlet 36 is extended outward to some distance from the duct. This is done by significantly reducing (or even eliminating) the lip radius to form, in effect, a corner or sharp edge 38 at the entrance to the duct inlet 36. Such behavior of the flow is a consequence of the viscosity as well as mass of the air which is now unable to accommodate to the sharp edge 38, and assumes a different, 'detached' form of flow. It should be mentioned that such behavior of the flow is always present at the outlet 40 of the duct (as well as at the trailing edges of airfoils and wings). The novelty in the means proposed herein is in the enforcement of an abrupt geometry change at an entrance or inlet to a duct or 'leading edge' side, in order to enforce a desirable flow field upstream of the duct inlet. Fig. 2c shows a further implementation applied to a duct 42 which takes the idea to a higher level by increasing the angle of a corner 44 formed between the upper horizontal edge of the duct and the peripheral wall of the duct to more than 90 degrees, i.e., the inlet end of the duct tapers outwardly from the inlet 46 to a larger diameter approximately mid-way along the height of the duct, which then remains uniform to the duct outlet 49. This arrangement, for a given induced velocity inside the duct, will create a region of even higher incoming velocities compared to the increased velocities generated in Fig. 2b at a distance from the inlet shown schematically at 48.

[0023] Fig. 3 shows qualitatively some preferable locations for a row or group of substantially airfoil-shaped inlet vanes as a function of inlet radius and duct angle. Fig. 3a shows the standard duct 12 with a 10-12% lip radius. Here, the velocity of the incoming flow still accelerates even while entering the duct inlet 18. The row or group of parallel, spaced inlet vanes 50 (which are substantially co-planar and which may extend parallel to the longitudinal axis of the VTOL vehicle V) is thus mounted in close vicinity to the duct inlet, because mounting them at a distance from the duct edge would subject the vanes to significantly lower air velocities, thus reducing the force that they are able to generate. Also shown in Fig. 3a is a row or group of control vanes 52 mounted below the duct's outlet 54 where the flow is substantially straight, retaining its high velocity even after exiting the duct.

[0024] Fig. 3b shows the duct 34 similar to that of Fig. 3a except that the radius of the inlet lip at the inlet 36 has been eliminated (or at least significantly reduced), as also shown in Fig. 2b. At a distance shown at 56, equaling approximately (or at least) one vane chord length (i.e., a length defined by a straight line connecting the leading and trailing edges of the vane), the incoming streamlines shown as 58 are not spread out as in Fig. 3 a and hence the average velocity across an area perpendicular to the streamlines (shown at 60) is similar to the velocity inside the duct. At the same time, because the row of vanes 62 is at a further distance from the vehicle's center of gravity, a larger moment arm and a gain in overall vane moment generation capability is facilitated. This increase in moment around the vehicle's center of gravity is increased further by also mounting the exit vanes 64 downstream of the duct outlet as described.

[0025] Fig. 3 c shows what is possible to achieve if the inlet shape is further modified to enhance a high speed region upstream from the inlet. The duct 42 is as shown in Figure 2c, but with the row or group of inlet control vanes 66 now located at an even larger distance 68 (i.e., greater than one vane chord length) from the vehicle's center of gravity 70, compared to the respective positions of the inlet vanes 50 and 62 in Figs. 3a and 3b. The distance between the inlet (upper) row or group of control vanes 66 and the center of gravity 70 is Ll and the distance between the outlet (lower) row or group of control vanes 72 and the center of gravity 70 is L2 and therefore, if a horizontal force P2 is generated by the outlet group of control vanes 72 and a force Pl is generated by the inlet group of control vanes 66, then a combined rolling moment equal to (PlxLl)+(P2xL2) can be generated by the groups of control vanes 66, 72. Note that, in this exemplary but nonlimiting embodiment the groups of outlet vanes 64 and 72 (Figs. 3b and 3c) are also located below the duct outlets a distance greater than one vane chord length. [0026] It should be appreciated, however, that the location of the inlet and outlet vanes may vary. For most if not all applications, the vanes should be located at a distance above and below the duct inlets and outlets, respectively, of at least one-quarter vane chord length.

[0027] It should be appreciated that in addition to the increased geometric distance between the inlet group of control vanes 66 and the center of gravity 70, the inlet vanes 66 are at the same time also experiencing higher air velocities which further increase the value of Pl attainable at the inlet side of the duct, thereby enhancing the magnitude of the (PIxLl) contribution of the inlet vanes to the overall moment generation capability.

[0028] It should also be noted that reducing the radius of the inlet lip (or eliminating the inlet lip) is also beneficial through the reduction in the magnitude of the upsetting moments caused by side winds and especially gusts. However, reducing the radius of the inlet lip has a detrimental effect on the thrust augmentation that can be obtained compared to a free rotor, so the proposed method would probably be less advantageous on low-power, ducted- fan vehicles that do not need to operate in high wind conditions. On the other hand, for ducted-fan vehicles that are characterized by high power loading, the method proposed herein could enhance considerably the practicability of such vehicles when operating in high wind conditions.

[0029] Figs. 4a-c illustrate a ducted fan 44 with vanes 74 about the inlet end and vanes 76 about the outlet end of the duct. The vanes consist of thin airfoils, membranes or other type of flexible surfaces, which unlike the typical vanes such as mentioned hereinabove which are substantially rigid and either pivotal or non pivotal fully or partially, are here statically fixed against rotation relative to the duct at their one end and movable through flexure of the vane at their other end. Fig. 4a illustrates inlet vanes 74 statically fastened fixed against rotation at points 75 to the duct 44 and connected by pivotal joints 79 to actuating rod 78, and outlet or exit vanes 76 statically fastened at points 77 to the duct 44 and connected by pivotal joints 81 to actuating rod 80 wherein the actuating rods are located at distance 90 from the duct openings which in this exemplary but nonlimiting example is also the cord length of the vanes. It should be mentioned that the vanes can be attached at their fixed ends 75 and 77 to members wherein such member may lie in a plane that is at distance from the said duct 44 inlet and exit respectively. [0030] Fig. 4c schematically illustrates view of the cluster of the inlet or upper vanes 74 pivotally connected by joints 79 to the actuating rod 78 which can linearly slide forward and backward to displacement 88 by means of a driving mechanism which may involve a ball screw, push-pull rod or other mechanical transition system not shown herein.

[0031] Fig. 4b illustrates the duct and vanes described in Fig. 4a where the actuating rods 78 and 80 are sliding to the right carrying with them the tips of vanes 74 and 76 which are hinged to the rods by joints 79 and 89 respectively causing the vanes which are fixedly attached to the duct at their other ends 75 and 77 respectively to bend sideward and become cambered with their cord line arched. Since the cord line lengths 90 are fixed for any specific types of vanes, their bending sideward as shown causes the distance between the activating rods and the duct to become shortened to length 92. It has been found that the present disclosure of cambered vanes is a good means to produce control forces similar to those which are produced by pivotal vanes when applied to mass flows such for example as mentioned hereinabove. Furthermore it has been found that the cambered vanes are advantageous especially but not limited to lower airflow velocities and smaller Reynolds numbers and smaller ducted vehicles, nevertheless they are applicable also in larger vehicles. It should be appreciated that the configuration of the vanes and their motions as disclosed hereinabove may vary to other preferred embodiments, and their flexure magnitudes and directions activated according to the relevant roll moments and lateral forces. For example, the vanes at the inlet and outlet sides of the duct may bend in opposite directions to yield a rolling moment, or each cluster of vanes can be split into smaller sections which are activated individually to specific directions and flexure according to the desired forces and moments. Furthermore, the current disclosure can be used also with vanes positioned at other locations and distances relative to the duct including within the duct fully or partially.

[0032] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.