BRIEF DESCRIPTION OF THE DRAWINGS
In order to understand the invention and to see how it may be carried out in practice, a number of embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:
FIG. 1 schematically illustrates typical stall patterns of some types of wing sections or aerofoils.
FIG. 2 schematically illustrates an aircraft comprising a wing according to one embodiment of the present invention.
FIGS. 3(
a) and 3(b) illustrate example design regions of long endurance UAV having high aspect ratio (AR=25) and moderate aspect ratio (AR=10), respectively.
FIGS. 4(
a) and 4(b) illustrate conventional mild-stall airfoils with moderate maximum lift, airfoils FX61-184 and NACA-4415, respectively.
FIG. 5(
a) illustrates lift coefficient distributions with angle of attack for conventional airfoils FX61-184 and NACA-4415 at Re=0.5*106; FIG. 5(b) illustrates lift coefficient distributions with angle of attack for conventional airfoils NACA-4415, NACA-6415 and NACA-6418 at Re=0.5*106.
FIG. 6 schematically illustrates contour geometry of a high-lift, mild stall airfoil designated herein as MS-18.
FIG. 7 illustrates lift coefficient distributions with angle of attack for airfoil MS-18, at Re=0.7*106.
FIG. 8 schematically illustrates the camber distribution of airfoil MS-18 along the chord thereof.
FIG. 9 schematically illustrates the thickness distribution of airfoil MS-18 along the chord thereof.
FIG. 10 illustrates pressure coefficient distributions along chord for airfoil MS-18 at stall and post-stall angles of attack.
FIG. 11 illustrates pressure coefficient distributions along chord for airfoil MS-18 at high post-stall angles of attack.
FIG. 12 illustrates the effect of Reynolds number on lift coefficient distributions with angle of attack for airfoil MS-18.
FIG. 13 schematic illustrates a Cl max boundary between conventional and high-lift MS-airfoils, as a function of Reynolds number.
FIG. 14(
a), 14(b) compare pressure coefficient distributions along chord of airfoils MS-18 and NACA-4415 at two post-stall angles of attack.
FIG. 15 illustrates a mission-adaptive, adjustable geometry, two-element airfoil according to an embodiment of the invention; one particular form of this embodiment, designated herein SA-19 is illustrated in FIGS. 15(a) to 15(e) in various modes of operation: FIG. 15(a)—cruise, loiter mode; FIG. 15(b)—decambering mode for high speed flight; FIG. 15(c)—landing flap mode; FIG. 15(d)—airbrake mode; FIG. 15(e)—aileron mode.
FIG. 16 compares lift coefficient distributions with angle of attack for airfoil SA-19, for a range of deflections of the second aerofoil element, at Re=1M.
FIG. 17 compares lift coefficient distributions with angle of attack for airfoils MS-18 and SA-19, at Re=0.8·106.
FIG. 18 schematically illustrates an aircraft comprising a wing according to another embodiment of the present invention.
FIGS. 19(
a) and 19(b) schematically illustrate the transformation of an MS-airfoil (MS-18) into a corresponding SA airfoil (designated MS/SA-18).
FIG. 20 compares lift coefficient distributions with angle of attack of mild stall airfoil MS-18 and slotted airfoil MS/SA-18, at Re=800K.
FIG. 21 compares maximum lift coefficient characteristics of airfoils MS-18 and MS/SA-18 as a function of Reynolds number.
FIG. 22 illustrates spanload lift coefficient distributions and stall pattern of representative MS/SA wing.
FIG. 23 illustrates aircraft lift coefficient variation with angle of attack, and operational limit of MS/SA wing of FIG. 22, up to which there is substantially unaffected aileron and elevator/rudder power for the aircraft.
DETAILED DESCRIPTION
According to aspects of the invention, high lift wings are provided for aircraft, in particular fixed wing aircraft. For the purpose of example, and referring to FIG. 2, such an aircraft is described herein as a fixed-wing aircraft, generally designated with numeral 1, of the regular subsonic/transonic configuration, having a fuselage section 2, main wings 10 (only the starboard wing (also referred to herein as a “wing half”) is illustrated in this figure), tailplane 3, vertical stabilizer 4, and a propulsion system (not shown). However, the present invention is applicable to other types of aircraft, for example: gliders; subsonic/transonic aircraft having canards rather than a tailplane; general aviation aircraft, and so on. Furthermore, while the present invention finds particular application in UAV aircraft, the invention may also be applied to manned aircraft, mutatis mutandis, in particular to general aviation, sailplanes, subsonic transport, naval aviation, and so on.
According to the invention, each wing half 10 comprises an outboard section 30 joined to or integral with an inboard section 40. The outboard section 30 may be defined as extending from the wing tip 22 to a transition plane 20, while the inboard section 40 extends from the transition plane 20 to the wing root 24.
In the illustrated embodiment of FIG. 2, and by way of non-limiting example, the wing has a substantially trapezoidal plan shape, the leading edge 52 of the wing 10 being substantially rectilinear and having a substantially zero sweep angle, and the wing having a taper of 0.6 to about 1.0, between the root 24 and the tip 22. In other embodiments of the invention, the wing 10 may have a different plan form, for example: swept-back or swept forward, and/or with a different taper ratio (along the full wing, or different taper ratios for the inboard section and the outboard section); and/or having a different plan form, including curved leading edges and/or trailing edges such as an elliptical form, for example; and/or the inboard section and/or the outboard section may have a positive, negative or zero dihedral angle; and so on.
The inboard section 40 comprises a high-lift mild stall (HL-MS) configuration, while the outboard section 30 comprises a slotted aerofoil (SA) configuration, and the slotted aerofoil section extends up to and including the tip 22. This particular configuration provides root stall and avoids tip stall. In other variations of this embodiment, other arrangements of one or more HL-MS portions and SA portions along the length of the wing are possible. For example, there may be an HL-MS portion at the wing tip and another at the wing root, sandwiching an SA portion.
By way of non-limiting example, the transition plane 20 may be located at about 30% to about 60% of the wingspan from the wing tip 22 to the root 24, with the outboard section 30 being 30% to about 60% of the wingspan from the tip, and the inboard section 40 being about 70% to about 40% of the wingspan from the root.
The inboard section 40 comprises an aerofoil design that provides mild stall characteristics at a high level of maximum lift. By way of non-limiting example, and referring to FIG. 6, such an aerofoil design may have a thickness-to-chord ratio (t/c)max of about 18% at about 22% of the chord, and a camber distribution and a thickness distribution optionally as illustrated in FIGS. 8 and 9, respectively, having a maximum camber of about 7.5% at about 40% of the chord. This example aerofoil design is generally designated herein as MS-18. The leading edge 52 of the MS-18 aerofoil is relatively blunt or rounded, having a low curvature leading edge radius, and aft portion 54 of the airfoil is cambered (FIG. 6).
In particular, the aerofoil design of the inboard section 40 is such as to provide, for a required aircraft mission, payload, application, and so on, high lift characteristics coupled with mild stall characteristics for that section of the wing, when the inboard section 40 is considered substantially in isolation (that is, as if the whole wing were designed in a similar manner for providing high lift, mild stall characteristics along the wing span (ignoring edge effects at the wing tip and wing root)).
High lift, mild-stall airfoils (HL-MS-airfoils) according to aspects of the invention are generally characterized by relatively high maximum lift relative to conventional MS aerofoils (see for example FIG. 13), while retaining and enhancing mild stall characteristics in the wide range of post-stall angles of attack. HL-MS airfoils according to the invention rely on the blunt leading edge that prevents formation of suction peak at high angles of attack and on the highly cambered aft portion of the airfoil that produce the phenomena of slowly creeping trailing edge separation. The combination of continuous lift build-up at the forward portion of HL-MS airfoils with slowly progressing trailing edge separation produces the feature of mild stall at high level of maximum lift and provides significant advantage relative to conventional MS-airfoils.
Referring again to the example aerofoil section (MS-18) illustrated in FIG. 6, such an aerofoil section may have a lift curve at design Reynolds number (Re=700K) such as for example illustrated in FIG. 7. FIGS. 10 and 11 illustrate nominal pressure distributions for this aerofoil section at stall and post-stall angles of attack α of 9°, 12° and 18°, and for high post-stall angles of attack α of 20° and 25°, respectively, for this aerofoil section. The variation of maximum lift and stalling characteristics with Reynolds number for the MS-18 aerofoil is illustrated in FIG. 12. FIG. 13 illustrates the variation of maximum lift coefficient (Cl max) as a function of Reynolds number for the MS-18 aerofoil, and compares this variation with the relatively inferior lift characteristics of a regular and standard mild stall aerofoil, such as the NACA-4415, the profile of which is illustrated in FIG. 4. FIGS. 14(a) and 14(b) compare the pressure distributions obtained with a MS-18 aerofoil according to the invention and the conventional mild stall NACA-4415 aerofoil, at angles of attack α of 15° and 17°, respectively, and it may be seen that the MS-18 aerofoil provides in each case more lift coupled with mild stall characteristics.
Furthermore, FIG. 13 delineates a schematic boundary 100 between the Cl max˜Re characteristics of reference, conventional mild stall airfoils and the corresponding characteristics of high-lift, mild-stall airfoils (HL-MS airfoils) according to the invention. This boundary thus defines a lower limit for Cl max obtained with a particular HL-ML aerofoil design at any particular Reynolds number (Re) between about 0.3*106 and about 2.0*106. The boundary 100 may be considered to suggest or approximate a linear minimal relationship between Cl max and Re between these upper and lower Reynolds number limits, and the boundary 100 can thus be described by the relationship
(Cl max)min=(0.35/(1.7*106))*Re+1.6, [0.3*106<Re<2.0*106] Eq. 1
The HL-MS aerofoil sections for the inboard section 40 may be designed in any suitable manner, such as to provide suitable Cl max˜Re characteristics at or exceeding that suggested by Eq. 1 above. For example, it is possible to start with a known MS aerofoil design that may be close in characteristics to that required, for example camber, thickness to chord ration, and so on. Alternatively, a baseline aerofoil may be designed using known methods. Then, the baseline aerofoil contour may be modified to provide a relatively blunt leading edge to obtain high lift, and a suction surface that is cambered and/or has a thickness distribution such as to provide slowly creeping trailing edge separation. CFD methods may be used to test the aerofoil, which can then be modified further, and again tested. A number of such trial and error iterations may be carried out until a suitable profile for the aerofoil, providing the required characteristics, is achieved.
According to the invention, and referring also to FIG. 15, the outboard section 30 is configured as a two-element slotted aerofoil (SA aerofoil), having a substantially static primary element 32 and a pivotable secondary element 34. The primary element 32 comprises the leading edge 38 of the aerofoil, and a major portion of the suction surface 31 and pressure surface 33 thereof. The secondary element 34 comprises the trailing edge 39 of the aerofoil, and a minor portion of the suction surface 31 and pressure surface 33 thereof. A slot 55 separates the leading portion 35 of the secondary element 34 from the trailing portion 36 of the primary element 32. For example, the slot 55 may have a width at least 2% of the airfoil chord in a non-deflected position of the flap element.
The precise form of the slot 55 generally depends on the particular mode of operation of the wing 10, as illustrated, for example, in FIGS. 15(a) to 15(e) which refer to cruise/loiter, maximum speed decambering, landing flap mode, airbrake mode and aileron configurations, respectively. Optionally, the hinge point 59 of the secondary element 34 is outwardly displaced with respect to the lower (pressure) surface of the secondary element 34.
By way of non-limiting example, and referring to FIGS. 15(a) to 15(e), such an SA aerofoil design may have a suitable thickness-to-chord ratio (t/c)max, a camber distribution and a thickness distribution, optionally similar to the corresponding characteristics of the HL-MS inboard section. This example aerofoil design is designated herein as SA-19, and the leading edge 152 of the SA-19 aerofoil may also optionally be relatively blunt or rounded, having a low curvature leading edge radius, and aft portion 154 of the airfoil may also be cambered.
Referring again to the example aerofoil section (SA-19) illustrated in FIGS. 15(a) to 15(e), the airflow through slot 55 provides enhanced efficiency and linearity of the aerodynamic characteristics of the aerofoil as compared to a similar non-slotted aerofoil, though more abrupt stall characteristics, and in FIG. 17 the variation of C1 with angle of attack α obtained with the SA-19 aerofoil is compared to that of the MS-18 aerofoil, at Re of about 0.8*106. Referring to FIG. 16, the variation of C1 with angle of attack α obtained with the SA-19 aerofoil is shown for a variety of angle of attack δ of the secondary element 34 (with respect to the first element) at Re of about 106.
In other embodiments of the invention, the secondary element 34 is spatially and/or rotationally fixed with respect to the primary element 32, and optionally, the secondary element 34 may comprise control surfaces such as ailerons, flaps and so on, that are pivotable with respect to the secondary element 34.
Referring in particular to FIGS. 19(a) and 19(b), in a first embodiment of the invention, the inboard section 40 comprises high lift, mild stall single element aerofoil sections, and the outboard section 30 comprises a slotted double element aerofoil configuration that is based on the single element aerofoil section of the inboard section 40. Thus, as will become clearer herein, the inboard section 30 and the outboard section 40 are provided with substantially similar profiles at least from the leading edge 52, 38 to at least a aft portion 54, 154 of the aerofoils thereof. By way of non-limiting example, the inboard section 40 may comprises a MS-118 aerofoil profile, as illustrated in FIGS. 6 and 19(a), and referring to FIG. 19(b), the outboard section 30 comprises a two-element aerofoil profile, designated herein as MS/SA-18, in which the primary element has a profile substantially identical to that of the MS-18 aerofoil for the leading edge, and the suction and pressure surfaces up to the slot 55. The profile of the suction and pressure surfaces of the secondary element may be similar to that of a corresponding part of the MS-18 aerofoil close to the leading edge thereof, but displaced with respect to said corresponding part of the MS-18 profile, indicated as phantom lines in FIG. 19(b). Thus, in this embodiment, the wing has a substantially smooth profile from wing tip to wing root.
The wing 10 according to this embodiment may be designed as follows. First, the basic HL MS aerofoil section for the wing is designed according to criteria such as aircraft mission profile, payload, cruise speed, and so on, for example, and the aerofoil section is scaled to provide wing tip and wing root HL MS aerofoils. A baseline wing is then defined by connecting wing root and wing tip HL MS aerofoils of a particular design (in this case the MS-18 design, as an example), via straight forming lines defining the leading edge and trailing edge of the wing, according to the taper ratio, sweep and so on. Then, a transition plane is defined along the span, for example about 30% to about 60% from the wing tip, essentially dividing the wing into the inboard section 40 and the outboard section 30. The aerofoil sections for the outboard section 30 are then modified to include a slot separating a trailing or secondary element that may be used as an aileron, flap, and so on, from a leading or primary element, of the now-two-element aerofoil. A hinge point is defined for the secondary element outside of the aerodynamic contour of the two element aerofoil, such as to enable the slot to be defined for a wide range of angles of attack, particularly positive angles of attack typical of low speed flight. Optionally, further modifications of the two-element aerofoil geometry may be made at the aerodynamic design and development stages of the wing, and such modifications may include, for example, shaping of the secondary element and fairing the trailing portion of the primary element of the aerofoil.
FIG. 20 compares the variations of C1 with angle of attack a obtained with the high-lift, mild stall, single-element airfoil MS-18 and with the two-element MS/SA-18 airfoil. While the C1˜α characteristics are substantially identical at low and negative angles of attack α, the MS/SA-18 airfoil shows a loss of mild-stall characteristics and an increase of its lift-carrying capabilities with respect to the single element MS-18 aerofoil.
FIG. 21 compares the variations of lift coefficient of an aircraft CL max with Reynolds number, the aircraft comprising with the high-lift, mild stall, single-element airfoil MS-18 and with the two-element MS/SA-18 airfoil. These figures can be regarded as illustrating a change in aerodynamic characteristics accompanying the transformation of an MS-type airfoil into SA-type airfoil. As may be seen in this figure, the effective operational limit (OL) for the aircraft is at an angle of attack α of about 17 or 18 degrees, with substantially unaffected aileron and elevator/rudder power at least up to this level of α.
Without being bound by theory, the resulting MS/SA-wing with the inboard MS section and the outboard SA section, according to aspects of the invention, provides mild-stall characteristics at post-stall angles of attack due to beneficial effect of HL-MS airfoils of its inboard portion and capability of SA-airfoils of the outboard wing part to retain attached flow and unaffected efficiency of the ailerons at post-stall angles of attack.
Spanload distributions of a typical high-lift MS/SA-wing at different lift coefficients and angles of attack are shown in FIG. 22. The mild decrease of the local lift from the wing root 24 to transition plane 20 towards the outboard wing is due to the chosen taper ratio of the wing 10, taking into account to the aerodynamics of the mild-stall/slotted aerofoil wing 10. Stall of the wing 10 develops through the following stages:
- at the linear range of lift coefficients (angle of attack α<about 12°), the wing 10 shows maximum sectional loading at its inboard section 40. This inboard section 40 first experiences the limit of maximum sectional lift defined by the characteristics of the HL-MS airfoils, triggering slow development of the stall at this wing portion.
- as angle of attack α is further increased, this induces mild stall of the inboard section 40, without substantially affecting the maximum lift of the wing 10. The outboard wing portion 30 maintains an attached flow, with significant margin between maximum sectional lift limit and actual local loading at this wing portion. Moderate taper ratio of wing 10 can contribute to increased margin (generally independent of the characteristics of SA-airfoils), producing a reduced loading of outboard wing portion.
- with a further increase in the angles of attack α, there is a gradual development of mild stall on the inboard portion of the MS/SA-wing. This is accompanied by continuous lift build-up on the outboard wing portion 30, until this part of the wing reaches its maximum lift limit defined by the characteristics of slotted airfoils of the invention. The resulting lift curve of the wing 10 may provide almost constant lift at post-stall angles of attack, up to the stall of its outboard portion, as shown in FIG. 23.
The attached airflow at the outboard wing at post-stall angles of attack facilitates sufficient, and preferably substantially unaffected efficiency of aileron operation and provides the required controllability for the wing with fully developed stall at the inboard wing section 30. This stall pattern of the MS/SA-wing according to the invention provides a capability for controllable flight at stall and post-stall angles of attack. Correspondingly, the design of the tail should be adequate to provide sufficient control power of tailplane 3 and rudder 4 at high post-stall angles of attack, and to provide sufficient elevator efficiency to the tailplane 3 to trim the increased pitch-down moments of MS/SA wing of the invention at post-stall angles of attack.
A second embodiment of the invention is substantially similar to the first embodiment as described herein, mutatis mutandis, and thus a wing 110 according to the second embodiment also comprises a first wing section 140 based on HL-MS aerofoils, and a second wing section 130 based on SA-aerofoils. However, in the second embodiment of the invention, and referring to FIG. 18, the profile of the outboard section is not based on that of the inboard section. For example, the inboard section may have a profile such as the aforesaid MS-18 aerofoil, while the outboard section has another profile, such as the SA-19 two element aerofoil for example. In such a case, there may be a discontinuity in the profile of the wing at the transition plane 120. Optionally, a plate 72 aligned generally orthogonally to the planform of the wing and also generally aligned with the direction of the flow over the wing 110 may be provided at the transition plane to separate the airflow between the HL-MS and the SA sections of the wing 110.
In the method claims that follow, alphanumeric characters and Roman numerals used to designate claim steps are provided for convenience only and do not imply any particular order of performing the steps.
Finally, it should be noted that the word “comprising” as used throughout the appended claims is to be interpreted to mean “including but not limited to”.
While there has been shown and disclosed certain embodiments in accordance with the invention, it will be appreciated that many changes may be made therein without departing from the spirit of the invention.
Patent Application
20070278353
