FIELD OF THE INVENTION

[0001] The present invention relates to aerofoils, and in particular an aerofoil adapted to utilise the lower/back surface of the aerofoil to increase lift.

BACKGROUND TO THE INVENTION

[0002] The invention relates to aerofoils, both propellors and wings, and is equally applicable to both, but will be discussed primarily in relation to aircraft wings.

[0003] An aircraft wing is an aerodynamic lift surface where the dynamic pressure is lower on the top surface and higher on the lower surface to produce lift. The net lift force is the differential force between the force on the lower surface and the force on the upper surface.

[0004] The actual force pushing in the direction of what is termed to be “lift” comes from the wing’s lower surface. Lift can be increased by either lowering the downward force on the upper wing surface by lowering the pressure, or increasing the upward force on the lower wing surface by increasing the pressure. Conventional wings primarily produce lift by having a curved top surface to lower pressure, with the lower surface playing little or no role in providing lift beyond that of a flat surface. Typical lower surface designs are aimed primarily at reducing drag.

[0005] It would be advantageous if existing wing designs could be adapted to utilise the lower surface to increase lift whilst maintaining the same lift/drag ratio. This would: reduce stall velocity and landing velocity as the minimum required lift could be produced at a lower velocity; reduce fuel consumption as equivalent lift could be generated at a lower angle of attack; reduce take off distance; allow a reduction in wing area thus saving weight; and increase the lift gradient towards the wing root, leading to improved flight stability and increased stall angle.

[0006] The object of this invention is to provide an aerofoil with a bottom surface adapted to produce increased lift without unduly increasing drag, or at least provide the public with a useful alternative. SUMMARY OF THE INVENTION

[0007] In a first aspect the invention provides an aerofoil for producing increased lift, comprising a lower surface is in the form of a convex section disposed towards a leading edge of the aerofoil and a concave section disposed towards a trailing edge of the aerofoil.

[0008] Preferably the convex section is in the form of a first hyperbola and the concave section is in the form of a second hyperbola.

[0009] In preference the first hyperbola has a first focal length, and the second hyperbola has a second focal length which is equal to the first focal length.

[0010] Preferably the aerofoil further comprises an inflection point between the first hyperbola and the second hyperbola located midway between the leading edge and trailing edge of the aerofoil.

[0011] In preference the aerofoil extends from a root to a tip, and the focal lengths of the first and second hyperbolas decreases from the root of the aerofoil to the tip of the aerofoil.

[0012] It should be noted that any one of the aspects mentioned above may include any of the features of any of the other aspects mentioned above and may include any of the features of any of the embodiments described below as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows.

[0014] Figure 1 shows a side view of a wing chord modified in accordance with the invention.

[0015] Figure 2A shows a wing modified in accordance with the invention. Figure 2B shows chords along the modified wing.

[0016] Figure 3A shows a root chord of a standard Zenith Zodiac wings. Figure 3B shows a root chord of a Zenith Zodiac wing modified in accordance with the invention.

[0017] Figures 4A and 4B provide comparative pressure traces for the bottom surfaces of standard and modified Zenith Zodiac wings.

[0018] Figure 6 comparative power requirements of a standard and modified Liang Chi cooling tower fan.

[0019] Figures 7A, 7B and 7C show improvements in efficiency, thrust coefficient and power coefficient across a range of advance ratio for the modified Flynano blades.

DRAWING COMPONENTS

[0020] The drawings include the following integers.

10, 10’ wing chords

11 leading tip

12 convex hyperbolic curve

13 inflection point

14 concave hyperbolic curve

15 trailing tip

20 wing

21 bottom surface

22 leading edge

24 trailing edge

25 root

26 tip

30 standard wing (Zenith Zodiac)

30’ modified wing (Zenith Zodiac)

31 , 3 T bottom surface

32, 32’ leading edge

33, 33’ trailing edge

34, 34’ root

35, 35’ tip

36’ increased pressure region

37’ decreased pressure region

38, 38’ top surface

42’ convex hyperbolic front section 43’ inflection point

44’ concave hyperbolic rear section

DETAILED DESCRIPTION OF THE INVENTION

[0021] The following detailed description of the invention refers to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts. Dimensions of certain parts shown in the drawings may have been modified and/or exaggerated for the purposes of clarity or illustration.

[0022] The present provides an aerofoil which utilises the lower surface adapted to produce increased lift combining the advantages of both lift and pressure type fluid dynamic devices by producing pressure as well as lift without adversely affecting the lift/drag ratio. The lower surface is in the form of a convex hyperbola towards the leading edge and a symmetrical concave hyperbola towards the trailing edge.

[0023] Fluid dynamic blades are devices to produce thrust, or lift, as a result of fluid flow over the blade surface. Typically, the thrust/lift is the sum of the positive pressure on the back and negative pressure on the front of the blade. The majority of thrust/lift is produced from the negative pressure component with only a small portion attributed to the positive pressure side. This results in high relative conversion efficiency compared to drag type surfaces which generate thrust by positive pressure only. Drag type devices do have better low velocity performance compared to lift type devices but max out due to friction drag at higher flow velocities.

[0024] The back side or pressure side of the profile of the invention is comprised of a contour constructed from hyperbolic geometry. The pressure side profile of hyperbolic sections with increasing focal length towards the air foil hub to. This provides relatively higher pressure increases towards the hub to provide greater stability, and to minimise the effect on any control surfaces which are typically towards the tip. The surface is divided longitudinally comprising a convex hyperbolic surface towards the leading edge and a concave mirror shape towards the trailing edge. The conjugate axis for the curve sections may be rotated along the centre line to achieve the appropriate blade twist in order to reduce the angle of attack from hub to the tip.

[0025] On the leading edge, where the profile is concave, air is accelerated due to higher tangential angle of attack. The concave curvature profile is like the geometry in lines of flow naturally occurring in nature when fluid follows the path of least resistance. Incorporating this path curve geometry within a three-dimensional contour result in a shape with the lowest possible drag coefficient that is practically achievable. Consequently, the fluid is accelerated over the convex curvature section of the profile with high efficiency and minimum drag. Past the centre line, the tangential angle of attack reduces to its lowest value in the concave section of the contour. This airflow velocity gradient translates into a corresponding pressure gradient where the pressure is the highest in the concave part of the contour. The use of hyperbolic curves optimises flow and minimises friction.

[0026] Figure 1 shows a side view of a wing chord 10 with a bottom section in accordance with the invention. On the bottom of the chord 10 a convex hyperbolic curve 12 extends from the leading tip 11 to an inflection point 13 halfway along the chord. A concave hyperbolic curve 14 extends from the inflection point to the trailing tip 15. The convex and concave hyperbolic curves are symmetrical in being the same length and with the same focal length. Tests have shown a larger focal length has a more pronounced affect, however the actual focal length that can be used in practise is limited by mechanical limitations on wing structure to ensure adequate strength and internal space. The actual form of the curves may also need to be compromised towards the leading and trailing tips to transition smoothly to the top section of the chord.

[0027] Figure 2A shows a wing 20 with a bottom surface 21 in accordance with the invention, whilst Figure 2B shows wing chords at intervals along the wing. The chord 10 closest to the root 25 of the wing 20 shows a bottom edge with a clearly discernible convex hyperbolic curve 12 towards the leading edge of the wing 22 and a concave hyperbolic curve 14 towards the trailing edge of the wing 24. Moving from the root of the wing 25 to the tip of the wing 26, the hyperbolic curves can be seen to have decreasing focal length until the chord 10’ at the wing tip 26 has no discernible hyperbolic curves. This gives maximum effect (i.e. increased lift) towards the wing root, and minimal effect towards the wing tip.

[0028] Whilst the teachings of the invention can be used to design an aerofoil from scratch, the advantages of the invention are best appreciated by examples in which an existing well-known aerofoil is modified in accordance with an invention and the performance of the original and modified aerofoil are compared.

[0029] In a first example a wing 30 from a Zenith Zodiac CH601 XL is modified in accordance with the invention to form a modified wing 30’. Root chords from a standard wing are shown in Figure 3A and a modified wing in Figure 3B. The standard wing 30 has a convex top surface 38 tapering towards the trailing edge. The top surface 38’ of the modified wing 30’ remains unchanged from the standard wing. The bottom surface 31 of the standard wing is flat, whereas the bottom surface 31 ’ of the modified wing has a convex hyperbolic front section 42’ extending from the leading edge 32’ to an inflection point 43’ midway along the chord where the surface changes to a concave hyperbolic rear section 44’ extending to the trailing edge 33’. In simulations of the wings at typical take off conditions, a speed of 70 km/h and an angle of attack (AoA) of 15 degrees, the modified wing 30’ achieved a 11% increase in the lift coefficient (lift - drag) and also a 1 .3% increase in the lift/drag ratio compared to the standard wing 30. In typical cruising conditions, a speed of 200 km/h and an angle of attack (AoA) of 3 degrees, the modified wing 30’ achieved an 8.5% increase in the lift coefficient (lift - drag) with just a 1 .2% decrease in the lift/drag ratio. Comparative lift and drag figures are presented below in Table 1 for a 15-degree angle of attack (AoA), and in Table 2 for a 3-degree angle of attack.

[0030] Table 1 . Lift and drag comparison for 15-degree AoA @ 70 km/h

[0031] Table 2. Lift and drag comparison for 3-degree AoA @ 200km/h

[0032] Figures 4A and 4B provide comparative pressure traces for the bottom surfaces 31 , 31 ’ of the standard 30 and modified 30’ wings at 200 km/h and a 3-degree angle of attack. For the unmodified wing 30, there is a slight increase in pressure from tip 35 to root 34 and from and slightly higher pressure at the leading edge 32 compared with the trailing edge 33. The modified wing 30’ shows a region with a marked increase in pressure 36’ towards the leading edge 32’ and root 34’, corresponding with the convex hyperbolic curve with maximum focal length, and a region with a marked decrease in pressure 37’ towards the trailing edge 33’ and root 34’, corresponding with the concave hyperbolic curve with maximum focal length. Comparative pressure traces for the top surfaces of the standard and modified wings show now discernible difference showing that the modification to the bottom surface of the wing has had little if any effect on the performance of the top side of the wing. Similar results are seen for pressures traces at 70 km/h and a 15-degree angle of attack (not shown).

[0033] Comparative flow trajectory traces (not shown) for the bottom surfaces 31 , 31 ’ of the standard 30 and modified 30’ wings at 200 km/h and a 3-degree angle of attack have also been undertaken. With the standard wing 30 the flow is reasonably uniform across from the leading edge 32 to trailing edge 33 with a decrease in speed toward the wing root 34. With the modified wing 30’ there is a region of increased speed towards the leading edge 32’ and root 35’ of the wing, corresponding to the convex hyperbolic surface with maximum focal length, and a region of decreased speed towards the trailing edge 33’ and root 35’ of the wing, corresponding to the concave hyperbolic surface with maximum focal length. Comparative flow trajectory traces for the top surface of the wings also show that the modification to the underside of the wing has little if any effect on the performance of the top side of the wing. Similar results are seen for flow traces at 70 km/h and a 15-degree angle of attack (not shown).

[0034] In a second example a NACA 1412 Aerofoil is modified in accordance with the invention. The modified aerofoil is also scaled down from a surface area of 8.37 m ² to a surface area of 6.12 m ² without limiting performance or efficiency. Additional 25% of lift/th rust is generated on the pressure side of the air foil. The modified aircraft wing delivering the same lift with reduced surface area, is overall more efficient due to the reduction in material and weight.

[0035] In a third example an aircraft propeller is modified in accordance with the invention delivering the same thrust with reduced diameter, achieving higher flow velocities before the critical sonic tip speed limit is reached. In the example, a 25% increase in efficiency is achieved.

[0036] In a fourth example a static fan blade, being a 1 .5 m diameter Liang Chi cooling tower fan, is modified in accordance with the invention. Figure 5 compares the power required to drive the two fans at different flow rates, with the modified fan achieving approximately 12% reduction in input power.

[0037] In a fifth example blades from a quadcopter are modified in accordance with the invention. The modified blade shows a 100% increase in efficiency at advance ratio of J 0.5 and a 66% increase in the thrust coefficient at J 0.

[0038] In a sixth example blades from a Flynano quadcopter are modified in accordance with the invention. Figures 6A, 6B and 6C show improvements in efficiency, thrust coefficient and power coefficient across a range of advance ratio.

[0039] The reader will now appreciate the present invention which provides an aerofoil with a bottom surface adapted to produce increased lift without unduly increasing drag. The examples given demonstrate the applicability of the invention across various aerofoils. Whilst improvements are maximised with the geometry shown, i.e. symmetrical convex and concave hyperbolic curves, compromises in geometry are often required to accommodate aircraft mechanics and simplify construction. Non-hyperbolic curves and non-symmetrical curves may be used with lesser results achieved.

[0040] Whilst the invention is applicable to the examples discussed above, it is equally applicable to any application where a surface is used to produce lift or thrust as it moves through a fluid, both air and water. Further example applications include: helicopter rotors; gyrocopter rotors; static fans; static pressure fans (hovercraft); Turbine blades; pump impellers; horizontal axis wind turbine blades; hydro power turbine blades; vertical axis horizontal take-off and landing aircraft; unmanned and manned drone propellers; ducted fans; ducted water jet impellers; and, marine propellers.

[0041] Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims, so as to embrace any and all equivalent devices and apparatus. Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in this field.

[0042] In the present specification and claims (if any), the word "comprising" and its derivatives including "comprises" and "comprise" include each of the stated integers but does not exclude the inclusion of one or more further integers.