Airfoils for rotor blades of rotary wing aircrafts

This application claims priority to European patent application No. EP 15400028.5, filed Jul. 3, 2015, the disclosure of which is incorporated in its entirety by reference herein.

(1) Field of the Invention

The invention is related to an airfoil for a rotor blade of a rotary wing aircraft, said airfoil having a predetermined maximum thickness to chord ratio. The invention is further related to a rotor blade of a rotary wing aircraft.

(2) Description of Related Art

Airfoils for rotor blades of rotary wing aircrafts are generally characterized by an extrados and an intrados that are arranged between corresponding leading and trailing edges of the airfoils and that are defined by predetermined intrados coordinate value pairs x/c, y int /c and predetermined extrados coordinate value pairs x/c, y ext /c. Therein, x is a distance from a leading edge to a trailing edge along a chord line of the airfoil, c is a length of the airfoil in chord direction, y int is a distance between the chord line and a respective intrados surface and y ext is a distance between the chord line and a respective extrados surface. The airfoils are usually further characterized by associated predetermined maximum thickness to chord ratios, as well as by leading edge radiuses r/c of corresponding leading edge circles and their center positions x/c and y int /c or y ext /c.

The documents U.S. Pat. No. 5,957,662 A and U.S. Pat. No. 6,315,522 B1 describe such airfoils and illustrate exemplary tables of values that comprise values for predetermined intrados coordinate value pairs x/c, y int /c and predetermined extrados coordinate value pairs x/c, y ext /c. More specifically, the document U.S. Pat. No. 5,957,662 A describes an airfoil with a maximum thickness to chord ratio of 8%, a leading edge radius r/c=0.00844 and corresponding leading edge circle center positions x/c=0.00842 and y ext /c=0.00064. The document U.S. Pat. No. 6,315,522 B1 describes an airfoil with a maximum thickness to chord ratio of 10%, a leading edge radius r/c=0.0096 and corresponding leading edge circle center positions x/c=0.0097 and y int /c=−0.0018.

Furthermore, the document U.S. Pat. No. 6,361,279 B1 describes airfoils with maximum thickness to chord ratios of 12% (Table 3) and 9% (Table 5). Nevertheless, for these airfoils no leading edge radiuses r/c of corresponding leading edge circles and their center positions x/c and y int /c or y ext /c are indicated. Instead, maximum camber positions and ratios of maximal camber to maximal thickness are used for further characterizing these airfoils.

However, these airfoils only provide for a restricted maximum lift capacity and a comparatively high aerodynamic drag. Furthermore, they only provide for restricted hover and forward flight performances.

It is, therefore, an object of the present invention to provide a new airfoil for a rotor blade of a rotary wing aircraft, said airfoil providing for a reduced aerodynamic drag, an improved maximum lift capacity and improved hover and forward flight performances of said rotor blade. It is a further object of the present invention to provide a new rotor blade having an airfoil that provides for a reduced aerodynamic drag, an improved maximum lift capacity and improved hover and forward flight performances.

This object is solved by an airfoil for a rotor blade of a rotary wing aircraft, said airfoil having a maximum thickness to chord ratio of 12%. This object is also solved by an airfoil for a rotor blade of a rotary wing aircraft, said airfoil having a maximum thickness to chord ratio of 9%. This object is further solved by an airfoil for a rotor blade of a rotary wing aircraft, said airfoil having a maximum thickness to chord ratio of 8%.

Advantageously, these inventive airfoils exhibit a reduced aerodynamic drag by means of extended laminarity. Furthermore, an underlying maximum lift capacity is improved. Moreover, more negative, i. e. nose-down, static pitching moments around the quarter-chord of the airfoils due to the applied modifications to the camber line are acceptable, since dynamic loads as seen in numerical simulations and wind tunnel measurements prove to remain within those of prior art airfoils.

More specifically, the inventive airfoils exhibit reduced aerodynamic drag compared to prior art airfoils due to extended laminar running lengths on the upper side, i. e. the extrados. This is achieved by an improved leading edge geometry and by introducing a well-defined main pressure recovery at a downstream position, i. e. towards the trailing edge of the airfoil. These measures further enable the reduction of suction peaks and thereby allow for higher maximum lift coefficients at moderate Mach numbers as well as the reduction of transonic shocks at high lift coefficients. Furthermore, the application of the inventive airfoils provides for improved hover and forward flight performances of a given rotor of a rotary wing aircraft.

Preferably, at least one of the inventive airfoils, more preferably at least two of the inventive airfoils and, preferentially, all inventive airfoils are applied to a single rotor blade of a given rotary wing aircraft. In other words, such a single rotor blade is provided with differing geometric shapes at different locations over its radial extension. Areas between these different locations are preferably provided with geometric shapes that result from interpolations between corresponding adjacent airfoils.

More specifically, according to one aspect of the present invention an airfoil for a rotor blade of a rotary wing aircraft that has a maximum thickness to chord ratio of 12% comprises, between a leading edge and a trailing edge, an extrados and an intrados. The intrados is defined by predetermined intrados coordinate value pairs x/c, y int /c and the extrados is defined by predetermined extrados coordinate value pairs x/c, y ext /c. Therein, x is a distance from the leading edge to the trailing edge along a chord line of the airfoil, c is a length of the airfoil in chord direction, y int is a distance between the chord line and a respective intrados surface and y ext is a distance between the chord line and a respective extrados surface. The predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c are defined according to Table 1 below.

A maximum error of each one of the predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c preferentially amounts to ±3%.

According to a preferred embodiment of the present invention, an airfoil for a rotor blade of a rotary wing aircraft with a maximum thickness to chord ratio within a range from 9% to 15% is provided that comprises intrados coordinate value pairs and extrados coordinate value pairs, which are determined on the basis of a reference airfoil that is defined by the airfoil having the maximum thickness to chord ratio of 12% as described above.

According to a further preferred embodiment of the present invention, an associated leading edge radius of a leading edge circle that defines an underlying curvature at the leading edge of the airfoil having the maximum thickness to chord ratio of 12% as described above amounts to r/c=0.01314. The leading edge circle has its associated center of circle located at x/c=0.01314 and y int /c=y ext /c=0.00000.

According to a further preferred embodiment of the present invention, a trailing edge tab is provided at the trailing edge of the airfoil. The trailing edge tab is defined by an at least essentially straight and parallel intrados and extrados.

According to a further preferred embodiment of the present invention, the trailing edge tab comprises a length in a range from 2% to 10% of the length of the airfoil in chord direction with a maximum inclination error of ±5° relative to the chord line of the airfoil.

According to another aspect of the present invention, an airfoil for a rotor blade of a rotary wing aircraft that has a maximum thickness to chord ratio of 9% comprises, between a leading edge and a trailing edge, an extrados and an intrados. The intrados is defined by predetermined intrados coordinate value pairs x/c, y int /c and the extrados is defined by predetermined extrados coordinate value pairs x/c, y ext /c. Therein, x is a distance from the leading edge to the trailing edge along a chord line of the airfoil, c is a length of the airfoil in chord direction, y int is a distance between the chord line and a respective intrados surface and y ext is a distance between the chord line and a respective extrados surface. The predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c are defined according to Table 2 below.

A maximum error of each one of the predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c preferentially amounts to ±3%.

According to a preferred embodiment of the present invention, an airfoil for a rotor blade of a rotary wing aircraft with a maximum thickness to chord ratio within a range from 7% to 12% is provided that comprises intrados coordinate value pairs and extrados coordinate value pairs, which are determined on the basis of a reference airfoil that is defined by the airfoil having the maximum thickness to chord ratio of 9% as described above.

According to a further preferred embodiment of the present invention, an associated leading edge radius of a leading edge circle that defines an underlying curvature at the leading edge of the airfoil having the maximum thickness to chord ratio of 9% as described above amounts to r/c=0.00481. The leading edge circle has its associated center of circle located at x/c=0.00481 and y ext /c=0.00008.

According to a further preferred embodiment of the present invention, a trailing edge tab is provided at the trailing edge of the airfoil. The trailing edge tab is defined by an at least essentially straight and parallel intrados and extrados.

According to a further preferred embodiment of the present invention, the trailing edge tab comprises a length in a range from 2% to 10% of the length of the airfoil in chord direction with a maximum inclination error of ±5° relative to the chord line of the airfoil.

According to still another aspect of the present invention, an airfoil for a rotor blade of a rotary wing aircraft that has a maximum thickness to chord ratio of 8% comprises, between a leading edge and a trailing edge, an extrados and an intrados. The intrados is defined by predetermined intrados coordinate value pairs x/c, y int /c and the extrados is defined by predetermined extrados coordinate value pairs x/c, y ext /c. Therein, x is a distance from the leading edge to the trailing edge along a chord line of the airfoil, c is a length of the airfoil in chord direction, y int is a distance between the chord line and a respective intrados surface and y ext is a distance between the chord line and a respective extrados surface. The predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c are defined according to Table 3 below.

A maximum error of each one of the predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c preferentially amounts to ±3%.

According to a preferred embodiment of the present invention, an airfoil for a rotor blade of a rotary wing aircraft with a maximum thickness to chord ratio within a range from 6% to 10% is provided that comprises intrados coordinate value pairs and extrados coordinate value pairs, which are determined on the basis of a reference airfoil that is defined by the airfoil having the maximum thickness to chord ratio of 8% as described above.

According to a further preferred embodiment of the present invention, an associated leading edge radius of a leading edge circle that defines an underlying curvature at the leading edge of the airfoil having the maximum thickness to chord ratio of 8% as described above amounts to r/c=0.00502. The leading edge circle has its associated center of circle located at x/c=0.00502 and y ext /c=0.00028.

According to a further preferred embodiment of the present invention, a trailing edge tab is provided at the trailing edge of the airfoil. The trailing edge tab is defined by an at least essentially straight and parallel intrados and extrados.

According to a further preferred embodiment of the present invention, the trailing edge tab comprises a length in a range from 2% to 10% of the length of the airfoil in chord direction with a maximum inclination error of ±5° relative to the chord line of the airfoil.

The above described object of the present invention is also solved by a rotor blade of a rotary wing aircraft that comprises at least one of the above-described airfoils.

Thus, new rotor blades and in particular new main rotor blades can be manufactured that allow for an improvement of an overall performance of a main rotor of a rotary wing aircraft, in particular of a helicopter, while maintaining favorable unsteady characteristics thereof, such as dynamic stall. Advantageously, such new main rotor blades can be provided with a particular geometric shape by application of the inventive airfoils at specific radial positions along these main rotor blades.

Preferred embodiments of the invention are outlined by way of example in the following description with reference to the attached drawings. In these attached drawings, identical or identically functioning components and elements are labeled with identical reference numbers and characters and are, consequently, only described once in the following description.

FIG. 1 shows a sectional view of a rotor blade of a rotary wing aircraft with an airfoil having a maximum thickness to chord ratio of 12%,

FIG. 2 shows a sectional view of the rotor blade of FIG. 1 with an airfoil having a maximum thickness to chord ratio of 9%,

FIG. 3 shows a sectional view of the rotor blade of FIG. 1 with an airfoil having a maximum thickness to chord ratio of 8%,

FIG. 4 shows a maximum lift coefficient diagram for the airfoils of FIG. 1 to FIG. 3 ,

FIG. 5 shows a maximum lift to drag ratio diagram for the airfoils of FIG. 1 to FIG. 3 ,

FIG. 6 shows a pressure coefficient diagram for the airfoil of FIG. 2 , and

FIG. 7 shows a rotor figure of merit for rotor blades having the airfoils of FIG. 1 and FIG. 2 .

FIG. 1 shows a rotor blade 1 of a rotary wing aircraft that is provided with an airfoil 1 a according to a first aspect of the present invention. The airfoil 1 a preferably comprises a maximum thickness to chord ratio of 12%.

Illustratively, the airfoil 1 a comprises a leading edge 3 a and a trailing edge 3 b , which are illustratively arranged on an airfoil chord line 2 of the airfoil 1 a. The airfoil 1 a illustratively further comprises an intrados 4 a and an extrados 4 b that extend between the leading edge 3 a and the trailing edge 3 b. The intrados 4 a is defined by predetermined intrados coordinate value pairs x/c, y int /c and the extrados 4 b is defined by predetermined extrados coordinate value pairs x/c, y ext /c. Therein, x is a distance from the leading edge 3 a to the trailing edge 3 b along the chord line 2 , c is a length of the airfoil 1 a in chord direction, y int is a distance between the chord line 2 and a respective intrados surface and y ext is a distance between the chord line 2 and a respective extrados surface.

According to one aspect of the present invention, the predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c of the airfoil 1 a that comprises a maximum thickness to chord ratio of 12% are defined according to Table 1 above. Preferably, a maximum error of each one of these predetermined intrados coordinate value pairs x/c, y int /c and these predetermined extrados coordinate value pairs x/c, y ext /c amounts to ±3%.

Preferably, the predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c of the airfoil 1 a are determined in the region of the leading edge 3 a on the basis of an associated leading edge circle 2 a that, thus, defines an underlying curvature of the airfoil 1 a at the leading edge 3 a. The associated leading edge circle 2 a preferably comprises a radius r with r/c=0.01314. The center of circle of the leading edge circle 2 a is preferably located at x/c=0.01314 and y int /c=y ext /c=0.00000.

Preferentially, a maximum error of each one of the above described leading edge circle dimensions amounts to ±5%. This likewise applies to all leading edge circle dimensions that are described in the present patent application.

According to one aspect of the present invention, the trailing edge 3 b is provided with an associated trailing edge tab 2 b. The latter is preferentially defined by an at least essentially straight and parallel intrados 4 a and extrados 4 b. Preferably, the trailing edge tab 2 b comprises a length in a range from 2% to 10% of the length of the airfoil 1 a in chord direction with a maximum inclination error of ±5° relative to the chord line 2 of the airfoil 1 a.

The predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c are exemplarily illustrated in an airfoil coordinate system 5 , wherein the x/c values are outlined on a x-axis 5 a and wherein the y int /c and the y ext /c values are outlined on a y-axis 5 b. Accordingly, a chordwise position of a maximum thickness 6 of the airfoil 1 a is illustrated on the basis of Table 1 above at the location x/c=0.284.

It should be noted that Table 1 above is illustrated by way of example with respect to the airfoil 1 a having a maximum thickness to chord ratio of 12%. However, this airfoil 1 a and, consequently Table 1, may also be used as a reference for defining other airfoils for rotor blades of rotary wing aircrafts with other maximum thickness to chord ratios. According to one aspect of the present invention, the airfoil 1 a is used as a reference airfoil for determining intrados coordinate value pairs and extrados coordinate value pairs for an airfoil that comprises a maximum thickness to chord ratio within a range from 9% to 15%.

More specifically, the maximum thickness to chord ratio of 12% of the airfoil 1 a at the location x/c=0.284 is obtained by dividing the sum of the values |y ext | and |y int | at this location x/c=0.284 according to Table 1 through the length c of the airfoil 1 a in chord direction. Therein, |y int | defines approximately 28.5% of the maximum thickness and |y ext | approximately 71.5% thereof. If now, by way of example, a maximum thickness to chord ratio of 15% should be realized, the values |y ext | and |y int | at the location x/c=0.284 according to Table 1 are rescaled such that the sum of the values |y ext | and |y int | at this location x/c=0.284 according to Table 1 divided through the length c of the airfoil 1 a in chord direction amounts to 15%, wherein |y int | still defines approximately 28.5% of the maximum thickness and |y ext | approximately 71.5%. Such a rescaling is then correspondingly applied to all other values of Table 1 for obtaining an airfoil with the maximum thickness to chord ratio of 15%.

It should be noted that the rescaling for obtention of the airfoil with a maximum thickness to chord ratio of 15% is described above only by way of example and can similarly be applied to all other airfoils and maximum thickness to chord ratios described in the present patent application. However, for brevity and conciseness, the explanation of such a rescaling for other airfoils and maximum thickness to chord ratios will not be repeated hereinafter.

FIG. 2 shows the rotor blade 1 of FIG. 1 that is now provided with an airfoil 1 b according to a second aspect of the present invention, instead of the airfoil 1 a of FIG. 1 . For brevity and conciseness, however, only the differences between this airfoil 1 b and the airfoil 1 a of FIG. 1 are described in detail hereinafter.

In contrast to the airfoil 1 a of FIG. 1 , the airfoil 1 b preferably comprises a maximum thickness to chord ratio of 9% and the predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c of the airfoil 1 b are defined according to Table 2 above, with c now being a length of the airfoil 1 b in chord direction. A chordwise position of the maximum thickness 6 of the airfoil 1 b is illustrated on the basis of Table 2 above at the location x/c=0.349.

Furthermore, while the predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c of the airfoil 1 b are determined in the region of the leading edge 3 a similar to the airfoil 1 a of FIG. 1 on the basis of the leading edge circle 2 a, the latter now preferably comprises a radius r with r/c=0.00481. The center of circle of the leading edge circle 2 a of the airfoil 1 b is now preferably located at x/c=0.00481 and y ext /c=0.00008.

Finally, it should be noted that Table 2 above is illustrated by way of example with respect to the airfoil 1 b having a maximum thickness to chord ratio of 9%. However, this airfoil 1 b and, consequently Table 2, may also be used as a reference for defining other airfoils for rotor blades of rotary wing aircrafts with other maximum thickness to chord ratios. According to one aspect of the present invention, the airfoil 1 b is used as a reference airfoil for determining intrados coordinate value pairs and extrados coordinate value pairs for an airfoil that comprises a maximum thickness to chord ratio within a range from 7% to 12%.

FIG. 3 shows the rotor blade 1 of FIG. 1 that is now provided with an airfoil 1 c according to a third aspect of the present invention, instead of the airfoil 1 a of FIG. 1 . For brevity and conciseness, however, only the differences between this airfoil 1 c and the airfoil 1 a of FIG. 1 are described in detail hereinafter.

In contrast to the airfoil 1 a of FIG. 1 , the airfoil 1 c preferably comprises a maximum thickness to chord ratio of 8% and the predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c of the airfoil 1 b are defined according to Table 3 above, with c now being a length of the airfoil 1 c in chord direction. A chordwise position of the maximum thickness 6 of the airfoil 1 c is illustrated on the basis of Table 3 above at the location x/c=0.353.

Furthermore, while the predetermined intrados coordinate value pairs x/c, y int /c and the predetermined extrados coordinate value pairs x/c, y ext /c of the airfoil 1 c are determined in the region of the leading edge 3 a similar to the airfoil 1 a of FIG. 1 on the basis of the leading edge circle 2 a, the latter now preferably comprises a radius r with r/c=0.00502. The center of circle of the leading edge circle 2 a of the airfoil 1 c is now preferably located at x/c=0.00502 and y ext /c=0.00028.

Finally, it should be noted that Table 3 above is illustrated by way of example with respect to the airfoil 1 c having a maximum thickness to chord ratio of 8%. However, this airfoil 1 c and, consequently Table 3, may also be used as a reference for defining other airfoils for rotor blades of rotary wing aircrafts with other maximum thickness to chord ratios. According to one aspect of the present invention, the airfoil 1 c is used as a reference airfoil for determining intrados coordinate value pairs and extrados coordinate value pairs for an airfoil that comprises a maximum thickness to chord ratio within a range from 6% to 10%.

In the following, the inventive airfoils according to FIG. 1 to FIG. 3 above are compared to conventional airfoils. By way of example, the airfoils with maximum thickness to chord ratios of 12% and 9% according to tables 3 and 5, respectively, of the document U.S. Pat. No. 6,361,279 B1 are selected as the conventional airfoils.

FIG. 4 shows a maximum lift coefficient diagram 7 with maximum lift coefficients C I, max outlined on a maximum lift coefficient axis 7 b, which are plotted over Mach numbers outlined on a Mach number axis 7 a. The maximum lift coefficient diagram 7 illustrates three exemplary rotor blade airfoil characteristic curves 8 a, 9 a, 10 a for demonstrating maximum lift coefficients that can be obtained in operation when applying the airfoils 1 a, 1 b, 1 c of FIG. 1 to FIG. 3 . More specifically, the characteristic curve 8 a illustrates maximum lift coefficients of the 12% airfoil 1 a of FIG. 1 , the characteristic curve 9 a illustrates maximum lift coefficients of the 9% airfoil 1 b of FIG. 2 and the characteristic curve 10 a illustrates maximum lift coefficients of the 8% airfoil 1 c of FIG. 3 .

The maximum lift coefficient diagram 7 further illustrates two exemplary rotor blade airfoil characteristic curves 8 b, 9 b for demonstrating maximum lift coefficients that can be obtained in operation when applying conventional airfoils. More specifically, the characteristic curve 8 b illustrates maximum lift coefficients of a conventional 12% airfoil and the characteristic curve 9 b illustrates maximum lift coefficients of a conventional 9% airfoil.

FIG. 4 is intended to allow a comparison between the characteristic curves 8 a and 8 b as well as 9 a and 9 b. As can be seen from this comparison, higher maximum lift coefficients can be obtained with the 12% airfoil 1 a of FIG. 1 and the 9% airfoil 1 b of FIG. 2 , than with the corresponding conventional airfoils in the illustrated Mach number range from 0.3 to 0.6 for the 12% airfoil, respectively 0.4 to 0.6 for the 9% airfoil. Furthermore, even the 8% airfoil 1 c of FIG. 3 allows to achieve higher maximum lift coefficients in the illustrated Mach number range from 0.4 to 0.6 than those that are obtained with the conventional 9% airfoil, as can be seen from a comparison of the characteristic curves 10 a and 9 b.

FIG. 5 shows a maximum lift to drag ratio diagram 11 with maximum lift to drag ratios L/D max outlined on a maximum lift to drag ratio axis 11 b, which are plotted over Mach numbers outlined on a Mach number axis 11 a. The maximum lift to drag ratio diagram 11 illustrates three exemplary rotor blade airfoil characteristic curves 12 a, 13 a, 14 a for demonstrating maximum lift to drag ratios that can be obtained in operation when applying the airfoils 1 a, 1 b, 1 c of FIG. 1 to FIG. 3 . More specifically, the characteristic curve 12 a illustrates maximum lift to drag ratios of the 12% airfoil 1 a of FIG. 1 , the characteristic curve 13 a illustrates maximum lift to drag ratios of the 9% airfoil 1 b of FIG. 2 and the characteristic curve 14 a illustrates maximum lift to drag ratios of the 8% airfoil 1 c of FIG. 3 .

The maximum lift to drag ratio diagram 11 further illustrates two exemplary rotor blade airfoil characteristic curves 12 b, 13 b for demonstrating maximum lift to drag ratios that can be obtained in operation when applying conventional airfoils. More specifically, the characteristic curve 12 b illustrates maximum lift to drag ratios of a conventional 12% airfoil and the characteristic curve 13 b illustrates maximum lift to drag ratios of a conventional 9% airfoil.

FIG. 5 is intended to allow a comparison between the characteristic curves 12 a and 12 b as well as 13 a and 13 b. As can be seen from this comparison, higher maximum lift to drag ratios can be obtained with the 12 % airfoil 1 a of FIG. 1 and the 9% airfoil 1 b of FIG. 2 , than with the corresponding conventional airfoils in the illustrated Mach number range from 0.3 to 0.6. Furthermore, even the 8% airfoil 1 c of FIG. 3 allows to achieve higher maximum lift to drag ratios in the illustrated Mach number range from 0.3 to 0.6, except for the Mach number 0.5, than those that are obtained with the conventional 9% airfoil, as can be seen from a comparison of the characteristic curves 14 a and 13 b.

FIG. 6 shows a pressure coefficient diagram 15 with pressure coefficients C p outlined on a pressure coefficient axis 15 b, which are plotted over an underlying chord length outlined on a chord axis 15 a. The pressure coefficient diagram 15 illustrates two exemplary rotor blade airfoil characteristic curves 16 a, 16 c for demonstrating pressure coefficients that can be obtained in operation when applying the airfoil 1 b of FIG. 2 . More specifically, the characteristic curve 16 a illustrates pressure coefficients obtained at the extrados 4 b of the 9% airfoil 1 b of FIG. 2 and the characteristic curve 16 c illustrates pressure coefficients obtained at the intrados 4 a of the 9% airfoil 1 b of FIG. 2 .

The pressure coefficient diagram 15 further illustrates two exemplary rotor blade airfoil characteristic curves 16 b, 16 d for demonstrating pressure coefficients that can be obtained in operation when applying conventional airfoils. More specifically, the characteristic curve 16 b illustrates pressure coefficients obtained at the extrados of a conventional 9% airfoil and the characteristic curve 16 d illustrates pressure coefficients obtained at the intrados of a conventional 9% airfoil.

FIG. 6 is intended to allow a comparison between the characteristic curves 16 a and 16 b as well as 16 c and 16 d. As can be seen in particular from a comparison of the characteristic curves 16 a and 16 b, suction peaks 17 b on the extrados 4 b of the 9% airfoil 1 b of FIG. 2 are significantly smaller than on the extrados of the conventional 9% airfoil, thereby allowing for higher maximum lift coefficients at moderate Mach numbers, as illustrated in FIG. 4 , and further allowing the reduction of transonic shocks at high lift coefficients. Furthermore, a geometrically defined main pressure recovery 17 a is introduced at a downstream position of the extrados 4 b of the 9% airfoil 1 b of FIG. 2 compared to the conventional 9% airfoil, thus allowing for a reduced aerodynamic drag due to an extended laminar running length on the extrados 4 b of the 9% airfoil 1 b of FIG. 2 .

FIG. 7 shows a rotor figure of merit 18 with figure of merit values FM outlined on a figure of merit axis 18 b, which are plotted over blade loading values C T /σ outlined on a blade loading axis 18 a. The rotor figure of merit 18 illustrates an exemplary rotor blade characteristic curve 19 a for demonstrating figure of merit values that can be obtained in operation when applying the 12% airfoil 1 a of FIG. 1 and the 9% airfoil 1 b of FIG. 2 in a single rotor blade, e. g. the rotor blade 1 of FIG. 1 and FIG. 2 . The rotor figure of merit 18 further illustrates an exemplary rotor blade characteristic curve 19 b for demonstrating figure of merit values that can be obtained in operation when applying conventional 9% and 12% airfoils in a single rotor blade.

When comparing the characteristic curves 19 a, 19 b, it can be seen that a gain 20 of approximately 1% can be achieved. Moreover, the single rotor blade with the 12% airfoil 1 a of FIG. 1 and the 9% airfoil 1 b of FIG. 2 can further be improved according to one aspect of the present invention by applying the 8% airfoil 1 c of FIG. 3 in a corresponding blade tip region thereof in order to alleviate transonic shocks at a respectively advancing blade side of the single rotor blade in operation in case of high speed forward flight. More specifically, replacement of the conventional 9% and 12% airfoils by the 9% and 12% airfoils 1 a, 1 b of FIG. 1 and FIG. 2 on an exemplary rotor—with identical planform shape, twist and radial thickness distribution—advantageously leads to an improved hover performance in terms of ‘Figure of merit’.

It should be noted that modifications to the above described embodiments are within the common knowledge of the person skilled in the art and, thus, also considered as being part of the present invention. For instance, according to FIG. 1 to FIG. 3 described above, the rotor blade 1 of a rotary wing aircraft is respectively provided with one of the airfoils 1 a, 1 b, 1 c. However, the rotor blade 1 may likewise be provided with two of the airfoils 1 a, 1 b, 1 c, as well as all three airfoils 1 a, 1 b, 1 c, as described above with respect to FIG. 7 .

1 rotor blade

1 a 12% rotor blade airfoil

1 b 9% rotor blade airfoil

1 c 8% rotor blade airfoil

2 airfoil chord

2 a leading edge circle

2 b trailing edge tab

3 a airfoil leading edge

3 b airfoil trailing edge

4 a intrados

4 b extrados

5 airfoil coordinate system

5 a x-axis

5 b y-axis

6 maximum thickness

7 maximum lift coefficient diagram

7 a Mach number axis

7 b maximum lift coefficient axis

8 a 12% rotor blade airfoil characteristic curve

8 b conventional 12% rotor blade airfoil characteristic curve

9 a 9% rotor blade airfoil characteristic curve

9 b conventional 9% rotor blade airfoil characteristic curve

10 a 8% rotor blade airfoil characteristic curve

11 maximum lift to drag ratio diagram

11 a Mach number axis

11 b maximum lift to drag ratio axis

12 a 12% rotor blade airfoil characteristic curve

12 b conventional 12% rotor blade airfoil characteristic curve

13 a 9% rotor blade airfoil characteristic curve

13 b conventional 9% rotor blade airfoil characteristic curve

14 a 8% rotor blade airfoil characteristic curve

15 pressure coefficient diagram

15 a chord axis

15 b pressure coefficient axis

16 a 9% rotor blade airfoil extrados characteristic curve

16 b conventional 9% rotor blade airfoil extrados characteristic curve

16 c 9% rotor blade airfoil intrados characteristic curve

16 d conventional 9% rotor blade airfoil intrados characteristic curve

17 a main pressure recovery location

17 b suction peak location

18 rotor figure of merit

18 a blade loading axis

18 b figure of merit axis

19 a rotor blade with 9% and 12% airfoil characteristic curve

19 b conventional rotor blade with 9% and 12% airfoil characteristic curve

20 1% gain in figure of merit