GB2474511A

GB2474511A - Variable pitch aerofoil blade

Info

Publication number: GB2474511A
Application number: GB0918258A
Authority: GB
Inventors: Nigel Kevin Harris
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-10-19
Filing date: 2009-10-19
Publication date: 2011-04-20
Anticipated expiration: 2029-10-19
Also published as: GB2474511B; GB0918258D0

Abstract

An aerofoil is rotatable through a large range of pitch angles relative to a hub body. The hub has a diameter that is comparable to, or smaller than, the aerofoil chord to expose the aerofoil inner edge to rotational flow local to that body at large pitch angles relative to the body centre-line. At this condition, up to three leading edges are seen, each with typical and similar or dissimilar aerofoil cross-sections along lines where the fluid flows. At small angles relative to the body the aerofoil seats against the body surface. At this condition one train leading edge is seen with a typical and similar or dissimilar aerofoil cross-section to the first condition along lines where the fluid flows. These allow differing aerofoil sections to be exposed at different pitching angles and rotation rates to enable a rotor to be used over a large range of operating conditions.

Description

Full Description

Background of the Invention

Traditional rotating blades (e.g. in a compressor or helicopter rotor) tend to be of constant two-dimensional or slowly varying cross-section with blade span (with helix).

Further, the blade tends to experience flow that is largely perpendicular to the blade leading edge. These blades tend to have a limited range of pitch angle due to the local curvature of the body that they rotate around, or have an area on the body that rotates with the blade when it pitches, or have a protrusion that provides a surface for the blade to rotate upon. Given the cylindrical nature of the body, this area can cause airflow disruption locally around the blade. This situation is made worse if either the body diameter approaches the chord length of the blade or the blade is mounted on a cylindrical cone where profile changes are significantly three-dimensional. Although these effects can be relatively minor when considering a blade that has a high aspect ratio, the effect on low aspect ratio blades can be considerable.

Rotating blades are designed to encompass a range of operating conditions from zero to high vehicle speeds sometimes by incorporating a helix (or twist) in the blade with span. This allows use of a constant rotation rate to be provided over a range of vehicle speeds by pitching the blade. However, speeds are limited by shock-induced drag closer to the transonic speeds, which in turn produces vibration, reduced performance and considerable noise (helicopters being an example where the blade rotates into then out of wind). Further, the blade cross-section tends to be designed with either a low speed rounded leading edge or high speed sharp leading edge, neither of which are optimal for the others speed conditions. This either increases drag or reduces stall performance.

Moving through the transonic range into supersonics, the blade experiences additional drag in a similar way that non-sweptback wings do compared to sweptback wings on aeroplanes.

Invention Details It is an object of the present invention to improve aerodynamic performance of an aerofoil over a range of vehicle flight speed conditions where the aerofoil rotates around a body of smaller or comparable diameter to that of the aerofoil chord.

To achieve the above object, the aerofoil is heavily three-dimensional.

According to the first feature of the present invention, the aerofoil root is profiled so that at one optimal flight condition where the minimum pitch angle relative to the body centre-line is required for that condition, only the centre-line of the aerofoil root is completely seated on the body whereby little or no fluid flow can permeate between the root and body (figure 2).

According to the second feature of the present invention and at the conditions of the first feature, there is one aerofoil leading edge. This leading edge may be swept back or not swept back as required at these flight conditions (figure 2: Line X-X'). The sweep back may vary with span in order to minimize drag at the appropriate local Mach number.

According to the third feature of the present invention and at the conditions of the first feature, starting at points on the leading edge and taking a cross-section through the aerofoil along a line where the fluid flows (e.g. figure 2: Line A'-A), the aerofoil cross-section is typical for that flight condition. Examples may be but not limited to a subsonic symmetrical or cambered aerofoil section, a supercritical transonic section, or a supersonic section.

According to the fourth feature of the present invention and at the conditions of the first feature, the aerofoil tip is shaped such that viewing from the front down the length of the central body, the tip chord shape appears circular with a centre of rotation at the centre ofthe body (figure 2: Line X'-Y).

According to the fifth feature of the present invention, in combination with the first feature, as the aerofoil is required to pitch away from the minimum pitch angle to attain another optimal flight condition at a large pitch angle relative to the body centre-line, the space between the aerofoil root and the body is open, allowing fluid to flow between the two (figure 1).

According to the sixth feature of the present invention and at the conditions of the fifth feature, the exposed aerofoil root profile is such that starting at points on the inner edge and taking a cross-section through the aerofoil along a line where the fluid flows (e.g. figure 1: Line A-A') the aerofoil appears as a typical aerofoil cross-section which may be similar or dissimilar to that stated according to the third feature.

According to the seventh feature of the present invention and at the conditions of the fifth feature, the leading edge (figure 1: Line X-X') profile is such that starting at points on this edge and taking a cross-section through the aerofoil along a line where the fluid flows (e.g. figure 1: Line B-B') the aerofoil appears as a typical aerofoil cross-section which may be similar or dissimilar to that stated according to the third or sixth feature.

According to the eighth feature of the present invention and at the conditions of the fifth feature, the aerofoil tip experiences rotational flow. Starting at points on the tip and taking a cross-section through the aerofoil along a line where the fluid flows (e.g. figure 1: Line C-C'), the aerofoil appears as a typical aerofoil cross-section that may be similar or dissimilar to that stated according to the third, sixth or seventh feature.

According to the ninth feature of the present invention and at the conditions of the fifth feature, leading edge stated in the second feature may now appear more swept to the local rotational flow (figure 1: Line X-X'). This is dependent upon the aspect ratio of the aerofoil and the original swept angle stated in the second feature.

According to the tenth feature of the present invention and at the conditions of the fifth feature, the aerofoil now has up to three leading edges of differing or similar cross-section. For very low aspect ratio aerofoils where the leading edge (figure 1: Line X-X' and trailing edge are extended further away from the pitching point beyond the locality of the body, the now exposed leading edge (as stated in the sixth feature) and tip (as stated in the eighth feature) may provide the predominant aerodynamic characteristics of that aerofoil. This is different to that stated in the second feature.

According to the eleventh feature, the aerofoil rotates about a point that is required that best fits the conditions for an acceptable torque, and may be required to provide a naturally restoring torque back to equilibrium conditions where larger pitch angles relative to the body centre-line are needed at lower body speeds.

According to the twelfth feature, rotating the aerofoil can provide more than one condition whereby differing aerofoil profiles are required at different pitch angles, rotation rates and body speeds. These include but are not limited to an aerofoil that appears to have low aspect ratio (and hence low drag) in high-speed flight, but high aspect ratio (and hence high lift/drag) in low speed flight.

Claims

Claims 1) A highly pitchable aerofoil that improves the aerodynamic performance of a Iraditional subsonic, iransonic or supersonic (or higher) aerofoil over a range of vehicle flight speed conditions where the aeroibil rotates around a body of smaller or comparable diameter to that of the aeroibil chord.
2)Ahighlypitchable aerofoil according to claim (1) wherein at one oplimal flight condition where the minimum pitch angle relative to the body centre-line is required for that condition, little or no fluid flow can permeate between the root and body.
3) A highly pitchable aerofoil at the conditions of claim (2) wherein there is only one major aerofoil leading edge exposed at this optimum condition, swept or unswept and with a cross-section set to that optimum condition.
4) A highly pitchable aerofoil according to claim (2) wherein the tip is shaped to provide minimal drag in rotational flow.
5) Ahighlypitchable aerofoil according to claim (1) wherein as the aerofoilpitches away from the minimum pitch angle to attain another flight condition at a larger pitch angle relative to the body centre-line, the space between the aerolbil root and the body is opened, allowing fluid to flow between the two.
6) A highly pitchable aerofoil according to claim(S) wherein the exposed root appears as a typical aerofoil cross-section to the local flow, which may be similar, or dissimilar to that stated in claim (3).
7) A highly pitchable aerofoil according to claim(S) wherein the aerofoil tip appears as a typical aerofoil cross-section which may be 5imilar or dissimilar to that stated in claims (3) and (6).
8) A highly pitchable aerofoil according to claim(S) wherein the leading edge stated in claim (3) appears more swept to the local rotational flow and is dependent upon the aspect ratio of the aerofoil and the original leading edge swept angle.
9) A highly pitchable aerofoil according to claim(S) wherein the aerofoil has up to three leading edges of differing or similar cross-section at that condition.
10) A highly pitchable aerofoil according to claim(S) wherein at low and veiy low aspect ratios, the exposed leading edge and tip provide the predominant aerodynamic characteristics of that aerofoil.
11) A highly pitchable aerofoil according to claim (1) wherein rotating the aerofoil can provide more than one condition whereby diflbring aerofoil profiles are required at difibrent pitch angles, rotation rates and body speeds.
l2)Ahighlypitchable aerofoil according to claim (1 1) wherein an aerofoil appears to havelowaspectratio(andhencelowdrag)inhigh-speedflighbuthighaspectratio(and hence high lift/drag) in low speed flight.Amendments to the claims have been filed as follows Claims 1) A highly pitchable aerofoil that is designed to provide lift (or thrust) in subsonic, transonic and supersonic local flow fields by exposing different leading edges and local cross-sections, where the aerofoil rotates around a cylindrical body (as part of or separate to the fuselage). As such the aerofoil has sweep and low aspect ratio (and hence low drag) in the high-speed configuration, but higher aspect ratio (and hence a higher lift to drag ratio) in the lower speed configuration.2) A highly pitchable aerofoil according to claim (1) wherein in high speed (e.g. transonic or supersonic) local flow fields where the aerofoil is pitched to a minimum angle relative to the fuselage centre-line, the following configuration is shown: a) There is one major swept leading edge exposed, and that the resulting downstream cross-section is seen to provide efficient thrust for that condition.Example cross sections to provide this are (but may not be limited to) a subsonic symmetrical or cambered aerofoil section, a supercritical transonic section, or a supersonic section.b) The leading edge is swept more along the span towards the tip as set largely by the change in the local flow field towards the tip.3) A highly pitchable aerofoil according to claim (1) wherein for a condition where the 1 aerofoil is pitched away from the minimum pitch angle stated in claim (2) to a larger pitch angle relative to the body centre-line as may be required for another lower speed condition, the following configuration is shown: a) The space between the aerofoil root and the body is opened progressively from the point of rotation to the root leading edge, allowing fluid to flow between the root and the body.b) From the aerofoil tip along the direction of the local flow field, the cross-section appears as a typical aerofoil, which may be similar to or dissimilar to that stated in claim (2).c) From the exposed root along the direction of the local flow field, the cross-section appears as a typical aerofoil, which may be similar to or dissimilar to that in claim (2).d) The aerofoil has up to three exposed leading edges of differing or similar cross-section at the condition stated in claim (2).4) A highly pitchable aerofoil according to and at the conditions stated in claim (3) wherein at low and very low aspect ratios the exposed aerofoil root and aerofoil tip (and downstream sections) provide the predominant aerodynamic characteristics of that aerofoil, whereas at the condition in claim (2), the swept section provides the predominant aerodynamic characteristics.5) A highly pitchable aerofoil according to claim (1) wherein the aerofoil tip is shaped to provide minimal drag in rotational flow at the conditions in claim (2) while acting as a leading edge at conditions stated in claim (3).