AEROFOIL/HYDROFOIL

Abstract: An aerofoil or hydrofoil (1) has means (2) for providing a high velocity jet of fluid directed from a nozzle (3) in a direction having a component of velocity in the same direction as the required direction of lift (L); a curved surface (5) convex on the side of the aerofoil or hydrofoil which is in the direction of the required lift, and which serves to direct the velocity of the jet in a direction which is substantially opposite to the direction of the required lift or thrust force, the primary jet being directed over the convex surface for attachment thereto; and a duct formed over a rear part of the convex surface remote from the nozzle, whereby the primary jet of fluid flowing over and attached to the convex surface causes an enhanced flow through the duct, the duct being directed at least at its exit end in a direction so that the flow therethrough has a component of velocity opposite that of the required direction of lift.

Description: The present invention relates to aerofoils or hydrofoils and, more particularly, to an aerofoil or hydrofoil having improved lift or thrust characteristics. Conventionally, aerofoils and hydrofoils are designed to provide maximum lift at a chosen forward velocity or narrow range of velocities, increased lift being provided, when required, by movable flaps, both leading edge flaps and trailing edge flaps. It has been proposed to utilise the so-called "jet-pump" principle to provide increased lift on an aerofoil or hydrofoil. A jet-pump comprises a high velocity but relatively low mass flow rate primary jet to induce a flow of secondary, ambient fluid in order to enhance the mass flow rate. DE-A-1951422 discloses an aircraft wing which has movable upper and lower flaps at its trailing edge. Air from a nozzle in the wing entrains ambient air, the resultant flow then being directed between the movable flaps. According to the present invention, an aerofoil or hydrofoil comprising: a curved surface convex on the side of the aerofoil or hydrofoil which is in the direction of the required lift; means including a nozzle for providing a high velocity primary jet of fluid directed from the nozzle to the convex surface to entrain a flow of secondary fluid; and, a duct formed over a rear part of the convex surface downstream of the nozzle; is characterised in that: the duct extends from the convex surface through the aerofoil or hydrofoil to the lower surface of the aerofoil or hydofoil remote from the nozzle, whereby the primary jet of fluid flows across the convex surface and causes an enhanced flow through the duct, the duct being directed at least at its exit end in a direction such that the flow therethrough has a component of velocity opposite that of the required direction of lift. Such an aerofoil or hydrofoil is thus able to make use of the so-called "Coanda" effect to produce increased lift in the required direction of lift by causing the primary jet to be attached either directly or indirectly to the convex surface in order to induce mixing between the primary jet and surrounding ambient fluid to improve the rate of mixing between the primary jet and entrained fluid such that there is an accompanying pressure reaction force from the solid boundary of the aerofoil or hydrofoil which results in an augmentation of thrust or lift in the required direction. If desired, in certain circumstances, a non-convex, for example concave, surface may be interposed between the nozzle and the convex surface. Preferably, the resultant of the forces acting as a result of the primary jet and entrained secondary fluid is substantially aligned with the axis of lift or required thrust. If desired, the duct may be adjustable to direct fluid flow to it either in a direction with a velocity component in the same sense as the direction of motion of the aerofoil or hydrofoil, thereby providing a means of thrust reversal, or in a direction with a velocity component in the opposite sense to the direction of motion of the aerofoil or hydrofoil, thereby providing a means of thrust enhancement. The invention will now be illustrated with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view illustrating the principles embodied in the present invention; Figure 2 is a diagrammatic cross-sectional view through an aerofoil according with the present invention; and, Figure 3 is a view similar to Figure 2 showing a modified aerofoil section. To illustrate the principle of the present invention Figure 1 shows a high velocity jet of fluid f exiting from a pressure vessel P, the jet being released close to a convexly curved surface s. At the rear of the curved surface s a diverging, diffusion duct D is formed, one side d of which is an extension of the curved surface s. A non-convex surface N can optionally be interposed between the pressure vessel P and the convex surface S. The primary jet of fluid f attaches itself to the convex surface s (the Coanda effect) and moves around the surface. By means of the well-known jet-pump effect it causes ambient fluid A to be entrained with it thus increasing the mass flow rate through the duct D. The balance of forces acting on the body forming the pressure vessel P, curved surface s and duct D are as shown in Figure 1, where F p is the reaction force of the high pressure jet f on the pressure vessel P, F v is the reaction force of the primary and entrained fluid in the vertical direction, F h is the horizontal reaction force on the curved surface (F h being approximately equal to F v and in turn greater than F p ), R is the resultant force on the body, and ϑ is the angle of the resultant force to the vertical. It will readily be appreciated that with F v and F h being substantially similar in magnitude, the resultant R will lie at some angle around 45° to the vertical. Advantageously therefore when applying this to the generation of lift in an aerofoil, the direction of R will be arranged to be in the direction of lift, that is to say, in general, substantially vertical. Figure 2 illustrates diagrammatically an aerofoil or hydrofoil 1 which has a vessel 2 or other means of supplying pressurised fluid to a nozzle 3 so as to release a primary jet 4 of fluid over a convexly curved surface 5, not necessarily of circular section, the primary jet 4 eventually or directly attaching to the convex surface 5. The means for supplying pressurised fluid may be a fan, pump or other conventional fluid moving device. The convexly curved surface may include discontinuities (not shown) in order to encourage mixing between the primary jet and the ambient fluid 6 and in some applications it may be appropriate to rotate the convexly curved surface in order to control the direction of the attached jet at its furthest extremity. Further improvement to the rate of mixing between the primary jet and entrained fluid can be obtained by increasing the radius of curvature of the convex surface so that it increases in the direction of flow. In some applications, on cessation of the primary jet, the convex surface 5 may be retracted partly or wholly into the aerofoil/hydrofoil main body 1 or otherwise closed in order not to impede the passage of the surrounding ambient flow. The profile of the convex surface is shaped so as to promote the efficient mixing of the primary jet and surrounding ambient fluid. It is the intention here that efficient mixing is promoted when there is an accompanying pressure reaction force from a solid boundary, which reaction force is substantially in line with the local flow direction which itself may be in the same or opposite sense to the flow direction. In this manner efficient mixing is achieved which results in an augmentation of thrust or lift force in the lift direction L and which can be accomplished within a relatively small increase in cross-sectional area between the exit of the primary jet and the section of the duct which delivers the mixed streams back to the ambient. By confining the overall cross-section of the flow, the distance in which the high velocity jet must spread in order to mix with the entrained stream is limited and this reduces the streamwise distance needed to achieve adequate mixing between the two streams. The reduced mixing length then results in reduced frictional losses at the solid boundary. An approach surface 7 on the aerofoil/hydrofoil guides part or all of the entrained ambient fluid 6 to the primary jet. This approach surface 7 may be made to move either linearly or rotationally by means (not shown) in order to form a smooth or near smooth tangential juncture with the upper region of the convexly curved surface 5 adjacent to or at its highest point, this movement being able to be performed preferably after the primary jet has been switched off. A duct 8 is provided rearwardly of the convex surface to direct some or all of the flow leaving the convexly curved surface 5 in a direction favourable to the resultant force required of the aerofoil/hydrofoil section. The cross-section of the duct 8 can be either constant or varying in the direction transverse to the majority of flow passing through it, but preferably the cross-sectional area increases slowly with flow direction as shown, as this is beneficial to the promotion of mixing between the primary jet and the entrained ambient fluid. The downstream end of this duct can have a system of guiding vanes to direct the release of the combined flow stream, thereby providing a further means of control of the direction of the resultant thrust. Figure 3 shows a diverter plate or plates 9,10 which may form one wall of the duct. The upper lip 11 of the aperture to the duct can be made to move generally in the direction M of the convexly curved surface (as indicated by the arrow in Figure 3) by either linear or rotational means (not shown) so that part or all of the combination of primary jet and entrained fluid can be redirected along the upper surface 12 of the downstream section of the aerofoil/hydrofoil. A diverter plate 13 may also be provided so as to deflect the exit stream from the duct in a direction so as to provide vectoring control of the resultant force. When not required, this diverter plate 13 may be retracted back into the main body of the aerofoil/hydrofoil by appropriate mechanical means such as a hydraulic ram (not shown). In certain applications, this plate may be orientated or constructed so as to cause an extreme deflection of the jet and thus provide a resultant force opposite the direction of motion of the vehicle, in other words, to perform thrust reversal. Figure 3 also shows how a circular cross-sectioned element 14 may be provided for rotation about a central axis A, whereby the convex surface 5 can be caused to rotate as described above.

Claims: An aerofoil or hydrofoil comprising: a curved surface (5) convex on the side of the aerofoil or hydrofoil which is in the direction of the required lift; means including a nozzle (3) for providing a high velocity primary jet (4) of fluid directed from the nozzle (3) to the convex surface (5) to entrain a flow of secondary fluid (6); and, a duct (8) formed over a rear part of the convex surface (5) downstream of the nozzle (3); characterised in that: the duct (8) extends from the convex surface (5) through the aerofoil or hydrofoil to the lower surface of the aerofoil or hydrofoil remote from the nozzle (3), whereby the primary jet (4) of fluid flows across the convex surface (5) and causes an enhanced flow through the duct, the duct being directed at least at its exit end in a direction such that the flow therethrough has a component of velocity opposite that of the required direction of lift. An aerofoil or hydrofoil according to claim 1, wherein a non-convex surface is interposed between the nozzle and the convex surface. An aerofoil or hydrofoil according to claim 1 or claim 2, wherein the resultant of the forces acting as a result of the primary jet and entrained secondary fluid is arranged to be substantially aligned with the axis of lift or required thrust. An aerofoil or hydrofoil according to any of claims 1 to 3, wherein the duct is adjustable to direct fluid flow to it either in a direction with a velocity component in the same sense as the direction of motion of the aerofoil or hydrofoil thereby providing a means of thrust reversal, or in a direction with a velocity component in the opposite sense to the direction of motion of the aerofoil or hydrofoil thereby providing a means of thrust enhancement. An aerofoil or hydrofoil according to any of claims 1 to 4, including means for deflecting the exit stream from the duct in a direction to provide vectoring control of the resultant force. An aerofoil or hydrofoil according to any of claims 1 to 5, wherein the convex surface is rotatable about an axis transverse to the flow direction of the stream of fluid flowing over it.

Inventors: ADKINS, RICHARD CYRIL
Applicants: BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1988
Main classification: B64C21
IPC codes: B64C9, B64C21