The wing section direction intended in this application and the upper and lower surfaces 13,14 of the airfoil are determined (cf. figure 2) by positioning the rear edge 4 of the airfoil at zero, the airfoil chord 15 that ends at the front edge 3 of the airfoil forming a negative x axis. In this context, the upper surface 13 of the airfoil implies the airfoil surface located above the plane formed by the airfoil chord 15 positioned as defined above, and the lower surface 14, in turn, implies the airfoil surface located below the plane formed by the airfoil chord 15. The mean line 16 of the wing section is an imaginary line running at equal distance from the upper and lower surface of the wing.

In this application, the front part 5 of the airfoil implies the portion of the airfoil facing the front edge 3 viewed from the thickest point of the airfoil (indicated with a broken line e. g. in figure 4A), and accordingly, the rear part of the airfoil stands for the portion facing the trailing edge of the airfoil viewed from the thickest point 18 of the airfoil.

The angle of attack A implies the angle A between the airfoil chord 15 and flowing liquid or air (cf. figure 3A).

The first aircrafts were developed in the 1890's. The first aircrafts or flying devices were characterised by a large wing area, a light weight i. e. low wing loading, and a low engine power. Due to its design, the flying apparatus of that time had poor aerodynamics and was consequently slow. These early flying devices relate to modem aircrafts only in terms of aerodynamic requirements, which the aircrafts have to meet in order to be airborne.

In the design of aircrafts, for instance, it is difficult to impart one and the same airfoil properties which are good both in a slow and a rapid stream. The main characteristics are: maximum lift combined with minimum air flow, in other words, the aim is to increase the lift force exerted on the wing without increasing the drag

excessively. On the other hand, there are efforts to increase the angle of attack of the wing to a maximum without the wing stalling. It is difficult to achieve these goals in one and the same airfoil; if the airfoil is designed so as to have good : creating features at a low flying speed, this usually results in a situation, in which the lift force of this airfoil deteriorates at a high speed due to higher drag. On the other hand, if the airfoil is designed so as to have low drag at a high speed, it usually has a poor lift force at a low speed, such as during take-off and landing.

Especially in these two situations, the wing may easily stall if there have been attempts to improve its lift force by increasing its angle of attack. Of course, stalling also occurs at higher speeds if the angle of attack of the wing is too much increased.

In order to achieve the mutually opposed goals mentioned above with one single airfoil, a number of different airfoil designs have been tested. The most common means is to use various turning and gliding leading edge slats and trailing edge flaps and the like.

Leading edge slats and trailing edge flaps are used to increase the camber of the wing mean line and the wing area, and thus to alter the lift force. At a low flying speed, they are turned down, thus resulting in an increase in the drag, however, the lift force generated by the wing grows simultaneously, thus allowing the flying speed to be dropped and possibly a small increase in the angle of attack. At higher speeds, the slats and the trailing edge flaps are brought into neutral position, and then the lift coefficient of the wing decreases, the cross-section becomes aero- dynamic and the drag is reduced. However, the effect of leading edge slats and trailing edge flaps on the increase in the lift force of the wing is confined within a relatively small range of angles of attack.

US patent specification 4 606 519 discloses an airfoil design, in which the upper surface of the wing has a profile divided into two parts, i. e. a first level and a second level, the second level defined by the trailing edge being located lower than the level starting from the leading edge of the wing. Between the levels there is a steep step which causes boundary layer flow separation on the upper surface of the wing.

Such a wing has appreciably improved stalling and lifting properties, but on the other hand, it has poor flight performance at high speeds due to increased drag.

US patent specification 2 419 161 depicts an airfoil, in which a control surface element has been placed between a structural opening between the leading edge and the trailing edge of the wing tip. There is an air flow through this opening. The purpose of the airfoil in this publication is to achieve a more controllable airfoil, and

not to increase the angle of attack the airfoil can withstand in order to reduce the stalling speed.

US patent specification 2 430 431 describes an airfoil design in which the stationary leading edge of the wing is followed by subareas, i. e. honeycombs, whose surface can be raised and lowered, and over which pressurised air or any other gas can be blown through pipes or ducts to generate low pressure on the upper surface of the wing. In the invention described in this publication, the airfoil part located on the rear side of the airfoil centre is lowered. The purpose of blowing air on the part of the airfoil is to increase the low pressure on the upper surface of the wing. In the practice, blowing air or any other gas on the wing surface requires too complex and heavy structures for such an airfoil design to be feasible or to enhance flight performance.

US patent specification 5 209 438 describes an airfoil comprising, in the vicinity of the leading edge of the wing, relatively small straps or components having a width of about 0.5 to 3% and a height of about 1% of the length and thickness of the airfoil chord respectively. By rapidly raising and lowering these arrays with a power unit at a specific rate, the air boundary layer on the wing surface is stirred, high- frequency low oscillation being generated in the boundary layer flow. The oscillation has the task of retaining the air flow on the wing surface. The stirring of the boundary layer flow is intended to improve the lift of the wing and to thereby allow higher angle of attack values. The arrays on the upper surface of the wing described in this publication are relatively small, the method being based on bringing the arrays into an active and rapid oscillatory state, and for this reason, the method does not notably increase the lift force of the wing nor the maximum angle of attack characterising the airfoil.

US patent specification 2 539 222 depicts a two-piece wing design, in which a tunnel-like construction has been provided through the leading edge of the wing or the torsion construction, the construction being ended by a valve. Air can pass through the tunnel-like construction when the valve is opened. The rear part of the wing is tuable. In practical operation, such a wing design increases the overall drag and is difficult to control.

The first objective of the invention is to provide an airfoil whose maximum angle of attack can be markedly increased without the body moving in a stream cavitating or stalling in its totality. The second objective of the invention is to provide an airfoil having good lift force when the body is moving in a stream at low speeds and with

high values of the angle of attack. The third objective of the invention is to provide a profile whose overall flow resistance can be reduced as the speed of the body increases.

The method of adjusting the airfoil of the invention is characterised by the features defined in claim 1 and the airfoil of the invention by the features defined in claim 4.

The dependent claims describe preferred embodiments of the invention. The invention is based on the idea that, as a body is moving at a low speed in a stream, a step is formed in its profile on the side where an increase in the low-pressure area is desired. The step is removed when the speed of the body increases, and then the overall flow resistance decreases.

The step generates a vortex on the surface of the body moving in the stream, behind the step, usually transversely to the central wing chord on the wing surface, the vortex preventing jointly with the step turbulent flow from proceeding from the step towards the leading edge (cf. figure 3C). This enables higher values of the angle of attack to be achieved with the profile of the body than with the same profile devoid of a step. The profile has good lift creating features as it is moving at a low speed in the stream, because the stream adapts better to the profile shape of the front part of the body at higher angles of attack than in conventional profiles, and what is more, the over-pressure field on the lower surface of the profile becomes stronger, thus increasing the lift further. The step is formed so as to avoid substantial flow-through between the leading and the trailing edge of the profile between the upper and the lower surface of the body profile.

In this application, a step implies a step-like or bump-like shape clearly differing from the remaining airfoil camber and provided on the side of the airfoil on which an increased low-pressure field is desired as the body is moving in the flow. Formed on the upper surface of the profile, the step slopes downwards, i. e. the profile is substantially higher from the step towards the leading edge than from the step towards the trailing edge. Similarly, formed on the lower surface of the profile, the step rises upwards.

When the step is formed in the front part of the profile, the profile is given a thicker shape at its front part, and when the step is formed in the rear part of the profile, the profile is given a narrower shape at its rear part. In this manner, a vortex is generated behind the step. There may be one or more steps in the same profile. The step is formed in a part of the profile which is attached to the remaining profile

surface at its hinged point of inflexion and which swings about this point of inflexion.

Thus, the operating principle and the design of the airfoil of the present invention differ substantially from the airfoil solutions described in the references cited above.

None of the cited references describes an airfoil which prevents stalling at high angles of attack of the airfoil by preventing a turbulent flow from proceeding to the leading edge of the airfoil and whose flow resistance can be reduced as the speed of the body increases. The angle of attack of the airfoil of the invention can be markedly increased without the body moving in the flow cavitating or stalling in its totality.

This solution differs from the airfoil solution disclosed by US patent specification 4 606 519 owing to the adjustability of the step. On the other hand, the method of the invention differs from the airfoil adjusting method of US patent specification 5 209 43 8, in which the step is rapidly raised and lowered, in that in the invention, a step is formed on the surface of the airfoil during slow flight, and it is removed during cruise flight. In addition, the method in accordance with the invention achieves higher values of the angle of attack than the invention claimed in the patent specification mentioned above.

In the method of the invention, the angle of attack and the lift force of the cross- sectional profile of an elongated body moving in a stream, such as a wing, are adjusted so as to increase the lift force and the maximum angle of attack of the body as it is moving in the stream at low speeds, its flow resistance being decreased as it is moving in the flow at high speeds. At low speeds, the angle of attack and the ; force of the body are increased by preventing the turbulent flow generated by stalling or cavitating from reaching the leading edge of the body and by generating a vortex behind the step, on the surface of the airfoil, the direction of the vortex being principally parallel to the direction of movement of the body, so that at least one step is formed on the side of the body surface on which a stronger low-pressure field is desired. As the step is formed, air or liquid cannot flow substantially through the upper and lower surface of the body profile. The step is formed either in front of the thickest point of the body profile or behind it by turning the profile part counter- clockwise about the hinged point if the step is desired on the upper surface of the body profile, and clockwise if the step is desired on the lower surface of the body profile as the body is moving from the right to the left in the stream, with the leading and trailing edge of its profile positioned as in figure 2. When the body is moving at

higher speeds in the stream, the flow resistance is reduced by turning a part of the profile about the hinged point in a direction opposite to the forming direction.

The method for adjusting an airfoil of the invention and the airfoil formed achieve great advantages. Thus, the airfoil can be excellently used during slow flight for instance, or at the landing or take-off stage of flying apparatus. At higher speeds (cruising speeds), the upper surface of the airfoil resumes its plane shape in order to reduce the drag. A profile with a varying surface can be adapted both to symmetrical and asymmetrical airfoils and also to airfoils used in the supersonic speed range.

We may mention as other benefits of the airfoil of the invention that it has proved to increase the stability of the flying device owing to the vortex generated behind the step. This creates a new potential in the design of the control system or auxiliary control systems of aircrafts. With the use of adjustable steps of the invention on the upper surface of the wing, the aircraft can still be controlled even though the automatic means and/or hydraulics for moving the control surfaces required for controlling the aircraft would be damage. In addition, the airfoil of the invention allows prevention of the oscillatory movement,"dutch roll", which starts easily and grows rapidly in current jet airliners, and is caused by damage to components of specific control systems, because the airfoil solution of the invention attenuates oscillation and stabilises the aircraft.

One of the advantages achieved with the step-lessly adjustable step of the invention is that, when the step in the front part of the airfoil is turned into its extreme position, it can be used as an efficient air brake. During landing run, the step enhances the effect of the wheel brakes, because the air flow presses the wing against the runway, thus appreciably shortening the braking distance on the runway.

The invention is described in greater detail below with reference to the accompanying drawings, in which figure 1 shows a partial perspective view of an aircraft with the airfoil of the invention, figure 2 shows a cross-sectional view of the aircraft wing (= airfoil), figures 3A-3C show a cross-sectional perspective of the flow patterns of a conventional airfoil and of the airfoil of the invention,

figures 4A-4B, 5A-5B, 6A-6B, 7A-7B show a cross-sectional perspective of optional airfoils of the invention without a step and provided with a step, figure 8 shows an airfoil with two steps, figures 9A-9B show optional airfoils of the invention, in which the step is formed in the rear part of the airfoil, figure 10 shows the airfoil of the invention equipped with a step and adapted to the mast of a sailing boat, figures 11A and 11B show hinging methods for the mast profile of a sailing boat, figure 12 shows a rotor or propeller profile of the invention equipped with a step, figure 13 shows the use of the airfoil of the invention as an air brake means.

The main features of the figures are briefly presented below.

Figure 1 shows an aircraft wing having a step 2 and an airfoil 1 (section 11-11). The wing has an upper surface 13 and a lower surface 14. The upper surface of the airfoil is usually more curved than the lower surface. The wing starts from the base 8 of the wing, i. e. the end of the wing facing the body, and it ends at the wing tip 9.

Figure 2 illustrates in greater detail the design of the airfoil 1, la as a cross-section 11-11 of the aircraft wing of figure 1. The (central) chord 15 of the airfoil is straight and extends from the leading, i. e. front edge 3 of the wing to its trailing i. e. rear edge 4. The main parts of the airfoil have been discussed above on page 1.

Figures 3A-3E show the maximum angle of attack A of a conventional airfoil (3A) and of an airfoil of the invention (3B-3C) in an air stream.

Figures 4A-4B, 5A-5B, 6A-6B, 7A-7B show more in detail the forming of step 2 in a number of optional airfoils of the invention. The step is formed in the front part 5 or the leading edge 6 of the front part of the airfoils described by turning the airfoil parts about the point of inflexion, i. e. hinged point 7. In some cases there is a cover plate 12, resulting in a bump-like 2b step.

Figure 8 illustrates an airfoil comprising two steps and two airfoil parts 5,11 hinged to be able to swing.

Figures 9A-9B show the forming of the step in the rear part of the airfoil. The step is formed with airfoil part 11, which is hinged by its rear edge 20.

Figure 10 shows a sailing boat with a turning airfoil part of the invention arranged in its mast to produce a step 2 between the mast 24 and the sail 23.

Figures 11A and 11B illustrate the structures and the operation of a turning airfoil part 11 mounted in the mast of a sailing boat. In figure 11A the airfoil part 11 is hinged at one point 7a, in figure 11B there are two turning airfoil parts 5,11 and two hinged points 7a and 7b.

Figure 12 shows an airfoil part 11 swingingly hinged in an aircraft propeller.

Figure 13 shows the use of the airfoil 1, la of the invention as an air braking means during the landing of an aircraft.

A conventional airfoil stalls considerably at lower angles of attack compared with the airfoil of the invention, which has the same profile, but is equipped with a step.

This difference is illustrated in figures 3A-3C.

In figure 3A comprising the angle of attack A, the turbulent flow has already proceeded from the high-pressure lower side 14 of the airfoil to the front edge of the upper surface 13, resulting in stalling of the airfoil.

By contrast, the airfoil of the invention shown in figures 3B-3C does not stall significantly in its totality even with higher angle of attack values A. This is due to the fact that the step 2 formed on the upper surface of the airfoil la results in the flow adapting better to the shape of the front part 5 or of the front edge 6,6b of the front part of the upper surface 13 of the airfoil with larger angles of attack as the angle of attack A of the airfoil is enlarged. This figure shows the low-pressure field (-) generated on the upper surface and the high-pressure field (+) generated on the lower surface of the airfoil 13.

Figures 3B-3C also show the vortex generated behind the step 2,2a to stabilise the flight of the device. In figure 3C the step 2,2a is formed in front of the front part of the airfoil with the relatively thick airfoil part 6b. The front part of the airfoil becomes thicker with the step formed in it. The figure shows the vortex generated behind the step 2,2a on the upper surface 13 of the airfoil to prevent the turbulent flow generated by the high-pressure flow on the lower surface 14 of the airfoil from proceeding from the step towards the leading edge of the airfoil as a stalling

situation is approaching. In figure 3C the step 2,2a has been formed by tapering the rear part of the airfoil. A vortex is generated behind such a step as well. An airfoil equipped with a step has good lift performance and withstands larger angles of attack without stalling than a conventional airfoil does.

In the performed wind tunnel tests a symmetric airfoil NACA 632015 (a variant with straight wings) stalled at its characteristic angle of attack of about 15 degrees, whereas the same airfoil equipped with the turning front part of the invention stalled only at an angle of attack of about 25 degrees.

The forming of the vital part of the invention, i. e. the step, on the airfoil surface is described more in detail below. The step is formed particularly advantageously in front of the turning front part or the front part of the airfoil, and then the turning airfoil part 5,6 starts immediately from the leading edge 3 of the airfoil 1, la, as in figures 4A, SA, 6A and 7A.

Figures 4A and 4B illustrate the turning of the front 6,6a of the swingingly hinged front part about the hinged point 7a on the lower surface of the front part of the airfoil. The steepness of the formed step 2 is steplessly adjustable by means of a hydraulically or mechanically operated push bar 10 as illustrated in figure 4B. The step is formed as in figure 4B starting from the discontinuity 19 shown in figure 19 so as to provide an airtight airfoil without any air flow between its upper and lower surface 13,14. The point of intersection between the discontinuity point 19 and the upper surface of the airfoil is approximately at the thickest point 18 of the airfoil marked with a broken line, which simultaneously forms the rear edge of the front 6 of the turning front part. The airfoil part 6a shown in figure 4A is swung a degrees about the point of inflexion 7a as shown in figure 4B.

Figures 5A and 5B illustrate how the front 6,6b of the front part, which is also swingingly hinged, turns about the hinged point 7a on the lower surface of the front part of the airfoil. The swingingly hinged airfoil part shown in the figure has the same operation and hinged point as the one shown in figures 4A and 4B. The swingingly hinged airfoil part 6,6b shown in figures 5A and 5B differs from the corresponding airfoil part 6,6a in figures 4A and 4B only regarding its shape.

Figures 6A and 6B illustrate the swingingly hinged front part 5 and the forming of step 2,2a by means of this. The rear edge of the front part hinged at the point of inflexion 7a located on the central airfoil chord 15 is located approximately in the same area as the thickest point 18 of the airfoil, indicated with a broken line.

The swingingly hinged front part 5 or the front 6,6a, 6b of the front part illustrated in figures 4A-4B, 5A-5B and 6A-6B is swung counter-clockwise about the hinged point of inflexion 7,7a, in other words, parallel with the negative y axis (coordinates shown in figure 2) over a desired angle a. In this situation, all the points of the turning front part or of the turning front of the front part of the airfoil move on a circular circumference, the radius of which is the distance between this point and the point of inflexion 7,7a, the final location of each point and hence the size of the formed step being determined by this radius and by the number of degrees over which the front part or the front has been swung from the xy plane.

Then the rear edge 18 of the turning airfoil part is located substantially higher than the part facing the trailing edge 4 of the airfoil step, viewed from the step towards the leading edge 3 of the airfoil.

The turning front part 5 of the airfoil starts from the leading edge 3 of the wing as in figure 6A. The trailing edge of the front part is located approximately at the same place as the thickest point 18 of the airfoil. On the other hand, the front 6 of the turning front part of the airfoil starts from the leading edge 3 of the wing and ends either at the thickest point 18 of the airfoil or at the trailing edge 21 of the front (figures 4A and SA). The front part or the front of the front part has been fixed to the remaining airfoil at the point of inflexion 7,7a. The hinged point of inflexion 7 comprises a hinging means or a similar construction, which acts as an axis of revolution for the front 6 of the front part or the front part 5. A discontinuity 19 is provided between the rear edge 18 or 21 of the turning airfoil part and the point of inflexion 7. As illustrated by the optional airfoils in figures 4A-4B, 5A-5B and 6A- 6B, the shape of the discontinuity 19 varies markedly. The step 2,2a is formed between the rear edge 18 of the turning front part or the rear edge 21 of the front of the airfoil and the part following this rear edge of the upper surface 13 of the airfoil (i. e. from this rear edge towards the trailing edge), as the front part or the front swings about the point of inflexion 7,7a.

The step 2 can be formed on the airfoil surface also so that it does not start immediately from the leading edge of the airfoil. Such a step does not prevent a turbulent flow from reaching the front edge of the airfoil as efficiently as does a step provided in the front part of the airfoil (cf. figure 3B versus 3C). This is due to the fact that the vortex generated behind such a step is weaker than the one generated behind the step formed in the front part. Figures 9A-9B illustrate airfoil designs, in which the airfoil part 11 forming the step does not start immediately from the front edge 3 of the airfoil. In figure 9A, the hinged point 7,7b of the airfoil part 11 is

located at the rear edge 20 of the part, which in this case is the same as the trailing edge 4 of the airfoil. In figure 9B, the hinged point 7,7b of the airfoil part is also located at the rear edge 21 of the airfoil part 11, but the airfoil part 11 does not extend all the way to the trailing edge 4. On the other hand, the step 2,2a is produced so that, as the airfoil is moving, the swingingly hinged airfoil part 11 adjoining its upper surface 13 is swung counter-clockwise about its hinged point of inflexion 7,7b over a desired number of degrees a in the xy plane, i. e. parallel to the negative y axis. Now all the points of the swingingly hinged airfoil part swing counter-clockwise about this point of inflexion with a radius of a circle equalling their distance from the point of inflexion 7b. The final location of each point is determined by their distance from the point of inflexion and by the number of degrees a over which the airfoil part has turned. At normal speed, the step is removed by turning the airfoil part 11 accordingly clockwise about this point of inflexion 7,7b when the airfoil is moving from the right to the left and its front edge 3 and rear edge 4 have been set as in figure 2.

Several steps can be formed in the airfoil, as in figure 8. The airfoil with several steps as illustrated in figure 8 has both a turning front part 5, which is hinged at the point of inflexion 7,7a, and a turning airfoil part 11 located partly in the rear part of the airfoil and having its hinged point 7,7b in the rear edge 20 of this airfoil part 11.

The structural shape of the step 2, such as its steepness and width, can be steplessly varied with a suitable mechanism, such as for instance the hydraulically or mechanically operated push bar 10 shown in figure 4B. When the flow rate is higher, the wing profile should be more aerodynamic in order to reduce the drag.

The profile is then turned to the same plane as the remaining wing at the point of inflexion so that the step is removed from the upper surface.

The front of the turning airfoil can be carried out either with a shell construction 6, 6a as in figures 4A-4B or relatively thick 6,6b, as in figure 5A-5B.

The step 2 formed on the surface of the airfoil is preferably similar to a steep step 2a, as in figures 4B, 5B and 6B, or bump-like as in figure 7B. The bump-like step 2a extends longitudinally generally over the total length of the upper surface of the wing, i. e. from the wing base 8 facing the body to the wing tip 9. However, it is conceivable that the step extends only over part of the wing length, which allows the wing properties to be more freely designed.

In some bump-like airfoil designs, the wing structures should have space to receive the rotating front part or front, as in the solutions ilustrated in figure 7A-7B. Figures 7A and 7B illustrate how such a bump-like step 2b is formed. The rear edge 21 of the turning airfoil part 6,6b continues as an elastic cover plate 12 fixed to the upper surface 13 of the airfoil. In such designs, the cover plate 12 on the upper surface is for instance an elastic or sliding profile plate, allowing a larger potential in airfoil designs. The bump-like step is advantageous in some thin airfoils because it increases the airfoil thickness whenever necessary.

The step 2,2b can also be formed for instance between the mast 22 and the sail 23 of a sailing boat as in figure 10 by providing a mast profile lb with a turning front part or front of the front part as shown in figures 11A and 11B. In such a mast profile, the front part 5 of the profile or a part of it can be turned to either side depending on the wind direction. The step 2,2b has then been formed between the mast 22 and the sail 23 (figure 10) and it is always formed on the side of the mast profile facing the sail 23 side on which an increased low pressure is desired.

Such a mast profile alters the speed characteristics of a sailing boat exposed to a vortex in varying wind conditions owing to an increase in the lift of the sail.

Using a turning airfoil part 11, a step 2c can also be provided in various propellers as in figure 12. The airfoil part is located on the propeller blades, at the opposite edges of the blades.

The step 2,2a formed with a steplessly adjustable airfoil part 6,6a of the invention located in the front part of the airfoil may also act as an air brake if the airfoil part is swung into its extreme position as in figure 13.

Besides the purposes of use mentioned above, the variable profile of the invention is usable also in the keels of ships or boats, helicopter blades, fans, windsurfers, windmills, propellers and turbines.