GB1584348A

GB1584348A - Devices to reduce drag

Info

Publication number: GB1584348A
Application number: GB24943/76A
Authority: GB
Original assignee: National Research Development Corp UK; National Research Development Corp of India
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1976-06-16
Filing date: 1976-06-16
Publication date: 1981-02-11

Description

(54) IMPROVEMENTS IN OR RELATING TO DEVICES TO REDUCE DRAG (71) We, NATIONAL RESEARCH DE VELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66-74 Victoria Street, London, S.W.1, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to devices to re duce the drag experienced by craft in motion relative to masses of fluid, and especially by aircraft, and in particular to reduce the drag resulting from the fact that in producing lift, various parts of an aircraft create local streams or flows whose directions are different from that of the free stream. This is particularly the case near to the wing tips of conventional aircraft.By "free stream direction" we mean the direction of the air relative to the aircraft, measured well ahead of the aircraft. It is well known to mount flat fins at or close to such tips to obstruct the prominent local flow of air which in flight passes around such tips from the lower side of the wing to the upper, but the usefulness of such fins has generally been confined to destroying this drag inducing local flow; the fins have seldom been shaped so as to generate any more positively useful forces by reason of their reaction with the flow.In a few cases it has been proposed to shape wing fins so that their reaction with local flows gives rise to such positive forces, but in such cases it appears that the barrier function of the fins was still uppermot in the minds of the designers and that the possibility of achieving much more substantial benefits by the correct shaping and setting of the fins was not appreciated.

The present invention results from considering such possibilities further, and in particular from considering the possibility of arrays of fins so shaped that they might generate thrust in a manner analogous to that of a close-hauled sail on a boat. The invention will now be described, by way of example, with reference to the accompanying drawings in which:: Figure 1 shows an array member in elevation; Figure 2 comprises sections on the lines A and B in Figure 1; Figure 3 is a plan view of an aircraft wing tip and tip tank fitted with an array of such members; Figure 4 is a front elevation of the parts shown in Figure 3; Figures 5 and 6 are graphs; Figure 7 shows a variable sweep array member diagrammatically; Figure 8 is a diagrammatic perspective view of an aircraft wing tip fitted with an array of four members; Figures 9 and 10 are diagrammatic plan views of alternative array members, and Figure 11 is a diagrammatic front elevation of an alternative array.

Figure 1 shows a rigid member 1 which will be referred to as a sail) having a root end 2 and tip end 3 and carrying at its root end a stub 4 by which it may he mounted upon an aircraft surface. The sail is of aerofoil section from root to tip. The tangents to the said centre (or chamber) line 7 at the trailing edge 5 lie in a common plane throughout the length of the sail, and the two transverse sections of Figure 2 in planes A and B show that the tangent 6 to line 7 at leading edge 8 in plane A makes a relatively great angle (about 20 in the example drawn) to the plane of the tangents to line 7 at the trailing edge 5, whereas at plane B close to tip 3 the corresponding angle is much less, only about 3" in the same example. Between root and tip this angle falls progressively.

Figures 3 and 4 show constructions according to the present invention, comprising an array of three such sails mounted on a tip tank 10, itself in turn mounted upon the tip of a conventional-type wing 11 of an aircraft. Track 12 illustrates the path of a local stream of air that is created close to the surfaces of wing and tank during flight.

This path leads from the underside of the wing with rearward and rotary components to its motion. It passes up and around the wall of tank 10 and then heads inwards and backwards toward wing 11, but is intercepted by the leading sail which turns it into the direction of the tangent 13 to the sail centre line at the trailing edge of the sail, which is so mounted that in a typical condition of flying, e.g. normal cruising altitude, the tangents along the whole trailing edge of the sail, from root to tip, lie parallel to the free stream. In practice the set of the trailing edge tangents will be chosen so as to offer the best overall contribution to the flight performance of the aircraft, having regard to the need to offer a substantial benefit in some flying conditions while avoiding creating a serious liability in any others.

In turning the stream the leading sail experiences a force in direction 14 after the manner of a close-hauled boat sail, and this force may be resolved into a forward component which helps to propel the aircraft.

The two other sails shown in Figures 3 and 4 work similarly upon the similar regions of the local stream that they intercept, and the sails are staggered as shown not only to avoid the forward sails spoiling the impact of the local stream upon the more rearward - for instance by the rearward sails lying within the wake of the forward -- but also possibly to create positively favourable interference effects.Tests with such a threesail arrangement have been conducted with the rear sail located at about 75% of the wing tip chord and projecting horizontally as shown. the forward sail at about 40O" wing tip chord and pointing upwards and the third said midway between the others in both wing tip chord position and inclination .Figures 3 and 4 illustrate that while the part of the local stream close to the wing and tank turns sharply around tank 10, and thus lies at a comparatively great angle to the free stream direction and needs a strong camber at the root of the sail to turn it, the weaker local stream 12a, which is further from the surfaces of the wing and tank, is less angled to the free stream direction and this only requires the lesser camber of the sail tip to straighten it.

Figure 5 shows the uncorrected drag coefficients for three tested aircraft wing designs, plotted graphically against life coefficient. Each tested wing carried a tip tank as in Figures 1 and 2, but one of the designs carried three sails arranged as in Figures 3 and 4, another carried only one with its span horizontal and located at about 75 O/o wing tip chord, and the third design carried no sail at all. Since the drag coefficients are uncorrected for rig tare drag the graphical zero must be considered a false one. At zero lift coefficient the sails increased the drag coefficient drammatically by eight drag counts per sail. This is not surprising since the sails were cambered and mounted in a manner appropriate to a lift coefficient of about 0.7.Flow visualisation tests at zero wing incidence showed that a severe separation occurred over most of the inner concave surface of the sails. However this disappeared quite rapidly with increase in wing incidence and at an overall lift coefficient of 0.8 the three sails per tank had reduced the drag coefficient by 108 counts whilst the single sail per tank had reduced the drag coefficient by 71 counts.

The effect of the sails can be seen more clearly in Figure 6 where the difference in drag coefficient between a lift condition and the zero lift condition is plotted against the square of the lift coefficient. This shows that the sails reduced the lift-dependent drag dramatically, the single sail increasing the effective aspect ratio of the wing by 23% whilst the three sails increased it by 46%.

Figures 1 to 6 all illustrate models of devices and aircraft wings, and tests upon them. Figures 1 and 2 are approximately to full scale, and the illustrated sail had a span to-mean-chord ratio of 3.5; the nominal thickness chord ratio was 15%, and the taper ratio 0.4. Figures 3 and 4 are to a scale of 0.4:1 approximately, and the area of each sail was 0.6 % of that of the aircraft wing. Figures 5 and 6 relate to tests of this model in an 8' x 6' wind tunnel at a Reynolds number of 1.1 x 10 based on wing means chord or about 1.1 x 105 based on sail mean chord.

Figure 8 shows another application of the invention to an aircraft wing tip, in a view taken from a position below the wing, and both forward and outboard of it. Because of the possible structural difficulty of anchoring the four sails 41 to 44 securely to an ordinary wing tip, the original tip of wing 45 has been cut away to receive a robust body 46 to which the roots 47 of the sails are anchored; a body such as 46 may of course be omitted if secure anchoring is possible without it. The profile of body 46 is such that when the body is in place it gives to the wing a tip profile as nearly similar as possible to the original, the small cylindrical tail 48 of the body having little aerodynamic effect other than that of fairing what would otherwise be a bluff body. Sails 41 to 44 are so mounted on body 46 that their trailing edges 49 all pass through the fore-andaft axis 50 of the body, which is effectively the chord-line of the wing tip. The line of each trailing edge 49 passes through the axis 50 and in tests for the reduction of the lift-dependent drag of the wing the best results were obtained when the difference in the angular setting of adjacent sails about axis 50 was in the region of 10-20". For the reduction of lift-dependent drag, best results were also obtained when the whole array of sails lay as nearly as possible horizontal, that is to say the trailing edges of sails 42 and 43 lay on opposite sides of the plane of the wing, and equally inclined to it.Sails with spans of up to about 50 /O of wing tip chord were tested, and results of the best overall promise were obtained when the span of each sail was about 25% especially 24% of the wing tip chord C, and when the root chord c of each sail was about 16% of wing tip chord C, so that the whole array of sails had a total chord of rather over C/2. Tests have suggested that with greater or lesser numbers of sails the ratio of the total of the root chords c of the individual sails to the wing tip chord C should still generally lie between about one-half and three-quarters, although possibly rising to full wing tip chord in certain cases.

A fixed geometry sail will have a camber distribution which is optimum for only one aircraft incidence. At other incidences the turning of the local flow by the sail is partly by incidence effects rather than by camber.

This tends to produce a pressure distribution with a much sharper "peak", and so with the danger of flow separation, particularly at high sub-sonic flight speeds. A camber distribution which varied with aircraft incidence might be an ideal solution, but this might be both costly and heavy in some instances.

A possible alternative solution is to mount the sails so that they can be turned, relative to the aircraft surface, about their span axes. This solution is illustrated in Figure 9, which shows one sail only of an array according to the invention. The span axis of sail 20 projects substantially horizontally outwards from the tip 21 of an aircraft wing 22. The sail is mounted on a shaft 23, which is rotatable by a motor 24 controlled by a device 25 responsive to aircraft incidence.

The reasons why such turning may enable the sails to act more effectively at incidences other than the original one may be summarised by stating that the ideal variation in camber, from root to tip, changes with aircraft incidence. Varying the root setting of a sail with a fixed spanwise camber distribution allows a compromise to be achieved between high thrust on the sail and low sail complexity.

Another alternative could be to mount the sails so that they are capable of being retracted into, or projected further from the aircraft surface. Such projection or retraction may help the sails to match changes of incidence more effectively than fixed sails can, because for wing tip-mounted sails the variation in camber from root to tip decreases with decrease in aircraft incidence as does the distance from the root over which a useful thrust force can be generated. By withdrawing part of the sail into the wing tip as the aircraft incidence is reduced, or extending the sail from the wing tip as incidence increases, an approximate matching of sail camber to local flow direction can be achieved. This solution is illustrated in Figure 10, which again shows only one sail of an array according to the invention.

Here the sail 30 is mounted on a rack 31 engaging with a pinion driven by motor 32, which is controlled by a device 33 responsive to aircraft incidence. By driving the rack 31, the motor 32 can move sail 30 between its fully extended position (as shown) and a fully retracted position in which it lies entirely within a recess 34 formed within the tip of wing 35.

Another possibility, for arrays mounted on a wing tip body and thus of the general kind shown in Figures 3 and 4, is to mount the body for rotation about a fore-and-aft axis. Such an arrangement is shown in Figure 13 where the body 60 on the tip of an aircraft wing 67 carries three sails 61, 62 and 63 and is rotatable about fore-and-aft axis 64 by a motor 65 controlled by a device 66 responsive to aircraft incidence. The sails are shown in full lines in the setting they may occupy for normal flight. For high lift, however, motor 65 may rotate body 60 through 75 , say, so that the sails occupy their dotted line positions.

A further alternative solution is suggested by noting that the angle of the local flow direction to that of the free stream, expressed as a fraction of the wing incidence, varies regularly with distance from the tank. Thus if the sails have variable sweep, the sweep angle may be changed with aircraft incidence in such a way that the active span of the sail, that is to say the span actually projecting from the wing, presents substantially the correct camber (for the chosen aircraft incidence) from one end to the other. As the speed of the aircraft increases, its incidence will decrease and the sails can be swept more. At extreme sail sweep angles the camber would tend towards an anhedral effect rather than a camber and thus would not be likely to induce high local velocities.

Figure 7 suggests a possible sweep variation; in this Figure the single illustrated sail 71, which again is only one of an array according to this invention, is rotated about axis 72 by motor 73 and to keep the sweep variation small, the most forward position corresponds to about 30 sweep and the most aft to about 70.

The forward thrust on such sails is likely to be a maximum when aircraft speed is low and incidence is high since the sails should provide a thrust proportional to the lift-dependent drag of the aircraft. A simple spring - 'g' feel control system for the sweep angle may be satisfactory.

Although the invention has been described with reference mainly to sails with their spans substantially at right angles to the surfaces on which they are mounted, it applies equally to wings of swept configura tion - both fixed sails and also sails that are swept in attitude but movable like, for instance, the sails shown in Figures 9 and 10. The invention applies also to arrays of sails mounted on a variety of surfaces of craft in motion within and relative to masses of fluid. For instance not only whole aircraft wings, but also parts of whole-span flaps which are deflected to give an aircraft extra lift at take-off and landing: in such a case the sails could be attached to such flaps at their outboard ends, or to the stationary wing near these ends.It is contemplated that it might be necessary to arrange that such sails became operative only when the flaps themselves were operative, and were retracted or otherwise made inoperative at other times to avoid then interfering with the proper flow of air over the wing. The inven tion applies also to surfaces of stationary craft dependent on gaseous movement (e.g.

the vanes of windmills), to the surfaces of boats, water wheels and other apparatus de pendent on the movements of non-gaseous fluids, to rotary and other moving surfaces of aircraft such as the blades of helicopter rotors, and to aerofoil surfaces on ground vehicles, e.g. racing motor vehicles. For helicopters the invention may have particu lar uses in reducing the effect of the vortices which form behind each blade, and which in the descent mode may cause adverse blade vortex interference. Another consequence of the "vortex unwinding" behaviour of the sails is that it not only reduces the life-dependent drag, but also reduces the initial strength of the trailing vortex and causes it to diffuse more rapidly as downstream distance increases.This suggests that use of the present invention may allow aircraft to fly more closely behind others than is now possible, thus reducing separation tines at airfields. A reduction in tip vortex strength is also likely to be of benefit to cropspraying aircraft, operating close to the ground.

WHAT WE CLAIM IS: 1. A body intended for motion within and relative to a mass of fluid, in which an array comprising a plurality of members of aerofoil cross-section is mounted upon a surface of the body so that the members project outwards spanwise from that surface into the space where a local stream of the fluid will form in use, whereby in such use the members will tend to divert that local stream back into the free stream and in so doing experience useful thrust, in which each said member is twisted from its root to its tip to allow for change in the direction of the local stream as distance from the surface increases, and in which the members of the array are staggered in the direction of the free stream so that the more rearward of the members avoid the wake of the more forward.

2. A body according to Claim 1 in which the direction of the flow of the local stream around the surface on which the array of members is mounted has a sense of rotation, and in which the staggered formation in which the members are mounted has an opposite sense of rotation.

3. A body according to Claim tin which at least one of the member of the array is adjustably mounted upon the body surface, whereby to modify the set of any such member as a whole relative to the local stream.

4. A body according to Claim 3 in which any such adjustably-mounted member is mounted to rotate about its span axis.

5. A body according to Claim 3 in which any such adjustably-mounted member is mounted so that it may move in the span wise direction, so as to retract into the surface or extend from it.

6. A body according to Claim 3 in which any such adjustably-mounted member is mounted to pivot about an axis lying substantially parallel to the body surface.

whereby to vary the sweep of such member relative to that surface.

7. A body according to Claim 1 is which the tangents to the camber line ol each member at all points on the trailing edge of the member lie substantially parallel both to each other and, in at least one operating condition of the body, to the nearby free stream direction also.

8. A body according to Claim 1 in the form of an aircraft wing, and in which the members of the array are miniature winglike members, in which the mountings of the roots of these members on the aircraft wing tip are staggered in a fore-and-aft direction, in which each member has a span of less than about 50% of the aircraft wing tip chord, and in which the total root chords of the members equal between one half and three quarters of the aircraft wing tip chord.

9. An aircraft wing according to Claim 8 in which the root chord of each member is between about 10% and 20% of the aircraft wing tip chord.

10. An aircraft wing according to Claim 10 in which the root chord of each member is about 16% of the aircraft wing tip chord.

11. An aircraft wing according to Claim 8 in which the mountings are also staggered in a spiral sense which is opposite to that of the normal local flow of air around the aircraft wing tip in use.

12. An aircraft wing according to Claim

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. with reference mainly to sails with their spans substantially at right angles to the surfaces on which they are mounted, it applies equally to wings of swept configura tion - both fixed sails and also sails that are swept in attitude but movable like, for instance, the sails shown in Figures 9 and 10. The invention applies also to arrays of sails mounted on a variety of surfaces of craft in motion within and relative to masses of fluid. For instance not only whole aircraft wings, but also parts of whole-span flaps which are deflected to give an aircraft extra lift at take-off and landing: in such a case the sails could be attached to such flaps at their outboard ends, or to the stationary wing near these ends.It is contemplated that it might be necessary to arrange that such sails became operative only when the flaps themselves were operative, and were retracted or otherwise made inoperative at other times to avoid then interfering with the proper flow of air over the wing. The inven tion applies also to surfaces of stationary craft dependent on gaseous movement (e.g. the vanes of windmills), to the surfaces of boats, water wheels and other apparatus de pendent on the movements of non-gaseous fluids, to rotary and other moving surfaces of aircraft such as the blades of helicopter rotors, and to aerofoil surfaces on ground vehicles, e.g. racing motor vehicles. For helicopters the invention may have particu lar uses in reducing the effect of the vortices which form behind each blade, and which in the descent mode may cause adverse blade vortex interference. Another consequence of the "vortex unwinding" behaviour of the sails is that it not only reduces the life-dependent drag, but also reduces the initial strength of the trailing vortex and causes it to diffuse more rapidly as downstream distance increases.This suggests that use of the present invention may allow aircraft to fly more closely behind others than is now possible, thus reducing separation tines at airfields. A reduction in tip vortex strength is also likely to be of benefit to cropspraying aircraft, operating close to the ground. WHAT WE CLAIM IS:

1. A body intended for motion within and relative to a mass of fluid, in which an array comprising a plurality of members of aerofoil cross-section is mounted upon a surface of the body so that the members project outwards spanwise from that surface into the space where a local stream of the fluid will form in use, whereby in such use the members will tend to divert that local stream back into the free stream and in so doing experience useful thrust, in which each said member is twisted from its root to its tip to allow for change in the direction of the local stream as distance from the surface increases, and in which the members of the array are staggered in the direction of the free stream so that the more rearward of the members avoid the wake of the more forward.

whereby to vary the sweep of such member relative to that surface.

12. An aircraft wing according to Claim

11 in which the difference in angular setting between adjacent members in the spiral stagger is between about 10 and 20 .

13. An aircraft wing according to Claim 12 in which the difference in angular setting is about 15 .

14. An aircraft wing according to Claim 11 in which the middle member or members of the array projects or project outwardly from the aircraft wing tip in a direction substantially the same as that of the span of the aircraft wing itself.

15. An aircraft wing according to Claim 11 in which the middle member or members of the array projects or project in a direction substantially at right angles to that of the span of the aircraft wing itself.

16. An aircraft wing according to Claim 8 in which the camber and cross-section of each member of the array varies so that close to the wing surface there is about a 20 difference in angle between the slope of the tangents to the camber line at the leading and trailing edges of each member, and in which this angle decreases with distance along the span of such member away from the surface, halving as one proceeds along that span by each increment of distance equal to about 6% of the aircraft wing tip chord.

17. A body according to Claim l, substantially as described with reference to the accompanying drawings.

18. An aircraft wing according to Claim 8 and substantially as described with reference to Figure 8 of the accompanying drawings.