The use of wing tip sails to reduce vortex drag

THE USE OF WING TIP SAILS TO REDUCE VORTEX DRAG

J . J . Spillman and J . E . A l l e n . >.-,. ,-i-, r^,-! ft-Cranfield I n s t i t u t e of Technology

TECHNISCHE HOGESCHOOL DELFT LUCHTVAART- EN RU!?.1TEVAAnTTECHNIEK

^ ^ f i H BIBLIOTHEEK

\'i- Kluyverweg 1 - DELFT SUHHARY

WindtunneJt. measurements of the flow around the tip tanks of a model of a Morane-Soulnler Paris aircraft have been used to design cambered and twisted auxiliary surfaces, each only 0"'^% to 0*6% of the wing area, which unwound the tip vortices formed at incidence and in so doing

e)q)erienced a thrust, effectively reducing the vortex drag.

Flight tests on a Paris aircraft showed that three such sails per tip tank increased the effective aspect ratio of the wing by over H0%. The increase in the overall lift-drag ratio at a lift coefficient of 0*35 was 21% and the maximum lift-drag ratio increased from 12«5 to 15.8.

More recent windtunnel tests have shown that sails have a similar effect when fitted to plain wing tips. The results suggest that three or four sails spiralled round the reeu? half of each wing tip will give best results.

These encouraging results suggest that far more windtunnel, flight and design work should be done to realise the potential savings in drag and fuel.

a E f f e c t i v e lift c u r v e s l o p e o f a s p a n w i s e s t r i p o f s a i l . c C h o r d o f s p a n w i s e s t r i p o f s a i l . ACj3 Z e r o - l i f t d r a g c o e f f i c i e n t o f a s p a n w i s e s t r i p o f s a i l . Cj) L i f t d e p e n d e n t d r a g c o e f f i c i e n t o f a i r c r a f t = •sj Cj^^ Cj) Z e r o - l i f t d r a g c o e f f i c i e n t of a s a i l b a s e d o n a i r c r a f t

°s wing area.

C L Overall lift coefficient of aircraft.

cj Tip chord of wing.

Cx Thrust coefficient of a sail based on aircraft wing area.

AD Incremental drag on a spanwise strip of sail, measured

parallel to local flow direction.

K Aircraft lift-dependent drag factor.

K^ Effective reduction in lift-dependent drag factor due to

a sail.

ki Effective lift-dependent drag constant for an element of

sail.

AL Incremental lift on a spanwise strip of sail, measured

normal to the local flow direction.

q Dynamic pressure of airstream.

r Distance from tip or tank centre-line measured normal to

surface.

AX Incremental thrust on a spanwise strip of sail, measured

into the aircraft flight direction.

Ay Spanwise length of an elemental strip of sail.

a Angle between the flight direction of the aircraft and

the no-lift line of the wing. Positive for positive lift.

^ The angle between the local flow direction and the

direction of flight of the aircraft.

i|» Angle between the tangents to the sail camber line at

the sail nose and the sail trailing edge.

4

INTRODUCTION

With the increasing need for fuel economy all possible areas of drag reduction need Investigating. One form which offers considerable promise is lift-dependent drag which for most transport aircraft is approximately 30% of the total drag in the cruise and more at lower speeds. A reduction in lift-dependent drag at a given flight speed would directly reduce the operating costs of existing aircraft. Even greater savings might be obtained for twin-engined airliners where the wing loading is fixed by the high lift-dependent drag at climb-out from the airfield. A method of increasing the effective aspect ratio of a wing would allow new aircraft of this type of have lower wing areas and weights in addition to higher cruise lift-drag ratios.

There have been many attempts to reduce vortex drag by modifying the shapes of wing tlps^. The use of end plates to reduce drag was involved in a patent by Lanchester as long ago as 1897 although the first experiments utilising end plates did not taike place until about 1921. Since then end plates have been proposed in various forms for many layouts, a recent one, known as 'Booster tips' has been suggested for light aircraft^. One proposal of particular Interest to the present investigation was that of Clements^ in 1955 in which he showed that a marked reduction in vortex drag could be obtained by canting the end plates outwards by S^^ and cambering then in the same sense by deflecting a trailing edge flap lb°.

In America Whltcomb and others'*»^»^»^ have Investigated special forms of end plates which they call 'winglets'. Windtunnel tests suggest that

If winglets were fitted to existing transport aircraft a saving of fuel consumption of about 7% would result and this would increase to 9% for aircraft designed specifically to use these devices, because of the reduction in engine and structure weight which could be achieved.

WHIG TIP FLOWS

The work at Cranfield arose from windtunnel studies of the flow

behind the tip of an unswept wing of moderate aspect ratio. These showed that the vortex type flow generally associated with the rolled-up vortex sheet will behind a wing tip existed just behind the trailing edge and seemed to be forming over the top region of the wing itself.

The authors have made studies of the flow about the wing tip of a windtunnel model of a Paris aircraft, with and without tip tanks, which have confirmed this impression. Small masts of hypodermic tubing carrying arrays of wool tufts were mounted normally to the surfaces of the tip tanks and studied to get a qualitative idea of the flow pattern variation with changes in wing incidence. Subsequently a two-tube

yawmeter was used to measure the local flow directions in a vertical plane above the tip or tank centre-line at a tunnel speed of 30 m/sec,

cozvesponding to a Reynolds number of U«5 x 10^ based on the wing tip chord.

At positive incidences there is a marked outflow component of velocity on the under wing surface near the tip which changes to a marked upwash at the tip or tank extremity to become a marked inflow on the top surface of the tip. As figure 1 shows the flow close to the surface of a plain tip or tip tank spirals round the tip from the lower surface to the upper surface, the angle, ^, between the local flow direction and that of the free stream. Increasing linearly with increase in the wing aerodynamic incidence, a. Figure 2 shows how the ratio, ^/a» decreases dramatically with radial distance from the tip surface. This decrease is more rapid than that predicted by assuming a simple line vortex along the centre-line of the tip half body. It can be seen that near the surface the value of •/(, is of the order of H'O at 60% of the tip chord behind the tip leading edge and even more further aft. It is worth noting that at distances from the tip surface greater than C l times the wing tip chord the ratio •/« is a function of incidence but is not very dependent upon the distance

aft of the tip leading edge.

Figure 3 compares the local flow angles around the tip tanks with those obtained for the plain tips. As on» might expect the local flow directions are less with the tip tanks and the linearity of ^ with o less good, particularly at distances from the surface greater than 0>1 of the tip chord. It is noticeable that both for tip tanks on and off, the flow angles at some distance from the surface decrease more rapidly for the low incidence cases.

In an attempt to correlate the results with the concept of a line vortex at the tip, for which ^ would vary inversely as the radius from the centre-line of the vortex, the results were plotted in the form

1. . £— against — • where r is the distance to the centre-line of a C"p Cy

the tank or the plain tip and Cf is the tip chord. Figure 1 shows that

the term ^ -— does not remain constant but decreases with distance from o CT

the surface in all cases, more rapidly with tanks on and at the more aft position. The results do not suggest any simple correlation.

WIMG TIP SAILS

If a little auxiliary surface having a chord much smaller than the tip chord, cj, is mounted from the wing tip, then its effective free-stream direction will be the local tip flow direction. Thus when ^ is positive, the resultant force on the small surface can have a component into the

aircraft free stream direction. Effectively the auxiliary surface is acting like the sail of a yacht in making use of a crosswind resultant airflow to obtain a thrust in the direction of motion.

The factors Involved can be seen more clearly by referring to figure 5. The local lift on a spanwise strip of the sail of width Ay is

AL = qcao('^ + l) Ay (1)

where a is the lift curve slope for that section of the sail

q is the dynamic pressure of the local air flow, and

c is the chord of that section of the sail.

The drag increment on the strip can be written as

AD = qc(ACn + kia^) Ay (2) "o

where ^Cn is the zero lift drag coefficient of the strip and

kx is an effective lift dependent drag factor for the strip.

Resolving these forces in the free stream direction of the wing gives

AX B AL sin ^ - AD cos ^ (3)

which for small to moderate values of ^ approximates to

Now since (^/g) decreases very ra]pidly with Increase in the distance from the tip surface, the term (•/a)(*/o + 1) "^^1 decrease even more rapidly.

Traa figure 2 its value will fall to less than 2% of the value at the root at a distance 0*2 07 from the surface. It follows that the sail span should not exceed this value If a simple strip theory approach is valid. However in view of the strongly three-dimensional nature of the flow over the tip this result should be treated with caution.

If the sail had zero camber and twist, an integration of equation (4) over the sail span must give an expression for the total thrust experienced by the sail which can be written in coefficient form as ,

. K I J L I - Cn (5)

where C^ is the thrust divided by dynamic pressure and wing area, and

C L is the overall lift coefficient

Any reduction in drag on the wing itself due to the effect of the sail will have the same form and can be included in the coefficients K^ and CDQ , Thus the effect of the sail on the drag of the aircraft will be to increase the zero lift drag coefficient by C D Q , which is proportional to

s

the sail area times the sail zero-lift drag coefficient and to decrease the lift dependent drag factor by an amount K^, proportional to the sail atrea. The effect is shown diagrammatically in figure 6.

The assumption Implicit in equation (2) is that the flow around the sail remains attached. However as the wing incidence, a, increases the value of i at the sail root increases significantly and since this is the local sail incidence there is a grave danger of the flow separating at this root section, changing the thrust on the sail to a drag. This is illustrated by the dotted line on figure 6 labelled 'sail root top surface separation'. To delay root separation to as large an incidence as possible a leutige nose down camber and twist is desirable allowing the flow to be turned towards the free stream direction over the full chord of the sail root, so minimising the peaklness of the pressure distribution. Thus a circular arc camber line is attractive, with the tangent to the centre-line at the trailing edge parallel to the free stream direction. Since ^ decreases rapidly with distance from the surface so should this cêuaber and twist.

A sail of fixed geometry with a significant camber and twist at the root is likely to have root separations from the undersurface at very small wing incidence because the angle between the local flow direction and the tangent to the centre-line of the sail at the root leading edge could be too large and negative. Again in this case the drag of the sail would be significantly Increased. As wing incidence increased the root flow would attach to the undersurface and the drag would decrease. This trend is Illustrated by the dotted line in figure 6 labelled 'root undersurface separation'.

To avoid both forms of separation the sail needs to have a restricted range of local flow direction or have varicüile nose camber or both.

One way of reducing the value of ^ at any point near the wing tip surface is to use a cascade of sails so placed that their mutual

interference effects the desired reduction. This is best achieved by spiralling the cascade of sails round the tip in a direction opposite to that of the air. Since the value of i. increases with distance from

a

the tip leading edge the front sail will be subject to the least variation in ^ and is least in need of a downwash effect from a sail ahead to keep the flow angles acceptable, whilst the rearward sails must have a significant iat«rfev«io» ' from those ahead to avoid flow separation.

The extent to which any sail can be cambered and twisted will depend upon the drag-rise critical Mach number and wing loading of the aircraft. Clearly the higher they are the less the camber any one sail can have and the more numerous and closer together the sails need to be. By spiralling in the sense described, favourable velocity interference effects should be achieved. The helical angle of the spiral must never be so small as to cause one sail to be in the wake of another since this will almost certainly lead to poorer lift and perhaps premature separation.

The total lift-dependent drag of the aircraft can be written in the fora

^DL

=

A ^ L '

^^>

The lift-dependent drag factor, K, will be reduced by nK^ by the addition of sails, where n is the number of sails. K^ itself depends upon the sail area, but since the sail span is dictated by the local tip chord, then Ki will be mainly dependent upon the sail chord to tip chord

ratio and the lift curve slope of the sail. The latter will increase with decrease in sail chord. Thus aerodynamlcally the best solution is a large number of small chord sails.

If the wing is highly tapered, the swirl velocities at the tip for a given lift coefficient will be less than those for an untapered wing. Sail effectiveness must be less therefore on highly tapered wings.

The effect of sails in unwinding the tip vortex will be to increase the lift generated on the outer parts of the wing, making them more nearly two-dimensional. Thus there will be an increase in the lift on the wing when sails are fitted and an increase in the distance of the

spanwise centre of pressure from the wing span centre-line. If the sails are essentially horizontal the lift on the sails themselves will increase the total lift and the spanwise centre of pressure.

If the sails are essentially vertical their effect on the main wing should be similar to that of 'the horizontal array but the force on the sails themselves will not contribute significantly to the overall lift and will only add a moment to the wing of the sail lift times a fraction of the sail span. Thus near horizontal sail arrays may give the lowest lift-dependent drag at a given total lift but near vertical arrays may give the lowest lift-dependent drag for a given wing root bending moment.

Since the sails affect the flow at the wing tip, unwinding the vortex, configurations which materially reduce the lift-dependent drag should also reduce the trailing vortex strength. This may allow aircraft to fly more closely behind others so reducing separation times at airfields. A reduction in tip vortex strength is likely to be attractive for crop spraying aircraft operating close to the ground.

WINDTUNNEL TESTS ON SAILS MOUNTED FROM TIP TANKS

A set of sails suitable for the one-seventh scale model of the Paris aircraft, illustrated in figure 7, was made to the geometry shown in figure 8. This corresponds to the local flow being parallel to the tangents to the sail leading edge camber lines throughout its span when the wing incidence is 6° and the sail is 60% of the tip chord behind the wing tip leading edge for the tanks on case.

Each sail had a span of 0*H1 of the wing tip chord, cj. This high value was chosen to ensure adequate span since it could be reduced easily when required. The root chord of each sail was 0>16 cj. Because the sails were so small it was difficult to ensure that their cross-sections were anything but "Aerofoil shaped".

It was decided to make the preliminary tests with tanks on since this provided an easier method of mounting the sails. Each sail had a circular root plug which was sunk into the model tank surface and locked in the correct setting by small wood screws. Whilst this method allowed a firm fixing for the sail it did not give much flexibility on the sail position on the tank. The sails were set such that the tangents to their camber lines at their trailing edges were parallel to the tank centre-line.

Two main configurations were tested. The first was with one sail on each tank at 75% of the tip chord aft of the leading edge and mounted horizontally in the plane of the wing, effectively increasing its span. The second had three sails per tank, the recu^most as described above, a second mounted vertically upwards at about 50% of the tip chord and a

third mounted midway fore and aft and at H5<^ to the other two sails. In all cases the camber at the root of the sails was inclined towards the spiralling flow from under surface to top surface.

Tests were made at 60 m/sec. in the 2*H m x 1*6 m low speed tunnel at Cranfield. The mean wing chord Reynolds number was 1*11 x lO^ but the Reynolds number based on the sail root chord was only 1*47 x lo^. Small transition wires were used on both the wing surfaces, the teunk noses and the sail top surfaces. It is of Interest to note that a coaparative test with no transition wires gave a lower zero-lift drag coefficient but the same lift-dependent drag factor.

Surface visualisation on the sails showed that at zero wing incidence thare was a separation on the under surface of the sails at the root, baeause of the large nose down camber. This disappeared as incidence

increased beyoxul 2° but at the same time a separation appeared on the top surface at the root at about U0% - 50% chord and remained at this position approximately up to the highest incidence tested, lO^. Surface flow H u e s on the tank showed evidence of the reduction in local flow angle, 4» due to the presence of the forward sails.

Force measurements were made at 60 è/sec,using a Warden six-component virtual-centre balance,over a range of incidences at zero yaw angle.

The results are shown in figure 9 as a plot of drag coefficient against the square of the lift coefficient. The drag values have not been corrected for tare drag since there was not time to make the special rig required and it was felt that it was better to leave the results

uncorrected for tare and intake effects and have obviously artificially high values than use estimated corrections.

It can be seen that the zero-lift drag coefficient increased by eight drag coxmts per sail, a much higher value than one would expect

for attached flow, even at the low Reynolds number of the sails. Clearly this is due to the flow separation on the sail under surface at the root. However, the sails make a significant reduction in the lift-dependent drag, one sail per side lowering it by 12% and three sails per side by 28%. The combination of higher zero-lift drag coefficient with lower lift-dependent drag factor resulted in the overall drag of the three sail configuration being lower than that for the no sail configuration only at lift coefficients greater than 0*12. However the zero-lift drag

coefficient increase was large because of the low Reynolds number of the sails. At higher Reynolds numbers one would expect the lift coefficient at which the sails would give a lower drag to be significantly less.

A full description of the tunnel tests on these sails is given in reference 9.

FLIGHT TESTS WITH SAILS ON TIP TANKS

In view of the doubts about the usefulness of windtunnel testing of models with sails; because of their low Reynolds nxjmber and their

approximate aerofoil sections, it was decided to test the same

configurations in flight on the Paris aircraft of the Cranfield Institute of Technology before attempting any further tests aimed at optimising the configurations. Supported by funding from the National Research Development Corporation sails were designed and fitted to the tip tanks in the same configurations as used in the tunnel tests. The sails had a N.A.C.A. 63012 thickness distribution about circular arc camber lines as shown in flgxire 10. A steel box spar of welded construction was used to mount ribs at the stations shown in the figure, the space between the

ribs was filled with plastic foam and the whole covered by a glass reinforced plastic sheet. An extension of the spar box spanned the inside of the tip tanks and provided a rigid fixing. Each sail was 0*62 m span and had a root chord of 0*25 m. The leading edge of the root of the rear sails was almost at the same fore and aft position as the trailing edge of the sail ahead of it. This can be seen from figure 11, which shows the rather crude root fairings and circumferential tank strap which were necessary to make good the tanks. The root fairings are fastened by hollow pop rivets and the strap by solid protruding rivets, a far from ideal installation but one necessitated by the difficulties of modifying existing tanks.

The Paris aircraft is used regularly for training students in flight testing techniques and is suitably instrvimented. In addition to the usual flying instruments a traversing probe is fitted in the jet pipe of the port engine. Earlier work by Klein^° and Allen had

calibrated the instr\unentation system. Because of some asymmetry in the jet pipe flow it was not possible to derive the engine thrust from an Integration of the probe traverse results. However, close agreement between the gross thrust and mass flows given by the engine manufacturer and that derived from the readings of the probe at a specific position

in the jet pipe was determined. Throughout the tests the thrust values were determined from the probe readings taken at this position, care being taken to ensure that the two engines had the same rotational speeds and jet pipe temperatures.

The aircraft was flown in steady, level, trimmed flight for two or three minutes for each test point. A never exceeded speed of 250 knots was observed for the single sail per side tests and of 180 knots for the three sails per side. This limitation was fixed by the strength of the tank attachment to the wing. Tests were made at 1,500 m, 3,000 m and tf,S00 m altitude over as wide a speed range as possible in a flaps up coxxf iguration.

The results of these flight tests are shown as plots of drag

coefficient against the square of the lift coefficient in figure 12. In spite of the care taken in the tests there is quite a large scatter in the results, probably due to errors in the engine thrust evaluation. However it is not difficult to draw mean straight lines through the

results. For comparison the results of the windtunnel tests for the same configurations are included in figure 12. The values of the lift-dependent drag factors are about 12% greater for the flight tests in all cases. This is not surprising when one realises that the windtunnel tests are for constant elevator setting whilst the flight tests are for trimmed conditions. Again Reynolds number effects and actual aircraft detail fittings must be significant. The flight test results suggest that a single sail per side reduces the lift-dependent drag by 9% whilst three sails per side give a reduction of 29%. This compares with the 12% and 28% obtained from the tunnel tests. Clearly the flight tests confirm the tunnel results and show that very significant lift-dependent drag reductions are possible using sails.

One significant difference between the flight and tunnel tests is in the apparent values of zero-lift drag coefficient. The single sail tests show an increase in value as would be expected from the tunnel tests and the rather crude tank modifications. However, extrapolation of the three sail results shows no such increase. Obviously a linear extrapolation is not valid and the dotted curve shown is more probable. The single sail is mounted well back on the tapering part of the tank behind the circumferential butt strap. It is probable that at low wing

incidences the effect of the drooped nose of the sail on the tank boundary layer causes a significant separation and consequently high drag. When two further sails are mounted forward in a thinner boundary layer and a less adverse pressure gradient the flow does not separate. However the flow over their cambered root sections changes the direction of flow approaching the rear sails and separation does not occur to such a marked extent. If no separation occurred it is estimated that three sails per tank would increase the zero-lift drag coefficient of the aircraft by only about 0.0006, and, within the experimental accuracy, the linear extrapolation of the no sail and three sail curves could give zero-lift drag coefficients as close as this. It is unfortunate thet the strength limitations did not allow the low lift coefficient region to be explored more fully.

Figure 13 shows the drag polars for the aircraft and indicates the significant reduction in drag achieved throughout the speed range of the aircraft when three sails per tank were fitted. Ihe mean curves

have been used to calculate the variation in lift-drag ratios with lift coefficient shown in figure lU. It can be seen that the lift-drag ratio increases by about 20% at C L = 0.3, by 25% at C L = 0-5 and by almost 30% at C L = 0-7. It is of interest to note that the design C L for the sails is about 0.5, that is the flow is parallel to the tangent to the camber lines at the sail leading edges at this lift coefficient.

It is worthy of mention that both the pilots who flew the aircraft with three sails per tank commented on the noticeable improvement in the performance of the aircraft during take-off and approach, that is at the higher lift coefficients. Some very slight changes in the handling characteristics were observed, particularly on aileron feel.

TUNNEL TESTS ON THE MODEL WITH PLAIN WING TIPS

Alternative wing tips have been fitted to the Paris windtunnel model to allow a more general exploration of the effect of multiple sails on wings without tip tanks. The new tips are essentially plain tips except that the extreme tip aft of the position of maximum thickness was removed and a body of revolution of a radius equal to the tip radius fitted, as shown in figiore 15. The body was made of a series of short cylindrical lengths of wood terminating with two tapered pieces which faired the rear end. A bolt extended forward from the penultimate piece, through the centres of the cylindrical pieces to screw into a metal fitting fixed to the wing tip. When this bolt was tightened all the pieces were clamped rigidly to the tip. The rearmost piece was a push fit into the aft end of the penultimate piece. Plasticine was used to fill and fair the gaps between the cylindrical pieces and the rest of the wing tip.

Some of the lengths of wood were bored to allow the sails used previously on the tanks to be mounted on them. This rig allowed the number, fore and aft position and angular position of the sails to be varied.

Figure 16 compares the drag of an array of one horizontal sail per tip with that with the same cylindrical tip array without sails. As before, the drag coefficients are uncorrected. The value of K, the lift-dependent drag factor, reduces from 1'16 to 0*9S simply with the addition of one sail in a horizontal position, i.e. extending the span,

at 75% tip chord behind the leading edge. This is a reduction of 14-5% as against the 12% obtained in the tunnel with tip tanks fitted. As before the zero lift drag coefficient increases by cibout 10 drag counts per sail.

A test was made using sails of the identical plan shape and mounting position but cambered only 15° at the root, that is they were designed for a wing incidence of U«5°. T-hese results are included in figure 16. As one would expect, the increase in zero lift drag coefficient is much less than that of the 20° sails, due presumably to a small region of under surface root separation and their effect on the lift-dependent drag factor at low wing incidences was almost as good as that for the more highly cambered sails. However, at the higher wing incidences the lift-dependent drag factor is no better than that for the no sail case and the effective zero lift drag factor is much worse, suggesting a severe separation on the sails. Clearly from the figure there is a discontinuity in the CQ 'V< C L ^ curve, corresponding to a wing incidence between H° and 6°, caused by a significant flow separation from the top surfaces of the sails. This confirms, at least for low Reynolds numbers, the suggestion made earlier that well cambered root sections to the sails are necessary for full effectiveness at the higher wing incidences. In comparing the results for sails with different amounts of camber one should bear in mind the difficulty that was experienced in making such small

sails accurately. It may be that the sails with lower camber had slightly different nose shapes.

A number of 20° camber sails were cast using the original sails as patterns, and a general investigation into the effect of sail array was made. It was found that arrays in which the sails were essentially horizontal, extending the span of the wing, gave significantly lower lift-dependent drag factors than arrays with the sails essentially verticali

Sails mounted from the rear half of the wing tip chord gave a bigger reduction in lift-dependent drag than those mounted in the same angular array but further forward. Arrays of sails essentially behind the tip chord trailing edge were tested, but gave results slightly less favourable than those mounted from the rear part of the tip chord.

Arrays of sails with a large spiral angle between successive sails gave poorer results than those with only 15° to 20° between successive

sails. In general the zero lift drag coefficients were slightly higher and the lift-dependent drag factors significantly higher, presumably because of the difference in interference effects. However, with sails in line, although the zero lift drag coefficients were the lowest

measured the lift-dependent drag factors were only slightly better than for the tips without sails.

Figure 17 shows the results for a selection of the arrays tested in the windtunnel. It can be seen that by increasing the number of sails up to four per tip the lift-dependent drag factor can be reduced, but that each additional sail has a progressively smaller effect. With four sails per tip the lift-dependent drag factor was reduced by 29%. The best three sail array gave a reduction of 26%, which is 2% less than that measured for the tip tanks on configuration, although in the latter case the angle between successive sails was considercdsly greater.

The effect of shortening the span of the sails was investigated, the outer portions of the sails being sawn off. Figure 18 compares the lift-dependent drag factors for a single horizontal sail at 75% wing tip chord. The shortening of the sail increased the lift-dependent drag factor by an amount which increased rapidly with progressive shortening of the sails.

The effect is significantly greater than a simple strip theory based on the measured flow directions would predict. However it seems probable that a compromise between low lift-dependent drag factor, profile drag and structure weight will suggest a sail span of about a quarter of the wing tip chord.

A series of tests of arrays of sails having a span of only 24% of the wing root chord were tested and the results are as shown in figure 19. The shorter sails had slightly less effect on vortex drag than the longer sails up to three sails per tip. However with four and six sails per tip the benefit from increasing the number of sails seemed to be lost. The accuracy of these results was less than for the longer sail results since the model was tested with sails on one tip only, the other being plain, and the incremental reduction in K doubled. One such array is shown in figure 20.

The results shown in figures 17, 18 and 19 suggest that there is little to be gained by exceeding four sails per tip and these need a span no greater than 0-3 times the wing tip chord. It is probable that the 2% reduction in lift-dependent drag factor gained by using four rather

than three sails per tip and the 1% reduction in the factor by using sails 0'3 C T span rather than 0*25 ex span are not worth while in view of the greater structural loadings they impose.

The rolling moment on the model with sails on one tip only has been measured in an attempt to determine whether the sails had a marked effect on the loading just inboard of the tip. Unfortunately the accuracy of the results precluded any definite conclusions but it appeared as if the main increment of root bending moment came from the loading on the sails themselves.

CONCLUSIONS

Small cambered and twisted surfaces, called sails, fitted to the wing tips of an aircraft can reduce its lift-dependent drag by up to 30%. Flight tests on a Paris aircraft show a 25% increase in maximum lift-drag ratio.

Three or four sails per wing tip, each having a span of about a quarter of the wing tip chord and a root chord of about 16% of that of the wing tip seem to give the greatest vortex drag reductions when fitted,near horizontally,outboard of the rear half of the wing tip.

The sails need to be set in a spiral array round the tip, each sail being further away from the top surface of the wing, by 15° or more, than the sail in front of it.

The sails need to tiorn the air smoothly from the spiral flow from lower to upper surface round the tip to a near streamwise direction. This requires the root sections of the sails to have circular arc camber lines bringing the tangent to the camber line at the nose of the sail more nearly into line with the local flow direction at positive wing incidences. The sails tested with good results had a 20° difference in the slope of the tangents to the camber line from leading edge to

trailing edge at their root sections. This angle reduced rapidly with distance from the root of the sail, halving approximately every 6% of the wing tip chord from the sail root.

Further flight tests with sails mounted from plain wing tips are needed to confirm at flight Reynolds numbers the results obtained in the windtunnel and to examine the effects on aircraft handling and trailing vortex strength and duration.

A much more detailed examination of the pressure distribution and flow about a wing tip, with and without sails is desirable at low and high subsonic speeds.

Whilst a fixed geometry sail array is probably satisfactory for low speed aircraft operating at lift coefficients above 0'3, there is a case for having the ability to vary the sail geometry, particularly the camber, in flight for advanced technology aircraft. This is because the need for low drag at high subsonic or supersonic speeds and the msucimuffi vortex drag reduction at the high lift coefficients,associated with full span flaps deflected,are not compatible with a fixed geometry sail.

Project studies of aircraft using sails should be made to investigate the ways in which they can be used with maximum benefit. It could be that quite marked changes in layout will result.

There is a need to explore other applications of the principle

behind the sails, for example in the design of rotorcraft and crop-spraying aircraft.

REFERENCES 1. HOERNER, S.F. 2. 'FLIGHT INTERNATIONAL' 3. CLEMENTS, H.R. 4. WHITCOMB, R.T. 5. FLECHNER, S.G., JACOBS, P.F. and WHITCOMB, R.T. 6. JACOBS, P.F. and FLECHNER, S.G. 7. ISHIMITSU, K.K. 8. WEEKS, V.D. 9. ALLEN, J.E. 10. KLEIN, V. Aerodynamic Drag. Published by author, 1951. February 28, 1976, p.460.

Canted adjustable end plates for the control of drag.

Aeronautical Eng. Review 14 No 7, July 1955.

A design approach and selected wind-tunnel results at high sub-sonic speeds for wing-tip mounted winglets. NASA TN. D 8260, July 1976.

A high sub-sonic speed windtunnel investigation of winglets on a representative second-generation jet transport wing.

NASA TN. D 8264, July 1976.

The effect of winglets on the static aerodynamic stability characteristics of a representative second-generation jet transport model.

NASA TN. D 8267, 1976.

Aerodynamic design and analysis of winglets.

AIAA Paper 76-940, September 1976. Flow studies behind a wing tip using a five-tube head aligned with the free stream.

Thesis, Cranfield Institute of Technology, 1971.

Vortex drag reduction using wing tip sails.

Thesis, Cranfield Institute of Technology, 1976.

Evaluation of the basic characteristics of an instrumentation system.

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