GB1573255A

GB1573255A - Drag reducer for land vehicles

Info

Publication number: GB1573255A
Application number: GB6526/77A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-02-19
Filing date: 1977-02-16
Publication date: 1980-08-20
Also published as: FR2341476A1; FR2341476B1; JPS52123021A; AU2230677A; SE7701650L; CA1084082A; JPS6115681U; AU516306B2; SE432085B; DE2705702A1; IT1077273B

Description

(54) DRAG REDUCER FOR LAND VEHICLES (71) 1, FRANK TIMOTHY BUCKLEY, JR., a citizen of the United States of America, of 17841 Pond Road, Ashton, Maryland 20702, United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a device for reducing the aerodynamic drag on articulated tractor-trailer combinations wherein the height of the trailer is greater than the height of the tractor.

Research has demonstrated that a substantial part of the aerodynamic drag experienced by a tractor-trailer combination moving over the highway is the result of flow separations that occur at the forward edges of the trailer. These separations result in large part from the inability of the flow that passes above the tractor roof and impinges on the front of the trailer to follow every contour of the trailer as it moves around its forward edges and onto its sides. The net effect of such separations is an increase in the average static pressure that acts on the front of the trailer with a corresponding increase in drag. It follows that significant drag reductions can be achieved with the aid of a device that will reduce flow separations in this region.

One prior art method of reducing the drag in the immediately described region lies in the design of devices which are attached to the front of the trailer and improve the manner in which the impinging flow moves around the trailer's forward corners. A disadvantage of such devices is the fact that those thus far designed have not been successful in the total elimination of flow separation. particularly of that part due to the flow which passes downward through the gap between the back of the tractor and the front of the trailer.

An alternate prior art method of reducing the drag on a tractor-trailer combination lies in the design of a device which attaches to the roof of the tractor and prevents the flow passing over the roof from impinging on the trailer. Past examples of this approach have been primarily in the form of flow deflectors designed to divert the flow about the portion of the front of the trailer which extends above the tractor cab roof. It is known, however, that the sizing of such devices for optimum drag reduction is a function of the size of the gap from the back of the tractor to the front of the trailer, that the drag reduction can decrease markedly when a device optimized for one gap is used at another, and that the performance of most such deflectors degrades very rapidly in the presence of winds that have a velocity component normal to the direction of motion.

The invention comprises a device that can be attached to the roof of a tractor for the purpose of reducing the aerodynamic drag on a tractor-trailer combination. The device is a fairing that is configured to prevent the flow that passes over the tractor roof from impinging on the front of the trailer. The fairing is designed to provide significant drag reductions in situations where the relative airstream is aligned with the direction of motion of the vehicle (0 yaw condition), and in the presence of winds that have a velocity component normal to the direction of motion (yawed condition).

Another important feature of the invention is that it provides a single device which achieves optimum drag reductions on a given tractor-trailer combination for all practical gap settings between tractor and trailer.

According to the invention, a streamlined fairing suitable for mounting on the roof of a vehicle comprises a nose disposed centrally of, and adjacent to, the front of the bottom of the fairing, a curved top surface extending upwardly and rearwardly from the nose, the top surface diverging in width from the nose towards the rear, and a pair of rearwardly diverging side surfaces, each side surface extending upwardly to respective diverging sides of the top surface.

The rear of the top surface, ideally, is substantially equal to the height of the front face or wall of the trailer above the tractor roof. The contour of the top wall of the device in the vicinity of the position of maximum height causes the flow that passes over the top wall to be rearwardly directed across the gap between the tractor and the trailer. and to re-attach smoothly on to the top of the trailer. Criteria are provided for the optimum design of the top wall or portion, together with a range of acceptable non-optimum design conditions that will produce nearly optimum drag reduction.

The side walls or portions of the device are generally vertical, and extend from the top wall to the roof of the tractor. The side walls are contoured outwardly from a position near the longitudinal centerline of the tractor to a width at a rearward position which, ideally, is equal to that of the trailer, or practically, as wide as the tractor roof will allow. The rearward portions of the side walls are configured to encourage the flow that passes along them to be rearwardly directed across the gap and to reattach smoothly onto the side walls of the trailer.

Examples of side wall contours that will encourage good yaw performance are cited, and a range of widths that will yield usable drag reductions is provided.

The primary object of this invention is the provision of an improved device that can be attached to the roof of a tractor used in combination with a trailer, or to the roof of the cab of a vehicle having a box-like van portion spaced behind the cab, and which by reducing wind resistance and aerodynamic drag under various conditions of yaw, will reduce fuel consumption, and will improve vehicle stability for purposes of reducing driver fatigue and enhancing safe vehicle operations.

Still another object of this invention is the provision of a device that will accomplish the foregoing obiectives in a manner which is different from and superior to those of previous devices intended for a similar purpose.

The invention may be carried into practice in various wavs, but certain prior art devices and various specific embodiments of the invention will now be described with reference to the accompanving drawings, the described embodiments of the invention being by way of example only. In the drawings.

Figure 1 is a perspective view of a portion of a tractor-trailer combination having one form of prior art drag reducing means mounted on the roof of the cab of the tractor: Figure 2 is a graph showing the effectiveness of the device shown in Figure 1 Figure 3 is a view like Figure 1 of another form of prior art drag reducing means: Figure 4 is a graph like that shown in Figure 2 showing a comparison of the effectiveness of the devices shown in Figures 1 and 3; Figure 5 is a view like Figure 1 of one form of the present invention: Figure 6 is a graph like that shown in Figure 2 showing a comparison of the effectiveness of the devices shown in Figures 1, 3, and 5; Figure 7 is a diagrammatic showing of various lengths of the drag reducing device of the invention which come within the scope of the invention; Figure 8 is a diagram illustrating the relationship between the width of the drag reducing device and the width of the tractor roof and the van; Figure 9 is a diagrammatic showing of means for extending the effective width of the drag reducing device; Figure 10 diagrammatically illustrates a means for extending the effective height of the drag reducing device; Figure 11 diagrammatically illustrates an instance where the height of the drag reducing device plus the height of the tractor roof are not equal to the height of the trailer; Figure 12 illustrates a situation where the upper edge of the drag reducing device is higher than the top of the trailer; Figure 13 illustrates variations in the side wall configuration of drag reducers of the form of the instant invention; Figure 14 is an envelope within which the plan view contour of the front and sides of drag reducing means is to be wholly contained; Figure 15 is a diagrammatic elevational view showing various contours which the drag reducing means of the invention can assume; Figure 16 is a view like Figure 1 of another form of prior art drag reducing means: and Figure 17 is a graph showing a comparison of the effectiveness of the devices shown in Figures 5 and 6.

In the discussion that follows, it will be useful to keep in mind the manner in which aerodynamic drag on a tractor-trailer combination is reduced with the aid of a device attached to the tractor roof.

When a device is attached to the roof of a tractor, the aerodynamic drag on the tractor is increased. However, because the front of the trailer rides in the low-speed wake produced by the device, the drag on the trailer is reduced. The net decrease in the drag of the combination, therefore, is equal to the decrease in the drag on the trailer minus the increase in the drag on the tractor. The net drag reduction that can be achieved on a given combination will be a function of the device employed, the gap involved, and the ambient conditions.

In Figure 1, a prior art tractor 51 and trailer 51' are shown with one type of prior art drag reducing device 50 attached to the roof 51" of the tractor. The height of the rear top edge 53 of the device above the tractor roof 51" is equal to the height of the top forward edge 55 of the trailer above the tractor roof. The drag reducer 50 is designed to encourage the flow of air that passes over the roof of the tractor to move in an upward and rearward direction, as indicated by streamline 52, so that upon passing beyond the rear top edge of the device, the flow will move across the gap 54 in a substantially downstream direction and then reattach at the top forward edge 55 of the trailer. Thus, with regard to its design and with regard to the effect it produces on the flow, device 50 can be considered as a two-dimensional drag reducer. While, for the zero yaw flow condition assumed in Fig.

1, the flow passing above the tractor is prevented from impinging on the trailer thereby substantially reducing its drag at that condition, a small part of the flow, illustrated by streamline 56, is widely diverted around the side edges 57 of device 50 giving rise to flow separations that increase the drag on the tractor 51. More importantly, these side edge separations increase in the presence of crosswinds, that is, for non-zero yaw conditions, and significantly reduce the effectivenss of device 50 at these conditions. This has been verified in experiments conducted in a lowspeed wind-tunnel, results from which are presented in Figure 2. As can be seen from Figure 2, while the drag is substantially reduced at 0 yaw, the drag reduction rapidly decreases in the presence of crosswinds, and even becomes negative, meaning that it increases vehicle drag, at yaw angles of interest. The average effectiveness of device 50 will be discussed later.

A second type of drag reducer is illustrated in Figure 3. The drag reducing device 60, attached to the roof of tractor 61, is a deflector whose optimum height from the roof of the tractor is less than the height of the roof of trailer 61'; and whose optimum width is less than the trailer width. The device 60 is designed to deflect the airflow passing above the tractor in upward and outward directions in a manner to avoid entrv of the air stream 62 into the gap 63, and to cause the flow to reattach at the forward top edge 64 and sides 65 of the trailer. The height and width of a deflector which causes the stream surface to optimally reattach at the leading edges of the trailer is, for a given vehicle configuration, a function of the size of the gap between the tractor and the trailer. In other words, a deflector optimized at one gap will deflect the flow too widely at larger gaps, causing the flow to reattach downstream of the leading edges of the trailer and to narrowly at smaller gaps, causing some of the flow to impinge on the front of the trailer. Consequently, nonoptimum performance will be realized at these other gaps. Further, the optimum drag reduction that can be achieved at one gap setting will be generally different from that which can be achieved at another gap.

Finally, the instability of the deflector's wake flow in the presence of cross-winds causes a significant reduction in the effectiveness of the device with increasing yaw angle. Wind-tunnel measurements of the performance of device 60 are compared to those of device 50 in Figure 4 where, while device 60 proves to be generally superior to device 50, the drag reduction effectiveness of device 60 is seen to decrease to near zero at yaw angles of interest. The average effectiveness of device 60 will also be discussed later.

A third type of drag reducer is illustrated in Figure 16. The drag reducing device 40, attached to roof of tractor 41, is the combination of a deflector whose optimum height from the roof of the tractor is less than the height of the trailer, and a fairing whose optimum width is equal to the width of the trailer. The rearwardly inclined upper surface 42 of the device is designed to deflect part of the air flow passing above the tractor in an upward direction so that the flow, after passing beyond the trailing edge 43 of the device, will continue to progress upward and onto the top of the trailing van.

The side surfaces 44 of the device are designed to direct the remainder of the air flow in an initially outward direction before causing it to move downstream near the vertical trailing edges. The flow separates from these edges and reattaches along the upper vertical corners at the front of the trailer. As in the preceding case, the separation stream surface produced downstream of the trailing edge of the inclined upper surface will, for a given deflector and vehicle configuration, be optimum at but one gap. At all other gaps the flow passing over the upper surface of the device will either be deflected too high or too low causing a decrease in drag reducing ability. The variation of average drag reduction effectiveness with gap size to be presented later will demonstrate the considerable magnitude of the decrease in performance that may occur at such non optimum operating conditions.

To summarize that prior art, there has been an evolution of designs for tractor roof-mounted drag reducers. With each new design there has come an improvement in drag reducing ability. However, in every case cited, the designs either suffer from the inabilitv to provide significant drag reductions in the crosswind situation or from having their design for optimum effectiveness being a function of the gap between the tractor and the trailer.

One form of the invention is illustrated in Figure 5. This form has been found to provide significant drag reductions at 0 yaw, to maintain its effectiveness with increasing yaw angle, and to have its geometry for optimum performance be independent of the size of the gap between the tractor and the trailer. At zero yaw the flow of air passing above roof 71' of the tractor 71 is encouraged to divide, with a portion passing over the top portion 72 of the device as shown by stream-line 73, and a portion passing around the side portions 74 of the device shown on the visible side by streamline 75. These flows are then encouraged, by the shape of the device, to change their outward directions to downstream directions which are substantially parallel to the roof and sides of the trailer before separating from the device 70 at its downstream portion 76. The flow then continues across gap 77 and smoothly reattaches itself to the trailer at its foward edges 78.

Ideally. to accomplish the latter effect in an optimum manner that is independent of the size of the gap 77, the height of the device 70 is substantially that of the vertical distance from the tractor roof 71' to the trailer roof 80, the width of the device 70 is substantially that of the trailer 82, the top wall is smoothly contoured upwardly and rearwardly from the front of the bottom of the device to the position of maximum height arwhich position the tangential plane of the top wall is substantially parallel to the top of the trailer, and the side walls diverge smoothly outwardly from the front of the device to the position of maximum width at which position the tangential planes of the side walls are substantially parallel to the sides of the trailer. The side surfaces also extend upwardly to the diverging sides of the top wall. In Figure 5 the longitudinal axis A of fairing 70 is shown. This axis A coincides with the center-line of the cab of tractor 71.

It can be seen that fairing 70 is symmetrical about a vertical plane passing through axis A, and has a nose disposed centrally of the front of the bottom.

General rules for the development of the top and side wall contours of the device to effect the described performance will be given later. Before doing this, however, it is of interest to comDare the performance of the instant invention with that of the prior art.

A comparison of wind-tunnel measurements of the variations of drag reduction effectiveness of devices 50, 60, and 70 with yaw angle is illustrated in Figure 6. As can be seen, device 70 provides superior performance not only at 0 yaw, but, more importantly, at non-zero yaw angles. Since a vehicle encounters a whole spectrum of wind-speeds and wind directions during highway operations, it is of interest to estimate the average drag reduction that might be provided by a given device. This can be done by taking appropriate values for the average wind speed and vehicle operating speed, assuming that the wind is equally likely to approach the vehicle from any direction, computing the relative air-speed and yaw angle for a number of wind direction angles equally spaced around the compass, and then using this information together with the drag coefficient versus yaw angle data to compute the average drag. This number would be indicative of the average drag that a vehicle could experience during long term operations over the nation's highways. The results of computations for a number of vehicle configurations, before and after modification by the addition of drag reduction devices, demonstrate that while the average drag is a function of the vehicle design and the design of the drag reduction device, the average percentage reduction in drag can be usually found in an approximate yaw angle range of from 5 to 8".

With reference once again to Figure 6, it can be seen that the average drag reduction of the device of instant invention, estimated from the data at yaw angles in the 5 to 80 range, is significantly better than that of designs 50 and 60 of the prior art demonstrating the benefit of its unique design. Similar results have been obtained in full-scale coast-down tests and in fuel econony runs.

Figure 17 presents a comparison of the variation of average drag reduction with gap size for device 70 of the instant invention with device 40 of the prior art. As can be seen, device 40 provides maximum drag reductions at a gap width of slightly greater than 50 inches. At gap distances less than this value, the device is not operating optimally in the respect that the flow is not sufficiently deflected to prevent its impingement on the front of the trailer. As a consequence, the effectiveness of the drag reducer decreases very rapidly with decreasing gap size. At gap instances greater than the optimum value, the flow is deflected too high so that flow reattachment occurs downstream of the trailer's leading edges. Again, the effectivenss decreases but this time as gap size increases.

In comparison, the effectivenss of device 70 increases with increasing gap size owing to the greater drag producing role played by the trailer face with increasing gap, and, hence, greater potential for drag reduction as gap size increases. Note that the performance of device 70 is significantly better than device 40 except near the gap size about which device 40 is optimized.

Since it is not unusual for the gap size on a given vehicle to be varied quite frequently, it is apparent that the average dragreductions that would be provided by the device of the instant invention are significantly higher than those that would be provided by the device of the prior art.

As has been mentioned, device 70 as shown in Figure 5 is a preferred embodiment of the present invention. It has been found that, while ideal performance is achieved with the streamlined fairings of the present invention that extend the full-length of the tractor roof, useful drag reductions can be achieved with shorter designs, as illustrated by shapes 85 and 86 in Figure 7.

The primary difference between the longer and the shorter fairings is that the longer fairing allows for the design of side wall contours that would provide better yaw performance than might be realized with a shorter design. Data have been obtained which indicate that acceptable performance can be achieved for fairings with lengths 1 as low as .2W, where W is the width of the trailer.

Ideally, the height of the streamlined fairing should be such that its height, h, be substantially equal to the vertical distance from the top of the tractor cab roof to the top of the trailer, H, that its width, w, be substantially equal to that of the trailer W, and that planes tangential to the top and side walls at their positions of maximum height and width be substantially parallel to the top and side walls of the trailer, respectively.

It is recognized that the width, w, may be limited by the width of the roof of the tractor to which it is attached, which is the usual case. This is particularly true in situations where the tractor is of conventional design as is illustrated in Figure 8. However, by designing the streamlined fairing 87 such that a shallow angle, a is formed between the tangential plane of its sides at their back edges and the centerline of the truck. useful drag reductions can be achieved at a width as low as .5W. A consideration of the behaviour of separated flows suggests that an angle a=tan-' [(W-w)/x], where x is the distance from the back of the device to the front of the trailer, would provide near optimum drag reductions at zero yaw angle. However, excellent drag reductions have been achieved in windtunnel tests with a nearly one-fourth the value-suggested by the equation above.

In addition, the width w of the fairing may be wider than the width W of the trailer.

However, in most situations the maximum width of the fairing is limited by legal restrictions to the width W of the trailer.

A means for adjusting the angle with which the flow separates from the back edges of the sides of a streamline fairing of lesser width than the trailer is illustrated in Figure 9. Here, trim tab 80 is hinged near the back vertical edge of the fairing 88, and can be adjusted to give optimum performance over a range of distances, x.

Since the trim tab, in effect, forms an extension of the width of the fairing, the width of such a modified fairing would w+2t sin y where t is the width of the trim tab and ?' is the angle between the trim tab and the centerline of the tractor.

It is further recognized that the height h of the streamline fairing may not always be equal to the vertical distance H from the roof of the cab portion of the vehicle to the top of the front wall of the trailing body. One instance where this can occur is illustrated in Figure 11. In this figure a tractor equipped with a fairing optimized for one trailer height indicated by top 91 is shown used in combination with a higher trailer as indicated by roof 90. Data have been obtained which demonstrate that, in spite of the fact that some flow will impinge on the trailer, the device can retain greater than 93 of its effectiveness for h1.9H and greater than 75% of its effectiveness for h > .8H.

Another instance where a height mismatch can occur is illustrated in Figure 12. Here a tractor equipped with a fairing optimized for one trailer height indicated by roof 93 is shown used in combination with a lower trailer indicated by top 92.

Again, research results have been obtained which indicate that the device can retain greater than 93O/o of its effectiveness for h < 1 .2H and greater than 81 /n of its effectiveness for h < l.4.

In situations where a fairing is to be designed that will be used with trailers of various heights, a compromise design solution can be sought. It is apparent that the loss in performance for a given height mismatch is greater when the fairing is of lesser height than the trailer than when it is of greater height. This suggests that the fairing be designed with a height that is closer to that of the higher trailer than the lower one.

A consideration of the results presented above indicates that a practically useful fairing be designed for a trailer whose height would be about equal to the height of the front wall of the lowest trailer above the cab roof plus about 60n of the difference in height between the highest and lowest trailer. For example, if the device is to be used with trailers that range in height from 12'6" to 13'6" above the ground, the fairing should be designed for a trailer height of about 13'1". A consideration of contemporary tractor heights suggests that the maximum value of h/H, corresponding to use with the 12'6" trailer, is equal to about 1.2, and the minimum value of h/H, corresponding to use with the 13'6" is equal to about .9. In light of the preceding data, it is apparent that the practical design solution provides optimum or near optimum performance for all trailer heights within the range considered in the solution.

Another possible practical design solution for the situation of use with trailers of various heights requires that the fairing be designed to have the smallest height possible. This would arise in situations where legal restrictions might limit the height of the fairing to that of the lowest trailer. In this situation, to regain the loss in performance with h < H, it would be advantageous to depart from the ideal design condition that requires the plane tangent to the top wall of the fairing at its maximum height position to be parallel to the top of the trailer. Thus, the fairing is provided with a slight inclination on the top wall near its trailing edge. In this instance a consideration of the behaviour of the flow separation from such a surface suggests that the angle of this inclination be less than or equal to about tan-1 [2(H-h)/xi.

However, the provision of such an angle for h much' less than about .9H would not be recommended because then the fairing may begin to exhibit some of the deleterious gap dependent performance characteristics of deflector type devices.

Another means for adjusting the flow that leaves the trailing edge of a fairing whose height h is less than H is illustrated in Figure 10. In this instance, trim tab 81 is shown hinged near the back horizontal edge of fairing 89. and deflected at a shallow angle , to optimize the performance of the fairing when used with the higher trailer.

To this point, discussion has been concerned ith ideal and permissible nonideal specifications of the height, width, and length of fairings to the instant invention together with similar specifications for the inclination of its top and side walls at their positions of maximum height and width with respect to the top and side walls of the trailer, respectively. Discussion will now be concerned with guidelines that can be used to develop the contours that fit within the prescribed dimensions and satisfy the specified tangency conditions.

Shown in Figure 15 are several examples of the profile view that the top wall of a streamlined fairing 97, 98, and 99 can assume and still function in a manner of performance consistent with the objectives of the invention. As can be seen, convex or concave-convex contours can be used. The surface need not be continuously curved, as shown, but can include straight portions as well. In all cases, at the position of maximum height, the tangent to the surface should be parallel to the trailer roof or should be inclined relative to the trailer roof at the appropriate angle to optimize performance when h < H. It is desirable, though not mandatory, that the surface be free of any areas where the slope is discontinuous, the reason being that such areas could cause flow separations to occur that might affect the angle that the flow separates from the top wall of the fairing, and could increase the drag on the fairing itself. For this same reason, gradual changes of direction with distance along the top wall are preferred over more rapid ones. Finally, it is desirable that the radius of curvature at the point of tangency be generous. For example, for fairings where l > h, the radius of curvature could be gradually incr view contour of the front and sides of the fairing lie wholly within a rectangular envelope 100, Figure 14 of length I and of width w that is intersected by a line 110 having one end point located .2w aft of one of the forward corners of the rectangle and the other end point located .2w inboard from the same forward corner, and another line 110' similarly located from the opposite forward corner. This envelope is sketched as dashed line 100 in Figure 14.

The drawings herein of the instant invention have shown its top and sides to intersect along a sharp corner. Experiments have shown that rounding the corner provides improvements in performance when compared to the sharp corner case.

The drawings have also shown the profile view of the rear of the fairing to be generally vertical; for example, as in Figure 7.

Experiments have demonstrated that useful performance increases can be achieved, particularly with fairings of shorter length, by tilting the plane of the rear of the fairing such that the rearward extremity 84' of the top wall in Figure 7 is displaced in the aft direction.

Finally, the drawings have generally shown the rear of the fairing in the same vertical plane as the back of the tractor.

However, other mounting positions on the roof of the tractor may be used. A rearward displacement of the fairing from the positions shown in the drawings may be limited by the requirement that the device not interfere with the articulation of the vehicle. A forward displacement of the fairing from that position may be limited by a legal length requirement that would mean that the fairing could not extend further forward than the front of the tractor.

From the foregoing, it is seen that there is herein provided an improved device for reducing the aerodynamic drag on tractortrailer truck combinations. Though the discussion above was specifically concerned with the use of the present invention on articulated vehicles, it is to be understood that the device would also have application for reducing the aerodynamic drag on single-chassis truck/van combinations.

Similarly, the discussion was concerned with van type trailers. It is to be further understood that the device would have application in any situation where the height of the front wall of the trailing body was heigher than that of the cab.

Finally, the discussion and illustrations have shown the plan view contour of the sides of the fairing to be symmetrical about the longitudianl axis of the fairing. While this is preferred in order to provide similar drag reductions at equal positive and negative angles of yaw, it is to understood that significant aerodynamic drag reductions could also be achieved with use of asymmetric contours developed in accordance with the guidelines set forth earlier.

It is to be noted that the embodiments of Figures 7, 8, 9, 10, 13 and 15 also conform with the invention in that each fairing has a nose disposed centrally of, and adjacent to, the front of the bottom of the fairing, a curved top surfaces extending upwardly and rearwardly from the nose, the top surface diverging in width from the nose towards the rear, and a pair of rearwardly diverging side surfaces, each side surface extending upwardly to respective diverging sides of the top surface.

WHAT WE CLAIM IS: 1. A streamlined fairing suitable for mounting on the roof of a vehicle comprising a nose disposed centrally of, and adjacent to, the front of the bottom of the fairing, a curved top surface extending upwardly and rearwardly from the nose, the top surface diverging in width from the nose towards the rear, and a pair of rearwardly diverging side surfaces, each side surface extending upwardly to respective diverging sides of the top surface.

2. A streamlined fairing as claimed in Claim 1 in which the nose is formed by the forward portion of the curved top surface and the forward portion of the side surfaces.

3. A streamlined fairing as claimed in Claim 1 or Claim 2 in which the rearmost and the uppermost part of the top surface is substantially horizontal.

4. A streamlined fairing as claimed in any preceding claim in which the nose is curved when viewed in plan.

5. A streamlined fairing as claimed in any preceding claimed in which the inclination of the curved top surface decreases towards the rear of the fairing.

6. A streamlined fairing as claimed in any preceding claim in which the rearmost and uppermost part of the curved top surface, in the direction of the width thereof, extends substantially horizontally.

7. A streamlined fairing as claimed in any preceding claim in which the rearmost part of the side surfaces is substantially vertical.

8. A streamlined fairing as claimed in any preceding claim in which a variable release angle tab is hinged to the top surface of the fairing.

9. A streamlined fairing as claimed in Claim 3 and Claim 8 in which the variable release angle tab is hinged to the substantially horizontal part of the top surface.

10. A streamlined fairing as claimed in any preceding claim in which variable release angle tabs are hinged to the side surfaces of the fairing.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. view contour of the front and sides of the fairing lie wholly within a rectangular envelope 100, Figure 14 of length I and of width w that is intersected by a line 110 having one end point located .2w aft of one of the forward corners of the rectangle and the other end point located .2w inboard from the same forward corner, and another line 110' similarly located from the opposite forward corner. This envelope is sketched as dashed line 100 in Figure 14. The drawings herein of the instant invention have shown its top and sides to intersect along a sharp corner. Experiments have shown that rounding the corner provides improvements in performance when compared to the sharp corner case. The drawings have also shown the profile view of the rear of the fairing to be generally vertical; for example, as in Figure 7. Experiments have demonstrated that useful performance increases can be achieved, particularly with fairings of shorter length, by tilting the plane of the rear of the fairing such that the rearward extremity 84' of the top wall in Figure 7 is displaced in the aft direction. Finally, the drawings have generally shown the rear of the fairing in the same vertical plane as the back of the tractor. However, other mounting positions on the roof of the tractor may be used. A rearward displacement of the fairing from the positions shown in the drawings may be limited by the requirement that the device not interfere with the articulation of the vehicle. A forward displacement of the fairing from that position may be limited by a legal length requirement that would mean that the fairing could not extend further forward than the front of the tractor. From the foregoing, it is seen that there is herein provided an improved device for reducing the aerodynamic drag on tractortrailer truck combinations. Though the discussion above was specifically concerned with the use of the present invention on articulated vehicles, it is to be understood that the device would also have application for reducing the aerodynamic drag on single-chassis truck/van combinations. Similarly, the discussion was concerned with van type trailers. It is to be further understood that the device would have application in any situation where the height of the front wall of the trailing body was heigher than that of the cab. Finally, the discussion and illustrations have shown the plan view contour of the sides of the fairing to be symmetrical about the longitudianl axis of the fairing. While this is preferred in order to provide similar drag reductions at equal positive and negative angles of yaw, it is to understood that significant aerodynamic drag reductions could also be achieved with use of asymmetric contours developed in accordance with the guidelines set forth earlier. It is to be noted that the embodiments of Figures 7, 8, 9, 10, 13 and 15 also conform with the invention in that each fairing has a nose disposed centrally of, and adjacent to, the front of the bottom of the fairing, a curved top surfaces extending upwardly and rearwardly from the nose, the top surface diverging in width from the nose towards the rear, and a pair of rearwardly diverging side surfaces, each side surface extending upwardly to respective diverging sides of the top surface. WHAT WE CLAIM IS:

1. A streamlined fairing suitable for mounting on the roof of a vehicle comprising a nose disposed centrally of, and adjacent to, the front of the bottom of the fairing, a curved top surface extending upwardly and rearwardly from the nose, the top surface diverging in width from the nose towards the rear, and a pair of rearwardly diverging side surfaces, each side surface extending upwardly to respective diverging sides of the top surface.

11. A streamlined fairing as claimed in

Claim 10 in which the variable release angle tabs are hinged to the rear of the side surfaces of the fairing.

1'. A streamlined fairing as claimed in any preceding claim mounted on the roof of a cab having a van or trailer situated behind the cab in which the nose of the fairing lies along the centre line of the roof of the cab with the fairing extending rearwardly with respect to the front of the cab.

13. A streamlined fairing as claimed in Claim 12 in which the uppermost part of the top surface is at approximately the same height as the top of the front wall of the van or trailer.

14. A streamlined fairing as claimed in Claim 12 or Claim 13 in which the width of the rear of the fairing is approximately the same as the width of the van or trailer.

15. A streamlined fairing as claimed in any of Claims 12-14 in which the length of the fairing is approximately the same as the length of the cab.

16. A streamlined fairing as claimed in any of Claims 12-15 in which the rear of the top surface of the fairing is disposed in the region of the rear of the cab.

17. A streamlined fairing as claimed in any of Claims 12-16 in which the rear of the side surfaces are located in the region of the rear of the cab at either side thereof.

18. A streamlined fairing as claimed in any of Claims 12-15 in which the highest part of the top surface is upwardly and rearwardly inclined at an angle which is less than or equal to approximately tan-1 [2(H-h)/x] and greater than or equal to zero, where 'H' represents the vertical distance from the roof of said cab to the top of the van or trailer, 'h' represents the height of said fairing and 'x represents the distance between the rear of said fairing and the front of the van or trailer.

19. A streamlined fairing as claimed in any of Claims 12-18, in which 'h', the height of the fairing, has a value greater than or equal to about .9H and less than

1.2H, where 'H' represents the vertical distance from the roof of said cab to the top of the van or trailer.

20. A streamlined fairing as claimed in Claim 19 wherein 'h' is less than 'H'.

21. A streamlined fairing as claimed in Claim 12 and Claim 8 or Claim 9 in which the uppermost part of the top surface is at a lower height than the top of the front wall of the van or trailer in which the variable release angle tab extends upwardly and rearwardly.

22. A streamlined fairing as claimed in Claim 12 and Claim 10 or Claim 11 in which the width of the rear of the fairing is less than the width of the van or trailer and in which the variable release angle tabs diverge rearwardly.

23. A streamlined fairing suitable for mounting on the roof of a vehicle and arranged substantially as herein specifically described with reference to Figure 5 of the accompanying drawings.

24. A fairing as claimed in Claim 23 but modified in accordance with any of Figures 7, 8, 9, 10, 13 and 15, of the accompanying drawings.