Note: Descriptions are shown in the official language in which they were submitted.
10840~2
This invention relates to a device for re-
ducing the aerodynamic drag on articulated tractor-
trailer combinations wherein the height of the trail-
er is greater than the height of the tractor.
Research has demonstrated that a substan-
tial 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 re-
sult 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 de-
sign of devices which are attached to the front of
the trailer and improve the manner in which the im-
pinging flow moves around the trailer's forward cor-
ners. A disadvantage of such devices is the fact
that those thus far designed have not been successful
in the total elimination of flow separation, particu-
larly of that part due to the flow which passes down-
ward through the gap between the back of the tractor
and the front of the trailer.
`~,
1(3 84(182
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
trac~or 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 de-
flectors designed to divert the flow about the por-
tion 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 config-
ured 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 re-
ductions in situations where the relative airstreamis 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
1084(~82
::.
which achieves optimum drag reductions on a given
tractor-trailer combination for all practical gap
settings between tractor and trailer.
The device consists essentially of a com-
bination of top and side portions or walls. The topwall or portion is contoured upwardly from the front
of the device, which is located at a forward position
on the top surface of the tractor roof, to a rear-
ward position whose height, ideally, is substantial-
ly equal to the height of the front face or wall ofthe 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 pas-
ses over the top wall to be rearwardly directed across
the gap between the tractor and the trailer, and to
reattach smoothly on to the top of the trailer. Cri-
teria 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 near-
ly 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 con-
toured outwardly from a position near the longitudinal
centerline of the tractor to a width at a rearward pos-
ition which, ideally, is equal to that of the trail-
er, 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
108~ 82
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 a device that can be attached to the roof of a tractor
used in combination with a trailer.
A further object of this invention is the provision
of a fairing which, by reducin~ wind resistance, will reduce
fuel consumption, and will improve vehicle stability for
purposes of reducing driver fatigue and enhancing safe
vehicle operations.
- ~till another object of this invention is the
provision of a device that will accomplish the foregoing
objectives in a manner which is different from, and superior
to those of previous devices intended for a similar purpose.
In accordance with the invention in one aspect there is
provided a fairing adapted to be mounted at the bottom portion
thereof on the roof of the cab of a truck with the longitudinal
axis of the fairing being parallel to the longitudinal
centerline of the roof of the cab, the fairing being adapted
; to reduce aerodynamic drag at the front portion of a truck
trailer positioned behind the cab of a truck and comprising
a nose portion disposed adjacent the front of the bottom
portion of the fairing at the longitudinal axis thereof, the
nose portion being adapted to be mounted adjacent the
longitudinal centerline of the roof of the cab with the
surface of the nose portion extending rearwardly with respect
to the front portion of the roof of the cab, a curved top
portion extending upwardly from the nose portion and rearwardly
with respect to the longitudinal axis of the fairing, the top
portion diverging in width from the nose portion toward the
rear of the top portion, the rear of the top portion extending
_ 5 _
1084(182
transversely with respect to the longitudinal axis of the
fairing and being adapted to be disposed above and adjacent
the rear portion of the roof of the cab, each opposite end of
the rear of the top portion being adapted to extend adjacent
a different opposite extremity of the rear of the roof of the
cab, the said top portion having a slope at its highest point
which is substantially zero relative to the horizontal, and a
: pair of oppositely disposed side portions diverging with
respect to one another, each side portion extending rearwardly
with respect to the nose portion and generally vertically to
the bottom portion of the fairing from a different oppositely
disposed longitudinal extremity of the diverging top portion,
each side portion terminating adjacent and below a different
end of the rear of the top portion.
In a further aspect there is provided a streamlined
fairing adapted to be mounted on the roof of a cab portion of
a land vehicle including a trailing body for reducing aerodyna-
mic drag comprising, a bottom portion, a rear portion and a
nose portion, and a continuous contoured surface extending
widthwise and upwards from said nose portion to said rear
portion and having a top portion and side portions which abut
at their respective boundaries, said top portion having at
least a segment the tangential plane of which is inclined with
respect to the horizontal 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 portion to a top wall of said trailing body,
h represents the height of said fairing, x represents the
distance between the rear portion of said fairing and the front
of said trailing body, and h has a value greater than or equal
to about .9H and less than H, and the said segment of said top
- 5a -
384~82
portion being located furthest from the said bottom portion.
In a still further aspect of the invention there is
provided in combination with a load carrying vehicle of the
type having a cab portion and a van portion spaced rearwardly
of the cab portion, said van portion having a front wall which
has a larger frontal area than the frontal area of the cab; drag
reducing means mounted on the roof of the ca~, said drag re-
ducing means comprising a streamlined fairing having at least
a top wall smoothly curved upwardly from a point on the center
line of the top surface of the cab roof to a rearward position
substantially equal to the height of the front wall of the van
at which rearward position a tangent to the top wall is
generally parallel to a top surface of the van and a pair of
oppositely disposed side walls smoothly curved outwardly from a
point on the center line of the top surface of the cab roof with
upper extremities of the side walls being joined to outer
extremities of the top wall, the side walls diverging outwardly
with respect to each other toward said rearward positio~.
In a still further aspect of the invention there is
provided a streamlined fairing adapted to be mounted on a roof
of a cab portion of a load carrying vehicle for reducing aero-
dynamic drag, said vehicle including a van portion havin~ front
and side walls, said front wall of said van portion extending
above said cab portion, said fairing having a longitudinal axis
and comprising: a bottom portion, a rear portion, and a
continuous contoured surface extending width-wise and upwards
from said bottom portion at the front of said fairing to said
rear portion, said surface having a top portion and side
portions on either side of the longitudinal axis of said fairing
which abut at their respective boundaries, said top portion
having at least a substantially horizontal segment a tangential
plane of which is inclined with respect to the horizontal at an
angle which is at least zero, said side portions diverging with
- 5b -
-`- 1084~8Z
respect to each other toward said rear portion and having sub-
stantially vertical segments which diverge with respect to the
: longitudinal axis of said fairing at an angle which is at least
zero, and said substantially horizontal segment of said top
portion being located uppermost with respect to said bottom
portion and said substantially vertical segments of said side
portions being located furthest from said longitudinal axis of
said fairing.
In a still further aspect of the invention there is
10 provided a streamlined fairing adapted to be mounted on a roof
of a cab portion of a load carrying vehicle for reducing aero-
dynamic drag, said vehicle including a van portion having front
and side walls, said front wall of said van portion extending
above said cab portion, said fairing having a longitudinal axis
and comprising, a bottom portion, a rear portion, a continuous
contoured surface extending widthwise and upwards from said
bottom portion at the front of said fairing to said rear portion,
and side portions on either side of the longitudinal axis of
said fairing which abut at their respective boundaries with said
contoured surface, said fairing further comprising a horizontalvariable release angle tab hinged to said contoured surface of
said faîring, the height of said fairing being less than the
vertical distance from the roof of said cab portion to the top
of the front wall of said van portion.
In drawings which illustrate embodiments of the invention:
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;
: 30
~` 1084a8Z
FIGV~E 2 is a graph showing the effectiveness of the
device shown in FIGURE l;
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 FIGU~E 2 showing
a comparison of the effectiveness of the devices shown in
FIGURES 1 and 3;
FIGURE S is a view like FIGURE 1 of one form of the
present invention;
- 5d -
1~8~Z
FIGURE 6 is a graph like that shown in FIG-
URE 2 showing a comparison of the effectiveness of
, the devices shown in FIGURES 1, 3, and 5;
FIGURE 7 is a diagrammatic showing of var-
ious lengths of the drag reducing device of the in-
: vention which come within the scope of the inventioni
FIGURE 8 is a diagram illustrating the re-
lationship between the width of the drag reducing de-
vice 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 reduc-
ing devicei
FIGURE 10 diagrammatically illustrates a
means for extending the effective height of the drag
reducing devicei
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 re-
ducing means is to be wholly contained;
FIGURE 15 is a diagrammatic elevational
view showing various contours which the drag reducing
1084~Z
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
5 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 in-
creased. However, because the front of the trailer
rides in the low-speed wake produced by the device,
l 5 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
20 will be a function of the device employed, the gap
involved, and the ambient conditions.
In Figure 1, a prior art tractor 5 1 and
trailer 51 ' are shown with one type of prior art drag
reducing device 50 attached to the roof 51" of the
25 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 de-
signed to encourage the flow of air that passes over
the roof of the tractor to move in an upward and
~084oBZ
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 ef-
fect it produces on the flow, device 50 can be con-
sidered 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 reduc-
ing 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 trac-
tor 51. More importantly, these side edge separations
increase in the presence of crosswinds, that is, for
non-zero yaw conditions, and significantly reduce the
effectiveness of device 50 at these conditions. This
has been verified in experiments conducted in a low-
speed 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 re-
duction rapidly decreases in the presence of cross-
winds, and even becomes negative, meaning that itincreases vehicle drag, at yaw angles of interest.
The average effectiveness of device 50 will be dis-
cussed later.
A second type of drag reducer is illustrated
in FIGURE 3. The drag reducing device 60, attached
--8--
108408Z
to the roof of tractor 61, is a deflector whose opt-
imum height from the roof of the tractor is less than
the height of the roof of trailer 61'; and whose opti-
mum 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 entry of the air stream 62 into the
- gap 63, and to cause the flow to reattach at the for-
ward 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 config-
uration, a function of the size of the gap between
the tractor and the trailer. In other words, a de-
flector optimized at one gap will deflect the flowtoo widely at larger gaps, causing the flow to reat-
tach downstream of the leading edges of the trailer -
and too narrowly at smaller gaps, causing some of
the flow to impinge on the front of the the trailer.
Consequently, non-optimum performance will be real-
ized 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 effective-
. ness 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 supe-
lQ~40az
rior 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 de-
vice 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 deflec-
tor whose optimum height from the roof of the tractor
is less than the height of the trailer, and a fair-
ing 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 up-
ward 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 pre-
ceding case, the separation stream surface produced
downstream of the trailing edge of the inclined upper
surface will, for a given deflector and vehicle con-
figuration, 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
--1 0--
lQ~8~
with gap size to be presented later will demonstrate
the considerable magnitude of the decrease in perfor-
mance that may occur at such non-optimum operating
conditions.
To summarize the prior art, there has been
an evolution of designs for tractor roof-mounted drag
reducers. With each new design there has come an im-
provement in drag reducing ability. However, in every
case cited, the designs either suffer from the inabil-
ity to provide significant drag reductions in the
crosswind situation or from having their design for
optimum effectiveness being a function of the gap be-
tween the tractor and the trailer.
The device of the instant invention has
been found to provide significant drag reductions
at 0 yaw, to maintain its effectiveness with increas-
; ing yaw angle, and to have its geometry for optimum
performance be independent of the size of the gap be-
tween the tractor and the trailer. One form of the
invention is illustrated in Figure 5. At zero yaw
the flow of air passing above roof 71' of the trac-
tor 71 is encouraged to divide, with a portion pass-
ing 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 encour-
aged, 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
-1 1-
82
downstream portion 76. The flow then continues across
gap 77 and smoothly reat-taches itself to the trailer
at its forward 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 sub-
stantially 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 trail-
er 82, the top wall is smoothly contoured upwardlyfrom the front of the device to the position of max-
imum height at which position the tangential plane
of the top wall is substantially parallel to the top
of the trailer, and the side walls are smoothly con-
toured outwardly from the front of the device to theposition of maximum width at which position the tan-
gential planes of the side walls are substantially
parallel to the sides of the trailer. 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 sym-
metrical about a vertical plane passing through axis
A.
General rules for the development of the
top and side wall contours of the device to effect
the described performance will be given later. Be-
fore doing this, however, it is of interest to com-
pare the performance of the instant invention with
that of the prior art.
A comparison of wind-tunnel measurements of
-12-
1084~82
the variations of drag reduction effectiveness of de-
vices 50, 60, and 70 with yaw angle is illustrated
in Figure 6. As can be seen, device 70 provides su-
perior performance not only at 0 yaw, but, more im-
portantly, at non-zero yaw angles. Since a vehicle
encounters a whole spectrum of wind-speeds and wind
directions during highway operations, it is of ir,ter-
est 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 in-
formation 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 opera-
tions over the nation's highways. The results ofcomputations for a number of vehicle configurations,
before and after modification by the addition of drag
reduction devices, demonstrate that while the aver-
age drag is a function of the vehicle design and the
design of the drag reduction device, the average per-
centage reduction in drag can be usually found in an
approximate yaw angle range of from 5 to 8D.
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
1084~2
data at yaw angles in the 5 to 8 range, is signif-
icantly better than that of designs 50 and 60 of the
prior art demonstrating the benefit of its unique de-
sign. Similar results have been obtained in full-
scale coast-down tests and in fuel economy 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 de-
flected to prevent its impingement on the front of
the trailer. As a consequence, the effectiveness of
the drag reducer decreases very rapidly with decreas-
ing gap size. At gap distances greater than the op-
timum value, the flow is deflected too high so that
flow reattachment occurs downstream of the trailer's
leading edges. Again, the effectiveness decreases
but this time as gap size increases.
In comparison, the effe~tiveness 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 poten-
tial 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 unusu-
al for the gap size on a given vehicle to be varied
-14-
1084~
quite frequently, it is apparent that the average
drag reductions 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 per-
formance is achieved with the streamlined fairings
of the present invention that extend the full-length
of the tractor roof, useful drag reductions can be
acheived 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 ~ 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 sub-
stantially equal to the vertical distance from the
top of the tractor cab roof to the top of the trail-
er, 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.
-15-
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 stream-
lined fairing 87 such that a shallow angle, c~ 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
behavior of separated flows suggests that an angle c~ = tan 1
[(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 reduc-
tions have been achieved in wind-tunnel tests with c~ 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 restric-
tions 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 (otherwise referred to as a
variable release angle tab) is hinged near the back vertical
edge of the fairing 88, and can be adjusted to g_ve optimum
performance over a range of distances, x. Since the trim
- 16 -
- - -
- tab, in effect, forms an extension of the width of
the fairing, the width of such a modified fairing
would w + 2tsin7 where t is the width of the trim
tab and ~ is the angle between the trim tab and the
; 5 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 im-
pinge on the trailer, the device can retain greater
than 9~ of its effectiveness for h > .9H and greater
than 75% of its effectiveness for h > .&H.
Another instance where a height mismatch
can occur is illustrated in Figure 12. Here a trac-
tor equipped with a fairing optimized for one trailer
- height indicated by roof 93 is shown used in combina-
tion with a lower trailer indicated by top 92. Again,
research results have been obtained which indicate
that the device can retain greater than 93~ of its
effectiveness for h < 1.2H and greater than 81~ of
its effectivenss for h < 1.4.
In situations where a fairing is to be de-
signed that will be used with trailers of various
1~
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 60~ of the difference in height be-
tween 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, cor-
responding to use with the 12'6" trailer, is equal
to about 1.2, and the minimum value of h/H, corres-
ponding 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
-18-
io8~82
;~
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 behavior 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)/x]. However, the pro-
vision 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 de-
pendent 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 con-
cerned with ideal and permissible non-ideal specifi-
cations of the height, width, and length of fairings
to the instant invention together with similar speci-
fications for the inclination of its top and side
walls at their positions of maximum height and width
-19-
:10840~2
with respect to the top and side walls of the trail-
er, respectively. Discussion will now be concerned
with guidelines that can be used to develop the con-
tours 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 objec-
tives of the invention. As can be seen, convex orconcave-convex contours can be used. The surface
need not be continuously curved, as shown, but can
include straigh~ 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 ~ptimize performance when h
< H. It is desirable, though not mandatory, that the
surface be free of any areas where the slope is dis-
continuous, the reason being that such areas couldcause 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 fair-
ing itself. For this same reason, gradual changes
of direction with distance along the top wall are
preferred over more rapid ones. Finally, it is de-
sirable that the radius of curvature at the point of
tangency be generous. For example, for fairings
where ~ > h, the radius of curvature could be gradu-
0 ally increased with distance from the front of the-20-
1~2
fairing, or simpler yet, a circular arc could be de-
scribed between the front and the position of maximum
height with the center of the arc selected to satisfy
the maximum height tangency condition. Or, for fair-
ings where ~ ~ h, the radius of curvature could begradually decreased to a value that preferably would
not be less than about .1W.
Figure 13 illustrates several configura-
tions that the plan view of the sides 94, 95, and
96 can assume. As before, convex, concave-convex,
or a combination of these and straight portions can
be used. The guide lines used in the development of
these contours within the length, width, and tangency
requirements for the side walls are the same as those
suggested for the top wall. Since the side wall con-
tours play an important role in achieving good yaw
performance, it is desirable that the width of any
flat portion 94' at the front of the fairing be small
compared with the width of the rear of the fairing.
A consideration of plan view shapes that should be
capable of providing adequate yaw performance sug-
gests that it is desirable to have the plan view con-
tour of the front and sides of the fairing lie wholly
within a rectangular envelope 100, Figure 14 of length
~ 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
108~)82
sketched as dashed line 100 in Figure 14.
- The drawings herein of the ins~ant inven-
tion 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 fair-
ing 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 dis-
placed 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 posi-
tions shown in the drawings may be limited by the
requirement that the device not interfere with the~ .
articulation of the vehicle. A forward dii~placement
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 tractor-trailer truck com-
binations. Though the discussion above was speci-
-22-
~0~4~82
fically concerned with the use of the present inven-
tion on articulated vehicles, it is to be understood
that the device would also have application for re-
ducing the aerodynamic drag on single-chassis truck/
van combinations. Similarly, the discussion was con-
cerned 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 higher 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 longitudinal 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 be understood that
significant aerodynamic drag reductions could also be
achieved with use of asymmetric contours developed in
accordance with the guidelines set forth earlier.
3o
