EPA-460/3-76-025
October 1976
ESTIMATION
OF VEHICLE
AERODYNAMIC
DRAG
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Office of Mobile Source Air Pollution Control
Emission Control Technology Division
Ann Arbor, Michigan 48105
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EPA-460/3-76-025
ESTIMATION
OF VEHICLE
AERODYNAMIC DRAG
by
Bernard Pershing and Mamoru Masaki
The Aerospace Corporation
El Segundo, California 90245
Contract No. 68-01-0417
EPA Project Officer: Glenn D. Thompson
Prepared for
ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Office of Mobile Source Air Pollution Control
Emission Control Technology Division
Ann Arbor, Michigan 48105
October 1976
-------
This report is issued by the Environmental Protection Agency to report
technical data of interest to a limited number of readers. Copies are
available free of charge to Federal employees, current contractors and
grantees, and nonprofit organizations - in limited quantities - from the
Library Services Office (MD-35) , Research Triangle Park, North Carolina
27711; or, for a fee, from the National Technical Information Service,
5285 Port Royal Road, Springfield, Virginia 22161.
This report was furnished to the Environmental Protection Agency
by The Aerospace Corporation, El Segundo, California, in fulfillment
of Contract No. 68-01-0417. The contents of this report are reproduced
herein as received from The Aerospace Corporation. The opinions,
findings, and conclusions expressed are those of the author and not
necessarily those of the Environmental Protection Agency. Mention
of company or product names is not to be considered as an endorsement
by the Environmental Protection Agency.
Publication No. EPA-460/3-76-025
ii
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FOREWORD
This report, prepared by The Aerospace Corporation for the
U.S. Environmental Protection Agency, Emission Control Technology Divi-
sion, presents a simple procedure for estimating road vehicle aerodynamic
drag using easily quantifiable vehicle shape parameters. The results of a
planimeter method for determining vehicle frontal area from photographic
enlargements are tabulated and discussed in an appendix. Also presented in
appendices are bibliographies on (1) road vehicle aerodynamic drag and pro-
cedures for its estimation, and (2) wind tunnel and full-scale testing
techniques.
iii
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ACKNOWLEDGMENTS
During the course of this study, Mr. Glenn Thompson of the
Environmental Protection Agency's Emission Control Technology Division,
who served as EPA Project Officer for the study, provided valuable guidance
and assistance. His efforts are gratefully acknowledged.
Mr. Mamoru Masaki and Mr. Bernard Pershing of The Aerospace
Corporation were principally responsible for the analysis effort reported
herein.
L. Forrest, Director
Vehicle Performance Office
Mobile Systems Group
Approved by
M.G. Hinton, Group Director
Mobile Systems
Environment and Energy Conservation Division
TT Iura, General Manager
Environment and Energy Conservation Division
v
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CONTENTS
FOREWORD iii
ACKNOWLEDGMENTS v
SUMMARY ix
1. INTRODUCTION AND BACKGROUND 1-1
2. DRAG PREDICTION METHOD 2-1
2.1 Approach 2-1
2.2 Drag Build-Up 2-2
3. DISCUSSION 3-1
4. CONCLUDING REMARKS 4-1
REFERENCES 4-3
APPENDICES
A. ROAD VEHICLE FRONTAL AREA AS DETERMINED
FROM PHOTOGRAPHS A-l
B. BIBLIOGRAPHY ON THE AERODYNAMIC DRAG
OF ROAD VEHICLES . B-l
C. BIBLIOGRAPHY ON WIND TUNNEL AND FULL-
SCALE AERODYNAMIC TESTING OF ROAD
VEHICLES C-l
vii
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TABLES
3-1 Effect of Rounded Edges 3-3
3-2 Comparison of Estimates with Full-Scale Data 3-4
3-3 Effect of Windshield Angle 3-5
3-4 Radiator Air Flow 3-9
A-l Road Vehicle Fronted. Area Summary A-6
A-Z ^Ref Comparison Similar Vehicles A-ll
FIGURES
2-1 Vehicle Dimensions 2-3
3-1 Effect of Edge Radius on Drag Coefficient of
Rectangular Configurations 3-2
3-2 Hatchback-Notchback. Drag Coefficient Ratio 3-6
A-l Passenger Sedan, Front View A-3
A-2 Schematic of Front View Photographic Projection A-4
viii
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SUMMARY
A simple procedure was developed for the estimation of road vehicle
aerodynamic drag based on easily quantifiable vehicle shape parameters. The
procedure is applicable to passenger vehicles, station wagons, and vans and
is based on a "drag build-up" method which includes the effects of the basic
body shape, underpanning, and cooling drag. Not included are the effects of
lift, sidewind, ground clearance, and certain shape details. The limitations
of the procedure are discussed and improvements and areas requiring further
study are identified. In a related activity, a brief investigation was made of
possible techniques for determining vehicle frontal area from photographs of
cars. Planimeter measurements of frontal area were made from photo-
graphic enlargements of approximately 80 cars. The results of this effort
are included as an appendix. Bibliographies on road vehicle aerodynamic
drag and on wind tunnel and full-scale road testing techniques are also
appended.
i x
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SECTION 1
INTRODUCTION AND BACKGROUND
The current method employed by EPA for exhaust emission
certification and fuel economy measurement of light-duty vehicles^ consists of
running the vehicles on a dynamometer through a prescribed duty cycle. The
power absorption unit of the dynamometer is adjusted according to a table of
50-mph road-load power settings defined for a discrete set of loaded vehicle
weights. Implicit in this procedure is the assumption that the aerodynamic
drag correlates with loaded vehicle weight. In reality, the aerodynamic
drag of road vehicles is a function of vehicle size and geometry and cannot be
adequately correlated by vehicle weight. The ability to estimate the drag of
individual vehicle configurations would provide an analytical basis for improv-
ing the accuracy of fuel consumption and exhaust emission testing, and for
normalizing Federal Test Procedure test results with respect to drag vari-
ations in a given vehicle weight category.
Analytical procedures and supportive test data for the estimation of
2, 3
road vehicle aerodynamic drag are limited. Two procedures ' have been
developed based on the "drag rating" method. A drag rating is determined
from the qualitative geometric characteristics of the vehicle configuration.
The drag rating is then interpreted as a drag coefficient through a correlation
curve. Both procedures indicate correlation curves which are linear with the
drag rating. These two procedures appear to be limited to the conventional
passenger vehicle configuration. Furthermore, since they are based on the
interpretation of qualitative characteristics, they lead to inconsistent esti-
mates even when restricted to the passenger class of road vehicles. There-
fore, a drag estimating procedure based on quantitative geometric character-
istics and applicable to the range of generic body shapes common to the
freeways was deemed to be desirable.
?Jc
Superscripts in the text refer to references listed after Section 4.
1 -1
-------
This investigation was undertaken to develop a simple, relatively
accurate procedure for estimating the aerodynamic drag coefficient of road
vehicles from easily quantifiable vehicle shape parameters. The method
developed consists of an aircraft-type "drag build-up" wherein the total drag
is considered to be equal to the sum of the contributions of the various com-
ponents of the vehicle. The preliminary formulation of this method based on
the available test data is presented in the main report. Appendix A describes
a study of methods for determining vehicle frontal areas from photographs.
Also presented, in Appendices B and C, respectively, are bibliographies on
the aerodynamic drag of road vehicles and on wind tunnel and full-scale road
vehicle aerodynamic testing techniques.
1 -2
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SECTION 2
DRAG PREDICTION METHOD
2. 1 APPROACH
The aerodynamic drag force of a body in motion with respect to the
surrounding air is given by
FD = 2 PU ARef CD ^
where
Fd = force, N (lb)
p = mass density of air, kg/m^ (slugs/ft3)
u = freestream (or vehicle) velocity, m/sec (ft/sec)
2 2
A^e^ = reference area, m (ft )
Cq = drag coefficient
A knowledge of C^ allows calculation of the vehicle aerodynamic drag over
all operating conditions.
Highly accurate and easily applied procedures have been developed
by the aircraft industry for estimating the drag of streamlined bodies. Since
road vehicle bodies typically are bluff and rather complex, considerable
aerodynamic knowledge and experience is required for satisfactory estima-
tion of their drag. Therefore, in the present study, a simplification as in
the manner of the drag rating method was sought. The basic approach
chosen was that of the "drag build-up" technique wherein the total drag coef-
ficient Cj-j^q,^, is considered equal to the sum of the contributions of the
various components of the vehicle
2-1
-------
n
%0T =Z^Cd i
1=1
(2)
where CDi is the contribution of the ith component including flow interference
effects. Each of the contributions is based on the quantitative geometric mea-
surements of the component and is restricted to the case of zero sideslip (no
crosswind) since neither sufficient time nor test data were available in the
present effort for the inclusion of crosswind effects. Both full-scale and
wind tunnel model data were used to establish the constants which define the
contributions of each component. It is noted that these data are limited
since much of the relevant work in this area is conducted by the automotive
industry and by the Motor Industry Research Association (MIRA) of England
and is treated by them as proprietary. As a result, these data were unavail-
able for incorporation in the present study. Therefore, the quantitative infor-
mation provided herein must be viewed as preliminary and in need of the
further substantiation which would be provided by the acquisition of a broader
data base.
2.2 DRAG BUILD-UP
The present formulation breaks the drag of a road vehicle into 11
discrete contributions. The reference area A_, which is used to normalize
XV
all the component drag contributions, is taken to be the projected frontal
area of the vehicle including tires and underbody details but excluding pro-
tuberances such as mirrors, antenna, and luggage carriers. The contribu-
tion of a component is a function of its size so that typically a representative
area A^ of each component, as well as appears in the estimates. The
relevant vehicle dimensions and areas are illustrated in Figure 2-1. The
details of the drag build-up are as follows.
2-2
-------
Figure
Vehicle Dimensions
-------
Front End Drag Coefficient, Cp^
CD1=°- 707 (^)j'-°-2-^(l)u+0-82(l)1-5-21(l)v
<3>
y
where
2 2
Ap = front end projected area, m (ft )
R = edge radius, m (ft)
E = running length of the edge radius, m (ft)
and the subscripts u, 1, and v refer to the upper, lower, and vertical edges
of the front end, respectively. The (R/E). are to be taken as 0. 105 when the
estimated values exceed this magnitude.
Windshield Drag Coefficient, Cpg
cV°-707fe)
where
2 2
= projected area of windshield, m (ft )
Y = slope of the windshield measured from the vertical, deg
P = 2V
and the subscripts u1 and v1 refer to the roof-windshield intersection and the
windshield posts, respectively. The value of cos p is to be taken as zero for
Y larger than 45° and the (R/E)^ are to be taken as 0. 105 for estimated
values exceeding this magnitude.
1.0 - 2. 79
(*L
cos (3 - 5.21
mi
2
cos y
(4)
2-4
-------
Front Hood Drag Coefficient, Cj}^
where
= projected area of body below the hood-windshield
intersection, (ft^)
L,^ = length of hood in the elevation or side view, m (ft)
and the quantity (A^ - -A-jp) is to be taken as zero if it is negative.
Rear Vertical Edge Drag Coefficient, C-p)^
where
= radius of rear vertical edges, rri (ft)
W = vehicle width, m (ft)
= length of rear vertical edge radius, m (ft)
H = vehicle height, m (ft)
Base Region Drag Coefficient, Cp,- _
-------
where
Ag = projected area of flat portion of base region
= projected area of upper rear or hatch portion of base
region measured from the upper rear roof break (or for
smoothly curved rooflines, that point where the roofline
slope is 15^ to the top of the flat base, (ft^)
C£)jj = drag coefficient of the flat base
= drag coefficient of the upper rear or hatch portion of
the base region
and the ratio (CDtj/CDt,) is shown in Figure 3-2 as a function of , the angle
ri ±5
of the line from the upper rear roof break to the top of the flat base as mea-
sured from the horizontal.
Underbody Drag Coefficient,
C„. = 0. 025 (0. 5 - x/L) —tL for 0 < x/L < 0. 5
6 \ R/ (8)
= 0 forx/L>0.5
where
x = smoothed forward length of the underbody, m (ft)
L = vehicle length, m (ft)
2 2
Ap = projected plan area of the vehicle, m (ft)
Wheel and Wheel Well Drag Coefficient, Cpy
CD? " °' 14 (9)
Rear Wheel Well Fairing Drag Coefficient, Cr>g _
C = -0.01 (10)
8
2-6
-------
Protuberance Drag Coefficient,
= —-V
D9 AR
A
PJ
(11)
where
2 2
A = projected area of jth protuberance, m (ft )
Pj
Bullet Mirror Drag Coefficient, q
CD =°-4AM
10 R
(12)
where
2 2
A^ = projected area of mirror with bullet fairing, m (ft )
Cooling Drag Coefficient, Cp^ ^
C°n--iSk)&
1.0 - 0.75 (^J
(13)
whe re
2 2
A^ = radiator area, m (ft )
u^ = exit velocity of cooling air from radiator
(u /u) = 0. 233 [l. 0 - k (u/100)2]
and
= 1 .146 (m/sec)"^ [or 0.299 (mph)"^]
2-7
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SECTION 3
DISCUSSION
The coefficients Cj)^ through Cj}^ comprise the bulk of the forebody
profile drag. The present method of estimating these quantities is based pri-
4 5 6
marily on the experimental results of Barth, Carr, and Saltzman for rec-
tangular configurations. An examination of these data reveals a dramatic
effect due to rounding the edges on the forward face of the models. Fig-
ure 3-1 shows these data plotted as a function of edge radius R normalized by
the vehicle width W. The data are insufficient to precisely establish the vari-
ation of drag with R/W or to determine all of the significant geometric param-
eters of influence. In the interest of simplicity two straight lines (shown
dashed in Figure 3-1) are used to represent the variation. It is assumed that
the shape of the variation is the same for any single edge or combination of
edges being rounded on a forward facing surface until further data become
available.
The effect of rounding the rear edges is considerably smaller than
that of rounding the forward edges. Saltzman's full-scale data with an R/W
of 0. 2 indicate that rounding all of the rear edges results in a drag coefficient
decrease of 0. 02. This decrease is taken to be that due to rounding the rear
vertical edges only, since Carr's results for the effect of rounding the various
rear edges did not definitely establish the contribution of the rear lateral
edges. Further work is necessary to more accurately determine the drag
increments due to rounding the rear edges.
5
Table 3-1 presents the drag coefficients obtained by Carr in a study
of the effects of rounding various front edges. It is noted that the squared
front lower lateral edge with all other edges rounded results in a higher drag
coefficient than for all edges rounded. However, Carr also presents data for
a simple automobile-like configuration which show the opposite effect. The
latter trend would be more consistent with the concept that small front
g
spoilers also tend to reduce drag as indicated by Hucho. The bracketed
3-1
-------
l.o .
O CARR5
4
0,81 \ y^PIECEWISE LINEAR D BARTH
\V FAIRING OF DATA
0.6
\
\
0.41-
0.2
0
V
0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32 0.36 0.40
EDGE RADIUS/WIDTH, R/N
Figure 3-1. Effect of Edge Radius on Drag Coefficient of Rectangular Configurations
-------
Table 3-1. Effect of Rounded Edges
F ront
Rear
Drag
Lower
Upper
Vertical
Vertical
Coefficient^-
-
0.875
X
0. 857
X
X
X
0. 531
X
X
X
0. 482
X
X
0.47
X
X
X
0. 29
X
X
X
0. 263
X
X
X
X
0.208
X
X
X
X
[0. 27]b
Width: Height: Length
R/W = 0.3
Carr
= 1:1:2.5
Rear lateral edges squared with all other edges rounded.
value 0.27 in Table 3-1, as measured by Carr for the squared rear upper
lateral edge with all other edges rounded, is assumed to apply for all edges.
Table 3-2 shows a comparison of the front end drag coefficient Cj^
computed using Eq. (3) with that obtained from the full-scale data of Saltzman.
In reducing the Saltzman data, the contributions of the wheels and wheel fair-
ings, underbody details, and the base were taken to be 0. 14, 0. 08, and 0. 14,
respectively.
The windshield contribution Cj)7 is assumed to be similar in form to
2
that of the front end with certain modifications. A cos y factor is applied
where y is the inclination angle of the windshield. This factor, which accounts
for the extent of secondary separation at the foot of the windshield, is based
9
on the data of Scibor-Rylski. Also, Carr's data indicate that rounding the
3-3
-------
Table 3-2. Comparison of Estimates with Full-Scale Data
Rounded Edges
Drag Coefficients
F ront
Lateral
F ront
Vertical
Rear
Vertical
Underbody
cd6
Estimated
Cdx
Measured1
CDj
00
o
o
1. 097
1.13
X
X
0. 08
0. 69
0. 68
X
X
X
00
o
o
0.49
0. 52
X
X
X
0
0.41
0. 44
X
X
0
0.43
0.46
Width: Height: Length = 1:0.793
R/W = 0. 2
: 2. 5
Saltzman, et al.
roof for windshield inclination angles greater than 45° has no effect on the
drag. The term for the radius at the roof-windshield intersection is multi-
plied by cos p to reflect this behavior. For (3 greater than 90°, the cos (3
term is taken to be zero. A comparison of measured and estimated wind-
shield drag values using Carr's data for the 45° windshield results is pre-
sented in Table 3-3. Although inaccuracies exist, further refinement would
detract from the simplicity of the estimating procedure.
Since the engine hood contribution appears to be relatively
small, the same approach as for the windshield is used. The radius terms
are omitted since Carr's data did not show a significant effect due to round-
ing the corners. A squared cosine factor appears in the windshield contribu-
tion to account for the windshield slope effect. Since a single geometric
slope for the hood is not available, an "equivalent squared cosine" factor is
employed. This factor is (A^ - A^,)/L^ , the difference between the cross-
sectional area at the hood-windshield intersection and the projected front end
area divided by the square of the distance between those two areas. This
3-4
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Table 3-3. Effect of Windshield Angle
Windshield Angle
(degrees)
Estimated
cL
Measured
35
0.478
0. 468
45
0.460
0. 460
55
0.439
0.451
65
0.421
0.434
a„ 5
Car r
results in an averaged, or effective, squared cosine term. For (A^ - Ap)
negative, this term is taken to be zero.
The formulation of Cj-jj., the drag coefficient contribution of the rear
7 8
portion, is based on the model test results of Hucho, et al. ' The rear con-
figuration is assumed to be either a hatchback or a notchback. Those con-
figurations with smoothly curved rooflines are treated as hatchbacks with the
roof break assumed to start at that point where the roofline slope is 15°. The
drag coefficient of the flat base CDg is taken to be 0. 15 while the ratio of
hatchback to flat base drag coefficient (C£)pj/C]T)g) is assumed to be a function
of <|>, the angle from the upper rear roof break to the top of the base area.
The variation of (Cnu/Cnn) with as obtained from the data of Hucho is
rl
shown in Figure 3-2.
Saltzman, et al. present the drag coefficient difference between a
smooth underbody and a detailed underbody of a vehicle as 0. 08. Taking the
plan view area A^ as being the significant area results in the contribution of
the detailed underbody above that of the smooth underbody as
c = 0.025 (A /A )
underbody "
3-5
-------
NOTCHBACK HATCHBACK FASTBACK
D
H
I— D
- D
B
100
HATCHBACK SLOPE, ~ deg
Figure 3-2. Hatchback-Notchback Drag Coefficient Ratio
-------
Beauvais, et. al., investigated the effect of smoothing the forward fraction
x/L, of the total length L, of the underbody. Linearizing the results for sim-
plicity modifies the above expression to yield the form for Cd^ given by
Eq. (8).
C
Test data on the drag of wheels and wheel wells are sparse. Carr's"
model investigation indicates a value of 0. 14. Kyropoulos, et al. , indicate
that the removal of wheels from a quarter - scale passenger automobile model
results in a decrease of 0.076. If 0. 14 is chosen for the wheels and wheel
wells, 0. 064 would be attributed to the wheel wells. Until further data
become available so that relative size effects can be included, Cj}7, the con-
12
tribution due to wheels and wheel wells, will be taken as 0. 14. Carr's
data for the rear wheel well being covered indicate that this contribution,
CjQg, is -0.01.
The contributions of protuberances to vehicle drag, and
are based on the data of Hoerner.^ The diameters of cylindrical protuber- _
ances such as antenna and luggage carrier rails are sufficiently small to
ensure subcritical Reynolds number flow even at high vehicle speeds. The
drag coefficient for this condition is about 1.1, the same as that for disk-
like mirrors and hood ornaments. The drag coefficient for cup-like shapes
convex windward is 0.4, the value applied to external mirrors with bullet
fairings. In general, the smaller protuberances, mirrors, and hood orna-
'ments will be mounted in regions of accelerated or locally separated flow
which will modify the magnitude of their drag contributions. However, such
refinements at present are unwarranted.
14
The cooling drag contribution is based on Olson's air flow
measurements through conventional front radiator configurations. Cooling
drag is taken to be the change in momentum rate of the flow through the radi-
ator. The mass flow rate rh through a radiator whose area is A is
r
m = p u A
rr r r
3-7
-------
where = air density behind radiator, and u = average air flow velocity
behind the radiator. Freestream values are denoted without subscript. The
change in momentum rate is the product of the mass flow rate and the differ-
ence between the freestream velocity and the exit velocity. The air from
behind the radiator passes over the engine with variations in the horizontal
velocity occurring due to flow c ros s - section area, heat exchange with the
engine, viscous effects, and mixing with secondary air flow. In order to
simplify the problem, the exit velocity is assumed to be 0.75 u^. Thus, the
cooling drag coefficient becomes
(u - 3 /4 u )
/"* * r
CD = m 2
cooling (1/2 pu A^)
With the added assumption that the radiator-to-freestream density
ratio is 0.9 due to heat transfer from the radiator, Eq. (13) is obtained for
j as a function of the velocity ratio (u^/u). If simplicity is of no concern,
the energy exchange and the losses through the radiator can be included in the
estimate. Olsen's data in modified form are presented in Table 3-4. It is
assumed that the incremental air flows due to configuration changes are inde-
pendent. Since the magnitudes shown at each velocity are nearly the same,
they were averaged for the two test vehicles and assumed linear with respect
to velocity to result in the variation of (u^/u) given by Eq. (14).
In the present formulation, the skin friction contribution is con-
sidered to be included in the profile drag coefficients, through In
general, the magnitude of the skin friction and its variation with Reynolds
number or vehicle speed are relatively small so that attempts to isolate these
effects are not warranted at this time.
3-8
-------
Table 3-4. Radiator Air Flow3,
Vehicle
Speed
(mph)
Flow Rate
(cfm)
Fraction Flow
Rate After
Losses
Fraction Initial
Flow Rate
Recovered
1974 Pinto
30
1340
0. 373
0. 559
A = 2. 08 ft2
x
60
3030
0. 545
0.258
1974 Mustang
30
1440
0.347
0. 656
A = 2. 36 ft2
r
60
. 3240
0.537
0.275
Losses due to bumper and grill
Recovery due to radiator fan and shroud
ani 14
Olsen
3-9
-------
SECTION 4
CONCLUDING REMARKS
Since the drag contributions of the various vehicle components are
not independent, formulations of aerodynamic force estimations are generally
limited to similar configurations where interference effects between compo-
nents are also similar. Nevertheless, it is believed that the present expres-
sions are applicable to conventional passenger vehicles, station wagons, and
vans. Only a few simple checks have been made since accurate configuration
data were not on hand for reliable comparison. Thus, the task remains for
the present drag estimation procedure to be tested for validity.
Several assumptions were incorporated in the present formulation of
a drag estimation procedure to retain the primary objective of simplicity of
application. Also, the limited test data available and the level of effort
allotted imposed restrictions on the scope and depth of the analysis. For
example, the drag reduction due to rounded edges is of concern because of
the large effect demonstrated by the experimental information. The assumed
variation should be redefined if necessary. Further effort is warranted to
determine the effective radius for edges of noncircular shape. Other shaped
corners, for example elliptical, should be compared with the circular to
determine the relative sizes for similar overall effect on the leading edge
suction and influence on flow away from the corners.
The drag coefficient for a flat base has been taken as constant in the
present formulation without regard to changes to the configuration. The
experimental base pressure coefficient measurements presented by Saltzman
13
demonstrate that variations occur due to rounding the rear edges. Hoerner
presents a base drag coefficient variation as a function of forebody drag, but
it is applicable only to the condition of well-behaved, nonseparated flow ahead
of the base of bodies of revolution. Since motor vehicles operate with a vari-
ation of flow conditions around the periphery of the base, a study of the effect
on the average base pressure would be desirable.
4-1
-------
Several factors which influence vehicle drag have been, by necessity,
omitted from the present analysis. Among the more important are drag due
to lift, crosswind effects, vehicle ground clearance, and certain detail fea-
tures of the geometry. Examples of the latter are front end treatment where
deep set headlights can produce two to three times the drag of flush-mounted
lights, strakes or side fences which fair the line between the rear window and
rear hood on notchbacks and which by their fencing action increase the base
drag, and chin spoilers and rear deck dams or upturns which, when properly
designed, have been shown to reduce vehicle drag. These factors deserve
detailed consideration and where found warranted should be incorporated in
the present method to improve its accuracy and scope of application.
Finally, it is considered necessary to apply the present method of
drag estimation to the available EPA roll-down data on current models.
These data are uncontaminated by wind tunnel interference effects and should
be of high quality. Such a comparison will provide the best opportunity for
refining and, where necessary, revising the details of the present method.
4-2
-------
1
2
3
4
5
6
7
8
9
10
11
12
REFERENCES
Federal Register, 40 (12'6) "Environmental Protection Agency —
Control of Air Pollution from New Motor Vehicles and Engines"
(30 June 1975).
R.G.S. White, An Experimental Survey of Vehicle Aerodynamic
Characteristic si MIR A Report No. 1967/11, Lindley, Nuneaton,
Warwick, England (September 1967).
J.J. Cornish III and C.B. Forston, Aerodynamic Drag Characteristics
of Forty-Eight Automobiles, Research Note No. 23, Mississippi State
University, Jackson, Miss. (June 1964).
R. Barth, Effect of Unsymmetrical Wind Incidence on Aerodynamic
Forces Acting on Vehicle Models and Similar Bodies, SAE 650136,
SAE, 400 Commonwealth Dr. , Warrendale, Penn. (~1966).
G.W. Carr, "Some Aerodynamic Aspects of Safety in Road Vehicles
Aerodynamic Lift Characteristics of Cars," Mech. Inst. Eng. Proc.
187, (30) (1973).
E.J. Saltzman and R. R. Meyer, Jr., Drag Reduction Obtained by
Rounding Vertical Corners on a Box-Shaped Ground Vehicle, ¦NASA
TM X -56023, NASA, Washington, D.C. (March 1974).
L.J. Janssen and W. H. Hucho, The Effect of Various Parameters on
the Aerodynamic Drag of Passenger Cars, Paper No. 14, Volkswagen-
werk AG, German Federal Republic.
W. H. Hucho, L.J. Janssen, and H.J. Emmelmann, The Optimization
of Body Details — A Method for Reducing the Aerodynamic Drag of Road
Vehicles! SAE 7 60185, SAE, 400 Commonwealth Dr. , Warrendale,
Penn. ("23-27 February 1976).
A.J. Scibor-Rylski, Road Vehicle Aerodynamics, John Wiley & Sons,
New York (1975).
F.N. Beauvais, S.C. Tignor, and T. R. Turner, Problems of Ground
Simulation in Automotive Aerodynamics, SAE 680121, SAE, 400 Com-
monwealth, Warrendale, Penn. (1968).
K. B. Kelly, P. Kyropoulos, and W. F. Tanner, Automobile Aerodynam-
ics, SAE Preprint No. 148B, SAE, 400 Commonwealth Dr. , Warren-
dale, Penn. (I960).
G.W. Carr, Reducing Fuel Consumption by Means of Aerodynamic
Add-On Devices, SAE 760187, SAE, 400 Commonwealth Dr. , Warren-
dale, Penn. (23-27 February 1976).
4-3
-------
13. Sighard F. Hoerner, Fluid-Dynamic Drag, Chapter XII, Published by
Author, Midland Park, New Jersey (1958).
14. M.E. Olson, Aerodynamic Effects of Front End Design on Automotive
Engine Cooling Systems, SAE 760188, SAE, 400 Commonwealth,
Warrendale, Penn. (23-27 February 1976).
4-4
-------
APPENDIX A
ROAD VEHICLE FRONTAL AREA AS DETERMINED
FROM PHOTOGRAPHS
A. 1 INTRODUCTION AND BACKGROUND
In EPA-certification testing of light-duty vehicles,^ the component
of the dynamometer power absorption unit setting which represents aerody-
namic drag is determined from a table of road-load power settings defined
for a discrete set of vehicle weights. . However, aerodynamic drag is more
accurately represented as a function of vehicle size and geometry. Various
drag estimation methods which reflect these two factors are being investi-
gated by EPA for comparison with current procedures.
In the prediction of aerodynamic drag, the size factor is repre-
sented by a reference area A^^, typically the vehicle projected frontal area,
while the effect of geometry is accounted for by the drag coefficient Cj-j
Methods for predicting C^ were developed and are reported in the body of
this report while the results of a study of methods for determining
from 35 mm negatives are presented herein.
In general, the contours of road vehicles are too complex to permit
direct measurement of and no detailed drawings or loft lines are provided
by the manufacturer from which this quantity may be obtained. In the field, .
measurements of A|^e£ are most readily obtained from front view photographs
of the vehicle. . In the present study, available methods were investigated by
which the projected frontal area of road vehicles can be determined from
3 5 mm photographic negatives and the preferred method was selected and
applied to a set of negatives provided by EPA. The results of this effort as
well as an assessment of the accuracy of the selected method are discussed
in the following sections.
^Federal Register, 40 (126), Part III, "Environmental Protection Agency —
Control of Air Pollution from New Motor Vehicles and Engines" (30 June 1975).
A-l
-------
A. 2
FRONTAL AREA DETERMINATION PROCEDURE
The 3 5 mm negatives provided by EPA consisted of front, side, and
rear views as well as three-quarter front and rear views of 77 road vehicles.
Included in this set were 56 passenger sedans, five station wagons, five vans,
eight pick-up trucks, and three sports cars. AO.2 mx 0.2m (0. 656 ft
x 0. 656 ft) square was attached to the front of each vehicle to provide a
dimensional reference. Glossy 8 in. x 10 in. enlargements were made from
the 3 5 mm negatives and were used for all subsequent analyses. A typical
front view is shown in Figure A-l. Apparent in this photo are the strong
effects of perspective which result from the proximity of the camera to the
vehicle and which cause a distortion of the projected area for all surfaces aft
of the plane containing the reference square. This effect, shown schematically
in Figure A-2, not only contracts dimensions in receding planes but also
causes a distortion of the front hood and roof projected area-due to differences
in elevation between the camera and these surfaces. Also of note in Fig-
ure A-l is the poor definition of the underbody contour which is almost totally
lost in shadow.
Measurement of the projected frontal area from the 8x10 prints
must be preceded by a definition of the underbody contour and a correction of
the windshield-roof contour for the effects of perspective. Once this has been
done, the physical measurements of the corrected vehicle contour and refer-
ence square can be made by some appropriately chosen technique and these
areas used in a straightforward fashion to compute t*16 present
study, the definition of the underbody contour was greatly facilitated by view-
ing the prints on a light table. The correction of the windshield-roof contour
and front hood and roof areas for the effects of perspective was accomplished
by utilization of all views of the road vehicle to pick up visual keys such as the
slope of the front hood and the ratios of windshield-hood intersection width and
rear window width to vehicle maximum width. These clues were then used to
construct on an overlay a best estimate of the true projected contour of the
windshield-roof line.
A-2
-------
Figure A-l. Passenger Sedan, Front View
-------
AREA
DISTORTION
/ ^ORTHOGRAPHIC
/ PROJECTION
^PHOTOGRAPHIC
IMAGE
VIEW A - A
PROJECTION
PLANE
FOCAL
PLANE
IMAGE PLANE
Figure A-2. Schematic of Front View Photographic Projection
-------
Two techniques were considered for the measurement of the
corrected projected frontal area: direct measurement by planimetry,
and use of a laboratory grade balance to determine the ratio of the projected
frontal area cut from the photograph to the area of the reference square.
Both techniques require the use of precision equipment, a planimeter or a
laboratory scale, so that no advantage is apparent here. Also, the accuracy
of the two procedures is considered to be equivalent. This point is discussed
further in Section A-4. The essential difference between these two methods
is that planimetry is nondestructive and allows preservation of the basic data
and repeatability checks of the perspective corrections and area measure-
ments. It is on this basis that planimetry was selected as the preferred
method of area measurement.
A. 3 AREA MEASUREMENT RESULTS
The procedure described above for the measurement of from
3 5 mm negatives was applied to the set of 77 road vehicles provided by EPA;
results are presented in Table A-l. The reference area is defined as
the orthographic projection on a vertical plane of the frontal area of the
vehicle including tires and underbody details but excluding protuberances such
as mirrors, antenna, and luggage carriers. The projected frontal area
excluding the exposed tires is also presented and is denoted The ^
is included as it may be found to be a useful parameter in future correlations
of predicted and measured aerodynamic drag.
The vehicles are grouped according to type and it is of interest to
note that while the Ap f of the passenger sedans and station wagons varies
2 2 2 2
from about 1.30m (14 ft ) to 2. 00 m (21 ft ), the Ap f of the vans is typi-
cally about 2.8 m (30 ft ). The large frontal area of the van in conjunction
with its large drag coefficient provides a good indication of the relatively
large power requirements of this type of vehicle, in highway travel.
A..4 MEASUREMENT ACCURACY
Two sources of error can be identified with the selected procedure
for measuring those associated with the photographic documentation
A-5
-------
Table A-l. Road Vehicle Frontal Area Summary
Total Area, Area Sans
Type, Make, License Number or -A-Ref Tires, Aj^ef
and Model Other Identification
(m2) (ft2) (m2) (ft2)
PASSENGER SEDAN
American Motors Gremlin
135 QN
(Ohio)
1. 386
14. 92
1. 298
14. 08
Racer
VKW 072
(Michigan)
1. 579
16. 98
1.495
16. 08
Buick Apollo
J7927
(Ohio)
1. 705
18. 38
1. 590
17. 14
Century
347 043
(Ohio)
1. 847
20. 11
1. 732
18. 87
Lie Sabre
J84 94
(Ohio)
1. 835
19. 70
1. 751
18. 80
Skyhawk
J7977
(Ohio)
1. 432
15.42
1. 344
14. 47
Skylark
J7974
(Ohio)
1. 631
17. 57
1. 513
16. 30
Special
J8619
(Ohio)
1. 798
19. 37
1. 674
18. 04
Chevrolet Chevelle
6178RU
(Ohio)
1. 702
18. 31
1. 577
16. 97
Vega
347 012
(Ohio)
1. 456
15. 69
1. 355
14. 60
Impala
199 QN
(Ohio)
1. 876
20. 20
1. 752
18. 87
Monza
347 081
(Ohio)
1. 427
15. 36
1. 334
14. 36
Chrysler Newport
11M228
(Michigan)
1. 865
20. 09
1. 794
19. 32
New Yorker
PHK134
(Michigan)
1. 865
20. 09
1. 757
i8. 88
Datsun B2 10
1350
(Michigan)
1. 384
14. 91
1. 298
13. 99
280Z
DTM82
(New Jersey)
1. 306
14. 06
1. 208
13. 00
Dodge Dart Sport
10M905
(Michigan)
1. 501
16. 16
1.402
15. 10
Fiat 128
MFR 22AZ(Illinois)
1. 461
15. 74
1. 384
14. 91
Ford Capri
EPA358
(U.S. Govt. )
1. 291
13. 89
1. 194
12. 85
Capri II GT
21M005
(Michigan)
1. 377
14. 81
1. 300
13. 98
Granada
PDF447
(Michigan)
1. 661
17. 88
1. 536
16. 53
Granada
Body Marking 844
1. 760
18. 97
1. 640
17. 68
Gran Torino
14M495
(Michigan)
1. 569
17. 95
1. 732
16. 56
Gran Torino
TFJ540
(Michigan)
1. 680
18. 05
1. 578
16. 95
LTD
NJG477
(Michigan)
1. 807
19.42
1. 697
18. 24
LTD
14M494
(Michigan)
1. 885
20. 27
1. 757
18. 89
-------
Table A-l. Road Vehicle Frontal Area Summary (cont'd)
Total ,
Area,
Area Sans
Type, Make,
License Number or
ARef
Tires,
ARef
and Model
Other
Identification
(m2)
(ft2)
(m2)
(ft2)
LTD
14M572
(Michigan)
1. 798
.19. 37
1. 676
18. 10
Maverick
21M067
(Michigan)
1. 620
17.49
1. 518
16. 39
Mustang II
21M056
(Michigan)
1. 565
16. 86
1. 440
•15. 52
Mustang II - Mach I
21M057
(Michigan)
1. 414
15. 23
1. 275
13. 73
Mustang II - Mach I
21M068
(Michigan)
1. 408
15. 15
1. 297
13. 96
Pinto
146QN
(Ohio)
1. 461
15. 72
1. 346
14.49
Thunderbird
21M113
(Michigan)
1. 747
18. 82
1. 617
17.42
Torino
17M842
(Michigan)
1. 756
18. 90
1. 636
17. 61
Honda CVCC
No. I. D.
1. 516
16. 31
1. 424
15. 32
Lincoln Continental
FVL183
(Michigan)
1. 981
21. 32
1. 856
19. 98
Mazda
416D61
(Michigan)
1. 526
16.41
1.437
15. 45
Mercury Marquis
1349
(Michigan)
1. 900
20. 28
1. 766
19. 03
Montego
Body Marking 733
1. 841
19. 82
1. 735
18. 68
Oldsmobile Cutlass Supreme
198QN
(Ohio)
1. 841
19. 81
1. 720
18. 51
Cutlass Supreme
7937RU
(Ohio)
1. 757
18. 90
1. 640
17. 64
Delta 88
170QN
(Ohio)
2. 020
21. 70
1. 900
20. 41
Plymouth Gran Fury
11M227
(Michigan)
1. 807
19. 44
1. 668
17. 95
Scamp
11M248
(Michigan)
1. 719
18. 52
1. 600
17.24
Valiant Custom
11M232
(Michigan)
1. 556
16. 76
1.450
15. 66
Valiant Custom
G1222746
(U.S. Govt. )
1. 624
17. 46
1. 527
16.43
Pontiac Firebird
506751
(Ohio)
1. 492
16. 10
1. 377
14. 86
Le Mans
164QN
(Ohio)
1. 774
19. 10
1. 640
17. 66
Le Mans
173QN
(Ohio)
1. 802
19. 40
1. 669
17. 97
Ventura
194QN
(Ohio)
1. 568
16. 84
1.467
15. 75
Saab 99 EMS
MFR 25C
(Illinois)
1. 323
14. 25
1.211
13. 05
-------
Table A-l. Road Vehicle Frontal Area Summary (cont'd)
Type, Make,
License Number or
Total Area,
ARef
and Model
Other Identification
(m2)
(ft2)
Toyota Celica
Corolla
MFR DTM9887(New Jersey)
No I. D.
1. 318
1. 401
14. 00
15. 10
Volkswagen Beattle
Rabbit
155QN (Ohio)
RCM581 (Michigan)
1.436
1.460
15.46
15. 17
Volvo 264 DL
HTE061
1. 746
19. 00
Area Sans
Tires, A^ef
STATION WAGON
Buick Estate Wagon J7971
Chevrolet Impala Wagon 139QN
Ford Gran Torino Squire 21M114
Torino Squire 21M112
Plymouth Fury Wagon 10M838
VAN
Chevrolet Beauville
Sport Van 30
Chevy Van 10
Chevy Van 20
Dodge Tradesman 100 van
Ford Econoline 150 van
PICK-UP TRUCK
Chevrolet Cheyenne 10 13M807
Cheyenne 20 13M845
Scottsdale Van 10 13M805
Datsun Pick-up 1343
Ford F-100 Pick-up 14M535
F-100 Pick-up 21M115
F -250 Pick-up 7378HV
(Ohio)
(Ohio)
(Michigan)
(Michigan)
(Michigan)
13M942 (Michigan)
13M818 (Michigan)
13M823 (Michigan)
19M589 (Michigan)
7928 HV (Michigan)
(Michigan)
(Michigan)
(Michigan)
(Michigan)
(Michigan)
(Michigan)
(Michigan)
1. 842
1. 921
1. 728
1. 758
1. 766
3. 048
3. 060
2. 972
2. 737
2. 758
2.440
2.470
2. 488
2. 431
2.452
2.486
19. 85
20. 68
18. 60
18. 91
19. 03
32. 82
33. 00
32. 00
29.47
29. 69
26. 25
26. 60
26. 80
(m2)
1. 219
1. 311
1. 341
1. 370
1. 727
1. 790
1. 609
1. 665
1. 650
2. 856
2. 900
2. 768
2. 658
2. 620
2. 254
2. 270
2. 323
26. 21
26.41
26. 80
2.251
2. 319
2. 331
(ft2)
13. 93
14. 13
14.44
14. 20
1.652 17.99
18. 61
19.27
17. 32
17. 91
17. 78
30. 75
31. 28
29. 80
28. 62
28. 2
24. 2!
24. 4!
25. o:
1.649 17.74 1.538 16.51
24. 2
24. 9
25. 1
-------
Table A-l. Road Vehicle Frontal Area Summary (cont'd)
Tyne, Make,
and Model
License Number or
Other Identification
Total Area,
ARef
(m2) (ft2)
Area
Tires,
(m2)
Sans
¦^¦Ref
(ft2)
Toyota Pick-up
No. I.D.
1. 650
17. 79
1. 53 5
16. 55
SPORTS CAR
Chevrolet Corvette Coupe
13M846 (Michigan)
1. 332
14. 35
1. 196
12. 89
Triumph TR-6 Top On
No I. D.
1. 309
14. 09
1.218
13. 11
TR-6 Top Off
No I. D.
1. 186
12. 78
1. 094
11. 78
>
I
s£)
-------
and image correction and those associated with the precision of planimetry.
The errors associated with planimetry of the corrected vehicle contour were
examined by performing a controlled series of planimetry checks on a set of
five vehicle contours. At various stages of the study, each of the five con-
tours was planimetered several times by three technicians to establish the
consistency with which the operation could be performed. A total spread in
A_ , of 1% or less was obtained in all cases.
Kei
The major sources of error were found to be photographic distortion
of the projected area due to perspective and poorly defined underbody con-
tours. This results because the corrections for perspective and obscured
contours are not precise; they require judgment, a good eye, and some
artistic skill. An indication of the magnitude of these errors is obtained by
comparing the data from Table C-l for those vehicles with the same body
style. There are seven such cases, four consisting of two cars and three
consisting of three cars. The comparison is shown in Table A-2, the last
column of which lists AA^ef/A^^, the ratio of the maximum difference in
measured A„ , to the mean A„ r for each case. The mean value of AA„ rl
Ref Ref Ref
A^e£ for the seven cases is 0. 0483. This quantity is representative of the
total error obtained and includes the effects of planimetry as well as perspec-
tive. However, these two effects are independent and would be combined
statistically by root-sum-squaring. Since the mean total error is larger than
the planimetry error by a factor of five, it is clear that the mean
is essentially a measure of the error due to perspective.
The errors due to perspective can be reduced by photographing at a
greater distance from the vehicle, and by use of reference squares and scales
at the vehicle base and at intermediate stations such as the windshield-roof
break line as well as at the front. Referring to the schematic of Figure A-2,
the deviation of the windshield-roof break line length from the true projected
length varies as (1 + X^j/X^)-1, where X^ is the distance of the break line
aft of the projection plane, taken here as the vehicle front, and Xp is the dis-
tance of the camera ahead of the projection plane. It is desirable to make
X„ at least one order (preferably two orders) of magnitude larger than X .
IT ri
A-10
-------
Table A-2. Comparison of Similar Vehicles
V ehicle
ARef
(m2)
. ARef
(m2)
AARef
(m2)
AARef
ARef
Ford Granada
1. 661
1. 760
1.711
0. 099
0.0579
Ford Gran Torino
1. 669
1. 680
1. 756
1 . 702
0. 087
0.0511
Ford LTD
1. 807
1. 885
1. 798
1. 830
0. 087
0. 047 5
Ford Mustang II
1. 565
1.414
1.408
1.462
0. 157
0.1074 .
Plymouth Valiant
Custom
1. 556
1. 624
1. 590
0. 068
0.0428
Pontiac Le Mans
1. 774
1. 802
1. 788
0. 028
0. 0157
Ford Gran Torino Squire
l
1. 728
1. 758
1. 743
0. 030
0.0172
AAR f
Mean Value of — — = 0. 0483
ARef
A second benefit of a larger stand-off distance is that the vehicle underside
will be more strongly silhouetted and more clearly defined.
An additional point to consider is placement of the reference square.
Typically, the vehicle'is largest at some station in the midsection and it is
the cross-sectional area at this station which must be used as the vehicle
reference area A0. It is imperative that the camera be placed far enough
K.
from the vehicle so that AD is imaged on the film rather than a more forward
K.
cross-section of lesser area. If A^ can be imaged on the film and if the
reference square is located at A , no error is introduced regardless of the
R
A-11
-------
vehicle-to-camera distance. The problem is locating the station of maximum
cross-section and the possibility that the projected area is greater than any
single cross-section. The latter case frequently occurs because of wheel
wells flared for tire clearance in the front fenders.
The effect of locating the reference square at an incorrect station is
determined as follows. From geometric optics
and from the thin lens formula
i = image size
s1 = image distance
o = object size
s = object distance
f = focal length of lens
(A-2)
where
Combining Eqs. (A-l) and (A-2)
1 = (s - f)
of
(A-3)
or, since s » f
(A-4)
A-12
-------
By logarithmic differentiation of Eq. (A-4)
di ds
(A-5)
is x
If Ad is to be determined to within ±2%, the linear dimensions must
K.
be determined to within ±1%. Accordingly, if the uncertainty in the station for
A_ is ±1 m (3. 28 ft), then to ensure a 2% error or less in area, Eq. (A-5)
XV.
states that the camera-vehicle distance must be greater than 100 m (328 ft).
From these considerations, it is seen that the camera-vehicle dis-
tance criterion is more appropriately two orders of magnitude greater than
either the uncertainty in the Station for A— or the separation between cross-
K.
sections where maxima of local projected regions may occur.
Photographing at large distances will require either a lens of very
large focal length, so that a normal size image will be produced on the nega-
tive, or extreme enlargement of a negative which is made using a lens of
more conventional focal length. Use of a large focal length lens is preferable
since sharper prints can be obtained. (The use of a light background such as
a portable screen of white paper will also facilitate a sharp definition of the
vehicle contour. )
By use of a large camera stand-off distance to minimize perspective
and by use of discretely placed reference squares and scales to minimize
errors in the correction of that perspective which remains, it is considered
possible to reduce these sources of error to the level of the planimetry error.
A-5. SUMMARY AND CONCLUSIONS
Available methods were investigated by which the projected frontal
area of road vehicles can be determined from 35 mm photographic negatives.
Planimetry of a photographic enlargement, the contours of which have been
corrected for the effects of perspective, is considered the most desirable
procedure. Application of this procedure to a set of 35 mm negatives indicates
the total error to be about 5% and to be due primarily to the effects of perspec-
tive and to the poor definition of the underbody contour.
A-13
-------
It is concluded that, for consistent results, the front view
photographs must be taken at distances at least one order of magnitude
greater than the vehicle length and that a light uniform background shotdd be
used to sharpen the vehicle contour. By use of these procedures, it is con-
sidered that the total error in vehicle measured projected frontal area can be
reduced to less than 2%.
A-14
-------
APPENDIX B
BIBLIOGRAPHY ON THE AERODYNAMIC DRAG
OF ROAD VEHICLES
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B -1
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Carr, G. W., "Aerodynamic Lift Characteristics of Cars, " Inst. Mech. Eng.
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(May 1957).
B -2
-------
Gottardelli, G. , "Visuallizzazione Delle Correnti Fluide Intorno a
Profilimodello di Autoveicoli (Visualization of Fluid Flow Around
Automobiles), " Termotecnica, 1 1 (11), pp. 525 (November 1957).
Grunwald, K.J., Aerodynamic Characteristics of Vehicle Bodies at Cross-
wind Conditions in Ground Proximity, Report No. NASA TN D-5935,
NASA, Washington, D.C. (August 1970).
Hoerner, S. F. , "Determining Drag of Automobile Under Actual Road Condi-
tions, " Automotive Industries, 103 (6) (15 September 1950).
Hucho, W.H., and H.J. Emmelmann, Theoretical Prediction of the Aero-
dynamic Derivatives of a Vehicle in Gross Wind Gusts, Paper
No. 730232, Volkswagenwerk AG (1973).
Kelly, K. B. , and H.J. Holcombe, Aerodynamics for Body Engineers, Paper
No. 649A, Automotive Engineering Congress (January 1963).
Kirsch, J.W., S.K. Garg, and W. Bettes, Drag Reduction of Bluff Vehicles
With Air vanes, SAE 730686, SAE, 400 Commonwealth Dr. , Warrendale,
Penn. (18-22 June 1973).
Klemperer, W. , "Investigations of the Aerodynamic Drag of Automobiles, "
Zeitschrift fur Flugtechnik,. 13 (1922).
Korff, W. H. , The Body Engineer's Role in Automotive Aerodynamics, Paper
No. 649B, Automotive Engineering Congress (January 1963).
Lay, W. E. , "Is 50 Miles Per Gallon Possible with Correct Streamlining?"
SAE Journal, 32 (4) pp. 144 (April 1933).
MacMillan, R. H. , and G. W. Carr, Importance of Aerodynamics in Primary
Safety, Fed Int des Soc d'Ing des Tech de l'Automob, 15th International
Congress, Section C, Paper 1. 18, Paris, France (13-17 May 1974).
Marcell, R. P. , and G. F. Romberg, The Aerodynamic Development of the
Charger Dayton for Stock Car Competition, SAE 700036, SAE, 400 Com-
monwealth Dr. , Warrendale, Penn. (January 1970).
B-3
-------
Marks, C.H. , F. T. Buckley, Jr., and W. H. Walson, Jr., An Evaluation of
the Aerodynamic Drag Reductions Produced by Various Cab Roof Fair-
ings and a Gap Seal on Tractor-Trailer Trucks, SAE 760105, SAE,
400 Commonwealth Dr., Warrendale, Penn. (23-27 February 1976).
Mason, W.T., Jr., Wind Tunnel Development of the Dragfoiler - A System
for Reducing Tractor-Trailer Aerodynamic Drag, SAE 750705, SAE,
400 Commonwealth Dr., Warrendale, Penn. (11-14 August 1975).
, G. Sovran, Ground-Plane Effects on the Aerodynamic Charac-
teristics of Automobile Models - An Examination of Wind Tunnel Test
Technique, General Motors Research Laboratories, Report No. GMR-
1378 (April 1973).
McCanless, G. F. , Jr. , Hood Scoop Designs for Cars Derived from Aero
Re sear ch, SAE, 400 Commonwealth Dr. , Warrendale, Penn.
(September 1970).
McLean, R. F. , An Aerodynamic Design in Plastics, SAE 317, SAE, 400 Com-
monwealth Dr. , Warrendale, Penn. (June 1954).
Milford Reid, J. P. , "Body Form, " Automobile Engineer, 44 (1)
(January 1 954).
Metz, L.D., "An Improved Technique for Theoretically Determining the Lift
Distribution on an Automobile, " Journal of Engineering for Industry,
95 (1) (February 1973).
Moller, E. , and H. Schlichting, "Windkanalmessungen an Kraftfahrzeugen
bei Seitenwind, " Automobiltechnische Zeitschrift, (4) Jahrang 53
(April 1951).
, H. Schlichting, "Luftwiderstandsmessungen am VW-Lieferwagen, "
Automobiltechnische Zeitschrift (6) Jahrgang 53 (June 1951).
Montoya, L.C., and L. L. Steers, Aerodynamic Drag Reduction Tests on a
Full-Scale Tractor-Trailer Combination with Several Add-On Devices,
NASA TM X-56028, NASA, Washington, D.C. (December 1974).
B-4
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Morelli, A. , Theoretical Method for Determining the Lift Distribution on a
Vehicle, Proceedings of the International Automotive Technical Con-
gress (F.I.S.I.T.A. ) Tokyo, 194, pp. 140.
"The Aerodynamic Forces on the Wheel of an Automobile, "
Giugno (Italian), pp. 281 (1969).
L. Fiovaranti, and A. Cogotti, The Body Shape of Minimum Drag,
SAE 760186, SAE, 400 Commonwealth Dr. , Warrendale, Penn. (23-27
February 1976).
Oda, N. , and T. Hoshino, Three-Dimensional Airflow Visualization by Smoke
Tunnel, SAE 741029, SAE, 400 Commonwealth Dr. , Warrendale, Penn.
(21-25 October 1974).
Ogorkiewicz, R.M., 11 Aerodynamic Drag," Automobile Engineer, 48 (10)
(October 1958).
Ohtani, K. , M. Takei, and H. Sakamoto, Nissan Full-Scale Wind Tunnel -
Its Application to Passenger Car Design, SAE 720100, SAE, 400 Com-
monwealth Dr. , Warrendale, Penn. (10-14 January 1972).
/
Schmid, C. , M Luftwiderstandes von Kraftfahrzeugen im Modellversuch, 11
Zeitschrift des Verein Deutscher Ingenieure, 82 (8), pp. 188
(19 February 1938).
Scibor-Rylski, A. J., "Aerodynamic Characteristics of the Wedge Shaped
Racing Car, " Automotive Design Engineering, 9^ (September 1970).
Steers, L. L. , L.C. Montoya, and E.J. Saltzman, Aerodynamic Drag Reduc-
tion Tests on a Full-Scale Tractor-Trailer Combination and a Repre-
sentative Box-Shaped Ground Vehicle, SAE 7 50703, SAE, 400 Common-
wealth Dr. , Warrendale, Penn. (11-14 August 1975).
Stolley, J.L. and W.K. Burns, "Forces on Bodies in the Presence of the
Ground, " Paper No. 1, Proceedings of First BHRA Symposium on Road
Vehicle Aerodynamics (1969, 1970).
Tremulis, A. S. , Aerodynamic Drag Characteristics of Land Speed Record
Vehicles, SAE 660387, SAE, 400 Commonwealth Dr. , Warrendale,
Penn. (6-10 June 1966).
B-5
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Turner, T. R. , Wind-Tunnel Investigation of a 3/8-Scale Automobile Model
Over a Moving-Belt Ground Plane, NASA TN D-4229, NASA, Washing-
ton, D.C. (November 1967).
Van Winkle, R.A., Jr., Overcoming Pressure Drag in Large Semi-Tractor
Trailer Trucks, AIAA Paper No. 75-216 (20-22 January 1975).
Von Frankenberg, Richard, "Streamlining Remains Unborn, " Automobile
Year, 1_2 (1964/5).
Von Sawatzki, E. , "Einfluss der Luftkrafte auf die Fahrtriehtungshaltung des
Kraftfahrzeugs, " Automobiltechnische Zeitschrift, pp. 515, 19 Heft
(October 1939).
Walker, G. E. Lind, "Car Aerodynamics," Automobile Engineer, 48 (6)
pp. 215-221 (June 1958) (7) pp. 262-270 (July).
White, R.G.S. , A Method of Estimating Automobile Drag Coefficients,
SAE 690189, SAE, 400 Commonwealth Dr. , Warrendale, Penn.
(January 1969).
Motor Industry Research Association (MIRA) Reports and
Bulletins (Lindley, Nuneaton, Warwick, England)
The Aerodynamics of Road Vehicles: A Survey of Published Literature,
Report 1958/1 .
The Aerodynamics of Road Vehicles: A Comparison of Cars and Their Models
in Wind Tunnels, Report 1958/9.
The Aerodynamics pf Road Vehicles — Road Measurements of Drag and Com-
parison with Wind Tunnel Measurements, Report 1958/10.
A Wind Tunnel for Full Scale Vehicles: Design Investigation with a 1 /24th
Scale Model, Report 1960/2.
B-6
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An Experimental Survey of Vehicle Aerodynamics Characteristics (First
Report), Report 1962/1.
Aerodynamic Effects of Modification to a Typical Car Model, Report 1963/4.
Aerodynamics of Basic Shapes for Small Cars: An Investigation of Three-
Eighth Models by Ford Motor Co. Ltd. Engineering Research in the
MIRA Model Wind Tunnel, Report 1963/10.
An Experimental Survey of Vehicle Aerodynamic Characteristics (Second
Report), Report 1964/4.
Correlation of Full-Scale Wind Tunnel and Road Measurements of
Aerodynamic Drag, Report 1964/5.
Aerodynamic Effects of Underbody Details on a Typical Car Body,
•Report 1965/7.
An Experimental Survey of Vehicle Aerodynamic Characteristics (Third
Report), Report 1965/13.
Correlation of Aerodynamic Force Measurements in Quarter and Full-Scale
Wind Tunnels (First Report), Report 1966/4.
An Experimental Survey of Vehicle Aerodynamic Characteristics (Fourth
Report), Report 1966/10.
Wind Tunnel Tests of Vehicle Models Using a Moving Ground Surface,
Report 1966/13.
A Study of Vehicle Aerodynamic Lift Correlation of Aerodynamic Force
Measurements in Quarter and Full-Scale Wind Tunnels (Second Report),
Reports 1966/16 and 1967/1.
A Rating Method for Assessing Vehicle Aerodynamic Drag Coefficients,
Report 1967 /9.
An Experimental Survey of Vehicle Aerodynamic Characteristics (Fifth
Report), Report 1967/11.
B-7
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The Aerodynamics of Basic Shapes for Road Vehicles: Part 1. Simple
Rectangular Bodies, Report 1968/2.
The Aerodynamics of Basic Shapes for Road Vehicles: Part 2. Saloon Car
Bodies, Report 1968/9.
An Experimental Survey of Vehicle Aerodynamic Characteristics (Sixth
Report), Report 1969/4.
Aerodynamic Characteristics of Exposed Rotating Wheels, Report 1970/2.
The Aerodynamics of Basic Shapes for Road Vehicles: Part 3. Streamlined
Bodies, Report 1970/4.
An Experimental Survey of Vehicle Aerodynamic Characteristics (Seventh
Report), Report 1970/11.
An Experimental Survey of Vehicle Aerodynamic Characteristics (Eighth
Report), Report 1971/8.
The Aerodynamic Effects of Fins on Cars, Bulletin No. 4 (1963).
The Development of a Low-Drag Body Shape for a Small Saloon Car,
Bulletin No. 2 (1965).
Airflow Measurements through Vehicle Cooling Systems, Bulletin No, 6
(1966).
B-8
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APPENDIX C
BIBLIOGRAPHY ON WIND TUNNEL AND FULL-SCALE
AERODYNAMIC TESTING OF ROAD VEHICLES
Beauvais, F. N. , S.C. Tignor, and T. R. Turner, Problems of Ground Simu-
lation in Automotive Aerodynamics, SAE 680121, SAE, 400 Common-
wealth Dr. , Warrendale, Penn. (1968).
Bettes, W. H. , "Aerodynamic Testing of High Performance Land-Borne
Vehicles — A Critical Review, " Proceedings of the AIAA Symposium on
The Aerodynamics of Sports & Competition Automobiles, 7 (20 April
1968).
. K. B. Kelly, Influence of Wind Tunnel Solid Boundaries on Auto-
motive Test Data, SAE 741031, SAE, 400 Commonwealth Dr. ,
Warrendale, Penn. (21-25 October 1974).
Buckley B. Shawn, Road Test Aerodynamic Instrumentation, SAE 741030,
SAE, 400 Commonwealth Dr. , Warrendale, Penn. (October 1974).
Carr, G.W., Correlation of Pressure Measurements in Model and Full-
Scale Wind Tunnels and on the Road, SAE 7 50065, SAE, 400 Common-
wealth Dr. , Warrendale, Penn. (24-28 February 1975).
Fosberry, R.A.C., R.G.S. White, and G. W. Carr, A British Automotive
Wind Tunnel Installation and Its Application, SAE 948C, SAE,
400 Commonwealth Dr. , Warrendale, Penn. (January 1965).
Gross, D. S. , and W. S. Sekscienski, Some Problems Concerning Wind
Tunnel Testing of Automotive Vehicles, SAE 660385, SAE, 400 Com-
monwealth Dr. , Warrendale, Penn. (June 1966).
Hamsten, B. , and F.M. Christensen, Correlation Tests in a Climatic Wind-
Tunnel, SAE 750064, SAE, 400 Commonwealth Dr, , Warrendale, Penn.
(24-28 February 1975).
C-l
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Hucho, W. H. , L.J. Janssen, and G. Schwartz, The Wind Tunnel's Ground
Plane Boundary Layer — Its Interference with the Flow Underneath
Cars, SAE 750066, SAE, 400 Commonwealth Dr., Warrendale, Penn.
(24-28 February 1975).
Kessler, Jay C. and Stanley B. Wallis, Aerodynamic Test Techniques, SAE
660464, SAE, 400 Commonwealth Dr. , Warrendale, Penn.
(February 1966).
Klemin, A. , "Moving Ground Developed for Tunnel Testing of Car Models, "
Automotive Industry, 71 (5) (4 August 1934).
Lindsay, Jerry P. , and Harold E. Lanktree, The New Chrysler Wind Tunnel,
SAE 730239, SAE, 400 Commonwealth Dr. , Warrendale, Penn.
(January 1 973).
Mason, W. T. , Jr., P. S. Beebe, and F. K. Schenkel, An Aerodynamic Test
Facility for Scale-Model Automobiles, SAE 730238, SAE, 400 Common-
wealth Dr. , Warrendale, Penn. (1973).
, Gino Sovran, Ground-Plane Effects on the Aerodynamic Charac-
teristics of Automobile Models — An Examination of Wind Tunnel Test
Techniques, Report No. GMR-1378, General Motors Research
Laboratories (13 April 1973).
Metz, L. Daniel, The Influence of Roughness Elements on Laminar to Turbu-
lent Boundary Layer Transition as Applied to Scale Model Testing of
Automobiles, SAE 730233, SAE, 400 Commonwealth Dr., Warrendale,
Penn. (January 1973).
Oda, Norihiko, and Teruo Hoshino, Three-Dimensional Airflow Visualization
by Smoke Tunnel, SAE 741029, SAE, 400 Commonwealth Dr.,
Warrendale, Penn. (October 1974).
Ohtani, Kenuchi, Michio Takei, and Hikota Sakamoto, Nissan Full-Scale
Wind Tunnel — Its Application to Passenger Car Design, SAE 720100,
SAE, 400 Commonwealth Dr. , Warrendale, Penn. (January 1972).
C-2
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Romani, L. , "Aerodynamic Tests on Cars and Car Models, " The Motor
(London), pp. A28 (7 January 1948).
Stalker, E. A. , "A Reflection Plate Representing the Ground, " IAS Journal
of the Aeronautical Sciences, (July 1934).
Walson, W.H., Jr., F.T. Buckley, Jr., andC.H. Marks, Test Procedures
for the Evaluation of Aerodynamic Drag on Full-Scale Vehicles in
Windy Environments, SAE 760106, SAE, 400 Commonwealth Dr. ,
Warrendale, Penn. (23-27 February 1976).
White,. T. F. , New Techniques for Full Scale Testing, SAE Preprint
No. 148E, SAE, 400 Commonwealth Dr. , Warrendale, Penn.
(March I960).
Motor Industry Research Association (MIRA) Reports and
Bulletins (Lindley, Nuneaton, Warwick, England)
A Wind Tunnel for Full Scale Vehicles: Design Investigation with a 1 /24th
Scale Model, Report 1960/2.
The MIRA Full Scale Wind Tunnel, Report 1961/8.
The MIRA Quarter Scale Wind Tunnel, Report 1961/11.
Wind Tunnel Blockage Corrections for Road Vehicles, Report 1971/4.
A Further Study of the Simulation Problem in Wind Tunnel Tests of Road
Vehicles, Report 1971/6,
The Use of Three-Eighths Scale Models in the Small MIRA Wind Tunnel,
Bulletin No. 3 (1962).
C-3
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TECHNICAL REPORT DATA
(Please read instructions on the reverse before completing)
1. REPORT NO. 2.
EPA-460/3-76-025
3. RECIPIENT'S ACCESSIO^NO.
4. TITLE AND SUBTITLE
Estimation of Vehicle Aerodynamic Drag
5. REPORT DATE
Oct. 1976 !
6. PERFORMING ORGANIZATION CQDE
7. AUTMOR(S)
Bernard Pershing
Mamoru Masaki
8. PERFORMING ORGANIZATION REPORT NO.
ATR-77(7359)-l
9. PERFORMING ORGANIZATION NAME AND ADDRESS
The Aerospace Corporation
El Segundo, California 90245
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-01-0417
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Air and Waste Management
Office of Mobile (Source Air Pollution Control
Emission Control Technology Division, Ann Arbor, Mich.
13. TYPE OF REPORT AND PERIOD COVliRED
Final Task Report
14. SPONSORING AGENCY CODE
15. SUPPLEMLNTARY NOT ES
16. ABSTRACT
A simple procedure was developed for the estimation of road vehicle
aerodynamic drag based on easily quantifiable vehicle shape parameters.
The procedure is applicable to passenger vehicles, station wagons, and
vans and is based on a "drag build—up" method which includes the effects
of the basic body shape, underpanning, and cooling drag. Not included
are effects of lift, sidewind, ground clearance, and certain shape details.
The limitations of the procedure are discussed and improvements and areas
requiring further study are identified. In a related activity, a brief
investigation was made of possible techniques for determining vehicle
frontal area from photographs of cars. Planimeter measurements of
frontal area were made from photographic enlargements of approximately
80 cars. The results of this effort are included as an appendix. Bibli-
ographies on road vehicle aerodynamic drag and on wind tunnel and full-
scale road testing techniques are also appended.
17. KEY WORDS AND OOCUMENT ANALYSIS
a. DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATl Field/Group
Aerodynamic Drag
Motor Vehicles
18. DISTRIBUTION STATEMENT
UNLIMITED
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
55
30. SECURITY CLASS (ThUpagt)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
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