Racecar Engineering - November 2005

Aero tips
But, as Carroll Smith also pointed out, it’s not
just the drag of the fastener themselves that
matters, but the wakes extending rearwards from
them. Just think about the shape of a wake you
can see easily, such as that from a boat moving
through water. Depending on the exact
circumstances, the wake spreads out downstream
and potentially affects the flow to other parts of
the racecar, as well as causing drag and local flow
separation. So to really offend an aerodynamicist,
just attach your Gurney to the underside of your
wing and use hex headed bolts to hold the thing in
place! If you do use nuts and bolts to hold a
Gurney on, at least use the dome-headed type
(wing trailing edges are generally too thin for
countersunk or flush fasteners) with the heads on
the underside, and the more obtrusive nut and
bolt shank on the upper surface where they sit
ahead of the vertical portion of the Gurney and
have minimal influence.
Moving on to protruding edges, borrowing
once more from Carroll Smith and Tune to Win.
Figure 5 shows the drag coefficients of various
sheet metal joints, and again the conclusions are
pretty obvious. Yet the occurrence of forward
facing edge overlaps is all too frequent, especially
so on the flat aluminium sheets used to panel in
the underside of racecars. Panelling in the
underside is aerodynamically a good thing to do
(providing cooling has also been carefully
considered), but leaving forward facing
protruding edges clearly negates some of the
effort. The designs in figure 5 point at the most
aerodynamically efficient ways of joining such
panels, and the small amount of extra effort will
surely be worthwhile.
There’s a tale told of a well-known racecar
C D
= 0.70
C D
= 0.70
C D
= 0.5
Velocity
Boundary layer thickness = .307 inches
.275”
–5°
–1° –7°
–2°
–10° +5°
.190”
.22”
.22”
manufacturer’s managing director who had the
habit of running his thumbnail across the joins in
bodywork after initial assembly to ensure they
were as tight fitting and smooth as possible – not
very scientific perhaps, but a valid inspection
technique nevertheless. And you can see his
reasoning – with all the effort put into CFD and
wind tunnel development programmes, it was
vital that there were no major tolerance problems
on the finished product. But a good fit between
body panels is vital whether or not you’ve
Relative surface drag
1.39
1.20
1.00
1.62
1.17
1.28
6.66
h
V
h = 1 / 2
b
V
h = b
Figure 6: relative drag
caused by different
shaped gaps in panels
Figure 7: thin tape over
gaps in bodywork can
help reduce drag
Below – figure 8: relative
drag caused by scratches
and ridges on bodywork
Relative surface drag
b
b
h
1
80
C D
= 0.11
C D
= –0.04
C D
= 0.13
C D
= 0.24
C D
= 0.01
C D
= 0.24
C D
= 0.16
C D
= 0.07
Figure 5: drag coefficients of all the major joint
types between sheet metal bodywork sections
“
CERTAIN GAP SHAPES
CREATE APPALLING
DRAG
”
developed your car on a computer or in a wind
tunnel. Figure 6 once again appears in Milliken
and Milliken, and originates in that 1963 paper.
Although this time the drag numbers are relative
to the third example from the top, we can see
from the second example from the bottom of
figure 5 that if a simple, shallow gap creates
significant drag, then it is probably fair to assume
that wider and deeper gaps will be worse. And
figure 6 tells us that certain gap shapes create
appalling drag.
An easy and frequently used way of improving
V
h = b
130
body fit at the track is to tape over the joins,
preferably with very thin tape. This at least will be
better than leaving large gaps. Similarly, where
body cut outs have been made, to clear
suspension legs for example, these can be taped
over to bridge the gap (see photo figure 7). Body
fasteners may beneficially be taped over, too.
Scratches and ridges have also been examined
to see their effect on skin friction drag, and figure
8 illustrates, this also coming from that 1963 paper
via Milliken and Milliken. Although actual
dimensions are missing in this figure, we can at
b
h
➔
56 November 2005 Racecar Engineering
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