Racecar Engineering - November 2005

Aero tips
Figure 15: ineffective wing mounting plate design
can disturb the airflow over a significant area of
the wing’s suction surface (Advantage CFD)
Above – figure 14: it might look like the
wake from the mirror on this Dallara
F305 could reach the airbox inlet,
however, it was tested and does not
Right – figure 13: the disturbance
caused by a mirror can travel well
downstream though, as CFD streamlines
of a Ferrari 550 show (Advantage CFD)
and at first glance it might appear that the
mirror’s wake could affect the engine inlet.
However, F3 support engineer Jos Claes reports
with typical thoroughness that wind tunnel tests
revealed the mirror’s wake to extend 500mm. The
distance to the airbox inlet is 700–800mm,
depending on engine, and the airflow at the inlet
is actually said to be ‘back to what it would be
without a mirror.’
A detail that has frustrated F3 designers and
race engineers alike in 2005 is that it is no longer
permitted to shroud the wheel tethers, so that the
aerofoil-section wishbones now have the cables,
clearly seen in figure 7, taped in place on their
leading edges. The safety angle – shrouds may
have had the potential for severing the tethers in
an accident – is evident though. It is perhaps
worth noting here that the wider section tube
Figure 17: oil streak
marks on this real wing
show the effects of the
mounting plates
used on the wishbones of the Dallara F305 was
apparently adopted for increased rigidity rather
than any benefit to the airflow.
And finally, another topic discussed by Carroll
Smith in Tune to Win was wing mounts, and their
potential for flow disturbance on the wing’s
crucial suction surface. Benefiting from the clarity
that CFD visualisation now offers, we can see
what he was getting at. Figure 15 shows the effect
“
THE LOCATION OF
ONE ITEM RELATIVE
TO ANOTHER CAN
BE THE CAUSE OF
PROBLEMS
”
Figure 16: a modified mounting design created
less disturbance to the airflow (Advantage CFD)
of the supports on the original wing assembly
used on the Prodrive Ferrari F550. Even these
relatively sleek-looking plates were causing
marked regions of separation, and their effect
became worse when yaw angle was introduced.
Figure 16 shows the modified mounting system on
the re-profiled wing developed by Advantage CFD.
The reduction in the separation caused by the
slimmer mounts is clear, and these mounts also
created less disturbance when the car was in yaw.
The separation zone at the rear of the new wing
was eliminated with a 6mm Gurney. Modifications
to the profile and the mounts produced a 2.5 per
cent reduction in drag for a similar level of
downforce – significant on this type of racecar.
For real life confirmation that the effect of
apparently ‘aerodynamically clean’ wing mounts
can be significant, look at figure 17, where oil was
used to visualise flow on the rear wing. The effects
of the mounting plates are clear to see.
And so it can be seen that small things,
cumulatively and even individually, can and do
make a genuine difference.
RE
References
C Smith – Tune to Win – Aero Publishers Inc, 1978
WF Milliken and DL Milliken – Racecar Vehicle Dynamics –
SAE, 1995
K Kelly and H Holcombe – Aerodynamics for Body
Engineers – SAE paper no. 640050, 1963
J Katz – Race Car Aerodynamics – Robert Bentley, 1995
S McBeath – Competition Car Downforce – Haynes, 1998
S McBeath – Competition Car Aerodynamics – Haynes,
publication due 2006
60 November 2005 Racecar Engineering
www.racecar-engineering.com