forwardsat55%frontsofrontenddevices
werethenexchangedorremoveduntilthe
aerodynamicbalancewasreinstated.This
processproducedsomeusefuldataonthe
effectofthediveplanes.Removalofthelower
diveplanelefttheupperoneinplace;removal
oftheupperdiveplaneleftnodiveplanes
inplace.TheresponsesareinTable 4 asthe
effectsofsuccessivelyfittingthediveplanes.
WecanseefromTable 4 thatfittingthe
upperdiveplanesontheirownproducedvery
efficientgains,with 41 countsofextrafront
downforceforjustsevencountsofextra
drag,yieldinga tangiblegaininefficiency
(-L/D)anda veryuseful3.3percentforwards
shiftinaerodynamicbalance.Addingthe
lowerdiveplanestotheupperdiveplanes
howeverwasinefficient, 12 countsmorefront
downforcebeingaccompaniedby 14 counts
moredrag,witha dropinoverallefficiencyand
justa 1.0percentshiftin%front.
Removingthediveplanesandleavingjust
thesplitterendfencesinplace,alongwiththe
newlowrearwingangleproducedthelow
dragbalancedsetuphighlightedinTable 1.
NextmonthAerobyteswillfeaturea brand
newwindtunnelproject.
Racecar’sthankstoPiersReid.exactly the same downforce increment as the
brake duct covers. Back on the front panel
again, covering the headlight gaps shaved
off a little drag and added a small amount of
front downforce while, curiously, reducing rear
downforce a little too. And most effective of
all, creating some prototype ducting to guide
air entering the upper grille more effectively
into the engine air airbox brought a small drag
reduction and another very useful increment
of front downforce. Overall the car gained over
10 per cent in aerodynamic efficiency, as given
by the –L/D figure, and with a 22.4 per cent
increase in front downforce the aerodynamic
balance was shunted sufficiently forwards that
the car was much closer to our target balance
range of 47-50%front. Might be time to buy
those shares in race tape manufacturers, then.
Rake in the gains
As the BMW did not feature a flat floor panel
or a rear diffuser at the time of our session,
expectations were not especially high that
increasing overall rake angle by raising the rear
ride height would yield much benefit. However,
Table 3 tells a different story, as rear ride height
was raised first by 10mm and then by a further
9mm using packers under the tyres.
Total downforce and drag both increased
more or less linearly with rear ride height over
this range, and reasonably efficiently too. That
both increased can probably be ascribed to the
increase in mass flow, and hence velocity, under
the car as the rear was raised, leading to more
head-on encounters between air and hardware
(= drag) yet reduced pressure underneath
the relatively flat underside (= downforce) of
this typical modern production car. However,
the story was not quite that simple, with front
downforce gains tailing off quite significantly
at the second adjustment. Splitter height had
dropped as rear ride height was increased, and
was now in the region where the wind tunnel
stationary floor’s boundary layer may have
started to interfere with flow under the front.
At the rear the initial decrease in rear
downforce reversed sufficiently to give a net
overall gain. This may in part have come from
the effective 0.4-degree increase in rear wing
angle, which according to our wing mapping
in last month’s issue could account for 11 or 12
counts of rear downforce gain, so there must
have been other gains and losses occurring as
well. Nevertheless, increasing rake by 10mm
brought the aerodynamic balance right into
our target range at 48.5%front, and with the
19mm rear ride height increase the balance
was identical and the downforce was the best
of the session in a balanced set-up.
Drag shaving
The last few steps in our session focussed on
drag reductions by initially dropping wing
angle to about two degrees shallower than
previously run. This put the balance too far
TECHNOLOGY – AEROBYTES
62 http://www.racecar-engineering.com SEPTEMBER 2019
CONTACT
SimonMcBeathoffersaerodynamic
advisoryservicesunderhisownbrandof
SMAerotechniques–
http://www.sm-aerotechniques.co.uk.
InthesepagesheusesdatafromMIRA
todiscusscommonaerodynamicissues
facedbyracecarengineersTel:+44(0)24-7635 5000
Email: [email protected]
Website: http://www.horiba-mira.comProduced in association with MIRA LtdTotal downforce and drag both increased
more or less linearly with rear ride height
over this range, and reasonably efficiently too
Table 3: The effects of raising rear ride height
∆CD ∆-CL ∆-CLfront ∆-CLrear ∆%front* ∆-L/D
+10mm RRH +3 +24 +30 -4 +2.6% +41
+19mm RRH +4 +21 +10 +11 n/c +32
Total as % +1.5% +6.4% +12.5% +1.9% +2.6% +5.0%
* Changesin%frontareabsolute,notrelative.Table 4: The effects of dive planes
∆CD ∆-CL ∆-CLfront ∆-CLrear ∆%front* ∆-L/D
Fitting upper DP +7 +40 +41 -1 +3.3% +67
Adding lower DP +14 +10 +12 -1 +1.0% -21Double dive planes were originally fitted to the BMWHere the lower dive plane had been taken off the carNo dive planes; as featured in the lower drag set-up