AEROBYTES
Maximising downforce
FIRST PRINCIPLES Miscellaneous modifications to the ADR3 Sports Racer
Simon McBeath offers aerodynamic advisory services under his own brand of SM Aerotechniques –
www.sm-aerotechniques.
co.uk. In these pages he uses data from MIRA to discuss common aerodynamic issues faced by racecar engineers
Produced in association with MIRA Ltd
Tel: +44 (0) 2476 355000 Email:
enquiries@mira.co.uk Website:
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busy session in MIRA’s full-scale wind tunnel was to produce the best downforce possible, together with a front-to-rear downforce balance to match the car’s static weight distribution with driver and half fuel aboard. The car started the session with very little front downforce, and in the previous two issues we saw how splitter and diffuser modifications at the front, opening up the sidepods and re-locating the rear wing enabled the desired balance to be achieved, while simultaneously
T Blank off
Rear three louvres
+Front three louvres
All louvres
his month we continue investigations on the ADR3 twin-seat Sports Racer. The aim of a
increasing total downforce and efficiency and reducing drag. Lots of other modifications were also tried, some of which were beneficial, some of which were not, but all produced valuable knowledge for the team.
BALANCE TUNERS The purpose of louvres in the top of wheelarches is to reduce lift. This is achieved by relieving pressure build up within the wheelarch, and probably also in the case of ‘positive’ louvres that protrude above the body surface, by killing some of the naturally occurring upper surface lift arising from the airflow accelerating over the wheelarch’s
Table 1: the effects of progressively blanking off the front wheelarch louvres, in counts and per cent
Δ CD Δ -CL Δ -CLfront Δ -CLrear Δ %front Δ -L/D +4
+13
(+0.7%) -4
(-0.8%) -5
(-1.0%)
(+1.3%) +5
(+0.5%) -14
(-1.4%) -6
(-1.6%) -8
(-1.7%) -66
(-14.4%) +20
(+3.1%) +13
(+2.6%) +52
(+10.2%) * abs = absolute change in per cent front (relative percentage change in brackets)
-1.03abs* (+3.1%)
-1.08abs* (-2.2%)
-6.23abs* (-13.1%)
+10
(+0.6%) +25
(+1.3%) -9
(-0.5%)
convex shape, which of course is accompanied by a reduction in local static pressure. So blanking off wheelarch louvres at the front would be expected to reduce front downforce, right? This exercise provided a clearer overall picture. The front louvres on the ADR
extended from well forwards to well aft of the top of the arches, so they were blanked off in three successive increments. First, the rearmost three louvres were taped over on each side. Then the front three louvres only were taped over. Finally, the whole louvred panel was replaced with a smooth blanking panel on each side. The results are shown in table 1, as changes relative to the previous configuration. The changes are shown in counts, where a coefficient change of 0.100 = 100 counts, and in percentages, and the Greek letter delta (∆) is used to indicate ‘change to’.
So the initial expectation
that these louvres, when opened up, would create more effective front downforce is borne out here by the decrease in front downforce when they were sealed off. And the effect on front downforce when all of the louvres were sealed was really quite significant. This, remember, is with non-rotating wheels too, and we might expect an even greater, speed-related effect with the wheels spinning within the arches, pumping air around. It’s interesting too to note
Blanking off all of the louvres on both front wheelarches had a significant effect on the downforce generated
that minor but similar balance changes were brought about by taping over either the front or rear three louvres on each side, but the manner in which these changes occurred was actually rather different. In all cases though, front downforce decreased and rear downforce
September 2011 •
www.racecar-engineering.com 21
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