TECHNOLOGY – AEROBYTES
tunnel has a fixed floor, albeit with a boundary layer control fence in place, and that the racecar’s wheels are stationary during testing, both of which affect the absolute downforce numbers obtained to an extent. But in each case the target for the session was to achieve an aerodynamic balance that was close to the static front: rear weight distribution with driver and half fuel aboard, together with the best downforce and
“the all- important
target ‘%front’ value was achieved”
efficiency attainable in the time available. As usual, and despite all the preparations that had been made in the expectation of what results might arise, the first runs on each machine served to drive the rest of the session. Let’s take another look at the numbers from the first run on each car. In both cases data were collected at 80mph (35m/s). And in each case the coefficients were based on an estimate of the cars’ frontal areas, which interestingly were very similar, despite the different shapes. As such, coefficients are directly comparable between the cars.
MORE EFFICIENT So the starting numbers probably reflected initial expectations in that the Arachnid created 21 per cent less drag than the Force, but it also generated 14 per cent less downforce. In simple terms that made the Arachnid 8.7 per cent more efficient, as indicated by the lift over drag (-L/D) figures. However, perhaps the most
striking thing about these first numbers was the difference in the front and rear percentages of total downforce. The Arachnid only generated 10.6 per cent of its downforce at the front and, unsurprisingly, this coincided with driver reports of understeer at ‘aero’ speeds. Indeed, the constructor was expecting the
The Force LM hillclimber is based on a single seater design with more aggressive aerodynamics…
Table 2 – finishing coefficients on the Arachnid, and the changes relative to the first run CD
-CL
Arachnid best Change, %
0.449 -15.9% 0.915 -15.6%
-CLfront 0.409
-CLrear 0.505
+255.7% -47.9%
% front 44.8%
-L/D 2.038 +322.6% +0.4% NB: the seemingly very large percentage changes to the front-end parameters are because of the relatively low initial values
Table 3 – finishing coefficients on the Force LM, and the changes relative to the first run CD
-CL
Force LM best Change, %
0.780 +15.4% 1.608 +27.3%
-CLfront 0.578
-20.3%
-CLrear 1.030
+91.1%
% front 35.9%
+37.5% -L/D 2.061 +10.3%
forward underbody may have been ‘over-filling’. That modification was
therefore carried out ahead of the wind tunnel session but, with limited time available in the tunnel, it wasn’t possible to test the ‘before’ as well as the ‘after’. However, as can be seen from the results, in this recently modified but un-tested form, the Force actually developed a somewhat excessive front downforce percentage, especially when taking into account that the car had a roughly 36 per cent front static weight split. Clearly then, assuming the earlier driver feedback was accurate, the balance had been shifted markedly forwards with the underbody modification, and so one of the main aims during the session with the Force was to shift the aerodynamic balance rearwards again. In recent issues we have
…that produced significantly more downforce, but also more drag
aerodynamic balance would show up as rear biased, and this served to validate the feedback from the track. So one of the primary session targets for the Arachnid was to shift aerodynamic balance forwards to be more akin to the static weight distribution of around 45 per cent front. The Force was something of
an unknown coming into the wind tunnel because modifications had already been carried out in an effort to prevent the front end going light at speed, as reported by the car’s owner, Graham Wynn, and guest driver Will Hall, a top ten contender in the British
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www.racecar-engineering.com • February 2012
Hillclimb Championship in a 1.6 Force single seater. An inspection of the car, which originally had a flat extension of the underbody that projected well forwards under the raised chassis, led to the notion that this projection, along with the relatively quite modest diffuser, might have been the cause of front-biased lift sufficient to overcome the downforce generated by the front wing. And the suggestion was made that shortening that forward underbody might overcome the problem. This might seem counter-intuitive, but the thinking was that the
delved into some of the details of how the balance of each car was significantly improved with subsequent configuration changes throughout the session, but we’ll end this project by summarising the numbers attained in the best configurations found on each car, as shown in table 2 and 3, relative to the starting numbers. In each case the all-important
target ‘%front’ value was achieved, ensuring better balance at aero speeds. This kind of precision refinement would be very difficult to achieve without the use of a wind tunnel.
Next month we start a new mini-series on a current specification LMP2 racecar.
Racecar’s thanks to CTR Developments, Force Racing Cars and Graham Wynn
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