TECHNOLOGY – AEROBYTES

Observations on two sports racers

Validating the figures for the Arachnid and Force LM

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

closed coupé circuit racer of CTR Developments and the Force LM hillclimber of Force Racing Cars, we conclude with a look at some interesting details divined in the MIRA full-scale wind tunnel. The first task with any racecar

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we test for Aerobytes is to run first at 60mph (approximately 26m/s) and then at 80mph (approximately 35m/s), which is the maximum speed available. If the flows around, over and under our racecar are similar at both speeds then the calculated coefficients will be the same at both speeds. Sometimes, though, the coefficients are different at the two speeds because attached flows have not fully developed at the lower speed, in which case the higher speed is used for subsequent runs. If the coefficients are the same then the lower speed can be used because it requires less power, and runs can be completed a little faster because it takes less time to accelerate and decelerate the air for each run. We generally use the maximum speed for Aerobytes, but the checks at the two speeds can prove interesting.

ontinuing with our examination of two distinctive sports racers, the Arachnid

The results at the two speeds on the Arachnid and the Force LM during their start of session baseline runs are shown below in tables 1 and 2. The changes to the coefficients are expressed in ‘counts’, where a coefficient change of 0.100 = 100 counts. The Greek letter ∆ (delta) represents the change to each parameter as a result of the configuration adjustments, in this case a speed adjustment, relative to the previous configuration. Clearly, something different was going on with the two cars here. The Arachnid produced lower negative lift (downforce) coefficients front and rear at the higher speed, whereas the Force produced higher negative

lift coefficients at the front and lower at the rear at the higher speed. Both cars produced slightly lower drag. The effect of different speeds on the Force was more akin to most cars we have tested for Aerobytes. And the most likely explanation here is that the flow under the front wing was probably not fully attached at 60mph, but it was at least better – if not necessarily fully – attached at 80mph. This effect can occur simply because of the sensitivity of front wings to ground proximity, and it may also be related to the development of the boundary layer, in spite of the wind tunnel’s boundary layer control fence, ahead of the front wing,

Table 1 – the coefficients on the Arachnid at 60mph and 80mph

CD -CL -CLfront -CLrear % front -L/D

60mph 0.5385 1.1480 0.1355 1.0125 11.80 2.132 80mph 0.5340 1.0835 0.1145 0.9685 10.57 2.029 ∆, counts

Table 2 – the coefficients on the Force LM at 60mph and 80mph

CD -CL -CLfront -CLrear % front -L/D

60mph 0.6850 1.2335 0.6860 0.5475 56.10 1.783 80mph 0.6755 1.2625 0.7250 0.5385 57.43 1.869 ∆, counts -9.5 +29.0 +39.0 -9.0 +1.33 +86.0

NB: coefficients are only reported to three decimal places. These figures are the means of two runs per configuration

-4.5 -64.5 -21.0 -44.0 -1.24 -103.0

The Arachnid’s coefficients changed as speed was increased

The Force LM responded differently as speed was increased January 2012 • www.racecar-engineering.com 55

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