This page contains a Flash digital edition of a book.
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


C


Tel: +44 (0) 2476 355000 Email: enquiries@mira.co.uk Website: www.mira.co.uk


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


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100