TECHNOLOGY – AEROBYTES
Table 1: the effects of opening up the rear wheelarch apertures alone, expressed relative to the previous configuration
Yaw, degrees CD 0 2 4 6
-CL
+20 +22 +22 +17
+34 +42 +46 +34
-CLfront +13 +17 +19 +19
-CLrear +21 +25 +27 +16
Table 2: the effects of opening up the front wheelarch apertures, expressed relative to the configuration in table 1 above
The smoke plume showed little smoke actually emerging from the front wheelarch aperture
Yaw, degrees CD 0 2 4 6
-4 -6 -2
+1 The first iteration of rear wheelarch aperture tested
Smoke can be seen emerging through the slotted rear panel, as well as spilling out of the rear wheelarch aperture
was more surprising, as figure 2 demonstrates. Here it can be seen that the pattern of total downforce reduction with increasing yaw was very similar with the wheelarch apertures opened up, compared to the baseline case with the engine cover fin, although the worst effect of yaw at four degrees was mitigated somewhat with the open apertures. The general impression here is that total downforce actually changed remarkably little by opening up the wheelarch apertures, but that paints a benign picture that belies reality. A clearer picture is revealed
by looking at the effect of the wheelarch apertures on aerodynamic balance, as given by the ‘%front’ figure, indicating the proportion of total downforce felt on the front axle. What actually happened is that front downforce decreased slightly, while rear downforce increased slightly, resulting in a more rearward aerodynamic balance to the downforce (less ‘% front’) across the yaw range tested, although the pattern of balance shift with yaw remains similar. We might speculate on the mechanisms involved here. Unlike louvres, which prevent air from directly entering from the front,
52
www.racecar-engineering.com • May 2012
the apertures evaluated here probably do allow air to enter from the front. So it would also seem probable that the front wheelarch apertures might have been allowing some air (or more air than previously) to enter the arches and generate an increment of front lift that reduced front downforce. But with open rear panels there is egress available from the rear arches so, if any air were to want to enter the rear arches, then it would find an easy escape route. In practice here, rear downforce increased slightly, which one might suppose could simply have been a mechanical leverage response to the front lift reduction, or it could have been that the rear wheelarch apertures actually allowed more air to escape from the rear arches than did the original louvres.
INDIVIDUAL EVALUATIONS Fortunately, the front and rear wheelarch apertures were also evaluated separately, so we are able to divine a little more information. The individual aperture tests were carried out at a different ride height combination though, so we will report their effects here as ‘∆’ or ‘delta values’. That is, the differences relative to the previous configuration. The rear wheelarch apertures were opened up first and the fronts second. The ∆ values are given in counts, where 10 counts equal a coefficient value of 0.010. So things were not as simple as the previous conjecture
-CL -14 -18 -5 -4
-CLfront -22 -24 -26 -17
-CLrear +9 +7
+11 +12
supposed. In fact, opening up the rear wheelarches alone was responsible for the additional drag we saw above. And perhaps surprisingly, additional downforce was created at the front as well as the rear, with the balancing actually shifting slightly more to the front at maximum yaw as the front downforce deltas increased with yaw and the rear downforce delta for the final yaw adjustment decreased. Going on to open up the front
wheelarch apertures produced the delta values in table 2. In this case, drag reduced slightly at low yaw but changed very little at higher yaw. Total downforce also reduced somewhat at low yaw but reduced less at higher yaw, while front downforce decreased across the yaw range, with maximum effect at four degrees. Rear downforce increased slightly across the range. So the effect of opening up
the front arch apertures fitted better with the mechanisms conjectured here than did the effect of opening the rear arch apertures, which seemed to have a more ‘global’ influence. Remember, though, that these tests were performed in a fixed floor, non-rotating wheel tunnel. As stated, this was only a first
iteration exercise to determine the extent of the effect of opening up the apertures. Subsequent development will undoubtedly mitigate some of the drag, downforce and balance changes.
Racecar Engineering’s thanks to Greaves Motorsport.
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