TECHNOLOGY – AEROBYTES
Figure 4
At yaw, the smoke plume can be seen to cross over the top of the engine cover fin before encountering the rear wing
was stalled at this flap angle in the straight ahead position, but was not stalled when running at yaw, which is an interesting fact to keep in mind. The other obvious pattern
is that downforce reduced in all cases as the first two degrees of yaw was applied, and then levelled off and even recovered slightly at the higher yaw angles tested. In most cases, this was the result of front-end downforce recovering as yaw increased. Generally speaking, rear downforce decreased in nearly all cases as yaw angle increased. But looking at how the engine
cover fin affected total downforce in each case, there was clearly a reduction in downforce at each yaw angle and flap angle tested, and the smallest effect of the engine cover fin on total downforce was at the minimum flap angle. In each case, however, there was a significant loss of total downforce and, after our part of the session ended, the team set about recovering the lost downforce arising from fitting the engine cover fin. Another way of looking at
the effect of the engine cover fin is to look at how aerodynamic balance altered across the same matrix of configurations, and three graphs, one for each flap angle, illustrate this most clearly. Again, there are a number
of patterns that emerge. Most obviously, the front percentage increased as rear wing flap angle was decreased (any other outcome would have been quite a surprise!). Secondly,
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in general the aerodynamic balance shift with increasing yaw angle is similar in each case, with an initial reduction in front percentage at the first two degrees yaw increment, which is then followed by a recovery in front percentage so that at six degrees the balance is not dissimilar to the balance at zero degrees yaw, even though as we saw above, total downforce at yaw is less than when straight ahead. Again, this ‘balance recovery’ is down to the front end of the car working better at six degrees yaw than at two and four degrees yaw. But now looking at how the balance shift was affected by the presence of the engine cover fin, we can see that at maximum flap angle the balance was slightly more front biased with the fin across all yaw angles tested. At medium flap angle the balance was quite similar with and without the fin, although where there was a difference there was very slightly more front percentage without the fin. At minimum flap angle the balance was again rather more front biased with the fin. In general one might have
expected the balance with the fin to have been more front biased, on the assumption that when the car was at yaw the rear wing would be adversely affected by the fin. It seems likely that this effect is somewhere in the mix, but there are clearly other factors involved here as well, as shown by the plot at medium flap angle, which did not seem
Figure 5
Figure 6
to conform quite to what one would have expected. Indeed, in the medium flap angle case it is hard to explain why there would be even a slight rearward balance shift at either zero or six degrees yaw by fitting an engine cover fin. Perhaps from the team’s viewpoint, one should be content that the impact of the fin on balance in this, the 2011 ‘preferred specification’, was relatively small. Unfortunately, because the
prototype fin the team had produced in such a short time was thought not strong enough
to withstand testing at higher yaw angles, we were unable to investigate whether the fin’s side forces and returning yaw moments would add yaw stability. Suffice to say, yaw moments (and roll moments) were slightly larger with the engine cover fin than without, but even at six degrees they were still miniscule. More testing next month
on some of the other 2012 mandatory modifications.
Racecar Engineering’s thanks to Greaves Motorsport
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