TECHNOLOGY – AEROBYTES
While much of the rear end is tightly regulated, the wheelarch flip ups and large rear body Gurney will be adding downforce
The swan neck-supported rear wing was well cambered and obviously fairly potent
The rear diffuser may be strictly defined, but is still reasonably voluminous
but it also generated over 17 per cent more total downforce, leading to a 13.5 per cent greater efficiency (-L/D) figure. One of the most interesting aspects is that despite running with the narrower (1.6m) span, reduced (250mm) chord rear wing mandated from the beginning of 2009, as well as the control floor and diffuser, the Zytek nevertheless achieved over eight per cent more rear downforce than the Radical. Certainly, the Zytek’s rear wing was a well- cambered, dual-element device, but, as Mike Fuller explained in last month’s issue, it shows that mandating the narrower span wing served predominantly to increase development cost for it clearly did not reduce downforce, at least not for very long. The
Zytek’s wing mounts were of the ‘swan neck’ type, which enabled higher downforce to be generated by more heavily cambered wings. Another key difference
between the cars was the level of front-end downforce, the Zytek generating an impressive 30 per cent more than the Radical, giving a more forward bias to the aerodynamic balance. However, these numbers must be viewed in relation to the cars’ static front-to-rear weight splits, which were roughly 39 per cent front for the Radical and 45 per cent for the Zytek. So the Radical’s aerodynamic balance was actually somewhat closer to its static weight split than the Zytek but, given that the Zytek was in its well-honed
Table 2: the effect of yaw angle on aerodynamic balance, as given by ‘%front’
0O 2O 4O 6O
‘%front’ at yaw angle Radical SR10 39.6 41.3 42.5 44.6
www.racecar-engineering.com • Le Mans
Zytek Z11 SN 41.7 40.6 40.9 41.4
For comparison, the lower downforce Radical SR10, as tested in 2008
‘preferred specification’, whereas the Radical was under-developed, we can assume that the Zytek’s ‘%front’ value in relation to its static weight split represented a balanced condition out on track. This assertion has two codicils – firstly, with the fixed floor and non-rotating wheels this wind tunnel underestimates the downforce of ground-level devices like front splitters and diffusers. Secondly, a car that has slightly less front aerodynamic percentage than static weight percentage is more likely to have a little understeer at high speed, rather than the inherently less stable alternative. So the ‘%front’ values should be looked at with this in mind and the Zytek provides a useful yardstick in this respect. One of the things that
emerged in the Radical session was how the balance shifted as a range of yaw angles was applied. The Zytek was tested at up to six degrees yaw angle, this maximum being used because it was the slip angle at which the tyres generated maximum grip, according to Greaves Motorsport’s race engineer, Alan Mugglestone.
The effect on the balance of the two racecars is shown in table 2. Clearly, the two cars showed
quite different responses to increasing yaw angle. The Radical’s aerodynamic balance became more front biased as yaw increased, which one would think would be a potentially unstable response. The Zytek showed an initial shift away from the front at two degrees yaw, but the balance then moved more to the front with the remaining yaw increments until, at six degrees yaw, the balance was similar to the straight-ahead position. This seems like an altogether more stable response. It must be remembered though that these numbers were recorded as steady-state readings with data averaged over minute long sampling intervals, and the actual dynamic transient response may not be the same. Nevertheless, the Zytek looks to have more benign characteristics when tested in steady state.
Next month we’ll look at the effects of the newly mandated bodywork modifications.
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