TECHNOLOGY – AEROBYTES
which could exacerbate this. The small gain in rear negative lift coefficient may just be a mechanical leverage effect but, as we saw in an earlier episode, the flow off the Force’s front wing impinged on the rear wing, so this may have been involved as well.
The change to the Arachnid’s numbers front and rear is probably also related to flow attachment, but here it is more likely we are seeing the effects of the flow over the upper surfaces becoming better attached at 80mph than at 60mph, which would create a greater increment of positive lift from the body, manifesting itself as slight reductions in negative lift coefficients.
In both cases, better attached
flows would explain the small reductions in drag coefficient.
ARACHNID ROOF LOUVRES One of the details added to the Arachnid prior to our session was a set of louvres in the cockpit roof (designed to try and improve cockpit cooling), along with others down the spine of the engine cover to aid heat rejection from the engine bay. The results of removing and taping over first the roof louvres and then the engine cover louvres are given in table 3, below. We can see that by removing
the roof louvres alone decreased downforce and drag, suggesting flow attachment was better, but the net effect was increased
body lift. But then removing the engine cover louvres as well caused a small net gain in downforce with very little change in drag. This suggests a combination of increased body lift and reduced body drag, together with an improvement in rear wing downforce and the attendant increase in induced drag. Perhaps then, this wing downforce increment slightly outweighed the body lift gain, but with total drag remaining practically unchanged overall.
FORCE REAR END Last month we saw that increasing the angle of the Force’s lower rear wing (a single element device) had a pretty modest effect. To evaluate whether the device was contributing anything significant at all, it was removed totally, which produced the results shown below in table 4. This certainly confirmed that
The Arachnid’s roof louvres influenced both downforce and drag
the lower rear wing was doing a very useful and very efficient job! Interestingly though, the significant downforce increment it generated was felt entirely at the rear, whereas it might have been expected to aid the underbody and hence benefit front downforce, too. Why this should be so is not entirely clear,
but it may be that the diffuser’s quite modest roof angle means that its exit is too far from the lower rear wing to enable any tangible interaction. Racecar Engineering regulars
will recall that we have carried out simple CFD studies on rear wing end plate depth in the past, which supported the perceived wisdom that deeper is better, up to a point. With minutes of this wind tunnel session left, a pair of cardboard end plate extensions that added approximately 250mm extra depth were hurriedly knocked up and evaluated. The effects are shown in table 5. The gain in rear downforce, which amounted to 7.8 per cent, was obviously very useful, and it came with an efficiency of 63.5 counts of total downforce to 19 counts of drag, or 3.34:1. Furthermore, the extra rear downforce increment allowed a last minute front flap adjustment that brought the balance back into the 34-36 per cent front target zone and yielded the highest downforce and highest efficiency, most balanced set up of the entire session – a very satisfactory conclusion!
Thanks to CTR Developments, Force Racing Cars, Graham Wynn
Table 3 – changes to coefficients (in counts) on the Arachnid when roof and engine cover louvres were removed and taped over
∆CD ∆-CL ∆-CLfront ∆-CLrear ∆% front
Roof louvres
+ E/cover louvres
Removing the Force’s lower rear wing made a big difference
Table 4 – the effects of removing the lower rear wing element
Remove lower r/ wing
∆CD ∆-CL ∆-CLfront ∆-CLrear ∆% front ∆-L/D -34.0 -195.5 +46.0 -241.5 +9.165 -178.5
-16.5 -23.0 -1 +7.5 -8.0 -3.0 ∆-L/D -15.5 +0.26 +24.5 +11.0 -0.71 +22.0
Table 5 – the effects of adding approximately 250mm rear end plate depth
∆CD ∆-CL ∆-CLfront ∆-CLrear ∆% front
Deeper rear end plates proved effective, despite the rapid prototyping… 56
www.racecar-engineering.com • January 2012
+250mm EP
+19.0 +63.5 -13.5 ∆-L/D +77.5 -2.245 +104.0
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