One of the most crucial aspects of any high-speed vehicle is its aerodynamic behaviour, and we appreciated this right from the start. We were fortunate to enrol the talents of Dr Carl Gilkeson at the University of Leeds, as the STAY GOLD programme became part of students’ potential studies.
You need a vehicle that behaves like an arrow in flight: you need the weight up front and the resistance at the back. To put it more technically, the best results in a straightline vehicle come when the centre of gravity (cg) is a long way ahead of the aerodynamic centre of pressure (cp) – so that the point through which all the weight acts is as far as possible ahead of the point through which all the aerodynamic forces act.
When I first showed Richard Noble a photo of STAY GOLD, his immediate reaction was “Beware an aft cg.”
Of course he’s right. Potentially that’s a recipe for instability on a straightline car. But where Thrust2 followed the aforementioned rule to the letter, thanks to the static weight of its Rolls-Royce Avon 301 engine and distinctive twin rear fins, I was interested to learn from chief designer Mark Chapman that BloodhoundSSC’s cg is just behind Andy Green’s cockpit. That’s about where GOLD’s is, but our problem initially was that the cp was ahead of it…
Computational fluid dynamics (CFD) studies by students at UoL, overseen by Dr Carl, have come up with some interesting findings.
The drag coefficient (cd) is almost bang on 0.5, as would be expected. GOLD doesn’t have the smoothest profile I’ve ever seen and is quite draggy with lots of bits sticking out here and there, but she is what she is and that’s all the budget permits her to be right now.
The good news is that while there is lift, as you would expect, both front and rear at very high speed, the current front wing produces healthy downforce.
And a tailfin will push the cp rearward by a significant chunk, thus improving stability as required.
Of course, we’ll monitor things carefully as our speeds rise, but the initial studies and our first tests at Kemble indicate that STAY GOLD is a stable car with benign dynamic behaviour which will be improved further when the tailfin is attached.
That means we should be able to add speed to a stable platform with a reasonable degree of confidence as we moved closer to the 300 mph mark.