Friday, August 03, 2012

A query about the BT42/44

Gordon Murray's place in the F1 design pantheon is assured, but a question arises over the aerodynamics of the iconic BT42 and BT44.

Speaking to David Tremayne some years ago, Murray explained his rationale as follows:

"I knew a lot about aerodynamics from practical experience. With any moving form you have a stagnation point where air meets it and decides how much is going to flow over, below or around it...I decided, instead of presenting some sort of parabolic-shaped bluff body to the air, I wouldn't give the air a chance." He sketches a triangular shape. "That way the stagnation point was there," he says, pointing to the leading edge of the triangle's base, which is very low to the ground. "So all the air had to go over the top and you had the minimum coming under the car," (F1 Magazine, May 2001, p140-141).

Murray spoke about this issue more recently on the BBC4 documentary, 'How to go faster and influence people':

"The BT42 was like an upturned very little air went underneath the car and most of it went over the top, because all the air that goes under the car produces lift, which counteracts the downforce you're getting from the wings."

This leaves me slightly confused, for a couple of reasons: (i) my understanding is that air going over the top of the car will be accelerated by the curvature, and will therefore produce lift; and (ii) the best way to generate downforce is to turn the region between the ground plane and the floor of the car into a mobile nozzle.

The greater the mass-flow beneath the car, the better; hence the presence of a diffuser, whose 'pumping effect' is maximised by increasing the ratio between the outlet area and the area of minimum cross-section at the leading edge of the floor. The raised nose on a contemporary F1 car also presumably contributes to increasing mass-flow under the car, although it's also designed to minimise the turbulent intensity of the air feeding the underbody.

But Gordon Murray is clearly no mug, so why does he think that it's important to minimise the air going under the car? My best guess is that Murray's idea was specific to cars from the 1970s, which lacked diffusers and raised noses. If there's no diffuser pumping air under the car, then perhaps excess underbody flow can be detrimental.


kenneth said...

Murray may also have been trying to dumb down his ideas for his audience and presenting a gross simplification. Trying to explain in 2 sentences what really requires 20 minutes. I didn't see this tv doco, he may imagine some young kids randomly watching some show about fast cars and inspire them to go study science and engineering. Reading his explanations reminds me of when I was back at school, getting my first basic understanding of why race cars have diffusers, and thinking "Wow, this race car stuff is even more fascinating than all these road car magazines I've been browsing!"

By the way, great blog mate. Love ya work.

Gordon McCabe said...

Cheers Ken.

Murray obviously understood and understands ground effect (see fan-car and v-shaped underbody skirts), so you may well be right.

Eddie said...

Great post, my understanding, as yours, is that the high mass flow/hi nose strategy works to mazimize flow both under and over the top of the diffuser. The BT 44s dont seem to have much/any rear diffuser, so I imagine that the low nose may play more into front downforce. If you can sweep all the air off the top of the track you can more effectively generate vacuum/low pressue under the wing. Hence the advantage in the low riding flexy Red Bull wings of 2010-11. The main differnce to me is that a modern car has a high nose to feed the diffuser and a low front wing line. Murray's comments make much more sense in the context of a car with the nose and front wing line in the same place.