Any aerodynamicists on here?
Discussion
Not quite the right forum I know but there isn't an "Aerodynamics" one is there 
.
I'm doing a bit of work on race car aerodynamics which has included reviewing an review paper which contains:
Why?
This is in the section refering to downforce, so we're talking negative lift coefficients. Therefore C of P ahead of the C of G wouldn't make the front lift thereby inducing (catastrophic) flight. I can see the effect on lateral stability during cornering. You wouldn't want the back stepping out too much would you, so more downforce there makes sense. However, that's not a reason why you absolutely MUST have the CofP behind the CofG.
Is there something I'm not seeing in relation to high speed stability? Something like a hammering along a straight and then all of a sudden the back overtakes the front?
Your thoughts please.
Charlie
.I'm doing a bit of work on race car aerodynamics which has included reviewing an review paper which contains:
Paper said:
...the aerodynamic centre of pressure must be behind the centre of gravity.
In the section refering to the effect of downforce on stability.Why?
This is in the section refering to downforce, so we're talking negative lift coefficients. Therefore C of P ahead of the C of G wouldn't make the front lift thereby inducing (catastrophic) flight. I can see the effect on lateral stability during cornering. You wouldn't want the back stepping out too much would you, so more downforce there makes sense. However, that's not a reason why you absolutely MUST have the CofP behind the CofG.
Is there something I'm not seeing in relation to high speed stability? Something like a hammering along a straight and then all of a sudden the back overtakes the front?
Your thoughts please.
Charlie
Oh, and I have just found out that it is possible, if somewhat time-consuming to post on PH without a functioning mouse.
Both. Lateral centre of pressure is more relevant to stability.
Think darts... big mass toward the nose puts the CG well forward, fins at the back move the CP toward the rear.
Try throwing a dart backwards...
That's the reason that LSR cars tend to have bloody great tail fins and why mid-engined road cars can tend to be a bit squirrely in crosswinds.
There is an argument for having the vertical centre pressure slightly behind the CoG too, though, since it is then self-compensating, to some degree ; the downforce will thus compress the rear springs slightly more.. which causes a slight nose-up rake... which tends to kill the downforce a little and bring things back toward natural equilibrium.
If it is the other way round (centre of downforce ahead of CoG), you get into a vicious circle where more downforce gives more nose-down rake, which gives more downforce... next thing you know you have no weight on the rear wheels and the car spins out of the corner.
As with most things, though, you can go too far. Some hillclimb single seaters have big rear wings cantilevered waaay out behind the rear axle line, to maximise downforce (hence grip) on the rear tyres, allowing them to squirt out of tight, slow bends effecively... if you ran the same set-up on a circuit car, which higher cornering speeds, the front tyres would be virtually off the ground and the thing would understeer horribly.
Think darts... big mass toward the nose puts the CG well forward, fins at the back move the CP toward the rear.
Try throwing a dart backwards...
That's the reason that LSR cars tend to have bloody great tail fins and why mid-engined road cars can tend to be a bit squirrely in crosswinds.
There is an argument for having the vertical centre pressure slightly behind the CoG too, though, since it is then self-compensating, to some degree ; the downforce will thus compress the rear springs slightly more.. which causes a slight nose-up rake... which tends to kill the downforce a little and bring things back toward natural equilibrium.
If it is the other way round (centre of downforce ahead of CoG), you get into a vicious circle where more downforce gives more nose-down rake, which gives more downforce... next thing you know you have no weight on the rear wheels and the car spins out of the corner.
As with most things, though, you can go too far. Some hillclimb single seaters have big rear wings cantilevered waaay out behind the rear axle line, to maximise downforce (hence grip) on the rear tyres, allowing them to squirt out of tight, slow bends effecively... if you ran the same set-up on a circuit car, which higher cornering speeds, the front tyres would be virtually off the ground and the thing would understeer horribly.
Sam_68 said:
Useful stuff.
Good post - just what i was looking for. Ta.I had come to terms with the lateral COP - makes sense intuitively, and the paper was very specific about it earlier on.
Vertical COP I wasn't so sure about and your explanation sounds more than plausible; the paper refers to "instability" and your explanation describes an unstable system (well, until the front wing hits the flat grey "boundary condition" of the tarmac anyway).
I'm happy, and seeing as that bit isn't the main thrust of the presentation I have to give tomorrow (more to do with CFD applications in race car aerodynamics) I should be ok.
Good analogy with the dart by the way - I may use that.
David Vizard mentions this in one of his books, he mentions that a rear-engined car travelling at high speed with a cross-wind could swap ends relatively easily.
Porsche have spent a lot of time and energy making this design work, just be careful if you've stuck a quick engine in a bog standard beetle
Porsche have spent a lot of time and energy making this design work, just be careful if you've stuck a quick engine in a bog standard beetle
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff