Diffusers non flat underneath.

[anonymous-user]

Symmetrical planiform wings can of course produce "lift" it just that they require a positive Angle of attack (AOA) to do it.

One of the big reasons for many OEM road cars not using tech like flat floors and diffusers is what happens when you get some yaw on the car! As Audi found out, when a typical road cars starts to yaw, it generates significantly more lift than when it is traveling straight. (and this is highlighted by things like diffusers only working within quite narrow angles of attack) You really don't want a road car that increases it's lift by 25% with 10deg of yaw.............

Very, very few road cars actually produce downforce, most use aero devices to reduce lift, but gennerally the net force they experience is still upwards at high speed. Most road car development is now heading towards reducing aero drag, rather than reducing lift for obvious reasons ;-(

[Sam_68]

Max_Torque said:

Symmetrical section wings can of course produce "lift" it just that they require a positive Angle of attack (AOA) to do it.

EFA.

Of course, symmetrical planform wings can generate lift too - it's actually quite unusual to find an aircraft with one wing a different shape to the other, in fact - but my spider-sense tells me that's not what you meant?

Max_Torque said:

One of the big reasons for many OEM road cars not using tech like flat floors and diffusers is what happens when you get some yaw on the car!

Not just yaw, either. Any bumps that cause variation in ground clearance can also cause dramatic variation in downforce so, for example, hitting a ridge or manhole cover whilst negotiating a corner at speed could have rather nasty consequences. Even worse, potentially, is anything that causes variation in pitch (braking or acceleration), as you're effectively changing the angle of attack. One of the reasons that modern race cars have suspension that would be intolerably stiff by road car standards is to maintain a stable aerodynamic platform.

But the plain, simple bottom line is that 'ground effect' just doesn't work to any worthwhile degree at normal public road speeds, with the amount of ground clearance you need under a practical road car.

With 1.5" clearance under a race car at 120mph, yes.

With 20 feet of clearance under an Ekranoplan (huge) at 400mph, yes.

With 7" of clearance under a Renault Clio at 40mph? Sorry, you're kidding yourself.

[egomeister]

Max_Torque said:

You really don't want a road car that increases it's lift by 25% with 10deg of yaw.............

if you hit 10 degrees yaw in a road car, the change in lift is the least of your worries!

[Sam_68]

egomeister said:

if you hit 10 degrees yaw in a road car, the change in lift is the least of your worries!

This is very true; particularly if you're travelling at the sort of speeds where you're getting any significant aerodynamic lift in the first place.

...but crosswinds can result in the same net aerodynamic effect as yaw, of course.

[egomeister]

Sam_68 said:

This is very true; particularly if you're travelling at the sort of speeds where you're getting any significant aerodynamic lift in the first place.

...but crosswinds can result in the same net aerodynamic effect as yaw, of course.

Yes indeed! I drove a Rover 200 for a while which was horribly floaty at motorway speeds especially if there was any wind, whereas my Clio 182 is remarkably stable at maximum leptons. The difference between the two is quite surprising. Anyway, enough drifting off topic (in a crosswind...)

[Sam_68]

egomeister said:

Anyway, enough drifting off topic (in a crosswind...)

There are two factors at play in crosswinds, mind you.

One is the aforementioned effect on lift (which inluences grip, but is pretty insignificant in terms of straight line stability as such). The other is simply the location of the centre of pressure in relation to the centre of gravity. A CoP located well behind the CoG is stable (like a dart; weight up front, fins behind). CoP ahead of the CoG is unstable and will be all over the place in crosswinds.

[cptsideways]

I have had a fascination with aerodynaics ever since I became a windsurfer, sounds like an odd one I know but you soon get an appreciation of the laws of physics when your directly connected to 5m square sail doing 30+ mph.

To give you some idea, an 8m aquare sail in a 20 mph wind generates suffient lift (the lift is used perpendiculary) to propel you & the board/rig along at 30mph. All whilst going 90deg to the wind at speeds greater than the windspeed. You can travel 1.5-2.4 X the windspeed at speeds of 10-25mph. However Drag increases with windspeed so you can only manage 45mph in 40mph wind for example.

A cars undertray area is about 8-10m sq so easily has the capacity to generate 100's of kg's of lift.

I'm sure there is some calculation for the amount of energy generated by an x size sail but its a fair few kw's ***see note below***

Having a mechanical physical knowledge of lift generation, wing angle of attack, wing stalling & drag gives a very interesting insight into the world of 3d physics & how it could be usueful to cars. Those that say aerodynamics have a little effect below 100mph etc are not right.




Interestingly someone has actually done some calcs on the windsurfing forums

w=Cf*q*A
w- wind force
Cf - aerodinamic coeficient for force
q- stagnation pressure (not completely sure if it's correct translation)
A- area
A=6m2
q(18,5;37,4;55,6 km/h) = 0.185;0,374;0,556kN/m2 --> For 10;20;30 knots
Cf= 1.3 for perpendicular surface let's put it 1.15 because it is not perpendicular to all areas

P - power = w*v

for approximate power generated in your 6.0 sail, if my calculations are correct

10knots=> P=1,221kW ==> 1,67Hp
20knots=>P=2,58kW ==> 3,52Hp
30knots=>P=3,84kW ==> 5,24Hp

[Sam_68]

cptsideways said:

A cars undertray area is about 8-10m sq so easily has the capacity to generate 100's of kg's of lift.

That area with multiplied by a shape that's capable of giving a suitably high coefficient of lift might have the capacity to generate 100's of kg's of downforce, but the simple fact is that cars have to be car shaped and take account of lots of other compromises.

You can generate substantial downforce from an underbody venturi if you can stop the air leaking in at the sides and ruining your low pressure (which is why ground effect race cars were run with flexible skirts that 'sealed' them to the ground until such devices were banned), but with a relatively narrow planform like a car, coupled to a relatively high ground clearance, you really are pissing into the wind (pardon the pun).

If you want to appreciate the difference that preventing ingress of air from the sides makes, read up on the Lotus 80, which suffered diabolical problems, 'porpoising' in a straight line due to a skirt system that didn't work consistently.

cptsideways said:

Those that say aerodynamics have a little effect below 100mph etc are not right.

I don't think anyone is suggesting that aerodynamic forces are insignificant below 100mph, just that the net lift/downforce that is acheivable with a practicable road car shape, running practicable road ground clearance, is not of a big enough magnitude to have a significant influence.

A sail forms a surprisingly efficient and 'perfect' aerofoil section with a good aspect ratio. A Renault Clio does not.

[Twincharged]

kambites said:

Give it a year or so and we'll have road cars blowing their exhaust gas into the diffuser. It wont do anything, but it'll be fashionable because they did it in F1.

Renault were using it on their road cars even before it was used in F1...