Aerodynamics

Kev · Post by **Kev** » Sun Dec 09, 07 5:40 pm

At what point does aero become significant on our 1/4 milers? The Street Eliminator cars seem to have wings and front splitters. At what speed is this necessary? My calculated speed at Milbrook was 130ish and it was floating around a bit. If you are still making big power at 130ish is the downforce taken off the back of the car? Small NASCAR style rear wing and a front splitter? Or under car tray? Discussion time!

latil · Post by **latil** » Sun Dec 09, 07 5:45 pm

I've heard many times that 70mph is when aerodynamics really start to work.

Guy · Post by **Guy** » Sun Dec 09, 07 6:41 pm

Where is the X student when you need him

Anonymous · Post by **Anonymous** » Sun Dec 09, 07 6:45 pm

Aerodynamics doesn't just start to work at 70 mph, but its at about 70 mph that you start to notice the negative effects of having too much drag etc.

The first thing to work out is the drag coefficient of the car (normally only attainable by wind tunnel testing or CFD) a Duster I believe has a Cd of 0.4 so you could use that.

The second is the front area of the car, that is the m^2 print or 'blockage' of the car when looking at the frontal aspect.

The third is the density of the air you are travelling in (1.2 kg/m^3 is normal air at sea level).

The last you need is the velocity of the car (in m/s).

Put them all together in the following equation to calcualte the force required to overcome aerodynamic drag at any given speed:

Fd = Cd*Area*(1/2*Density)*(Velocity^2)

You should be able to produce a graph showing the relationship of aerodynamic drag versus speed.

You will actually find that the force required to overpower the rolling resistance of the tyres is also significant, as is the force required to ovverpower resistance (friction) in the system, i.e. brakes dragging, bearing clearances too tight etc.

Anonymous · Post by **Anonymous** » Sun Dec 09, 07 6:56 pm

That Force is given in Newtons (N).

To get the power relationship:

((Aero drag force*Velocity)/(Efficiency in system))*10^-3

That comes out in kW.

Efficiency in the system is the gearbox efficiency * axle efficiency. Those numbers can be anything so be more conservative with them, you might expect and axle to be 0.9 or 0.95 efficient but a gearbox could be as low as 0.6.

Dave-R · Post by **Dave-R** » Sun Dec 09, 07 7:06 pm

They design modern cars in wind tunnels because every little bit of drag hurts performance at even quite low speeds. This is one of the reasons modern cars get such good mileage and why they sound so quiet.

Wind noise is turbulance.

The 1970 Challenger has 370lbs of force lifting the front end at 100mph. It also has 340lbs of drag. One reason why I fitted the T/A type spoilers. I doubt they will get rid of it all but they will help a bit.

Kev · Post by **Kev** » Sun Dec 09, 07 7:13 pm

Yeah! Great answers

. What I need to know is if I'm going through the 1/4 at 150mph, will the Taxi stay on the track or loop, or whatever?

Anonymous · Post by **Anonymous** » Sun Dec 09, 07 7:24 pm

It'll stay on the track. Unless you do something silly like put a Cessna wing on the roof!

I thought for a minute there a Hot Rodder was actually thinking about trying to improve the aerodynamics of his car - perish the thought!

Holly · Post by **Holly** » Sun Dec 09, 07 7:26 pm

Probably not, I'd guess a chin spoiler would be needed (dunno if the back needs anything, but the 9 sec Subaru running at york, with no spoiler on the back, seems to do ok at the top end with a 1/2" nascar style baffle on the back of the boot).

andyrob · Post by **andyrob** » Sun Dec 09, 07 7:48 pm

my mates jap crap has similar power to wieght ratio as my roadrunner,always beat him off the line but as we go over the line he is passing me like im standing still, ie over 100 & the old b body is bollocked
spoilers & wings are good for the circuit but no help for drag racing, better off taking the mirros off etc
put a superbird nose on a fast b body like dave billys & it will make a diference but the spoiler will stabiler the car but slower it down
cannnot expain why but rear diffusers are said to work, but ground effect is only good for corners
duck taping up the gaps on the cars is said to work
i think all are old mopars would cross the line another 5 mph + in an 11 sec if they had good areos
now shoot me down

Pete · Post by **Pete** » Sun Dec 09, 07 8:00 pm

I can honestly say that the 'Bird nose and spoiler work at high speed, but the S&M 'Birds never did well at the track - too heavy & too much drag.

IMHO the biggest issue with a drag car is stability. I think it is essential to get the ride height correct at the front and to stop as much air as possible getting under it - quite scary at the tope end otherwise.

Cannonball · Post by **Cannonball** » Sun Dec 09, 07 8:15 pm

better yet, less drag at 150 if you go through the top end with just your back tires on the ground

Pete · Post by **Pete** » Sun Dec 09, 07 8:31 pm

Yes, we have noticed, I am NOT playing that game

Anonymous · Post by **Anonymous** » Sun Dec 09, 07 8:39 pm

Remember to button down your hood in future Dunc

Aero is a big part of a successful race car, regardless of the series involved.

Stability is a big player as is overall drag.

Downforce is over rated in my opinion. The only thing that down force does is increase the available traction between the tyres and the road. Now if you have a top Fueller or are trying to take a corner at 2g then I can see the point in achieving large downforce. Otherwise, I think your car is heavy enough, you can tune some extra rearward force in chassis setup.

Downforce creates drag, this is why you don't see wings on cars at Bonneville. The winged Mopars arguably gained very little from having the horizontal wing at the back, they probably achieved more by having the large vertical stabilisers.

Stability like that is often a symptom of another problem though and generaly it a far more fundamental problem.

Golden rule!

Keep the centre of mass of the car IN FRONT of the centre of aerodynamic pressure.

That means the point in the car where the aerodynamic pressure acts. Think of it like this, what happens if you throw a dart backwards?

Now with a heavy front end you might assume that the centre of mass is further forward, but if you have ever had the car suddenly snap out on you at high speed (as guys driving '32 coupes at bonneville regualrly do) then think again. When in doubt, shift some mass forwards.

Second golden rule!

Attached flow, turbulence is created when flow detaches from the surface of the car. This happens at points where there is a significant change in direction of the bodywork (i.e. back of roof down rear screen). This creates turbulent flow and thus more drag. Think of ways to keep the flow attached as much as possible. Also Filling the wake at the back of the car can help a little.

mopar_mark · Post by **mopar_mark** » Sun Dec 09, 07 8:45 pm

Bratfink wrote:Remember to button down your hood in future Dunc

Aero is a big part of a successful race car, regardless of the series involved.

Stability is a big player as is overall drag.

Downforce is over rated in my opinion. The only thing that down force does is increase the available traction between the tyres and the road. Now if you have a top Fueller or are trying to take a corner at 2g then I can see the point in achieving large downforce. Otherwise, I think your car is heavy enough, you can tune some extra rearward force in chassis setup.

Downforce creates drag, this is why you don't see wings on cars at Bonneville. The winged Mopars arguably gained very little from having the horizontal wing at the back, they probably achieved more by having the large vertical stabilisers.

Stability like that is often a symptom of another problem though and generaly it a far more fundamental problem.

Golden rule!

Keep the centre of mass of the car IN FRONT of the centre of aerodynamic pressure.

That means the point in the car where the aerodynamic pressure acts. Think of it like this, what happens if you throw a dart backwards?

Now with a heavy front end you might assume that the centre of mass is further forward, but if you have ever had the car suddenly snap out on you at high speed (as guys driving '32 coupes at bonneville regualrly do) then think again. When in doubt, shift some mass forwards.

Second golden rule!

Attached flow, turbulence is created when flow detaches from the surface of the car. This happens at points where there is a significant change in direction of the bodywork (i.e. back of roof down rear screen). This creates turbulent flow and thus more drag. Think of ways to keep the flow attached as much as possible. Also Filling the wake at the back of the car can help a little.

Downforce is over rated in my opinion. The only thing that down force does is increase the available traction between the tyres and the road

When my mates pro mod rear wing collapsed just before crossing the 1/4 line at 200mph, it was the loss of the wing which caused loss of down force & major contributor to cause the car to roll & total the car, so think he may disagree