CFD on LSR website

[Pinacate]

Dreamweaver,

The software is about $20k, and you'll want a  $5k pc. A more expensive version will run multiple computers so you get your results faster. There are a few programs in that same price range, and several that are very much more expensive. If you are running a $100k car then it may make a lot of sense...

[Pinacate]

In the time span of a week and half I've had two LSR racers tell me that they were adjusting their downforce to improve aerodynamic yaw stability. This idea comes from the classic airplane sketch showing the center of pressure behind the center of gravity which provides stability. They both had the idea that stability and center of pressure location were global, singular items. As most of you know the stability refered in the sketch is pitch stability. The airplane also needs yaw and roll stability. The pitch, yaw, and roll axis all have there own separate centers of pressure which can be in very different locations from each other.  In light of this, an additional stability section has been added to the web site that illustrates why adjusting downforce won't affect aerodynamic yaw stability (it does affect mechanical yaw stability through tire grip). Here is the link.

http://www.racecaraerodynamics.com/pages/airpane.html

This is a bit off topic, but a friend asked me to look at the new "swan neck" rear wing mounts that have popped up on the LeMans LMP cars. It shows how the mounts affect flow separation and overall downforce.

http://www.racecaraerodynamics.com/pages/2009%20LeMans%20LMP%20wing%20mounts.html

Hopefully, I'll get the anti-fly stuff written up next weekend.

[manta22]

Pinacate;

Thanks for all your efforts; this is good information.

Regards, Neil  Tucson, AZ

[Pinacate]

We finally have some analysis posted of devices that help keep the cars from flying when they are sideways, like roof rails and rounded bottoms. These simulations were done on a very simple car body without tires. The flow under the car is very important and tire will certainly affect it, especially when the car is sideways. Soon, we'll be using realistic car bodies with rotating tires and this entire topic will get re-visited. Anyway here is the link:

http://www.racecaraerodynamics.com/pages/anti-fly_configurations.html

[High Gear]

Lead creates downforce with zero aero drag.

Just a thought...

Gary

[Blue]

Lead creates downforce with zero aero drag.

Just a thought...

Gary

Ahh, but weight creates longer acceleration time/distance.

Many LSR builders say that light weight leads to faster acceleration and this is a better trade vs. "mass-based" downforce.  Maybe the best compromise would be light weight and low power at low speeds coupled with higher power and higher downforce at high speeds.

[jl222]

Lead creates downforce with zero aero drag.

Just a thought...

Gary

Ahh, but weight creates longer acceleration time/distance.

Many LSR builders say that light weight leads to faster acceleration and this is a better trade vs. "mass-based" downforce.  Maybe the best compromise would be light weight and low power at low speeds coupled with higher power and higher downforce at high speeds.

  My ideal would be low weight and high power at low speeds with high downforce and just enough downforce at high speed to maintain traction.

[A2WindTunnel]

Lead creates downforce with zero aero drag.

Just a thought...

Gary

Lead can be a fix but is not the necessarily the solution and could lead to other problems.  As I’m sure most of you know.. lead placement is very important and if you are looking for more traction and want to take a lead approach instead of adding downforce, then you could end up with a Center of Gravity (CG) too far back if you don’t balance the car out right.  If you do balance it out with lead (front to rear) the car will become extremely heavy so there is a trade off of what is "safe" for driving and how heavy you must make your car to solve your traction issue.

Most of you might have seen this video and I have talked about it in another post, but it illustrates what happens when the CG is too far back and the Center of Pressure (CP) is forward. 

http://www.youtube.com/watch?v=LYV0GiNDr6E&feature=channel_page

The car is 1989 Pontiac Firebird driven by Gary Eaker.  They went into Bonneville knowing that the car had more front and rear lift then they would like from wind tunnel testing performed at GM.  However, this was kind of a project in conjunction with Pontiac, and some of the criteria was the car to appear stock, and Pontiac wanted the stock spoiler for that production year.  They only way they could solve the traction issue now was to add weight to the rear but keeping total weight to a minimum (as BLUE stated for the acceleration issue).  The split was 1600/2800 (front/rear) making the CG further back then ideal, and the car had a CP further forward then ideal (now they are fighting 2 issues). This is what they had to work with, and with out going into too many details.. On a 300mph attempt the car had a slight tail wiggle, and with the CG far back it acted as a giant pendulum and the fish tail got amplified very quickly.  The result was the car getting airborne, but the heavy lead in the rear did keep the tail on the ground.  So you could say lead helped Gary from total lift off but Lead did NOT solve an aero problem that this particular car had, and actually made it unsafe to drive when it fish tailed due to the pendulum affect the added rear weight caused.

Dave

[jl222]

  There are plenty of cars that spin without a lot of weight in the rear. Street roadster are probably the worse caused by rules not allowing engine setback.
   Most spins are caused by tire spin caused from lift , not enough weight, too high wheel rate, not big enough contact spot on tire, to high air pressure, bad track or just to much hp.
   But don't always equate wheel spin with hp '' Carroll Smith''.
   
   Look at all the rear weight on a top fuel car and how they flatten the tires on a run and how small the contact spot gets when they blow the tires away.
   
    ALL rear drive cars want to come around when the tires spin.


          JL222

[Blue]

The next problem with adding ballast (we call it that in the aero industry) is IF we add lead to the front and rear, we may get a decent CG, but have now increased the polar moment of inertia for both yaw and pitch.  This means that it's harder to get a spin or loop started, but once started it takes a LOT more force to stop it.

Try the light weight and traction control first with some nice tails to keep it straight at speed.  Keep all of the weight in the center and add wing as necessary.