OptimumLap – Vehicle Dynamics Simulation, Simplified.

[Lorenzo Pessa]

Originally posted by FsaeSpec:
Link doesn't work, would like to read

You can find some stuff here: http://blog.axisofoversteer.co...p-will-be-13753.html

[HksUsa]

Hello,

How are the downforce/lift and drag calculated if the only inputs are the lift and drag coefficients ? Wouldn't OL need the total area of the car to be able to calculate these values ?

Thank you, Kevin

[OptimumG]

Hi Kevin,

You are correct, indeed we do. The aerodynamic inputs for OptimumLap are constant downforce and drag coefficients, as well as the total frontal area (reference area) and the air density.

All of these inputs are located together under the heading of "Aero Inputs".


--
Pete
pete.ringwood@optimumg.com
OptimumG
Vehicle Dynamics Solutions
http://www.optimumg.com

[HksUsa]

Thank you Pete for your answer.

I was aware of the frontal area input, but I have trouble figuring out how you can calculate the downforce/drag of a whole car by simply using its frontal area ?

When calculating these numbers for a wing, you need the total area of the wing, wouldn't it be the same for a car ?

[Claude Rouelle]

HksISA,

Just to be sure you understand (because I have seen students being confused), in simple terms, we are speaking about the area of the car (or the wing) PERPENDICULAR to the direction of the movement.

For a car that is the frontal area, we do not worry about the top view area. That is unless you want to car to move up and down like aircraft do.

[HksUsa]

Originally posted by Claude Rouelle:

Just to be sure you understand (because I have seen students being confused), in simple terms, we are speaking about the area of the car (or the wing) PERPENDICULAR to the direction of the movement.

For a car that is the frontal area, we do not worry about the top view area. That is unless you want to car to move up and down like aircraft do.

Well I'm not sure I understand... An aircraft also moves forward, just like a car does and we do consider the top view area of the wings for the lift/drag calculation, even at an angle of attack of zero degree.

Based on what you said, we should only consider the leading edge (Bord d'attaque) area to calculate the lift and drag for a wing, which would be incorrect.

[Claude Rouelle]

An aircraft moves backward?

Fortunately not the one I am flying in....!!

[JulianH]

HksUsa,

it doesn't matter which area you use. It is just a convention to use the frontal area of a car to calculate the aerodynamic forces (for an aircraft you use the area of the wings, I guess).

If you go to a wind tunnel and measure the downforce of the car, you get the force, now you divide by the dynamic head and your result is cLift * Area. If you change the area to only the drivers head or so, than your cLift must change too but the force stays the same.


Just accept that for calculations in cars, the frontal area is used and then calculate the coefficients cLift and cDrag from the forces and the surroundings (density of air and velocity of the car).

Regards,

Julian

P.S. Claude, a team member of our 2012 team desgined and tested an model aircraft that could fly forwards and backwards and won a lot of prices with it

[HksUsa]

Julian

Thank you for your answer. My concern here is only about the mechanics behind de OL aero calculation method.

So, if I understood correctly, it says frontal area in OL, but it is totally arbitrary ? I can use the area I want ? (As long as I have the corresponding Cl/Cd of course)

[OptimumG]

Originally posted by HksUsa:
So, if I understood correctly, it says frontal area in OL, but it is totally arbitrary ? I can use the area I want ? (As long as I have the corresponding Cl/Cd of course)

Kevin, that is correct. You can use which ever values you like, although it is common to set the frontal area to 1 and to just use the corresponding Cl*A / Cd*A values.

As Julian mentions, as long as you account for it, it will be fine. It's the total lift force and drag force that is important for this simplified point mass vehicle model.


--
Pete
pete.ringwood@optimumg.com
OptimumG
Vehicle Dynamics Solutions
http://www.optimumg.com