Aerodynamic pressure testing video

[JulianEdgar]

https://www.youtube.com/watch?v=YDirvbQeGbc

[freebeard]

Two quibbles:

• No mention that a positive or negative value has differing effect on the opposite ends of the vehicle.
  
• No mention of moment arm. A given lift at one end will have a different effect than at the center of the roof. IOW 10lb of downforce at each end would/might counter 25lb of lift at the roof.

Am I wrong once or twice?

[JulianEdgar]

Originally Posted by freebeard

Two quibbles:
No mention that a positive or negative value has differing effect on the opposite ends of the vehicle.

I figured that was obvious when you apply the forces developed by the pressures, as I do in the video.

No mention of moment arm. A given lift at one end will have a different effect than at the center of the roof. IOW 10lb of downforce at each end would/might counter 25lb of lift at the roof.

Only if you're differentiating front and rear axle lifts ie CLr and CLf. If you're looking only at Cl, it doesn't matter where those lift / downforce forces occur. So no, 10lb of downforce at each end does not counter 25lb at the roof.

[freebeard]

I'll take one out of two, obviously.

I can't see being sucked forward by the edge of the hood. I suspect it's negligible.

[JulianEdgar]

Originally Posted by freebeard

I'll take one out of two, obviously.

I can't see being sucked forward by the edge of the hood. I suspect it's negligible.

And the leading edge of the bonnet / hood, and (in some cases) half the windscreen, and the A pillars, and usually the outer edges of the front bumper.

Look at the pressures, multiply by the areas, use the triangle of forces to calculate the horizontal component.

Certainly not negligible.

[freebeard]

So the Beetle's bluff forebody is a liability?

[JulianEdgar]

Originally Posted by freebeard

So the Beetle's bluff forebody is a liability?

No idea - do some pressure measurements and find out. The days of guessing are long gone.

[JulianEdgar]

Originally Posted by freebeard

I'll take one out of two, obviously.

I can't see being sucked forward by the edge of the hood. I suspect it's negligible.

I've run this image here before I think, but look at it in the context of what you said above.



Jaguar XK

Blue = thrust
Red = drag
Green = neither lift nor drag

Now we can't see the stagnation zone at the front of the car, nor the base area in the wake at the back (and both will be red), but you can see why car makers try to optimise front curves to give thrust.

[freebeard]

Are they factoring thrust and drag out from the normal?

[JulianEdgar]

Originally Posted by freebeard

Are they factoring thrust and drag out from the normal?

Yes, look at the legend - it's in Cd.