What causes car drag?

[Vman455]

Originally Posted by aerohead

If I have to explain it to YOU, there's no reason for us to ever communicate directly again.
I can't believe you're even asking such a question after all you've written!

I was just curious about your method of calculation because, as I said, I don't see enough information here or in the book to produce the numbers you did. Thanks in advance for your answer.

[aerohead]

Originally Posted by Vman455

I was just curious about your method of calculation because, as I said, I don't see enough information here or in the book to produce the numbers you did. Thanks in advance for your answer.

These forces, as all aerodynamic forces, vary as the square of the velocity.
All one need do is, calculate the differences of the squares, then use the fraction as a multiplier to the original data points, for any given velocity.
The new data reflects the existing air density, at time of original measurements.
If original measurements were not normalized to standard SAE atmospheric conditions, the new data will reflect the same discrepancy.

[AeroMcAeroFace]

"How did you calculate those figures without a complete pressure map of the cars in question? You know as well as anyone here that drag force on a three-dimensional body cannot be calculated from a few centerline pressures, and Julian has never claimed such."

this was the question asked by Vman455, I don't think that has been answered. We know how you calculated the pressures at different speeds, but if you only have measurements along a centreline, how do you somehow get total drag?

From a 2D plot, somehow you have created a 3D plot, the insight is not a body of revolution.

[aerohead]

Originally Posted by AeroMcAeroFace

"How did you calculate those figures without a complete pressure map of the cars in question? You know as well as anyone here that drag force on a three-dimensional body cannot be calculated from a few centerline pressures, and Julian has never claimed such."

this was the question asked by Vman455, I don't think that has been answered. We know how you calculated the pressures at different speeds, but if you only have measurements along a centreline, how do you somehow get total drag?

From a 2D plot, somehow you have created a 3D plot, the insight is not a body of revolution.

Didn't have to. I was only relating the original values, from the original locations measured, at the original, lower velocity, to a new, higher reference value, consistent with other pressure profile reporting in the literature. Which allows comparison at equal velocities, something lacking in the original presentation. Apples to apples!
The car's contour doesn't change, nor it's influence.
The obvious discrepancy between delta-P, from forwards, to rearwards stagnation points, and the implication, as to drag coefficient is interesting to me, being 180-degrees out of phase with convention.

[JulianEdgar]

Originally Posted by aerohead

Didn't have to. I was only relating the original values, from the original locations measured, at the original, lower velocity, to a new, higher reference value, consistent with other pressure profile reporting in the literature. Which allows comparison at equal velocities, something lacking in the original presentation. Apples to apples!
The car's contour doesn't change, nor it's influence.
The obvious discrepancy between delta-P, from forwards, to rearwards stagnation points, and the implication, as to drag coefficient is interesting to me, being 180-degrees out of phase with convention.

As has been pointed out by the other posters, it's quite impossible to calculate comparative drag values between different cars by viewing only their centreline pressures. Your claim to have done so just indicates that your knowledge and understanding of car aerodynamics is quite poor.

As I say - I don't mind if you don't understand; it's when you mislead others that it bothers me.

[aerohead]

Originally Posted by JulianEdgar

As has been pointed out by the other posters, it's quite impossible to calculate comparative drag values between different cars by viewing only their centreline pressures. Your claim to have done so just indicates that your knowledge and understanding of car aerodynamics is quite poor.

As I say - I don't mind if you don't understand; it's when you mislead others that it bothers me.

1) the only value of the measurements for the audience would be their comparisons to those vehicles of different architecture.
2) presenting all data at a common velocity allows for convenient comparison.
3) as pressure drag dominates aerodynamic drag, it's of the utmost interest.
4) and pressure drag has to do with the differential between fore and aft.
5) your fore and aft pressures raise extreme challenges to interpretation.
6) if you don't want us to work with your numbers, stop posting them. It's a public forum.

[JulianEdgar]

Originally Posted by aerohead

1) the only value of the measurements for the audience would be their comparisons to those vehicles of different architecture.
2) presenting all data at a common velocity allows for convenient comparison.
3) as pressure drag dominates aerodynamic drag, it's of the utmost interest.
4) and pressure drag has to do with the differential between fore and aft.
5) your fore and aft pressures raise extreme challenges to interpretation.
6) if you don't want us to work with your numbers, stop posting them. It's a public forum.

Ok, so now we're dealing with a car that is only two-dimensional? Why am I not surprised?

Seriously, this has gone from the sublime to the ridiculous.

[aerohead]

Originally Posted by JulianEdgar

Ok, so now we're dealing with a car that is only two-dimensional? Why am I not surprised?

Seriously, this has gone from the sublime to the ridiculous.

You're providing centerline pressures. And implying things solely based upon the limited data. Speculating.
I await the thousands of pressure readings from every square-inch of each panel, recorded simultaneously, under identical conditions, and your interpretation of their cumulative effect.

[JulianEdgar]

Originally Posted by aerohead

You're providing centerline pressures. And implying things solely based upon the limited data. Speculating.
I await the thousands of pressure readings from every square-inch of each panel, recorded simultaneously, under identical conditions, and your interpretation of their cumulative effect.

Sorry, what has this got to do with your claimed ability to compare the drag of different cars based solely on measured centreline pressures? Are you retreating from this absurd claim?

[freebeard]

Originally Posted by aerohead

These forces, as all aerodynamic forces, vary as the square of the velocity.

I'd love to see you debate this guy:

ecomodder.com: Lift at red lights