Chrysler Airflow and it's aerodynamics

[aerohead]

Originally Posted by JulianEdgar

I don't understand. The K value doesn't appear to have thing to do with Cd for flat plates.

"Drag coefficients are shown as 'K', which is in lb sq feet (mph)^2"

Why aren't they talking of the frontal area of the car? ie lbs drag per square foot of frontal area * (mph)^2.

I am no mathematician but I would have thought the conversion of that to Cd would be fairly straightforward.

Aeronautical engineers don't typically use projected frontal area-based drag coefficients.
* They may use wetted-area-based coefficients.
* They may use volumetric displacement-based coefficients.
* All components of an aircraft can be calculated individually, then added together, plus an estimated interference drag component.
* Typically, they resolve total aerodynamic drag into that of an equivalent flat plate, flat into the 'wind.'
* Sometimes they've used flat plate area sliding parallel to the streamlines, both sides.
* All calculations are for 'flight conditions,' maybe cruising at 41,000-feet altitude Above Ground Level/ Above Sea Level.
It's a nightmare!

[JulianEdgar]

Originally Posted by aerohead

Aeronautical engineers don't typically use projected frontal area-based drag coefficients.
* They may use wetted-area-based coefficients.
* They may use volumetric displacement-based coefficients.
* All components of an aircraft can be calculated individually, then added together, plus an estimated interference drag component.
* Typically, they resolve total aerodynamic drag into that of an equivalent flat plate, flat into the 'wind.'
* Sometimes they've used flat plate area sliding parallel to the streamlines, both sides.
* All calculations are for 'flight conditions,' maybe cruising at 41,000-feet altitude Above Ground Level/ Above Sea Level.
It's a nightmare!

We're talking about a car, not an aircraft. Why do you assume that Chrysler engineers were using an aircraft approach? There's nothing in the paper to state that, as far as I am aware.

[JulianEdgar]

I have been doing some maths and I realise I made an important mistake earlier.

Looking closely at the Chrysler paper, the equation they use for their K value is in fact:

K = drag force (lb) / [frontal area (square feet) * (mph)^2]

That makes much more sense, as it's the standard drag equation with air density missing (but obviously assumed for a standard atmosphere).

I have now been trying to convert that to drag force (newtons), frontal area (square metres) and speed (metres/second) so that we can convert to our normal Cd values.

Using an air density of 1.2 kg/m^3, and taking into account the 0.5 multiplier on the bottom of the Cd equation, I get a conversion factor of 398.746 (ie multiple Chrysler's K value by that number to get Cd).

But math is not my strong point so it would be great if someone can check.

(Addition: I've been checking that against probable Cd values from the cars shown in the paper, and it seems to largely make sense. I've asked a few of the experts I know to check my calculation, so we'll see how wrong I am!)

[JulianEdgar]

One of my experts has got back to me.

He used a density of 0.002378 slugs/ft^3, which was the standard density in old imperial units. (Equivalent to 1.225 kg/m^3).

Using that density, his conversion factor is 391.0. That is, multiply the K factor from the Chrysler paper by 391 to get Cd in a modern context.

[California98Civic]

YouTube just suggested this video to me. It is a historic film from the 1930s created by the Chrysler Corporation to promote the Airflow. Full of details about their attitude and process in designing the car, it is really interesting and really quaint.

https://m.youtube.com/watch?v=AG55Bf9nWHk

[freebeard]

What should one call that frame, a birdcage? I thought Nash was first with unit construction:

For the 1941 and 1942 model years ....[t]he Nash Ambassador 600, built on a 112-inch (2,845 mm) wheelbase, became the first popular domestic automobile to be built using the single-welded "unibody" type of monocoque construction that Nash called "Unitized", rather than body-on-frame. From 1941 through 1948, Nash Ambassador models placed this unibody structure on top of a conventional frame,[15] thus creating a solid and sturdy automobile. It was also one of the first in the "low-priced" market segment with coil spring suspension in front and back, "giving it the best ride in its class."

The Chrysler Brothers were consummate engineers, but some of it never made it to market. I never saw Airflow on a used car lot in the 1950s, but I wish they'd been full of Star Cars:


www.hemmings.com: SIA Flashback - Mopar's Star Cars

It looks like the original article 404s, but here's something squirreled way in my albums:



Front-wheel drive 5-cylinder radial.

[California98Civic]

That long decked rumbleseat coupe is a beauty. Here is another video of an Airflow on today's roads. Great panning shots of the dash. And the guy seems to daily drive this super rare coupe version. Must have bank!

https://m.youtube.com/watch?v=YkzRl-1IMPs

[JulianEdgar]

Originally Posted by California98Civic

YouTube just suggested this video to me. It is a historic film from the 1930s created by the Chrysler Corporation to promote the Airflow. Full of details about their attitude and process in designing the car, it is really interesting and really quaint.

There's another video (also on YouTube) that shows all their wind tunnel test models. Again it's interesting, in part because you quickly realise that they had little idea of what Paul Jaray had been doing in Germany about a decade earlier.

[AeroMcAeroFace]

I think that is an optimistic flow attachment angle

[California98Civic]

Originally Posted by AeroMcAeroFace

Attachment 30808

I think that is an optimistic flow attachment angle

Me too. I thought it was interesting that they sorta started from scratch in wind tunnel design and that they claimed to have worked with one of the Wright brothers (among other people). It show at once how serious they were and how little they yet knew.