Lowest possible coefficient of drag?

[tallguy]

I apologize if this has already been discussed, but here goes anyway.

What is the absolute lowest possible coefficient of drag that is achievable -- at least, in theory?  Based on some research
I have recently done, it looks like an infinitely thin plane slicing through the air would be very, very aerodynamic.  But
most people I've met aren't quite that thin.

I can imagine some kind of narrow "teardrop" shape, of course.  If it is extremely long, then its shape, between the "nose"
area and the "tail" area kinda resembles a cylinder.  If it's extremely (for example, miles) long, it will have more drag than one that is shorter, right?

We all know that there are practical limits on what is buildable, transportable, drivable, and affordable.

Yes, narrower is better (from a strictly aerodynamic point of view).  But the body of a streamliner land speed vehicle still must be large enough to contain the driver or rider, as well as the hardware to propel and control the vehicle.

Based on a small amount of research I did, a (relatively advanced) F104 jet aircraft has a Cd of about .021.  This did quite impress me, especially since that aircraft has been around for quite a long time . . . decades, in fact.

Rob Freyvogel's car also quite impressed me.  I wonder what its (claimed) Cd was.  And I also wonder what "tricks" are
available to minimize disturbance of the local air as a vehicle passes through that air.  Some of the ideas that tend to come to
mind include utilization of sound, temperature, and even perhaps magnetic (or electrical) fields.  Yes, I'm kinda shooting
from the hip.  It's a few months before Speed Week 2024 (if the weather cooperates, that is).  I'm trying to kinda "stay in
touch" with y'all between now and then.  And by the way, good luck to all racers.

[tortoise]

In calculating drag coefficients, the reference area used in the equation may be defined differently. In automobiles it is conventionally the frontal area. In aircraft, this is not typical. Often, the plan view area; the plane as viewed from above, is used. This area, you will see, is, on an airplane, much larger than the frontal area. Thus, the reported drag coefficient of an airplane will be much lower than if it were calculated as on an automobile. 

[Interested Observer]

An object?s drag is generally composed of two components which are often described as ?form drag? and skin friction.  Your thin sheet may have minimal form drag but could still have significant skin friction.  Same with the long tubular.

[tortoise]

it looks like an infinitely thin plane slicing through the air would be very, very aerodynamic.

If it had any drag at all, having zero frontal area, it's Cd would be infinite.
[Or maybe it's undefined; there seems to be some controversy about this.]

[ack]

Here is the wind tunnel data for the ACK Attack.

[tortoise]

ack, the chute doors reduced drag quite a bit but you appeared to run without them. Did they cause a stability problem?

[Jack Gifford]

I believe that the term "frontal area" creates confusion. The area term in the equation (DRAG=AREA x Cd) is the total cross section in a plane perpendicular to the object's trajectory. I suspect that "frontal area" is sometimes used to reference the area of the front of an object.

[tallguy]

I believe that the term "frontal area" creates confusion. The area term in the equation (DRAG=AREA x Cd) is the total cross section in a plane perpendicular to the object's trajectory. I suspect that "frontal area" is sometimes used to reference the area of the front of an object.

I agree that it could be sometimes used that way, but I'm not convinced that this is always a good idea.  Since objects
intending to be relatively aerodynamic (or even hydrodynamic, such as a fish) are usually not ideal cylinders, there is typically
some taper in the shape.

[tallguy]

Although I don't like making assumptions, or encouraging others to make assumptions, I think maybe we can agree that
minimizing frontal area is generally desirable in land speed racing.  But of course, this tends to compromise things like
cushioning -- such as cushioning on a roll bar -- for safety.  I am a big fan of safety.  I haven't yet met anyone who wanted to die in a crash during a land speed run, even if that run was part of a two-way average that set a land speed record.

[WOODY@DDLLC]

Remember the only difference between theory and reality - is reality! 
https://en.wikipedia.org/wiki/Fineness_ratio

[tortoise]

I think maybe we can agree that minimizing frontal area is generally desirable in land speed racing.

If form drag is brought to low values, skin friction becomes the major source of drag. This makes reducing skin surface area a priority. A sphere packs a given volume of stuff in the smallest surface area. Sometimes drag is reduced by increasing frontal area.

[ack]

ack, the chute doors reduced drag quite a bit but you appeared to run without them. Did they cause a stability problem?

Yes and no. Adding the doors did cause instability but were not the root cause. We did a lot of studies at Swift Engineering using their, at the time, new Cray super computer and CFD software trying to get at the root of the problem. Studies showed almost no diffrence in stability doors on or off. Just as my freind Ken Mort predicted. Ken was a brilliant guy, top of his class at Standford, and was one of the people in charge of the 130 foot tunnel at NASA Ames for 15 years. It took me a long while to figure out. The Swift data is much too large to post here if anyone is interested maybe I could dig it up and put it on a Google drive link.

[tallguy]

ack, the chute doors reduced drag quite a bit but you appeared to run without them. Did they cause a stability problem?

Yes and no. Adding the doors did cause instability but were not the root cause. We did a lot of studies at Swift Engineering using their, at the time, new Cray super computer and CFD software trying to get at the root of the problem. Studies showed almost no diffrence in stability doors on or off. Just as my freind Ken Mort predicted. Ken was a brilliant guy, top of his class at Standford, and was one of the people in charge of the 130 foot tunnel at NASA Ames for 15 years. It took me a long while to figure out. The Swift data is much too large to post here if anyone is interested maybe I could dig it up and put it on a Google drive link.

Mike, when do you next plan to have Ack Attack attempt to set a new record?

[tallguy]

I think maybe we can agree that minimizing frontal area is generally desirable in land speed racing.

If form drag is brought to low values, skin friction becomes the major source of drag. This makes reducing skin surface area a priority. A sphere packs a given volume of stuff in the smallest surface area. Sometimes drag is reduced by increasing frontal area.

I'm aware that a spherical shape is the most "efficient" for enclosing a given volume with minimal surface area. 

I can also imagine that something shaped like a brick (say, 2 inches thick, 3 inches wide, and 6 inches long) headed down a race track with a small face (2" x 3") leading the way . . . would be less aerodynamic than a nice "teardrop" shape with a frontal area slightly greater than 6 square inches.

Regarding a "teardrop" shape, though . . . how is the most efficient (strictly from an aerodynamic point of view) shape
(for example, dealing with length of the body) determined?  Is a race car with an overall length of 20 feet going to be
more aerodynamic than a race car with an overall length of 30 feet (with all other things being kinda equal)?  I suppose computer programs dealing with fluid flow can tell us the answer.  Not that other considerations -- such as mass -- should be ignored.  A carbon-fiber body is likely to be less massive than a steel body with the same stiffness.

[Frank06]

There's a lot of airfoil shapes to be considered.  Have you ever had a look over at ecomodder.com?