Cd and blunt front ends

[aerohead]

Originally Posted by JJW

The thread has progressed somewhat but I have a question which is somewhat related to the original question. I got to talk to an aerospace engineer once when I was a kid, and I asked how a rounded shape could be as aerodynamic as a cone shape (I think I said "more pointy" at the time) on the nose of model rockets. His response was that the round shape presented the same amount of area to the oncoming air as the pointy one, and slower than the speed of sound, they both did about as well to push the air to the sides of the rocket body.

Can anybody elaborate on if my memory from when I was about 11 is any good? If that is correct, and knowing a bit more now, it seems to me that the blunt shape does the job of getting the air attached to the sides via less surface area, and therefore less viscous drag than a long pointy cone-like shape which would have the same cross sectional diameter. (I'm a software engineer, not a mechanical engineer, so please excuse any confused terminologies!)

It is spelled out in text that in subsonic flow,all streamlining basically occurs at the rear of the body(presuming flow is attached in the fore-body).At MACH 1 or higher,everything has to do with the nose.A convex hemisphere nose is considered more than adequate to guarantee fore-body attached flow in subsonic flow,and the air prefers the acceleration profile it provides over that of a straight walled conical nose section.Hucho's book demonstrates how little smoothing of leading edges is necessary for attached flow in the VW Rabbit development.A few chips off the ol' block is all that's necessary.

[Cd]

Let me ask a new question to all of you then :
Since apparently a brick like front end that has attached flow can be just as slick as a car that has a rounded nose, what good does it do to extend the nose out on our cars with coroplast extentions ?
I would like to keep my car as stock looking as possible ... but I also want a car that is as slick as wet soap.
I understand that since I have a hatchback, I will always have a large wake.

In a perfect world, you could have a huge boat tail extention on your car and no one would laugh at you - but this is real life.
I have been mocked by my own parents for my hobby, and I just want to have a car that I can tinker with that will not make me the butt of everyones joke.

So... without installing a boat tail on my car, what would you say I might be able to get my .Cd down to ?

[whokilledthejams]

Originally Posted by .Cd

In a perfect world, you could have a huge boat tail extention on your car and no one would laugh at you - but this is real life.
I have been mocked by my own parents for my hobby, and I just want to have a car that I can tinker with that will not make me the butt of everyones joke.

I think being mocked by loved ones comes with the territory. Of course, my dad, and my girlfriend's stepdad were both intrigued that I actually saw modest gains from putting cut up election signs on the front of my car. Every gained mile per gallon counts!

On the other hand, my girlfriend, I think, thinks I'm a dumbass. Of course, I remind her that she has no right to complain about $4.25 gas if she continues to drive like a jerk. Then, she gets angry. You wouldn't like when she's angry. She could save a car payment a year by just driving more sensibly.

[MechEngVT]

Originally Posted by texanidiot25

Possibly this weekend I'll grab an old magazine article I have at my home, I think from hotrod mag of an 89 NASCAR Monte Carlo vs. a Lemans Porsche LMP car. Despite the Monte Carlo having a near stock-car body and obviously a boxier car, was able to reach higher top-speeds and had less of a Cd. thanks to it's smaller frontal area.

NASCAR Monte Carlos have next to zero downforce. An LMP car should produce tremendous downforce at high speeds. Increasing downforce will increase Cd (I guess the opposite of lift/drag ratio....anti-lift/drag ratio?) so at say 170mph the stock car is probably weight-neutral and pulling through the air but the LMP car's tires are about flattened and suspension bottomed and drag-limited which isn't a problem in their racing series since the fastest those cars ever go is down the Mulsanne at LeMans but typically go much slower through tight turns without banking. Different cars for different purposes.

[MetroMPG]

Originally Posted by .Cd

what good does it do to extend the nose out on our cars with coroplast extentions ?

Assuming the stock configuration has attached flow, I don't think there's any benefit of doing that, unless your goal is to mess around with the stagnation point.

EG: from Phil's "book illustrations" album:



The "optimum" nose has the lowest stagnation point - ie. less of the total flow will go under the car vs. above/around it.

[Shawn D.]

Originally Posted by MazdaMatt

Cd is area-irrelevent, right? Cd*A is what calculates out the drag coeff, right? Cd is simply a product of shape and surface imperfections, so a 1/25 scale model should have the same Cd, but a much smaller Cd*A.

That is correct, assuming that the Cd for both were measured at the same Reynolds number.

Originally Posted by texanidiot25

Possibly this weekend I'll grab an old magazine article I have at my home, I think from hotrod mag of an 89 NASCAR Monte Carlo vs. a Lemans Porsche LMP car. Despite the Monte Carlo having a near stock-car body and obviously a boxier car, was able to reach higher top-speeds and had less of a Cd. thanks to it's smaller frontal area.

Originally Posted by MechEngVT

NASCAR Monte Carlos have next to zero downforce. An LMP car should produce tremendous downforce at high speeds. Increasing downforce will increase Cd (I guess the opposite of lift/drag ratio....anti-lift/drag ratio?) so at say 170mph the stock car is probably weight-neutral and pulling through the air but the LMP car's tires are about flattened and suspension bottomed and drag-limited which isn't a problem in their racing series since the fastest those cars ever go is down the Mulsanne at LeMans but typically go much slower through tight turns without banking. Different cars for different purposes.

Correct! Again, as MazdaMatt says, drag is not solely based on Cd, and don't forget the power and weight differences.

Originally Posted by whokilledthejams

On the other hand, my girlfriend, I think, thinks I'm a dumbass. Of course, I remind her that she has no right to complain about $4.25 gas if she continues to drive like a jerk. Then, she gets angry. You wouldn't like when she's angry. She could save a car payment a year by just driving more sensibly.

She gets angry because women don't like their emotionally-based opinions to be trumped by logic. Been there, done that, still have the scars!

Originally Posted by .Cd

Let me ask a new question to all of you then :
Since apparently a brick like front end that has attached flow can be just as slick as a car that has a rounded nose, what good does it do to extend the nose out on our cars with coroplast extentions ?

Because you are increasing the fineness ratio.

[Cd]

Originally Posted by Shawn D.;


Because you are increasing the [URL="http://en.wikipedia.org/wiki/Fineness_ratio"

fineness ratio[/URL].

Thanks !

The article makes it sound as if a high fineness ratio is good when you need to break the sound barrier.

I rarely break the 70 MPH barrier ( when I even get around to driving. )

[Shawn D.]

Originally Posted by .Cd

Thanks !

The article makes it sound as if a high fineness ratio is good when you need to break the sound barrier.

I rarely break the 70 MPH barrier ( when I even get around to driving. )

Yeah, yeah, it's great for the transonic and supersonic regions, but don't fixate on that part of it. As I've mentioned in other threads, pressure drag is the main component of drag for cars, and reducing the fineness ratio for a blunt object like a car will make a significant difference in pressure drag.

[ebacherville]

the more air you push around and ove the car the better.. real test that any one can do is this , take a 1 foot by 1 foot by 1 foot box, hold it out the window at 60mph.. then place a cone on its front end then hold it out the window.. what one seems to cut through the air better..

I can say that my nose on my car made huge diffrences when getting blasted bu a big rigs air shock wave, its very solid when passing a semi.. however the added length of the nose and tail is noticeable in crosswinds.. crosswinds effect the car more since there is more side profile to catch the wind..

[Cd]

Originally Posted by ebacherville

the more air you push around and ove the car the better.. real test that any one can do is this , take a 1 foot by 1 foot by 1 foot box, hold it out the window at 60mph.. then place a cone on its front end then hold it out the window.. what one seems to cut through the air better..

I can say that my nose on my car made huge diffrences when getting blasted bu a big rigs air shock wave, its very solid when passing a semi.. however the added length of the nose and tail is noticeable in crosswinds.. crosswinds effect the car more since there is more side profile to catch the wind..

Interesting.