Aerodynamics

[jacksoni]

Unh, I guess I'll have to ask you to better define "bigger in the back than in the front" a bit better.  Yes, this is a competition coupe -- but it has put more than a few folks into the Club, so - - - what am I missing in the discussion?

Though I guess implied by my comment I did not mean that there are no successful liners or comp coupes that have this configuration (big back) but from an aero standpoint, as Howard and others point out, it is far from optimum stability wise not to mention speed (Cd etc). Though I have no actual knowledge of the circumstances and what SCTA eventually figured out, I think we are missing a good guy (John Beckett) because of this sort of aero issue. Sort of like the add weight to the back of the car deal for traction.  Bad for dynamic stability. Cars fly when going fast and get out of shape, good aero as outlined by Howard  help reduce/prevent that stuff from happening.

[slopoke]

.

[Seldom Seen Slim]

Yeah, I know -- I was just hasslin' you.  And I happened to have a photo of Tom's car handy.  It hasn't been seen here recently -- thought I'd give it another moment in the sun.

[Freud]

One of Howard's principles came into discussion when Treit's car was designed.

The discussion of length. Dr Seals wanted it shorter but this was the minimum length that would allow

the needed equipment to fit.

FREUD

[Rex Schimmer]

A good comparsion for the long back vs short back is being worked on right now. The people that bought Jr. Kurtz's old 6969 stream liner, Roy Fastjia (spelling??) originally built the liner. This car has (had) a cut off back end, aero guys call this a "bluff body" and when Junior ran it the best time was around 311mph with a 500 inch+ Van Dyne big block and held the AAF streamliner record for a while. This car is now being modified by the new owners and it will have an extended and tapered rear body work. So we can see the difference. I hope it will be at B'ville this year.

Rex

[Howard]

 8-) Hi guys,
I was reviewing this topic and ran across thee post I put up. I owe you guys an apology. I came across as arrogant and I did not intend that.
I am always in a hurry and I typed that in and took off for the shop without really thinking about how I wrote it.
Again, If I came across as arrogant I did not intend that at all.
Sorry,
Howard

[JimL]

I really like Howard comments.  I'd be curious about the rear lower end of a tank....should it carry the bottom line back, kind of like a tapered fin, from the lowest part of the belly?  I've been enjoying a little "mental exercise" on that subject. 

It would certainly provide some useful space, if the aero was not diminished.  It would look a little like the tapered mounts on the bottom of some of the wing tanks (such as turboprop transports and all), except that the "free air" surface would be above, instead of below, the tank body.

Thanks for any comments.

JimL

[Peter Jack]

Howard:

No apology needed.

Your last paragraph in the original post covered the subject well and the knowledge imparted probably caused many of us to think further on our own theories. Isn't that what these forums are supposed to be about?

Thanks for your input.

Pete

[interested bystander]

Had a laugh Rex, at your attempt to spell Roy's last name - DON'T They teach spelling in engineering school? LOL!

You aren't the first.

Years ago when Roy was one of the PREMIER West Coast dragster chassis builders an alledgedly knowledgeable enthusiast magazine captioned a photo or Roy hard at work fabricating something or another calling him Roy FIESTA!

Ole!

I think it's Fjastad, but don't quote me on that!

A great guy and Landspeed hall-of-famer, no matter how you spell the last name.

[Elmo Rodge]

I really like Howard comments.  I'd be curious about the rear lower end of a tank....should it carry the bottom line back, kind of like a tapered fin, from the lowest part of the belly?  I've been enjoying a little "mental exercise" on that subject. 

It would certainly provide some useful space, if the aero was not diminished.  It would look a little like the tapered mounts on the bottom of some of the wing tanks (such as turboprop transports and all), except that the "free air" surface would be above, instead of below, the tank body.

Thanks for any comments.

JimL

JimL, that's the way I'm headed on my Tank. Wayno

[Rex Schimmer]

Interested Bystander,
Your spelling of Roy's name is much closer than mine, I just knew that there was a J in there someplace. Good guy and still going fast!


Rex

[rebelce]

There are so many variables that in the end it's the overall combination that counts.  You can have a shape that has a very low CD but is unstable. You can make a stable shape unstable by placing the CG too far aft. In the end, you start with an idea and refine it with what you have available until it does what you want (or not!).   

http://en.wikipedia.org/wiki/Kammback   

gives you another approach like Rex pointed out.  I like to call it working out the weak links, one at a time.

[Blue]

From Wikipedia:
"While the realities of fluid dynamics dictate that a teardrop shape is the ideal aerodynamic form, Kamm found that by cutting off / flattening the streamlined end of the tear at an intermediate point, and bringing that edge down towards the ground, he could gain most of the benefit of the teardrop shape without incurring such a large material, structural, and size problem. The airflow, once given the suggestion of the beginning of a turbulence-eliminating streamlined teardrop tail, tended to flow in an approximation of that manner regardless of the fact that the entire tail wasn't there. This is called the Kamm effect."

Whoa, whoa, WHOA!!!

This is so misleading as to be dangerous.  IF (and only IF) the blunt tail forms a stable recirculating vortex with minimal momentum exchange with the surrounding flow can drag be less than if it is fully separated and disorganized.  A correctly shaped tail with no separation will still have MUCH less drag than any cut off "Kamm" rear end regardless of speed.

Just as important for LSR, while a long and fully aerodynamic tail may be mechanically inconvenient, it helps create aerodynamic stability.  Something the cut off tail can't do.

In less technical terms, the wiki article is BS.  In the professional aeronautics world, we fill in separation.  Chopping things off and saying it's lower drag went away 50 years ago.

[Peter Jack]

Thanks Blue!

Pete

[4-barrel Mike]

Oh, no!  BS???



"Kammback is not just another fancy word.  It refers to the aerodynamic shape of our little wagon.  A shape that contributes meaningfully to both the stability and the handling of the Kammback."

Shame on GM for misleading the world!!!  LOL

Mike