Streamlining. More important, the front or the back?

[freebeard]

Originally Posted by aerohead

I'm not sure how to interpret the numbers.
The Dymaxion Car was previously estimated at Cd 0.25.

From the article:

* Results based on a simplified CAD model of the Dymaxion car which contains no internal, engine bay or cooling flow. Also absent are the suspension geometries and bodywork detail (hinges, louvres etc). Other simplifications include basic wheels and wheel arches.

Double sounds about right. What jumped out at me was the crosswind performance.

[aerohead]

Originally Posted by freebeard

From the article:

Double sounds about right. What jumped out at me was the crosswind performance.

In 1987 the Arivett Brothers of Southern California tested a model of their Top-Fuel streamliner which had Cd 0.20 @ zero yaw and then Cd 0.18 @ around 12-degrees of yaw.
It's the only other report which specifically delved into crosswind Cd performance,which makes me kinda the FNG when it comes to this sort of thing.

[3-Wheeler]

Originally Posted by sendler

Thanks for the replies. My question stems from discussions of whether streamlining the front or the back of a motorcycle will make the most improvement if you were only to do one or the other......

I like to think about the front versus back conundrum this way:

"If you don't have attached flow at the front, will improving the back matter?"

After asking that question, the starting place becomes more obvious.

Jim.

[justme1969]

If the dynamic load bearing surfaces are 18 to 20 degrees the quantified latent load with subtracted cross vector shift over the varying surface irregularitys would be equal to the cd?
LOL I love ya aerohead. better check my points for clarity now.

[sendler]

Originally Posted by 3-Wheeler

I like to think about the front versus back conundrum this way:

"If you don't have attached flow at the front, will improving the back matter?"

After asking that question, the starting place becomes more obvious.

Jim.

The pressure zone in the front of a vehicle tends to form a bubble around any irregularities. Reattachment can then form again at any time as the pressure drops into the low pressure zones. The wake drag is the biggest loser. Start at the back.

[Grant-53]

Yeah, what he said. There is a variation of drag with angle in incidence. Same with angle of attack.

[aerohead]

Originally Posted by justme1969

If the dynamic load bearing surfaces are 18 to 20 degrees the quantified latent load with subtracted cross vector shift over the varying surface irregularitys would be equal to the cd?
LOL I love ya aerohead. better check my points for clarity now.

Yep,I'm gonna need some expanded data to have any chance at an intelligible response.
*The entire structure is exposed to dynamic loading,whether positive or negative,so we need to be really specific.
*The 18 to 20 degree angles need a context as to flow orientation as there should be curvature in the forebody,and angles tangent to any segment of the curve will vary as a function of there specific position.
*'Latent load' is a psychrometric term applying to total enthalpy of an air mass.Would we be talking about kinetic energy potential,'equipotential' of the discreet streamline flow filaments at a given body position?
*And the term 'cross vector',are we talking about spanwise,transverse,flow induced by the yawed,crosswind 'relative wind' flowing at angle to the body centerline?
*I'm not certain about surface irregularities.On an aircraft we'd be experiencing transverse contamination of the forebody's laminar boundary layer,triggering early transition to T.B.L.,increasing drag;as aircraft drag is dominated by skin friction,which would really suffer from the T.B.L..
On a car or bike we'd always be in T.B.L. and irregularities might not really aggravate drag.'features-drag' would already be integrated into the Cd and if build quality were uniform it might not matter which direction the flow was coming from as far as drag.
*The body form,in cross-flow could induce a yawing moment which would not be present in zero wind due to the center of pressure moving around.Today,this would at least show up in the wind tunnel during yawing tests,whether as model or full-scale.
*Since 1978,directional stability has been a non-issue,after Morelli's work at the Pininfarina tunnel.The low,rounded nose,and reflexed tail,with stabilizing fins/rear-wheel fairings produce a zero-lift body of remarkable side-wind stability.
*As far as Dymaxion Car is concerned,no automaker would build such a design today.It was absolutely fabulous in 1934,but is eclipsed by later technology.
*help me with more data.A sketch would be great!

[aerohead]

Originally Posted by 3-Wheeler

I like to think about the front versus back conundrum this way:

"If you don't have attached flow at the front, will improving the back matter?"

After asking that question, the starting place becomes more obvious.

Jim.

Absolutely!
Walter lay's research bore this out.
A HUMMER H-1 with full boat tail would not best Cd 0.24.
A HUMMER H-3 might see closer to Cd 0.12 with its softened leading edges and windshield,n some gap -filling.

[Frank Lee]

Originally Posted by aerohead

Absolutely!
Walter lay's research bore this out.
A HUMMER H-1 with full boat tail would not best Cd 0.24.
A HUMMER H-3 might see closer to Cd 0.12 with its softened leading edges and windshield,n some gap -filling.

Yeah, but to answer the question, would an H1 and H3 with a bubble front end best .24 and .12?

[Cd]

Something like this monster, but with a rounded edge to the windshield right ? :