Maximum angle for boat tail?

[aerohead]

Here's a link to a photo out of Hucho's book.It's from Mair's research on boattails.You'll notice that the body undergoes a very gentle narrowing,with very little "angle" initially,growing steeper in inclination as it proceeds to the rear,and scaling from this coordinate grid,you'll be able to figure where the maximum 22-degree incline begins,and continues.Also,if you'll print this image,blow up the image scale,then add in additional grid lines,you can add finer resolution to the Cd values/Length.If you curve your cars body more abruptly than what bis shown,you'll more than likely experience separation.-------------------------------------------------------------- Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - aerohead's Album: Book illustrations - Picture

[abcdpeterson]

Originally Posted by Daox

At sub sonic speeds the flow around a object really doesn't change all that much. At least thats what Hucho says. And since he knows more about aerodynamics than this whole board combined, I tend to trust him.

I plan on staying sub sonic, so that is good news. lol

[abcdpeterson]

Originally Posted by aerohead

Here's a link to a photo out of Hucho's book.It's from Mair's research on boattails.You'll notice that the body undergoes a very gentle narrowing,with very little "angle" initially,growing steeper in inclination as it proceeds to the rear,and scaling from this coordinate grid,you'll be able to figure where the maximum 22-degree incline begins,and continues.Also,if you'll print this image,blow up the image scale,then add in additional grid lines,you can add finer resolution to the Cd values/Length.If you curve your cars body more abruptly than what bis shown,you'll more than likely experience separation.-------------------------------------------------------------- Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - aerohead's Album: Book illustrations - Picture

If I understand correctly that would make sense. a gentle / rounded transition before starting the 22 degree incline is needed. Correct?

I can see in the photo the transitional portion is, and then the 22 degree incline portion.

I am not sure what is being shown with the graph at the bottom.

[MetroMPG]

A useful template is this image:

Align the highest point of your roof to the highest point of the half teardrop, and you have a decent approximation of what you can get away with in terms of initial curve & the curve much further out:



(Ironically, the EV1 is farthest from fitting the template, yet it's the most slippery production car built, at 0.195.)

[Bicycle Bob]

Some stuff on Reynold's # effects, separation, and speed:
Drag of Blunt Bodies and Streamlined Bodies

[aerohead]

Originally Posted by abcdpeterson

If I understand correctly that would make sense. a gentle / rounded transition before starting the 22 degree incline is needed. Correct?

I can see in the photo the transitional portion is, and then the 22 degree incline portion.

I am not sure what is being shown with the graph at the bottom.

The values along the bottom,reflect the ratio of the boattails length,as compared to the diameter of the body.At the far left,with "zero" boattail,there is zero drag reduction.---------As you move to the right on the graph,the length of the tail is increasing as compared to the diameter.If you follow all the way to the right,you're at the point where the tail comes to a point,a full boattail.------------ From the Cd values,you'll notice that drag continues to drop as you lengthen,with the curve kind of flattening out as you near the full tail.This is why many will suggest that the law-of-diminishing-returns would have you chop the tail,well short of it's full length.Dr.Kamm and Dr. Korff suggested that we chop it off at the point where the wake area would be 50 % of the vehicles Frontal Area,to help with manuevering in traffic and parking.For racing and record vehicles,they don't have to contend with these issues,so they're more free to run the full enchilada.

[winkosmosis]

Originally Posted by MetroMPG

A useful template is this image:

Align the highest point of your roof to the highest point of the half teardrop, and you have a decent approximation of what you can get away with in terms of initial curve & the curve much further out:



(Ironically, the EV1 is farthest from fitting the template, yet it's the most slippery production car built, at 0.195.)

I don't understand the use of that picture. It seems pretty arbitrary to overlay it on a vehicle. The entire flow is affected by the front of the vehicle, which never matches the shape of an aerofoil. The overlay isn't an aerofoil to begin with-- the point of an aerofoil is to have a curved front that allows the wing to function at a range of attack angles.

[Bicycle Bob]

That looks very oversimplified to me, too. The "ideal" shape is impossible, having no ground clearance. The Corvair Greenbrier wagon, which looked like the offspring of a VW van married to a Chinese Pagoda, still lucked out with a phenomenally low drag.

[aerohead]

Originally Posted by winkosmosis

I don't understand the use of that picture. It seems pretty arbitrary to overlay it on a vehicle. The entire flow is affected by the front of the vehicle, which never matches the shape of an aerofoil. The overlay isn't an aerofoil to begin with-- the point of an aerofoil is to have a curved front that allows the wing to function at a range of attack angles.

the overlay is generated from a circular body of revolution with a thickness ratio of 2.5:1.This is the lowest drag form that exists."cars" with the lowest drag all have "half" of this shape.This is because of the "ground-reflection" or "mirroring" phenomena which exists for road vehicles in ground-effect,first reported on by Prandtl in the 1920s.------------------ What you see of your car,the "air" see's two of.One above the ground,and one below,just as if you'd parked your car on top of a mirror.Due to this phenomena,the "fineness ratio" of your car ( it's length divided by it's height ) is only half what you think it is.------------- To get to the ideal 2.5:1 fineness ratio,your car's length must be 5-X it's height.------------- Getting back to the overlay,most modern automobiles have attached flow up to their point of maximum cross-section,their "Frontal-Area".Where cars get dirty,is in the region behind the point of maximum cross-section,defined on the overlay as the high point of the car's roofline,wherever that occurs.------------------------------ If you scale your vehicle to fit under the overlay as we recommend and then construct any aft-body modification which respects the outline of the overlay,both top AND sides,your new "body" should experience no flow separation whatsoever,until it gets back to the "end" of your new body.This is exactly what Kamm was doing with his K-Cars.This is exactly what Jaray did to achieve Cd 0.13.This is exactly what Lay did to achieve Cd 0.13.This is what GM did with Sunraycer and what Honda did with Dream-2.If you extend the tail all the way to the point,you will have virtually eliminated all your profile drag ( 55 % of your aero drag ) and you've got a shot at Cd 0.10 as with Honda's car.

[Bicycle Bob]

"the overlay is generated from a circular body of revolution with a thickness ratio of 2.5:1.This is the lowest drag form that exists."
Could you provide a reference for that? What are the qualifications? It seems odd that the Zeppelins would have used a shape with a higher surface to volume ratio.