View Single Post
Old 01-24-2013, 06:55 PM   #412 (permalink)
Master EcoModder
aerohead's Avatar
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 12,467
Thanks: 20,049
Thanked 6,226 Times in 3,845 Posts

Originally Posted by Tesla View Post
I thought about that initial slope being steeper, but there are a number of reasons I don't think it will be too much of an issue.

1 / As per your comment above, the overall picture in this field seems to be much more important than the sum of it's parts.

2 / No disrespect here, I think nearly everyone here would still be floundering without your imense input, but I still have a bit of a question about the angles attached to the template, they do not seem to conform to a continuous curve, this suggests to me that some errors have been introduced in the process of transcription. So we know the template numbers quoted do get a positive result, but as there is discrepancy sugests that there is some room to play around these numbers, how much?, who knows?

3 / Many so called "streamlined", & proven vehicles seem to fall below the template curve enough to suggest they shouldn't be as good as they are.

I haven't got access to Hucho's book, but if I continue this hobby it may well get on the Buy list.

Copiers are great for a single copy without magnification, once you start magnification & copy of copies, the errors start to multiply out, fine for a visual representation, but not for a scale model, but as you said, if that's all you have to work with then you do what you can.

The whole math noodling thing, thanks, but I do it all through excel, PITA working out all the formula's and error checking, but once it's done, all you need to do is adjust paramaters and dimensions and it does all the calculations. Be great if I had some of those flow programs, but this takes too much of my time anyway, so probably better of without them.

If you wouldn't mind, can you elaborate on how the template angles were derived
Your opinion on how to apply template to sides of vehicle, as I have found it produces a very aggressive vertical profile which would suggest seperation.
If you'll go back to permalink#1 on page 1 of this thread,underneath the 'Template' are a number of images.
The 3rd image from the left is Hucho's drag table,and if you'll look towards the bottom you'll find his image of a streamline body of revolution.of L/D= 2.5 and Cd 0.04.
*From Jaray's research,this body,in ground reflection,as a 'half-body' would have Cd 0.08.
*Adding narrow wheels/tires would bump the drag to Cd 0.12.
*Wheel fairings would allow sub-0.12 drag coefficients,as the wheels are better integrated into the body.
* This represents the drag minimum for ground effect for a door-slammer which you could walk up to,open a door and get into.(the 2012 Bochum University solar racer would be a fast-roofline version,@ Cd 0.14).
From boundary layer theory it is crucial that aft-bodies taper very gently to protect the fragile boundary layer which is operating in an adverse pressure environment,and can detach if the pressure increases too rapidly since it is already at zero velocity,and Bernoulli's Theorem would require this flow to decelerate even more as pressure increases.
Since the 'Template' is based on a streamline body of lowest drag,and cannot produce flow separation,it satisfies Hucho's requirement for attached flow and pressure regain which he says is the entire premise for automotive streamlining.
Since the 'Template' also respects W.A.Mair's maximum boat tail tangent angle of 22-degrees for attached flow with a bluff body it represents the shortest,lightest,strongest,and lowest drag,generating an automobile of L/H= 5,which is L/D= 2.5 in mirror-image,effective bluffness/effective fineness ratio,which Hucho reports as the ideal for low drag.
Any shorter and pressure drag goes up,any longer and surface friction drag goes up.
If you had to fail,you'd want it to be on the side of skin friction,as it plays such a minor role with road vehicles.So 'longer' vehicles would be preferable to 'shorter.'
There are shapes which will yield lower drag,but you can't walk up to them and open a door,get inside and go.And these shapes don't lend themselves to production automobile streamlining whereas the 'Template' does.
The 'Template' is rough although with a number of major bulkhead stations established,a 20' length of 1" scedule-40 PVC pipe is all that's needed to produce a near-flawless contour.Sure it's low-tech but it will get you in the winner's circle.
Photobucket album:
  Reply With Quote