Quote:
Originally Posted by 3-Wheeler
Phil,
The more I dig into this, the more interesting it is becoming.
When you talk about the optimum length/width ratio being 2.5:1, I automatically assumed that the Streamlining Template had these ratios.
Well from the drawing below, you can see that the 2.5L dimension is at this location while drawn on top of the "template" and the yellow line representing this length is much shorter than the rendering???
If one were to extend the tail of the rendering all the way to the ground level, I'm guessing the length/width ratio would be closer to 3:1. Whoops, I'm off a little bit. Just fired up the CAD and checked. It's 3.58:1. And this length/width ratio does *not* include the nose.
If the rendering *was* actually drawn 2.5:1 in length from the maximal height, the 22 degree angle would be the predominant *curve* from the ground up towards the maximal height. Not sure what to make of it?
Unless.... your 2.5:1 ratio is for a fully revolved shape above the ground, and when this revolved shape is cut in half and brought to the road surface, the length/width ratio then becomes 5:1. If one includes the shape of the nose, then maybe the rendering is pretty close to this ratio. It's getting late, and I will check tomorrow.
Since it can be assumed that Honda did a good job designing the roof of the Insight, I went ahead and worked up the geometry of the roof shape, stopping where the tail starts.
Here is what that geometry looks like. You can't tell from this small picture, but I zoomed in real close with the CAD program to make these curves/angles, so they are very close to the actual shape of the roof.
By the way, this particular picture was shot from about 100 feet with a zoom lens, so the perspective is at a great distance, minimizing parallax error.
Jim. |
*There are a range of streamline bodies of revolution which in free flight have a frontal area-based drag coefficient of Cd 0.04.
*The shortest streamline body which will produce Cd 0.04 has a L/D ratio of 2.1:1.Hoerner identifies this as the 'ideal' although there are reasons not to use it in aviation.Hoerner says the 'optimum' will be a bit longer.
*W.A.Mair's research on boat tails established the rear slope tangent angle of 22-degrees as the maximum angle which would maintain attached flow.
*Later,Buchheim figured 23-degrees might be accepted as the 'steepest' angle we'd want to use.
*When I put the 'Template' together I tried to respect a 'minimum' streamline body of revolution which was just long enough to also include no more than a 22-degree rear slope tangent angle.
*The 'original' 'Template-C" shows the 2.5:1 streamline body which I chose,in 'ground-reflection' as per Jaray,Prandtl,and Rumpler.This Cd 0.04 body is shown in Hucho's book.
*Technically,the body of the vehicle would have exactly the same profile in plan as in elevation.Working with 'production' automobiles,we'd have something more narrow.
*Our cars,and also one underneath the ground is what the air 'see's'.This is borne out in the original 'Template'.
*Using the 'above ground' portion of the 'Template' produces a minimum drag body of Cd 0.08 in the ground proximity as identified in the wind tunnel.
*This minimum drag body has L/H=5 at the ground plane.
*If the aft-body portion is 'shorter' it will suffer separation and higher pressure drag.
*If the aft-body portion is 'longer' it will suffer a drag increase due to increased wetted area and it's skin friction.
*The rear contour of the 'Template',according to wind tunnel investigations will guarantee attached flow and produce the lowest skin friction.
*When you add wheels to the 'Template' body the drag increases to about Cd 0.13 (from wind tunnel investigations).
*If we fair in the wheels,we can get drag coefficients below Cd 0.13.
*From the empirical data derived in the wind tunnel,if we want the lowest drag we need to stay close to the 'Template' profile.
*If you do anything,go longer.The added skin friction will pale in comparison to increased pressure drag from flow separation.
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I reduced the forebody curvature of the streamline body since all the major aerodynamicists say that a convex hemisphere -based nose is plenty good for below 250 mph.