Honda Insight Tail Extension Project

[3-Wheeler]

Originally Posted by ecomodded

Looks like it is moving while sitting still, You really did a excellent job.
Amazing really,have you factored the cd of her yet?

Regarding the Cd, I tried for quite some time to collect data from coast down testing, and ultimately came up empty handed.

It turns out the method used to collect data, like speedometer with a camera, or hand held GPS is not accurate enough to get good coast down readings. My coast down numbers were all over the place after two years of trying.

Oh well, we gave it good shot and now know how NOT to do it.

I would think that a sample rate of let's say 10 Hz or something similar would be needed to get good coast down data, and ultimately a good Cd of the car.

Jim.

[sheepdog 44]

The wheel strakes and deflectors could be lengthened. On concept cars they have a very shallow angle that extends farther out in front and behind. I always thought they looked a little blunt on the stock car.

[3-Wheeler]

Originally Posted by aerohead

I found what I was thinking about.It was drawn over Mair's boat tail 'template',not the 'Template.'
Mair's wind tunnel model,if the rectilinear flow portion is removed,and only nose and boat tail combined,forms a 'template' with a fineness ratio of 2.52:1.Very similar to the 'Template.'
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To duplicate a 'Template'esgue form :
*drop 3.35-H from max camber point and revolve a line for 12.5-degrees,stop.
*drop to 5.294-H and revolve 12-degrees,stop.
*drop to 6.588-H and revolve out 7-degrees,stop.
*drop to 7.96-H and revolve until you strike the ground plane.
This image is very similar to the 'Template' and shaves a little length off Mair's overall length,as his 'template' has a constant 22-degree line of convergence once that angle is achieved.
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As to the 'Template',it is modeled from 'Streamlined body l/D=2,5,Cd 0.04,which appears in Table 2.1 Drag coefficients for different bodies-----------,Hucho,2nd Edition,Page 61.
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The drag curve for Streamline bodies of revolution is asymptotic at around L/D= 2.1 but the 2.5:1 is the 'shortest' body which just fulfills Mair's 22-degree maximum boat tail slope angle for protecting boundary layer attachment.Rolf Buchheim thought 23-degrees would be the limit.I'm just playin' safe with 22.

Phil,

I took your dimensions from above and worked them with a 2D CAD program and here's what came out...

The shape does not look right, and does not match the "template" very well.

Any comments?



Jim

[JethroBodine]

How about if you stayed on the vertical and took the angles from there?

[NeilBlanchard]

JB, that may be correct; though it looks like Jim has used the tangents correctly. As a draftsman myself, I know the vagaries of hand drafting vs CAD.

It might be a partial ellipse that is the most accurate, but the three radii come very close? I think the tangents need to be "fudged" a bit...

[aerohead]

Originally Posted by 3-Wheeler

Phil,

I took your dimensions from above and worked them with a 2D CAD program and here's what came out...

The shape does not look right, and does not match the "template" very well.

Any comments?



Jim

Jim,what I did was to keep all the centers of rotation directly on the same vertical line below the point of max camber which bifurcates the car into 'fore' and 'aft.'
See if that works better.
I just kinda pencil-whipped this out of curiosity and never re-visited it.And by brain is known to go haywire,so it's a good thing to check my work.

[aerohead]

Originally Posted by 3-Wheeler

Regarding the Cd, I tried for quite some time to collect data from coast down testing, and ultimately came up empty handed.

It turns out the method used to collect data, like speedometer with a camera, or hand held GPS is not accurate enough to get good coast down readings. My coast down numbers were all over the place after two years of trying.

Oh well, we gave it good shot and now know how NOT to do it.

I would think that a sample rate of let's say 10 Hz or something similar would be needed to get good coast down data, and ultimately a good Cd of the car.

Jim.

I printed off an enlargement of one of your photos (fabulous!) and compared to the Template and some values I assigned to the Template.
The original car has about 32.3% of Template aft-body.The Template would predict Cd 0.242 at this length and HONDA published 0.25.This is a 3.3% deviation.
From your photo it looks like the new tail brings the car out to 56% of Template aft-body.
At this length the data curve predicts Cd 0.1625.If we throw back in a 3.3% deviation it gives a drag coefficient for your car on the order of Cd 0.167.
This is just an estimate of course.
With a steady speed mpg baseline we could compare 'before' and 'after' mileage and use GM's Cd/mpg relationship to reverse-engineeer the new Cd.
0.167 doesn't seem unreasonable.This would have you in Daihatsu UFE III and GM Precept PNGV territory,a smidgeon below AeroCivic.Fun!

[sheepdog 44]

Originally Posted by aerohead

The original car has about 32.3% of Template aft-body.The Template would predict Cd 0.242 at this length and HONDA published 0.25.This is a 3.3% deviation.
From your photo it looks like the new tail brings the car out to 56% of Template aft-body.
At this length the data curve predicts Cd 0.1625.If we throw back in a 3.3% deviation it gives a drag coefficient for your car on the order of Cd 0.167.

How long is the template aft body? Could you estimate the Cd with the length of the aft body at 100%, 87.5%, 75% 62.5% 50% etc?

So people can see exactly what can be gained by adding another foot to their car. I'd like to know what length is needed to go "all out" and adjust for what i'm comfortable with. Maybe people would go for a complete aft body if they knew what it required and the benefits over a short Kammback.

[aerohead]

Originally Posted by sheepdog 44

How long is the template aft body? Could you estimate the Cd with the length of the aft body at 100%, 87.5%, 75% 62.5% 50% etc?

So people can see exactly what can be gained by adding another foot to their car. I'd like to know what length is needed to go "all out" and adjust for what i'm comfortable with. Maybe people would go for a complete aft body if they knew what it required and the benefits over a short Kammback.

There are numbers out there somewhere but no tellin' where,so I'll put some up.
If you'll take some graph paper you can construct a curve from which to extract discrete values for any length:
*0% = Cd 0.503
*10% = Cd 0.39
*20% = Cd 0.314
*30% = Cd 0.255
*40% = Cd 0.209
*50% = Cd 0.177
*60% = Cd 0.153
*70% = Cd 0.14
*80% = Cd 0.133
*90% = Cd 0.13
*100% = Cd 0.13
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From the 'Template' you'll notice that some of the length is lost to ground clearance and also that there is no appreciable drag reduction for tails extended beyond about 80%,as the 'phantom' tail is beginning to operate at this point,with the wake itself acting as a solid boundary for the active outer flow.
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With wheel fairings the Cd can be lowered a bit more.

[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.