Designing a tail extension for the first-gen Insight

[aerohead]

Originally Posted by 3-Wheeler

AeroHead,

I see what you mean about the slight separation over the hatch. Does anyone have a larger format picture of the template that I can use?

I would like to overlay this template in a CAD file and generate a matrix of X-Y moves for this curvature. That way I can place the Insight sketch in the CAD and overlay the X-Y coordinates over the tail of the car to design a best-fit extension.

Metro,

Thanks for posting the underside angle of your extension. I will plan on going at least this steep in angle. The compound angular shape of hatch and by the wheel skirts will just make the design that much more interesting to work out. Similar to art.

Jim.

Jim,I apologize,but I'm useless to everybody on this one.While I have Photo Shop on a desktop I've never found time to get into it.I default to photo-enlargements here at the copy center to get some better resolution to work with and measure off of.Cave-dweller Tech!

[aerohead]

Originally Posted by RobertSmalls

That's what Honda said too, and we ended up with a Cd of 0.25. I'm inclined to go all the way, since I think it will be departure angle and visibility, not overall length, that make the tail cumbersome to drive daily.

The rear view mirror looks entirely through the upper hatch glass. The lower hatch glass is very helpful while reversing, but not useful on the highway. I'd be okay with blocking the view through the lower hatch glass, but a rear view camera would become almost required, adding to the cost of the project.



I do see the widening gap between the hatch and the streamline. But isn't that normal, as the falling pressure after the apex causes streamlines to move farther apart? I can't really picture disorderly, reversed flow in the few cm between the illustrated streamline and the glass.

And, this picture has me wondering where the stagnation point is on a stock car, and on one with a grille block. Do I want to send air over the car where there's more room for air, or under the car to fill the vacuum under the tail?

A look at Kamm's K-Car under smoke ( in the FLOW-IMAGES ) shows the smoke hugging the top of the roof until his roofline exceeds the template curvature and clearly separates,beginning it's tumble into the wake.
I believe Honda's car is doing the same.A smoke rake would have been helpful,giving us more lines to interpret.
Short of that,and with the smoke NOT hugging the hatch,I can only interpret it as un-attached flow.And my guess is that like GM,it is a concession to outward rear vision for the driver.
The stagnation point should be directly in front of the grille opening.
Considering concept cars with perfect full bellypans,to achieve the lowest drag,the cars are always lowered,some deploying a moveable chin spoilers to prevent airflow underneath.This also showed up for me on the CRX.

[3-Wheeler]

Originally Posted by aerohead

Jim,I apologize,but I'm useless to everybody on this one.While I have Photo Shop on a desktop I've never found time to get into it.I default to photo-enlargements here at the copy center to get some better resolution to work with and measure off of.Cave-dweller Tech!

Hi AeroHead,

I noticed that RobertSmalls put a larger picture of the Honda Insight on this thread, but I'm interested in a blowup of your template!!

I want to make sure to get detailed curvature from it and keep the tail extension the same.

Thanks, Jim.

[3-Wheeler]

Originally Posted by RobertSmalls

...Do I want to send air over the car where there's more room for air, or under the car to fill the vacuum under the tail?...

Hi Robert,

Yes, the idea is to give the air plenty of room to expand away from the body shape as it passes through the air.

The air under the car, unfortunately gets compressed and it not allowed room to move away from the body since the road is there. This compression takes energy and creates more air drag compared to all the other outside surfaces of the shape.

My tail is planned to be 48 inches long, unless I hear back from the DOT before hand.

Metro made his tail 51 inches I believe, and his results speak for themselves.

The overall car length will still be shorter than a typical short-bed pickup in length, so I'm going to go for it, and keep the full length!!

With the tail extension, I am looking for at least 90mpg results (at my typical speed). OK, maybe throw summer temps in there as well, for good measure.

Jim.

[RobertSmalls]

I made a pair of creepy images (attached), overlaying the Insight on the 2010 Prius. I scaled the two images to line up the ground and the apex of the roofs, though I should have rotated the Prius image slightly counterclockwise.

I was hoping the Prius' roof would extend much farther than the Insight's, giving us an idea of how much curvature is acceptable. But as it turns out, the Prius' roof is just as long and very slightly less steep.

I'll buy some yarn tomorrow and see if the Insight's rear glass has a problem or not. I sure hope not, because I'm planning to leave the hatch glass in place and go steeper from there.

Also, while we've got Photoshop warmed up, does anyone want to try to line up the streamlining template (or a Prius roofline) with the top view of the Insight?

[RobertSmalls]

Originally Posted by 3-Wheeler

My tail is planned to be 48 inches long, unless I hear back from the DOT before hand.

Metro made his tail 51 inches I believe, and his results speak for themselves.

The overall car length will still be shorter than a typical short-bed pickup in length, so I'm going to go for it, and keep the full length!!

Jim,

Darin's tail is 62" long.

If 48" is achievable without cutting the end of the tail off, then your car will be as long as a minivan.

As for fuel economy, I suppose my biggest goal is to be able to do lean burn at 74mph instead of the 68mph I can do now.

[ChrstphrR]

Originally Posted by RobertSmalls

And, this picture has me wondering where the stagnation point is on a stock car, and on one with a grille block. Do I want to send air over the car where there's more room for air, or under the car to fill the vacuum under the tail?

You want to send most of the air over the top, and around the sides (like what aerohead usually extolls, and Basjoos mentioned in his post too.

For the most part, you wouldn't want a LARGE increase in airflow under the car, or it may have an unsettling lifting body effect, which could affect handling at highway speeds or higher. Not that this is likely to happen with the shape of the Insight's nose; but it was an issue with the Stingray Corvettes of the mid-60s.

[aerohead]

Originally Posted by 3-Wheeler

Hi AeroHead,

I noticed that RobertSmalls put a larger picture of the Honda Insight on this thread, but I'm interested in a blowup of your template!!

I want to make sure to get detailed curvature from it and keep the tail extension the same.

Thanks, Jim.

Sorry Jim,MetroMpg(Darin) put the template you see together,based on a conversation we had a while back.
I can't even use it!
Perhaps one of the photo-shop-savvy members can do the enlargement.

[aerohead]

Originally Posted by 3-Wheeler

Hi Robert,

Yes, the idea is to give the air plenty of room to expand away from the body shape as it passes through the air.

The air under the car, unfortunately gets compressed and it not allowed room to move away from the body since the road is there. This compression takes energy and creates more air drag compared to all the other outside surfaces of the shape.

My tail is planned to be 48 inches long, unless I hear back from the DOT before hand.

Metro made his tail 51 inches I believe, and his results speak for themselves.

The overall car length will still be shorter than a typical short-bed pickup in length, so I'm going to go for it, and keep the full length!!

With the tail extension, I am looking for at least 90mpg results (at my typical speed). OK, maybe throw summer temps in there as well, for good measure.

Jim.

Jim,if you can compare the area at the back of your new tail to the area of the back of the Insight,it will give you an idea of your new Cd.The profile drag varies directly with wake area.

[3-Wheeler]

Originally Posted by RobertSmalls

Jim,

Darin's tail is 62" long.....

Hi Robert,

Thanks for the clarification. I should have looked directly at Metro's picture on the first page of this thread.

He has the dimensions laid out a clear as glass, and somehow I missed this.

I do remember Metro quoting the 51 inches in his tail extension post however, and that stuck in my memory!!

If the Insight ends up the same length as a minivan, then parking after this summer will be more interesting.

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Metro, did your extended tail drag on the pavement when going up driveway aprons? Just trying to get a feel for how much clearance is needed.

Jim.