Designing a tail extension for the first-gen Insight
 

At least three members here have expressed an interest in building a boat tail for their 2000 Honda Insights, so here are a few of my notes on what such a boat tail would look like.

I started with the side-view image from the InsightCentral encyclopedia, and checked the existing angles of the car's rear end. It's about 8° in the vicinity of the antenna, 12° around the middle of the hatch glass, 14° for the rear third of the glass, and it could plausibly be 18° at the spoiler.

http://ecomodder.com/forum/attachmen...1&d=1265253883

Here's a picture where I extended the roof at 18° and the bottom of the rear bumper at 7°, and truncated where it is 15.5cm tall for the license plate. This results in a lengthening of 1.48m, making the car 5.42m long. For comparison, a 2010 Accord is 4.93m long, and a F150 short cab short bed is 5.41m long.

The 18° and 7° angles are from EM conventional wisdom, which may be from Hucho's book. However, I won't believe it for certain until I see it in a wind tunnel or on a prototype. Particularly, what's the relationship between the angle of the bottom of the tail and Cd?

The design of the top is easy, and the bottom is fairly straightforward, too. But what will the sides look like? How far forward will the tail extension need to start? The rear is rounded, and it's not clear exactly where flow becomes detached.

http://ecomodder.com/forum/attachmen...1&d=1265253883

Btw, the rear of the tail will be about .8m wide, which is almost large enough for stock tail light housings and a license plate, but I was really hoping to leave the stock tail lights where they are.


My next step is to create a 3D model of the Insight, add a few different styles of boat tail, and see what happens in the SolidWorks FloWorks simulation.

I found a program to convert a .JPG into a .DXF that SolidWorks can import as a sketch, and I made .DXF's of the top, side, and front views. I'll combine them into a model some time soon, and I'll make the files available if anyone is interested.

Hi Robert,

It's funny, but I have that same sketch from Honda, and was going to use it in a similar manner...

The 7° from the underside of the car might be more of an issue.

For this large of a taper, there has to be a large amount of air entering the stream under car, and then expand to fill the void created by the 7 degree taper. I hope your model shows that you have enough air volume to play with there.

The stagnation point at the front of the car typically does not let that volume of air underneath, and thus could "starve" the taper and not expand fully, thus causing a partial vacuum and resultantly cause unwanted turbulence there.

Hopefully the model will show if can expand at that rate on the bottom side or not.

__________________________________________________ ________

I also wrote an email to my DOT safety inspector, and asked if there are any either State or Federal restrictions for the extended length of the tail on the back side of the car.

If I spend that much time planning and building an extension, I want to be darn sure that all bases are covered legally, and not get a big surprise if I am pulled over for an adhoc inspection.

I know for Class II motorcycles (3-wheeler's), there are very specific requirements for tail lights, reflectors, running lights, etc.

Jim.

Two things to keep in mind that might allow you to shorten that tail. Using a gently curved rather than a straight line allows the air flow to remain attached to a somewhat higher angle. Also, if the tail is tapered in from the sides as well as vertically, this reduces the volume of air that is needed to fill in under the tail.

I believe it's supposed to be 2.5˚ max, no?

You can also chop the end 1/3 off to keep it shorter and still retain roughly 90% of the benefit.

Cool! Subscribed.

The underside of the Flea's boat tail angled up to 6.5 degrees, and retained attached, if turbulent flow to the trailing edge, documented in the tuft testing video.

http://ecomodder.com/forum/attachmen...1&d=1264546553

I think basjoos is right: you can get away with steeper angles if you have adequate radii between sides/top and sides/bottom plus taper in both plan and profile. That would permit pressure recovery from the sides, and the bottom angle can be increased.

My taper used predominantly straight lines on the top & sides, but if you arc gently through the taper, you can go up to 22 degrees and retain attached flow, says Mair (says Phil). The Insight has decent enough upstream aero that I bet it's possible on your cars.

The challenge in construction for you guys will be the unusual cross-sectional shape at the rear of the car. You've got a lot more taper already in progress in the greenhouse at the transom than you do at the height of the rear wheels.

Insight
 

If you'll go to "FLOW-IMAGES" Photo-Gallery-2,on the second row,you'll see your car in the wind tunnel under smoke.
If you'll notice,the flow is actually separating a smigeon over the hatch.
If I were to do a tail,I believe I'd decrease that angle a bit.
Darin put the Insight under the first template and you can see that the back of the car is a little too "fast."
I agree with the others about gently curving the tail.It will be stronger.Because of that she can be lighter.And she'll aerodynamically superior.
The template can also be used to help design the sides.

Link for the above ^^

http://ecomodder.com/forum/showthrea...uft-11504.html

http://ecomodder.com/forum/attachmen...p;d=1261259019

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.

http://ecomodder.com/forum/attachmen...1&d=1265337618

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.

http://www.insightman.com/mag_photos...windtunnel.jpg

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?

Hello Robert
Just an idea...remove the rear bumper facia to anchor your tailcone.

http://yves.fungiart.com/images/insi...r/rear12_2.jpg


http://yves.fungiart.com/images/insi...per/rear11.jpg

This is a link where I got the scale drawings

InsightCentral.net - Encyclopedia - Honda Insight Scale Drawings

There is a photoshoped image of a red Insight with a streamlined trailer. i'l post when I find it

More aero
http://www.insightcentral.net/_image...ar-airflow.jpg

Good luck!:thumbup:

troglodite aero
 

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!

normal
 

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.

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.

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.

Not as useful as I expected
 

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?

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.

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.

blowup
 

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.

Cd
 

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.

Hi Robert,

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

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.

__________________________________________________ ____________

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.

Re: the length of my cardboard boat tail:

The tail as you see it only adds roughly 4.5 feet to the overall length of the car, beyond the OEM bumper, including the tail lights glommed on the tail's transom.

The 62.5 in. measurement you see in the diagram was just there to provide a reference to another Metro owner who inquired - it shows the length from the forward edge of the rear taillight assembly to the tip of the tail.

There's still a fair amount of "car" underneath the forward end of my boat tail as it appears in profile.

True, but we don't (and maybe can't) know the contributions of profile drag vs skin friction, nor how much of the profile drag happens at the rear of the vehicle.

Come to think of it, maybe you can estimate skin friction's contribution to Cd, by looking at the Cd of a flat plate in turbulent flow. That would give Cd=0.03 from skin friction, for any object that is roughly shaped like a compact car, assuming 100% turbulent (and perhaps also 0% detached) flow.

If a tail increases surface area by a third, you'd be looking at Cd=0.04 from skin drag, and the rest is form drag. But don't expect to reduce profile drag to zero by reducing transom area to zero. Basjoos achieved Cd=0.17 with a tail and other mods, and any car shaped just like GM's race prepped Impact with tailcone will have a Cd=0.153.

Darin, did you record before and after transom area and CdA on your Metro?

I didn't try to measure the "after" Cd.

But you can get the transom area of the boat tail from this photo:

http://ecomodder.com/forum/attachmen...1&d=1264546553

I'll take a photo tomorrow to estimate the transom area of the car without the tail.

Also, I doubt you're going to see any reversed flow on the end of the hatchback (upper) glass. Did you get a chance to try it (tuft testing)?

The smoke trace shows boundary thickening, but I can't see how there would be separation/reversal/recirculation there.

Note also the smoke trace at the front of the car doesn't start on the surface of the hood, so one would expect a gap at the end as well.

There is energized airflow here. The whole length of my rear window stays clear of rain at speed.

http://us1.webpublications.com.au/st...0/3061_8mg.jpg

Five AirTab vortex generators were placed across the trailing edge of the rear hatch.

Julian Edgar


Also a little information about Insight bellypans and rear tire airdams.
autospeed.com

http://us1.webpublications.com.au/st...110872_7lo.jpg

I tuft-tested today. The angles of the stock glass are fine. Yarn mostly lays straight back at 64mph, but the tufts near the left and right edges of the hatch glass dance around more than the others. The mirrors might be partially to blame for this.

I will build something similar, but a fairing BEHIND the tire would have a larger effect.

I agree that behind the wheels would be better. I think those wedges have a few problems. One is they would create vorticies on either side if they were parallel to the airstream. Two is they are probably not parallel to the airstream. Three, they probably create a restriction, forcing more air between the tires but not in a good way.

drag
 

No doubt things have changed since the 1970s.Back then,profile drag constituted 55% of drag,skin friction was 7-12%,the remainder locked up in cooling sys. drag,interference drag, and induced drag.
With respect to the rear,if you're finished modifying the front,then the rear is where the rest of the drag is.The only way you can eliminate it is with a full boat tail.If you reduce the transom to zero you will reduce profile drag to zero if you stay on the template.
With respect to basjoos car,it can still go 0.08 lower.
The EV-1 could go 0.063 lower.

AeroHead,

Now I'm curious...

Would you care to elaborate on what you would do?

Thanks, Jim.

elaborate
 

Jim,this was one of my quirky thought experiments and I hope basjoos won't be offended.This is what I did.
From a side shot that basjoos provided I drew in a ground plain under the tires.With a French-curve I extended the apparent hatch line rearward and projected a prolate ellipse forward to capture the nose of the Civic to ground level.
This established a 'template' around the car,as Jaray,Kamm,Lay,and others had done with development vehicles.
Locating the 'top' of the roof camber I was able to establish the position of frontal area at 44% of overall body length,aftbody constituting 56% of body length and L/D= 2.416.Exit angle at zero ground clearance is 35-degrees,and the exit angle at the boat tail 'hatch' is 26-degrees.
If I stretched only the aft-body out to 1.78-D ( D is 2H ) which is the aft-body of the 2.5:1 streamline body,the new point at ground level to define the 'effective' tail would produce Mair's 22-degree angle.
The intersection of this new template with the line of the undercarriage projected straight back,would define a body with Cd 0.12 for a standard Civic.
NOTE: The diffuser portion would not respect SAE's 10-degree break-over angle and this part would have to be 'active',or the tail unit would have to comprise a trailer rather than a full cantilever tail.
I've brought nothing new of my own here.Kamm's full tail produced Cd 0.15.Jaray's,with a front airdam got Cd 0.13.Walter E.Lay produced three cars with Cd 0.12 this way.AeroVironment got Cd 0.089 with wheel fairings on the Sunraycer,Honda Dream-2 got Cd 0.10.
I believe that a convex windshield,careful cooling system ducting,fairing of the tires and the tail mods would push the Cd to,or below 0.10.
Practical?

How about removing any extra weight from the car such as the hatch, most of the interior, dash, A/C, Seat/s with light one, door panels, carpet, battery in the back with all the components? How much weight will come of those parts?

Although that would save weight, it would not affect aero, so it really is off topic. But if you want to start a thread about it, I'll weigh in on how much weight you'd save and why some of those components pull their own weight.



SAE J689 Curbstone Clearance, Approach, Departure, and Ramp Breakover Angles?Passenger Car and Light Truck - IHS, Inc

Shouldn't a standards document be in the public domain?

Phil, do you know the rest of the relevant angles and clearances recommended by the SAE? And how low do you think the Insight's Cd could go within those limits, and without any drastic reconfiguration of the windscreen, etc?

Are you planning to respect the SAE recommendations?

I think they're just that - recommendations. (Look at any super high performance car for examples clearance/approach/departure angle "violations".)

Violations would require a more vigilant driver, and would restrict the car from some roads/driveways.

standards
 

Robert,I think they're in the seminars or sticky on mod data.
They're easy so here goes: Approach--------- 16-degrees
Breakover--------- 10-degrees
Departure--------- 10-degrees
As Darin has commented,these are minimums so angry consumers won't be going Alaina Bobbett on the automakers should parts of their cars get knocked off or scrubbed away.If we're ever watchful,we can cheat the mins a bit.
P.S. You would want to 'ballast' the car to at least 300-pounds when measuring to simulate its ride height when loaded.

0.04
 

Robert,I looked over Hucho's 2nd Ed. He gives Skin friction component of profile drag for typical car @ 0.04,so your value is dead on.He goes on to say that it's basically a constant due to Reynolds Number effect.
Cutting drag by lengthening is the only thing he says makes a difference,and form drag benefit far out weighs the skin friction penalty until you go beyond the aftbody dimensions of the 2.5 L/D tail stucture.

Overlaying the AeroHead Aero Template on the Insight, and then importing into a small CAD program, the following angles of curvature were measured...

http://ecomodder.com/forum/attachmen...1&d=1266371501

Since Robert did some tuft testing, and the yarn stuck cleanly to the rear glass, that means that our "angle-of-departure" can increase to match the Insight rear hatch angle and still be called good, compared to the rate of change for the Aero Template.

http://ecomodder.com/forum/attachmen...1&d=1266371501

After measuring the rear deck angle on the Insight, I was somewhat surprised to find out that the angle does not approach 18° as first thought. It does instead, match the angles of the Aero Template rather well.

When starting to layout the angles-of-departure for the tail extension, I will make a master template from foam board and use this to layout angles for the top and sides of the extension, and the underside will certainly be stepped to achieve at least 6.5° as Metro has previously mentioned.

Jim.

Well, it didn't stick cleanly to the glass. It fluttered around a little, especially near the left and right edges of the glass. But yes, the angles will do just fine.

Jim, would I be correct to say Honda used a more aggressive template than Aerohead's? I see you've properly aligned the template with the ground and the apex of the roof. But you should be able to get away with sliding it forward and upward until the 14° part of the template lines up with the 14° part of the hatch glass.



I made a CAD model in Pro/E by projecting the Insight's top, side, and rear views. As expected, the result is roughly the shape of the Insight, but lacking detail in a few areas. It should be fine for looking at flow over the top of the car, but less so in the vicinity of the tires and beneath the car.

http://ecomodder.com/forum/attachmen...1&d=1266376535
http://ecomodder.com/forum/attachmen...1&d=1266376535
Adding a boattail to the model is as easy as drawing it in the top and side views, then radiusing the edges.

300-pounds
 

Jim,don't forget to borrow 300-pounds of family or neighbors for the front seats when you do your parametrics,so the 'rake' will mimic its driving ride inclination and not throw off your angles.
Or sand bags,bar-bell weights,large napping German Shepherds.
Nice photo-shop work!

I forgot to add, could someone post (a link to) the template? I'd like to trace out a tail with it.

template
 

Robert,it should be in the search engine above at Aerodynamic Streamlining Template Part-C