Designing a tail extension for the first-gen Insight

[discovery]

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?

[RobertSmalls]

Originally Posted by discovery

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

Purpose—The purpose of this document is to provide minimum static design guidelines for curbstone clearance, approach, departure, and ramp breakover angles. This is to minimize damage, if any, in normal vehicle use conditions. This document also encompasses all current worldwide regulations and requirements.

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?

[MetroMPG]

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.

[aerohead]

Originally Posted by RobertSmalls

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?

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.

[aerohead]

Originally Posted by RobertSmalls

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?

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.

[3-Wheeler]

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



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.



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.

[RobertSmalls]

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.



Adding a boattail to the model is as easy as drawing it in the top and side views, then radiusing the edges.

[aerohead]

Originally Posted by 3-Wheeler

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



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.



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.

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!

[RobertSmalls]

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

[aerohead]

Originally Posted by RobertSmalls

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

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