Ford Escort ZX2 Aerodynamics

[freebeard]

The stock height for a Escort ZX2 is 52.3 inches, from Wikipedia. For easier math, I am going to assume that I have a 2" drop along with 1" smaller tires(0.5" drop from stock), giving me a ride height of ~50". So I need to shoot for 200" or 16' 8". The car is already 175.2" long (14' 7"). According to Hucho, I need to add only 25" of boattail! The full length boattail puts me closer to 5X.

Fineness ratio seems like the roughest possible guide to optimum ...For an ideal half-teardrop. As you get away from that shape, reality intrudes. The biggest deviation from ideal is the width over the rear tires. Ignoring upstream influences, you'd want the rear 70% of the teardrop after that point. :yikes:

Compare Mair's torpedo with Hucho's teardrop.

Here're examples of fitting the (admittedly squashed and stretched) teardrop, with it's truncations, to the body above and below the beltline, separately:



You could consider a Kammback/boxcavity for the greenhouse (for sightlines) and a truncated teardrop for the tail.

[BabyDiesel]

Originally Posted by freebeard

Fineness ratio seems like the roughest possible guide to optimum ...For an ideal half-teardrop. As you get away from that shape, reality intrudes. The biggest deviation from ideal is the width over the rear tires. Ignoring upstream influences, you'd want the rear 70% of the teardrop after that point. :yikes:

Compare Mair's torpedo with Hucho's teardrop.

Here're examples of fitting the (admittedly squashed and stretched) teardrop, with it's truncations, to the body above and below the beltline, separately:

You could consider a Kammback/boxcavity for the greenhouse (for sightlines) and a truncated teardrop for the tail.

70% after the rear tires?! Umm, that can't be road legal if I add that much to the rear of the car... lol.

I could not find Mair's torpedo. Do you have a picture?

That is an idea! I am curious to know what kind of drag reduction I would get with a setup such as the one you describe. I would hope for less than 0.20 Cd.

I made some *epic-fail* front skirts today. I am embarrassed to post them, they are that bad

I am doing coast down testing tonight! We will see just how much improvement I have done to the ZX2. Stock Cd is 0.33, I am hoping for >0.3 I will be using the method spelled out here -> Measure the drag coefficient of your car

[BabyDiesel]

Very crude work with Microsoft Paint. Is this what you were describing in your previous post, freebeard?

[freebeard]

That's good as far as it goes. Remember you're dealing with a 3D shape. Here's what happens when you put a teardrop over a Beetle (the only examples I have ):



Two blisters added to the teardop would cover the roof areas, but look what happens below the doors.



This is what I have on Mair. I think it's ambiguous. Is l the distance to the change from curved to straight or to the truncation?

In any case whereas the teardrop has a constantly changing tangent, Mair was making torpedoes with a henispherical nose cap, and arbitrary body length (it could be a train), and a tail end that goes conical when it reaches 22°.

Good luck with the tests.

[BabyDiesel]

What software do you use to show 3d images such as your Beetle? I would love to use it to get a better idea of how the rear is going to play out. 2D only gets me so far.

It appears that L starts when the curve is beginning. So as far as a bluff body vehicle as my ZX2, L would start at the ends of the bumper, perhaps after a Kammback to get the body shape? The distance of L is what is added on IMO. The 22* conical tail end would be the sides from looking from the top I presume?

I have the coastdown testing completed! I will post the results later in a different thread, and add a linkback to this forum.

[freebeard]

I use Wings 3D (free) to model the aeroform and any unrendered pictures (you can see a grid on the ground and polygons), and Poser 7 (2nd hand) for the renders such as above. The Beetle model was $25 years ago, but there are frees one as well.

One free program that will do it all (with better lighting) is Blender; but boy howdy, does it have a learning curve. Wings has a right-click/fiddle/left-click/repeat work flow that is understandable by me.

There are free models of the Ford Escort available:

Google search
https://www.yobi3d.com/#!/search?q=f...xplorer%252098 They're not labeled but it's the fifth one in the third row, not sure how to download.
Ford 3D Models | 3D CAD Browser Ten different Escorts to choose from, you have to register to download, not sure what the 'box' requirement is

What you might do is take the line of separation at the rear and extrude it back and shrink it to follow the template curve, rather than the half-circular shape of the template itself. Add Dodge Charger sail panels for the Kammback.

Mair was working with torpedoes but the 22° would apply to the top and sides.

[aerohead]

Originally Posted by BabyDiesel

70% after the rear tires?! Umm, that can't be road legal if I add that much to the rear of the car... lol.

I could not find Mair's torpedo. Do you have a picture?

That is an idea! I am curious to know what kind of drag reduction I would get with a setup such as the one you describe. I would hope for less than 0.20 Cd.

I made some *epic-fail* front skirts today. I am embarrassed to post them, they are that bad

I am doing coast down testing tonight! We will see just how much improvement I have done to the ZX2. Stock Cd is 0.33, I am hoping for >0.3 I will be using the method spelled out here -> Measure the drag coefficient of your car

Here is a depiction of Mair's boat tail research model without the cylindrical section.

Here is the full monty

Along the 'with skinny tires' line you can read off estimated Cd as a function of tail length.
Here you can see the relationship between fineness ratio and drag.Half-bodies based upon streamlined bodies would have about 2X the drag in ground proximity.

Here you see how the drag varies as a function of tail length on the M-B C-111 III

Here is Fachsenfeld's 'extensible' boat tail of the mid-1930s which Hucho felt the automotive community ought to revisit.

Here is the Land Speed Record EV1:lowered,taped up,flush wheel covers,and boat-tailed.The car was reported at Cd 0.14.

Here,Buchheim et al. has used a bit more aggressive contour with their Cd 0.14 VW-Flow body (long-tail)

The same car with truncated tail doesn't gain that much drag

[aerohead]

Originally Posted by freebeard

That's good as far as it goes. Remember you're dealing with a 3D shape. Here's what happens when you put a teardrop over a Beetle (the only examples I have ):



Two blisters added to the teardop would cover the roof areas, but look what happens below the doors.



This is what I have on Mair. I think it's ambiguous. Is l the distance to the change from curved to straight or to the truncation?

In any case whereas the teardrop has a constantly changing tangent, Mair was making torpedoes with a henispherical nose cap, and arbitrary body length (it could be a train), and a tail end that goes conical when it reaches 22°.

Good luck with the tests.

In Mair's diagram,the vertical hashed line is the beginning of the curvilinear portion,so the value of length is denoting the distance to the truncation from that point,as from the max camber point with the template.

[freebeard]

I was questioning the rightward extent of l —truncation or transition to conic section? Both? According my little brass caliper on my screen the l/d illustrated is 11 over 13 eighths or 1.18. That puts it at about the rightmost dash after the rightmost hollow dot.

If the dashed triangle is ineffectual, just chop it off? Or round it off like a Dymaxion?

[aerohead]

Originally Posted by freebeard

I was questioning the rightward extent of l —truncation or transition to conic section? Both? According my little brass caliper on my screen the l/d illustrated is 11 over 13 eighths or 1.18. That puts it at about the rightmost dash after the rightmost hollow dot.

If the dashed triangle is ineffectual, just chop it off? Or round it off like a Dymaxion?

The table shown by Mair does not include skin friction,so it kinda misrepresents reality.
*With zero boat tail mair's body was Cd 0.204
*With a tail truncated at the hashline Cd 0.122
*With the full tail, Cd 0.066
--------------------------------------------------------------------------
Without the skin friction,the casual observer would imply that the rest of the tail is of little value,however,in free flight,it would mean almost a halving of the drag.