Ford Escort ZX2 Aerodynamics

[freebeard]

That's a lot of skin friction. Time to break out the shark skin.

[BabyDiesel]

I read redneck's boat tail thread and saw something interesting. He stated that 50% of the length will give 90% of the results. Was he correct in saying this? The above models of the truncated tails seem to point to yes.

Speaking of shark skin, would having trip strips laying perpendicular to airflow along the boat tail have any positive effect? Is a TBL wanted at this point, or are we teying to have the airflow go closer to laminar?

[aerohead]

Originally Posted by BabyDiesel

I read redneck's boat tail thread and saw something interesting. He stated that 50% of the length will give 90% of the results. Was he correct in saying this? The above models of the truncated tails seem to point to yes.

Speaking of shark skin, would having trip strips laying perpendicular to airflow along the boat tail have any positive effect? Is a TBL wanted at this point, or are we teying to have the airflow go closer to laminar?

I have scans ready for NASA's boat-tail research.They're full boat-tail netted Cd 0.238,while their truncated tail netted Cd 0.242,not much of a gain in drag.
This is getting into what Prof. Alberto Morelli referred to as 'phantom' tail phenomena.
Once the rear flow is cambered down,inwards,and upwards with the boat tail,there is a length at which you can truncate the tail with zero drag penalty.
The quasi-annular mass of boundary layer,when shed at the truncation stuffs the wake with a mass of air which the inviscid outer flow just skips over,as it would with a locked-vortex.
Dornier Aircraft Corporation was the first to discover and patent a wing based upon this principle in 1920.Technically,you don't want to remove more than 5% of the overall length to enjoy the zero penalty.
here's the formula for wing truncation drag

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With respect to trip strips,if the aft-body is to the 'template' contour,any roughening would only increase drag.The boundary layer is already fully turbulent which allows fully laminar flow in the inviscid layers 'above.'
If the contour is hypo-template,then some sort of boundary layer stimulation could help with separation.This has been researched in Europe for the U.S.Air Force with respect to the C-130 Hercules' fuselage radical up-sweep separation issues.Sub-boundary layer-thickness VGs,placed from empirical testing allowed some good effect.

[aerohead]

Full-tail = Cd 0.238

Truncated tail = Cd 0.242

Here you can see the phantom tail with a streamline body of revolution in free flight

[BabyDiesel]

Aerohead, I found the scholarly article that mentions the C-130 fuselage. They were working with scale models - http://enu.kz/repository/2009/AIAA-2009-90.pdf

From Kosin & Lehmann's phantom tail research, my math showed that 70% of the original length gave a 12.12% increase in drag. I'm trying to figure out how exactly Hoerner, 1951 got the Cd of 0.158 with what appears to be only ~40% of the tail cutoff, when it takes 70% of the tail to be cutoff to equal a Cd of 0.169 in Kosin & Lehmann's data?

It seems that the more "aggressive" contour receives a greater drag penalty with a truncated tail than the less aggressive contour. The more aggressive contour is better for street driven vehicles due to length. Would there be a happy medium between these, or is the Aero Template the best to use for our daily drivers?

[BabyDiesel]

Need to mention that I have made progress on my front wheel skirts and they should be on today

Pictures!






It is an simple design for testing purposes. The tabs are 1/8" aluminum bar pieces bent and drilled into the fender. They will act as stoppers to the skirt, keeping it from contacting the tire and hopefully making them flush with the fender. I have the tabs arranged in such a way that the front sits inside the fender more than the back. This leaves a gap that low pressure air can escape from IIRC. The skirts are nothing more than painted coroplast (have been repainted & they are my old rear fender skirts!) and aluminum bar. The bar is positioned to be the same height as the notch on the sides of the fender. This is where the tire will contact the skirt at, so I am using plastic cutting board that will be attached to the aluminum bar. This should provide a slick surface that will protect the tire and skirt while still being inexpensive

FYI, I will perform coast-down testing to determine any aerodynamic benefit when I get back from vacation next week. Hoping that they will put me in the 0.25 Cd range

[aerohead]

Originally Posted by BabyDiesel

Aerohead, I found the scholarly article that mentions the C-130 fuselage. They were working with scale models - http://enu.kz/repository/2009/AIAA-2009-90.pdf

From Kosin & Lehmann's phantom tail research, my math showed that 70% of the original length gave a 12.12% increase in drag. I'm trying to figure out how exactly Hoerner, 1951 got the Cd of 0.158 with what appears to be only ~40% of the tail cutoff, when it takes 70% of the tail to be cutoff to equal a Cd of 0.169 in Kosin & Lehmann's data?

It seems that the more "aggressive" contour receives a greater drag penalty with a truncated tail than the less aggressive contour. The more aggressive contour is better for street driven vehicles due to length. Would there be a happy medium between these, or is the Aero Template the best to use for our daily drivers?

You might need to help me out with images.
One thing to bear in mind,is that the Lehmann/Kosen streamlined body is of larger fineness ratio with Cd 0.066,whereas Hoerner might have been using a 2.1:1 streamlined body of Cd 0.04 in free flight.
In ground proximity,as half bodies,the longer body could have no lower drag than Cd 0.132,while Hoerner's would be Cd0.08,due to the 'ideal' pressure drag/friction drag bias of the shorter body.
When wheels are added it alters the drag/length relationship.
The drag trend of the truncation should behave the same as a function body length,but the dissimilar surface friction qualities and overall drag differences of the two bodies aggravates the comparison,not to mention the effects of the wheels.
As to the 'happy medium',Kamm and Fachsenfeld advocated a streamlined body which was truncated at a 'practical' length (where wake = 50% frontal area).This would allow for moving in traffic,as well as parking and driveway ramps.
Then Fachsenfeld came up with the idea for the extensible tail which would only deploy on the highway.
It just comes down to how much length you can live with,or add when needed.
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The 'template' is primarily for aft-body streamlining.Since it is based upon the lowest drag streamlined body which also respects down-slope, in-slope ,and up-slope contours incapable of producing separation it's a safe bet for modding.
23-degrees is the absolute maximum tangent angle which can sustain attached flow according to PhD Rolf Buchheim et al,from VW research.
Here is their profile

I'm playing safe at 22-degrees,according to Mair's research.It can't fail.
Again,just chop off as much tail as you like.

[aerohead]

Ah! I just did a big head-slap!
That table is for 2-dimensional flow wing sections, not 3-dimensional flow bodies.
Wings can't tolerate more than a 5% truncation without serious drag penalty.
I did this table for the motorcycle guys primarily.If you shorten a wing,never truncate it,go ahead and do a 'fast' curve to a point.
Here's something to compare to

But don't do this

Do this

[freebeard]

Skirts on and off for testing should be OK, but I wouldn't go trying parallel parking with them on. Any impact will tear the tabs off the fender. An easy upgrade would be to connect the four tabs to the skirt with a 4-bar linkage. The bars only need to be longer than the amount you need the skirt to move. So, 1 to 1+1/2 inches? Two fasteners on each bracket so it won't rock.


...if I hadn't lost this model, I'd go back and finish it.

About the 'virtual' boattail I suggest the Coanda nozzle. A zoomy example here:


embodied here, lower left:


If you're working with exhaust, a bell-mouthed exhaust tip in a concave stainless steel bowl in/as the truncation.

[BabyDiesel]

Thank you for all the explanations aerohead!

freebeard, I will plan out this 4-bar design while I am on vacation this week. I won't be back till Sunday or Monday.

The wheel skirts failed beyond miserably As soon as I turned onto the road, the back fell off the rear tab and it was in contact with the sidewall for 1/2 a mile, aluminum rod and all. If I had done my 4 mile test, the tire would have blown out!

It turned out that the hinge is flexing and not keeping the skirts within its tolerances. So this idea is done, and I will be driving to the beach with my same 0.269 Cd lol.