Aeromodding a Yugo (Zastava Koral)

[yugomodder]

Hi I'm a fairly new member here, but I've been a lurker for a while. I started a thread over in the Fossil Fuel Free section about converting a Yugo to an EV, but I think I got a little too far into the aerodynamics there, so I'm starting this thread. Here is the other thread if you'd like to check it out (http://ecomodder.com/forum/showthrea...ugo-20862.html)

Ok, so the big problem areas I see on a Yugo starting from the front are it's front end which slopes down toward the ground, then cuts off abruptly so that the air hits the ground, and then bounces back and forth in a turbulent manner with the underside which has no bellypan(what I imagine happens, haven't actually seen a CFD simulation or tuft testing in that area), then the back end cuts off at an awkward angle with no Kammback, the rear has no diffuser probably because it doesn't have a bellypan, and obviously the usual smooth wheel covers, rear wheel skirts, etc would be put on to reduce drag.

I downloaded a model of a Yugo in google sketchup to try and model some of these modifications. *note model of Yugo not most geometrically accurate model of any car ever*

I took some pictures of different Kammback, nose, bellypan, diffuser, etc models. I'll post these in the next posts so it's a little easier to read and digest.

[yugomodder]

This is the unmodified model I downloaded



Here is the model with a kammback according to template, as well as a bellypan, and diffuser. The nose has also been lowered to the level of the rest of the car. Note: white areas are where modifications have been made.







Next post will include different nose designs.

[yugomodder]

Here are different nose designs I was considering. Note: sharp edges would be rounded, it's just difficult for me to model them in google sketchup.


So this design is just sloping directly upward from furthest point forward on the car to the wipers.



Note: once again all edges would be rounded off.

Next post discusses how to apply the template to this car
This next design is a squared off nose which gives the most room for batteries, and doesn't slope the wrong direction or have an air intake like the original nose



This next design is simply curved



Looking at it a bit more I thought that this next design would give me the best aero/storage space

[yugomodder]

Ok, so I know the idea of an aerodynamic top for a car has been discussed but largely dismissed, but just hear me out. The template tapers slowly at first, but then later speeds up. If it's just a few degrees for the first part I could build an aerodynamic top that is just slightly taller than the existing roof, but then taper it down, and the accelerating taper coming of the back end would leave a smaller wake than the current Kammback design. I'm not sure how this could affect CdA and I guess I could test it eventually, but for now it's just a though experiment and I'd like to hear some theories as to what the effect would be.

This is a picture of the current Kammback with the template overlayed. I think it matches fairly well.

This is a picture of the current Kammback with the template overlayed as if the windshield were slightly higher and the taper started closer to the front of the car. As you can see, the template in this case would go below my current Kammback.


This last image is what the car would look like with a streamlined top according to the template.


Now to me this seems like it would at least reduce Cd, but I'm not sure about CdA. Also I'm not sure how I would get the hatch to work properly in this configuration if there even is any aerodynamic benefit. I just thought it was an interesting thought experiment, and I would like to share it with everybody.

Note: The aerodynamic top adds 1.75-2inches to the height of the car, but the tail comes down to 3.75-4 inches below it's position at the other Kammback. This means the top would add about 2% to the frontal area which means if drag is the only concern it would have to have a Cd that is an additional 2% less than that of the original Kammback. You could possibly get some more headroom if you re-did the interior a bit but you would also have slightly less glass out the back to see through, so there are trade offs either way.

Now that 2% less isn't just two percent, if the normal Kammback nets a 6% gain, then 2% is 1/3, meaning the new Kammback must be at least 1/3 more effective. The original Kammback cuts about 2in off the height of the wake area, where as the new one cuts 6inches off the height of the wake area. If it's all about the amount of wake area then the new Kammback should give three times the Cd reduction of the original one. Now there are other factors, but that's promising. If the frontal area actually increases by about 2% and the Cd is reduced by about 18% (6%*3) that would give a total new CdA which is 84% of no Kammback (1.02*0.82=0.8364) and about 89-90% of the other Kammback.
The front part coming off the windshield may help alleviate any small separation that occurs at the top of the roof, along with providing smoother airflow from which to streamline which could possibly net a small drag reduction.
*warning all of this information is deduced through logic based off of numbers which may or may not apply to this or your situation. Your mileage may vary*

Please let me know what you think!

[yugomodder]

Please let me know your thoughts as to what changes I could make, as well as your estimates of how much these modifications would reduce drag.
According to the 65+ modifications; even a simple grill block should decrease drag by 5% or so, a simple belly pan should do the same, as well as a diffuser, and a kammback should give 5-10%. This nose redesign should give considerably more drag reduction than a simple grill block, especially considering what I'm starting with, the bellypan should be able to be very flat considering there is no longer an exhaust or other ICE parts, and I should be able to make a reasonable diffuser and Kammback. Now considering a stock Yugo has a drag coefficient of around 0.4, this isn't going to be the most streamlined car ever. But then again I'm not going for a 0.17 Cd like the aerocivic or anything like that but some drag reduction would be extremely helpful.

If I could get the drag coefficient down to normal car territory it should really help alleviate load from the batteries.

[aerohead]

Originally Posted by yugomodder

Ok, so I know the idea of an aerodynamic top for a car has been discussed but largely dismissed, but just hear me out. The template tapers slowly at first, but then later speeds up. If it's just a few degrees for the first part I could build an aerodynamic top that is just slightly taller than the existing roof, but then taper it down, and the accelerating taper coming of the back end would leave a smaller wake than the current Kammback design. I'm not sure how this could affect CdA and I guess I could test it eventually, but for now it's just a though experiment and I'd like to hear some theories as to what the effect would be.

This is a picture of the current Kammback with the template overlayed. I think it matches fairly well.

This is a picture of the current Kammback with the template overlayed as if the windshield were slightly higher and the taper started closer to the front of the car. As you can see, the template in this case would go below my current Kammback.


This last image is what the car would look like with a streamlined top according to the template.


Now to me this seems like it would at least reduce Cd, but I'm not sure about CdA. Also I'm not sure how I would get the hatch to work properly in this configuration if there even is any aerodynamic benefit. I just thought it was an interesting thought experiment, and I would like to share it with everybody.

Note: The aerodynamic top adds 1.75-2inches to the height of the car, but the tail comes down to 3.75-4 inches below it's position at the other Kammback. This means the top would add about 2% to the frontal area which means if drag is the only concern it would have to have a Cd that is an additional 2% less than that of the original Kammback. You could possibly get some more headroom if you re-did the interior a bit but you would also have slightly less glass out the back to see through, so there are trade offs either way.

Now that 2% less isn't just two percent, if the normal Kammback nets a 6% gain, then 2% is 1/3, meaning the new Kammback must be at least 1/3 more effective. The original Kammback cuts about 2in off the height of the wake area, where as the new one cuts 6inches off the height of the wake area. If it's all about the amount of wake area then the new Kammback should give three times the Cd reduction of the original one. Now there are other factors, but that's promising. If the frontal area actually increases by about 2% and the Cd is reduced by about 18% (6%*3) that would give a total new CdA which is 84% of no Kammback (1.02*0.82=0.8364) and about 89-90% of the other Kammback.
The front part coming off the windshield may help alleviate any small separation that occurs at the top of the roof, along with providing smoother airflow from which to streamline which could possibly net a small drag reduction.
*warning all of this information is deduced through logic based off of numbers which may or may not apply to this or your situation. Your mileage may vary*

Please let me know what you think!

*if the windshield header has that sharp of a corner,then the roof addition would help maintain attached flow back to your new zero-point.
*the flow from there on back would have the benefit of the camber which Kamm would give the thumbs up to.
*my thought about the front is to leave the hood alone,but go after the actual nose of the car.
*from the hood top leading edge,you could project forward and down as you see with contemporary cars,with maybe at least(I want to say) 80mm radius.
* the nose should enclose the bumper,and the outer edges should wrap around with at least the 80mm radius as well.
*you could project the bottom of the nose down to the elevation where it intersects an imaginary line projecting from where your tire tread leading face hits the ground,up and forward at a 16-degree angle,as measured from the horizon.This will prevent a ground-strike with curbs and driveway ramps.
*rear wheel skirts aren't mandatory,but they'd help your Kamm boat tail work better.
*your roof/boat tail extension is dead on,so you're good there.
I don't see any reason why you couldn't match ,or come in lower than current production car Cds.

[yugomodder]

Thanks for the input Aerohead!
I went ahead and modeled what I think you are describing as far as a nose extension.

Here is at 16 degrees from the ground.


Here is shortened up a bit


The 16 degrees looks a bit long, but it shouldn't really give any problems. It looks to me like I could lower the whole thing a bit and get rid of some frontal area where the tires are, but still build a good nose extension.

This last picture is of another design I was playing with before.

I'm not sure exactly how much aerodynamic benefit this design would have, but it seemed to give more room for batteries. Obviously building a nose extension is going to provide far more room though.

As far as wheel skirts, yeah I'm planning on outfitting it with wheel covers and rear wheel skirts. I don't think it'd be worth it to put front wheel skirts on it before it's more streamlined. Also yes all of these sharp edges would be rounded off significantly, it's just difficult to model that in google sketchup and still be able to change around the nose design.


Next thing would be to lengthen the boattail if I'm lengthening the front end.

[yugomodder]

Here is how the car would look with a boattail that is slightly longer than the length of the nose extension.

This is it lined up with the template


This is with nothing behind it


If I'm going to lengthen the nose I'm also going to lengthen the back like this. I'm not sure as to exactly how long of a boat tail would be reasonable, but this seems like a good start.

Now I'd like to be able to mount this so that the hatch is still useable. I figure that I can create a "platform" by moving the bumper back like shown. I could build it with a frame and then either plastic, sheet metal, fiberglass or something over top. I'm not sure whether to build the sides so that they are part of the hatch or the rest of the vehicle. If they're part of the rest of the vehicle that should make the hatch significantly lighter. I'll try to model what I'm talking about to give a better idea. I'd probably want to move the rear glass to the back of the hatch instead of buying something else clear, and then knock out where the previous back of the car had been. This would add tons of space to the rear for cargo, batteries, or both. I'd have to come up with some sort of rubber gap sealer so that water doesn't get into the car, and to reduce road noise. Does anyone have any idea what I could use?

[yugomodder]

Note:sharp edges (except for those at the back of the kammback) will be heavily rounded.

This is what the side view would be like.

This is the top view showing where I think the hatch should be.


This is a rear view


I will also taper in the sides slightly (just a few inches or so) in this configuration to help reduce drag and prevent vortices from forming between the linear and angular flow.

[GRU]

It all depends on how much money you want to spend on your aero mods. The front is not a big deal, i wouldn't do too much cutting/welding to make the wedge shape you had in the 3rd post , you can just add a piece of plexiglass from the tip of the front bumper to do the tip of the hood so you don't have that "scoop" at the front but the biggest mods should be done to the underbody and back. Maybe make a adjustable kammback and do some tests to see what works best. I would cover both the front and rear wheels and add lawn edging all around the car