Aerodynamic Streamlining Template: Part-C

[Tesla]

Originally Posted by hotrodfeguy

This all makes a ton of sense, and I will give you the wind tunnel it would be money in the bank. All of your data is spot on gentlemen !!! But in real world how much is to much? My point is this, lets say your doing 55 and have a 30MPH side wind. It kinda goes to $hit, the long side, boat tail then becomes a negitive impact on areo. Not trying to be a killjoy. Just that I see some holes in some of this. Why double the side area or even at 25% of the car to gain, front to back areo? cause when you introduce a side wind it then becomes more area for drag. look at the car form angles like a diamond if you will, It should not be areo view from just one angle. wind soes not blow form one direction.

On my Driver metro I was going to do a kamback with no sides like a naked edged Kamback to get data then put sides on it to see if I have a real issue here with sidewind or not. I know the more improvements I have made(flat hubcaps, mirroe delete passenger side, wheel well covers, grill block, front air dam) the more the car tends to get shoved off the road by cross wind. And at the last few pages thats where this 22* may be affected, In my humble opinion it should be more agressive to cheat or be more universal to cross wind. I know there is a perfect forula for front back, but I am saying maybe in daily winds it should be more general purpose?

Maybe I am all out in left field. I could be totally wrong.

The first thing is you are not driving the vehicle sideways into a crosswind, there is no argument regarding crosswind effects, but you also need to consider the overall effects of crosswinds.
Remember, even without the tail you have a wake behind the vehicle that contributes it's own forces to vehicle handling, as soon as a crosswind hit's it blows the wake out and the vehicle rear jerks in response. There have been lots of reports of improved crosswind handling with boattails, this is probably due to the fact that the boattail has a smaller cross sectional area than the wake that was present without it.

There is a significant issue with getting too aggressive on rear taper as you may only get the added weight penalty without any aero benefit, you need to do a lot of reading around on things that have been done to get a gauge of how far you can go.

It seems to me this thing works in stages,
1 / Stuffing the wake, basically any "closed" volume in the rear, any shape within the template profile will reduce the size and intensity of vortexes, this may have some benefit to Aero by increasing the pressure in the rear.
2 / Short aggressive taper, this utilises the tendancy of the air to wrap around the vehicle, the airflow will seperate fairly quickly depending on how aggressive it is, but it will give the benefit of reducing wake area and also taking the turbulance away from the rear of the vehicle if a cavity construction is used. There is a guidline ratio of cavity depth to minimum width of around 12%, any less and the turbulance is still pulling on the rear surfaces, any more is of no further benefit.
3 / Borderline steeper template profile, can work very well, but is prone to seperation in adverse conditions, so creates wavering as air attaches and dettaches.
4 / Template, 22° and other rules observed is almost 100% guaranteed to give a positive aero result consistantly.
5 / Less taper, will perform well too, but is more than required, and some aero benefits have been forgone in not using a steeper angle, extra weight is added as well, but extra skin friction is so minimal it is a non issue.

Interesting your observations on crosswind handling, in my experience, I have gained improved handling, but the FE gains have been so minimal that I don't consider an improvement in that area yet. I feel that they are all part of a jigsaw puzzle and at some point I will have enough pieces in place and the FE benefits will fall into place.

[Sven7]

Originally Posted by ennored

Don't use the 0176 above QWERTYUIOP, use the numpad. I use it all the time at work, since I use temperatures a lot. But * is faster and we know what is meant.

Tried that on my laptop here (where the numpad is over letter keys) and it opened a History tab and a "save as" window. They say my generation is good at computers but sometimes I just don't know...

Originally Posted by tesla

if the body is 'tall' let width be the dominant factor for the taper

Exactly. That's why the width dominates my illustration. I know you weren't trying to cut it short, either

Tropfenwagen, top. Too bad for the unfaired wheels and other 1920's accessories that dragged it down.

Originally Posted by freebeard

If that doesn't work, press Shift-Option-8.

Is that Mac?

Originally Posted by aerohead

For roofline use of the 'Template',the full body height is the only parameter necessary,pin-pointed at the bodies location of max. camber.

I know we've been over this before, but isn't it important on wagons that the top of the rear portion doesn't "stick out" of the template? Ie, that we have to move it back until it forms a tangent with the trailing edge. Otherwise you have a vertical step before the boat tail starts.

[quote=aerohead]If the 'interference is so great as to be able to closely match top and sides we streamline the main body separately from the greenhouse,creating the 'combination form'.[/aerohead]

What do you mean by interference? Is that another way of saying pressure differential?

"advocated the tail truncation at 50% frontal area wake area to ease driving in traffic and parking issues."

How about you cut the tail in half and the smaller half moves in to nest inside the larger one when you need to park. Kinda like aero Russian nesting dolls.

[Tesla]

Originally Posted by freebeard

Yup.
My modelling skills don't support the blending you seek. That's the thought experiment, though; how to minimize the additional length.?

I actually meant that as to when you get to apply it in the real world on the vehicle, as for the model it is fine, my modeling is even cruder, but I can visualise the final result.

Originally Posted by freebeard

Edit: Tesla, weren't you talking about a trailer? That could spin off as a new thread.?

Yes, it is in the pipeline, just don,t know how long the pipe is at the moment.
The trailer idea was why I got onto this binge, the more I thought about it, the more I realised how much I didn't know, so I think I am starting to get a better picture of the guidelines and restrictions now.
I still need to finalise the design idea, then consider materials and construction methods, tie that in with utility and run it all back through the design again before I even consider picking up any tools, but it will be a seperate thread when I start the project.

Originally Posted by freebeard

No Coanda nozzles for you! How about a compound curve cap that hinges up like a trunk lid, over sliding van doors that meet each other at the apex of the boat-tail?

Everything I have read with regards to the nozzles puts them a couple of notches ahead of VG's, they do work, but careful design, testing and implementation is critical for a positive result.
As for the access to trailer load area, this is a major stumbling block with the boat tailing on a trailer as convention and practice dictates a big door at the rear for loading, so looking at different solutions for this and allowing some time to "look outside the box".

[Sven7]

Here's what I was talking about as far as preserving the radius. Really hastily sketched up in photoshop


wagon form by Tyler Linner, on Flickr

time for a movie... later!

[hotrodfeguy]

ok I want to bounce a idea off you areo experts. What if one was to make a scaled down wind tunnel out of a used fish tank(or equalvalant plexi) and take the ends off. Place small fan on one end. Use tufts and or smoke with small coffee straws to highlight flow patterns? By use of a air pump, for smoke output? This way we could rotate car at angles(5*,10* ect) to see how side wind would effect things. Cause when you drive when it's snowing out you can see how the cross wind changes the angle the wind comes at the car. Instead of coming right at the front it comes in at a angle sometimes up to 30*. Questions I have would be:

1) whats the right angle with cross wind?
2) maybe flat on opposite side is better if you were in a all out MPG race with cross wind from one side
3) Maybe a round bubble/bomber butt works at the end?

All crazy ideas I was thinking sorry if these offend.

[Tesla]

Originally Posted by Sven7

Tried that on my laptop here (where the numpad is over letter keys) and it opened a History tab and a "save as" window. They say my generation is good at computers but sometimes I just don't know...

I got it to work, make sure the Numerals are locked (Num Lock), otherwise the keys have shortcut functions, then pres Alt key and 0176.


[QUOTE=Sven7;353004]I know we've been over this before, but isn't it important on wagons that the top of the rear portion doesn't "stick out" of the template? Ie, that we have to move it back until it forms a tangent with the trailing edge. Otherwise you have a vertical step before the boat tail starts.

Originally Posted by aerohead

If the 'interference is so great as to be able to closely match top and sides we streamline the main body separately from the greenhouse,creating the 'combination form'.[/aerohead]

What do you mean by interference? Is that another way of saying pressure differential?

I think this is a case where a little knowledge is dangerous, Aerohead would intuitively know where you can cut corners and where you can't because of his vast experience in this field, he has said numerous times he put the template up as a safe, simple guideline to address rear aero issues, many vehicles fit the model well, some, particularly wagons and vans have the issue where max camber is usually at same point, around the drivers head position, but they have an extended roof line with a very shallow curve back.
I have no doubts that the template needs to be pushed back, the max camber dimension must still be used for the overall length, but the template should be slid back until it is just touching the rear roof line. How else could you apply it to truck trailers and rail cars etc.
Also in his reply a few posts back he mentioned about starting the side tapers at different points, depending on max width, so the greenhouse (window) portion may well begin at a different position in relation to the roof and the lower sides, this may result in 3 or more different scaled tapers, starting at different points, then the challenge is to merge these together as they converge by using a generous radius on corners and trying to observe the 22° rule.
The easiest and safest solution is to base everything on the largest dimension, but this also results in the longest tail, and does not provide minimal cross section if cut short. The ideal is the more complex multi taper, but you run the risk of breaking attachment with poor design, so mayme going for the ideal, and padding out a safety factor for the shorter tapers.
Don't know regards the inteference comment, he is deep within his knowledge and it sometimes sounds like code when he writes, I will understand one day.

Originally Posted by Sven7

"advocated the tail truncation at 50% frontal area wake area to ease driving in traffic and parking issues."

How about you cut the tail in half and the smaller half moves in to nest inside the larger one when you need to park. Kinda like aero Russian nesting dolls.

I have thought about that telescoping idea, maybe incorporating semi solid segments with an inflatable bag inside to expand them??

[Tesla]

Originally Posted by hotrodfeguy

ok I want to bounce a idea off you areo experts. What if one was to make a scaled down wind tunnel out of a used fish tank(or equalvalant plexi) and take the ends off. Place small fan on one end. Use tufts and or smoke with small coffee straws to highlight flow patterns? By use of a air pump, for smoke output? This way we could rotate car at angles(5*,10* ect) to see how side wind would effect things. Cause when you drive when it's snowing out you can see how the cross wind changes the angle the wind comes at the car. Instead of coming right at the front it comes in at a angle sometimes up to 30*. Questions I have would be:

1) whats the right angle with cross wind?
2) maybe flat on opposite side is better if you were in a all out MPG race with cross wind from one side
3) Maybe a round bubble/bomber butt works at the end?

All crazy ideas I was thinking sorry if these offend.

Firstly, Crazy Ideas is otherwise known as thinking outside of the square, 99% are crazy, but 1% are absolute Gem's, so keep em coming.

As for the wind tunnel idea, may be fun, but as I understand there is a whole lot of scaling issues, tunnel dimensions, vehicle scale, airspeed, laminar airflow etc. and even with full size facilities, there is still a lot of trial and error, things work in tunnel testing, but fail dismally on the road.

From my readings for best all round performance, a bluntish rounded front, generous radius all round, no sharp edges and a tapering rear, not exceeding 22°, if not doing full tail, then truncated with a small lip at rear to promote clear seperation.

[Tesla]

Originally Posted by Sven7

Here's what I was talking about as far as preserving the radius. Really hastily sketched up in photoshop


wagon form by Tyler Linner, on Flickr

time for a movie... later!

Looks great, but realistically I don't think I'd get away with it at full length, best I could or would probably do is cut at the very first section, basically a short Kamback cavity, about 12-18", but if you add a trailer, then this provides a great launch point for an extended tail by working to streamline the gap between vehicle and trailer.

[Tesla]

Originally Posted by aerohead

In the near future I'll have a better answer,but for today,let me say the following:
*research on boat-tailing was conducted on bodies of revolution of which all body camber originated from a common location at the point of maximum cross-section.
*if you build a mirror-image half-body based on the body of revolution,the plan taper would also originate at the point of maximum roof camber.
*so in a 'literal' translation of a half-body car,the plan taper would be identical to the roofline taper.
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Then there's the real world!
*All contemporary automobiles will have some side body camber.
*Some,like EV1 and Insight Gen-1 will also have measurable plan taper.
*Every car should be taken on a case-specific basis,utilizing whatever the manufacturer provided,and building upon that.
*So a 'CUBE' won't be like a 'MG EX 181' when streamlining.
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*For roofline use of the 'Template',the full body height is the only parameter necessary,pin-pointed at the bodies location of max. camber.(It's good to ballast the car with at least 300-pounds to get it to settle first,if it'd going to do that).
*For body sides plan tapering, we first need to determine the location of max body width.
*And it could be different for the lower body and greenhouse.
*From these locations we've got to analyze what we're working with,and see how to morph the sides as best we can to mimic what the roofline would be doing.
*If the 'interference is so great as to be able to closely match top and sides (which will be the case more than not),then as Paul Jaray did in 1921,we streamline the main body separately from the greenhouse,creating the 'combination form'.[I had to do this with the T-100]
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Technically,it would probably require a wind tunnel and hundreds of pressure taps to tune the body.
Short of that,perhaps these 'proven' separation-free bodies suggest contours which will maintain attached flow.
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I'm working on a plan-taper thread but always seem to be months away from posting.
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*Examples of 'Template' cars which include plan taper have already yielded Cd 0.12 or lower.
*If the plan taper is excluded,drag can be as high as Cd 0.21 (Kamm)
*Pseudo-Jaray 'fast', 'fast-back' bodies of low effective fineness ratios yielded Cd 0.20 - 0.186 (Heald- Schl'o'r)
*Bochum University's solar car has a 'fast' roofline,with Cd 0.14.
*GM's Sunraycer showed Cd 0.089 with wheel fairings in model form,Cd 0.12 without the fairings @ full-scale as raced.
*HONDA's Dream 2 solar car is Cd 0.10
*Without the PV arrays,these cars would have lower drag and easier ingress/egress.
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These vehicles are looooooooooooooooooooooooooooooong! And it's why Kamm and Fachsenfeld advocated the tail truncation at 50% frontal area wake area to ease driving in traffic and parking issues.
Fachsenfeld also envisioned inflatable boat tails to add length for highway travel.
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I'll keep chipping away at future threads.

Thank you so much,
Confirming some things I suspected and adding more.

Please don't dangle the carrot for a plan taper thread,
As you've probably picked up I am particularly interested in the "Plan taper" and how this is applied in a real sense, don't want to pre empt too much on the upcoming thread,

but just two more things and I'll stop bothering you.
1 / Please confirm your views on how to apply template to plan taper, as I understand it, it is applied as a symetrical wing view centred on vehicle centre line, resulting in a much shorter tail than the roof taper side view a lot of the time, there seems to be some conflicting views on this.

2 / I have read some comments regarding possible issues with crosswinds by using the full template plan taper and that less angle is better on the sides. Is this actual fact or does it relate more so to poor merging between roof and sides resulting in angles >22°?

Going back to read through again.

[freebeard]

It's hard to keep up. I don't think I'd get the attributions right so take this as a general comment. I interpret it this way, until we hear from aerohead himself:

Originally Posted by aerohead

*For body sides plan tapering, we first need to determine the location of max body width.
*And it could be different for the lower body and greenhouse.
...
*If the 'interference is so great as to be able to closely match top and sides (which will be the case more than not)

Draw an equivalence between the bolded items and your station wagons are included. You'll notice even the Tromphenwagen has a 'shoulder' at the beltline.

I, too, am looking forward to any forth-coming plan taper thread. The Template is a good fail-safe fool-proof fallback position, but it's a ball and chain in real life.

Originally Posted by Sven7

Is that Mac?

That was the joke. Why press 5 keys *in the correct order* when you can press 3 at once.

hotrodfeguy -- It's a fair question. Even with a 1/4 scale model you'd need like 1500hp to move the air. ...through your aquarium?