REAR SPOILERS: Length-'Angles'

[aerohead]

Originally Posted by Pendragon

I am pretty much in agreement, but I do believe that placement of the max height of the roof line further aft than is commonplace (such as the Daytona Coupe) is something we should be seeing more of from the car designers.

I have my doubts about the wind tunnel smoke visualizations shown in commercials...I suspect that the positioning of the smoke wand is chosen to look good rather than show the problem areas. I suppose you can't blame them too much for turning the marketers loose.

As an aside, I have seen several things that the designers are "tweaking" that are probably beyond what is reasonable for all but the most committed modders. The shape of the headlight area of the nose of some cars has been shaped to modify airflow to try to reduce drag from the outside mirrors. The mirrors themselves have been seeing more attention to lower drag shapes as well. There are some other flow modifiers on the front fenders of some cars which attempt to reduce drag from the wheel well and wheel. I doubt that we will see the wind tunnel work for these items on commercials, but we should...it would demonstrate the attention to detail that helps make more efficient cars.

Cheers

P.S. When are we going to see flush mounted windshields and "hidden" windshield wipers?

I GOOGLED the Shelby Daytona coupe.State of the art would suggest that this shape would do better 'backwards,' as Chrysler attempted to illustrate in 1934 with their Airflow series.
The 1984 Ford Probe-IV concept was the last car I can recall which relied primarily on the fore-body for drag reduction.The car achieved Cd 0.154,pretty good.
Then Ford pushed the roof forward,extended the tail to create the Probe-V which came in at Cd 0.137.
'Pumpkin seeds,' like the 1922 Jaray,1933 Lay,1957 MG EX-181,1987 Sunraycer,and lately,NUNA-3 demonstrate Cd 0.13,0.12,0.12/0.089,0.077 respectively.
All these cars respect the basic aerodynamic streamlining tenant,that for low drag,a car must use its 1st 1/3rd of body for penetration,and the last 2/3rds for pressure recovery.
I've tried to illustrate this with the Aerodynamic Streamlining Template.It's based on sound empirical science borne out of the best wind tunnels over the last century of investigations.
Of course,automakers are free to produce whatever they want,within DOT and EPA guidelines,but for low drag and maximum mpg,the 'Template' demonstrates the lowest drag form.
-------------------------------------------------------------------------
As to the wind tunnel photos( Madison Ave. aside ),the images at 'Flow-Images' appear unadulterated to me.
Since this sort of thing is peer-reviewed there is no reason why an automaker would risk loss of credibility within the auto community by fudging on the photographs,knowing,that like 'cold-fusion,' other tunnels would be duplicating the tests for verification.
-------------------------------------------------------------------------
And yes,we see subtle massaging of the fore-body.And that's good.As long as they don't forget the elephant in the living room( the tail ).
As far as the windshield mounting and wipers,these were looked at during development of GM's Precept PNGV car ( CD 0.163 ).In the wind tunnel,they demonstrated no measurable drag,so things may be okay as the are with respect to contemporary manufacturing technology.

[California98Civic]

Originally Posted by aerohead

As far as the windshield mounting and wipers,these were looked at during development of GM's Precept PNGV car ( CD 0.163 ).In the wind tunnel,they demonstrated no measurable drag

Point taken, but the Precept also featured a cowl for the wipers, no? That's what I see in the images on ecomodder:





james

[aerohead]

Originally Posted by California98Civic

Point taken, but the Precept also featured a cowl for the wipers, no? That's what I see in the images on ecomodder:





james

The car was tested with wipers.They showed zero on the load cells.I can't tell you why they did the cowl cover.
GM has had 'hidden windshield wipers' on production cars dating to perhaps the late 1960s.
Typically,the wipers are embedded within the stagnation bubble which resides at the windshield base.The air just skips over this region.You'll see it in hundreds of wind tunnel smoke photos.
Here's the caveat: if you live in Europe and have access to the Autobahn or Autostrada,or race at LeMans,etc.,during inclement weather,when the wipers are running and away from their stowed position,it has been seen where the airflow at these elevated velocities can easily lift the wipers off the glass.And you may notice that cars like Porsche will rivet foils to the wiper arms to spoil this lifting.
Event horizons at these speeds are very brief.You better be able to see where your going,and what traffic around you is doing.
As to the Precept,it was designed by stylists.Perhaps it was 'retro'.

[Pendragon]

aerohead,

Windshield wipers tend to be in a high pressure region at the base of the windshield which is less affected by airflow than other areas, but there is anecdotal evidence of windshield wipers lifting at speed. It just all depends. Hiding them under the lip of the hood became popular some years ago for styling reasons as much as anything. (A Ferrari abused by the lads of Top Gear had to have the springs on the windshield wipers replaced with upgraded ones to keep the windshield wipers from lifting at undisclosed speeds.)

You may recall that "cold air induction" systems used to be located at the base of the windshield of a number of cars to take advantage of the pressure there. Most NASCAR Sprint Cup Cars still have the inlet to the air cleaner located there for that reason.

Nevertheless, attention to small details will be useful as the windshields are increasingly laid back to improve the airflow IMO.

Peter Brock designed the body for the Daytona Coupe for 'Shel. He was interviewed for a recent documentary about it and explained that he based the design on a combination of intuition and information from aircraft aerodynamics, specifically attempting to utilize the shape of a laminar flow wing profile on the roofline which resulted in the highest point of the roof being further aft than on many contemporary car designs. (Who can forget the Gurney bump in the roof?) They did some tuft testing and so on to refine the shape and rushed to get the cars built in time for the racing season. At the track one of the drivers (I can not remember just who at the moment) told them that the car was undriveable and would kill someone unless changes were made. There was some sheet metal available and so the "spoiler" was fabricated and the car taken out again. Upon returning to the pits he suggested taking off "about an inch" which was done and the car was raced that way.

Complications arose when the additional bodies were being manufactured because the constructor took liberties with the dimensions when converting them from standard English (SAE) measurements to metric measurements to which they were accustomed. It took some doing to correct the situation.

By today's standards the Daytona Coupe is probably crude, but the tuft testing showed that it worked and the performance was distinctly better than the open car (20+ MPH on straights).

Anyway, getting to the actual reason I was actually posting back, I was not intimating in any way that the wind tunnel video in commercials was "doctored", simply that they chose a position of the smoke wand that gave a nice, smooth image. They all do it.

Cheers

[euromodder]

Covering the wipers doesn't have to cost anything when done smart.
It keeps pedestrians away from the (non working) wipers in a collision, reducing their injuries.

If it gives even a minute aero advantage, that'd be all the better.

Lots of European cars have airfoils on their wipers to keep them from lifting @ speed.
On my car, the blades themselves are shaped like a Gurney-Flap, with the aft edge turned up 90°.

[brucepick]

Originally Posted by aerohead

brucepick,it does function exactly like the raised tailgate.
If the trailing edge is close to the template the spoiler captures the low-pressure vortex and isolates that low pressure in front of it,preventing it from communicating that pressure to the wake behind which would otherwise lower the base pressure,increasing the delta-P across the car,increasing drag.
--------------------------------------------------------------------------

When the outer flow does separate at the trailing edge of the spoiler, it has decelerated to a lower velocity/higher pressure ( that evil Daniel Bournoulli ) leaving the base pressure of the wake higher,the delta-P lower,an concomitant lower drag.
--------------------------------------------------------------------------

If you tilt the spoiler up at 30-degrees from the back of the trunklid( boot),it will hit the 'Template' sooner,and it can be 'shorter'.
--------------------------------------------------------------------------

If you were to also extend the C-pillars back along the 'Template' line as I did with the CRX,you can tailor the vortex even finer with this buttress,and in so doing,foster even better side flow towards the rear by,eliminating a potential breeding ground for longitudinal attached-vortices on each side.
You'll have a bit of new blind-spot,but you can compensate for that with your mirrors.
Anyone with a 1st-gen Insight would be familiar with this.
--------------------------------------------------------------------------
If you can't reconcile yourself to the 'Template' try to set 22-degrees as your maximum local tangent angle.Air cannot follow an angle steeper than this,and typically,you need to have close to a body height worth of length in progressive curvature before you can get to 22-degrees.

It took a while but I'm making plans now. Hope will be in the driveway cutting and gluing tomorrow, based on what aerohead wrote above.

My plan is for a spoiler that's also a false raised trunk lid, starting at a line about half way up the rear glass, going to a level meeting the template, as far back as the rear bumper. I plan to also extend the C-pillars rearward, and taper the extensions inwards slightly.

The C-pillar extensions will use mount brackets that will be silicone-glued to the window glass, or will attach those brackets with heavy duty double stick foam tape. That should get me out of gluing, duct-taping or screwing to the sheet metal.

A question to aerohead:
Re. the max height of the C-pillar extension panels.
Do you prefer Option 1 or 2 below?

Option 1: Start the forward end of the vertical panels at the full height of the C-pillar, measured at the point where the extension panel begins. It would taper down to the rear, ultimately to the level of where the spoiler/false trunk lid meets the template curve. This leaves a sort of chamber between the two extended side panels, above the spoiler.

Option 2: Have the side panels only go as high as the level of the spoiler/false trunk lid, wherever that turns out to be. Something like 6" above the trunk lid. So, no chamber between them, at least not above the horizontal spoiler panel.

Visibility: one more "feature" to the design:
The forward edge of the spoiler at the rear glass, will be at a height so that the whole spoiler should appear only as a horizontal line when viewed via the rear view mirror. So I hope to keep good rear visibility that way. Likely this will mean the forward edge will be raised a few inches vs. its rear edge. However I won't know exactly where it will end up until I put it together and look in the mirror.

Aerohead, you already mentioned that I'll get a bit of a blind spot from the side panels. I'm considering eventually making them from Lexan, but they will be coroplast or even cardboard for starters.

[radioranger]

Old post i know but trying to decide on spoilers for my 98 escort 4 door sedan I remember the mid window spoilers on the Merkur back in the day or should i just go with the factory spoiler and that's that , I have a 3 mile long hill here for testing and a bit of fab skills, learning anyway. money is a big issue with mods but thanks to all you guys for keeping me interested. and relatively sane, great site

[Sven7]

Hi, ranger. Could you post a side view photo with the spoiler? It's hard to say without seeing it.

[oil pan 4]

We have a spoiler wiki.
Rear Spoiler - EcoModder

[aerohead]

Originally Posted by Pendragon

aerohead,

Windshield wipers tend to be in a high pressure region at the base of the windshield which is less affected by airflow than other areas, but there is anecdotal evidence of windshield wipers lifting at speed. It just all depends. Hiding them under the lip of the hood became popular some years ago for styling reasons as much as anything. (A Ferrari abused by the lads of Top Gear had to have the springs on the windshield wipers replaced with upgraded ones to keep the windshield wipers from lifting at undisclosed speeds.)

You may recall that "cold air induction" systems used to be located at the base of the windshield of a number of cars to take advantage of the pressure there. Most NASCAR Sprint Cup Cars still have the inlet to the air cleaner located there for that reason.

Nevertheless, attention to small details will be useful as the windshields are increasingly laid back to improve the airflow IMO.

Peter Brock designed the body for the Daytona Coupe for 'Shel. He was interviewed for a recent documentary about it and explained that he based the design on a combination of intuition and information from aircraft aerodynamics, specifically attempting to utilize the shape of a laminar flow wing profile on the roofline which resulted in the highest point of the roof being further aft than on many contemporary car designs. (Who can forget the Gurney bump in the roof?) They did some tuft testing and so on to refine the shape and rushed to get the cars built in time for the racing season. At the track one of the drivers (I can not remember just who at the moment) told them that the car was undriveable and would kill someone unless changes were made. There was some sheet metal available and so the "spoiler" was fabricated and the car taken out again. Upon returning to the pits he suggested taking off "about an inch" which was done and the car was raced that way.

Complications arose when the additional bodies were being manufactured because the constructor took liberties with the dimensions when converting them from standard English (SAE) measurements to metric measurements to which they were accustomed. It took some doing to correct the situation.

By today's standards the Daytona Coupe is probably crude, but the tuft testing showed that it worked and the performance was distinctly better than the open car (20+ MPH on straights).

Anyway, getting to the actual reason I was actually posting back, I was not intimating in any way that the wind tunnel video in commercials was "doctored", simply that they chose a position of the smoke wand that gave a nice, smooth image. They all do it.

Cheers

I apologize for the very late reply.
With respect to the cowl/wiper area,this is one of the few things I altered at Bonneville with the T-100.
I have the blister hood fairing which ends perhaps 6-inches away from the glass to enable clearance for the wipers when they deploy.
I covered this entire region with cardboard and duct tape,completely sealing this area airtight.From the timing slips the modification registered zero change to the velocity at 1-mile.Nothing pushed the speed beyond 105 mph.
I had tried the same modification in 1990 with the CRX.Again,no change in speed.
I have to surmise that if this area makes no difference at 100+ mph,then its probably not going to show up at the gas pump either.Hucho and other messengers provide us no hope that it would.
I'm not opposed to it when I see,but it looks like we can't even measure 'counts' from the modification.
I do agree strongly that when the windshield is laid over very steeply,that lifting of the wipers is certainly an issue at speed.