Reattaching wake and drag penalty
 

I'd like to ask all of you here about what kind of drag penalty that results from using a spoiler ( or trunk lid ) to reattach airflow on a car with a rear taper that is not the ideal angle.

http://farm4.static.flickr.com/3113/...503bc37a_b.jpg

Note that the Camaro had a worse Cd without that spoiler, even though it kicks up at an angle ( The same results can be found in upward tilting spoilers on the R&T project cars - the Crisis Fighter Pinto and Crisis Fighter Datsun Z car.

http://farm3.static.flickr.com/2284/...8c695430_o.jpg

On cars such as this Fusion, if you follow the roof line out at a 15 degree angle, you eventually meet up with the edge of the deck lid spoiler. I am guessing that this acts as a way of grabbing the airflow and bringing it back into the ideal trajectory.

For those folks on this site not willing to create a huge boat tail on the end of their prized car, why not create a large shelf spoiler that is easily removed ?
From the examples above, it seems as though adding a spoiler can sometimes have a drastic reduction in drag.... but what are the penalties ?

Thanks !

Isn't that something? I've been thinking of this exact thing now, ever since Metro put up that F150 aero thread.

I've been outside pondering the best aeromods for my Tempo. Even have a crude mock-up on it.

Boattail doesn't appeal to me for several reasons.

No top chopping; I don't want to drop the seats onto the floor because I want to retain rear seat functionality too, and I need the headroom.

No fastback window and bodywork; too much work for too little gain.

And so on... until I settled upon the notion of a "Bonneville Style" spoiler remarkably similar to the one pictured on the Fusion! So I've been reviewing my Hucho book and surfing for aero articles. Yes, that's why my recent posts reference some Hot Rod magazine stuff.

I want my spoiler to go out rather than up to achieve the imaginary 14 deg drop from the roof. This minimizes the area of the body that leaves a wake zone. The Camaro would have been even more slippery with a "Bonneville Style" spoiler.

Yo
Frankie, whyoncha extend it all the way out and tilt it down like the trunk on the ESX ?
Not only would the airflow connect, but it could follow the angle downwards and make your wake even smaller.
Your back glass is really steep. I wonder if a small lip spoiler would be helpful in convincing the air to meet the big spoiler without too much turbulence.

http://farm4.static.flickr.com/3437/...7a64921158.jpg

I thought the reason a spoiler (can) reduce drag on (some) so-called "fastbacks" is that the lift generated by the attached air following the slope of the body created vortecies at the sides of the roof...and at some angle the benefits of filling in the vacuum behind the car (with air coming down over the fastback) were outweighed by the drag penalty of the vortexes (see, I can spell it either way). I don't have my Hucho handy, but I'm surprised the optimum angle is as low as 14 degrees...not the first time wind tunnel results have surprised me, but still.

On a lot of sedan race cars we add spacers under the hood hinges to let the high pressure area in front of the firewall loose. It often ads 4 or 5 mph down the straightaway. Anybody tried it for mileage?

Dave

Ummm... what? :confused:

I'm told what you'll have by raising the rear of the hood is flow into the engine compartment.

I think you are on the right track- thanks for the workup!

I need to think about how far out the back I'm willing to go with these appendages.

I found a wedge-shaped spoiler in a book and posted the translation here. I guess it's the next best thing to a full length roof extension.

penalty
 

I'm guessing that the Fusion is FWD? The Camaro is RWD,yes? Since these are both Bonneville cars and they're running parachutes,then they're both looking at at least 175-mph.Some Bonneville cars have enough horsepower to spin their tires when they hit an "aerodynamic wall" ( the car has more power but cannot attain higher speed due to air drag) and can drift into a most dangerous spin.--------- I think these spoilers are a compromise between drag reduction and downforce and not necessarily the best designs for "street" cars.------------------ Any design which has an exit tip which falls along Kamm's roofline profile,whether an up-kick or straight back,will both do the job.------------- All spoiler setups pay a mpg price due to the attached vortex directly above them since the air is making more than one "pass" over the body.And you have to pay for this circulation.-------------- If you do the straight back shelf be mindful of unsuspecting pedestrians walking up from behind.Maybe cushion the trailing edge as Porsche did with the Carrera Whale-tail.------------- I'm partial to Ford's bi-wing spoiler which appeared on the Sierra concept car and Merkur XR4Ti.It gets the job done and with less drag.

Hmmm... good idea. I will make mine razor sharp.

How high do you space the hood? Is it high enough to direct some of the air flow upward so it gets a head start flowing over the roof or is it just high enough to bleed off some of the high pressure air into the engine compartment?

Kamm extending spoiler
 

This thread has piqued my interest because I have been playing around with an extending Kamm-rear spoiler idea. The critical downstream rear angles could always benefit to a long-tail configuration. The idea is very similar to reconfigurable/extending airplane wing flaps. The move-able wing extend & become flush/fixed to the fastback surface & additional wingtips deploy downwards to clean up some side flow. I've considered these add. tips to deploy outside the rear light assembly but would compromise the safety equipment. The wingtips always could be made w/ clear polycarbonate to passed safety reqs. Lower speeds & parking maneuvers needs retracting the wings. 'Active' wing flap angles become too complicated.
Also another idea, a model specific (Prius 2010) rear wheel covers extend to very clean flows sharply detaching at the bottom & rear edges...


http://i429.photobucket.com/albums/q...extensions.jpg

OK, this is something I'm working on for my Escort. And one of the limitations that I have is that TOWMBO will not tolerate any public display of eccentricity. :eek:
So, a belly pan is acceptable, but a boat tail is not. Wheel discs are borderline acceptable, rear wheel pants are clearly not. So, I might get away with vg's, but...
So, my back glass is ~30* from horizontal, and a line from the back of the roof line to deck height at 14* says I need a 8"+ shelf attached. :(
What does the collective mind know about the +/- of rear window dust deflectors? I know, those are usually only for "wagon" backs, but a deflector with an included arc of 25* would certainly be a place to start. Yes, this would cause lift at high speeds, but at econ speeds, what would be the down side? Would a smooth deflector blade cause more drag than 8 vg's across the back edge?

deflectors
 

What I've read about them is that the act of directing air towards the wake is good,but air separates off the back side of the deflector for a net drag increase overall.--------- simple flat plates extending straight back,which capture vortices along a quasi-Kamm profile have been demonstated to help,although you have the "length" issue.

Cd: "damned pressure bubble"... thanks for the laugh. :)

"Out damn bubble, out!" Seems to be common running joke 'round these parts...so has anyone done the obvious & 'attacked' it with something like this...

http://www.team-bhp.com/forum/attach...s-dsc02307.jpg

Looks like bug deflectors moved backyards but smoke tests & tuft testing would be interesting to see.

I hope I'm not misunderstanding you... if I attached a short Kamm profile lip to the top of my rear windshield area, I might gain a little benefit?

Both Aerohead and Basjoos created air deflectors for their wipers. I'll let them eloborate on any gains. I tufted my car and noticed that the turbulent bubble on my car only extended a couple of inches past the arm of the wiper.

http://farm4.static.flickr.com/3280/...d8d709c8_o.jpg


.

Also look here. This is a very good article on tufting the wiper area : http://autospeed.com/cms/title_Silen...5/article.html

I'll let Aerohead answer this one.
I added a small lip spoiler to the edge of the roofline on Franks' car due to the fact that his back glass has such a steep angle. His car is almost like a hatchback with a trunk.

( I hope I understand your question. )

Usually the rear edge of the hood is 1" higher. Much more and latches and such get hard to use. I also pull the gasket off the rear edge of the hood to really open up the gap.

Dave

short Kamm
 

It's the only thing I would have great confidence in as to real drag reduction.Profile drag is the largest component of aero drag,and it is governed by the pressure differential between the stagnation pressure at the nose,and base pressure of the wake.------------- If flow is "clean" (without significant separated flow) up to the point of the vehicle's maximum cross-sectional area,then it's the aftbody which dictates where the flow will ultimately separate,and where the flow separates dictates the base pressure,which dictates profile drag.-------------- Anything you do to lengthen the body,while simultaneously reducing wake area,is,by defination,streamlining,and the only known path to lower drag.------------------- Any benefit can be calculated as a direct function of wake area reduction.------------- This is why I developed the aerodynamic streamlining template.If you follow it,you get guaranteed drag reduction as a function of your new wake area.Size matters!

"Bonneville-style"-origins?
 

I was data mining for spoiler info and crossed paths with the old article by CAR and DRIVER on the 'Crisis Fighter Pinto'.
In the article they mentioned how,in March,'74,Porsche had already changed the 911 Carrera tested in February,'74,from a 45-degree upswept rear spoiler,to a very long nearly-horizontal wing-like surface with capping plates, extending from the decklid.
They surmised that the new spoiler for this 911 Carrera RSR provided the same downforce as the Carrera but without the drag penalty of the steep up-sweep.
The RSR finished 2nd at Le Mans,won at Daytona,won the Trans Am Championship,and also won the Targa Florio that year.
================================================== ========
I do not have a 'true-length' photo of the car,so I can't say how far the spoiler extended,but it was VERYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY long!
No risk to pedestrians though as this was strictly a full race car.
--------------------------------------------------------------------------
I do not have a large dry lakes racer database but this 1974 RSR may be the progenitor of all the 'Bonneville'-type foils.

911 spoilers
 

Aerohead, are these the 911 spoilers you refer to?

street vs race
 

focus,the three images we see here are what we could have bought.The 1974 911 RSR was a track car only and its spoiler made these whale-tails look like tinker toys!
I've only seen the RSR in a book.

Was it this one?

File:Porsche RSR am 19.05.1974.jpg - Wikipedia, the free encyclopedia

This:

http://i52.tinypic.com/dxlj44.jpg

Looooooooooong enuf?

Shooda just stuckit on the roof above the window!

that's it!
 

Tim,that's the animal I saw in the book.It's hyper-'Template' so maybe not a candidate for low drag as we might need,but it may have morphed into the 'horizontal' plate we're seeing with drag racing and dry lakes events.Thanks!:thumbup:

pretty loooooooooooooooooooong!
 

Yeah,I don't think it for us.
With a 6-cyl engine cantilevered back beyond the transaxle they might have needed all that downforce.'Don't think it's a good fit for us.
Maybe inverted liquid-fuel rocket boosters are next for Porsche?:eek:

They also added another rear window, stacked on top of the original but with a shallower angle. That supposedly helped with both downforce and drag. And the front splitter was required to give reasonable aero-based grip at the front to match that at the rear.

-soD

Cool! I hadn't noticed the shallower rear glass from the photo. Thanks for pointing that out.

They did all kinds of things with the 935 once they picked through the rule book. Like they found they could raise the floor (originally written in so front-engine cars could pass the exhaust pipe under the side or some such) so they raised the whole thing. Then lowered the car back down again so the floor was at the original height. Result: Lower car overall, lower CG, etc. The engineers worked that rule book over with a lead pipe! ;)

-soD

good eye!
 

soD,you've got that x-ray vision! I didn't even pick up on that.Raptors will have to watch their backs when your in town!:thumbup:

Oh, I didn't get that from the picture. :D That's from reading endless articles about the 935 racers over the years. I'm a wee bit of a Porsche nut, you see... (Just got back from dinner in the wife's 911SC!)

-soD

nut
 

having driven four different Porsche products,and been a passenger in a few others, I definitely see the attraction.
Shift gates a bit mysterious but other than that,real driving pleasure!