Drag penalty for having a shallow rear window angle, and using spoiler to re-attach ?

[gone-ot]

The buttress area was "filled-in" ala' Daytona 500 models:

[ennored]

Not quite the case you're after, but one that does point out the value in getting the rear shape correct.

A Pontiac Solstice Convertible has a Cd of .44 or .45 with the top up (hard to find "real" numbers). The Coupe version is 10% better, say about .41 (again VERY hard to find good numbers).



Top up side view doesn't really show what it's like:





I've been told the airflow never reattaches on the convertible, but that the tail on the coupe is just high enough to get some reattachment. (Note that they both use the same door glass, rear fenders, tailllights, and rear bumper/fascia.)

[aerohead]

I believe that the first year model was reported at Cd 0.45.Then Cd 0.44 for the following year(s).
It's an exquisite example of an aft-body drag penalty.Thanks!

[tomi_k]

In context of the this thread - Assuming e.g that rear window angle is 18degree at the maximum (lets say a fastback type rear end shape) and flow over the rear window is attached without a spoiler (not a wing). With a spoiler and still assuming that flow over the rear window and the spoiler is attached, can the spoiler reduce the drag caused by 18degree rear window?

[aerohead]

Originally Posted by tomi_k

In context of the this thread - Assuming e.g that rear window angle is 18degree at the maximum (lets say a fastback type rear end shape) and flow over the rear window is attached without a spoiler (not a wing). With a spoiler and still assuming that flow over the rear window and the spoiler is attached, can the spoiler reduce the drag caused by 18degree rear window?

* It's very contextual.
* On a simple prismatic body, with simple angles and no curvature, optimum backlight angle is a function of the percentage of the length of the aft-body, compared to the overall length of the vehicle, expressed as a ratio.
* On complex bodies, where curvature is used, the optimum backlight angle will also be a function of the percentage of aft-body.
* Some cars, designed for maximum interior useful volume, won't have any appreciable backlight angle at all, they'll just be vertical.
* In order to anticipate some hypothetical, we really need to specify a few more details of the body proportions.
* It's best to just pick a specific vehicle, and one in which blueprints are available, depicting all surfaces and proportions as they appear in reality.
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* And we need to be very careful how we use the term 'attached flow'.
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A few specific vehicles:
* Volkswagen's 2013 XL1, Cd 0.189, rear downslope angle = 17-degrees maximum.
* GM's 1996 EV1 rear slope maximum = 20-degrees, then reflexed to 18-degrees, and continued to reflex as low as 14-degrees at the trailing edge.
* GM's 1993 Impact land speed record car's boat tail extension, relaxed the rear reflex to only 10-degrees at the trailing edge, for Cd 0.137.
* Ford's 1985 Cd 0.137, Probe-V concept had a maximum 11.5-degrees.
* GM's 1987 Cd 0.089 Sunraycer rear slope angle never exceeded 17.5-degrees.
* HONDA's Cd 0.10, 1993 Dream solar racer's maximum rear slope was 11-degrees.
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I'll bring Rolf Buchheim's prismatic drag table next Wednesday. It may contain some data for an 18-degree backlight.

[tomi_k]

Thanks for the respond, Aerohead.

The 18degree was a reference which I gathered from some of the Hucho's article and seems that the rear angle (whole rear end portion) somewhere between 15-18degree seems to be the maximum / critical angle before major drag increase (if the angle is made shallower)...

What I try to understand is that can a spoiler be used at the end rear end shape to reduce drag? Ex. if the rear end shape maximum angle is e.g. 18degree and one installs a spoiler at the end in a way that airflow over the spoiler is "attached", impact to wake is minimal but air flow pressure on front of the spoiler (in larger area) would be changed from negative to positive pressure. If these conditions would be met in a same vehicle, would that lead to reduced drag? Or is this too general description?

E.g. if the rear end angle is made, lets say maximum of 22degree (reduced wake area), so the rear end shape exceeds the critical angle.... But installing the spoiler in away that the tip of the spoiler would still form "artificial" angle of 18degrees or less (from tip of the spoiler to earlier point of the shape, tangent point), and the flow would remain still attached over the spoiler, flow pressure would be changed from negative to positive - would this lead overall reduction of drag?

Ultimately I try to understand that if "wake area" is bad and that would be minimized by creating the rear end shape which has shallower angle than e.g. 18degree, would
(any) spoiler shape / angle / size be able to recover drag penalty caused by the angle which exceeds so called "critical angle" (somewhere between 15-18degrees)...

[aerohead]

Originally Posted by tomi_k

Thanks for the respond, Aerohead.

The 18degree was a reference which I gathered from some of the Hucho's article and seems that the rear angle (whole rear end portion) somewhere between 15-18degree seems to be the maximum / critical angle before major drag increase (if the angle is made shallower)...

What I try to understand is that can a spoiler be used at the end rear end shape to reduce drag? Ex. if the rear end shape maximum angle is e.g. 18degree and one installs a spoiler at the end in a way that airflow over the spoiler is "attached", impact to wake is minimal but air flow pressure on front of the spoiler (in larger area) would be changed from negative to positive pressure. If these conditions would be met in a same vehicle, would that lead to reduced drag? Or is this too general description?

E.g. if the rear end angle is made, lets say maximum of 22degree (reduced wake area), so the rear end shape exceeds the critical angle.... But installing the spoiler in away that the tip of the spoiler would still form "artificial" angle of 18degrees or less (from tip of the spoiler to earlier point of the shape, tangent point), and the flow would remain still attached over the spoiler, flow pressure would be changed from negative to positive - would this lead overall reduction of drag?

Ultimately I try to understand that if "wake area" is bad and that would be minimized by creating the rear end shape which has shallower angle than e.g. 18degree, would
(any) spoiler shape / angle / size be able to recover drag penalty caused by the angle which exceeds so called "critical angle" (somewhere between 15-18degrees)...

* I'll bring more materials Wednesday.
* If flow was attached at 18-degrees you'd have pressure regain, and when flow finally separated at the end of the body, the wake would at as high a pressure as you'd obtain.
* The 'size' of the wake is not as important as it's pressure.
* If the flow was separated at 18-degrees, it would be the job of the spoiler to do exactly as you describe.
* Flow reattaches on top of the spoiler, and captures a locked-vortex against the body, as high as the spoiler.
* Allowing slower, higher pressure to act against the upper rear surface, killing lift, while allowing slower, higher pressure air in the wake, reducing pressure drag, the major component of drag.
* If you know the streamline path over the existing profile, then it's just a matter of placing the top of the spoiler where the optimum profile ought to be.
* HOT ROD Magazine's Chevy Camaro Bonneville land speed record car does exactly this.

[freebeard]

* It's very contextual.

An example opposite the prismatic body would be the GM EV-1. The cabin appears too steep, but the generous taper in plan compensates.

Consider also the Gurney flap or wickerbill.

The device operates by increasing pressure on the pressure side, decreasing pressure on the suction side, and helping the boundary layer flow stay attached all the way to the trailing edge on the suction side of the airfoil.[4] Common applications occur in auto racing, helicopter horizontal stabilizers, and aircraft where high lift is essential, such as banner-towing airplanes.[5]

It does what I think you want the spoiler to do, on a smaller scale.

[tomi_k]

Thanks, Aerohead & Freebeard ! Just trying to get my head around the topic what matters the most... and learning. And thanks for being patient to respond (probably basic and silly questions) but gotta start from somewhere....

<Quote>
* Flow reattaches on top of the spoiler, and captures a locked-vortex against the body, as high as the spoiler.
</Quote>

From drag perspective, the air flow before the spoiler what happens for the overall drag if

a) flow on front of the spoiler is attached? If flow is attached, is the spoiler directed air flow direction reducing the drag (i.e. surface pressure direction (at the end) -> without spoiler the surface pressure is against the direction of vehicle movement vs. with spoiler the surface pressure is in same direction of vehicle movement)?
b) flow on front of the spoiler is not attached -> and creates a "air bubble" (turbulent)... is this what causes overall drag increase if wake increase is not necessary the dominant factor?

According one of Hucho's document (see picture attached), if I understood correctly, regardless of attached or separated airflow, when the critical angle of rear portion of the shape is exceeded, overall drag will increase. Unfortunately, I found this information a bit too late but glad I found it.

The other attached picture is an illustration of what I am trying to understand... If Hucho's document info is correct and valid... is there a way to recover "penalty of overall drag" caused by exceeded rear slanted angle as it sounds like increased drag is evident regardless what type of flow over the rear shape one has and/or and if wake area is reduced?

[Vman455]

Re: your second picture--

Pressure always acts normal to a surface. The arrows in the lower image are not correct.

A spoiler affects flow upstream because pressure is transmitted through air (fluid) at ~340 m/s. Fitting a spoiler may increase pressure as far upstream as the roof, as I found on my Prius with a small lip spoiler:


Gauge pressures with no lip spoiler


Gauge pressures with lip spoiler


Difference


Lip spoiler for reference

Les than $100 worth of equipment and a couple hours and you can measure yourself and find out what a spoiler does on your car.