Spoiler or not to spoiler

[Meph]

Moving onto the read end of my Z, trying to determine if my hatch spoiler would in fact decrease Cd as wind tunnel tests have shown in this post here

Windtunnel Test Data - HybridZ

tests #9 to #12

I own two hatches, one with and without. Im currently finishing the spoiler-less to be installed pending what everyones thinking.



Take a look at the data, pics are lower down in the same post. Would deleting the spoiler in theory, assist mileage?

[COcyclist]

Am I missing something? I only see results for tests 19-28. It looks like the spoiler helps, at least the #28 test with 90% of the front covered has the lowest drag. It should be even lower if it were not a race car with the windows open. Make yours like the blue car.

[Meph]

theres two images of results, maybe the first failed to load, check again, tests 9-12, they show a erduction in drag but maybe someone can look at the shape and curves and tell if this is possible, or if id be better off with a standard hatch.

[lunarhighway]

the hatch is the wrong angle for the flow to staty attached. the spoiler virtually decreases this angle. cars like a porche 911 or the saab 900 also use a big poiler at the rear, that actually gives them a profile more like a common sedan. and than theres also the pinto crisis fighter that used a similar spoiler like the one pictured. cars like the prius or insight all have fairly high rear ends, this is because the ideal slope of the rear for the flow to remain attached is not very steep so with a normal lenght this will result in a high rear.

the reson why a lot of cars don't do this is because it usually results in a split rear window.

the position of the spoiler is another matter of course as it can work in two ways. a full horizontal spoiler would extend the car and change the virtual angle of the rear but angling the spoiler up would reduce the length needed to get the same virtual angle, it would also make more downforce but also more drag (still the net effect would be an overall drag reduction).

since you already have some interesting data and i assume this car is not intended to become the ultimate aero car, i'd figure out what configuration of spoiler reduces drag the most and go with that. if you add an undertray and perhaps even tire dams you can further reduce the drag, and enhance the performance, without making any real visible changes the the classic shape.

[aerohead]

According to CAR and DRIVER magazine's experiments with Z-Cars in 1974,removing the spoiler will cause increased drag and loss of mpg.
The backlight angle is just too fast,and as lunarhighway has mentioned,the spoiler tricks the air into thinking that the hatch has a more gentle slope.
The 1978 ZX has an aft-body of 41.5% of 'Template.'
If the back of the Z-car was fleshed out up to the 'Template' curve and it had a decent windshield,you could get the Cd to 0.202.

[COcyclist]

Meph, If you could get the rest of the wind tunnel data to post that could be interesting. I am curious what the whale tail does to things and what is a Pantera Hatch? It looks like it could be a recessed hatch with a vertical rear window like some mid engine cars use.

It sure seems counterintuitive that something sticking up at the back of the car could reduce drag and improve mpg.

[Kyleyadon]

It sure seems counterintuitive that something sticking up at the back of the car could reduce drag and improve mpg.


I cite an episode of Mythbusters where they tested whether it was more efficient to drive a pickup with the tail gate open or closed. They found that it was actually more efficient to drive tail gate closed. Which as you state here, is counter intuitive. They did further testing to see why this is and found that with the tail gate up it creates a small and beneficial vortex inside the bed of the truck. This bubble of turbulent air creates a cushion that the main bulk of air flowing over the truck rides over. In essence it creates a sort of virtual kamback in the bed of the truck. With a vertical, or near vertical spoiler on the back of a car you should have a similar phenomena, if only in a smaller scale.

[some_other_dave]

When a spoiler like that is beneficial, it is almost always a band-aid, because the aero of the original design is relatively poor. The older Porsche 911s were this way, and for a very similar reason--the rear slope was too great, so adding a spoiler would create a flatter "virtual" rear slope that not only reduced lift (the primary goal for the 911 and likely the Z as well) but also reduced drag slightly.

-soD

[AeroModder]

The lowest drag of all those tests has a wing and VGs on the rear:

http://www.youtube.com/watch?v=u-kyYCHWQhU

It looks like the wing helps straighten out the airflow from the steep rear end. This combination has the lowest CD and the lowest front/rear lift from their tests.