drnick

porken,

that 90 degree bend on the trailing edge cant be good for airflow. ive never seen a trailing edge like that on anything with aerodynamics involved.

why not simply extend by a couple of inches the section before the 90 degree trailing edge and have a clean edge.

i love the airbrake idea!

PorKen

I think I answered my own question with some quick net searching.

Each grade of aluminum needs to be bent with a certain radii, measured in multiples of the thickness of the sheet.

I reckon for the fixed (no airbrake) prototype, I'll use steel sheet for the cover with a replaceable aluminum flap so I can change the height.
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drnick,

It appears to be a counter-intuitive, but it does seem to work. There is a diminishing return in regards to drag, but the flap has been proven to increase a given wing's effectiveness.

I didn't want to make my wing too obtrusive, ie. larger than the S-wing, so I decided to go with the flap.

Did you see these links on the beginning of the post?
Race Tech 104: Gurney Flap
All American Racers: Gurney Flap

I hope I can make the airbrake work, I really like the concept. And regardless of braking ability, it ought to look cool.

drnick

hey porken, i just checked those links - very interesting. one point to consider is whether the 'S' spoiler and an extension to it is comparable to the gurney wing. the gurney wing shows airflow over both upper and lower surfaces and both articles describe the effect generated in terms of the re-attachment of laminar flow to the underside/suction area.

does your extrusion allow flow over both upper and lower surfaces?

PorKen

drnick,

The cover will be attached at the front, in both fixed and moveable designs, with no airflow underneath.
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I realized that I have been improperly referring to the S-spoiler as a wing.


Build Your Own Race Car | Tips: Aerodynamics

A spoiler has no underside area, and is attached to the car. The car body is the airfoil.

A detached wing is more effective in creating downforce with less drag.

The flap is used on NASCAR type trunk mounted spoilers, where a wing is disallowed.

drnick

porken, i am sceptical that a 90 degree bend is usefull on the rear fixed spoiler lip - i dont think its the same condition as when added to a wing, are you sure its used on fixed rear spoilers?? the articles you linked to both illustrate wing type spoilers.

my pseudo qualification in this field comes from distant memories of the 'scouts', a kind of boys club and our scout group were 'air' scouts! extending the fixed spoiler surface area that the high pressure acts on will increase the downforce, and the angle that it projects at will determine the drag. if you plan to travel very fast then a smaller angle of attack and greater width (backwards, into the air stream) is ideal and slower speeds the opposite aplies. IMO, from the illustrations in the linked articles it seems that the lip added to the 'flat' side of the wing is in effect changing the overall wing profile in much the same way (but not as elegantly) as flaps do on an aircraft - not so mysterious??

mark kibort

the old rear stock wing (or lip) is used only for breaking up the laminar flow to get better flow effeciency, and certainly no down force. this can be tested at 80mph. pop the rear trunk lid at this speed and watch the lid open and kind of float. same thing happens with a stock S4 wing. Now, give the stock S4 wing some tilt, and it provides some good downforce. raise it up out of the boundary layer , even only 2-3" and it is pretty impressive. I measured 80lbs of downforce at 120mph with a spring scale and a video of the action. i posted the downforce numbers, and I think it started out at 20lbs at 60mph or something.

no, it would be interesting to see the gurney lip on the spoiler and see if that helped, but I imagine it will not. Fast test, would be to activate the rear hatch release or just get on the hyway with it ajar, and see if it pushes down at faster and faster speeds.

mk

mark kibort

think of the 90 degree flap on the rear S4 wing, as the new rear point for which the angle of incidence can be shown for a given wing.( i.e. for a little .25" flap, that changes the effective angle of attack of the wing by X degrees.) However, at a cost of much more drag than just a tilting of the wing. simular characteristics occure with the flap put on a rear spoiler.
The reason for the gurney flap was to keep downforce at high angles of attack , where normally, the wing could be stalled.

I do like the air brake idea. To bad that idea was toosed out and the rules dont allow controllable aerodynamic devices on race cars (all sanctioning bodies) Now, think of it., dive planes that retract, and wings that straighten out at speed!!!! However, its not allowed in most all forms of auto racing, except maybe those dirt circle track racers!!

the only way to slow a nascar down with a adustable flap, is when the car is going backward, and those flaps pop up!!

MK

QUOTE]Originally posted by drnick
porken,

that 90 degree bend on the trailing edge cant be good for airflow. ive never seen a trailing edge like that on anything with aerodynamics involved.

why not simply extend by a couple of inches the section before the 90 degree trailing edge and have a clean edge.

i love the airbrake idea! [/QUOTE]

PorKen

OK, forgoing the 90, perhaps an adjustable extension.


GermanCarFans.com | Hamann 996 news
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mark kibort said: "just get on the hyway with it ajar, and see if it pushes down at faster and faster speeds"

Excellent idea.
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Regarding the airbrake - what about a pop-up (curved) air deflector, to force more air onto the factory spoiler? It could be mounted to the factory holes for the ski rack at the top of the hatch.

I really want to do something with those ski rack bolt holes.

Jim_H

Ken,
You have PM

SharkSkin

Regarding the "open hatch" experiment... Before I fixed my hatch, it would open sometimes at speed. With my windows closed, the hatch would float open. If I opened the windows, it would pull the hatch closed. Just another variable you may want to consider in your experimentation. Otherwise you might attribute some downforce to your mod when in fact the difference is caused by the side window.

PorKen

SharkSkin,

But how am I going to flash my gang signals with the windows up?
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Jim_H,

I got the info, thanks. I'll probably do the 'back nine'.

Clean out your box, man. Did you get the reply to your regular email? (PM me.)

SharkSkin

Out the sunroof, if you have one?

PorKen

Clips, double-sided taped to glass
Lower bracket, slid onto clips, and bolted to hatch
Spoiler double-sided taped down to lower bracket
Slightly high when stopped but settles down onto the foam spoiler over 40MPH

Results of Mark's hatch open test (windows closed) viewed in the mirror:
*When unlatched from outside when stopped, the hatch is about 1" open.
*With the stock S-spoiler, the hatch never lifts any higher. It starts to close noticeably over ~80, and at 100 closed to ~1/4".
*With the accessory spoiler added, it stops being jiggled by bumps over ~35, closes to ~1/2 at 80, and shuts tightly at 100.

I was worried the accessory spoiler might get blown off, but it does not want to lift, or pull away (even during a 140 mile round-trip 80-90+ rainy highway run).

PorKen

Results of the official fish-scale test:
*6lbs to 'seat' the new spoiler (pushed down onto the foam spoiler)
*30lbs to close the hatch to 1/4"
*40lbs to flush close the hatch

So ~10lbs (or more) additional downforce at 100?

Ispeed

What would Chris Lockhart say? "WWCLS"

His wing also keeps the sun off the car at the right angle, and can be seen from space. But I'm sure it has a lot of downforce. Race ready stuff. I know the stock S4 wing doesn't do much at all.