SVX spoiler = downforce?!

[oab_au]

The function of the spoiler, is as the name suggests. It is to spoil the air flow off the back of the body. Without it the air will curve down over the rear to form flow eddies close to and behind the body. This causes a drag on the car and reduces its speed potential.

As this car was designed to be a high speed tourer, body slipperness is more important than down force. The spoiler straightens and accelerates the air flow off the boot, to extend the the eddies far behind the body to reduce the drag.

Downforce is provided by the shape of the underside of the body. The curve of the front under tray is airfoil, to accelerate and reduce the air pressure under the body. This prevents the pressure rising under the body to cause lift, it actually reduces it to pull the car down.

Anyway thats the way I read it.
Harvey.

[Chiketkd]

Originally posted by oab_au
The function of the spoiler, is as the name suggests. It is to spoil the air flow off the back of the body. Without it the air will curve down over the rear to form flow eddies close to and behind the body. This causes a drag on the car and reduces its speed potential.

As this car was designed to be a high speed tourer, body slipperness is more important than down force. The spoiler straightens and accelerates the air flow off the boot, to extend the the eddies far behind the body to reduce the drag.

Downforce is provided by the shape of the underside of the body. The curve of the front under tray is airfoil, to accelerate and reduce the air pressure under the body. This prevents the pressure rising under the body to cause lift, it actually reduces it to pull the car down.

Anyway thats the way I read it.
Harvey.

Thanks Harv. Great explanation. I have heard of under trays helping to reduce lift at high speeds.

-Chike

[Chiketkd]

...when I got home from work, I took some initial measurements of the USDM spoiler. It was 31* so what I focused on was finding out whether our spoiler was an airfoil.

Measured at the center (directly behind the pivoting point of the rear wiper), and the circumference of the spoiler was almost exactly 15". When I measured the top and bottom halves, both were equal in length (7.5")...

Therefore the SVX spoiler is not shaped like an airfoil, and wouldn't produce downforce in this way.

-Chike

[Chiketkd]

...to determine whether our spoiler produces downforce at speed.

If our trunk was unlatched, but closed, how much force would be needed to push it down into the lock and latch itself? Tried pushing it down with one hand in the center of the trunk and I had to lean into it a bit before I heard a click - probably 30-40lbs of force.

Decided to try this on my way home tonight on the smooth interstate roads. My trunk was closed but unlatched and I accelerated evenly up to 75mph...at around 70mph, I thought I heard a 'click' so I didn't go any higher, however, when I pulled over later it still wasn't locked.

However, if we were able to determine a speed at which it does lock and could get an accurate measurement on the force needed to do this, we could then extrapolate these figures to determine the approximate downforce at higher speeds.

-Chike

[Sidewinder]

Couple things:

First, the purpose of the spoiler is to create downforce. Think of a dragster. The very reason why they have these wings on the front and rear is to create downforce. Of course, the body has a great deal to do with the downforce as well, as Harvey noted. For an example, look at the venturi effect.

As Chike noted, the spoiler appears to not be in the shape of an airfoil. Nevertheless, if there are pressure differences between the upper and lower portions of the spoiler itself, then we have some forces present.

With an airplane, lower air pressures are found above the wing while higher pressures are under the wing. The high pressure "lifts" the wing and the airplane flies. The same effect is used to roll the airplane. When the airplane rolls to the right, an aileron on the left wing drops down, changing the pressures while the aileron on the right wing rises, again changing pressures. The right wing develops less lift while the left wing develops more lift. This rotational effect allows the airplane to roll on it's lengthwise axis.

Anyway, back to the spoiler. All we need is higher pressure above the spoiler and lower pressure beneath it and we could have downforce. I am willing to bet that as the air stream that flows off the rear window and beneath the spoiler is travelling faster than the air flowing on the stream higher (the stream that flows over the spoiler). Therefore, despite the fact that the spoiler is not shaped like a normal airfoil, we could still get downforce.

[dcarrb]

Want downforce?

Crack open your sunroof so that the leading edge remains below the roofline, with the trailing edge a couple of inches high.

To muddy the issue of airflow over/under a vehicle as relates to downforce, it's noteworthy that full-bodied "stock" race cars cover front grille openings with tape to increase downward pressure on the nose of the car by routing more air over the hood, the flow of which, at speed, literally presses down the front of the car.

dcb

[Chiketkd]

Originally posted by sidewinder
Anyway, back to the spoiler. All we need is higher pressure above the spoiler and lower pressure beneath it and we could have downforce. I am willing to bet that as the air stream that flows off the rear window and beneath the spoiler is travelling faster than the air flowing on the stream higher (the stream that flows over the spoiler). Therefore, despite the fact that the spoiler is not shaped like a normal airfoil, we could still get downforce.

Interesting hypothesis. I wonder if there would be an elegant way of demonstrating this on our cars?

Maybe the streamer/ribbon test that MotorTrend used on the Evos they tested would work???

-Chike

[ssnsltd]

[QUOTE]Originally posted by NapaBavarian
[B]I figured I'd post a picture of an airfoil for anyone who isn't familiar...



Pardon me if this has been brought up before but:

With the factory wheels mounted as they come from the factory- basicly each spoke is a reverse wing.
does that meen they force air under the car at speed?

[Sidewinder]

Well, the wheels are unidirectional, so the answer is yes. I believe the most immediate reason for the wheels shape is to bring air in and cool the brakes. But then, that air will flow under the car too.

[oab_au]

There has been a lot of magic attributed to the wheels. That they scoop the air in to the brakes. But centrifugal force has the air, being expelled out, to the outside of the rim. Just like an centrifugal blower.

Harvey.

[lee]

I thought maybe I'd jump in here and see if I can confuse things just a bit.

The idea that the length of the bottom of an airfoil, when compared to the top, generates lift (or downforce - I'll use lift as it's easier to type) is the bunk they give you in high-school and non-technical college degree programs.

One thing about science teaching that always makes me angry is they tell you one story, then when you go to the next level they say, "Well, that wasn't exactly true..." From elementary school science, to high-school physics, to college physics, and then to grad school physics the story kept changing.

Now that I've started I should admit: I am not an expert, and I don't even understand the theory very well (only 1 grad course in fluid dynamics - so it's tough to explain), but lift is generated by a rotation of the fluid (air in this case), not a pressure differential due to one side being longer than the other.

Even see a "sail" boat that moves by use of a rotating cylinder instead of a sail? Trust me it works if you haven't. Or, ever see the rotating contrails coming off a wing (usually some humidity helps). Might help to think of the lift being the equal & opposite force (torque in this case) to the contrails.

Before I get myself in too deep, I should point out that aerodynamics remains today a controversial subject (usually settled in a wind tunnel) open to different interpretations. Nonetheless, use of pitot tubes on a wing surface has shown Bernoulli's equations most closely represent what's going on (rotation produces lift).

Most aerodynamic wings (in cross-section) have arcs on both top & bottom). Look at the SVX wing and it too has arcs top & bottom. Now as to the practical question of how effective the SVX wing/spoiler is.....I'm a physicist, why would you expect me to know anything about the real world?

[Chiketkd]

Originally posted by lee
Now as to the practical question of how effective the SVX wing/spoiler is.....I'm a physicist, why would you expect me to know anything about the real world?

-Chike

P.S. I was reading a NASA document about lift and read about what you were stating. Interesting stuff, but not sure what measurements to take to determine the effectiveness of the SVX's wing...

[lee]

Originally posted by Chiketkd

...snip... but not sure what measurements to take to determine the effectiveness of the SVX's wing...

drop by your local university and convince them you NEED some pressure transducers and data recorders...find an undergrad mechcanical and aerospace engineering student in a fluid dynamics class, convice him a good project would be to measure downforce of several different wing styles...

....gotta be a riced out Honda or two around there...I seem to remember a few when I was at Darden taking a class or two.

[NapaBavarian]

Originally posted by lee
I thought maybe I'd jump in here and see if I can confuse things just a bit.

The idea that the length of the bottom of an airfoil, when compared to the top, generates lift (or downforce - I'll use lift as it's easier to type) is the bunk they give you in high-school and non-technical college degree programs.

...snip...

Most aerodynamic wings (in cross-section) have arcs on both top & bottom). Look at the SVX wing and it too has arcs top & bottom. Now as to the practical question of how effective the SVX wing/spoiler is.....I'm a physicist, why would you expect me to know anything about the real world?

I'm with ya on school, and teachers hated me because I would question them (in grammer school) and learn years later that I was right but they didn't want to teach the right way because kids couldn't understand it... I learned to sit quiet and shut up, they were right because they were.

I actually learned it in flight school So I ran a Google image search to see what was up and got this-



Here it is in context

http://www.grc.nasa.gov/WWW/jp/ptchi...ualization.htm

Thanks, now I can spread the word

Originally posted by Chiketkd
...to determine whether our spoiler produces downforce at speed.

If our trunk was unlatched, but closed, how much force would be needed to push it down into the lock and latch itself? Tried pushing it down with one hand in the center of the trunk and I had to lean into it a bit before I heard a click - probably 30-40lbs of force.

Chike, remember that "impact" forces are much different than forces applied more slowly. Did you push down on the trunk quickly or slowly?