[Nik]

You're right, a flat airfoil does create a lot of drag, that's why curved airfoils are used as the air flows a lot smoother over them and thus creates less drag. Endplates keep the air from spilling off the sides and increase the downforce by keeping the air perpendicular to the surface. The force acting on the airfoil is the opposite of the change in momentum of the air hitting the airfoil over differential time. F=dp/dt=ma Air flowing parallel to the airfoil cannot exert a force on it!!

We're really just arguing physics principals versus engineering models here. Same thing with Ptolomy, everyone says he was wrong, but he never said his model was how things actually were (blame the media for that), just that it was good for calculations. We could have stuck a man on the moon within 4 ft of where we did using his model with the earth at the center of the universe.

http://www.physicsmyths.org.uk/bernoulli.htm

 

[zanny]

My vote is, my mk1, with a plumley modded "thing" is a wing.
Why do i say this? Take a look at this pic.
http://www.carpron.com/multisite/v/Upload/zanny/aw11aero.jpg.html

By relocating my wing back, I am into the blue smoke.
The red smoke has to pass under the wing.
Now, look at the yellow smoke. Why don't we see any coming up through the engine lid and rear buttress area? We've been told, it flows from the bottom of the car through the engine, and out the vents.
The first pic in this thread, shows a"spoiler". Is it a spoiler or a gurney flap?

 

[Nik]

A gurney flap is a small spoiler attached to the trailing edge of a wing.
http://www.allamericanracers.com/gurney_flap.html

 

[2stripes]

you have a lot of info on this aero stuff Nik....i was so excited when i saw this section open beacuse it was my favorite part of my motorsports minor....cant wait for my school to start the masters program next fall.

 

[Nik]

you have a lot of info on this aero stuff Nik....i was so excited when i saw this section open beacuse it was my favorite part of my motorsports minor....cant wait for my school to start the masters program next fall.

A masters in motorsports? I might have to go to graduate school after all... Seriously, email me more info or something.

 

[2stripes]

i got ya....check your email.

 

[trdny]

Can you use both at the same time or will the spoiler work against the wing? I would guess that the spoiler would have to go if you wanted a wing.

 

[Guest]

^ yes, for a wing to work well, it needs to have a significant amount of airflow beneath it, and the spoiler disrupts this airflow, ruining the effectiveness of both.

 

[2stripes]

sorry to tell you RavenR6....but you're not totally correct. We tested the Chase Daytona Prototype for a few days in the wind tunnel doing various setups. That car runs both a wing and spoiler as do many other daytona prototypes....

Having the rear spoiler decreased the overall drag of the vehicle and is primarily used as a tuning device. By changing the height you can achieve more downforce where needed....as in the front or rear. No matter what though we had more than enough downforce everywhere....in fact there were times we went over our horsepower limit. Thats when we changed other parts and settings.

 

[2stripes]

also to point out in that pic you can see the gurney flap on the trailing edge of the wing. The tapered design has been proven to be very beneficial. Just though Id add that comment as it came up earlier.

 

[Guest]

sorry to tell you RavenR6....but you're not totally correct. We tested the Chase Daytona Prototype for a few days in the wind tunnel doing various setups. That car runs both a wing and spoiler as do many other daytona prototypes....

See, the difference there is that the wing is elevated quite high off the back end of the car, giving it enough clearance so airflow works for both. If you moved it closer to the rear decklid like it'd be on a normal car, it wouldn't be nearly as effective. I was assuming a fairly close clearance between the two.

 

[2stripes]

ok....ill give you that one :smile:

 

[JuMpA]

^ yes, for a wing to work well, it needs to have a significant amount of airflow beneath it, and the spoiler disrupts this airflow, ruining the effectiveness of both.

so would the toms wing and spoiler be a complete waste to have both?

and just to add another question, would the effects of a aerodynamic device(spilers, wings, etc) be affected if we were to drive into a strong winds blowing in our direction?

 

[2stripes]

yes the effects would be affected....thats where cross wind design comes into play. A race car wants to go as fast as it can at all times so if it is getting pushed around by wind it cant do that. This is why millions and millions of dollars are spent on aero testing by race teams.

As I was washing my 2 the other day I realized that we kinda have a wing and spoiler setup. Granted the spoiler on our cars barely comes up, but it is raised from the trunklid and if looked at from a rear side view it has the right shape.

 

[fosley]

I can't find the site where I read it, but I think it was one of those "general information on building your racecar" type sites. They said that a spoiler is designed to reduce lift by creating turbulence behind the car, and has nothing to do with creating downforce. Now, vectors add, so reducing lift is essentially the same as adding downforce, but it's done by a different principle than a wing. Also, it reduces drag, unlike most methods of adding downforce.

Because it just requires a small, sharp edge on the back of a car, reduces lift, and decreases drag, it's a very cheap way to improve gas mileage and improve higher speed performance.

Bear in mind that this is only what I've been led to believe. Maybe I'm wrong, but I thought I'd throw it out there and see what other people have to say.

 

[Brock]

Interesting bit of information I got from the Nov. '05 copy of EVO magazine is that when Lotus was designing the turbo Esprit the designers put a spoiler on the back. When their drivers tested the car they found that the spoiler made the car unstable at high speeds. The bosses there liked the look of the spoiler and demanded that it stay on the car, a small "trip strip" was put at the top of the rear window to make the air turbulent and render the spoiler ineffective.
This thinking would lead me to believe the wing on the MR2 probly does nothing due to the turbulent airflow across the rear deck.

 

[Spots]

The bosses there liked the look of the spoiler and demanded that it stay on the car, a small "trip strip" was put at the top of the rear window to make the air turbulent and render the spoiler ineffective.

Mmmmm, I'm not sure about this.

There's laminar flow, where you have a nice and smooth, low-energy airflow over an object. This reduces skin-drag.
Then there's turbulant flow, where the flow is turbulant and high-energy. Turbulant flow has higher skin drag.
And there's seperation, which is Really Bad and isn't really a flow at all.

Laminar flow is great, until you want it to flow around a corner - laminar flows do not like to change direction. When it encounters a tight turn, the flow will seperate immediately and create a wake in the "shadow" of the object. For example, the wing of a stalled aeroplane.

Turbulant flows, because of the higher energy contained in the flow, find it easier to change direction and hug a complex shape. You can see this on golfballs - because most of the drag comes from the wake behind the ball, and very little drag from the skin of the ball, dimples are added to make the flow go turbulant at just the right point to help minimise the size of the seperated wake.)

Turbulator spars, or 'trip strips' as above, are used on aircraft wings and the like to help deliberately force a flow to go turbulant earlier than normal and in doing so, help keep the flow attached to the wing at steeper angles of attack or on high-camber wings.

Golfballs, wings, cars too... turbulators are used to help prevent a flow from seperating from the skin of an object and forming a draggy wake. In the case of aircraft wings, if the flow seperate significantly then no lift will be generated and the pilot will be an unhappy man.

Aerodynamic devices still work just fine in a turbulant flow, but cannot function inside of a seperated flow (eg: deep within the wake of a car, in the "shadow" of a fully-stalled wing, etc)

So, I'm not sure about the "turbulator was used to make the spoiler ineffective" thing. You're right about the MR2's sharp notch-back probably doing bad things to the wing on the rear deck, yes. (Though our little roof spoiler of the Mk1 should help to guide airflow downwards)

 

[Guest]

Not quite... The lift/downforce of a wing is often attributed to Bernoulli, but that's actually not how it works. If it were, planes couldn't fly upside down, their wings would start to behave as they do on racecars and they would accelerate faster that just gravity towards the ground, and symetrical airfoils would be completely worthless. Bernoulli's equations do work as a model for calcuations though, and give accurate approximations.

What actually happens is almost too obvious, they push the air up (cars) or down (planes). You can use a flat airfoil as long as you give it an appropriate angle of attack, which is a lot easier to visuallize.

I must respectfully disagree. Bernoulli's principle is EXACTLY how it works. Air flowing over the top of the car is moving faster than the air going underneath the car, thus creating lift. It has NOTHING to do with the movement of air behind the car. The movement of the air behind the car only has to do with how much induced drag is created from the act of creating lift.

You are correct in that the only way to create lift from a symmetric airfoil is to ensure the chordline of the airfoil is at a positive angle to the relative wind. This angle is angle of attack. A positively cambered airfoil (almost all vehicle shapes) does not require AOA to create lift. All that is required to create lift is that the air molecules on the top of the airfoil be moving faster than the air molecules on the bottom of the airfoil. A negatively cambered airfoil, which is what is on the back of the 91-95 MR2, always pushes down at 0 AOA, given a clean airstream. The MR2s?, along with almost all other automobile 'wings', are also set at a negative angle of attack, thus creating even more downforce. All that is required for an automobile wing to be effective is that the size of the net 'down' vector plus gravity must be greater than the size of the net 'up' vector.

Whether the airflow on an MR2 makes the thing on top of the trunk a spoiler, (i.e. the linear flow off of the roof is not able to get underneath it), or a negatively cambered wing (i.e. the linear flow off of the roof IS able to get underneath it), I suspect is more a function of speed than anything else. Either way, the net effect is to create a downforce, whether by killing lift by acting as a spoiler, or by creating 'lift' downwards by acting as a wing.

As far as a plane flying upside-down, all that has to happen is to give the airfoil an angle of attack that creates enough lift to overcome gravity. If you think in velocity vectors, it is easier to understand. Even a normally positively cambered airfoil can create a net velocity vector away from the earth when upside-down, IF you give it enough angle of attack. However, this would require a lot of power to overcome the induced drag that is created, and the airfoil would have a very high stall speed.

Conclusion:
The act of an object pushing air up or down doesn?t create lift, either up or down. The act of air flowing around that object creates a force which then acts on that object.

 

[Tomg]

A word on wings/spoiler combos.
The understanding is also that the proximity of either to the diffuser at the rear of that car, affects how well they work.
You do want to have a clean seperation at the end of the body work(hence the spoiler) and these are generaly speaking a little different on prototypes than on say a nascar. The spoilers on prototypes reduces drag, and doesnt realy do much for downforce. But they do the same thing that the diffuser does, and that is to introduce the air that flows over and under the car into normalized air as neatly as possible. Without creating to much drag(low pressure).
On nascars the spoiler almost works as a huge gurney flap, and does produce downforce, but at the cost of a huge drag shadow behind the car, which your opponent will use to his advantage as seen doing the bumper riding thing.

Also the seperation of flow, is easyly overcome by the addition of "strakes" to have more surface area, to make it stay attached to the aerodynamic structures.
They also found that having exposed drive shafts, suspension arms in the tunnels actually help the flow to stay attached.

Now one thing I want to know, if there is some sort of equation or rule of thumb, to decide what angle vs length on front diffusers, and it's relation to the rear diffuser channels in terms of length/width.
Now I know there is a thumb rule for the rear in relation to speed, between 7 - 16 degrees, lower for faster speeds, and higher for lower speed.

Ofcourse there is the rake angle of the underside too, but that is not realy doable on a street car, well it is, but probably not worth the trouble.

T