Car Aerodynamics: Experimental trials not agreeing with lift theory, help

russ_watters

The two methods are both correct - they are just two different ways of calculating/measuring the same thing and depending on your situation one may be more suitable than the other.

Important question: Is the rear of the car body higher than the front? Most cars make such terrible airfoils that ground effects are a very significant fraction of the equation.

Stingray

Besides the issues Fred mentioned, a small model like that tends to be pretty inaccurate. Things like the airflow through the engine compartment can have a very large effect. I heard a story in the design of the 3rd gen Corvette (in the late 60's) that it initially had terrible front-end lift. That was largely cured by adding some slats behind the front wheels to give the air flowing through the radiator an easier path out of the engine compartment.

Anyway, your small wind speeds are also a problem. In these experiments, things can usually be called "comparable" if something called the Reynolds number is similar. In your case, think of it as velocity times length. So a 1/24 scale model at 25 km/hr acts like a full-size one at ~1 km/hr. As I understand, drag and lift coefficients are usually doing odd things in that range. They're certainly not constant.

I agree with Fred that you should just explain the problems with your setup. Even though it didn't quite work, it sounds like a fun experiment.

saplingg

Thanks everyone for the advice

arildno

Remember that in the take-off phase, the primary lift source on a plane is made by lowering the flaps in order to force the air on the DOWNSIDE of the wing to go into centripetal motion&be pushed downwards (and hence, push the wing upwards in accordance with Newton's 3.law)

vivesdn

For those who see tha car as an airfoil: what would the angle of attack be? Most cars have a low front and high rear, so the angle of attack is negative. This explains the effect our coleague is measuring with the balance.

Another point is that the bernouilli effect explains why pressure decreases with air speed (basically, a mass/energy balance). But you should not guess the air speed just by the increased path.

dunlon

The car should be like an upside down plane wing. A greater the speed should increase the down forces which should keep the car on the road. However, a rear spoiler could have the capability to lift the front end of a car (at high speed) and make it uncontrollable.



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donate car

russ_watters

Welcome to PF, dunlon.

Most cars, are shaped like right-side up airfoils, flat on the bottom, rounded on top. That's why spoilers are necessary on racecars.

Stingray

Shaping a car's body like an inverted airfoil doesn't work. It was one of the first things that was tried with racing cars, and it failed miserably. The proximity of the ground completely destroys the flow properties that you'd expect.

This observation is the reason that the first wings appearing on successful race cars were placed on very high supports. The wing needed to be in "clean" air to work properly. These designs were eventually banned for safety reasons, leaving engineers to figure out how to improve downforce in more subtle ways.

rcgldr

The model presents a couple of issues. At 1/24 scale, you run into a Reynolds number issue. The "chord" length (distance from front to trailing edge) times the relative air speed is small, and this significantly affects the results. The other missing component is air flow going through the radiator which is then re-directed under the car, which contributes to lift. Other's have already posted about AOA and the possible issue with non-horizontal flow in your wind tunnel.

As mentioned, most sedans generate lift at high speed, mostly at the rear end. For example, the Audi TT in it's first year of production had a severe read end lift issue, leading to bad accidents on the Autobahn. The cure was to add a rear spoiler and to adjust the suspension to be more understeery. This is also the reason that most high-powered sedans are speed limited to 250kmh.

Here is a video of a modified RX7 experiencing rear end lift at Bonneville Speedway: while going about 215mph, the rear end lifts, resulting in the car rotating, which eventually flips over, the driver was OK:

rx7215.wmv

Methods used to create downforce in race cars:

Nascar - Air dam at the front end blocks most of the air from entering below the radiator, reducing air pressure underneath at the front. The radiator is taped to adjust airflow through the radiator and under the car depending on temperature. More tape means less air flow through the radiator and more downforce. Air is drawn in from the sides of the car, so the cars have a negative angle of attack both below and above, the cars are about 1.5 inches higher in the rear than the front. In addition, a rear spoiler, or for the new cars, a rear wing, is used to generate downforce at the rear. On the cars with the spoilers, the size of the spoiler is limited by rules depending on which brand body style. On the newer cars with the wings, all cars are running the same body shape, so they run the same wing size.

Indy Racing League and Champ Cars use underbody tunneling to channel air inwards (horizontally), then outwards and upwards to get underbody effects. This isn't allowed in Formula 1 race cars which have a skidboard that is measured before and after a race to see if the car was too low. These cars also use wings and upper body shape (including winglets) to generate downforce. Even the mirrors on an IRL car are used as winglets. When a high downforce car like a Formula 1 car is raced in the rain, there's a very visible, huge "rooster tail" of water vapor emitted upwards from the rear of the car, a good indicator of the volume and upwards acceleration of air by these cars at speed.

Regarding hump / Bernoulli theory, that mistkanely states that air has to flow faster over the hump than the flat section of a wing, I offer this picture of a flat top, curved bottom flying body glider, with an apparent zero angle of attack (if you go by the flat surface or by leading / trailing edges, which is misleading, I prefer to use the "effective" angle of attack, where zero EAOA means no lift).

flat top, curved bottom glider.jpg

1969 Nascar version: dodge daytona.jpg

1969 Formula 1 version: lotus 49 with high wing.jpg

vivesdn

You do not need an airfoil to create lift/downforce: a simple flat surface is able to do that provided it forms a non zero angle of attack with the air current.
Most cars are designed in a way that they have a low front and a high rear, so they're forming a negative angle of attack -> negative lift, or downforce. This is why you see a nonzero downforce also on the control car. So, why you need spoilers? Basically, to increase this effect in high performance cars, or just to have more sex appeal.

About bernouilli: it is false that bernouilli explains lift in airfoils. It is not the increased path that generates higher velocity and thus lower pressure. It is the angle of attack of an airfoil that will compress lower flow (higher pressure) because it forces it to deviate downwards-> acceleration. And the upper flow will find a greater volume to fill so according to mass balance (which is the base for bernouilli eqs) will increase its speed, reduce pressure and also deviate downwards -> acceleration. According to Newton, the airfoil will accelerate upwards. It is not the increased path, but the angle of the airfoil that generates an increased speed, which is the consequence, not the cause.

If the car has a symetric profile and a zero angle of attack, it shouldn't generate lift/downforce, just drag. But as the space below the car is limited, at high speeds (or low car) the lower flow will have higher pressure than the upper flow generating sobe lift. This is why front spoilers are also effective: you're trying to limit the flow below the car, forcing it to pass over it, creating a low pressure below the car.

Idjot

I admittedly haven't taken the time yet to read this whole thread. Rather I just kind of skimmed through it and I hope I'm not repeating anyone here.

I just want to point out that most vehicles experience a downward force due to wind because there is more surface area exposed to the wind on the top side of the car.
The purpose of a spoiler is to reduce drag and/or to reduce lift. Most street cars that have a spoiler don't actually "need" one because they'll never drive at a speed where it's effects will be noticeable. And some spoilers aren't capable of changing anything other than the appearance of a car. It depends on the car itself, really whether a spoiler can be helpful or not. They were designed for racing vehicles in the practical sense and introduced to the general population to capitalize on the "My car will look faster and I'll look cooler driving it" sense.

rcgldr

More surface area doesn't translate into downforce, a typical wing has slightly more surface area above and it generates lift. It's the effective angle of attack that determines lift, and the shape of the airfoils determine efficiency for a specific range of air speeds.

correctionSpoilers may reduce drag.

At the front of a car, "spoilers" are called air dams, and used to prevent air flow from entering from in front of and below the car reducing air flow under the car at the front.

Idjot

The surfaces on a car are pushing against the air. The additional friction on the upward side of the car contributes to drag which is essentially trying to stop the linear motion of the car. But not all of the drag's opposing force is in a perfectly linear direction. The angles of the body parts on a vehicle can cause downward forces too. That's all I mean by pointing out the additional surface area. That there is more for the air to "drag" on and "push" on and therefor more force from above than below.

As to your statement about spoilers never reducing drag, here's a nicely packaged, easy to read definition for you, which you should have read before you tried to correct me "by definition".

http://en.wikipedia.org/wiki/Spoiler_%28automotive%29

rcgldr

Sorry, I sit corrected, apparently there are multiple definitions for "spoiler" in the automotive world (it's more consistent for aircraft, they spoil lift and increase drag slightly). Wiki's definition differs from the terminology used for race cars. In the link below, note that "spoiler" only refers to the angled wedge at the rear decklid of a car, and those "spoilers" increase both downforce and drag. A wing can produce the same downforce with less drag (as used on the "cars of tomorrow"). Air dam is the term for the wedge at the front bottom of a car. Other than Wiki, I've never heard the term "spoiler" used in reference to the "air dam" at the front of a car, or for wings at either end of a car.

http://www.stagefronttickets.com/nas...car_terms.html

Deflector or turbulator would be a better term to use for what Wiki is including in it's definition of spoiler.

Aerodynamic forces are due to displacement of air, and friction only contributes a small component to the the overall displacement of air.

Mostly pushing the air forwards. The forwards acceleration of air corresponds to drag. As a car passes through a volume of air, it leaves a void behind that the air accelerates towards from all directions, except the air can't accelerate backwards through the car, and the close proximity to the pavement means there's very little air to flow upwards from below the void created by a car (so ground effects have to occur at the car, not at the void behind the car). The net result is a mostly forwards (drag) and some downwards (lift) acceleration of air as a typical passenger car passes through the air.

The only vertical flow through a car is through the radiator and then down underneath a car, creating lift. Nascar race cars tape up the radiator based on the ambient temperature to minimize radiator air flow.

Even though air is deflected upwards by the forward facing surfaces of a car, the air will continue to follow the car's profile, and flow downwards past the rearward facing surfaces, and the net result is some lift. From the windshield and back, the profile of a typical sedan approximates an air foil that generates lift.

Since most sedan body shapes generate lift, the high powered ones are typically speed limited to 250kph = 155mph to prevent excessive rear end lift. This was an issue with the initial year release of the Audi TT, where several accidents occurred on the Autobahn because the rear end lifted and the cars spun out. They added a "spoiler" to the rear to increase rear end downforce (with the cost of some drag) and adjusted the suspension for more understeer. Here's a link to a video of rear end lift on a RX7 doing 215mph at Bonneville, the rear end lifts and the car starts yaw rotation and then flips over on its hood (driver was OK):

rx7215.wmv

Downforce can be created by pitching the overall body downwards, using air dams, spoilers, wings, ground effects (underbody tunneling). All this results in increased drag though, with the exception of ground effects combined with powerful fans at the rear of a car (like Chaparral race cars).

Idjot

You've definitely done your "homework" on this topic. Looks like we've both learned something here, me mostly. Now what about the person that started this thread... any of this stuff helping you ?