Drag coefficient

[eloise]

From what i understand , drag coefficient show how drag an object is ,with lower number mean the object is less draggy . What i alway thought is that a blunt object will have bigger drag coefficient than a pointy object ( hence why missiles and aircraft have pointy nose )
However after have a look here :
http://chrisoncars.com/2010/09/drag-coe ... -nonsense/
it seem that i was wrong , but it doesnt make sense to me at all :
how come :
this :


have better drag coefficient than this



And this

have much better drag coefficient than
this :


doesnt make any sense to me at all
even simple shape doesnt make sense :



so why fighter and sport car have pointy shape while it seem here that the blunt shape have better drag coefficien

[sprstdlyscottsmn]

It is about a lot more than being pointy. Drag Area is the drag coefficient multiplied by the frontal area (for cars, wing area for planes). Overall drag coefficient has a lot to do with smoothness of the airflow. Look at your second picture. See how the back smoothly tapers and the rear wheels are covered? That reduces drag. In the first picture you see that most of the front of that car is designed to divert airflow to the sides. Reduces drag.

Meanwhile the sports car (Jag XJ220?) the lower 1/3 of the nose is grillwork and splitters. further up the hood you have vents. in between the two you have air rushing in, hitting the engine to cool it (if that is the layout), and diverting out. Open wheel wheel wells that are over half the height of the car? Increase the coefficient (relative to frontal area remember?) and those big air scoops in front of the rear wheels? That air plows into the brakes/engine depending on vehicle layout. Then you have a flat dropoff that is over half the total height of the vehicle. This last bit is a big contributor to why pickup trucks have poor drag.

Now look at the bus. flat front and flat rear, but smooth top and sides with relatively small wheel wells.

Formula one? look at all those open sections! Wheels, struts, fins, intakes... that's drag.

Fighter jets and missiles have something else to contend with. Wave drag. To reduce wave drag you need a smooth and uniform increase in cross sectional area and a smooth decrease. Missiles really just try to minimize the frontal area entirely. Blunt object have a lot of wave drag. This is why a fast magnum pistol round has MUCH less range than even a slow rifle round.

Hope that helps.

[eloise]

It is about a lot more than being pointy. Drag Area is the drag coefficient multiplied by the frontal area (for cars, wing area for planes). Overall drag coefficient has a lot to do with smoothness of the airflow. Look at your second picture. See how the back smoothly tapers and the rear wheels are covered? That reduces drag. In the first picture you see that most of the front of that car is designed to divert airflow to the sides. Reduces drag.

Meanwhile the sports car (Jag XJ220?) the lower 1/3 of the nose is grillwork and splitters. further up the hood you have vents. in between the two you have air rushing in, hitting the engine to cool it (if that is the layout), and diverting out. Open wheel wheel wells that are over half the height of the car? Increase the coefficient (relative to frontal area remember?) and those big air scoops in front of the rear wheels? That air plows into the brakes/engine depending on vehicle layout. Then you have a flat dropoff that is over half the total height of the vehicle. This last bit is a big contributor to why pickup trucks have poor drag.

How about this car?

, it doesn't have its wheel taped, and also have shape that disrupt air flow ( like the intake, the 3 part separation of the nose, open wheel.. etc and it still have smaller coefficient than the sport car

Now look at the bus. flat front and flat rear, but smooth top and sides with relatively small wheel wells.

Formula one? look at all those open sections! Wheels, struts, fins, intakes... that's drag.

So Fins, intake, struts, wheel will cause more drag than a simple perpendicular surface? why aircraft have intake, wing, pylon, missiles but still have smaller drag coefficient than a car?

Fighter jets and missiles have something else to contend with. Wave drag. To reduce wave drag you need a smooth and uniform increase in cross sectional area and a smooth decrease. Missiles really just try to minimize the frontal area entirely. Blunt object have a lot of wave drag. This is why a fast magnum pistol round has MUCH less range than even a slow rifle round.

Hope that helps.

what is the difference between wave drag and normal drag?
btw I thought that object with smaller drag coefficient will have smaller total drag for the same frontal area? if that true then why bother with wave drag? as long as your total drag is smaller then it better? ( like the super fast thing like a Space shuttle use a blunt nose )

[mrigdon]

Cars have to deal with lift, as well. It's not just a matter of reducing drag absent any other forces. You need a shape that generates downforce, as well. You also have to figure in various airflows in and through the car. The engine itself needs air (just like a jet, most missiles don't). In addition, you have to divert airflow over the brake pads to cool those and avoid fade. If you're going to track a car, you often have to have modifications made to increase that brake airflow (or end up in a wall).

[eloise]

Cars have to deal with lift, as well. It's not just a matter of reducing drag absent any other forces. You need a shape that generates downforce, as well.

But aircraft have to generate enough lift to fly, so isn't it just opposite of a Car? why they still have very drag coefficient?

You also have to figure in various airflows in and through the car. The engine itself needs air (just like a jet, most missiles don't). In addition, you have to divert airflow over the brake pads to cool those and avoid fade. If you're going to track a car, you often have to have modifications made to increase that brake airflow (or end up in a wall).

The Boeing 747 for example also need air for lift and engine and it still have significant smaller drag coefficient than a sports car , why is that?
or the bullet is completely smooth , no air needed for cooling, down force or engine or brake, but it still have bigger drag coefficient than a Boeing 747, or the old car why is that?


drag coefficient = 0.031

vs


drag coefficient = 0.295


vs

drag coefficient = 0.212

[mrigdon]

Eloise, I've seen you take part in a quite a few technical discussions. I'm surprised you haven't gone over to wikipedia. http://www.wikipedia.org/wiki/Drag_coefficient, unless they don't have a page for your native language?

At any rate, because of a difference in the way reference area is calculated for planes and cars, you can't compare those numbers. From the article:

As noted above, aircraft use their wing area as the reference area when computing CD, while automobiles (and many other objects) use frontal cross sectional area; thus, coefficients are not directly comparable between these classes of vehicles.

You're comparing apples and oranges.

As for the bullet, I think this explains why the CD is so much larger.

For a streamlined body to achieve a low drag coefficient, the boundary layer around the body must remain attached to the surface of the body for as long as possible, causing the wake to be narrow.

The boundary layer should be detaching quite abruptly at the blunt end of the bullet, increasing the drag coefficient in that case. Keep in mind, CD also depends on the direction of flow. So the same shape can have different CD depending on that.

CubeCD.jpg (14.84 KiB) Viewed 20595 times

Also, the F-4's nickname "Flying Brick" is a misnomer. According to Wikipedia, the CD of a brick is 2.1 while the CD of the Phantom is 0.021 subsonic and 0.044 supersonic.

[johnwill]

As has been stated, cars, bullets, buildings, road signs, etc. use cross section area as the reference area for calculating aerodynamic drag coefficients. Yet, airplanes use wing area. In case you are wondering why the difference, I'll try to explain.
For cars, bullets, etc., drag force is the primary aero force of interest, so cross section area is a convenient way calculate the coefficient.
Airplanes, however, have many, many coefficients of interest. Every control surface deflection results in a six-axis load on the airplane, lift, drag, side force, pitch, roll, and yaw moment. That means an aileron deflection results in a lift force, drag force, side force, pitch moment, roll, moment, and yaw moment. If there are seven control surfaces, that's forty two different coefficients. Then each of the six axes of rate motion, vertical, axial, lateral, pitch, roll, and yaw rates, develop six-axis load coefficients. For example, lateral motion of an airplane causes aerodynamic lift, side force, pitch moment, roll moment, and yaw moment on the airplane. That is thirty six more aero coefficients. Each of the six-axis accelerations develop six inertial coefficients, for thirty six more coefficients. Angle of attack and angle of sideslip are very powerful load sources and have six-axis coefficients that are inter-related. That means sideslip coefficients vary with AoA and vice versa.
External stores each have their full set of coefficients to contribute. Ever wonder what a "drag count" is for an external store? It is the drag coefficient of the store at 0.80 mach x 10000. There are thousands of pressure coefficients to describe the air pressure over the entire area of the airplane surface for every flight condition.
Don't get the idea these multitude of coefficients are constants, far from it. Every one of them varies with mach number, angle of attack, sideslip angle, etc. Someone has to calculate, measure, estimate, derive, guess at, etc. every one of them. It is a big job. Then, as flight test progresses, test data is used to revise the coefficients and build a complete model of the airplane.
So you begin to see the aerodynamics and motion of an airplane are completely defined by coefficients. To calculate the total effects of control surfaces, motion rates, motion accelerations, external stores, and on and on, coefficients are summed. That means for example, the roll moment coefficients from many different sources are added to get the total. For that process to work, all coefficients must use the same reference area and reference length. Force coefficients use area, while moment coefficients use area and length. The most convenient ref area is wing area (S) and the most convenient ref length is mean aerodynamic chord, MAC.
I hope that explains why the 747 has a ridiculously small drag coefficient (0.031) compared the a bullet (0.295).

[sferrin]

Someone has to calculate, measure, estimate, derive, guess at, etc. every one of them.

If there were a hell on earth. . .

[johnwill]

It does get to be tedious, but much of the dog work is automated, as with wind tunnel data, computational fluid dynamics, flight test data processing, etc. One thing I did not mention, original design coefficients are for a rigid airframe. But all of those have to be "flexible-ized" to describe a real airplane. More fun, because until the airplane flies, no one really knows for sure what the flexible effects are.

[eloise]

thanks everyone, you guy are awesome finally get it now

[rheonomic]

For a streamlined body to achieve a low drag coefficient, the boundary layer around the body must remain attached to the surface of the body for as long as possible, causing the wake to be narrow.

This is also why, for example, gold balls have dimples. These trip the flow into turbulence, which delays flow separation and thus reduces the pressure drag compared to separated flow. As a similar example, for a cylinder immersed in laminar flow, separation will occur approximated 80 degrees from the leading edge, whereas in a turbulent flow field separation will be pushed back to about 110 degrees.