For airplanes, it's generally calculated. What's measured is fuel consumption. Then thrust is calculated based on engine performance. Then CD is calculated from the thrust.

However, it can also be measured in model tests. And it can be calculated analytically.

It all depends on what problem you are trying to solve.

For cars the answer might be different. It's easier to measure actual cars in full-scale windtunnels.

 

^ ok, that's what i expected. thanks!

 

Interesting stuff, Mike. I would have guessed that using models was almost universal, but that's probably because it's a highly visable process. I've watched wind tunnel tests, and have always just assumed that it was a common practice. Shows how easy it is to come to a wrong conclusion when watching a process you only partially understand. Good post Mike; It's always nice to learn something new.

One method used with automobiles simply lumps all the sources of drag togehter. We take the car up to speed, and then measure the coast down rate. I now use accelerometers or GPS logging to get several samples per second, but we use to make the measurements with a stop watch at much lower resolutions. Once you have measured the coast down rate you can make changes to the car, and the change in the coast down rate tells you how effective the changes were. Works for aero changes, tire changes, etc. I guess it's a pretty crude approach, but it works quite well.

 

I'm reading a review of AP1 in a respectable auto mag, and it says that with the top up Cd is 0.43

 

and which mag is this?

 

For the physics guys, a question:

The pricinple that explains how airplane wings generate lift is well known, however, why is it that airplanes are able to fly up-side down when that is actually a contradiction to the same principle?

I never understood this.

 

actually the reason how airplanes generate lift is not at all well known. the very common misconception gives credit to the assymetric airfoil shape, which then follows that a plane can only fly in 1 orientation

what matters is the angle the wing is presented to the medium.. called the angle of attack. a plane can fly and even climb upside down..just not as efficiently as it would normal-side-up

 

also the effect of the airfoil will be altered by the movement of the elevators and ailerons affecting the action the fluid (air) takes over/under that surface

 

http://www.av8n.com/how/ <--- more than youll want to know

 

Russian Autoreview.

they tested AP1 s2000 in the wind tunnel at 144km/h and the results were:
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