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Name: Zé Martins
Status: Student
Grade: 9-12
Location: N/A
Country: United States
Date: August 2008

My question was to do with air resistance. I noticed that a question on this had previously been answered about roller coasters. however, what was not mentioned was how to determine the air drag coefficient. I am writing an essay about a pull back car. My aim is to determine the heat lost inside the engine.

I plan to achieve this by taking the energy conservation principle that input=output and making the total energy input Fd=(1/2mv*v)+(Friction*distance)+(Air resistance*distance)+(heat lost inside engine). I have gathered all equations and apparatus. I am using Air drag=1/2CA(v*v)d where A is S.area, d is air density, v is velocity and C is air drag coefficient. My question is: Experimentally, with apparatus and the desired equations, how do i find a) the air drag coefficient and b) the density of the air?

Hi Zé,

Unfortunately real world situations with something as complex as a vehicle, do not come close to being able to be described by the simple equations you stated.

It is impossible to calculate the drag coefficient by simple calculations. There are two common ways presently used: one is to simply measure it using the actual vehicle in a "rolling road" wind tunnel, and the other is to use sophisticated FEA (Finite Element Analysis) software to analyze an accurate 3D CAD model of the proposed vehicle. The latter, in spite of its sophistication and complexity, will generally result in close, but not particularly accurate, results. In real life, the published drag coefficient (Cx) of a vehicle is determined by actual measurement. The values determined using FEA software (and lots of supercomputer time) is generally used as a comparative design guide. The CAD-generated results are not particularly accurate, but one can see the CAD-derived Cx value change, as changes are made to the CAD design. In this way, one can see if the change was beneficial or detrimental.

You asked how to determine air density. This can easily be looked up in published tables. Regarding your intention to determine heat lost by the engine, it seems that your proposed method of determining this is far too simplistic. It should also be noted that there are many more significant sources of heat loss than just the engine itself, such as the transmission, the differential, wheel bearings, the tires, and even air resistance itself. In fact, in a typical car motor that is rated at (let us say) 200 HP, the actual power delivered to the driving wheels will be less than 150 HP. The 50 HP or more difference (in this case) is a very significant source of heat loss that does not occur inside the engine and would not be taken into account in your suggested equation.


Bob Wilson

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