

Finding Air Drag Coefficient
Name: Zé Martins
Status: Student
Grade: 912
Location: N/A
Country: United States
Date: August 2008
Question:
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?
Replies:
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 CADgenerated
results are not particularly accurate, but one can see the CADderived 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.
Regards,
Bob Wilson
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