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KDR exam question

  • lucy_simkin94@hotmail.com
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lucy_simkin94@hotmail.com created the topic: KDR exam question

Hi Bob and Friends
A question on the Drag equation
DRAG=Cd 1/2pV^2 S
What does S represent?
According to the online exam S=frontal area, the area presented to the airflow/the hole in the airflow.
In the exam I believe S=max plan area as I have it on my KDR's despite being almost certain it should be frontal area, I googled it and it seems there can be either in the equation depending on the desired reference for the drag, so if comparing to lift the S=max plan area but if comparing to AoA or something I suppose frontal area is used, do you know how to decipher or what is the accepted equation to our dearest at CASA?
Looking at the MOS Unit 1.3.1, 2.4.1 (c) S - defines surface area, must be referring to Max plan area, if this is the case maybe the online exam question should be revised to reflect this because I chose that on the practice exam and was told frontal area where upon I lost a mark in the actual exam if that is the case.
Not that it matters because I passed so its no gripe of mine.
Cheers Lucy
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Richard replied the topic: KDR exam question

Hi Lucy,

The S in the Lift equation does indeed refer to the plan area of the wing, since the plan area is that which is being used to produce the lift.
The S in the drag equation is the frontal area, and for the CASA exam, my understanding is it refers to the mean frontal area (not the maximum frontal area). The frontal area is the surface area being exposed to the oncoming airflow and as such is responsible for the drag on the aerofoil in the wind tunnel.
Cheers,
Rich
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  • lucy_simkin94@hotmail.com
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lucy_simkin94@hotmail.com replied the topic: KDR exam question

Hi Richard, that is most likely where i went wrong probably hit max frontal instead of mean frontal, thank you very much for clarifying i was a bit perplexed :)
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  • John.Heddles
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John.Heddles replied the topic: KDR exam question

Useful point to keep in mind is that the main purpose of S in the equations is to balance units and make CL and CD non-dimensional. It doesn't really matter a toss just what area is being referenced, only that there is an area .. only changes the numbers which come out of the calculations. Most important to check just what S is defined for what ever document you are reading if you want to do the numerical sums (and they aren't difficult .. one only has to be careful of using consistent units).

Engineering specialist in aircraft performance and weight control.
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