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• barbz_dee
• Topic Author

### barbz_dee created the topic: ADY cyber exam FJ

The 'S' in the drag formula is? max Frontal area or max plan area?

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 388

### John.Heddles replied the topic: ADY cyber exam FJ

You need to think back to what the coefficient is doing for us. By using non-dimensional coefficients, such as Cd, we are able to compress vast amounts of engineering test data into small sets of functional information.

In developing Cd, we needed to include something with dimensions of (length) squared to make Cd dimensionless. (Length) squared we normally think of as an area. It doesn't really matter what area you use in developing Cd data, you just need a convenient area to get (length) squared into the equation.

As a result, you should see, in any reference engineering text which is providing numeric data for Cd, information as to what reference area has been chosen for the data. It matters little, really, what area you use .. just changes the numeric values of Cd presented in the data. If you wanted to be very simplistic, you could just use 1.0 unit of area in whatever system of units you might be using .. doesn't matter one iota.

In answer to your specific question, though, the use of a frontal cross sectional area or the wing plan area would be two reference areas which would make sense for aircraft work.

So far as exam questions are concerned, if the examiner wants you to play with numbers, the particular reference area has to be specified unless you are only looking at proportions, in which case you are unlikely ever to need a specific value for the reference area used in the Cd data.

Again, whatever data you might be using should include some information as to just what reference area has been used in its derivation.

Engineering specialist in aircraft performance and weight control.

• Posts: 2098

### bobtait replied the topic: ADY cyber exam FJ

CASA do not expect you to actually do any calculations to obtain a numerical answer. The idea of the formula is simply to present the factors that affect drag or lift. As far as the exam is concerned, the answer they expect is the maximum frontal area for drag and the plan area for lift,
The following user(s) said Thank You: barbz_dee

• barbz_dee
• Topic Author

### barbz_dee replied the topic: ADY cyber exam FJ

Hi, some students sat last week Friday and got a question in the cyber that asked:
What is the 'S' in the drag formula?
Options were
A.max frontal area
B.mean frontal area
C.max plan area
D.mean plan area

They put A. but in their KDR it states that it was wrong.
So, Im confused. What should it be?

• Posts: 410

### Stuart Tait replied the topic: ADY cyber exam FJ

If we think of drag as being a resistance to the flow, a logical choice would be the frontal area (Af) of the body which is perpendicular to the flow direction. This is the area shown in blue on the figure

I would think that is Maximum frontal area
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• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 388

### John.Heddles replied the topic: ADY cyber exam FJ

So, Im confused. What should it be?

Unfortunately, it is whatever area the examiner might like to prefer, ie which book did he read ? ANY of the answer alternatives would be more than acceptable and OK for the Cd formula. Indeed, absolutely ANY area will do.

However, somewhere it MUST be defined if you want either to play with numbers or, as here, state what area is used.

The ONLY way your question will be resolved is for someone to run the question by the examiner and find out what area he wants to be nominated for Cd.

It really is that simple .. it doesn't matter an iota what area you use, but it must be defined if we are to make any use of Cd data. Choosing one area over another has ONLY ONE effect .. the Cd numbers change. We use the numbers in exactly the same way. This is the reason that the (oft-seen in adverts) Cd data for motor vehicles are very different to what we normally see quoted for aircraft ... the area specification generally used is different in the two cases.

So far as having this sort of question in an exam ... just plain silly and pointless.

Engineering specialist in aircraft performance and weight control.