[m_ridzon]

I am doing airfoil work, calculating lift and drag coefficients with traditional lift and drag equations from fluid mechanics. To date, it has been my understanding that the planform area to use in the calculations should be the respective area in the specific direction; i.e., use the projected frontal area for drag calculation and use the projected top area for the lift calculation. Someone recently told me that was wrong for airfoil work. They said to instead use the projected top area for both calculations. Oddly enough, when I made this change in my work, I noticed more sensible results. Is this standard convention for airfoil work?

Thanks in advance!

[praveen]

It does not matter which area you use as long you as you stick to one convention and everybody uses this convention, both experiments and numerics, to tabulate the non-dimensional results. That way you can compare the results.

The purpose of non-dimensionalization is to generate results which apply to not just one airfoil but to a class of airfoils (at same Re) which are self-similar, that is they have same shape but scaled up or down (usually down because in a wind tunnel you use smaller models to fit in the tunnel but you dont have this restriction in numerics).

Sometimes it can make physical sense to use a particular area. Wetted area is more relevant if drag is predominantly viscous. For high mach flows, drag may be dominated by pressure drag, then frontal area is more relevant for this phenomenon.

Whatever area is used for drag/lift coefficients must be clearly stated and you must use same area to compare your results with other results.

[m_ridzon]

Originally Posted by praveen

It does not matter which area you use as long you as you stick to one convention and everybody uses this convention, both experiments and numerics, to tabulate the non-dimensional results. That way you can compare the results.

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Whatever area is used for drag/lift coefficients must be clearly stated and you must use same area to compare your results with other results.

Thank you, but this is not a very useful reply. You gave a very vague, general answer to a rather specific question. In my paraphrase, you basically said, "use whatever area you want, but be sure to document it." My question was to a very specific niche of engineers who work on airfoils. I believe this area of engineering has a "standard convention" in terms of using the planform of the projected top area (i.e., view from above) when calculating both lift and drag. And published data I've run into, whether online or in books, has often omitted any detailed mention of the exact planform used in lift versus drag (they usually report the chord length and nothing else in terms of dimensions). This leaves the reader to believe a "standard convention" exists, in that a certain planform area is typically used for both the lift and the drag data being reported. In a very exact technical sense, this is incorrect to use the lift's planform area for calculating drag. But if it's standard convention in this field, then it becomes acceptable because it's assumed and understood by anyone working in the field.

I believe the overhead planform is used (view from above) for reporting both lift and drag, but I'm looking for confirmation from someone who has extensive airfoil background.