PDA

View Full Version : Parasitic Drag Coefficient Controversy

Pugachev Cobra
29th Mar 2010, 14:23
Enormously technical guys and gals, hi!

Remembering back my lessons on aerodynamics, I recall our teacher telling us that the parasitic drag coefficient - 1.28 - was accepted by the industry, but he mentioned there was some controversy and some different lines of thought.

I have tried to research and dig into this coefficient disagreement, trying to find different lines on it, but haven't found anything significant.

Would anyone here at least try to elaborate on it?

Maybe some reference material (even better if it's found online).

Thanks again!

Mr Optimistic
29th Mar 2010, 16:19
http://aae.www.ecn.purdue.edu/~aae251/Lectures_Fall02/class14.pdf (http://aae.www.ecn.purdue.edu/%7Eaae251/Lectures_Fall02/class14.pdf)

The parasitic drag for the long range high capacity passenger transport Boeing 777-300 was found to be 0.0106. The airplane wing has an aspect ratio of 8.66 and an airplaneefficiency factor of 0.88. The complete drag polar for the airplane is,CD = 0.0106 + (1/_e AR) CL2 or,CD = 0.0106 + 0.0417 CL2

http://pages.slu.edu/student/paisd/research/dragproj.pdf

...didn't see 1.28 (or much I understood)

Pugachev Cobra
29th Mar 2010, 18:14
Actually that's interesting material, but doesn't really help about my question. I think I didn't expressed myself properly.

For reference, this basic NASA link depicts it:
Shape Effects on Drag (http://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html)

My doubt is about the drag coefficient of the equivalent flat plate, which most textbooks seems to always refer to as 1.28.

Are there other studies challenging this fixed value?

Mr Optimistic
29th Mar 2010, 18:35
Wapedia - Wiki: Drag coefficient (http://wapedia.mobi/en/Drag_coefficient)

has the table you refer to but before that is the statement

'The overall Cd of a real square flat plate perpendicular to the flow is often given as 1.17.'

Since it has 2d and 3d values this looks like a potential theory calculation with the 2d result representing a semi infinite plane. I will have a look around but hopefully some expert will turn up soon.

Can find things like this

http://staff.fit.ac.cy/eng.fm/classes/amee202/amee202_cn_chapter08.pdf

but it isn't clear where the numbers come from (measurement or calculation).

I have just remembered how much I hated those complex plane transformations !

Oh, found another one.

http://ftp.rta.nato.int/public//PubFulltext/AGARD/AG/AGARD-AG-336///08chap06.pdf

This has real measurements.

Pugilistic Animus
31st Mar 2010, 20:43
I'm not sure I understand are you refering to section characteristics cd or to wing characteristics Cd?,...the sections along the span have different individual section characteristics,...when using an equivalent flat plate comparison, the number obtained [whatever it is from the calculation] is for use in comparing actual airfoil characteristics with the flat plate model in order to gain insight as to the wing's i.e. improve section characteristics so the numerical value is not a important as what it stands for-a simplified system with which on compares actual data,...the text's value is most likely correct,...I really try not to get bogged down in numbers,...but rather consider the system first,..check numbers later:)