# How much friction is caused by a 10x10 cm window on the side of an airplane?

Say that the airplane is going in 1000 km/h. On the side of the airplane, there is a 10x10 cm window. How much friction would this window cause. For the sake of the calculation, imagine that the airplane is infinitely long. The window is also perfectly level with the rest of the wall, and there is no turbulence on the side of the plane. The outside air is also standing perfectly still. The airplane is travelling at sea level.

I know it is neglectable compared to the turbulence etc. caused by the form of the airplane, but I really need the answer to the question.

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I have to wonder, why do you really need the answer to this question? – David Z Dec 25 '10 at 22:46
If the window wasn't present, what would be? Painted or bare aluminum fuselage wall? Is this all about the friction coefficient of glass? What are you asking? – sigoldberg1 Dec 25 '10 at 22:54
Unfortunatelly, I have no idea what it would be if the window was not present. I guess the friction coefficient of glass would be relevant for answering this question, but I really just need to know the energy loss on glass due to air passing by the glass. The reason for the question is an invention of mine. – David Dec 25 '10 at 23:03
@David: and do you think an answer that assumes the plane being infinitely long will really be relevant for your invention? :-) – Marek Dec 25 '10 at 23:19
Given the vagueness of your question, you really want to ask a practicing aeronautical engineer. This is not really a physics question, more engineering. Try using linked up to find a guy/gal at Boeing, or visit the MIT dept. or NASA or somesuch. You will really need a series of wind tunnel experiments on your invention before you know. – sigoldberg1 Dec 26 '10 at 21:32

## 1 Answer

If the window is pefectly flush, so the the surface geometry is the same as without it, why would we expect there to be any effect whatsoever? I think all that matters is geometry and surface roughness. Also note that 1000KM/hour is pretty fast, airliners might travel that fast a stratospheric height, but drag would be a few times higher at sealevel -because the density is maybe 3 times as high as at cruising altitude.

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I'm trying to understand your last sentence...is "days" a unit of measurement for drag? – Justin L. Dec 26 '10 at 9:16
I cannot assume that the surface is perfectly flush. It is a standard kind of glass (I don't have any specific in mind, just not a type of glass made utilizing nano technology to make it flush). Do you think friction might be 0 under these conditions? – David Dec 26 '10 at 9:29
Well, David, it seems to me, under ideal conditions, that is the window is perfectly shaped, the friction would be 0. If you can provide non idealities, like the window actually has a slight angle with the normal of the side of the plane(which i assumed to be perfectly parallel to the velocity vector), or the window has a rough surface causing a total of 1% of its area to be perpendicular to the velocity vector, it would actually be a very easy question. However this form of the situation seems to have no friction at all. – Cem Dec 26 '10 at 10:01
Surface roughness isn't necessarily a bad thing. Airflow over an airplane is strongly turbulent. Commercial airplines often have a special paint coating with "riblets", which on a microscopic level influence the finer wavelengths of the turbulence spectrun, and reduces drag. I suspect the CFD analysis of this is very difficult, and the results are probably highly dependent upon the low level details interacting with the larger scale flow. – Omega Centauri Dec 26 '10 at 14:42
To my knowledge, there really isn't any measurement for arbitrary roughness for a glass. This is not because we are incapable of calculating it, but I believe rather it is so trivial(and believe me when I say this, so vastly trivial) compared to other forces acting upon the aircraft that it is completely negligible. – Cem Dec 26 '10 at 16:05