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The oceans are becoming less viscous as they are heated. I'd imagine a similar effect is likely occurring in the atmosphere as well. What, if any, effect would this reduction of viscosity have on aeroplanes? Would they be more efficient and use less fuel?

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As far as I remember viscosity of the gases, unlike liquids, increases with increasing temperature. So in order to decrease viscosity, You would have to cool the air (which is already cold at some 10 km).

When it comes to it's effect on planes, viscosity is responsible for creating pressure gradient between top and bottom side of an airfoil or wing. That pressure gradient creates lift. On the other hand viscosity affects drag - generally the more viscous fluid, the greater drag.

There is no easy answer to Your question, but if You would like to improve flight conditions, I think it would be easier to start with the plane construction and materials, instead of the air viscosity.

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  • $\begingroup$ It is not clear (to me at least) whether or not an increase in viscosity would increase drag. An increase in viscosity would decrease the Reynolds number causing the location of transition from laminar-to-turbulent flow to move towards the trailing edge. $\endgroup$ – OSE Mar 24 '14 at 18:31
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    $\begingroup$ The viscosity of (an ideal) gas does indeed increase with temperature. $\endgroup$ – John Rennie Mar 24 '14 at 19:14
  • $\begingroup$ First of all, there are different kinds of drag and "the effect of viscosity is that the lift is reduced and a total drag composed of skin-friction drag and pressure drag is present. Both of these are detrimental effects. (...) No drag would result if the airflow were frictionless (inviscid). " $\endgroup$ – Wojciech Mar 24 '14 at 19:48
  • $\begingroup$ @Wojciech Not only would no drag exist if it were inviscid, but no lift would exist either (technically). $\endgroup$ – tpg2114 Mar 24 '14 at 20:24
  • $\begingroup$ Thank you all for the interesting discussion. It was just something I was pondering over while I try to learn a bit more about fluid dynamics. That is interesting about the ideal gas becoming more viscous with increasing temperature. I'll look into it further. $\endgroup$ – Seanosapien Mar 24 '14 at 23:46

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