We should find a way to attract aerodynamics guru in this thread. I vaguely remember of Navier-Stokes equation from my generic engineering school, 25 years ago, not enough to give a bit of movement to this thread.
Peter wrote:
It would really surprise me if it did. Does airflow break from laminar to turbulent at different temperatures?
I’m not aerodynamics expert but I don’t remember temperature is ever mentioned in this context.
Not an expert, but my humble logics tell me: the characteristics of a wing vary according to air density.
Air density changes with variations in atmospheric pressure (L or H weather isobars or altitude increase or decrease), temperature and humidity. Therefore a change in temperature alone will affect air density which in turn will affect the characteristics of said wing…
Now different wings (platform, profile (polar), aspect ratio, chord, etc) will be affected differently, and obviously not to the same extent, the same as laminar to progressive turbulent flow separation differs on different wings.
With a few equations it seems that lift = volumetric_mass x surface x (speed^2 – speed_far^2) / 2 (speed is the speed of air on some representative point on he wing, speed_far is the true airspeed, speed of air far away from the wing). Assuming constant speed for a given wing, and assuming the airspeed profile along the wing does not depend on temperature (remember the true airspeed is constant, not indicated), this means that lift is always proportional to density. Having a smaller or bigger wing loading changes the relative weights of surface vs (v^2 – v_far^2) in the equation above, but the effect of temperature (i.e. density / volumetric mass) is the same for both.
Taking a representative point is no problem since the equation can be repeated and integrated over (and under) the wing with the same result. If this is correct, only marginal non-ideal / viscous behavior could have an impact.
Of course I’m no expert either, I might have gotten some assumptions wrong.
Peter wrote:
Does airflow break from laminar to turbulent at different temperatures?
Yes. Well, it’s dependent on Re, and in the atmosphere, Re will change with temperature (if the velocity is the same). But, is the effect even noticeable compared with the corresponding effect of change in density (in the atmosphere)? Hardly I would say, but definitely measurable. A break from laminar to turbulent is dependent on 3 things:
For all practical purposes, skin roughness is the most important one. Flying in rain vs a dry wing could be like night and day due to this. The Atec Faeta has a laminar flow wing. The stall speed is about 30 knots. In rain the stall speed is 50+ knots. Which is surprising since the Alphatrainer also has a laminar flow wing, and has no noticeable such effect. A normal GA NACA profile has no such effect, unless covered in 10 cm of snow or something 
I think this has more to do with the surplus power of the engine. If it climbs at 4-500 fpm optimally, then on a hot day, it will hardly climb at all, or at least that is what it feels like. On a clear winters day in -20, it will be like a rocket, climbing at perhaps 700 fpm
I think this has more to do with the surplus power of the engine. If it climbs at 4-500 fpm optimally, then on a hot day, it will hardly climb at all, or at least that is what it feels like.
I think you’re correct and would add that even with enough excess power at a higher density altitude, higher TAS results in a shallower climb angle at the same airspeed and climb rate.
@LeSving but surplus power is more related to engine power output, that is related e.g. to air density. And not to airfoil “efficiency”.
Mathematically there should be a difference in between airfoils, as per what Peter was asking, but I question whether this is noticeable in any terms that are relevant to us. I think it isn’t.
