Hi all,
Think of airplane wings - aysemetric wings are used on low speed aircraft. High performance wings (less drag) are fully symmetrical.
The only serious use of symmetric foils on aircraft is for aerobatic aircraft, because they want to have similar flying characteristics in normal and inverted flight. I know of not any modern aircraft, even if high performance, supersonic or whatever which uses symmetric foils. And that for a good reason. Especially at transsonic speeds the differences are significant.
the relationship is very similar (Reynolds numbers). Generating the same lift, a symmetrical foil will provide less drag through the water
The Reynoldsnumber is nothing constant at all, double speed - double Reynoldsnumber. Furthermore there are other differences (e.g. Mach number). Generating the same lift an asymmetric foil has less drag than a symmetric one.
However, if you compare aircraft with sailing boats, you have to consider that the wetted area of the daggerboards and rudders is small compared to the wetted area of the hull(s), while on aircraft wing and empennage have a significant contribution to wetted area. Hence on boats, the non-induced drag is dominated by the hull and not the foils, while on aircraft the foil contributes strongly to drag.
A bit off-tobic, but there is also a common misunderstanding for induced drag: The wrong saying is, that iunduced drag depends on apsect ratio. The correct one is induced drag depends on span, and induced drag coefficient depends on apsect ratio.
To get an understanding how different water and air actually is, just have a look at an aircraft propeller and a ship propeller. Other number of blades, blade aspect ratio and sweep angle and -if you have a closer look- totally diffrenet foils.
However you can use low speed airfoils, Reynoldsnumber 500000-6000000, in water. But you have to be careful, that the lowest cp value isn't below the cavitation limit. Actually I have seen a lot of rudders on boats which start to cavitate before reaching their lift limit.
Finally if you want to use the full benefit of better foils, you have not only to 'exchage' the profile, but also to adapt the area of rudder and daggerboard. Example: You have a NACA 0012 with let say cLmax of 1.4 and low speed (Reynoldsnumber) and now you get a purpose designed section (which need not to be asymmetric) with a cLmax of 1.7, than you could reduce the area with ratio 1.4/1.7 and you would still have the same manoeuvrability.
Cheers,
Klaus
