I am referring to the Kaman K-max and the Kaman Huskie here. They both use NACA 23012, which is an asymmetric airfoil. Any reason as to why this is? Is there a different mechanic in play when intermeshing rotors are considered? Does Reynold's number change due to factors that don't affect normal helicopters?

Edit - I can see how my question can become very broad. My question would be answered if somebody could explain what factors come into play in intermeshing rotors that affect the choice of airfoil.

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    $\begingroup$ Pretty sure most helicopters use asymmetrical airfoils, not just the ones you mention, so I'd question if there is a valid premise underpinning this question. $\endgroup$ – AEhere supports Monica Mar 23 '20 at 9:22
  • $\begingroup$ Yeah, I edited the post. $\endgroup$ – FerrumYanda Mar 23 '20 at 9:37
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    $\begingroup$ Yep, I'd say the simple answer is, because they all do. The only flying machines that don't use them have special reasons not to. Intermeshing creates mechanical and control issues, but nothing related to the airfoil. $\endgroup$ – Guy Inchbald Mar 23 '20 at 17:48
  • $\begingroup$ Helicopters may use either symmetrical or more commonly asymmetrical airfoils. It is not the symmetry that is important but the low pitching moment (Cm). The reason for using airfoils with low pitching moment is to avoid blade twist because unlike thick airplane wings helicopter blades are thin and relatively flexible. If you look at NACA 23012 you will see that the pitching moment stays around 0 at useful alpha angles (note the range between 5 to 15 degrees). As such this is not a bad airfoil for heli blades $\endgroup$ – slebetman Mar 23 '20 at 18:05
  • $\begingroup$ Cambered airfoils started to appear on helicopters in the 80s, when as slebetman says, new composite materials made blades torsionally stiff enough to tolerate them. This allowed higher Cls and a bit more efficiency from the blades. $\endgroup$ – John K Mar 23 '20 at 18:27

Helicopters in general use asymmetric aerofoils because they are not required to fly inverted or produce negative lift. Intermeshing helicopters are no different in this regard.

The exception would be aerobatic model helicopters, which use symmetrical aerofoils because they are designed for similar performance either way up.


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