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Feb 7, 2009

Which Wing

Pictures: Ice build-up on the leading edge of a wing.

Pictures: Leading edge slats and trailing edge flaps (single and double slotted).

Which wing is the most optimal? Let's say I have to have fixed landing speed for various reasons (FAR, JAR, etc.). We set the landing speed to 61 knts for example.

Let's also say that I have the trailing edge flaps to lower landing speed (let's choose double slotted for maximum lift). In most airfoils the trailing edge is anyway turbulent, so the flap hinge line does not add any cruise drag.

Now the question is: Do I have any benefit of the NLF airfoil without leading edgen devices when I could also have the leading edge slats with not so laminar airfoil? Obviously the leading edge device destroys the flow there and I have a none laminar case.

1) With the more turbulent airfoil I get smaller wing (A1) and the L.E. device still lets me land at the desired speed (61 kts).

2) With the NLF airfoil I can not use L.E. devices to keep the laminar flow there, so I have to have a larger wing (A2) to keep the landing speed the same (61 kts).

So the question is now: Is the cruise drag for the wing 1 with the area A1 less or more than for the wing 2.

Using the wing area calculator in the next link:

I get the following results for some small airplane (same weight 4750 lbs) and both have the same landing speed (61 kts):

1) Leading Edge Slats, A1 = 126 ft^2

2) No Leading Edge Devices, A2 = 152 ft^2

I save 25 ft^2 if I use L.E. Slats. Is that smaller area enough to win the benefit of the NLF airfoil at cruise? Maybe sombody has allready done this calculation somewhere for some configuration. I have to investigate (or do it my self).

Additional details are that the laminar flow can be hard to keep if the wing is flying in snow, rain, etc. and that the L.E. device could be used to crack the L.E. ice. Other point is that the L.E. device adds some cost, weight and complexity.

I will return to this question later (with an answer)... or send me one!

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1 comment:

  1. I am assuming that the wing 1 is better since it is smaller. But if it really has smaller cruise drag is still not known.