Expert: Paul Soderman - 9/24/2006

Question
Hello Paul -

Regarding winglets on transport aircraft, I have a question on maximum crosswind components. The airline I work for lists a recommended crosswind component of 35 kts. for a non-winglet 737, as opposed to 33 kts. for one with the blended winglet configuration.

Why is this? I've got a hunch it has to with center-of-gravity and rotation about the yaw axis, but am having a problem visualizing it.

Thanks in advance - Matt


Answer
Hi Matt

I hadn't heard of this restriction on winglets, but it makes sense.  Winglets are designed to reduced aircraft induced drag by modifiying the vortex distribution in the tip area, which reduces the wing downwash, which reduces induced drag.  The winglet lift is slightly in the aircraft thrust direction, so some of the winglet drag is negated.  And of course, the wing span is less than the wing would have to be for the same induced drag, so airport congestion is reduced.  What a nice aerodynamic improvement, thanks to Whitcomb, a NASA engineer.

In crosswind, the loading on the two winglets would be different - higher loading on the upwind side.  That would affect yawing moment due to the asymmetric lift forces (plus or minus depending on where the force lines pass the center of gravity).  And a rolling moment would be induced.  With different winglet loading, the lift of the left and right wings would be different, which induces a rolling moment.  The aileron performance also depends on the local flow induced by the winglet.  And the difference in induced drag (left and right wing) would affect yaw moment.  I am not sure which is more import (roll or yaw) without seeing some wind tunnel data and knowing where the center of gravity is relative to the winglets.  As you can see, it is a complicated problem.  But, in any case, the effects are relatively small since the crosswind restriction (winglet versus non-winglet) is only 2 kts.

Paul