This sensitivity to protrusions also gives rise to one of the most serious considerations about using laminar-flow airfoils: The wing must be perfectly smooth.
#BOUNDARY LAYER AIRFOIL SKIN#
This means that the wing skin must be entirely free of protruding fastener heads or other similar features in the portion of the chord where laminar flow is expected. Protrusions and ContaminationĪny obstacle to the flow will trip the boundary layer. There have been cases where a spanwise paint stripe was enough to trip the boundary layer. This means it will not tolerate lap joints between metal skins or any skin joint that is not completely filled and faired.
Laminar flow will also be tripped by any significant step protrusion or discontinuity in the skin. Any significant roughness will lead to premature transition to a turbulent boundary layer. In order to sustain laminar flow, the surface of the wing skin must be smooth. Trying for the drag reduction of laminar flow is worthwhile for fast cross-country airplanes like this Lancair Legacy. Both the surface condition and the shape of the wing are critical to maintaining laminar flow. Laminar boundary layers are very sensitive and easily “tripped” into becoming turbulent. Attempting it imposes some significant requirements and restrictions on the airplane. This drag reduction is not easy to achieve. The best laminar airfoils can have drag levels of about half that of airfoils with full-chord turbulent boundary layers. Airfoils designed to sustain laminar flow can have much lower parasite drag than turbulent-flow airfoils. They produce significantly less skin friction drag than turbulent boundary layers. Laminar boundary layers flow more smoothly over the skin than turbulent boundary layers.
Once the flight conditions the airplane will operate at have been defined, and the designer knows what lift coefficient to optimize the airfoil choice at, it’s time to consider what type of airfoil to use.Īirfoils break down into two general classes based on the behavior of the boundary layer, which is the thin layer of air right next to the surface of the wing.
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