The boundary-layers that occur on wings of flying aircraft have both laminar and turbulent-flow regions. Laminar-flow starts at the leading-edge and is felt on the forward part of both wing surfaces. Depending upon the local Reynold's Number, the degree of free-stream turbulence, the surfaces’ curvature and their roughness, the boundary-layer flow will eventually change to being turbulent. The skin-friction drag of these surfaces depends on the way that boundary-layers grow and transit. The turbulent boundary-layer causes the greatest amount of local skin-friction and the postponement of the transition region to aft locations on both surfaces is a useful way to reduce the total drag.
The boundary-layer transition region can be delayed by stabilizing the Tollmien-Schlichting Waves, which are responsible for the transformation. This may be done passively by controlling the boundary-layers with a device to damp out these waves, and to reduce the resulting turbulence as it forms. An early investigation was made to determine the kind of a compliant surface which would be needed to absorb the energy of these transitional waves, before they grow to an unstable size. The necessary physical properties of a flexible skin were calculated, when it behaves as an elastic and damping medium to the waves. It was found that an extremely light surface is required--even a thin latex rubber skin, supported on sponges does not meet this requirement and this solution was seen as being impractical for boundary-layer control. However, it is possible to replace the compliant surface by a less obvious kind of energy absorber, as proposed below.
The effect of the compliant surface is produced by a large number of "blades of grass" or thin streamers, laying parallel to the flow, see figure. Each blade is supported by an elastic "stem" having a circular cross-section, which projects at right-angles from the wing at intervals along it. The stem part of the streamer bends, allowing the blade to become parallel with the external flow.
By suitably designing the length and flexibility of the stems and blades, they can produce the necessary physical properties that the continuous rubber skins could not provide. Due to the controlled mostly laminar-flow outside the effective surface, there would be only a low-velocity air-flow inside and around the supporting stems. Consequently the associated form-drag from these would not add much to the drag developed on the outside of the effective compliant surface.
When a patch of turbulence occurs, the forces on each blade cause it to bend to the shape of the disturbance. Then it would locally rub on nearby blades, as it is displaced from its steady-flow position. Also, when a blade is displaced from the average effective surface, the modified cross-flow around its edges will damp out the motion of the patch of air that caused the displacement to occur. Both of these effects will damp out most of the turbulence, keeping the boundary-layer thin and causing it to produce comparative less drag.