![]() This gave researchers an idea of what level of wing surface quality was required to actually get the benefits of laminar flow airfoils. In flight, this configuration was found to have a profile drag representative of boundary layer transition at 60% of chord. Surface waviness was then measured and found to be no more than 0.005 inches (0.13 mm). After the paint was dry, it was sanded in a chordwise direction, using sanding blocks, whose curvature matched the local surface curvature. The wing was then sprayed with two coats of primer paint and a coat of paint type filler. To reduce the waviness, RAE personnel stripped the wing to bare metal. The standard waviness criteria shows the critical wave height to be 0.0053 inches (0.13 mm) for this application. This showed peak wave amplitudes, above the mean, of approximately 0.011 inches (0.28 mm) over a two-inch (5.1 cm) span. Measurements were made of the surface waviness. Reducing the surface roughness reduced the drag at low lift coefficients to a level representative of laminar flow to 35% of chord. In the "as delivered" configuration, a profile drag was measured which was representative of the wing section with boundary layer transition at the leading edge (0% laminar flow). The wing airfoil was designed to support laminar flow to 60% of chord. The RAE first tested it in an "as delivered" configuration. British testing of its wing suggests that the build quality of the period's metal wings was insufficient to maintain laminar flow. The Bell P-63 Kingcobra used early laminar airfoils, the NACA 66(215)-116 at the root and the NACA 66(215)-21 at the tip. Most important for its low drag, however, was the very smooth wing surface of the P-51 with no gaps ahead of the spar. The suction peak near the nose of older airfoils will lead to local supersonic flow at a lower flight Mach number, and increased drag from the shocks which would follow. ![]() The "rooftop" distribution of the 6-digit NACA airfoils did help, though, because it gives them a higher critical Mach number than the "peaky" distributions of earlier airfoils. At higher Reynolds numbers it needs progressively steeper gradients to keep the boundary layer laminar, such that the range of angles of attack where a long laminar boundary layer is possible on both sides of an airfoil (the laminar bucket) gets smaller and smaller. How laminar was the P-51 wing?Īt the flight speed of the P-51 very little laminar flow was left the full effect of laminar airfoils can only be exploited at Reynolds numbers below 5,000,000. The graph in your question is misleading because the lower side flow of the traditional airfoil should be as laminar as that of the P-51 airfoil if the surface smoothness of both is comparable. Upper side flow past the suction peak near the leading edge is a prime candidate for transition, and that is what causes flow around the "traditional airfoil" to become turbulent earlier. On the other hand, a pressure rise in flow direction corresponds to a deceleration in flow direction, so any movements perpendicular to the flow direction will grow relative to the flow speed, and as a consequence the turbulent transition occurs rather quickly. On modern gliders the lower surface is laminar in excess of 80% chord at higher angles of attack, which can correspond to a Reynolds number of 5,000,000 or more when transition eventually occurs. If the flow is accelerated, all speeds in flow direction increase while cross flow will not be affected, so a laminar boundary layer in accelerating flow is stabilized. disturbances like bugs, rivet heads, waviness or turbulators.įlat plate flow (without pressure changes) normally transitions at a Reynolds number of around 400,000.How soon it transitions to a turbulent boundary layer depends on: In still air every boundary layer starts laminar.
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