Forces Acting on an Airplane
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were finished to exacting tolerances. An absolutely smooth surface was necessary due to the fact that any surface break or rough protrusion would interrupt the airflow and detract from the laminar flow theory. Because of the exactness required, the foil had been shelved by other manufacturers due to the clearances and tolerances which are used in mass production.
The engineers at NAA approached this problem with a plan to fill and paint the wing surface to provide the necessary smoothness. The foil which was used for the Mustang had a thickness ratio of 15. 1 percent at the wing root at 39 percent of the chord.
The tip ratio was 11. 4 percent at the 50 percent chord line. These figures provided the maximum thickness area at 40 percent from the leading edge of the wing and resulted in a small negative pressure gradient over the leading 50-60 percent of the wing surface.
The B-24 bomber's "Davis" airfoil was also a laminar flow airfoil, which predates the Mustang's. However, the designers of the B-24 only knew that their airfoil had very low drag in the wind tunnel. They did not know that it was a laminar flow airfoil.
There were several aircraft modified by NACA, in the late 1930s, to have laminar flow test sections on their wings. Hence, such aircraft as a modified B-18 were some of the first aircraft to fly with laminar flow airfoils. The boundary layer concept is credited to the great German aerodynamicist, Ludwig Prandtl.
Prandtl hyposthesized and proved the existence of the boundary layer long before the Mustang was a gleam in anyone's eye. Example: First, lets get more specific about what laminar flow is. The flow next to any surface forms a "boundary layer", as the flow has zero velocity right at the surface and some distance out from the surface it flows at the same velocity as the local "outside" flow.
If this boundary layer flows in parllel layers, with no energy transfer between layers, it is laminar. If there is energy transfer, it is turbulent. All boundary layers start off as laminar.
Many influences can act to destabilize a laminar boundary .Скачать