I am told that on larger aircraft, suction will be needed to maintain laminar flow because of the larger leading edge radius. Why so? Is it because the air has to travel over a longer distance on the skin, so slows more, and thus piles up toward the trailing edge, causing adverse pressure?
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asked May 3, 2020 at 9:50
Abdullah is not an AmalekiteAbdullah is not an Amalekite
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$\begingroup$ The flow path is longer, so transition happens relatively earlier. Also, wing sweep and gaps from leading edge devices will quickly trigger transition. The LE radius is not to blame. $\endgroup$– Peter KämpfCommented May 3, 2020 at 10:39
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$\begingroup$ "relatively earlier" as in percentage of chord? $\endgroup$– Abdullah is not an AmalekiteCommented May 3, 2020 at 15:59
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$\begingroup$ wait why would LE radius not have an effect? since the LE slows things down/causes an adverse pressure gradient? $\endgroup$– Abdullah is not an AmalekiteCommented May 3, 2020 at 16:07
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$\begingroup$ Every nose has a stagnation point, regardless of radius. The larger radius only makes the suction peak near the lading edge at higher angles of attack less peaky. $\endgroup$– Peter KämpfCommented May 3, 2020 at 19:53
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Sort of interesting that very slow, lower Reynolds number airfoils, such as models, actually have to trip the airflow to form a boundary layer.
Surface friction causes the air near the wing surface to move at the wings speed. In a wind tunnel this is slower than the free stream, in the air, the moving wing speeds up the air around it. In relative terms, the effect is the same.
Boundary layers are draggier than laminar flow, but much less draggier than turbulent flow. The boundary layer is like "grease", helping reduce drag and create lift.
At higher subsonic Mach numbers, early efforts at a "laminar flow wing" for long range escort fighters resulted in something resembling an airfoil flown backwards, with a sharp leading edge, a very gently increasing thickness (to keep laminar flow), maximum thickness further back towards the trailing edge, and a much shorter taper to the trailing edge. This is reminiscent of the blunt "boat tail" approach being tried these days, also found in bullets.
Though successful, low speed handling and stall characteristics led to some needed modifications.
But chord length, and airspeed, as well as airfoil type and surface smoothness, are very important parameters in predicting airflow around a wing. Airfoil Tools, on the web, is a great place to begin to explore airfoil types for various applications. Particularly of interest are Coefficient of Lift vs AOA, and Lift/Drag ratio.
answered May 3, 2020 at 15:19
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1$\begingroup$ when you write "boundary layer" you sure mean "turbulent boundary layer". There is always a boundary layer, and it is either laminar or turbulent. $\endgroup$ Commented May 3, 2020 at 15:54
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$\begingroup$ Thats what i was wondering after reading this answer. so far as i knew, there are laminar boundary layers - the best kind - then turbulent. The only thing that goes beyond that is separated layer, where flow at skin is moving upwind. Am i correct? what's going on in this answer? $\endgroup$ Commented May 3, 2020 at 15:57
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$\begingroup$ @ABJX if only we could see the actual flow, "laminar flow" would be near impossible near the surface, where air struck by the wing surface is accelerated and deflected. Terminology aside, flow separation is what we do not want. Much depends on Reynolds number, and Mach number as well. As Peter pointed out in his link, laminar flow is a reality for the underside of the wing, and supercritical design sadly parts with camber that serves so well at higher subsonic Mach numbers. When I began studying airfoils, I thought battleships had it wrong. Now, perhaps not! $\endgroup$ Commented May 3, 2020 at 23:04