OK, this post is for people like me who are stirred into wide eyed wonderment when clever whizz-kids with big HP scientific (nogal) calculators start outsmarting each other with terms like AoA, boundary layer, viscosity, lift, thrust, Von Karman vortices, etc. And this post is in connection with flaps, slots and slats. And this stuff is just slapped together from the books on my shelf. And this is just because I would also like to know what's going on and what the whizz-kids are getting their undies in a knot about. So this is meant for guys like Pottie and me. So why do whizz-kids stick flaps and slats and slots and stuff onto perfectly beautiful solid wings which work so well while cruising along? Answer: Because the wing is not large enough to ensure comfortably low landing and take-off speeds; and because the wing is not large enough for serious manoeuvres in the case of the Gripen. How large is large enough? To provide a lift force, the wing needs to be able to deflect a stream of air; a larger stream of air deflected at a larger angle needs to be pushed harder by the wing. In turn, the air pushes harder back at the wing, providing lift, which is precisely the trick that the whizz-kids want to play on Mother Nature. But, Mother Nature has a trick up her sleave too: to their horror, the whizz-kids discovered that if they take their cleverly conceived tiny wing and try to bend the air down far enough for their purposes, Mother Nature let go of the wing and just rushes straight over it, leaving a large wake behind it which tugs the whizz kids’ wing back instead of pushing it up. Then the wing is nothing more than a tree trunk being blown backwards in a strong gale; not very impressive huh? Why? Ludwig Prandtl gave us the answer in 1905. He was a German, of course. And he deserved a Nobel Prize for his insights but got none because the Nobel committee obviously think that small things, like boundary layers, amuses small minds not worthy of Nobel prizes. Ludwig said that air is almost inviscid, and not at all sticky like syrup. Obviaas! But that’s the whole point. It is almost inviscid. Almost, but not quite. Therefore it still sticks to a surface. While Pottie and me and most other people does not give this another thought, Ludwig said that because it still sticks to the surface, of a wing for instance, the air that actually touch the wing sticks to the wing, like in motionlessly attached to the wing! This happens while other air rushes over the wing at a crazy speed just a tiny distanc away from the wing. So this means that in a VERY short distance above the wing, the airspeed can differ from zero on the wing surface to Mach 0.99 just a millimetre away. Ludwig called this tiny region on the wing surface, where the air moves at such hugely different speeds from one point closer to the wing surface to tne next point further away from the wing surface, the boundary layer. This boundary layer allows the airstream to follow the curves of the wing. Why? Because if there is no boundary layer which allows the airspeed to differ from zero on the wing surface (because of the viscosity of air) to a crazy speed just a tad away from the wing, the only other way nature can deal with its own viscosity in the presence of a wing rushing through it is to cause a jumble of mini-tornadoes (vortices) on the wing which allows the freestream rushing over the wing to slip past the wing. These vortices make it unnecessary for the airstream rushing over the wing to follow the contours of the wing. Now, back to the boundary layer case. The craftily shaped wing curves dictate the tiny speed variations and consequently, the tiny pressure variations, above and below the wing. And if you get the shape of the wing right, the pressure below the wing is generally higher below the wing than above the wing. This pressure difference causes the pushing force of the air on the wing. The key here is that the airstream must follow the curves of the wing. And, this can only occur if the boundary layer of the wing exists; the boundary layer being the region where the orderly increase of speed occurs from zero on the wing surface to the crazy speed of the onrushing air just a tad away from the wing surface. If the boundary layer breaks down into an incoherent jumble and tumble of vortices (like mini tornadoes mixed together), the airstream breaks away from the beautiful high-tech wing curves and cast off into its original straight path. In effect, the air now behaves as if it is flowing around a tree trunk. (Sorry for repeating myself here, but if you are like me, you need a lot of explanations to understand nature.) So the key here is Ludwig Prandtl’s boundary layer. No boundary layer, no lift, just huge drag like a tree trunk being blown in the wind. (If the Gripen was on fire at the time, it would be like a candle in the wind) And as the whizz-kids discovered, a boundary layer is like a woman: extremely fickle. She needs constant attention or she leaves. And that is where slats, flaps and slots come in. For fast cruising speeds, you need a smallish wing. But for low speeds, the smallish wing needs to be angled more and more against the slower onrushing air to cause a larger deflection of the slower moving air in order to generate enough lift force to keep the Gripen in the air. This higher incidence angle, which the whizz-kids call the Angle-of-Attack (AoA), causes an ever increasing pressure recovery towards the back of the upper side of the wing. This pressure increase tend to slow down the boundary layer as it moves towards the trailing edge of the wing until it is eventually pushed off the wing. And, as explained above, to deal with the speed difference caused by the zero speed air attached to the wing surface (due to the air’s viscosity) and the feestream air rushing over the fast moving wing, nature needs either a boundary layer or a jumble of vortices on the wing. And if the boundary layer is pushed off the wing by a too steep Angle of Attack, nature causes a jumble of vortices on that part of the wing which make the wing looks like a flying tree trunk to the onrushing air. And even Pottie and I know that a tree trunk has difficulty staying in the air, let alone relying on the tree trunk to keep a whole airplane in the air. By the way, the whizz-kids’ code name for this phenomenon when the boundary layer is separated from the wing surface like this, is “flow separation.” So there we just busted another secret whizz-kid code. So, to prevent flow separation (you hear that whizz-kids) it is important to help the boundary layer along when you want to fly slowly. Flaps, both leading edge and trailing edge flaps, make the wing look less like a flat plate and more like a smoothly curved plate. Boundary layers, like women, like smooth. When the flaps are moved slightly away from the wing to allow a slot between the flap and the wing, some of the higher pressure air below the wing slips through the slat and modifies and stabilises the boundary layer over the top of the wing to allow higher AoA’s which, in turn, allows slower speeds (or higher lift at high speeds). And this is my 2c worth. And admittedly, I could not sit still long enough to edit this post because I need another cup of Java.
_________________ Stay foolish; stay hungry
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