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#41
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Dimples On Model Aircraft Could Greatly Extend Range
On 9 Nov, 23:22, "Highflyer" wrote:
"Gregory Hall" wrote in message ... snip | * * * * * I've noticed that when we paint an airplane with the really | shiny smooth urethane paints, it seems to lose a little cruise speed. Depending upon the size of the aircraft paint can add a considerable weight burden. However, when the paint is really smooth it can drag along with it an increasingly thicker layer of turbulent air building toward the aft end of the aircraft. Dragging this thick turbulent boundary layer causes increased drag which seems counter-intuitive to smoothness. This is why various small surface patterns (seems to me somebody should try fractals) often decrease drag - they decrease the extent of the turbulent boundary layer thus the drag caused by it. -- Gregory Hall During WWII they decided to repaint the Spitfire with a flat matt finish paint so it would be harder to see at night. Eh? Virtually all RAF combat aeroplanes in WWII were painted in a matt finish. Why on earth would you paint a Spit so it was hard to see at night? They did not fly at night - or do you have loads of pics of Spits in a matt black finish? Guy SNIP |
#42
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Dimples On Model Aircraft Could Greatly Extend Range
Highflyer wrote:
During WWII they decided to repaint the Spitfire with a flat matt finish paint so it would be harder to see at night. It worked, they were harder to see at night. They lost 20mph because of the increased drag of the matt finish paint. If you noticed a loss in cruise speed with a slick paint job, I would suspect your data collection procedures. You're thinking of Special Night, the extremely matt "anti-searchlight" black finish specified for night fighters from 1940, and the underside of bombers from 1939. Standard camouflage paints at this time had been improved from the pre-war, biplane era standard by the use of more finely ground pigment to give a smoother surface. The paints were still matt, but of a sheen we could call eggshell. These are the "Type S" paints. Special Night was applied in two stages, an undercoat of smooth Night and a topcoat of Special Night. In January 1942, de Havilland performed speed trials with a Mosquito before and after the application of the Special Night topcoat, and discovered a loss of 26 mph. Special Night was replaced by "smooth" Night on Mosquito night fighters within a couple of months. In August 1942 the Night Fighter scheme was redefined to be Medium Sea Grey overall with a camouflage pattern of Dark Green on the upper surfaces. This followed complaints from the squadrons that the night fighters could be detected as dark shapes on most nights, and that a lighter colour would be better. The only single engined night fighters in RAF squadron service during the "Night" period were Hurricanes and Defiants. |
#43
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Dimples On Model Aircraft Could Greatly Extend Range
Hi m
On Nov 9, 2:26 pm, wrote: On Thu, 6 Nov 2008 14:35:21 -0800 (PST), in sci.engr.mech "Ken S. Tucker" wrote: Well I think we're nit-picking sematics, my quote, "My understanding is the theory is NOT _well_ understood, but is evolving, along with applications, by experimental feed-back, aka trial & error, (I'm using SM board). " Note the word "theory" What theory do you mean? It seems it's well understood how to design the riblets to minimize drag. If you mean the theory of the underlying fluid dynamics then I wouldn't hold your breath. We still don't have adequate turbulence models to begin with. Yes, well turbulence is a problem. Also after the original riblet research was performed similarities to shark scales/skin were observed. http://ntrs.larc.nasa.gov/search.jsp...de%20matchall&... Yes! Thanks for those links. Those papers are experimental results and testing, AFAIK, there is NO generally accepted theory of the "riblets effect", though it appears to be evolving. (If you have a ref to a General Theory of Riblets, I'd would appreciate a link). Not sure by what you mean by this. The requirements to design the riblets are known. What more do you want? We build and fly models (wingspan ~ 24", speed 20 mph) what riblet would you recommend? We could glue a sandpaper to the surface of one wing, balance, and test fly to observe yaw anomally. I'm guessing: At a molecular level the riblets control the turbulent interfacing between fluid and surface and inhibit the integrated formation of macroscopic turbulence, such as Eddy's. That micro control is certainly a quantum relation between molecules in the fluid and the interacting solid surface, whereby the micro turbulences are quantized. Fluids as it effects most every situation we deal with, including this one, deal with the macroscopic properties and not the microscopic or molecular properties of the media. There is no quantum relationship between the fluid and the surface in continuum flow which we are addressing here. That only becomes important in rarefied gas dynamics. That is where turbulence begins, a vacuum like back suction. Pardon the poopy ascii, of a circulation, Good Air === (going above wing) /\---------- | | | | o--------\/ Surface "o" is pulled up and cycles around. Regards Ken |
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