Quote:
Originally Posted by aerohead
1) Three different researchers reported optimum drag reduction only with a 'long' diffuser of 'small' upsweep angle ( around 2.8-degrees )
2) Drag reduction for a 'short' diffuser occurred at a 4-degree upsweep.
3) W.H. Hucho mentions A.R. George's research of high underbody angles from June 18-20 ASME-CSME- Conference, 1979. These steep angles induced a counter-rotating pair of vortices, which induced an upwash, as a prismatic fastback would create a 'downwash'. If the vehicle already had an upper set of counter-rotating vortices, the lower set could be tuned to mitigate some of the vortex drag of the uppers. Any vorticity kills any chance of pressure recovery. It all becomes heat. Softened edges as on Orbywan's boat tail will prevent vortex formation in the first place, according to Carver-Funderburk at Texas Tech University.
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4) As to 'angles', they're completely contextual. If you're doing a simple angle, then, the 'optimum' angle will be a function of the proportion of the length of the tail, in comparison to the overall length of the vehicle. A percentage.
5) If you're doing a 'curved' boat-tail, then any tangent angle, at any point along the contour, will be a function of the location along the contour, as a percentage of the 'total' 100% tail. As Koenig-Fachsenfeld/ Kamm investigated at the FKFS, and W.A.Mair.
6) Technically, one would not exceed the degree of tapering seen on the aft-body of a streamline body of revolution, of 2.37:1 fineness ratio. This ratio was appropriate in the context of airship envelopes, of thousands of square-feet frontal area, where the turbulent boundary layer was so enormous that, the TBL sloughing off the rear, actually created 'phantom' tail surface.
7)As a scaredy-cat, I'm not comfortable recommending anything more aggressive than Mair's contour. It's basically what NASA used on their streamlining, except they exceeded 22-degrees beyond the 'natural' separation line, and the added stinger was embedded within turbulence, unable to provide any meaningful pressure recovery.
8) The particular2.37:1 contour WOULD exceed Mair's recommended 22-degree maximum for any part of the tail.
9) And using Mair's 22-degrees is 100% predicated upon using Mair's 'lead-in' contour to 22-degrees. Top and sides would have identical contour.
10) The 'diffuser' needs to respect the SAE' 10-degree 'departure' angle at a minimum, so as not to be shorn away ascending or descending ramps. A hinged, 'moveable' bottom section could get around clearance issues.
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11) A fairing can be added to exposed axles. I've done this on Viking. The lowest drag is with a 4:1 aspect ratio symmetrical section enclosing the axle.
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Good stuff ...thanks
1-3)Considering the length of the "tail"(axle to bumper) of this MH...even with the fuel tank back there... I can see a 2.8-degree diffuser in my future.
4) LOL...the angle verses tangent is all fun conversation
5-6) a curved boattail would likely be inflatable... which I'm struggling with - backing into campsites(seeing where I'm going), ladder, etc obstacles to usability. I'm not giving up here yet.
7+) limitations are important. The "rocketail" or a version of it(yes there is a patent), still appeals to me. 11 degrees of the 1st surface, 14 degrees of the 2nd. & only 27" long. connecting the "corners" would be a creation... almost artsy.
Is there somewhere I can get the 2.37:1 fineness ratio definition clearly? of course I'm not shortening the MH ...only adding(something store-able)to the rear.
11)A smoooth belly pan, including axle & pumpkin fairing I know I can resolve.
In other threads I see concern with coroplastic, it seems many use it with success & others have worry of longevity.