Quote:
Originally Posted by sregord
I'll keep posting...see if what I find is helpful.
It may be redundant ...but where I have found the EcoModder Forum Link(s) I'll include them.
I did a quick read thru the Orbywan's RV mods(Boattail & belly pan), very nice builds, a true craftsman.
https://ecomodder.com/forum/showthre...d-e-18151.html
...surprised that the underside smoothing didn't have more impact.
https://ecomodder.com/forum/showthre...n-s-18427.html
Being familiar with racecar aerodynamic history, and changes from the early 60s(if it LOOKS aerodynamic) to Jim Hall's Chaparrals of the late 60s...and then the crazy downforce numbers of the late 80s(Nissan blowing tires & Spice collapsing suspension components) Mulsanne's Corner: Road Atlanta 1992, Rough and Tumble, attributed mostly to the underbelly's relationship with a diffuser. Today's endurance motorsports, is designed for "enough" downforce, and more efficiency(limited fuel capacity) than ever. https://www.racecar-engineering.com/...ta-ts050-2019/
How to connect any of this to a rounded corner big box/brick is a challenge.
We are working with something that is not <3" off the ground, or moving at over 75mph. ...but efficiency is what the goal is, not making the vehicle aerodynamically heavier.
Regarding the BellyPan: "pulling" air thru the underside with a <15*(degree) angled diffuser at the rear, with strakes ...extended into the(now revised bottom) boattail?. The other issue IS the "in the way" straight axle & rearend. Mulsanne's Corner: 1991-1993 Toyota Eagle MkIII
Don't you wish you could afford carbon fiber?
Regarding the Boattail addition, nice, clean, updated... obviously functional. Again Orbywan, a real craftsman.
It would be interesting if a measurement of the constructed angle from the sides & top is available. For reference... the NASA data:
https://www.nasa.gov/centers/dryden/...ain_H-2283.pdf
...doesn't reference a "best" boattail angle from the side or top. I did a little more checking and found:
https://doi.org/10.1155/2020/7580174
and
https://www.vortaflow.com/consulting...0reduction.pdf
The angle needs to be around 16* before separation of airflow. Funny there's that 15-16* again. I saw Aerohead had mentioned this number. 20-22* is too much but maybe less important at <75mph? Question: Does a curved side/top "inflated" boattail basically compound that 15* to a point? Has the group found more info about this? |
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.