Thread: Discussion XC#10 in a series, Giant Icon?
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Old Jun 30, 2010, 12:38 AM
G Norsworthy is offline
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United States, CA, San Jose
Joined Sep 2008
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better lift

No guarantees the spreadsheets are without error but they are giving me reasonable answers. Same course, 26 miles, inverted, but stronger lift and sink. Here are the assumptions:

Lift 1 m/s
Sink .33 m/s
Course length 26 miles
Distance between thermals 1 mile
Speed to fly using sink rate and no expected thermal
Final glide in sink at speed to fly

Result is in minutes

#8 83
150 span 83.9
170 span 86.2
12 chord 87.3
14 chord 85
150 span 85.8

#8 V2 airfoils 81
150 span V2 82
170 span V2 83
12 chord V2 79.8
14 chord V2 83.4
TrekBiker wing V2 81.3

With stronger lift now the 12" chord thin airfoil wins based on higher speed to fly because of its aspect ratio and wing loading. #8 with thin airfoils and TrekBiker wing not far behind. The big losers are the thicker airfoils, with #8 as built the maximum of that subgroup. Looks like the thinnest wing that will hang together on course is a good place to start, with camber change to modify the polars.

The lesson here is if you are going to fly on inverted days against good pilots, the plane has to be optimized to win these particular races. The pilot can't do much unless with a bit of luck the inversion layer breaks through and with a less competitive plane restarts higher or takes advantage of upper level lift on course. I distinctly remember flying an unballasted #8 on this type of day and watching two MXC's pull away from us, and there was nothing we could do about it. Now I understand why.

Next step is to get the polars off the recent Supra build and see how that does on various courses with camber change.
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