Thread: Discussion XC#10 in a series, Giant Icon?
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Old Jun 25, 2010, 07:23 PM
G Norsworthy is offline
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I'll just release this as I generate it. I redid the curve fit for TrekBiker's plane polar, not sure what happened the first time but it was not right. Here is a whole pile of planes run in a 26 mile course on an inverted day with the following assumptions:

Lift 0.4 m/s, adjusted down from the last model because XFLR5 is optimistic for wing only
Sink -.13 m/s
Max altitude 500m
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

#8 218.5
150 span 261.4
170 span 201.4
12 chord 235
14 chord 221.7
14 chord 150 span 260.5

#8 V2 airfoils 219.1
150 span V2 262.4
170 span V2 200.2
12 chord V2 231.9
14 chord V2 227.1
TrekBiker wing V2 215.6

The clear winners here are the stretched spans, with the losers being the clipped wings. The 13.25" chords are more or less maxima in the design space with 12 and 14 dropping off. Not much difference in airfoils, trading places depending on the planform. The planes get spread out pretty far here, with the more efficient climbers getting ahead in the weak lift.

I've flown in this kind of day. Once you recognize it there isn't much you can do except know when the thermal is topped out and bail in time. The lift is regular, and the good teams are all going to fly the same race. Assuming the top 4-5 teams are equivalent pilots, the plane will win the race. This is in direct contrast to the boomer day when the team with the fat chord that dares fly the highest will most likely win. Recent memory shows about 4 boomer days for every inverted day, and sometimes if you are patient the inverted day turns into the boomer day. Spreadsheets are getting easier to manipulate.
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