In light of that Mike, I guess most people would fill it up and hope for the best :)
> -----Original Message----- > From: [EMAIL PROTECTED] [mailto:aus-soaring- > [EMAIL PROTECTED] On Behalf Of Mike Cleaver > Sent: Wednesday, 11 July 2007 12:59 AM > To: Discussion of issues relating to Soaring in Australia. > Subject: Re: [Aus-soaring] Block speeds and wing loadings > > At 16:50 10/07/2007, Todd wrote: > > >As for the weight to fly stuff. just taking a stab in the dark here > >but i would presume that the polar moves directly proportional to > >weight of your glider. so if you increase the weight by 10% then the > >sink rate will be 10% higher at a 10% faster speed than your polar > >says. so taking that into account you should be able to do a little > >maths that takes into account your new sinkrate and works out the > >new speeds to fly. then optimise that again and you should have your > >optimum weight. anyway thats just a guess. debate that if you want. > > Actually that is not the case. The sink rate and speed for each point > on the polar curve moves down and to the right in proportion to the > square root of the increase in wing loading. This means that the > glide angle at each point on the 'stretched' polar stays the same. > You would need to increase the wing loading - i.e. the total mass - > by 21% in order to increase speeds by 10%. > > If a glider had a glide angle of 40:1 at 100 km/h (a sink rate of > 2.5 km/h) at a weight of 600 kg, then increasing the weight to 705 kg > would move that 40:1 glide angle point to 110 kg, at which point the > sink rate would be 2.75 km/h. > > The same glider is likely to have a sink rate of 8 km/h at 200 km/h > (i.e. a 25:1 glide angle) at its original 600 kg all up weight. At > 705 kg it would achieve 25:1 at 220 km/h, when its sink rate would be 8.8 km/h. > > The polar curve might indicate 100 km/h as the best speed to fly to > reach the next thermal if that thermal produces a climb of 2 kt at > the 600 kg weight. However, at 705 kg that same next thermal will > only yield a climb of 1.7 kt due to the larger circling radius > combined with the increased sink rate. That is not enough to give a > faster X/C speed than at 600 kg, even though the inter-thermal speed > is now 110 km/h. > > However, if the next thermal is going to give 7 kt climb at 600 kg, > it will give say 6.7 kt at 705 kg - and at that smaller proportional > loss of climb, the gain of flying to the thermal at 220 km/h > outweighs the loss of the lower climb rate. > > What is more, flying faster at 220 km/h while keeping the weight at > 600 kg will give you a greater sink rate of say 9.5 km/h (a glide > angle of around 23:1 instead of 25:1) so you will lose out by having > the glider at a lighter wing loading. > > Apologies for the mixed and unusual units, but it makes the > arithmetic simpler! The effect is what happens in real life - you can > use a real polar and stretch it to account for varying wing loading, > but there is still a bit of SCWAG ** in the achieved climb rates > unless you also calculate the circling polars using appropriate > models of the lift distribution across the thermal - which is what > the different handicapping models do. > > > Wombat > > > (**) SCWAG = Scientifically Calculated Wild - Arsed Guess > > > _______________________________________________ > Aus-soaring mailing list > [email protected] > To check or change subscription details, visit: > http://lists.internode.on.net/mailman/listinfo/aus-soaring _______________________________________________ Aus-soaring mailing list [email protected] To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring
