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
