Talk:Gliding competition

Error in water ballast section

The statement "If, for example, a glider's weight is increased by 50% by water ballast, then the forward speed together with its sink rate will also increase by about 50%." is inaccurate. The speed and sink rate go up by the square root of the increase in wing loading, so if you double the gross weight (thus doubling the wing loading (weight/wing area)), the sink rate and speed only go up by a factor of 1.41 (or 41%). I don't have the references handy, but I know this to be true. It's used throughout soaring both full scale, and radio controlled.

24.8.219.134 (talk) 09:16, 16 February 2009 (UTC)

Yes. I agree that it is highly misleading and I have made the wording more general. When this edit appeared, I left it alone for some unaccountable reason but I should have thought about it. The whole subject could have an article on its own. Ballast obviously affects both the straight glide performance and the circling performance. However reading off the sink rates from the polar curve in the handbook of my ASW27, for 36 kg/sq m and 56 kg/sq m loadings, at 55 knots, the heavy glider sinks at about 16 feet more per minute, about an 11% change. At 70 knots the lighter glider sinks at about 16% faster and the difference widens even more as the glider speeds up. Any increase in the wing loading also affects the size of the circle and so the pilot may have to change the bank angle to stay in the thermal. The blessèd Helmut Reichmann quotes an ASW15 with a wing loading of 5.75 pounds per square foot (I guess he used metric in the German original). If the wing loading were increased to 6.5 pounds per square foot, the sink rate for a 450 foot circle increases by only about 0.3 feet per second but for a circle of 150 feet the sink rate increases by about 1.5 feet per second. In other words it depends on how wide the thermals are. An increase in the sink rate will also have a large percentage effect on the climb rate in a weak thermal and lesser percentage difference in a strong thermal. It is therefore impossible to attach numbers to the benefits of ballast without specifying the exact conditions. The pundits at my club claim that their cross country speeds are about 10% better with water than when dry.

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BetacommandBot (talk) 15:47, 8 March 2008 (UTC)

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