On Wednesday, February 1, 2017 at 10:15:05 AM UTC-8, Chris Davison wrote:
A questions that I should know the answer to but don't...in a thermal, all
things being equal, will a 15m glider and an 18m glider with the same wing
loading climb at the same rate?

Thanks
Chris

Not if their polars have different sink rates in a bank at min sink speed and the same wing loading. If they were the same it would be a coincidence.

To convince yourself of this imagine two 15/18M gliders of the same type - identical in every way except one has 15M tips and the other has 18M tips. They are ballasted to the same wing loading. In the thermal they are in identical lift. Beyond this the only difference is that the 18M glider has spoilers deployed. Will one glider climb faster? Yes!

Okay, I think your question probably assumes both gliders keep spoilers closed and are identical in every other way, but the same basic idea holds. The 18M glider has different span, aspect ratio and wetted area and therefore a different mix of form and induced drag at any given speed and lift coefficient. Go get a polar for a 15/18M glider and do the ratio of the square root of the wing loadings transformation to equalize the wing loading and you'll see the polars are still different. The 18M ought to have a lower sink rate because of lower induced drag attributable to the higher aspect ratio.. Above a certain speed the polars may cross over as form drag goes up with V-squared. There's actually a lot more going on than just these simple effects, but that ought to give you a basic understanding.

Shorter answer: There's a lot more to aerodynamics and aircraft performance than just wing loading.

Andy Blackburn
9B