On Wednesday, February 1, 2017 at 8:42:35 PM UTC-8, Bruce Hoult wrote:
On Thursday, February 2, 2017 at 5:12:27 AM UTC+3, Eric Greenwell wrote:
Chris Davison wrote on 2/1/2017 10:14 AM:
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?

I'm told thermal climb rate is related to "span loading" (weight/span),
while high speed performance is related to wing loading (weight/wing
area). In your example, the 18m glider will climb better.

Told by who, I wonder? :-)

Span is important to minimize induced drag, but that's a waste of time unless you have enough wing area to give an acceptable coefficient of lift or AoA at desired circling speeds and radii.

There is probably an intermediate cruising speed range where the dominant factor is wing loading / wing area / wetted area / span*chord. At a guess that might be from midway between min sink and best L/D speeds out to maybe 1.4 or 1.5 times best L/D speed.

At higher speed I'd have thought the dominant factor would be minimizing span*wing thickness, i.e. frontal area. That's what kills the 1960s 40:1 ships at high speed -- or newer short span ones such as the PW5.

In classical aerodynamics, induced drag dominates at low speeds (high lift coefficients), and that is inversely proportional to aspect ratio. However if you work through the math, area and wing loading cancel the wing chord out, hence the term span loading (which normalizes for wing loading in effect).

At high speeds profile (and parasitic) drag is dominant. Wing thickness is loosely related to profile drag, but the main thing that kills the 60's ships is the bad behavior of the laminar flow, not the thickness per se. Fuselages have gotten a little cleaner, but it is the wing sections and understanding of laminar flow that is the biggest difference I think.