Hey guys! I’m new here and I have been thinking about how sailboats move faster than the wind, by engaging in crosswind flight.
I wanted to get your thoughts on if there is a physical restriction preventing standard gliders from doing this? It’s clearly possible to fly crosswind with a tether, and I can’t see why it wouldn’t be possible without a tether. I am new to this area and I could be wrong.
TIA!
Hi @Samosa
Welcome to the forum
If you want to see just how perfomant a glider can be
Look through YouTube (real study) for dynamic soaring
Yeah gliders doing dynamic soaring is insane. I just havent seen anyone talk about doing flying faster than the wind w/ cross winds with a glider before and wanted to know if that’s because it’s not possible or if that’s not popular, etc?
Welcome to the forum @Samosa
About dynamic soaring:
On the forum: (Dynamic) Soaring, Untethered airborne wind energy systems.
As you are indicating the tether is important for crosswind flight. If you don’t have the grounded tether, you will eventually drift downwind at the speed of the air.
You can get away with air moving at different speed then moving between those. The moment of inertia acts as a tether for some split seconds. This is dynamic soaring as others mentioned.
Anyways, you need to points with different airspeeds to fly crosswind, either the two airflows in close proximity, or an airflow and a tether anchored to the ground, or some moving cart/vessel.
For vessels you can extend this thinking to want a submerged wing/foil to keep you from drifting downwind, as the water will not act as a fixed anchor rather just slow down drifting. When the submerged wing flows through the water it can produce lots of lift to keep you from drifting downwind and only produce a little [undesired] drag force.
Such a vessel is in reality just two foils acting in their separare medium/flow, connected by a tether. This concept could of course be extended to kite plus kite or vessel plus vessel, as long as each part experiences different flow.
Perhaps a point before examining all the intricacies of dynamic soaring.
A glider (without tether) gets its speed from its glide number (lift to drag ratio, L/D ratio) as a main parameter. As a result, high L/D ratio rigid gliders fly faster than paragliders. Wind speed adds to or subtracts from the speed of the glider, as with an airliner. Flying crosswind or not crosswind is not the question.
With a kite, things are different. To fully exploit the L/D ratio of a kite and make it flying as fast as possible, a tether allows it to take advantage of the energy of the true wind, and then the apparent wind when flying crosswind, just like the blades (fixed on a mast, without tether) of a wind turbine.
Wing loading is just as important as glide ratio in determine flight speed of a free flight aircraft. Think RC glider vs human glider. Or placing a heavy person in the same paraglider.
For the same wing loading and and lift, the L/D will only determine how far you get 
Yes of course. It was the reason why I wrote “a main parameter”. Rigid gliders fly faster than paragliders because both glide ratio and wind loading are higher.