These are specialized gliders with large wingspans, all over 80 feet. Typical glide ratios of more typtical modern gliders range from 40 to 50 to 1, while racing and aerobatic gliders are a bit less, around 35 or so.
It's mostly due to huge wingspans and high aspect ratios (long thin wings) which accelerate more air but at a lower speed than smaller wings, so the same momentum change (f = ma), but less kinetic energy change (v^2 factor), so less work done, less energy required, and therefore better lift to drag ratios. Note the Nimbus 4 has a 60 to 1 glide ratio at 68mph, so it's also covering range at a descent pace. The large narrow wingspans are possible because of the strength of composite materials.
Note there aren't a lot of places to launch or land a "open class" glider with an 80+ foot wingspan. 15 meter (49 foot) class gliders are more common and these gliders have L/D in the 40 to 50 range.
I'll be very glad if anyone can help me to find any information about the relation between the wing span and total length of a glider or a sailplane, I have found that this value is between 30 to 50% but there are any mathematical expression to find this value?
Thank you very much!
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