Casinos can make money because most patrons usually end up losing it. Flying a glider has a lot in common with going to a casino. You typically start out with a little boost, and sometimes, with favorable conditions, you can even increase your height. If you stay in the game long enough though, you will eventually lose all that you started with. Only the most skilled, and lucky, pilots can stay aloft as long as they desire, but even that is changing as we turn our technological sights towards perpetual flight.
A prime example: researchers at Lehigh University are attempting to build a pilot-less glider that would be capable of endless flight. Their aspirations hinge on the concept of “dynamic soaring,” which can be traced back to an 1883 Nature paper by polymath Lord Rayleigh (ofwhy the sky is blue fame) entitled, “The Soaring of Birds.” The new meets the old in mankind’s long-standing quest to perpetually stay aloft.
Simply stated, dynamic soaring involves making use of non-uniform wind to gain lift or momentum. It sounds interesting, but that doesn’t actually tell us much about how to do it. One might expect that there is a good theory out there that tells us how to proceed — a “physics of soaring” if you will. The problem is, we don’t even have any such theory for sailing, and that’s only two dimensions. Most websites and popular textbook descriptions of sailing account for the ability to tack upwind by a nebulous appeal to largely ineffectual Bernoulli forces. These accounts then usually trail off into increasingly fractalized, and largely irrelevant, partial differential equations which serve only to obscure the lack of fundamental insight. How then are we to sufficiently understand complex 3D flow effects in the air?
In order for a perpetual glider to stay aloft by dynamic soaring, an on-board control would likely need to translate sensor data into flight adjustments according to some algorithm. Clues as to what form such an algorithm might take could perhaps be harvested by observing those aerial champions who make sustained-flap-free flight routine — the turkey vultures. Many large birds of prey, like hawks and eagles, can soar the odd thermal until their heart is content. Typically though, they choose to boost their flight with amply-powered wingbeats on a regular basis.
The reason that most birds that are well-equipped for soaring will still laboriously pound the wind, is that they usually have some place to go. That’s typically not the case for all large birds of prey though. In 2010, there were almost two million turkey vultures in the US. At four million square miles of US turf, that gives just two square miles per bird. In other words, these guys really have no place pressing to be. Instead, they need to be able to hang out efficiently just above the treetops to scan the highways for any opportunity in their competitive roadkill marketplace.