by Jay » Sat May 16, 2015 6:02 pm
The concept is feasible, but the aircraft would not likely resemble anything you’ve ever seen flying. The wings would have to be very, very long and slender (extremely high aspect ratio). Propellers would be ineffective in the vanishingly thin air at “ultra-high” altitude unless the blades were also very, very long and slender. With a conventional airplane configuration, just getting such a flimsy and ungainly contraption off the ground would be difficult, to say the least. Tiny (hobby-scale) jet engines are available, however, so there is still hope.
Quad-copters have become popular only because they are so much easier to control than conventional helicopters. Unfortunately, they are also far less efficient, both in terms of aerodynamic efficiency and power-to-weight ratio. The highest altitude achieved by a conventional helicopter specially designed to operate at such elevations is 40,820 feet (12,442 meters). In this case, 25,000 meters is definitely “ultra-high.” Even a quad-copter designed to function in such thin air is likely to run out of fuel (or electricity) long before reaching your goal.
One possibility is a torqueless “rotary flying wing” configuration, in other words a helicopter that is all rotor with little or no fuselage. Torqueless rotation, driven by small jet engines at or near the tips of the “wings” or rotor blades, eliminates the need for the multiple counter rotating rotors and multiple motors of a quad-copter as well as the need for the tail rotor and the complex, very expensive and comparatively heavy power train of conventional helicopters. Centrifugal tension in the rotating wing would provide stiffness that would otherwise be absent in such an extremely lightweight and elongated structure. The centrifugal effect would also deliver fuel from the hub to the engines at remarkably high pressure using only a small cheap conventional fuel pump. Such high pressure would be welcome in tiny high-performance high-altitude jet engines. I’ll leave the design of the unique flight controls up to you (and Rube Goldberg)!
The concept of torqueless tip-driven helicopter rotors has already been proven by major manufacturers and several variations have been produced, although with little commercial success. In the 1950s, using “cold pressure jets” (compressed air jets rather than jet engines), the Djinn by Sud-Ouest was the most successful. Small jet engines mounted on the rotors of other designs worked exceptionally well when the engines were running, but produced too much drag in autorotation, such as during an engine-out emergency, and so were abandoned as a risk to the safety of passengers. This ought not be a concern with a “telenautic” aircraft on the scale of a (very) large model.
Telenautic? Yes, I made that up (from the Ancient Greek tele = far away or remote, and nautes = sailor or pilot). What do you think? Will it fly?
Jay