Radio-Controlled Gliders Edging Close to Mach 1

This stuff blows my mind:

The fastest remote-controlled airplane flight ever recorded took place in 2018, with a top speed of 545 miles/hour. That’s 877 km/h, or Mach 0.77!

What was the limiting factor, preventing the pilot-and-designer Spencer Lisenby’s plane from going any faster? The airstream over parts of the wing hitting the sound barrier, and the resulting mini sonic booms wreaking havoc on the aerodynamics. 

...Dynamic soaring (DS) was first harnessed to propel model planes sometime in the mid 1990s. ... But dynamic soaring is anything but new. Indeed, it’s been possible ever since there has been wind and slopes on the earth. Albatrosses, the long-distance champs of the animal kingdom, have been “DSing” forever, and we’ve known about it for a century.

...When wind blows over a hill, it has to go up and over, with emphasis on the “up”. Glider pilots have used this ridge lift to their advantage for a long time, and the model airplane practice of gliding around indefinitely in the ridge lift on the front side of a hill is called “slope soaring”. With an infinite source of lift, model gliders can do all sorts of fun acrobatics using very little power, as long as they don’t hit the hill or fly up and over to the back side of the mountain.

On the back side, the air pulls back down again, and there’s a strange turbulent zone between the moving air up-top, and the wind-shadow in the valley where the air is still. The back side was known to eat model airplanes, first pulling them downwards a little, then tumbling them around, and then leaving them to slowly sink in the still air. 

...Until one day, when glider pilot extraordinaire (and aeronautical engineer) Joe Wurts flew up and over. He realized that he couldn’t make it back against the strong wind, and didn’t want to hike down, so he aimed the plane at the valley floor. It picked up speed, crossed the turbulent zone, and entered the still air where he was able to fly it back to the crest of the hill because he wasn’t fighting the strong headwind. When it climbed and re-entered the wind stream, he noticed that it had picked up speed. Then he started doing it on purpose.

...And fast is fast. The progress in speed records stalled for a number of years due to the fact that there were no commercially available radar guns that would measure above 300 mph ....

When the sport was young, the highest achievable speeds were limited by the stiffness of the wings: Flutter is the wing-killer. If an oscillation in the wing starts up at high speed, there’s a lot of energy to keep it amplifying, and the result is that the wing tears itself apart or away from the body of the plane.

...When Spencer manages to design and build a plane that can approach the sound barrier, the next limiting factor may be the human in the loop. At speeds already in excess of 200 m/s, a human’s reaction and decision time of around 250 ms to 500 ms translates to 100 m of plane travel. If a plane hits bad turbulence at those speeds while only 20 m above the ground, it could be carbon-fiber dust before any meatbag even has a chance to blink. To quote Spencer, “With each new speed, you get this feeling like you’re in over your head and your brain can’t keep up with what’s happening.”