You go through this world as most of us do, believing that to eclipse the max speed of a real P-51 Mustang by more than 161 km/h with an R/C plane, you’d need to employ some form of propulsion, maybe a little turbine. But then, out of nowhere, you learn that actually no, the fastest R/C planes are gliders.
At some point, you were going to have to learn about “dynamic soaring.” It’s an inevitable part of life, I’m sorry if you’re just hearing about it now, from me.
I recently made this discovery myself via Hackaday who describes the technique used to attain these speeds, the aforementioned “dynamic soaring,” at some length:
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 aeroplane 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 aeroplanes, first pulling them downwards a little, then tumbling them around, and then leaving them to slowly sink in the still air. If you’re lucky, it’s a long walk down to pick up your intact plane. If you’re unlucky, you can collect your pieces off the leeward side of the hill.
Basically, the pilots position the plane on the leeward side of a hill and flies it in a semi-vertical loop, catching the wind blowing up and over the hill at the top, then diving through the turbulent boundary air and into the calm air behind the hill then looping back up into the fast-moving air. The plane gains speed when its belly is exposed to the fast-moving air coming over the hill. There’s a good diagram here.
If you’re interested in a surprisingly engaging 1-hour long lecture on the topic, here’s current record holder Spencer Lisenby:
Building and flying high-speed dynamic soaring gliders looks like a fun hobby — it’s amazing to watch these guys build little R/C planes that cope with the incredible stress of flying at these speeds, maybe more amazing that the pilots can fly them remotely.
Is there a practical application? Lisenby posits Mars exploration, but I don’t know if I’d file that under “practical applications.” Maybe it’s better if it’s just a pure, pointless pursuit of speed.