In sailplanes we add up to around 200 litres of water to wing tanks in single-seat gliders (empty weight typically around 280kg) to improve the high speed performance (typically up to 270 km/h these days) and ability to fly comfortably in turbulence.
replyIt's surprising what you can fly formation with, even without an engine [1] :-)
https://www.youtube.com/watch?v=G0icOICQLTc
Dumping (most of the) water before landing:
https://www.youtube.com/watch?v=I4Yv-V7eozk
[1] this particular glider does have a small engine for takeoff, but maximum speed with the engine extended is 180 km/h and here they are flying at 280 km/h.
What a rabbithole this is now :D
replyThat glider next to the world's fastest aircraft (in 1953) dressed up for a fight, how about that.
And they can water the grass...
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So several thousand $ to get trained and certified and finally be ready to get towed and soar...
but just a few hundred (California) to try for half an hour behind a pro in the cockpit--oh yeah!
...omg they let you fly the thing for another couple bucks!!!