In 2012 Jonker Sailplanes penetrated the Open Class market with the introduction of the 21m version of the JS1. The outboard wing and wing-wingtip junction was redesigned to accommodate the higher wing loads and a higher maximum take-off mass of 720kg [1587 lb] (increased from 600kg). The 21m wingtips have integral water ballast tanks with a capacity of 17 litres [4.5 gallons US ] each with cockpit control allowing the pilot to jettison the wingtip water ballast separately to the main wing and tail fin tanks.
The JS1C-21 not only displays an excellent glide ratio at moderate speeds, but also at high speeds and maintains its outstanding climbing performance.
It has proven its popularity during the 2017 WGC in Benalla with 23 out of 35 competitors flying the JS1C in the Open Class. Seven of the first 10 places were claimed by JS1 pilots with Russell Cheetham and Andy Davis taking the gold and bronze medals respectively.