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JS3 Rapture

February 7, 2017




A New Class of Glider

On 12 December 2006, Jonker Sailplanes flew the maiden flight of their first glider, the JS1 Revelation. It immediately established Jonker as manufactures of world championship winning sailplanes. 10 years later, to the day, they took to the sky for the first time in their latest feat of gliding engineering – the JS3 Rapture. 


Jonker Sailplanes was originally a small team of three engineers and three technicians who combined their knowledge to produce a sailplane that would win competitions and fly beautifully. These six were the beginning of the original JS1 success story. There are now 16 qualified engineers and more than 110 employees working in the Jonker factory in Potchefstroom, and with M+D Flugzeugbau in Germany strengthening the team, the company is now a force to be reckoned with.

The impact our local manufacturers are having worldwide cannot be overstated. They have already produced over 100 sailplanes, a major slice of the available market, and their JS1 gliders, which come in 18 and 21-metre configurations, have won major international championships and constantly feature among the leaders worldwide. The JS3, a 15-metre racing class aircraft, is not just a revamp of this older model; it’s a completely new sailplane, with a new, fuselage, cockpit and modified wing. 

It was less than two years ago that brothers Attie and Uys Jonker and Johan Bosman, designers of the JS3, considered developing a 15-metre sailplane. The goal from the start was to set a new standard in this class of competition gliding. Specifically, they wanted to better the performance of the Diana 2, which was designed purely for competitions and flown with marked success by Sebastian Kawa. 

Traditionally gliding competitions were won or lost on days with little lift, because the old airfoil designs couldn’t climb well when heavy. New designs only pay a small penalty when climbing heavily loaded with water ballast, but running with a high loading significantly improves speed. Thus, the Jonker brothers and Johan understood that the key to developing a championship winning glider was an all-new hull, combined with a wing that would still climb well despite having a small surface area and a particularly high wing loading – no small feat. If that wasn’t enough, also on the agenda was uncomplicated flight behaviour, improved crash safety and improved cockpit ergonomics.



A goal was now in place, but a game plan had to be established before any tools could be lifted. The team instituted a new integrated development approach. Each aspect, component, function of a large or small part, the manufacturing process, the optimum material strength and the required tools were defined and developed together. Progress was somewhat slower than previous designs during the initial phase, but it saved time in the overall production and increased the precision considerably. The new process made it possible to seamlessly exchange the wings, fins or trunk of different aircraft without further adjustment.

Furthermore, the usual wind tunnel tests were replaced by highly effective CFD analysis (computational fluid dynamics, numerical flow mechanics) and FEA computer-aided numerical design. This allowed for fast and efficient simulation of flows to calculate the effective forces and thus the impact of design characteristics on the plane’s performance. The end result is a glider with superior aerodynamics and low mass, but with no compromise on rigidity, structural strength and safety.

The JS3 Rapture was built almost entirely in the Jonker Sailplanes factory. Even moulds were manufactured in-house with the use of a CNC-milling machine. The perspex canopy and instrumentation were the only two items that were procured from external suppliers. 



In early 2016, an executive decision was made at Jonker to get the JS3 Rapture ready for the World Gliding Championships in Benalla, Australia. This meant completing the first aircraft by December 2016, giving the team 10 months to complete the design and test fly the JS3, an unheard of tight development schedule for a completely new sailplane. 

It wasn’t long before rumours abounded in the gliding community around the world that Attie and Uys Jonker would be flying a new sailplane at the Championships, but no one imagined it would be a 15-metre racing class ship. A sailplane of that span did not exist in the Jonker’s stable and no one thought a clean-sheet design could be completed in time – except the Jonker team. 

But they underestimated the challenge. Uys Jonker said, “Completing the JS3 in time to celebrate 10 years since the company’s memorable first flight, and in time for the World Gliding Championships, seemed like an easy prospect for Jonker Sailplanes, but as time passed, employees needed to work round the clock to ensure the glider was finished on time.”



The JS3 is remarkably slender, with a wing that, at first glance, could be mistaken for a Schempp-Hirth design. But the distinctive wing-fuselage junction is definitely new and is especially striking. The optimised wing-fuselage transition, combined with the internally shortened wing taken from the JS1, plus the new, very slim trunk and the small stabiliser, all adds up to a spectacular increase in performance in the 15-metre class. 

There is no horn on the rudder, and all pressure sensing probes are on the tailplane extremities instead of the fin post to reduce interference drag. The canopy seals with car type door seals, ensuring no airflow/pressure gets lost at the joints. Then there is an electrically operated air vent in front of the instrument panel to prevent air flow coming into the cockpit when you don’t want it. 

As final proof of the bothers’ unwavering dedication to streamlining, they even made the tailwheel retractable. The water tanks are forward and aft of the main spar on the small wing, to allow the JS3 to adhere to the stipulated 525 kg in competitions. Lastly, the wingtips are interchangeable with the JS1 Evo, so the JS3 is also available in the 18-m configuration. 

Adding some luxury to a thoroughbred competition sailplane, there is an electrically operated back rest, so the pilot can quickly get comfortable for hours of soaring. And to reduce the pucker-factor associated with the possibility of an out-landing, the JS3 comes with the option of a retractable full FADEC sustainer jet engine on the fuselage, or a Front Electric Sustainer (FES) system.

The JS3’s maiden flight was carried out by Attie Jonker. These were his first impressions:

“Performing a maiden flight is an amazing experience, but to be the Chief Design Engineer and also do the test flight is really special. Normally I’m on the calculation and design side, leaving the flight testing to Uys and AP, but I managed to secure this honour. Not being used to flight and certification flight testing and all the nervousness that goes with it, my heart was racing as I climbed into the cockpit, while the test pilots around me were smiling and having fun. However, when rolling, I had immediate aileron control followed by good elevator response when the tail lifted – this immediately settled me down.” 

Uys Jonker, who took the JS3 for its second flight, this time at maximum all up weight, continues: “The aircraft is controllable, agile and easy to fly and gives you goose bumps when you climb into the cockpit. With a wing loading of 60 kg/m2 (compared to 58 kg on the 21 m JS1), the JS3 transforms into an agile racer when fully loaded, yet it still climbs wonderfully, almost keeping up with a loaded JS1 in the 21-m configuration. And the cruise at high speed is something to take note of.” 

As an indication of the JS3’s impressive speed, Justin de Reuck, who was taking photos from the C182 cameraship, noticed that while doing 100 kt in the C182, the JS3 was shooting past on the outside of the turn.



Development of the JS3 is the product of a well-established plan undertaken by an exceptionally dedicated team. More than 20,000 engineering-hours went into designing the first prototype. A considerable amount of time and money was invested, but the return on investment will be off-set with the start of production. Jonker Sailplanes already have a number of orders for the JS3 Rapture and they can currently produce two sailplanes per month in their factory. SACAA certification is expected early this year, after which they will apply for EASA certification.

With the birth of the JS3, Jonker Sailplanes has two fuselages and three sets of wings, giving them four different types of gliders – something every other manufacturer envies of the JS team. At time of writing, after the ninth day of racing at the World Championship in Australia, the Uys brothers were both sitting in the top half of the field. They are competing against the Diana 2, ASW27, ASG 29, and Ventus 2, all of which have a long history of successes. It’s a remarkable achievement for a first race in a brand new plane. There’s no doubt everyone there is keeping a close eye on the JS3.

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