A Revelation indeed!


At the 10th anniversary of the JS1 Revelation, the Jonker brothers share their tale about how the dream of two young boys came true.  They developed a world class sailplane, barely 100 years after the Wright brothers cemented their names in history in the same manner!

Jonker Sailplanes.... a family affair Attie Jonker in the font of the Ka7 with his boys

The creation of a dream.

Brothers and aviation… a proven recipe amongst a few renowned pioneers… In the late 1700’s the Montgolfier brothers were the first humans to leave the face of the earth in their hot air balloon.  In 1903 the Wright brothers made the first controlled, sustained flight of a powered, heavier-than-air aircraft.  Then in the new millennium, in 2006, the Jonker Brother’s JS1 Revelation does its maiden flight. 

Almost ten years after the first JS1, the Jonker brothers delivers their 100th sailplane.  The path to this milestone came with lots of obstacles, but that only gave this one of a kind South African sailplane company, more reason to strive in creating one of the world’s best gliders!

Uys landing the 100th JS1 Revelation after the successful test flight on 10 December 2015



The Beginning…

Tienie Jonker, the father of Attie and Uys, was born in 1943 and grew up on a farm in Christiana in the North West Province.  The Jonker descendants inherited their entrepreneurial genes from their grandfather, At Jonker, who was a prosperous farmer, diamond digger, businessman and even a coal mine manager in his younger days.  As a kid, Tienie’s exposure to aviation was limited to the aircraft overflying their farm.  He recalls running behind these aircraft until they disappeared over the horizon.

Tienie studied Science and Chemistry at the North West University in the 1960s. He expressed his affection for aviation by building and flying model aircraft and constructing his 4-channel transmitter and servos.

Tienie' was the first pupil to go solo on the Orient airfield – Magalies Gliding Club

History for the Jonkers took a turn when Tienie saw a Tiger Moth towing a glider one afternoon. He followed the flight path on his motorcycle and ended up at the Air Force Gliding Club’s Summer Camp at the Potchefstroom airfield. Tienie had his first flight in a state-of-the-art Ka-7 a couple of hours later. Apparently, he returned home that night with a huge smile despite his bruises and scars. (The scars caused by Tienie’s first wing-run attempt which ended in a spectacular “speed-wobble”, thankfully just before the Ka-7 got airborne.)
In the 1970’s Tienie started his career as a scientist at one of the mines in Rustenburg, which gave him the opportunity to join the Magalies Gliding Club. On the same weekend that the Magalies Gliding club relocated to their current location, Orient Airfield, Tienie went solo – the first pupil on the new airfield.

1973 Tienie traded his career as a scientist for education while moving to the small town of Bloemhof. Naturally, the Bloemhof Gliding Club got established within a few years of Tienie’s arrival. The Bloemhof Gliding club boasted with one keen instructor, a Ka-2, two little boys, who could barely see over the steering wheel of a car, and a few interested locals. Suddenly Bloemhof was on the worldwide gliding map!

The first day of gliding operations at Bloemhof. TIenie instructs, with Uys next to the glider

Tienie instructing a Bloemhof local gliding pupil from the back seat of the club's Kranich III

Attie and Uys were barely 5 years old when Tienie strap them in the back seat of the Ka-2.  While he was controlling the rudders, Uys and Attie took turns flying the glider.  According to their dad, the boys could make survivable landings at the young age of 9.

History seems to repeat itself…. In the backset of a Ka-7, Attie’s two sons, TIenie and Christiaan, stapped-in exactly as their dad was 35 year ago

With the salary of young teacher raising a family, it was wishful thinking to import a “new” glider from Germany.  Tienie decided that if he cannot buy it, he will build it!  In 1975 the construction of the Tern started in the Jonker Family’s backyard.

Tienie Jonker, dad and main inspiration of the Jonker Brothers’ dream, constructing his own glider in the 1970’s

The unpainted Tern on its way to the airfield for its maiden flight

Tienie cutting the ribs of his homebuilt Tern

The Tern’s ribs being laid out

Fuselage progress

Tienie test flew his Tern in 1980, and soon Tienie and his son’s filled the skies with their flying voyages in a glider they, they not only owned but also built themselves. The Jonker family moved to Potchefstroom in 1987. Tienie joined the North West University and became an active member of the AKA gliding club at the Potchefstroom Airfield. Nowadays Tienie is a very successful tour operator, enjoys life with his wife and grandchildren, flies his Motorfalke, builds model steam trains and is still an inspiration for everyone at JS and the Potchefstroom Airfield.

The brothers… and the Revelation

Attie and Uys’s earliest memories are filled with great times of building and flying model aircraft, as well as hours spent on the airfield with their dad.  From a very young age,  the two brothers were fascinated with their father’s endeavor and became engrossed in model aircraft projects in and around the workshop; a hobby that their own sons would also later enjoy.  

Attie building his first model kit

The first recorded Jonker joint venture….

Like father, like son….. Attie and Uys’s boys already keen enthusiasts

The club’s Zugvolgel single seater was really popular amongst the youngsters. Attie is preparing for take off while Uys assists with the checks

Attie after his Silver C 5 hours duration and 50 km cross country in the Tern

“It was good fun on the airfield being a youngster.  In those days things were much less regulated.” Uys explains. “Today, at our gliding club, it takes six months and a few exams to be qualified as a winch driver.  I remember being solo on the winch before turning 10, after a single demonstration followed by a supervised launch.  It was a thrilling experience for a ten-year-old to operate an enormous V-8 machine, launching gliders on weekends!”

As their pilot skills started to improve, they yearned for better-performing gliders.  Attie started fiddling with the design of the Tern’s boxy cockpit, trying to streamline it.  He learned the hard way that streamlining meant more than just mere cosmetic changes and that no streamlined shape fits around a “fat” structure!  Thus the need to study engineering to design a “better” glider became apparent.   As a young high school boy with his interest in Engineering and high-performance gliders,  Attie was fortunate to acquire and rebuild a damaged Standard Austria (SHK).  Flying the Tern and the Austria, the brothers participated in their first national competition in the Sports Class in the early nineties.

Attie finished 2nd in his first SA Nationals- Sports Class, with the Standard Austria (SHK) that he rebuilt in his high school years

By the mid 90’s both Attie and Uys completed their Engineering studies at the North West University. Uys started his career at Denel Aviation (South Africa’s military aircraft manufacturer), while Attie decided to lecture full-time at the North West University’s Engineering School. Uys enjoyed his career at Denel, progressing quickly while gaining lots of experience in aerospace manufacturing. During this time, Uys, an aspiring engineer in training, would also meet Gideon Coetzee, who was a technician on the Rooivalk project, both of them not knowing that Gideon would later become the JS quality and excellence manager. However, at this point, Uys and Attie had other plans of their own, so Uys decided to move back to Potchefstroom.
Uys joined Attie in 1999 to start a composite glider repair facility, as well as a wind turbine blade factory, both of these on the premises of the North West University. One of their first big challenges was the restoration of a severely damaged Schempp-Hirth Standard Cirrus wreck. They pooled together all their spare cash and purchased a serviceable Standard Cirrus to mold the missing cockpit. Within 18 months of working every weekend, they were the proud owners of two refurbished Standard Cirrus Gliders.
The Jonker brothers enjoyed this new plateau of performance and were selected in the South African National squad for the Club Class WGC in Australia (2001) and Mushbach (2002). During the World Championships in Leszno (2004) where Attie finished 5th in the 15m Class and Uys 13th in the 18m Class the image of a new 18m Class glider unraveled between the two brothers.

Attie was always working on sailplane designs since childhood.  His position as lecturer at the university thus gave him the perfect platform to fulfill his lifelong dream.  He researched Prof Loek Boermans’ (Delft University) design methods and verified X-foil designs in wind tunnels – thereby gaining confidence in profile and wing design.  Uys, on the other hand, put his focus on composite construction techniques and production.   Together the two brothers made significant progress with the initial designs and manufacturing processes, but they needed not only more hands but also more brains, entering Johan Bosman.  “I cannot believe how it was possible to complete the design without the aerodynamic support of Bossie,” Attie recalls.

The JS1 prototype took its maiden flight, piloted by Uys, on 12 December 2006 – a date nominated by the Jonkers’ six months earlier.  “It was a hectic last few weeks to complete the glider in time.  Even on the day of the test flight we were still sanding surfaces, fitting instruments, and sealed Mylar gaps.   But it was all worthwhile – you cannot describe the feeling of the first flight after so many years of work!”

The experience gained from their earlier endeavor with the glider repair shop came in handy with the movement from design to fabrication.  Here Peter and Uli Kremer, CEO’s of Alexander Schleicher, showed lots of help by allowing the brother's unrestricted access to their manufacturing plant.  Starting a student flying group (“Potchefstroom Akaflieg”) they ordered an ASH 26 fuselage, with the intention to pair this with their new developed wings.  When a set of damaged ASH 26 wings became available, they realised that rebuilding a complete ASH26 will benefit their manufacturing experience.   A new fuselage based on the ASH26 / ASW27 design was developed to meet the Jonker Brother’s design requirements.    “Obviously, Schleicher was not initially too happy that our idea of a once-off Akaflieg project transformed into a highly desirable commercial product.”, Uys admits. “But Schleicher was fantastic in the whole process assisting us and to settle differences without serious complications.”   

Since 2006 Attie and Uys have become regular participants in the 18m class during World Gliding Championships, always participating with their self-designed JS1 Revelations, with Uys taking the Silver Medal in Hungary in 2010.  “We function very well as a team flying our JS1s.  It’s great fun to fly together with a partner, who not only happens to be your brother but are also your lifelong gliding companion ”, the two heartily explains.

Attie, JS’s Chief Design Engineer, completed his Ph.D in sailplane design in 2014.   Using the Akaflieg concept that originated in Germany, Attie developed a mean R&D team consisting of 16 young engineers and 6 technicians.  His team is quite secretive about their next exiting project but one can expect that something special must be brewing.

Uys, CEO of JS, manages the business and focuses on the marketing and flight testing departments.

The Aero-Guru

During the late 90’s, a talented graduate student, Johan Bosman (also known as “Bossie”) met Attie in the classroom.  Bossie, intrigued by airplane design from a young age also continually tried to perfect his model aircraft performances.  “Attie was the perfect mentor for me, as we share the same perfectionist need for performance,” Bossie explains.  He completed his M.Eng in the aerodynamic design of the new 18m glider - which would later become known as the JS1.    

Bossie called the JS1’s profile the JJB44. The JJB44 airfoil had its heritage from the original T12 airfoil and is, in fact, a series of six profiles.  Each profile of the JJB44 is located at a specific spanwise location and is optimised to a specific Reynolds number requirement.  Bossie gives a glimpse of the secret of the JS1 profile: “The JJB44, or T12 airfoil, is the result of hundreds of iterations and also recommendations from Prof Boermans, the profile guru from Delft University.  We managed to maintain very low profile drag, while also keeping the enlarged laminar drag bucket for low drag at high lift coefficients. The angle of attack of the tip airfoil was modified to be greater than that of the baseline T12 airfoil, and it also has a higher capable lift coefficient, resulting in less profile drag.   Although aerodynamically advanced, Attie had structural implications using such a thin optimised aerofoil.  At the time the T12 airfoil was the thinnest main airfoil used on modern sailplanes, and it meant some real challenges for Attie”.

When Bossie completed the aerodynamic design, he developed CAM-CAD software for JS’s CNC milling machines cutting out the moulds.  “Designing the molds and programming the CNC machines is no walk in the park.  It seems straight forward to select off-the-shelve programs, but these programs rarely meet our tolerance requirements” Bossie explains.

Bossie specializes in CFD (Computational Fluid Dynamic) to design the latest products.  This is the direct result of his 3 years Ph.D. studies titled Evaluation of new aerodynamic concepts using CFD for the improvement of a glider fuselage.  “Profile design is no longer a black art.  However, not too many people can optimize complex 3-D shapes and predict laminar to turbulent transition accurately,” Dr. Bosman explains and add, “This is the future of glider design.”   

Between all the design work Bossie enjoys life with his wife and 2 little daughters (Who also hopefully inherited their father’s passion for aerodynamics.), flies model aircraft with his father-in-law, and enjoys the experience of flying his self-designed Revelation.

Johan Bosman, Aero-Guru at JS, enjoying his JS1

The Revelation in the Open Class

When Uys was asked about the 21m version of the JS1, he told the story with a small grin:  “The JS1-C is just one of the examples where it paid off to take the customer’s input seriously.  In April 2011, three-time world champion Andy Davis approached us, requesting the increase of the maximum all-up weight of the JS1-18m, to compete in the Open Class in Uvalde the following year.  Oscar Goudriaan had a similar request nearly the same time: ‘What are we going to do for Uvalde?’  It took our team a couple of weeks to outline the basic specifications of the JS1-C.  Andy and Oscar were undoubtedly thrilled by the idea of a 59kg/m2 Open Class machine, immediately placing their orders.”

“We revealed the secret to only a few people:  Andy, Oscar and Laurens Gaudriaan, along with Bruce Taylor and Brad Edwards from Australia.  We code named the project U505, after the well known U-boat disappearing in the 2nd World War.  Keeping the project a secret caused some other problems – we had to design modifications to the Cobra trailers to fit the 21m tips – this must have annoyed Alfred, as the 15m Cobra trailer is a bit tight for both the wings!”

“We managed to keep the secret tight, and it must have been a revelation when Bruce Taylor landed his JS1-C 21 at Uvalde after launching from Houston.  After the first competition day at the 32nd WGC, it became quite apparent that the JS1C-21m was a force to be reckoned with in the Open Class. On this day, Bruce Taylor finished second, and all four JS1C-21s managed the nerve-wracking 100km final glide in dead air.  Oscar obtained the 3rd place overall in the Open Class with his new JS1-C, and JS’s new 21m order book soared!

The new JS1C-21m sailplane immediately gained popularity and is our most successful product to date.  During the 33rd World Gliding Championship in 2014, the JS1C- 21m took 2nd and 3rd place, with overall five positions in the top ten.  The following year during the 18th FAI European Gliding Championship, the JS1C-21m took the top four positions and overall seven out of the top ten.”

First 4 placed to the JS1-C 21 during FAI EGC 2015

Uys landing after a demostration at the EGC 2015 in his JS1C-21m with a Jet Engine installed

“The JS1-C revived the Open Class” Dick Butler, designer of the Concordia, commented recently. 

One can now compete in the Open Class with the budget and ground handling of an 18m glider, unmatched high-speed performance while not giving a dime in weak surviving weather. 

“And when you arrive with your 15m trailer and start rigging, you don’t scare away your best friends!”

The Jet Sustainer

In 2011 JS and M&D Flugzeugbau joined forces with the JET project.  JS was searching for a suitable turbine manufacturer to meet their Sustainer requirements, and M&D was already testing a turbine designed for the sailplane market.  In 2007 M&D identified the potential benefits of installing a small jet turbine sustainer in gliders, but the range of jet turbines available for model aircraft had its limitations. The available sustainer engines were not certified, and in-flight starting was not a design feature considered in these engines.  M&D decided that the best outcome would be to develop and certify an entirely new jet sustainer engine for modern sailplanes.

M&D applied for EASA Type Certification for their jet engine, the MDTJ-42, in 2009.  “Certifying the turbine was no easy process – M&D and EASA had to establish the special conditions, as no requirements for jet turbines exists in the current CS-22 standards” Tim Markwald, CEO of M&D explains.  During the six-year certification journey, tons of fuel was burned during the testing of approximately 50 test engines, and more than 180 test flights were conducted.  M&D was awarded the Type Certification of the MDTJ-42 in December 2015. 

In parallel with the M&D’s turbine journey, JS also had their Jet fun. “To install the MDTJ-42 sustainer engine, a full compliance demonstration according to CS-22 had to be done.”    JS designed the integration of jet system in the current JS1B and JS1C fuselages – their system includes the retraction mechanism, fuel system, power supply and display unit. The latter was designed and manufactured by LXNAV in Slovenia.  The demonstration of compliance to the SACAA was done while M&D was certifying the turbine in Europe.  Six months after the EASA approve the turbine, the SACAA approved the amendment of the JS1 Type Certificate to include the Jet turbine.

“The Jet sustainer is an ideal solution and fits a modern glider like a glove.  Using a Jet as a sustainer has many advantages.  Firstly, there is no increase in workload -  just a single switch and a throttle dial, like the FES.  There is also no noticeable performance loss with the Jet extended – once a customer completed a competition flight unawared that the engine was extended!  The weight without fuel is only 17kg, and the pilot can select fuel the quantity of fuel on board.  When fully loaded ( 32kg of fuel) a retrieve distance of up to 250km is possible”, explain Uys.

One of their customers said the following after roaring past at VNE:   “I may not have a Citation, but I still have a Jet!”

The JS1 Jet sustainer engine

The Team

Team JS is mentored and led by their chairperson of the board, Niels Sundberg, owner of Sun-air Scandinavia, makes for a winning formula, guiding their company to new frontier’s year after year

Uys Jonker, CEO of Jonker Sailplanes demonstrate the Jet Turbine

Gideon Coetzee , Quality and Excellence Manager

The Future

When Uys was asked about their future products, the excitement in his voice was eminent. 

“We currently have a real powerful R&D team.   We use robust design tools and model all our designs in detail before entering the prototyping phase where all moulds are CNC milled.  Our design tools have fantastic modeling features, including built-in FEMM analysis tools.  It is mindblowing to observe to progress when 10 engineers work on the same model in different sections simultaneously …”

“We like to surprise the market, but then we also struggle to keep our secrets ...  It has been very rewarding to work for a goal, and we design our goals around World Gliding Championships...  In 2010 we arrived in Hungary with a hand full of JS1-B models, in 2012 it was the JS-C 21m, and in 2014, Attie and I revealed the JS1-C EVO.”

But contests being only 2 years apart, put a lot of pressure on the design team, as new projects generally take more than 3 years.  So on the question, if JS will arrive with something new in Benalla, Uys replied: “Time will tell… it would be nice, but we also have to be realistic….maybe we …. just change the name to JS3!”