Nostalgia Motorglider JT-10
for local soaring
The beauty of the old wooden (motor)gliders!
In the late sixties three new single-seat motorgliders were developed in Germany, all of which were similar in basic concept, involving a high aspect ratio low wooden wing and the engine on the nose. They were the Schleicher ASK-14, Sportavia RF-4 and Scheibe Flugzeugbau SF-29. At the time, they inspired me to consider the design of a similar plane but it never materialized. However, I flew my first ever soaring hours in an RF-4 and later bought an ASK-14 which I flew for three years.
Recently I took a look at my old sketches and decided to do some 3D-modeling work on a motorglider design which would have the flavor of those old designs. Another reason for the modeling was the following e-mail I received:
"I saw your photo of the ASK-14 you previously owned. I am currently researching the possibility of acquiring a set of plans from the factory to begin a homebuilt of that beautiful craft. Do you know of any earlier gliders of the ASK-14 type that plans are available for? Can you supply any copies of info, specs, manuals, etc. for the ASK-14. Would you be interested in drawing up a set of plans for a homebuilt ASK-14 type craft using modern design concepts but remaining true to the original in style? Wood is my material of choice and a cockpit sized for larger pilots. The aircraft is so beautiful!"
Something of all the above-mentioned motorgliders is included in the JT-10 concept. It has a steel tube fuselage like the SF-29, a vertical tail style and retractable landing gear like the ASK-14 and an engine cowling resembling the one in RF-4. The engine chosen for the design is the Japanese HKS 700, simply because Ihad already made a 3D-model of it. Actually this engine is too big and powerful for the design; a 35 hp unit would be quite enough. There is an alternative available soon, the Hexadyne Aviation P60.
Cockpit size in the original model was not "for larger pilots". It was of my size, for pilots about 177 cm tall (5 ft 9 in). However, Bob e-mailed me again that he is 188 cm. (6'-2'') tall and 113.5 kilos (250 lbs.) in weight. He was a professional athlete in his youth and built like a heavyweight wrestler still.
So I scaled up the original 12 meter version by a factor of 1.1, resulting in a 13.2m span and enough cockpit space for a 195 cm tall pilot. The engine, of course, is of the original size but the propeller was increased in size to 1.6 m in diameter for better efficiency in takeoff. In the extended version, the fuselage length is 6 meters.
For the lightest weight, wing and tail surfaces should be made of wood. If modern composite construction is considered, the problem is the enormous work required to make the plugs and molds. One possibility is to make simple molds of sheet metal (aluminum) which would allow the use of thin sandwich structure by vacuum bag molding. The final wing would not be as smooth as the wings of modern gliders but hopefully better than wooden wings at least.
Another method to be considered could be the Burt Rutan "positive lay up" approach. First make the spar, bond foam panels on the spar and hot wire cut the panels to airfoil shape using suitable metal or plywood templates. The skin lay up, filling and painting require the utmost care, otherwise the wing weight could jump to an unacceptable level. Shown below is one possible wing construction using this method.
Landing Gear Variations:
A retractable main wheel landing gear combined with retractable outrigger wheels (operated with air brakes) would be my choice for the machine and is shown in the cut away picture above in both gear up and down positions. This necessitates a side stick since there is no space for a central control stick when the gear is retracted.
A retractable gear would be OK for a single owner, but in flying clubs with dozens of pilots with varying experience, this type of landing gear combined with a feathering prop would be an expensive combination. I know flying clubs in Finland where belly landings with a RF-4 were so frequent the propeller was practically a disposable item.
Therefore, I have modeled three other landing gear alternatives as well; a Scheibe Falke type single wheel, a normal tail wheel type landing gear and a retractable dual wheel type landing gear. All are pictured below:
Click the pictures to view the bigger size!
The soaring performance of this type of glider is not for the record hunters. Anyway, JT-10 would beat hangliders, paragliders and hot air balloons in penetration regardless of the landing gear configuration and propeller chosen. L/D of 25 could be expected with a retractable gear and feathering prop.
I know several glider pilots who only fly local soaring and for them a machine like this would be as good as anything else. In good soaring conditions, one could try to fly a 100 km triangle or even more.
The JT-10 model was created and rendered using formZ RenderZone software in a PowerMac computer.
Comments: If you have any suggestions or comments regarding this design, please, send me e-mail!
I would like to encourage you to work further on the JT-10 design. It would fill a void in the aviation community in this country. I'm a big believer in the lower powered end of aircraft. Your JT-10 would be a fun project and a good little flyer. It's unfortunate that the RF4 and ASK14 never quite caught on in America...hopefully yours' can
I was surfing the web looking for motorglider related topics and came across your great web site. Your mention of the older wooden single seat European motorgliders such as the ASK-14, and ,of course, your JT-10 concept struck me as the PERFECT solution for what I see as a great need of the sport craft market here in the USA. Is there any possibility that the JT-10 will be put into production or sold as detailed plans? What a great combination as a glider/cross country/touring plane. Well done. Well done.
Tom Norton - Houston, Texas
Hueva Paive, Mr Tervamaki,
Please excuse my poor Finnish it has grown rusty since my visit to Finland in 1987.
I have long enjoyed your gyrocopter designs but this JT-10 concept resonates well with me. Do you plan to work this one out?
The 990cc Generac motor has been susesfully striped down to about 32 kilos for direct drive flight operation and peforms as well or better than a 1/2 VW. This might be a suitable poweplant for your concept. Please stick with the monowheel format.
Best regards, Matt Naiva
I recently viewed your web site and saw the article regarding the JT-10 motorglider. I was very excited to see someone else who was interested in this simpler motorglider.
I, like many others, would like to encourage you to further your design work to include either a set of plans or even a kit of this fine design. Thank you for all the designs and models you have made public for us to enjoy. You are truly a master in your chosen fields.
Respectfully, Matthew Poleski, USA