The History of Electric Flight
Part II: The steady evolution of electric RC gliders at Graupner.
Although not a prerequisite, you may want to read Part I before proceeding with this article. — Ed.
In Part I of this series I covered the early history of electric flight in Germany, led by Fred Militky, Chief Designer at the German Graupner model aircraft company. As a footnote to that early development of free-flight models, in the March 1964 Aeromodeller magazine, it was reported that Militky had been able to put single channel radio in a stock Silentius using the remarkable Bentert 3.5g receiver and a magnetic actuator. The Bentert receiver was a super — regenerative design on 27mHz and was completely sealed in epoxy resin. Together with the magnetic actuator it gave proportional control of the Silentius’ rudder although only one model could be flown at the time without interfering with each other. A long cry from today’s frequency hopping and, essentially, interference free radio systems where we take 3.5g receivers for granted!
This time I will follow Graupner’s developments a bit further, starting with the Militky designed Hi-Fly model that I showed last time held by the designer in front of his pioneering full-size electric motor glider. The Hi-Fly was introduced in 1973 and could be built in three versions: a pure glider, with an IC (internal combustion) power pod over the wing and also with twin electric pusher motors mounted on the wings. The geared motors were designated Jumbo 2000F and were made by the German Marx company. The nominally 12V brushed motors were geared 5:1 for driving folding pusher propellers. The motor mounts were corrugated, presumably for cooling and, if you want to recreate the appearance of them on a modern build there are files for a 3D-printed replica on Thingiverse — see Resources for the link. The 3D-printed version will house a modern brushless motor. There is contact information on the Thingiverse page to obtain a CAD version of the Hi-Fly plan if you want to laser cut a kit. There is also a scan of the original plan on Outerzone which is also linked in Resources.
The Marx motors were high quality and efficient but not capable of very high power hence the need for two of them to power the Hi-Fly. Militky’s next design overcame this problem by making use of a inexpensive Japanese brushed motor with decent brushes that could handle a lot more power. Graupner designed their own gear box and a neat extension shaft to a nose mounted folding prop for the Mosquito model that was introduced in 1975. One of the problems at that time was that there were no electronic speed controllers so motors were switched on and off by a servo driven microswitch. The sudden torque of start up to full power could damage the gear box so a neat shock absorber was built in the drive train using O-rings to absorb the loads. The Mosquito was the first of many designs to use a similar drive set up and would not look old-fashioned on today’s flying fields. I have linked the plan in Resources, below.
The motor used in the Mosquito was the Mabuchi RS 540, later better known by the Graupner designation of Speed 600. This 6:1 geared version drove a 14.5in diameter folding prop and was designated Jumbo 540 FG6. It used a seven cell NiCad pack for power. Until the advent of inexpensive brushless motors from China, the Speed 600 and its smaller brother, the Speed 400 were the workhorses of economical electric flight around the world and I will return to them in a later article.
The Mosquito was reviewed by Nick Cook in the British Radio Modeller magazine. Nick was very impressed by the quality of the kit and said:
“all the pre-shaped components fitted exactly and all the rough cut parts were just oversize. That’s what properly engineered kitting is all about.’’
Graupner kits were quite a contrast to the cottage industry UK kits of the time. He was also impressed by the performance of the model which took about three minutes to climb to about 150 metres. Three climbs from a single charge gave consistent flight times of around 25 minutes without thermal assistance. Practical — and economical — RC electric gliding had arrived!
In 1979 Graupner reissued the Hi-Fly with a single geared motor, this time mounted in the nose, and renamed the model Ultra-Fly. The shock absorber was now incorporated in the nose spinner which made for a neater and easier installation. The new, lower KV, motor ran on ten cells and was designated Jumbo 550 FG3. The two packs of five cells each slid sideways in to the base of the fuselage and an electronic relay switch was used for motor control. The installation diagram from the kit shows how this all fitted together and how down and right thrust was built in to the model. I suspect that the climb performance with the new motor and cell combination was significantly better than the Mosquito but I have not been able to track down a review online. I have linked the Ultra-Fly plans below.
For the 25th anniversary of Fred Militky’s original free-flight Silentius kit release Graupner brought out the two metre wingspan Silentius 86 electric RC glider. This used a newer version of the geared Mabuchi motor running on seven cells, simpler construction than its predecessors and a modern lower cambered wing section. Altogether, a very practical model. At the first electric World Championships in Belgium in 1986 the Graupner display team had a fleet of these models in identical colour schemes and displayed them to great effect. Graupner distributed a poster at the event to show off the Silentius 86 which also had a thumbnail picture of Militky launching the original free-flight Silentius which I featured in Part I of this series. I was flying at the event and brought a copy of the poster home which now refuses to lie flat to be photographed. I could only track down a low resolution image of the poster but it does show the link between the two models and the colour scheme of the display models.
By 1989 Graupner had rationalised their electric glider range and all three models were offered with the same drive train of a geared Speed 600 motor and seven 1.4Ah NiCad batteries.
Unlike the other models above, the plans for the Silentius 86 do not appear to be on Outerzone at this time. If anyone has a plan tucked away please let me know so that it can be scanned and made available to the community.
Next time the story moves across the Atlantic to the USA and the pioneering work of a pair of brothers — the Bouchers. Please leave your comments in the Responses section below, which you can access by clicking the little 💬. I would love to hear what you think! Thank you so much for reading and see you next time.
©2023 Mike Goulette
Resources
- Graupner Hi-Fly scanned plan and photos from Outerzone. — “from Graupner kit #4239. Note the Hi-Fly was available in two versions: as a glider and as a twin-engined electric model…”
- Graupner Jumbo 2000F Motor for Hi-Fly from Thingiverse. — “This is the motor for the Graupner Hi-Fly electric RC soarer of the 70s. This is a wooden soarer with 2.30m span, which flew extremely well and I wanted to build the model again, after about 40 years…”
- Graupner Mosquito scanned plan and photos from Outerzone. — “Electric glider model. Wingspan 250cm … A very good model manufactured by Graupner. I have three…”
- Graupner Ultra-Fly scanned plan and photos from Outerzone. — “Radio control 2.3m electric sailplane model. Graupner kit #4257.…”
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