Thermy 4: Design Modifications

As already mentioned at the start of the construction report, I’d cast an intermediary eye on the Lentus Thermik by Multiplex. I especially like its semi-scale appearance, that gives it a similar look as modern soaring planes. Most notably the bulbous nose and the T-shaped tailplane are spectacular and provide a great silhouette during flight.


Since the Thermy 4 is continuously built from wood, design variations seem the thing to do. And I’m not alone with that thought, Wolfgang Werlich, the Thermy-family’s designer, kindly responded to my request on the topic and finds the idea possible and interesting. That encourages me to go ahead.

Nevertheless, a sturdier construction of the vertical stabilizer and the rear fuselage are called for, which means more weight behind the centre of gravity. The horizontal stabilizer will need a continuous elevator and on top of that the original linkage which controls the elevator will have to be altered.

So the fuselage will need some adjustments, too, not least for physical appearance because the bowden cables exiting the fuselage at its flanks and the broad mounting for the horizontal stabilizer won’t be needed any more. I even thought about implementing a separable fuselage in order to swap the standard tailplane for the T-tailplane. However, that would mean even more weight and add an unnecessary weakness to the whole design.


Quasi as a counterweight to the heavier tail I’m planning a semi-scale nose to achieve a somewhat prototypical appearance. It will also have the room for additional ballast to balance the tailplane, if need be.

I will try to base the nose on the ASW-28: bulbous, with a large, forward-tilted cockpit. There are quite a few models of this prototype with 4 m wing span, so it’s possible to purchase a deep-drawn canopy. However, I want to try and build my own canopy first. It seems that many aircraft modelers build their canopies with the means of PET bottles, a form and a hot air gun.

That means I won’t have to first find a suitable canopy and then adjust the nose to fit it – instead, the canopy can be adjusted to the existing cockpit, which I prefer very much.


My EasyGlider‘s control surfaces are made from elapor, reinforced with steel rods in order to avoid torsion. Quite a reasonable decision considering the soft material; so it’s rather odd that no such reinforcements can be found in the Thermy 4’s diagrams, for the control surfaces are merely made from triangular balsa slats. I will surely add some reinforcements, however I haven’t decided on the material, yet. I can choose between carbon fibre and red cedar, though I tend to the latter. Some experiments and the total weight will tip the balance for this decision.

During summer and autumn it happens every now and then that my EasyGlider clips the odd reed on the meadows during landing. Those leave quite pronounced marks on the leading edges. Because of that I’m thinking about reinforcements here, too. Red cedar slats are conceivable again, or perhaps fibreglass reinforcements. As with the control surfaces, a few experiments are called for in order to make an informed decision.

Planning with FreeCAD

With all these design variations I will have to do some thorough planning. And after that, I will have to precisely transfer my diagrams to the components. Of course, one can draw manually, scan the sheets and print them any number of times, but this time I’d like to benefit from the additional precision and capability to make modifications afterwards which are offered by CAD programs. So while planning the fuselage I will familiarise myself with FreeCAD.

Naturally progress will be painfully slow at first, I was at odds with myself for quite some time about this being worthwhile. However, with all those ideas and wishes that I plan to tackle all at once, good starting conditions are paramount. And at any rate, this project isn’t supposed to develop quickly.

In the meantime the workshop is prepared for this new project. More about that next time.

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