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Thermy 4: Make a Wish!

Since I’ve braced up and actually want to build the Thermy 4, I want to begin with getting clear about one thing: What do I actually want?

The question isn’t quite as trivial as one might think. I’m planning big on this airplane. That is, I want to incorporate as many features and wishes as possible because I hope to keep this bird flying for the rest of my life. So today is about the demands that I’ve got for this model.

Boundary Conditions

First of all it’s clear that I will fly above level country. That’s simply owed to my residence. So no special toughness for slope soaring is required.

Then I’m most probably going to spend most of my time in the green countryside. I’m not much of a club person and don’t need to interchange a lot with like-minded people. I, my plane, some buzzards or storks as company – good enough! (Right now I’m looking out of the window, watching a buzzard grabbing the thermal updraft near the ground…)

My passion is thermal soaring. Aerobatics are fleeting variety, but to me that’s accessory, not requirement. The model is going to be flying slow, rather than fast.

Necessary Requirements

Based on these boundary conditions, some requirements ensue. (I won’t elaborate on rudder and elevator.) Without a club or regular buddies, I will depend on self-launchability. Possible techniques include catapult, winch, rubber-rope and electric drive. For me, only the electric drive with electronic speed control (ESC) is realistic, all other options require vast amounts of space and increase the demands to the model’s toughness. The Thermy is constructed as an electric sailplane anyway. On top of that, the electric drive helps to avoid off-field landings.

From my point of view, thermal soaring enforces particularly actuated flaps. They extend the speed range and improve the turning capabilities within the updraft. At the same time, they can support the ailerons. Furthermore, flaps allow for butterfly or crow (flaps down, ailerons up) to assist landings or braking during mandatory descents. Cambering and butterfly necessitates particularly actuated ailerons.

I chose the Thermy 4 especially because it meets all these requirements. So one can see that such considerations greatly help to narrow down the field of possible aircraft models.

Desired Features

My first enhancement to the original model is to include lift dumpers. Butterfly is not very prototypical. And when landing off-field, the flaps are endangered if they aren’t raised prior to touchdown.

Although I will fly in the green countryside, I’d like to add an undercarriage. On the one hand for the looks, on the other hand for those occasions when I might fly at a model aerodrome. More of a dalliance is the wish to include a wheel brake. However, I will accumulate some experience with runway landings before I decide whether or not an R/C brake is worthwhile.

For flying in twilight, I’m thinking about illumination. So I’m plannig to include a set of navigation lights and an ACL strobe.

This model is going to be very expensive by my standards. I’ve never spent more than 250 € for a single model, this time it’s going to be five to six times the sum. Because of that I’d like to try and work in a whole aircraft rescue parachute system. That is definitely going to be a scratch-build since commercially available systems are either limited to small models, or very expensive.

Finally, I will reserve the option for a tow coupling. I don’t want to rule out that eventually I might get in touch with other R/C pilots, so towing might become an option.

Unwanted and Unneeded

Conceivable, but incompatible with the Thermy’s wings would be a wing with six control surfaces. A second set of ailerons only makes sense with a really large sailplane.

I’m aiming at thermal soaring, so I will exclude a retractable engine. I’d love to try such a thing, but the extra weight isn’t worth it to me. And since my model will never be finescale, it’s going to be a folding propeller at the nose.

In order to save even more weight, there won’t be a retractable undercarriage, only a fixed one.

It would be really appealing to implement a water ballast system. Ballast allows for higher speed, but I won’t use the Thermy for thermal races, nor for slope soaring. With the latter, ballast would really pay for itself, allowing swift flying with high wing loads and lower landing speed after the water is jettisoned. I will keep this option in mind in case I consider a slope soaring plane or a large sailplane in the future.

Channel Configuration

These requirements and features lead to the amount of R/C channels that I will need:

  • 1 channel for ESC
  • 2 channels for elevator and rudder
  • 2 channels for camber- and butterfly-able ailerons
  • 2 channels for flaps supporting the ailerons
  • 1 channel for lift dumpers
  • 1 channel for wheel brake
  • 1 channel for illumination
  • 1 channel for whole aircraft rescue system
  • 1 channel for tow coupling

So, a total sum of 12 channels, fortunately what my transmitter can barely offer.

That concludes staking out my technical needs and wishes concerning the Thermy 4. Next time, I will talk about possible design variations.

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