Tuesday, February 5, 2019

Sig Cub Electric Free Flight

For me there is something fun and challenging about trying to push something of lower performance to perform at close to an optimum level. The people that build and race the full-scale solar cell powered cars would be a good example of this. My challenge has been to build and fly an electric powered free flight model using the STRIX motor timer, 7 mm coreless electric motor, and one cell lipo battery of around 200 mah.

Sig Cub Electric 

Constraints of the timer dictate a motor run time of 10 seconds or less, and the size of motor and battery are limited also. I have personally limited myself to not using a gear drive. The weight of the electrical components is around 12 grams, if I remember correctly. From my experiments with adding the electrical system to various rubber models I believe optimum wingspan is about 22” to 24”. The most recent plane that I constructed just for this electric system is the Sig Cub. Several years ago I had built a Sig Cub for the intended rubber power and lost it to a thermal with just hand winds.

Flying at 23 Degrees F Motor would cut out

To cut the weight and the drag down further of electric system I left off the landing gear and cut the height of the pylon down. For the wing I used 3 mil mylar and then tissue just on the bottom for some color. The tail surfaces are covered with Esaki tissue and sprayed with an art spray which someone suggested might help the warping problem. It has not worked well and at the right conditions the stab starts to look like a potato chip. I did not pre-shrink tissue, but I think some type of plastic will be used to replace the tissue on the tail.

Launched Too Steep

Beyond the specifications of the airplane, I try to improve the flight by constantly tweaking what I can easily adjust on the airplane. That is position of the wing, rudder trim tab, and angle of launch. To get a longer flight you want it to climb as steep as possible without stalling and the plane transitioning from power to glide as close to level as possible. With this rather weak power package a real steep climb results in bad stalling and a very short flight. Other times I notice the plane climbing and diving in a very gentle manner. No doubt it is stalling but by a smaller amount. The best flights are in a constant climb angle with no hint of a stall with a fairly large turn radius because too tight a spiral wastes energy also.

This Looks to be Good Altitude for this Power System

Straight Pins Used on Rudder Trim Tab

What I like about electric is it is possible to get a large number of flights in because time is not needed to wind a rubber motor or fill an engine with fuel. Batteries need to be swapped out fairly quickly however because performance goes down with multiple runs. What I like about this weak electric flight is the plane is relatively close in and easier to observe.  A negative I have found is that the electric system does not work well below 30 degrees F, but then that is not great flying weather either.

Bill Kuhl

Related Links

https://sigmfg.com/products/sigff1-sig-cub-kit  Sig Cub on Sig Website



In the video the climb is not consistent all the way up and duration is not as good as it could be. Probably turning a little tight also.


  1. Great things Bill! I wish you all the best. It is important to push the boundaries on anything. After all, what are the boundaries? It would be impossible to determine if we don't push ousrselves to such.

  2. Thank you for the comment. I am limiting myself on the areas that can be improved on. If a more powerful motor was selected, it would climb higher for sure and then glide longer. With the more powerful motor the battery would drain quicker.

  3. Have you considered a supercap instead of a battery for cold weather?