Intro

My adult start in free flight started after I built a Sig Cub rubber powered model. After launching the Sig Cub on hand winds it caught a thermal and flew out of site. Although a little bummed on losing the model, watching it circle higher and higher was so cool.  I built a couple more Sig Cubs; one for electric power and another rubber power model for a postal contest.

More recently I noticed that the Sig Cub was not available on the Sig company website.  As this is a very good rubber model to build after the very simplest models, I decided to design a similar model. this model has slightly more wing area, shorter pylon, and different tail construction. Another reason for designing a model of this size is that I found a source for inexpensive 7″ diameter propellers if you purchased 40 propeller lot.

Design Ideas

Inexpensive to build  – I am trying to use a small amount of different wood sizes. Part of 1/16″ sheet, 1/8″ square strips, and 1/4″ square strip is all that is needed. I used gift wrap tissue which is readily available and less expensive than hobby grade tissue.

Upgradeable – I want start with basic rubber model and then present more features such as dethermalizers and electric power.  This model is large enough that it can carry a little extra weight.

Basic Structure

This is the start of building my prototype airplane.

First Flights

My first flights of my model were from a frozen lake near my house. I noticed right away that the propeller was shaking badly, I had forgotten to check this before flying the model. I added clay to the light side of the propeller, the shaking stopped and the model climbed better. Other adjustments were taking out some of positive wing incidence and tweaking the CG. The plane climbed nice and smoothly transitioned to glide.

Link to YouTube Short video of plane shaking from unbalanced propeller.

https://youtube.com/shorts/hE2tW89M5Z0?si=mZUz0-wrK4PzxCvT

My next item to change with my plane was to change the propeller to a 3D printed propeller that had wider blades and more pitch. The propeller also had a better hub design that kept the prop shaft from wobbling in the glide. The climb might not be as fast as the first propeller but the cruise appears better as does the glide.

Viscous DT

Like my first Sig Cub, this plane could easily fly away in a thermal air current, chances of this happening might be early morning or in the evening. I wanted to start with testing a relatively simple and inexpensive method for bringing the plane down after a selectable time period. The burning fuse method fits that but I did not want young people using fire. There is a good deal of science that can be learned from the “viscous DT”, that is the method I will show first. In case you did not know what “DT” stands for it is “dethermalizer” which is a method of bringing the airplane down fairly quickly. For my plane I choose to raise up the front of the wing at least 45 degrees, this should bring the plane down vertically without damage.

Rotary Damper used to create timer

Attached to fuselage with glue and toothpicks through holes.