Download BK ROG Plan PDF by Paul Bradley

Download BK ROG Assembly Guide by Paul Bradley

Download .STL File for 3D Printed Prop

Download .STL File for 3D Printed Prop Hanger

Intro      Page Under Construction

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.

About the Name

To come up with a name for my plane I just used my initials “BK” and added ROG which stands for Rise Off Ground. From a hard surface this plane should be able to takeoff easily.

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, 1/16″ x 1/8″ strip, 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.

For this project I want to give a large amount of detail but make it easy to navigate to amount of detail you need but not at the start, just trying to get into the webpage.

Basic Structure

Printing the Plans

After downloading the plan PDF file it should be printed at 8.5″ x 15″ size paper, it will print onto four sheets. If you do not have a printer cable of printing this large it can be printed at local printer such as Insty Prints for a reasonable price. Two sheets will be for the fuselage, vertical fin, and stabilizer. The other two sheets will be for the wing halves. There are crosses on one side of page that will help align the pages for taping together.

Paul also created an Assembly Guide that will print on 4 pages of 8 1/2″ x 11″ paper after downloading.

Additional Notes for Assembly Guide

• After taping the plans together the plans will need to attached to a surface that pins can be pushed into like ceiling tile. The plan will have to be covered with a clear covering that glue does not stick to such as Saran Wrap or parchment paper.
• The parts after cut will have to be held down over the plan while the glue dries. It is best not to put a pin through the balsa.
• The cuts of the balsa should match the plan as close as possible, the strength of the glue joint is almost directly proportional to the area of the glue joint. The different glued components should not be removed until the glue has setup.
• Step #4 is to sand the leading and trailing edge before gluing the wing halves together. One thing I am learning with building models is some steps absolutely have to be done in a certain order and some steps go more smoothly if done in a certain order. Balsa models have to be handled very gently or there will be broken parts. It is much easier to sand the wing halves before joined together.
• Step #5 where the wing halves are glued together is a critical one. The wing panel that is raised up 4″ has to be exactly 4″ across the entire width of the wing (chord). It also has to be straight across the front and rear. Inaccuracies will result in unwanted turns in the flight path.
• Covering the wing and tail will be explained in more detail. My prototype planes have been covered with gift wrap tissue rather than a model grade tissue as it is cheaper and readily available. It is attached with gluestick. I have not used dope over the covering to keep things simple.
• Step #7 & #8 are critical as if the wing is higher on one side it will cause a turn also.

Materials Needed

1 sheet 1/16″ balsa

1/8″ square balsa strip

1 – 1/4″ x 36″ Square hard balsa

1 – 1/16″ x 1/8″ balsa strip note 1/8″ wide strips can be cut from the sheet of 1/16″ balsa

1/32″ music wire

Supplies for Building

Supplies for Flying

Lubricant for Rubber Motor	Mechanical Winder

Covering Material tissue paper

7″ propeller

Building Board, something that pins can be pushed into like ceiling tile. Clear material to cover the plans like Saran Wrap on Parchment Paper.

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. The 3D printed propeller is heavier.

Lighter Injected Molded Propeller	High Pitch Heavier Propeller

More About 3D Propeller from Paul Bradley

“Most of the work done buy the prop is in the outer 25% of the blade. Since the Chinese prop is flatter in that area it is able to spin faster using a given motor. That normally translates into more thrust even though the pitch is reduced. Faster turning props usually make it easier to trim a model as well.

A lower pitch prop will definitely produce a faster climb. The higher pitch prop produces a longer motor run with less initial thrust. It is always a challenge matching the prop to a given air frame to get the best performance. For outdoor models some people prefer to give up some motor run time in favor of a more aggressive climb to help get the model up and away from ground turbulence. The nice thing about 3D printed props is you can tweak the pitch value to one that gives the best result for your model. Something between the commercial lower pitch values and the higher pitch values used for longer motor runs.”

After installing the high-pitch 3D printed propeller I observed exactly what Paul had described, the plane would climb slowly but continue climb and hold level flight for a longer time than with the prop with less pitch.

Switching to More Rubber

All of my flights with the lower pitch orange propeller had been with a single loop of 3/16″ rubber braided.

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	Rotary damper with arm added

Attached to fuselage with glue and toothpicks through holes.

Fuselage = Motorstick

The fuselage or also called a “motortick” is a single stick plus a pylon that attaches by small rubberbands. This stick needs to be as straight as possible and also rigid as the rubber motor is pulling hard on it trying to bend it. If it bends it down it changes the angle of the stabilizer causing the model to dive. The stick can also bend sideways causing the model to turn. Originally I used a stick of 1/4″ square hard balsa which worked fine while using a single loop of 3/16″ rubber. As I changed to two loops of 1/8″ rubber and increased the turns I noticed first the plane would climb really slow and when I increased the turns further the plane dived into the ground after launch breaking the motorstick at the pivot point for DT system

.

At this point I realized that the present motorstick was not stiff enough for this amount of rubber wound tighter. Further adding to the issue of the bending was the long distance between the prop hook and the rear hook. My first idea was to look at similar airplanes using 7″ diameter propeller, the Sky Bunny and the AMA Beta. On both models the motorstick was larger and the distance between the hooks was much less. I moved the rear hook ahead two inches for the new fuselage, I might try it farther forward.

At this point I looked at the Sig Cub plan again. the distance was 17″ but I noticed the motorstick was 5/16″ x 1/4″. As this is a rather odd size to purchase I decided to laminate a strip of 1/6″ balsa to 1/4″ square stick. Someone informed doing the lamination was shown on the original Sig Air Modeler magazine.

For the 1/4 square stick I had selected a rather light piece of balsa thinking it would be plenty strong with the lamination. After just the second flight the model stalled and crashed on the nose breaking the balsa just past the propeller. it was easily repaired. The CG location has moved rearward since using a lighter propeller again, this means moving the wing rearward which should give less stability than a more forward wing. I had also covered the other side of vertical fin and stab which adds a small amount of weight but it was at the rear of the plane. Moving the rear hook forward 2 inches should shift the weight of the rubber.

	Shaft was too tight through tube

Another issue I had noticed since building the new fuselage was the prop hanger I built using an aluminum tube glued to balsa and wrapped with thread covered with glue was not turning as freely as it should. The plane had really short flights, I had thought the fit of the prop hanger that is included with the propeller might not be fitting tight enough and the propeller was wobbling during the freewheel phase, this needs further testing. I filed the prop shaft slightly for a looser fit, it did help performance. It was suggested I should have brass washers between the aluminum tube and the propeller to reduce friction more.

Cutting Balsa

To cut relatively thin balsa it is common to use single edge razor blade or Exacto knife. I found the razor blades at a hardware store. It is important to change blades often before getting dull. It is best to create a template of thin plywood or cardboard to cut around. It is best to make multiple cuts around template instead of cutting through in a single pass.