Preparing a Plan for Your Model Airplane Design
Tips on an initial draft plan for prototype model plane flights
In the previous section on determining the parameters for the Robin I came up with an initial set of dimensions for this new micro airplane design.
I started with the model’s wing. I felt a 20 inch wingspan would be a good starting point, based on the 15 inch wingspan of the 4-Site biplane and the known power of the 4-Site’s electric motor. Once the wingspan of a model plane design is established, the remainder of the measurements can be figured out.
Following are the dimensions for the Robin's first iteration:
1. Wingspan: 20 inches
2. Wing chord: 5 inches
3. Flat airfoil
4. Fuselage length: 15 inches
5. Nose length: 3 inches
6. Stabilizer 5 inches behind the wing (aileron) trailing edge
7. Tail surface area: 33 square inches
8. Fin and rudder area: 18 square inches
We always aim for as light a weight as possible for any model airplane design, and especially so for an indoor electric flyer. A good approach is to select a construction method that is likely to make your design employ the lightest structure that will support normal flight loads. Typical, for indoor models this would require the use of foam with some carbon rod reinforcement.
I have always preferred the look of balsa wood in my RC model aircraft. I decided not to use foam. Instead, I constructed the Robin primarily out of square balsa wood and finished with a lightweight iron-on covering. I employed a profile fuselage for minimum structural weight and sufficient strength.
Balsa frame test
This will be the first time I have used a balsa frame structure to build a profile model of this size, so I ran a trial to ensure the 1/8 inch square balsa wood had enough strength to withstand the shrinking of the iron-on covering. The test rectangle I made and covered is shown below. The 1/8 inch square balsa construction approach proved strong, looks great and will work out just fine for the Robin.
There is no sure way to determine an aircraft’s final weight other than to build a prototype model and weigh it. Use of 1/8 inch square balsa with the minimum internal structure needed for rigidity will be the guiding principle on the initial plan for the Robin.
I estimate that a total flight weight of two ounces should work out well for the finished aircraft. The fight tests will let us know for sure if initial estimates are correct or not.
Now it is time to draw up the outlines of the Robin prototype. TurboCAD is indispensable for this task. I start with drawing a rectangle 20 inches by 5 inches for the wing top view. I added a line 3 inches back from the wing’s leading edge. This line divides the wing (3 inches wide) and the ailerons (2 inches wide).
I saw no need for a wing center spar. Rather, the 1/8 inch square balsa will be used for the wing leading and trailing edge, and I’ll include a small carbon rod in the center section of the wing’s leading edge for strength. Draw a line down the center of the wing to represent the fuselage and extend 3 inches in front of the wing leading edge for the Robin’s nose section.
Using the tail dimensions determined earlier, I made the stabilizer 11 inches long and 1.75 inches wide. The elevator will be 1.25 inches wide. This will give a total tail surface area of 33 square inches. I located the stabilizer 5 inches behind the aileron trailing edge.
Robin prototype goals
As the aim of the prototype is to validate aircraft construction methods, total weight and flight characteristics, I see no need for a complex aircraft outline.
Following this approach, I made the wing and tail simple rectangle shapes. I used TurboCAD to draw just the outside dimensions for this initial plan of the Robin plane design, and I will “fill in the structure” as I build the test aircraft.
This takes some amount of building experience, as you will have to account for things like the cut out in the elevator for the rudder to move left and right as you construct the first model. These considerations will become second nature after you complete several plans built models.
Using the same aircraft plan outline approach, I next draw the Robin’s fuselage side view using the top view as a reference for wing and stabilizer location.
I sketch in a fin and rudder shape that provides around 18 square inches surface area. I also look for generous fuselage side area, as this will help with both aircraft strength and stability.
Note I thought ahead to include some 1/8 inch balsa sheet in the fuselage for wing and stabilizer mounting. I will figure out details of the motor mount in the nose once the fuselage is built. There is no need for detailed drawings at this point.
Now that the Robin’s initial drawing is complete, print out a full size plan for the prototype build. In the next section, I will go over details of using this outline plan to construct a prototype model.
Author: Gordon McKay