Read time: 12.3 minutes (1230 words)
Building an Indoor Model¶
We do not actually have a plan to work from yet, but pretend we do. How will you proceed in building this model? Let’s walk through a typical building process, just to get a feel for the way we design indoor models. This discussion will help us figure out a reasonable way to create our 3D model later.
1. Clear off your workspace¶
Simple! We start off by picking some flat surface we will use for building. I use a sheet of tempered glass to do my building work. I will place the plan on this glass surface and cover it with parchment paper so glue does not stick to the plans.
2. Lay out your plan¶
For now, we will imagine placing a clean white paper page big enough to handle the complete airplane. We will draw two lines on this page. One runs from the nose of the airplane to the tip of the tail. The fuselage of our airplane will be placed on this line. We will call this the X axis. Perpendicular to this line, we draw another one that runs long enough to reach from wing tip to wing tip (with the wing flat - no dihedral. We will call this one the Y axis. There will be a third axis, one we cannot draw, that runs perpendicular to both of these two axes. We will call this one the Z-axis. The origin of this coordinate system is at the tip of the nose where all three axes come together. If this all sounds familiar, it is probably because you took geometry in high school! All measurements we will need to build our model will be defined in this system.
3. Gather your materials:¶
Balsa wood sheet stock
balsa wood strip stock (maybe you cut this from sheet stock with a suitable stripping tool)
Wire for the prop shaft, bearing, and rear hook
Tissue to build paper tubes for mounting
Mylar film for covering
Glue (We will not be gluing anything for a while)
Cutting and sanding gadgets
An accurate measuring tool - ruler, digital caliper, etc.
I digital scale for measuring weights. (Mine is accurate to one milligram.)
4. Build the Fuselage¶
We will might start off by building the fuselage of our model. This involves a motor stick, tail boom, and mounting posts for the flying surfaces.
4.1. Motor Stick¶
Cut a rectangular strip for the basic stick.
Trim it as needed using a knife blade
4.2. Tail Boom¶
Again, cut a basic strip.
Trim it as needed using a knife blade
4.3. Mounting posts for the wing and stab¶
Cut square posts with rounded tops where we will mount the flying surfaces
Sand the top part round for mounting tubes which will make the connection
4.4. Glue the wing and stab posts in place¶
This involves identifying where each post will go. On my design the posts are in front of the leading edge and behind the trailing edge of each flying surface.
Note
You might notice that having the wing and stab available when we do this will be handy. What that means is that we might be building things in a somewhat random order. There is not one right way to do this!
4.5. Attach front bearing and rear hook¶
This will involve bending the wire parts, and attaching then to the motor stick centered to handle the loads imposed by the rubber motor.
4.6. Attach tail boom to motor stick¶
Some builders use a butt joint for this, but I will use a bit of overlap and glue the tail boom to the side of the motor stick.
5. Build the Wing¶
The wing is fairly easy, unless you really want to get into weight savings by using double-tapered spars. For now, let’s just use strip stock with constant thickness. Our wing will have a flat rectangular center section with five ribs, and wing tips tilted up to provide needed dihedral for stability. The wing will be constructed flat, covered, then dihedral will be added as a final step. We will use a simple circular arc airfoil, common in many indoor models. These may be formed by bending strip stock, or cut from sheet stock.
5.1. Wing spars¶
We will use a rectangular center section meaning that the leading and trailing edges are straight in the center section. The wing tips are going to be rounded at the forward tips. That means we will need to bend the spars around a template of some sort. (We will generate this template in our work with OpenSCAD.) Some builders like to use a full wing template to assemble the wing
5.2. Wing Ribs¶
The ribs are all identical circular arcs. Each rib will have a specified chord length, and camber. It will also have a specified strip height and thickness. We will not worry about how these ribs are formed. They could be cut from sheet balsa, or bent around a form.
6. Build the Stabilizer¶
The stabilizer is going to be designed in the same was as the wing. However, we will use fewer ribs (three) and there i no tip dihedral in this design.
7. Build the Vertical Fin¶
This part looks exactly like the tip of the stabilizer, except we will add another rib to close the outline. Once covered, this will be glued to the tail boom.
8. Build the Propeller¶
For the Limited Pennyplane class, the propeller must be constructed using sheet balsa. Normally, the builders cut the bladed of the propeller and sand these blanks down so they have somewhat of an airfoil cross section, and are lighter. These blades are then wetted and formed over a form of some sort to give then something of a normal propeller form. Quite often that form is a simple can or tapered coffee cup.
Note
Obviously, we cannot do this in OpenSCAD, but I came up with a simple way to create a propeller blade that will be close to what we might build. We will see that in the actual model design notes.
Finally, these blades are glued to a wood spar to provide the desired pitch, and the wire drive shaft is glued in place to complete the propeller.
9. Cover the Model¶
In this step, we use a thin mylar film to cover the flying surfaces. Actually doing this is something of an art form involving thin mists of spray contact cement and gingerly dripping a pre-glued structure onto a piece of film attached to a wooden frame. Then the covered model structure is cut loose using a razor blade of soldering iron. (I said it was an art form!)
Generating a 3D model of this covering turned out to be fairly simple once I found a neat piece of OpenSCAD code created by Justin Lin. More on that later.
10. Assembling the Model¶
Assembling the completed model is pretty easy. The propeller drive shaft fits into the support bearing on the motor stick. The wing and stab are connected to the mounting posts using paper tubes that slide tightly over the mounting posts. Once all of these connections are made, all that remains is to create a suitable rubber motor, wind it up and turn it loose to watch it fly.
Well, actually, it is not quite that simple. There is always a lot of fine tuning to get a model to fly the way it needs to, and picking the right combination of propeller and rubber motor is yet another piece of art work in this kind of competition event.
Wrapping Up¶
Seasoned modelers can put together a new model in a matter of hours, most of that time spent in cutting strips of wood and gluing things together. I have even seen it done in the middle of a multi-day competition event!
Our goal here was just to get a feel for the parts we need to design and how those parts are combined to create the final model. We will need to generate code for each part and generate more code showing how those pars are connected to form the major components of the model. On last count there were over 70 parts in my Limited Pennyplane design, so we have a fair amount of work to do!