I know I promised at the end of the "Uniflow" article that I was going to discuss strength of materials, but having read an article in Model Aviation, I thought it was high time for some discussion about Cox reed valve engines. That topic covered, it is now on to a very much introductory article about the strength of materials. This time I will not be using math, so some will be disappointed and I am sure some will be happy. Here it goes …
When I build a model plane I like to sit and look at the plans first, and when I am doing this, I, like many, am checking so see how the parts fit together, what each part is made of, if the parts are well made, and if the parts are actually provided. I go one step beyond this, I ask a simple question of each part: Is this part in tension, shear, or compression? Using some simple symbols, I will explain them thusly:
Shear, ^ I v
Tension and Compression are probably the most familiar to the control-line modeler, as they represent something we see a great deal with tight lines and a new motor. Parts within the model also undergo these same forces. Note I use the word "forces," as theses all require a force to be applied to the model in question.
So, before I go into each, I need to set some parameters for the reader. One is that, in all our examples, we will be discussing forces in static equilibrium. To put this into an example, let’s think of a well-made model undergoing a pull test. The lines get “tight,” however the bellcrank does not pull out of the plane; it stays statically mounted to the model and provides an equal and opposite force to balance the force applied by the test. If this were not a “static” test then the bellcrank might pull out of the aircraft, or the aircraft might pull over the tester.
So, let’s talk about each in turn:
Tension is created when two forces are applied, to a point, in exact opposite directions. This is what happens with your line at the pull test above. The pull tester pulls and the model owner pulls, too. Parts of a plane can go into tension as well, certainly the if the bellcrank is suspended -- then the two platforms holding them can be in tension when a force is applied to the model’s lines, leadouts and bellcrank. Parts put under tension in a model will in most cases need a mechanical lock of some sort to remain static. I like to think of it this way: If you butt-glue two sticks together, then pull, how well do they hold? It would be a surface area of the gluing surface issue. However, with a small stick, some sort of mechanical lock is needed to bring the “spliced” stick back to its full strength. In many cases the modeler will splice two sticks together at an angle to increase the gluing area, then wrap them with thread and soak it with glue to bring the strength back to that of the unspliced stick.
Now let’s think about compression for a moment. This is what happens when you put your palms together and push inward, as in the old isometric exercises the astronauts once did. For our purposes on a model, compression happens on the inboard side of the bellcrank bolt, when tension is applied. Having parts of a model in compression may actually be a good thing, as parts in compression will tend to stay put, and not break. I strive for this when I mount a bellcrank, by putting a T-shaped platform through the center rib, of the model. This creates a mechanical lock for the platform and means the long leg of the platform is under tension, while the short legs are held firm, in compression, to the center rib, and don’t pull through.
Shear is two forces of equal but opposite directions, applied at some distance apart, to a part of the model. The most familiar sort of shear in models happens when the model flies, and in a more extreme way, when the wing goes through a loop or corner maneuver. Let’s look at this example in some detail.
Most wing designs (except those I design) have a top and bottom spar at the highest point of the rib, and then the ribs hold them apart, and on some designs, there are sheets of wood applied between the ribs and spars, they are called shear webs, or shear webbing. So, a model is flying around, the pilot pulls hard up, the tail drops the nose goes up, the wing flexes up at the tips and down at the fuselage. The top spar is under compression, and the bottom spar is under tension. Since the top spar is being forced by the compression to be shorter, and the bottom spar is getting stretched longer by the tension, and these two forces are displaced by the distance of the wing rib, the ribs and the web are undergoing shear.
Portland modeler Leo Mehl did a great deal of experimenting with shear webs in the late 1990s and early 2000s and he found that the best way was with the grain running perpendicular to the spars. Without the experimentation, this would seem to be correct as it is always how they are shown on the plans. But, since I am a heretic, I have to ask — Why? The answer is found in the good old activity of splitting wood. If you have split wood, then you know a splitting maul is really not that sharp, it is designed to drive between the grain of the wood and force it to split along the grain. On the contrary, if you are trying to split wood perpendicular to the grain, an ax is used, and the blade is really quite sharp. An ax must cut the wood, as wood has vastly different characteristics depending which way the grain is running. So the grain in a shear web must be placed perpendicular to the shear force applied, just as Leo found out.
Now I know this seems like a bit of an esoteric exercise this far, but virtually every part on a model should be looked at and the modeler then needs to ask, is this part under tension, shear or compression? And then, regardless of the plans, make the appropriate adjustments to the design to improve the durability of the model. As something of an experienced builder and designer of CL models, here are my recommendations for a fastening parts together. For parts under compression, glue works great. For parts under shear , glue that is flexible works best. Parts that are under tension need a good mechanical lock — which means parts that are shaped to fit in a way to turn the tension into compression, or just a good-old fastener.
Now that I have introduced the reader to tension, shear and compression, I will next tackle how to express the strength of materials — tensile, shear, and bulk modulus!
This page was upated Nov. 7, 2017