Scale Matters

The electric P-51 prop to be used on the P-3 during static judging at contests. All photos by Orin Humphries

Mock-ups, Basics: Woods and Glues, Stooge, Second Checking

By Orin Humphries
July 2024

This is a teaser about my present project that has been moving right along, my Lockheed P-3 Orion. The electric P-51 prop shown above will be the basis for all four of the Scale props that it will use during Static Judging at contests.

(The P-3 is now paused for a bit so I can make a simpler entry for the 2025 Nats.)

The size of the P-3 project.

Mockups



P-3 mockup parts.

Building my P-3 from true scratch has been an odyssey of some 50 years, having begun it in 1973, but putting it aside for a long time. This is a complicated design, with a bomb bay and nose wheel well forward of the wing. Those cavities present structural weaknesses if the nose wheel encounters a hard landing sometime. That aside, there are LOTS of parts to be fit together, tabs and slots to arrange, and drawing mistakes are a certainty.

I decided to check my drawings by making a mockup of the bomb bay, the wing saddle, and the nose well before touching any balsa stock. My wing must be removable for transport, as it has a 74.5” span, a bit too much for my Mustang to haul around.



Checking fits and line-ups.



The wing saddle, the bomb bay and the nose wheel well.

Sure enough, I found spots in the drawings that needed correction, but the cardboard assemblies gave proof of principle for my approach. Setting the mockup on the already-constructed top half of the forward fuselage led me to build the bomb bay.



To save finishing time and materials later, I lined the bay and doors with thin styrene sheet from the hobby shop.

Next came the nose wheel well and cockpit deck.


There were a number of places where the mockups showed me errors in tabs vs. slots for them. It is too much to keep straight without something to look at.

Box Project


I wanted to make a smaller line box for less heft when called to the line at a contest.



Plywood is expensive; you have to get it right.



I found a measurement error, too short, in the parts for the front an back of the box. The mockup also helped me understand the puzzle piece nature of how the sides, ends, and two floor pieces fit together.

Basics: Woods and Glues

Part 1, Woods


A visit with my friend, Ben Olson, in Gig Harbor, Wash., brought up a good topic for a new sub-series, Basics, meant for newer people to the hobby, but actually a little bit for everyone to some extent. Ben is our mechanic at the Historic Flight Foundation where I am a lead docent.


This is Orin sitting in the cockpit of the museum’s P-51B. He is looking forward through the gunsight (Yeah, someone's gotta do it!)

Ben’s maintaining all of the historic aircraft at the museum has kept him from having time to pursuing his control-line modeling hobby. As we visited, he asked for and update on glues today. Well, that had to first review things about woods that I had to learn while at the University of Washington getting my Aero degree. I was the lead designer of an UAS (in today’s parlance), an Unmanned Aircraft System. The Professor whom I worked for wanted a drone that could fly autonomously between Paine Field and Grant County Airport. I selected a 12-foot span Telemaster and I got to purchase a Saito (?) four-cylinder horizontal opposed four-cycle engine for the power plant. (Spending somebody else’s money is a kick!)

Part of the task was to measure the efficiency of the propeller I had selected in the UW wind tunnel. The head of the tunnel was solicitous about the tunnel’s blades and made me show that the blades on my model prop were going to stay intact and not go rattling around his tunnel!

I went to the library and got a book on the strength of woods. I wanted to know the tensile strength of the birch in this model prop. But I could not find a tensile strength listing for any wood. But I did learn something very useful to all of us. Let’s look at two blocks of the same size, one representative of balsa and one representative of spruce. The spruce block will weigh three times what the balsa block will.  Accordingly, the spruce block is three times as strong as the balsa block. (Here, I am referring to the shear strength, and we will see how this plays out.)

Puzzled, though, that I found no tensile strength listing, I had to think with a microscopic view of wood fiber.

As we all are aware, wood has two primary parts. These are the cellulose tubules throughout their length, and the tubes have these fuzzy, Velcro-like hairs all over them. The interlocking of these hairs holds the tubes together. This is the heart of it.

Say you have a wing and there is a crackup. You look at the broken ends of the failed balsa. Have you ever seen a truly clean, square tensile failure? That is, sharp, blunt ends of the piece at the break? Or, were the broken ends always kind of strung out over a bit of length? The ends are always ragged, aren’t they?

This is classic Shear Failure, not Tensile. What happened, if we look at the break happening in slow-motion with a magnifying glass, is that the little hairs started letting go at random places throughout the over-stressed volume. As the hairs slipped past each other, individual tubes started to snap. These tube failures are distributed randomly, presenting the classic ragged break face.

This is why woods all fail in shear, not tension.

Finally, as you know, some woods are loosely put together, like pine and balsa, while others are very dense with high fiber counts per millimeter of depth into the surface. This is where selecting the right adhesive truly matters.

Basics, Part Two, Glues



First, Never glue things like ballast weights or wing struts to a butyrate-painted surface. Butyrate has a low adhesion strength. Always build in a plywood pad before painting an area and then later screw the next component into the plywood.

There are several kinds of glues: cements, resins, epoxies, CA glues and RTV silicone rubber.
  • Cements are some plastic or other, like cellulose acetate or stearate, that is dissolved by a brew of nasty organic solvents. The solvent mix evaporates, leaving the plastic resolidified.
  • Resins started off long ago by boiling cow hooves and diluting the goo with water or something. Many resins today are the chemical developments of organic compounds, the great- grandchildren of those hoof molecules. (Is this why we sometimes hear a clopping sound when some old, vintage model taxies?) (Sorry, I couldn’t resist.)
  • Epoxies are two different chemicals, one being made of a polymer and the other being a catalyst that joins the polymers together.
  • Cyanoacrylate glues are yet another class of compound that reacts within itself if it is prevented contact with air, specifically, the humidity in air.
  • I am leaving off heat glues for our modeling subject as they need no further explanation and are not commonly used by us.
  • Canopy glue, now called RC 560, is the ONLY glue to put canopies on with, folks. You must stir it up first as it settles on the store shelf. It is in the dictionary under “Tough!” It is white in the bottle but it dries clear.
  • Finally, there is silicone glue. (When silicone glue sets up, it gives off a reaction product, acetic acid, (which evaporates,) as in vinegar. That’s the odor you smell. I have used silicone glue to shock mount fuel tanks, putting a dab under four corners of the tank. This is on my Staggerwing, for instance.

Which Glue to Use?

Your choice of an adhesive comes down to two things: Is the wood very loose internally like balsa fibers, or does it have densely packed fibers, like our plywoods? That is, is it a soft wood or a hard wood? You see, this is a critical consideration, because almost no glue deeply penetrates a piece of wood. Any glue is bonding only to as many surface layers of wood fibers that it can reach.

Recall that the bonds between layers of micro-tubes in any wood comes from only the entanglement of the little hairs. These hairs can be pulled apart by excessive load levels. For strength, a wood must have very many tightly packed fibers per millimeter.

So…

(1): if you are gluing together a soft wood, you must select a glue which can penetrate as many layers as possible. The more layers that you engage, the stronger the joint will be. Cements like Sigment, and resins like Tite-Bond, are thin and can penetrate the layers of fibers. Epoxies are thick and cannot penetrate. In crashes, more epoxy joints in balsa break that cement joints.

(2): For hard woods, which have very strong bonds per millimeter of depth between layers, compared to balsa, you need grip only the surface layers to get good adhesion. Glues do not need to penetrate harder woods to get strong joining. Epoxies are very appropriate for harder wood joints.

CA glues are another whole class, due to the wide range of consistencies these glues come in. The thin version penetrates very well, but leaves not as much adhesive actually at the interface of the pieces being joined. Still, many joints are fine using thin CA.

Thin CA has a great secondary use. If you need to strengthen the surfaces of two pieces of wood you are about to join, first coat each separately with thin CA. Then, with strong surfaces, join them together with whatever. I do this all the time. If I get a looser screw hole, I put a drop of thin CA glue in the hole to toughen the area around it.

Medium CA is our familiar “gap filling” variety. This is for joints that do not fit together perfectly. You can force the gap-filling version to complete the bond faster if you apply Zip-Kicker to the area. I do find, though, that the use of Zip-Kicker tends to yield bonds that are not quite as strong as those joints without its use. This is a minor effect, however, and such hastened joints give yeoman service.

There is a thick viscosity version of CA glues that I, myself, have never found use for in my building efforts. Gapped joints are weaker than good-fitting joints.

Lastly, there is a version of CA glue that is safe for use on foam structure.

Sigment may be gone forever. Testors glue for wood, green tube, dries a bit differently than Sigment did, if memory serves. Testors, if put on in the same size bead as I did Sigment, dries to a stand-off film as in a fillet. I like a glue bead to hug the joint. Put Testors on in several thin coats. For all drying glues, put on three light coats for the best joints.

To recap, then, soft woods need penetrating glues to develop sufficient, resilient strength in your joints. Hard woods have very strong surfaces and do not need thin glues; thick ones work fine.

Stooge

Stooge made from car door locks.

Gene Pape in one of his columns showed us a great product. It is a set of remote car door locks, for projects, available on Amazon for $56. That’s right, only $56. The set comes with four actuators since it was for cars, and the fob comes already-coded to the enclosed receiver. We range-checked the fob at a club meeting and that is far beyond our needs as Control Liners.



The stooge in use at the field.

Second Checking

Getting the right threads.

I recently saved my tail feathers doing a reality check before threading an acorn spinner nut for my new ARF A-26. The motors’ tech sheets stated that the shafts have metric threads and my stash could only come up with one acorn nut for the pair. I wanted these nuts for Scale appearance.

The spec sheets said the shaft threads were M8x1.00. No acorn nuts were available anywhere I could find. I went to Tacoma Screw and bought a tap for that thread. This would be the only time in my life I would use this tap, which cost me $20+. I had hoped that I could find an acorn nut in my stash that had a hole smaller than ¼ inch, so it would have enough meat for the larger thread. I found two acorn nuts with a hole that would just allow a 3/16inch diameter drill shaft.

The tap charts said to use an H-size drill for tapping this metric thread. I set the acorn nut up in my hobby-size mill and drilled the hole. I then put the new metric tap into a handle and just about started the threading procedure. But I said to myself, “I should do a second check, having failed to do so for my F-84 had cost me grievously.”

So, I set out for my hobby room with the tap and my new Micro-Mark bolt and nut threads gauges to see if the motor shafts fit the stated size. I was smiling for a “What a good boy am I” for doing this second check in the face of trustworthy factory spec sheets.



The thread checkers.

To my great surprise, the threads were M8x1.25! I stopped the plan to thread my acorn nuts with the tap I had bought, and headed back to Tacoma Screw. They kindly exchanged the tap for the one I really needed, actually bending back a bit to accommodate me. I really like the company. Thus, my M-M gauge set had paid for itself on its first usage. You really should check this kind of gauge set, which Tacoma Screw also had by their cash registers. Other fastener companies around the country no doubt have them as well.
Never trust anything! Nuthin’!

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This page was updated July 5, 2024