Scale Matters

Orin's NS2 flying. Fred Cronenwett photo. All other photos by Orin Humphries unless otherwise noted.

A collection of topics:

LED lights and color perception, Koverall and rib stitching, turbulators, finishing resin surfacing, hotter glow plugs, and the last of the 4BK Carb notes

By Orin Humphries
March 2024

LED lights and color perception

At the U.S. National Championships one of the judges came up to me on the second afternoon and we talked. He opened with the red color on my Monocoupe was too bright. I agreed with him and explained something I had recently learned on my Stearman. I shared that under the LED lights in my workshop the yellow shade had looked one tone darker than it does outside and I had worried that I’d used the wrong shade somehow. However, when I took the Stearman outside for a photo, the shade looked correct in natural sunlight. It turns out that the modern LED bulbs we buy are labeled, “bright white,” as opposed to “soft white,” a yellowed shade. These bright white bulbs are a close approximation to sunlight, but they are not “true sunlight.” Hence, my Monocoupe’s red shade had been chosen for what looked right inside my workshop. I could not see repainting and marking it for a Fun Scale model.

The judge then asked me what fabric I had used to cover my Stearman.

Koverall



Covered with Koverall and ready for finish.

I said that I had used Sig Koverall. I went on to explain this great material and its application. You see, it is a polyester fabric identical to the covering used on the big full-size planes, except that it is the thinnest variety. It says on the sheet that comes with it: "ou may stick it down using any method that you like."
  • The wingtips are Sig’s gift to modelers: This is a free fabric, not adhesive backed, so it goes to the bias when you pull it on a compound curve.
  • Grasp the fabric at 45 degrees around from the leading edge, that is, halfway to the center of the tip, tug it, and wiggle the fabric side to side once. Now the fabric is on the bias and the surface is wrinkle-free. Stick that down. Next, grab the fabric 45 degrees forward, around from the trailing edge, tug it. Wiggle it side to side once to smooth it, and stick it down.
  • I got 100% perfect tip coverings on my Stearman.
  • Now, shrink the covering with an iron per the usual.
  • Another gift: You don’t have to wait a day for the covering to dry. Paint it immediately!
  • You must put on two sealer coats using only nitrate dope, as butyrate does not adhere well with this fabric.
  • Second gift: You may now apply any other paint finish you choose over the nitrate.

Rib Stitching and Taping

Look again at the photo above, please, noting the rib detail.

I made a stitch spacing template with a strip cut from a file folder, penciling the dots on this strip per my scale on the Stearman. I would then lay the template along a wing or tail rib and make pencil dots on the Koverall fabric. For a 45-inch span wing it took 55 minutes to do both upper and lower surface, one wing.

My scale on this bird was 1/8 so I applied dots of Elmer’s Glue for the stitch bumps. The bumps were the right height for this bird. The bumps were applied using a small squeeze bottle with a fine spout.



Small squeeze bottle for use in applying small glue dots to simulate stitching.

I tested strips of Koverall for the taping over the dots/stitches. Two things came out of this. First, at my 1/8 scale, Koverall is too thick and stiff to simulate taping. My flying buddy, Jeral, let me have some of his Japanese silk. Strips of this were just right, but sticking it down didn’t work with just the sealer coats of nitrate dope on the wing and tail.

The second thing I found, then, was that I had to paint on strips of additional dope over the ribs for the tapes to adhere to, straight out of the can. To stick down the quarter-inch wide tapes, I laid them over the rib and brushed on… (paint thinner works, but it takes longer and needs more attention.) I used acetone with a fan in my window to soften the extra strip of clear dope. (That’s a NASTY chemical; be forewarned. Use thinner if you are not comfortable with acetone. All dopes have nasty solvents in them, too, so decide what you are going to use.) I rubbed the softened clear dope into the silk tape with my finger, feeling the dope move up through the fabric. This dried the tape in place as a ridge as I rubbed it. My soft fingertip molded the tape around the line of dots, which at my scale simulated the thread that runs between the stitches perfectly.



Rib stitching simulated with glue dots.



Flying in Arlington, Wash., in January 2022. Geral Godfrey photo.

Turbulators

As shown in F. W. Schmitz’ "Aerodynamics for Model Airplanes," the atmosphere treats model-sized wing and tail chords oppositely, compared with the familiar ways it treats full-sized airfoils. (I gave a copy of my Schmitz to AMA’s Model Aviation Museum in Muncie, Ind. Ask them for a reprint.) I first ran afoul of this different behavior with my Carrier birds in the early 1970s. At our lowest speeds, my wings would alternately stall, rolling the plane side to side. This gave additional worries to keeping the plane under something resembling control in that diciest flight regime. Thanks to NASA’s giving me my Schmitz’ book on microfiche, I corrected this random stalling by affixing turbulators to the leading edges of my wings. The turbulators are just a single line of heavy thread or fishing leader just behind the leading edge. This was a competitive advantage and I set my records with them. (Carrier Pilots, please ignore this and forget I said that…)



Turbulators near the leading edge of the Staggerwing.

Note the fishing leader along the leading edge.

Early in my Staggerwing’s flights at the Eugene Regionals, I encountered wind which I hadn’t flown in back home. Note that my “Stag” is a Profile model. When the model came around to the wind-on-my-face part of the circle, the inboard wings stalled sharply and snapped the model toward me, dropping a few feet. The flights continued but were not pleasant experiences. This stalling behavior occurred because with that momentary crosswind on the model, the profile fuselage blanked off the inboard wings. (I always wondered if that would happen were the fuselage built up instead of profile.)



Orin flies the Staggerwing at the Northwest Control-Line Regionals in Roseburg, Ore. Flying Lines photo.

After two such contests in the wind I stopped competing the model and just flew it at home when conditions were better. I got the idea to add turbulators to the inboard wings of my Beechcraft using fishing leader as seen above. This eliminated 90% of the crosswind stalling. It is now too small to notice by others. I placed the upper wing’s turbulator on the underside of the leading edge as that area is where the stall was occurring.