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.

Styrene stripping used for turbulators on the leading edges of the wing and stabilizer of a Carrier plane.

At the 2023 Regionals I saw a man with his stunter to which he had added a row of large turbulators to his wings’ upper surfaces. These appeared to be from a full-sized bush Cub or some such. As an aero engineer, I would like to just offer the following: The height necessary for turbulators is pretty much dependent upon the length of the chord of the surface the devices are attached to and the speed of the local flow. For model-sized chords, the thickness of heavy thread or fishing leader has served me faithfully for some 50 years, now. The extra height of the devices on that stunter are causing a lot a drag that could be factored in. Maybe the whole package works the way he wants, though.

Finishing Resin Surfacing

There are many ways to get to a glass-smooth finish, and many of us have settled on our favorite way. Each has its pros and cons. The way I like arose in the '70s in the stunter community. They squeegeed epoxy onto their flaps with an old credit card. They sanded that, and then they put on a second coat. It was a real timesaver over dope filler coating.

This approach morphed into a way shared by many, using ¾-ounce fiberglass and finishing resin. This is my favorite, which I have now tweaked over many applications.

I first seal the grain a bit with a thinned coat of nitrate dope. The glass and resin sticks best to nitrate.
Lay on the fiberglass and squeegee on a coat of finishing resin. Do not thin the resin.
When cured, sand only lightly, 400 grit or finer. Squeegee on a second coat.

The first coat just brings the resin up to the top of the glass. The second coat fills the weave and should leave a margin for the surface you want.

This is the latest tweak, and it will save you weeks: Do not use a grit coarser than 400! You WILL sand through the cloth to the wood. And then have to build that patch up from scratch.

On my next covering job, I will use 600 grit. This takes more time and requires patience. However, the payoff is that I expect to avoid going through the cloth. Put the time into sanding now, get the payoff from weeks saved from maddening re-dos.

Many finishing systems can be used. My favorite is to use Klass Kote epoxies. This is very expensive, but my 1980 Beech T-34C with K&B Super Poxy looks as good today as it did way back then. No more paint erosion or humidity blushing from contact with nitro.

Hotter Glow Plugs

All of my career and that of legions of other modelers, I have carefully avoided using hot glow plugs. I imagined that these were for fire-breather contest styles like Speed, etc. I thought that Scale and my level of Carrier would have balky, troublesome engine starts. Two years ago, MECOA offered the cat’s meow of glow plug filaments. They alloyed the element, Rhodium, with the common Platinum, or whatever is used. The rhodium in filaments ignites better, but the price of it went into translunar injection orbit! It rose to $28,000 per ounce. This caused a price jump that only national-level competitors would buy.

The first lot sold out quickly and I missed out. The next year I bought some and just gave them a try. I got the BEST starts of my looong career! I got my flying buds to try more affordable brand, Sig and OS. They got the best starts you can have, as well. We didn’t have a problem using hot plugs that season, and winter starts were like none before.

This season, I did have two starts on my Carrier bird where the engine ran forward for a bit and then ran backwards, I bumped it with my starter, dead, and the engine ran forward for the flights.

I note now that MECOA no longer offers the HOT plug rating, and offers MED-HOT. I am using them now. The price fell significantly, but it is still double the price of common plug brands. For myself, you get what you pay for. Winter starts are so much easier. I saw this again Feb. 9 at Arlington. It was 41 degrees F and 90%RH. The engine fired off quickly.

Last visit for the 4BK Carb Notes

Staggerwing throttle setup.

My Staggerwing’s bad throttle setup, as I said in the last column, was fighting my attempts to throttle back and land. Really, it was like I was back fighting a “barn-broke stable horse” trying to get the plane just to land. On my Staggerwing’s OS .40SF with its 4BK, I shortened the throttle rod 1/16th of an inch to get more throw out of the bellcrank. (The idle stop fitting was already low enough.)

As you know, throttling down rotates the barrel and spirals it to the left. For review, this drags the idle needle along with the barrel to the left. But that moves the idle needle into the LEAN range a lot. I backed the idle needle out about 80.

I then flew the plane at Arlington solo off my Gene Pape stooge. It gave me a great improvement but it still wasn’t enough of an adjustment for a good ground game in the contests. My idle mixture, though, seemed really great.

A couple of days later, I did a driveway run at home with the plane tethered. This confirmed the idle mixture was spot on. I then did my air test of the idle mixture that I first showed you a photo of last time. Blowing through a tube into the fuel nipple on the carb calibrated my mouth’s air pocket routine. This carb at idle is tighter than the typical Perry carbs, which are super.

Testing the carburetor's flow.

Thinking that I might need to back the idle need out because of this, I turned the needle out about 10. Upon restarting, I found that my idle mixture was now ever so slightly rich, but okay for a cold, humid day (I think maybe it will be okay on a warmer, dryer day. We will see.) Upon restarting the engine, I took the throttle down to maybe 5% of throttle range at my handle, and I pulled on the plane’s tether to get a little slack. The plane crept slowly, not standing still. I pulled the throttle further to 0% and again pulled some slack in the tether. This old bird finally just sat there and purred! I feel I have finally mastered these two-needle carbs, and I can get back to having a life! I hope some of you have found some help with yours by sticking with me through all of this.

There will be one more of these catching up articles in the near future.