Scale Matters

Orin's Stinson SR-6 ready for finishing. All photos by Orin Humphries unless otherwise noted.

Lore, spraying and paint runs, J.Roberts throttles and handles

By Orin Humphries
May 2025

Lore

A long time ago I had a few columns wherein I could share some humorous anecdotes that I’d picked up while being a docent at a museum. The source of the hilarity was our mechanic, Ben Olson, of Gig Harbor, Wash. These were tales of hijinks from his early days as an aircraft mechanic. Later on, Ben was for around 22 years associated with Bob Hoover, “The best stunt pilot in the world,” according to many, as Bob’s mechanic on his famous Yellow Mustang and Aero Commander Shrike.

This is another story from those Days of Yore.

Bob and Ben were at the Reno Air Races, and after a busy day, races now over, took a stroll up the line. Bob, by the way, was the race official flying the pace plane and watching over the racers from higher above them during the races. Up near the end of the line they found a newly finished Sopwith Pup and its proud owner. (No, there wasn’t a class for Pups at Reno…) Bob walked around the biplane, checking it over, and then asked the owner, “Mind if I get in it”? Hesitatingly, the owner let Bob climb in and look around. This was followed by, “Can I take it up”?

The owner was askance, having never let anyone even sit in the cockpit! The owner asked Ben, “Can he fly?” The owner didn’t have a clue who Bob was! Ben answered, “He can fly anything!”

Ben said to Bob, “I’ll spin the prop for you.”

Bob took off and did his signature climb out. That means that between 200 and 300 feet, Bob did a roll!!! The owner was seriously in danger of having a stroke!

Bob put the plane through every stunt maneuver in the book. When Bob landed and parked, the owner didn’t say a word. As Bob and Ben headed back to Ben’s motor home that he always brought to Reno, Bob asked, “He’s pretty mad, isn’t he? Tomorrow morning early, go get him and tell him that I would like him to come with you.”

The next morning Ben went up to the Pup owner and said, “Mr. Hoover would like you to accompany me.”

“What’s he want?”

“Please accompany me, sir.” When Ben and the owner reached Bob’s yellow Mustang, Ben began outfitting him with jacket, helmet, and goggles. Ben then strapped him into the back seat. Bob took the owner for a Mustang ride with aerobatics!

Upon landing and exiting the Mustang, the Pup owner was the happiest man In Nevada!

Spraying

The SR-6 pictured at the top of the column is my current project, the last J. Roberts kit ever produced!

Bob Smurthwaite, J. Robert Smurthwaite, was the man behind the J. Roberts Model Company. Bob and I met at the Northwest Regionals in the early 1970s and had become fast friends, corresponding into the early 1990s. In September 1992 I wrote my friend whose factory had closed two years previously, and asked if he could put together one of his kits for me as a remembrance. He went out into his factory and he put it together with his own hands, sending it to me. He passed away two months later, in November. I am building and will compete this piece of history so that it cannot somehow later fall into the hands of someone else who knows nothing of my friend and his great contributions to control-line.

The bane of painting in any mode is getting ugly paint runs. As I began spraying my Stinson SR-6 project, I was soon in deep trouble. I am using a great brand of paint, Klass Kote epoxy, but doing something terribly wrong. (Looking ahead, if you are getting runs, one of two things: You have too much thinner, or you are trying for one-coat coverage.)

Arghh! Paint runs.

It took a week to get with the two people who could coach me in how to do this properly and avoid runs. The first was my great friend, Ben, above, who is a leading expert in painting full-size aircraft and other things. Ben paints aircraft wings with them vertical during restorations, a great pallet for runs if you don’t know how to avoid them. He shared the following:

Start with rubbing everything down with a tack rag, Always. There is a lot of dust just in the air. (A tack rag, BTW, is just cheesecloth with bee’s wax, of all things, which will not mess up any kind of paint.
Let me explain a crucial term: “Flow.” The perfect mix ratio has to show flow when the cover coat is applied. That means that you will spray on a less-than-full, even paint film. If you have the light on it right, you will see a slightly orange peel-like surface. As you look at it closely, you will see the little bumps flattening out. Their edges will spread out, i.e., flow, until they join with each other into a smooth film. It doesn’t mean that the film will flow around the edge of the piece like a waterfall.
Do some test sampling to get your thinner ratio where you want it. Start with a fog coat on the hard-to-reach places, corners, and edges. What is a “fog coat”? (Some call it a “mist coat.”) It is really thin and bumpy, on a very small scale. “Fog” means fog.
Wait a few minutes and test for a tacky surface with your finger. When it reaches the tack condition, spray on the intended coat, never trying for a complete, one-coat coverage! Look for flow. Keep it a little light. Intend to be repeating this exact procedure the number of times required to get the color density that you want. Red is quite thin and requires more coats than most blues. Yellow, well, I’m sure all paint manufacturers at least mix yellow paints where their vat is near something yellow…

What’s with this “tack coat”? How does this prevent paint runs? The light had not come on for me as yet. Ben referred me to another past master of spraying, a fellow also from our former museum, Dave Helland of Mukilteo, Wash. Dave is in model hydros. Dave repeated Ben’s procedure with one addition. Dave and I spray epoxy paints, which are temperature-driven chemical reactions. First, though:

1B. Dave warms the epoxy parts A and B to 75 degrees F. This will speed the reaction and beat out a tendency for the paint to run.

I do this warming my way; yours may vary. I put my cans of A and B paint in my laundry room, which has an overactive furnace heat vent, even when shut off “completely.” I close the doors and let the warm air do its work on the paint cans. I bought a great thermometer off Amazon for checking things. In my house the temperature gets higher, to 76.8F, with the house set at 72F, so, when I’m warming epoxy paints, I set my house to 71F to get the right 75F in the cans.

Check out this thermometer. These are cheap on Amazon.

“How”, you ask, “do I check the paint temperature without getting my thermometer dirty?” I set a glass of water next to the cans, and I check the water.

Tack Coat

The reason for doing the tack coat is that this first layer will be sticky, hence its name, and it will glom onto the next coat. It captures the next coat and holds it in place. Hence, I am dubbing this next coat, “The capture coat.”

Don’t think you are free to gob the capture coat on, however. The stickiness of the tack coat can only reach up so far into the next layer of liquid. Any thicker a layer, and the top portion of the new paint layer is free to slide off. This next, real coat, is to be less than a final coverage thickness, as said above.

FYI, for the record on my current project, I am using Klass Kote epoxy paint and thinner warmed to 75 degrees F, a very basic Badger external-mix airbrush, and a compressor that is delivering 33psi. My mix ratios for K-K white and red have worked well at: 5 parts A, 5 parts B, and 16 parts thinner.

On my first try, however, with my specific paint color and thinner ratio (varies with the color), I tested the surface at eight minutes and it was already drying past tacky. I immediately applied the intended coat, thicker than the tack coat, of course, and it worked out; I got no runs.
On my next spraying session, I checked for tack within a couple of minutes and found it to be just right. Note, Dave had told me that with his mixing, he was getting tack around 15 minutes to that condition. So, the time-to-tack will vary with you, too.

Epoxy Paint Run

Before I was mentored in this method, I got a bunch of paint runs on my fuselage. This was with a white base coat over an earlier silver coat. The silver had already cured completely, of course. I was really dismayed when I discovered these runs some three hours after spraying the white, which was dry to the touch by then. Just to see, I grabbed a piece of paper towel and put thinner on it. I then rubbed the thinner over the runs, and to my amazement, the runs vanished. The white paint had not cured enough to resist the thinner. Then I called my friends for help, which is given above.

Butyrate

This works for spraying butyrate dope as well. However, Ben does not warm the paint first, as is done with epoxy paints. Warming is not required, and butyrate dries so fast that pre-warming would probably foul up the work.

Ben’s trick

When spraying butyrate and rarely getting a paint run, Ben gets a strip of masking tape, and he touches the sticky side of the tape to the paint run. The tape picks up the run without affecting the area. This also works for epoxy paint if the surface under the run is good and cured.

Verschlimbessern

I am strongly advising you not to alter the above process, thinking that you know so much already. This German word that I learned last year means, “Making something worse when you were trying to make it better.” (EDITOR'S NOTE: This is pretty much how I do all my projects.)

J. Roberts Bellcranks

Varieties of throttle bellcranks, all J. Roberts’ invention.

There are two primary versions of these units, Upright and Inverted. Sub-varieties are: elevator pushrod on the right, elevator pushrod on the left, elevator portion long span, and elevator portion short span. The throttle portion of the system never varies.

Nota Bene: Unlike Veco style two-line bellcranks, you may NOT flip these over from their designed orientation. They then will not work with the J. Roberts three-line handle.

The idea is that there are three control lines instead of the familiar two. The handle that is to be used with these bellcranks has three leadouts.

Normal J. Roberts handle.

These bellcranks come in many variations, all built around the same approach. They are available from Brodak, and are a bit expensive. The two primary versions are upright and inverted.

The two outer control lines operate the elevator portion of the system. The central line operates the throttle. However, the elevator lines and the throttle line work in conjunction with each other. They do not affect each other, though, and the beauty of Bob Smurthwaite’s design is that throughout the full range of operation, there is always equal tension in all three lines. No line is supposed to become slack by making any change in throttle setting.

(NOTE: The bellcrank can put out way more throttle range of motion at the handle than any model engine’s carburetor arm can utilize. When the carb arm reaches a limit, the handle will tell you by your feeling a “bump” on the lines. If you force the throttle portion of the handle past this point, you will have slack in some line or lines. Keep the trigger (the handle’s throttle part) within the range your carb can absorb. You will feel the endpoints.

Connecting the Bellcrank

Besides the above-mentioned mounting the bellcrank in its designed orientation, this is also crucial to operation of the system. There must be NO, repeat NO, friction in any part of the throttle linkage hardware, nor in the motion of the parts of the bellcrank on its platform. The system will not operate if there is.

This bellcrank is a complex system of levers. In science class, this design is called a “third class lever system.” The bottom line is that any friction in operation of three-line bellcrank will be multiplied against you. The rod from the bellcrank to your carb’s arm swings spanwise some at the forward end of the travel of the bellcrank’s throttle portion. As the rod passes through the firewall to the engine, the rod can rub against the side of the hole. The hole has to anticipate this and be wide enough.

Please refer again to the photo of the varieties of these bellcranks. In the lower right corner, you will see a package wrapper of how the base plate of any of these bellcranks must sit on your platform. There is a red arrow drawing your attention to the fact that the slot in the frame is not in line with the “alignment hole.” The centerline of the slots is in line with the aft edge of the hole, not the hole’s center. There is a riveted washer on the bottom of the base plate under this slot. Since the washer has to move spanwise when you operate the throttle, you must make an adequate groove for it in your bellcrank platform.

You will mount your bellcrank on 1/8” thick aircraft plywood. You must make a groove in the plywood for clearance with the rivet that’s on the bottom (or top, if it’s an inverted one) of the slot in the bellcrank’s frame. Note this: The slot is not in line with the inboard bellcrank mounting hole. The slot is a little aft. Don’t let the rivet drag against the plywood.

Setup and Leadouts

For two-cycle engines I like to connect the throttle arm of the bellcrank to the carb arm with the carb arm all the way back, at idle/stop. The throttle portion of the bellcrank system is full aft. This corresponds to the trigger on the handle’s being all the way back toward you.

I like to adjust the carb’s throttle arm so the carb arm and the throttle rod are as close to being at a right angle as your plane’s propeller will tolerate. That is, when the carb arm is at full throttle, then, the linkage is as close to the back of the spinner or prop as you are comfortable with. Having the throttle rod and the carb arm close to 90 degrees at Idle gives you Vernier Idle control! You do NOT need Vernier at high power; that doesn’t exist, as you already know.

Now for the leadouts.

Measuring the leadouts.

With the throttle rod is connected to the carb and the carb is at full idle, and the rod is adjusted for the throttle arm in the crank to be at full aft,
Set your handle’s trigger full back, toward you.
Measure the difference in length between the elevator leads and the throttle lead.
Factory is 2-3/16”; trigger full forward is 3-13/16” (but these vary a little).
Make your model’s leadouts to be a mirror image of the handle’s leadouts. Be accurate.
Use a 1/8” diameter wire as shown for measuring.
Note carefully: provide enough minimum clearance between the elevator leadouts and the wingtip, when the throttle is at full power setting, so you can still move your elevator up and down! The elevator portion of the system moves spanwise when you are throttling.

Special note to first-timers: The model’s leadouts will not be even with each other at the wingtip or wherever they come out. This system is not like your familiar two-line system leadouts.

You will need a three-line set of control lines, of course. I suggest you look for factory-made sets at Brodak. If you are going to tie your own terminations for your desired line length, you must get all three lines to the same exact length to 1/16” or less.

My best advice is to start a practice set of leadout endings with extra-long cables. It is tough getting them within 1/16” of perfect. Then, you can shorten the leadouts to final length. I like to use a leadout tying method that I can undo and correct. I prefer tubes to be crimped, but copper wire wrapping works, too. When using tubes, don’t select the smallest possible. It’s impossible to make a correction with the smallest tubing, so you should use a more generous tube diameter.

Mnemonic

(I have trouble remembering how to spell that word…)

All two-cycle model engines these days go to high power with the throttle arm moving forward. For you at the handle, this is, “trigger-forward.” Remember it this way, perhaps: It’s like how the gas pedal in your car moves.

Throttle Lever

When flying, you can move the handle’s trigger by placing your finger in the hole, or you can grasp the lever that comes out the top of the handle and move it fore and aft. This gives rather coarse throttling. You can accidentally kill your engine when trying to taxi. Again, the carb arm cannot use all of the travel that the throttle portion of the bellcrank can generate. The portion of the available trigger motion that is actually used by your carb is a fraction of what’s in the handle’s capability. Your trigger won’t be moving full range. Don’t force it past where it “wants” to stop.

I invented a modification last year to the trigger in the J. Roberts handle that provides for Vernier action when throttling. See photo below.

Trigger hole is filled to prevent use of the trigger. Orin recommends use of the lever.

This tubing spreads the carb arm motion out over a much longer range at the handle. You must grasp the top end of the tube to move it to get this great advantage. Nota Bene: If you grasp the tube down at the top of the handle, you will eliminate all reason for adding this modification. Look at how my flying Buddy, Jeral Godfrey, is demonstrating this (photo below). There are two things to note carefully.

Only two fingers are touching the rod, and they touch only at the very top of the rod.
His hands are locked together at the wrists. Do not move the rod by moving your arm; that is too coarse. Move the rod with only your fingers.

I use this Vernier setup on my Carrier bird as well as Scale models, and people remark how uncommonly smooth and stable my slow speed flight path is compared to every other Carrier model’s path is.

However, after a lifetime of moving the throttle with my finger in the hole, when things got dicey in a flight, I’d jam my finger in the hole instead of using the top of the rod. That always made things worse. To head myself off at the pass, I have blanked off the windows as shown in the second picture up from here.

Jeral Godfrey's handle modification.

My flying buddy, Jeral Godfrey, came up with a variation of this Vernier setup. He wanted to have a stronger grip of the handle, wherein he could have all four fingers on the grip. He cut away the back of the trigger hole so he didn’t have to blank off the window as I do.
If I ever finish my huge P-3, I will need to use Jeral’s tweak of this approach for any chance of hanging onto it. Hats off to him.

Lever between the carb and the wing.

I’ve published this before, and I show it again for “one-stop shopping.” Not all models have room for this. Put this lever between the bellcrank and the carb. It spreads out the usable range of the handle’s trigger movement. This is further Vernier action in throttling for very competitive taxi routines, whether in a contest or for great sport flying.