Scale Matters

The author flying his profile Staggerwing at the 2023 Northwest Regionals. Flying Lines photo.

Two-needle carburetors, the future, and Stearman yellow shade

By Orin Humphries
January 2024

Two-Needle Carbs

I never had call to adjust the idle of a two-needle carburetor. There is one on my perpetual Staggerwing, a 4BK type on the OS .40 SF engine. I got this engine on eBay so there was no factory poop sheet with it.

The O.S. .40 with 4BK carburetor. Orin Humphries photo.

The engine for my Stearman, now, I blundered onto when I cleaned out a storage space, a new-in-box vintage SuperTigre .45. Its carb from that era has a wider spiral slot in the carb barrel and an anti-backlash spring in it. I have long suspected that we in control-line cannot use these carbs with our J. Roberts three-line handles and bellcranks. You see, the bellcrank, for you Physical Science types, is a Third Class Lever System. The bottom line is that such lever setups multiply any resistance in the system against you. That’s why when you set up a model that you’re building with a J. Roberts crank; you must not allow the slightest drag to be in the connection to the carburetor. Otherwise, moving the trigger in the handle will be difficult.

So, in the old SuperTigre carb, that spring makes them unusable by us. The carbs are fine in systems having a servo to run the throttle, but poison to a three-line crank system. I proved this to myself when building my Stearman by connecting a handle to temporary leadouts on the plane. I could not move the trigger aft of the engine’s midrange throttle position.

Digging through my stash I found that an OS 4BK carb that I had from eBay would fit the crank case’s carb boss the best. (It still required a thin brass shim.) This carb, having no internal spring, when situated on the ST engine did satisfactorily move full range with the handle’s trigger motion. (Fast forward to model completion and its inaugural flights.) The model is at the field for flight tests and experience. This was less than a month before I was due to take the model to the Nats in Muncie. Everything went okay except that the idle was too high for a standing start as required for having a competitive ground game in a contest.

I adjusted the idle setting screw as I have on single-needle carbs for about 60 years. What a disaster! I completely lost my low-end operation. The engine would just die as I approached idle. Two days before my departure for Muncie, my pitman and I spent the middle of the day trying to adjust the idle stop screw to no avail. I would have to take my Stearman there with no ground game. And this was the plane I wanted to use to try for a spot on the USA World Team for 2024.

Trying to adjust something I didn’t understand at the Nats would not be a smart thing to do. After flying two relays with the Stearman and its having no taxi routine in the Team Trials, I figured that trying another adjustment could result in my engine dying as I came out of a wingover. I passed on a third flight so I could take home my plane in its present one-piece arrangement. When I originally examined the 4BK prior to installing it showed that the spray bar has a high-speed metering hole in the middle, and there is a very thin low speed metering slot straddling the hole. The idle needle is threaded into the carb’s barrel. As you work the throttle the barrel rotates as in all other throttle designs. Except: this barrel moves right and left as it is rotated either way. The bottom line is, the lateral movement of the barrel drags the idle needle slavishly along with it.

The “idle stop screw” is a multi-function affair. As the barrel moves to the left as you throttle back, the idle needle is blocking more of the idle metering slot in the spray bar. Since the slot is so very thin, any small leftward travel of the barrel to get a lower setting makes a major change in the percentage of the idle slot that is blocked by the needle.

Bottom line: If you do adjust the idle screw, you must make a significant change in the idle needle setting in the opposite direction. Don’t let the idle needle blank off a lot more of the tiny slot.

A Starting Point

Testing air flow through the carburetor.

Once back home I finally wrapped my mind around how these carbs work as described above. One day, after flying, Jim Zevely and I had a go at adjusting the 4BK on my Stearman. In the 1970s when I was learning throttle carbs, I wondered how one can tell if the idle setting in a carb is in the right ballpark. I was trying to use Perry carbs in my Carrier birds and I had an Idea one day. I set an unmounted, new carb at idle and put a length of clean fuel tubing on the carb nipple. I formed a marble-sized pocket of air in my mouth by trapping some between my tongue and the roof of my mouth. I then forced the air through the carb and counted how many seconds it took to exhaust that amount of air. It took five seconds. I mounted the carb and ran it, needing no adjustment of the idle disk. This, then, is the O. Humphries carb setup routine.

How hard do you force the marble-sized air pocket out? Well, don’t give yourself a stroke, but be assertive. This routine gets you in the ball park. The Stearman was tied down on the bench. I disconnected the fuel line and put the air tube onto the 4BK’s fuel nipple in my Stearman. I adjusted the idle needle until I got to that starting point as mentioned above. I then reconnected the fuel line. When I started the engine and throttled back, the mixture was way rich. Follow me on all this, please.  I shut the engine down by pinching the feed line and I turned the idle needle in a full 180°(!) in. That was a whopping adjustment compared to what I am used to with single needle carbs. Restarting the engine, the mixture was still very rich but showed a significant improvement. I shut it down and turned the idle needle in yet another 180°. The engine idled so much better that I knew I was close. I turned the idle needle in “just” 90° more and had found the sweet spot. My ordeal was over with the return of a competitive idle ability.

“But Wait…If you order now, we’ll give you TWO mysteries to solve.”

Just when you think you have something knocked, this hobby baffles you. Enter stage left, my trusty Staggerwing with its 4BK carb. This is really wild. I kid you not.

The next time I went to the field I took the Big Red Bipe. I set it up on the work table there just to check its fuel feed and needle setting for the day ahead. (I had been having trouble with a clogged needle valve caused by residual soldering resin in my homemade tank.) I hooked my handle to the leadouts directly (no lines yet) per this usual routine. Once the engine was warmed up, I pulled it down to idle. Just where I thought it should purr, it died smartly. Well, that’s a fine kettle of fish! I could accidentally shut it down in a taxi routine at the Regionals. NO Bueno. I wondered why a controlled idling seemed to come and go from time to time. Anyway, I adjusted the idle setting a bit per the above.

With the handle directly on the leadouts, the throttle trigger in the handle moved with no resistance in either direction. Note this.
I set up the plane in the pits as Jeral arrived at the site that morning. When it was my turn, I put the Staggerwing up for a practice of its contest routine. We can’t do the whole ground game on our field as it’s not a paved surface and there are some spots. After I shot my missed-approach option, the next on the list are Touch and Go. I had trouble idling the engine low enough to get it properly on the ground. There seemed to be an unknown force in my throttle system fighting me and moving the throttle toward Open. It felt like there was some spring in the carb that opposed going to dead idle. That had definitely NOT been encountered on the work table at the beginning of this session. On the table, shutting it off was too easy, effortless.

The next “fun” appeared when I tried to land. I pulled the throttle back, watched the plane slow down, and I let go of the trigger. That is no problem with my Carrier bird and its OS 40FP engine. But my trigger moved up by itself! The plane was doing a wave-off. So, I grabbed the trigger and had to force it a low enough speed for the plane to stop floating and land. On the ground, rolling on its mains properly, I relaxed back pressure on the handle’s trigger. The model accelerated to “mushoff” speed and floated low around the circle a bit. I had to fight the triggered to a full stop landing and shutdown.

I talked this over with Jeral and asked why I could shut down too easily with just the handle on the leadouts and couldn’t get it to stay down when flying. He solved that by saying “You were now on 60 feet of stretchable cable.”

Back home I put on my Aero Engineer’s cap and analyzed what was going on in the carb at idle setting. Picture the carb barrel closed to where there is only a tiny aperture still letting air in. The air has to really squirt through that pin hole to get inside the barrel’s big hole. Professor Bernoulli tells us that when air speeds up it loses sideward pressure (“static”, or sideward pressure). When it expands within the large cavity and slows down, the static pressure increases. Okay, simple enough, the air squirting through the tiny opening into it is at low pressure there, but once inside the big cavity, it slows down and regains pressure. That is the gremlin, folks.

Imagine you are very tiny and now inside the big cavity in the carb barrel. You can look around at the sides of this bored hole. In your mind’s eye, you will see the air, now at higher pressure as it moves slower toward the other side of the hole where the exit pinhole will be, is pressing against the front and back walls of the chamber. This pair of opposed forces cancels each other. But pressure is in all directions. There is also pressure to the right against that side of the chamber, trying to force the barrel in that direction. Is it also canceled by the opposite direction that the air is pushing, the left, on the inboard, side?

You might think so, but the weird behavior felt in my handle’s trigger says maybe not. The barrel has a hole in the right end of it that gloves the spray bar, right? That is not a perfect fit or the barrel couldn’t rotate. There has to be a tiny gap. That, sports fans, is all it takes. Some of the pressure to the left escapes along the spray bar and is actually pushing against the carb body, not entirely on the left of the chamber. Thus, the sideward pressures are not equal and opposite. The leakage prevents a balancing of forces on this sideward axis.

We are there; thanks for sticking with me. I said earlier that any unusual force in the throttle system of the J. Roberts bellcrank design will be magnified against you. The unbalanced lateral pressure in the carb barrel is pushing the barrel to the right. As it moves because the control lines at 60 feet in length are somewhat elastic, the barrel can move to the right a tiny bit. As the barrel does so, it must rotate open, too, thanks to the spiral slot in it for the idle screw. The engine speeds up.

Mystery solved. Is this behavior a bad thing? No, actually, I value it for this carburetor design. This means I cannot accidentally shut this engine, or the one on my Stearman, down during my ground game at the contest. I just have to remember to be assertive in my trigger work with the handle.

The future of piston engines

At the Nats a good fraction of the models was powered by electric motors and controlled by radios. With electric propulsion there is no such thing as no idle control. There is no such thing as needing a different needle setting on a different day. Each of those models has a certainty of getting a good ground game, compared to piston powered models. Both of my entrants finished down from where they could have been with their abilities to do maneuvers because my carbs at that time would not idle controllably.

We are on the cusp of seeing piston power fade away. I do not mean it has barely started; it is already happening before our eyes. Yes, electric power is very expensive, but compared to what, these days? A gallon of fuel at my local Hobby Town USA is going for $35. What does a fine OS engine cost today? What do kits cost today?

Electric power leads to more operational systems that are beyond piston powered, mechanical CL Scale systems. More points. At the very least, you have control of your throttle when your lines go slack.

Okay, 'nuff said about throttles.

Stearman colors

My Stearman is painted in the Navy version, affectionately known as “The Yellow Peril.” My research into its colors and markings was quite an odyssey! I wound up buying two of the set of “The Official Monogram US Navy & Marine Corps Aircraft Color Guide.” This set of four, actually, is by John M. Elliott, Maj. USMC (Ret). He spent 25 years compiling this tome. As you know this is “the bible” for these aircraft. I was referred to them by the kind folks at the National Museum of Naval Aviation at Pensacola, Florida. They told me the color was Lemon Yellow per Appendix G of Volume I.

In this part of the work Elliott states that in 1940 the Navy decided to change the name of lemon yellow to light yellow. “The shade stayed the same.”-ibid. Then the name, lemon yellow was moved to a different shade. Before the end of the war, however, the names were reshuffled once again. “Lemon yellow” was returned to our familiar such shade. Later on, another shade yet of yellow was created and “light yellow” was assigned to that one.

It gets far worse. In the next volume of Maj. Elliott’s set, Vol. II, he misquotes himself. He calls the Navy shade, “orange yellow.” That let the horses loose! Mind you, please, I will never let myself be seen as critical of Elliott’s works as he compiled this material over a period of 25 years! NO ONE born of Woman can keep thousands of factoids straight over a quarter of a century!

Lemon yellow is quite different from the shade that is on the Navy Stearman, an N2S, at the Mid Atlantic Aviation Museum in Reading, Pa. They let me copy a set of photos of their plane for my scale doc notebook. It looks orange yellow, but that is after the vagaries of photography, picture printing, phone displays not being color calibrated, etc.

The contest rules for Color, Finish, and Markings state that a contestant must authenticate the colors used on their model from “accredited sources.” For Stearman builders, then, all they have to do is select just which paragraph of Maj. Elliott’s works to copy and present to the judges. They can then use that shade. Having said all of this …

There is a trustable source. The National Museum of Naval Aviation in Pensacola, Florida has a crew of archivists. In my extensive communications with them, they have assertively stated that the correct shade “…for Navy N2S Stearmans is Lemon Yellow.”
Technically, any Navy Stearman that is not Lemon Yellow is the wrong color.

This does not speak to Army liveries, please note.

A Scale Deviation

I have for several years been advocating that tail-dragger models could benefit from relocating the main gear a bit forward of Scale location. This I supposed would lessen the tendency to ground loop when landing on some surfaces. I figured that the loss of a few scale points if the judges spotted the non-scale alteration (as maybe a half-inch) would be more than made up by a good landing. It would also make rolling on the mains during takeoffs and landings more secure.

Yes, the judges at the Nats this year spotted this move and took off a tad on the static score. (What the first one saw was that this gear move necessitated moving the base of the forward cabane struts forward as well. The forward struts now slanted where they were supposed to be vertical. He pointed this out to the other judges.) The surface at Muncie is paved and there is not as big a chance at ground looping. Back home before going to the Nats, I had made seven practice flights on our field. The rougher than ideal terrain ground looped the plane three times. What good does the scale deviation from gear location really get me, then? (But tell that to my Monocoupe at the Nats. It had scale main gear location. See previous Nats article.)

I no longer suggest moving the main gear forward a bit.

Canopy Plugs for Vacuum Forming

I may be repeating myself, but if you want to vacuum form your own canopy for the first time, please read my previous article(s) on the subject. The part that people are missing from the verbiage I have used is about how smooth the surface of the “plug” you are making has to be. Take to heart that if you can see wood grain or any slight unevenness in the surface, it will show up faithfully in your molding.
Fred Cronenwett made a video of me discussing this subject, as said in my Nats article, and he posted it on some public media. I suppose that could be Facebook.

The plug has to look like a glass blower made it. Send your plug to Ziroli Plans and have them assess the surface. They will adjust things for you for a nominal fee. Study it carefully when you get it back. Never order less than three copies of the plastic canopy. Trust me.

Props, Revisited

This is My Goldberg Shoestring Stunter from decades ago. (Also with it is a preview of my present project, a P-3 Orion. The Orion’s horizontal tail is as big as a couple of my other models’ wings!)

One of the hardest to let go of in some false beliefs that we modelers have is our notions about prop selection. The Shoestring has a K&B .28 Sportster engine. I had originally flown it with a 10x6 prop, but it was a bit too slow which made it soft overhead. To increase power and speed, I switched to a 10x4.

I know what you are thinking: that was a change in the wrong direction. That should have slowed it down. But the 10x4 picked up a couple thousand rpm and made it a powerhouse. The engine was now higher up on the power curve.

But this improvement was too much. It was too fast for the type. I switched to a 10x5 to slow it down. Yep, you probably think that speeded it up. Wrong, again. The plane now was faster than when it had the 10x6 but not too fast as with the 10x4.

Why is our concept of prop dimensions and performance backwards? As said in past articles on props, our mistake is leaning too much on full-size aviation factors. Not all that is true for full-size is true for us. In this case, the full-sizers’ engines turn around 2,100 rpm no matter what prop they hang on it. Our engines, when tuned to a proper mixture, run at very different rpms when different props are tried on them. Change either of the prop dimensions, diameter or pitch, and you will see a 1,200 rpm difference.

Too bad, though, people who have believed one way for decades are usually not able to change their trusted dogma. “Dogma: a belief that has been held for so long it is no longer questioned.” With all due respect, please, stop thinking about this and, go fly; “Try before you buy.” Good Science is done when one doesn’t just take someone’s word for it; repeat the experiment and see what you get.  Fly a x6, a x5, and a x4 pitch on the same airplane, being careful to tune to a proper mixture for each prop. Do not tune to a particular rpm.

I say this to you with two years of prop research behind it, “Let go of the 6-pitch as the best.”

I leave this with a favorite quote from Reader’s Digest: “No one ever truly hears something until they say it to themselves.”


While I was struggling to get my larger Stearman ready for the Nats, I had to let writing articles go. Now that the crush has past the backlog will be caught up; there will be several in a relatively short time. Thank you for your time.

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This page was updated March 20, 2024