Control Line: Aerobatics
Wynn Paul
That old bugaboo—"My engine won't run right!"—came up again the other night when I got a call from Beaumont, TX regarding a kit-built Gieseke Nobler with a built-in tank that had the engine go lean every time the plane got into the air. So, let's go through the complete process of attempting to clear up this engine problem.
Initial diagnosis and simple checks
Since adjusting the needle valve did not keep the plane from going lean in the air, the easiest thing to do first is substitute engines to see if the problem persists. If it does, then the problem can be the tank, fuel, fuel line, or filter. For this article, assume the problem still occurs, as it did with the Beaumont Nobler.
- Check the fuel line and filter for pinholes and check the filter for a leak or clogged screen.
- The easiest fix is to replace the fuel line entirely with new tubing. Watch how you bend the fuel tubing—don't kink it.
Cowling and cooling considerations
One item to consider is the cowling. If your cowling fits very closely around the engine (less than 1/8 in. clearance), it may not allow enough air to circulate, causing the engine to heat up and run lean; this is especially true if the entire head is not projecting from the cowl. On the other hand, if the cowl is a very loose fit around the engine head with a half-inch or more clearance, it may not force enough circulation of cooling air around the fins and the engine could overheat, although that is less common.
Dave Rees discussed airflow through cowlings in his "Dazzler 40" article in Flying Models, November 1976, p. 31. He speculated that air travels in a helical flow clockwise when viewed from the front of the plane, and that a cowling should be designed to take this into account—putting the intake on the inboard side and the exhaust on the outboard side.
- If your plane runs lean consistently, try flying it without the cowl to see if things improve.
- You may also try the "Rabe engine shroud": a piece of tin stock about 3/8 in. high by 3 1/4 in. long (or long enough to leave about 1/3 of the circumference of the fins open at the front). The shroud is held on by a piece of wire. At the back of the engine, cut a hole about 1/8 in. by 1/8 in. to let the air out after circulating around the fins. This shroud is meant to evenly distribute cooling air around the fins.
Built-in tank problems
If the engine is going lean it probably means that excess air is getting into the mixture. This assumes the engine runs lean in both level and inverted flight. If the tank goes rich when the plane is inverted then you have different problems besides air in the fuel.
- Check the engine height relative to the center line of the tank pick-up tube. Moving the engine away from the engine bearers will make the run leaner in inverted flight (for inverted engine installations).
- The pick-up tube may be cracked or the entry points of the various tubes (pick-up and vents) into the tank may have leaks.
- With a built-in tank you may be able to replace the pick-up tube by going through the firewall, although most pick-up tubes are soldered to the rear of the tank. Try installing a new pick-up tube to see if that helps.
If you didn't clean out a commercially bought tank (shame on you), solder, flux, or other foreign matter may have broken loose and lodged in the pick-up tube, restricting fuel flow.
- Try forcing fuel or thinner under pressure back through the pick-up tube and out the vents. Put tubing on the vents and point the ends away from your face. Use a fuel bulb or a high-pressure line to force fluid through the pick-up tubing.
- Try a detergent cleaning agent.
- Be certain to fill the tank full of model fuel and then drain it completely before refilling and attempting to fly again.
- If you mix your own fuel, don't be afraid to admit you might have mixed a bad batch—try one or two different types of fuel to see if that helps.
Too-rich conditions
If you have a too-rich run, possible causes include:
- Incorrect tank height relative to the carburetor.
- Excessive muffler pressure (quite rare).
- Consider using an idle-bar plug; idle-bar plugs seem less likely to load up compared with regular plugs.
Suspect engine faults
If you suspect the engine itself for the lean-run syndrome, inspect for:
- Loose head or loose backplate.
- Clogged spray bar (a frequent occurrence).
- Burrs on the inside of a Super Tigre spray bar.
- Spray bar orientation: the fuel metering orifice (the hole in the spray bar) should point down the venturi; with a Fox .35 the holes should be at right angles to the venturi barrel.
- Ringed engines: too-small ring end gap can cause the ring to expand, increasing friction in the sleeve and causing a big rise in operating temperature; this can produce a very lean run or even seizure.
Last resort
If all the above ideas fail, you'll just have to cut out the tank. It is better to cut than to hang the plane on the wall.
For information on PAMPA or stunt contact Wynn Paul, 1640 Maywick Dr., Lexington, KY 40504.
Transcribed from original scans by AI. Minor OCR errors may remain.
