Radio Control: Pylon Racing

Radio Control: Pylon Racing

Bill Hager

RETALIATION! The talk at this year's Championship Race from some of our country's top Formula One fliers was about the effort they planned on putting out for FAI Pylon.

Yes, it seems as though we got our ears pinned back in F3D this year's AMA Nationals and the World Championships. We don't like it. So what do we do about it? We work harder, and maybe next time it will be our turn. Come on, guys. Let's get going.

For those of you who are interested in FAI Pylon (F3D), here are some tips passed on from Art Arro.

Tuning the Pipe

First lesson in tuning the pipe is to learn there is no single optimum pipe length for all engine sizes and brands. What works for someone else may not work for you. Fuel, prop, aircraft, exhaust duration, blowdown time and compression ratio are factors that influence TP/TPM (tuned-pipe / tuned-pipe muffler) length. Proper TP/TPM length can only be obtained by flight testing and experimentation — just like propellers.

1. To begin the process you need a good tach and a cutting tool such as a tube cutter or hacksaw. Select the flying prop, or one slightly smaller, to duplicate the unloaded condition encountered in flight. I hesitate to recommend a "test" prop since some rpm addicts use a tiny prop — just enough to provide airflow for engine cooling.

• Run the engine with the TP/TPM in stock condition on a test stand and record peak rpm.
• Remove the TP/TPM and cut 1/4 inch off the header pipe. Always cut the header pipe unless special mounting geometry (angles, cowl clearance, etc.) requires otherwise.
• Carefully remove any burrs or particles from the cutting operation — debris left in the header or TP/TPM will usually find its way into the engine later.
• Reassemble, run the engine again and record peak rpm. You should note a slight increase.
• Continue shortening the header in 1/4-inch increments until no rpm increase is noted. If the pipe is too short, the engine will surge on peak.

Note: The optimum length is slightly longer than the surging condition. If you are surging, lengthen the TP/TPM by inserting one or more cutoff pieces within a silicone coupler and reattaching to the pipe and header.

1. TP/TPM length is an arbitrary measurement — there are no universal standards. Some measure the distance from the end of the divergent cone (apex) to the glow-plug centerline; others measure from the exhaust flange (or piston face) to the apex, or measure overall length including the stinger outlet. When you obtain a known pipe length, be sure you know the standard used for the measurement. This inconsistency has caused a lot of grief.

1. As TP/TPM resonance is reached, rpm will increase and needle-valve sensitivity will be reduced. This is normal — the TP/TPM dominates the engine's running characteristics when properly tuned. The needle valve then mainly controls engine running temperature. Always back down on the rich side to avoid cooking the engine. I generally back down 300–700 rpm from absolute peak; you can detect the absolute peak quickly by pinching the fuel line and noting the rpm rise.

1. When operating with a TP/TPM, standardize your flying setup with respect to fuel (especially nitro content), props, glow plug, and flying course. You can compensate for weather changes by changing the prop or the length of the TP/TPM. As air density drops you may have to lengthen the TP/TPM, reduce the prop load, or increase nitro to compensate. How much change is needed is a product of experience. (Remember that no nitro is allowed in F3D Pylon.)

1. An alternate mounting method is to slide the TP/TPM into a large-diameter mini-pipe (such as those made by K&B or Prather Products). Use a high-temperature red RTV sealant (GE or Dow) to secure the TP/TPM to the mini-pipe, and a silicone coupler to seal the junction between TP/TPM and mini-pipe.

• Mount the mini-pipe/TP/TPM assembly to the engine via the exhaust manifold and an O-ring.
• For extra security I use two music-wire hooks and a spring retainer secured to the mini-pipe and stretched around the engine cylinder.
• The disadvantage of this method is the pipe length is relatively fixed and cannot be optimized easily for weather conditions, fuel, or prop load. However, it is an excellent and economical mounting method when you have a known pipe length and a couple of mini-pipes available.

Most TP/TPMs on the market have a fairly broad rpm range; you should be on-the-pipe statically or just after takeoff. You don't want a "just on" pipe that requires diving steeply to achieve resonance. Short pipe lengths are used for RC speed attempts, but in pylon racing the engine will probably fall off the pipe when lugging down in tight turns.

Engine Modification

The best performance increase from a TP/TPM comes from optimizing the engine for this device. Some engines perform better initially with a TP/TPM, while others show only a small gain in stock configuration. Consider these engine modifications to realize maximum performance with a TP/TPM:

1. Lower the compression ratio by adding head shims or gaskets to account for the supercharging effect of the TP/TPM. Deck clearance generally should not be less than .010 inches in most instances. Recommended deck clearances are in the .012–.018 range, especially if you have some nitro in the fuel. Blown plugs and running hot are sure signs of too-high compression. Ideally, increase combustion-chamber volume while retaining a deck clearance of about .012.

1. If you have access to a lathe or a good machinist, produce a new combustion chamber — a deck bowl or a high-roof top configuration. For reference, note the K&B 7.5 long-reach combustion chamber (part no. 9157) used by some ducted-fan pilots. This head shape reduces detonation and generally improves plug life over a stock head.

1. Plugs: I strongly recommend OPS RC 300 and Rossi R-7 or R-8. These plugs have proven extremely durable in high-rpm situations with a TP/TPM.

1. Increase exhaust duration to fully accommodate the supercharging effect of the TP/TPM. A practical limit is about 180°, after which the engine becomes peaky and very hard to set.

• Changing exhaust timing involves skilled machine work and is best left to true engine experts. An easier method is to replace the stock sleeve with a pipe-timed sleeve.
• The K&B 6.5 front-rotor engine has about 150° of exhaust duration; performance on a TP/TPM can be increased by substituting a sleeve from the F-1 version with about 160° of exhaust duration.
• Some K&B Formula One fliers may have piston/sleeve assemblies that are no longer competitive in F-1 due to a loose fit; these are ideal replacements for the 6.5 front-rotor when operated on 15% nitro and a TP/TPM.
• For unlimited pylon racing, K&B offers a performance update package for their 7.5 ducted-fan engine. This includes a special sleeve with 180° of exhaust duration and "eyebrow" porting. The update also dispenses with the Perry pump and carburetor, which sometimes give reliability problems in field use. Some combinations of OPS, OS, and ST engines may yield similar results with a pipe-timed sleeve substitution — check with the importer.

Included with this segment of the article is a drawing of several TP/TPMs available from MACS Products, presented to give the reader a general idea of dimensions and weights when selecting a TP/TPM for a particular application. The next installment will discuss some of the newer performance-added silencers, such as the Magic Muffler, Wizzard, and V-Tec mufflers.

See you next month.

Bill Hager 706 Glen Haven Dr. Conroe, TX 77385

Transcribed from original scans by AI. Minor OCR errors may remain.