by loucrane » Fri May 22, 2015 2:26 pm
Hi, Stringjobs,
It’s been a while since your post. Since then the "need" for a 4-2-4 run in Stunt has lost a lot of favor.
The new-metal engines do run happily at higher RPM than the older iron-piston-in-steel-sleeve types. Winning stunt fliers are less concerned how it sounds than how it works in flight. If it works better, it is better. So we launch at around 11.5 KRPM on a 4" pitch prop, or something like that... It lopes along smoothly. lightly loaded until we pull a maneuver. Then, prop load increase is easily met by the somewhat richer needle setting - with no dramatic change in exhaust sound. And when the loads are released the engine had not gone to a very different heat and mixture condition.
The simplest answer for the Schneurle, ABC engines, first designed for RC, has been to accept a rich 2-cycle run at higher RPM, on a combination of prop, plug, fuel and needle setting that works, with a uniflo tank or pipe or muffler pressure fuel feed. The right steady, rich 2-cycle combination is harder to upset than an over-rich 4-cycle that changes firing mode, torque output and heat, then drops back to the previous conditions.
The RC-based engines have a long RPM range in 2-cycle operation. They do gain power when loaded to near the max they can put out with the successful prop/fuel/plug/load combination. They even can actually gain RPM under load, but it is harder to hear that change in 2-cycle. In 4-2-4, the engine might not actually gain RPM instantly, when it shifts from firing every other to firing every rev. We hear about twice as many exhaust noise pulses as soon as load drives the engine into firing every rev. That DOES NOT mean RPM has doubled.
Yes, the Bob Hunt/Dean Pappas articles were great! I'll miss Flying Models magazine...
Best timing for a 4-2-4 run means less these days because we've tamed the beast of the hi-RPM RC engine.
My favorite timing for 4-2-4 engines like the old Fox 35 was 130°exhaust/120° bypass; 10° total blowdown. That allowed longer use of combustion pressure before venting AND the longest practical bypass flow duration. The 4-2 break could be moved to higher RPM - think higher power - in 4-stroke before break. The RPM after the break was still only slightly above the highest 4-cycling RPM when the setting was right. Prop power applied relates to RPM when the load conditions are the same.
I believe Lew Woolard's Silver Fox 35s and Fox schneurle 40s use the same time and blowdown. He recuts certain pieces a certain amount to get that. Given Fox's old reputation for dimensional variances, I measure each one and adjust what and how much I cut to get "my" numbers in that specific example. Not much difference, usually, but I feel better about it.