The ground electrode doesn't heat up enough to produce much emission, although it is certainly more at 300F-400F than in a car at 220F. The emission from the hot center electrode acts in series with the applied spark voltage. So if the mag at low rpm is putting out -10,000V and the emission is -1000V, then there is 11,000 V available to jump the gap. But if the applied voltage is +10,000V then the resulting pressure across the gap drops to 9000V! At normal pressure, the required voltage to jump a gap with rounded electrodes is about 50,000V/inch. This increases somewhat linearly with increasing pressure, and decreases with sharp edges, which concentrate the voltage pressure over a smaller point. At the start, the cylinder pressure is at a maximum since the low flow rate into the cylinder while cranking does not produce much of a pressure drop across the throttle valve so the cylinder fills almost completely. That's why you'll sometimes hear pinging, detonation, while starting a car. So if your cranking pressure is about 150 psi, you'll need about 12,500V to jump the gap with used, normal, somewhat-rounded electrodes, even more if there are contaminents on either or both electrodes.
I designed blades for a three-blade CS or GA prop but there have been some manufacturing difficulties. Also, Jim Smith of Wichita is flying his recently-acquired three-blade, carbon-over-wood FP on his RV-6. Initial performance data from a quick test based on IAS is somewhat questionable, so he will be setting up to do a full-blown GPS test as soon as the weather quiets down. The TAS based on IAS was somewhat on the high side, but the rpm was somewhat on the low side. Three RVs I've had people do tests on have shown TAS from IAS to be 8mph to 11mph high as compared to GPS-derived speed. I hope I didn't goof and get the pitch a little too high. If that is the case, it should perform better with 160HP or 180HP rather than Jim's 150HP. He's going to a fly-in in NM where you can see the prop.
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