My RV-7A uses an O-360 with a Sensenich fixed pitch prop. This is a cruise prop, which means there is virtually no aerodynamic braking when power is reduced to idle for landing and rollout. By experience, I've learned that a 2000' paved runway is about the shortest I can land on comfortably even when setting the idle speed at 600 RPM and flying an "over the fence" short final approach speed of 60 to 65 knots without burning through brakes and flat spotting tires. Since the PMag apparently cuts out at 800 RPM, is a dual PMag system really a wise choice for this plane? I say no. What say you?
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Is PMag for a fixed pitch O-360 really ideal?
- Thread starter alcladrv
- Start date
PMag doesn't cut out at 800 RPM. Below about 800 RPM it relies on the ships power and above about 800 RPM it transitions to internal power. As long as you have ship power, it will not cut out below around 800 RPM. Any of the other electronic ignitions will also require ship power so I'm not sure that is a deciding factor. There might be other reasons which I'm sure will fill this thread!
jmra72a
Well Known Member
I have two P-mags on my RV-7A IO-360 M1B with a fixed pitch propeller, and wouldn't have any other ignition. I find on final when pulling the power completely off that I cannot get below 800 RPM. As mentioned above, below 800 RPM the P-Mags use 12 volts from aircraft power so i is not an issue.
Rdog420
Well Known Member
Every engine, prop, fuel system will be different. The Emags are supposed to "spark" down to 700 RPM per the manual... However, is the spark powerful enough to ignite the mixture in your cylinder is the question you have to answer.
per their manual
The Cut-Out Test (ground run) should be done after initial installation, power plant maintenance, and at annual inspection. It checks ignition condition, and the entire engine system, in challenging conditions. Internal alternator output will drop at low engine speeds. Forcing the ignition to its low-speed low-power limit demonstrates the system’s capacity to operate below your in-flight idle – typically 1000 to 1100 rpm. Cut-Out Test a) Operating on one ignition, lower engine speed to 1200 rpm. Then turn bus power OFF to that ignition. A slight rpm dip may be expected due to the lowered spark energy. b) Very slowly lower the engine rpm until the engine reaches low idle limit or quits. A log-book entry will help you track Cut-Out trends over time.
per their manual
The Cut-Out Test (ground run) should be done after initial installation, power plant maintenance, and at annual inspection. It checks ignition condition, and the entire engine system, in challenging conditions. Internal alternator output will drop at low engine speeds. Forcing the ignition to its low-speed low-power limit demonstrates the system’s capacity to operate below your in-flight idle – typically 1000 to 1100 rpm. Cut-Out Test a) Operating on one ignition, lower engine speed to 1200 rpm. Then turn bus power OFF to that ignition. A slight rpm dip may be expected due to the lowered spark energy. b) Very slowly lower the engine rpm until the engine reaches low idle limit or quits. A log-book entry will help you track Cut-Out trends over time.
