Full static RPM is the actual rpm the engine turns when the plane is held still by the brakes. The minimum static rpm is important to having the engine make proper power on take off, but it is critical to preventing the engine from detonating on take off and climb.
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On a Corvair flight engine, the absolute minimum static rpm is 2,700. If the engine is built and running correctly, the sole factor determining the static rpm is the propeller selection and setting. If the number is lower than this, the engine will not make expected power on take off and climb. This will extend the take off roll and reduce the rate of climb, but the really critical issue is the same as other direct drive aircraft motors: A moderate but significant reduction in the static rpm at full load leaves the engine vulnerable to detonating under a full load.
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Many people ask “How much difference can 100 rpm make?” In reality, it will make a lot. An engine being held back to 2,550 rpm and run on 93 octane fuel with the timing at 30 degrees has almost no margin against detonation on a 90F day. The exact same engine with a lower pitch prop with the static rpm at 2,700 will have vastly increased margin of safety. I have said this many times, but still builders persist in believing the myth that lower rpm is somehow ‘easier’ on the engine, when just the reverse is true.
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To demonstrate that this doesn’t just apply to Corvairs, below is an excerpt from the FAA Type Certificate Data Sheet for a Piper PA-22 “Tri-pacer.” Notice the phrase “Not under“, this is the manufacturer specifying an Absolute Minimum Static RPM for a combination of engine and prop on their plane. The number is different, but the concept is identical. On a certified plane this is the law, in the case of Corvairs, I can’t force anyone to use my number, I can only point out that an engine failure on take off is the eventual result of too low a static rpm.
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“2. Propeller (with Lycoming O-290D or O-290-D2 engine) – fixed pitch metal (a) Sensenich M76AM-2 or +25 lb. (-50) (b) Sensenich M74DM +30 lb. (-50) Airplane Flight Manual shall be revised to reflect the subject propeller and limits. Landplane: Static r.p.m.: Not over 2450, not under 2150 Diameter: Not over 74 inches, not under 72.5 inches Seaplane: Static r.p.m.: Not over 2450, not under 2350 Diameter: Not over 74 inches, not under 72.5 inches”
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The primary group violating this Critical Understanding of Corvairs are builders who have made their own props and builders using a ground adjustable props with too much pitch in the setting. There will be pushback on my number on discussion groups, where people will say “So and So has done flown with a static rpm of 2,450 for years” Great, that is a testimony to the toughness of the Corvair, but his engine is likely protected by a retarded timing setting or a very rich carb setting. Notice how people repeating a BS endorsement of low rpm don’t even understand that the ignition timing, the A/F ratio and even the camshaft profile and timing marks play critical roles in the minimum acceptable static rpm. Consider that Grace flew B. H. Pietenpols’s personal Aircamper, and it’s static rpm was near 2,800; consider that no person on earth has spent more time running Corvair flight engines on dynos and developing their engine installations than myself, and Ernest Jones was my mentor in aircraft propulsion at Embry Riddle. Yet some people think that having a “a big, good looking” wood prop on their plane or taking the advice of a local ‘expert’ outweighs my recommendation.
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It is a free world, they can make that choice, but when their engine breaks, I have a right to expect them to ‘own it’ and tell everyone that they chose not to listen to my experience. Ironically a lot of these people develop amnesia after an engine failure or a crash, and they have absolutely no recollection they ever failed to follow any recommendation of mine, or at least that’s what they tell the people in their EAA chapter, the internet, the FAA and their lawyer.
