Critical Understanding #2, Absolute Minimum Static RPM.

Builders:

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.

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(Make Vx line 2.1 in your Hand book)

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(Make Vy line 2.2 in your Hand book)

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-ww.

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About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at www.FlyCorvair.com and in more than 50 magazine articles.

10 Responses to Critical Understanding #2, Absolute Minimum Static RPM.

  1. Mike, thanks for calling me paranoid. Instead of looking for some dark secret, how about going with the most basic of explanations, the one I wrote, simply saying that too many people are doing dumb things out of ignorance and poor decision making, and I would like to reduce that. Why you find that alarming I don’t understand. I find people having accidents that were entirely preventable alarming.

  2. Dan Branstrom says:

    As I remember on my course that covered Aerodynamics for Naval Aviators, there is a theoretical, but quite accurate, measure of the pressure that a cylinder experiences during operation for a given power output.

    For example, at a given rpm, in order to develop a certain horsepower, each stroke of the piston develops a certain pressure in the cylinder. If you halve the rpm, in order for the engine to develop the same horsepower, the pressure in the cylinder has to double.

    To develop the same horsepower at a slower rpm, the pressure has to rise. If the octane rating of the fuel isn’t adequate, the mixture will detonate. Also, because the engine is working slower, there are fewer cooling cycles that the cylinder experiences, so it runs hotter. A hot engine also makes detonation more likely.

    If you’ve ever driven a car with low octane gas and opened the throttle wide open at a low rpm, you can often hear the engine knocking because car engines are muffled.

    • Dan Branstrom says:

      To add to what I just wrote, it’s a good idea, in my opinion, to use 100 octane low lead when making your first flight. It gives additional protection against detonation in case the ignition advance is slightly off or the mixture isn’t right.

      It is not a guarantee that you won’t have detonation if mixture and timing are off, but it puts the odds more in your favor.

      • Dan Branstrom says:

        If your wide open throttle static rpm is less than 2,700, either your engine isn’t developing enough power or your prop is pitched too high. It’s better to have the rpm about 2,800.

        If your static rpm is much higher, the prop isn’t pitched high enough.

      • Dan, I concur. Over the years I have always said using 100LL makes a lot of sense in the 40 hour test period

    • Dan, the measure of work load is brake mean effective pressure or BMEP. It is standard measure of load on an aircraft engine

  3. jaksno says:

    I am not interested in opinions re flight corvair engines other than your own….possibly a second would Mr. Weseman’s. The end.

  4. Joseph H Horton says:

    William, Just catching up a bit on your reading materials. I’m Sorry that Jaksno missed the line about open mind. The Static RPM of a flight ready corvair is a number that I either did not know or remember over the past 15 years that I have been involved. My first prop/engine combination resulted in a 2850 rpm static. I only flew with it a short time like that and tried triimming the prop. That resulted in 2900 static rpm. I was much more comfortable with that in climb. The prop was replaced not long after that with the Senich 54/58 which gives the combo a 2950 static. and winds up to 3350 rpm straight and level. There are so many other variables to consider and I only offer this as data. I perfer to be on the higher side of static.

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