Tolerance on prop RPM

Agreed, it’s Dimensional Analysis

A force has units of Mass times Distance divided by Time squared.

The Mass comes from air density, some power of the prop diameter cancels out all the various Distances and RPM squared takes care of Time.

and since MP will obviously (?) improve with more RPM (more suction), we should get an overall square relationship i.e. 1% extra rpm gives 2% extra power.

Once you’re at wide open throttle, manifold pressure won’t change any more with RPM, it’ll be pretty much as close as you get to 29.9in at sea level and will stay there as you increase prop RPM. Airflow (and fuel flow) will increase as RPM increases, but without a turbo once you’re at wide open throttle, you’re done as far as MP is concerned.

Power = torque times omega, or some factor times torque times rpm in any chosen unit. At WOT and increasing the rpm at 2500 ish rpm. You are passed the peek torque. The torque is decreasing with increasing rpm. Hence 1% increase in rpm will give less than 1% increase in power. With a turbo anything can happen depending on how the turbo is set up.

The torque curve (versus rpm) is pretty flat on a mildly tuned aircraft engine meaning at around peak power rpm power varies roughly linearly with rpm. You typically lose a little less with reducing rpm then you gain with increasing rpm.

Aircraft engines that are frequently above redline usually suffer in terms of longevity, for instance an A-75 Continental is an A-65 that’s run faster (different prop and tach redline) to make 10 HP more. A-75s last about half as long between overhauls. An 0-200 that’s run at 4200 rpm lasts about one race. But that doesn’t mean if you run 10 minutes at 100 rpm over redline you’ll suffer any ill consequence.

- meaning you have to look at the torque curve of your engine, and you will see exactly how much increase in power you will get.

“Hence 1% increase in rpm will give less than 1% increase in power. "

Yes, partly due to the fact that manifold pressure decreases (slightly) with increasing RPM. But 1% – 1% is a very useful approximation for the small changes we are talking about.

“and since MP will obviously (?) improve with more RPM (more suction), we should get an overall square relationship i.e. 1% extra rpm gives 2% extra power. Concur?”

No … MP does not change (much) – it just decreases very slightly as RPM is increased. You have noticed that hundreds of times when you go from cruise to climb – when you advance the prop lever from say 2200 til max RPM there is a corresponding MP drop of about ½ inch or may be a little more..

“Actually MP probably won’t increase with extra RPM, because more suction will mean less MP. But MP is really an inverse proxy for how much air is available to the engine, and increasing the suction by 1% must increase the air by 1%.”

Hmm … remember that MP is not suction, it is pressure – absolute pressure. With stopped engine it shows atmospheric pressure (29-30") and just acts a rude barometer. When you start the engine, the engine acts as an air pump, and because of the throttle’s restriction the pressure in the intake manifold drops to below ambient pressure, say 10" at idle power. You could call that 20" of suction, but no instrument is calibrated that way. Wide the throttle wide open, the intake manifold pressure is quite close to ambient pressure, usually a Little lower, although ram effect could bring it up to or slightly above ambient pressure on some aircraft (I know of some older Mooneys).

With a given throttle position, if you increase RPM, it works just like increasing RPM on your vacuum cleaner, it “sucks” more, i.e. MP drops (a little).

It’s just that Lyco don’t appear to publish one.

Are you sure the limit is the engine and not the propeller? At least Hartzell publishes extensive information what you should do after overspeeding how much for how long for my propeller.

If that redline is a propeller limit, why should there be a tolerance? VNE is VNE without a tolerance attached, too.