Question for "Tech- Pilots"

[Kobradelta1000]

Hi there!

I fly since many years Flightsims (msfs 5.1 and c64´s solo flight:-) , i think i have a bit of basic knowledge collected since then, to answer the ~most~ questions from friends and other intrerested people regarding of the tech behind (iam a bit of petrolhead too, planes or cars- where i could- and get practical experience- btw.- long live the OLD flat six ), and basic flying itself of aeroplanes- in an amateur way, of course.

Now, i have a question about plane engines- what the rpm gauge in the cockpit shows, are the actual revolutions for the prop- but what revolutions does the engine itself (a plane with a constant speed prop, of course; and ISA,msl, no wind, lvl flight). I searched for example the Packard Merlin and found no info for that.

For ex.: the pilot select 2500 prop rpms;
My idea is, the governor regulates the propeller as long the engine needs to come on 2500 crankshaft rpms- then the prop AND the engine would have the same rpms- right or wrong

Lets say the P-51(civ) does 2500RPM on the gauge- what rpm´s does the crankshaft of the engine?

Any information on that topic would be very interesting and a very welcome further education for me-
Thank you!

[ft]

The gear ratio between the engine and the propeller on the P-51D is 0.479:1. In other words, as the engine turns through 1000 revolutions, the prop only turns through 479. At 2500 RPM on the prop, the engine is at 5220 RPM.

Cheers,
/Fred

[Pistonpilot]

Anyone who has owned an American V8 engine will be familiar with the 6000-6400ish RPM red line. I've noticed that, in my limited experience, the math works out that when the prop hits the high RPM stop, big airplane engines are doing about 6000-7000 RPM.

I'm sure someone here can explain the physics behind it, but I just assume that 6000 RPM is where big engines like to work!

-Ian

[Kobradelta1000]

thank you very much for your answers, guys !!

[pilotgod]

Anyone who has owned an American V8 engine will be familiar with the 6000-6400ish RPM red line. I've noticed that, in my limited experience, the math works out that when the prop hits the high RPM stop, big airplane engines are doing about 6000-7000 RPM.

I'm sure someone here can explain the physics behind it, but I just assume that 6000 RPM is where big engines like to work!

-Ian

You are spot on. The engine needs to produce torque to keep those big props spinning, so the gear ratios are designed to allow the engine to work at the top of it's torque band for METO power. This means that for cruise, they are a bit below the peak, but if you look at the torque band of most of these engines, you will see it flattens out in the 4500-5000 range through to about 7200 RPM (varies for engines, of course).