As I’ve related elsewhere, I’ve been experimenting with a new prop on my Hatz. I don’t want to dilute that discussion, which is more about data collection, while here I want to talk more generally about rpm, pitch, and performance. First, the engine, which is a Lycoming O-290-D which has a “rated power” of 125HP at 2600 rpm. The manual also lists a “takeoff power” of 130HP at 2800 rpm, but nowhere in the manual does it say what “takeoff power” means. I’ve heard of “5 minute takeoff power” ratings for other engines, but there’s no mention of anything like that in the Lycoming manual. (Edit, it's in the TCDS Dan posted, strange it's not in the manual). So, the prop: The original prop on the plane is a 74x50 Sensenich metal prop. Static rpm is 2480, around 2500 in a 60kt climb, I cruise 70kts at 2250, and it maxes out at 2600 rpm WOT at 80kts in level flight. The new prop is a 74x47 Sterba wood prop. Static rpm is 2440, 2425 at 60kt, and it maxes out at 2565 at 85kt in level flight (the full throttle rpm averages 75 lower over most speeds). Even though the new prop has less pitch, rpm is down and speed is up… but takeoff and climb performance are worse. My goal with the new prop was to get a little close to the 2600 rated rpm for more power during climb, hopefully without losing too much airspeed in cruise. That the reduced pitch didn’t achieve this points to the other differences between the props. The wood prop has a different blade shape (it’s wider than the metal prop near mid blade but narrower closer to the hub) and a thicker airfoil section. I will be sending the prop back to Sterba for repitching, but how much? According to Ed an inch of pitch is good for about 50 rpm. I’ve seen references to pitching the prop so it’s at redline rpm at full throttle in level flight, but what is “redline”? The manual makes no mention of any limiting rpm. The old prop maxes out at the 2600 “rated rpm”, but of course I won’t be cruising at full throttle. I’m thinking reducing the pitch by 3” to pick up, say, 150 rpm (so it’d make 2575 during climb) would be about right. It would bring the pitch down to 44” which is what two other prop makers recommended (Sterba recommended 50 but I thought 47 would be a better place to start). That would (presumably) lower the cruise speed at the same rpm, or raise the rpm needed to get the same speed, which is OK if it’s not too drastic. It also means the rpm would go above 2600 (some might say “overspeed”) at full throttle in level flight, but the answer to that is “don’t do that” (or maybe it’s OK for 5 minutes?). I’m not looking to get up to the “takeoff power” rpm of 2800, I suspect that a fixed pitch prop that allowed that would perform very poorly in cruise.