I have a question regarding prop pitch and RPM vs power output. The factory book for my AA5 shows different RPM/power settings using the factory stock prop (a 72 X 59 correct?).
>>If you had a cruise prop it is. (57 inches was the climb prop)
At sometime in the past, the prop on my plane was re-pitched and is now a 72 X 57. How does this effect the numbers in the table. My assumption is that the power produced with my re-pitched prop at a given RPM is something less than the power shown in the book.
Power at rpm is not changed. The more rpms the more power regardless of prop. Repitching the prop just makes it harder for the engine to reach those rpm (if pitched coarser - larger pitch). That is why your engine is rated output of 150 hp at 2700 rom. Many of the serious racers use low pitch props on their engines to spool up to 3100 or so rpm and develop more power. Bob Arnold and Harvey Wilcox both have the fastest 180 hp planes around. (Cheetahs with the 0-360 engine) and 61 pitch props. Most tigers have 63-65 pitch props.
The reason I bring this up is that I just checked my tach accuracy using one of those precision optical tachs I borrowed from a friend. I found out that my mechanical tach is reading 200 RPM low at 2150! That means that when I've been cruising around at 2400 RPM at 4500 feet thinking a was below the 75% power point and could lean per the book, I've actually been running at 2600 RPM. The book shows 2600 @ 4500 feet as 81% power! My fuel burns have never matched the book an now I think I know why!
I found the same thing years ago with my AA5. Tach was low. I now fly for given fuel burn (fuel flow computer).
Is there a rule-of-thumb regarding power settings after a re-pitch, or do I need to have a manifold gauge added temporarily so I can derive a new RPM/power table?
I have a Cheetah with a climb prop PLUS (pitched a little more than normal). Most of my hours are in Tigers (300+) and I can tell you that with the climb prop on the Cheetah it takes off and climbs like a Tiger. Of course my cruise speed suppers until I get above 8500 ft. Up that high the extra RPM I can get makes up for the prop. When I first got the Cheetah my thought was to re-pitch the prop back to cruise but after flying it awhile I do not want to do that. The few mph I lose down low is more than made up by the climb performance.
I do not have the figures in front of me so will respond based on memory. The Tiger prop is pitched at 61 or 63. The Cheetah cruise prop is pitched at 59 and the Grumman approved climb prop is pitched at 57. I have flown a Cheetah with a prop pitch of 53!! It out climbs a Tiger. Cruise suffers of course unless you get up high. No STC is required. The authorized prop shop can re-pitch the cruise prop to your specifications and a notation entered in the engine logbook. I believe you are allowed to re-pitch the prop two times.
I had my Traveler repitched to 56 inches around 5 years ago. Sensenich [sic] repitched and refurbished the prop for around $250. I now get an additional 100RPM at full throttle with the breaks held and between 75 to 100 ft per min increase on the climb. I have not noticed any differences at cruse speed either + or -.
What pitch did you end up with? If I'm reading the propeller part number correctly (1C172/BTM7359), it's a 73 inch diameter with a 59 inch pitch.
I have a Cheetah with a climb prop PLUS (pitched a little more than normal). Most of my hours are in Tigers (300+) and I can tell you that with the climb prop on the Cheetah it takes off and climbs like a Tiger. Of course my cruise speed suppers until I get above 8500 ft. Up that high the extra RPM I can get makes up for the prop. When I first got the Cheetah my thought was to re-pitch the prop back to cruise but after flying it awhile I do not want to do that. The few mph I lose down low is more than made up by the climb performance.