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Confused about prop pitch

cgoldy

Registered User
Moogerah Queensland Australia
I think I need to re- pitch my prop

Wide body cub
31" bush wheels
o-375 engine
Macaulay 84 / 43 prop
Heavy cub

RPM. MP Knots. MPH
2300 19 82. 94
2400. 21 85. 98
2500. 23 90. 103
2550. 25 95. 109

Can't spin the prop faster than 2550 rpm. The last 25% of my throttle throw there is no response except for slight increase in MP. I fly in the mountains and want to make sure I can get out of the places I get into. I was thinking about re- pitching to 42" . Am I on the right track???????



cgoldy
 
8.5:1

Running on mogas but get similar results with avgas
 
Perhaps your current 43 pitch is not true. You might have a prop shop check the pitch out. It does sound like you might be over-pitched a little.

Bill
 
What Bill said, have it checked, maybe two different shops. I had a prop COME OUT of a prop shop. Supposed to be pitched 42 and found out later they pitched it 47.
 
What Bill said, have it checked, maybe two different shops. I had a prop COME OUT of a prop shop. Supposed to be pitched 42 and found out later they pitched it 47.
Hey, I paid good $ for that gag and it lasted 3 years!
 
Are you using a mechanical or digital tach? Are you sure you tach is accurate. I had a false reading on my C-85 J-3. Used a Tru-Tach meter to determine the problem. My tach was off by 200 RPM in the upper ranges.
 
As a general rule of thumb with this propellor family, you will pick up about 75 rpm for each inch of pitch reduction. If you are currently getting 2550 rpm at wide open/run-out, I'd be dropping 2 inches of pitch. The part about the last 25% of the throttle travel is normal. As with almost all gasoline engines, they're somewhat over carbureted. One question, are you leaning it properly? In addition to the effects of altitude, the mixture will richen even more as you approach full throttle due to the basic design of the carburetor. They're set up to over fuel somewhat at full throttle to keep the valves/heads cool under heavy loads, but at anything over a couple thousand feet you don't need or want this extra rich mixture. It just costs you power and money.
 
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I'm not that confident with the tach. It is electric. I will check it before i re-pitch it. It was the fact that I had all that extra throw of the throttle that makes me think it is far too course.

Funny you should say that about checking the prop pitch. The 44" stamp on this new prop had been stamped out and re stamped with 43" I thought that that was a bit weird for a new prop!

I leaned aggressively for the test but did not re-lean for each power setting
 
After flying around to warm it up. Tie the tail down and run it wide open static. If you can pull 2450 rpm static, it's pitched about right.

I'm running the same prop on an O-360. It'll pull 2450 static, fly 105 mph @ 2450 rpm, 95 mph @ 2250 rpm.

This was taken when my engine was new (375 hours).

Crash

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A couple of years ago I had a local prop shop bend my 8242 to 8240 in advance of my buying a new prop. I wanted to see for myself what advantages I might get from a flatter pitch. The prop shop told me to expect around 40 RPM gain per inch of bend at full throttle. I got 80 RPM so they were spot on. The more interesting thing I noticed was zero impact on static run-up RPM (optical tach). And no apparent advantage in take-off or climb performance, although admittedly I was not prepared to recognize minor changes, just seat of the pants impressions. My take on fixed pitch props is that I prefer a prop that nets me redline or very near redline engine speeds in full throttle level cruise. That provides the best all-around performance for me.

Are you seeing redline with a full power shallow descent?
 
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"Are you seeing redline with a full power shallow descent?"

I haven't tried. What would that show?
 
Hey, I paid good $ for that gag and it lasted 3 years!

They didn't know him or it would have been free!

Not all shops are the same, my guess is SB took his to George.

New does not mean correct. And the big props can actually increase in performance by cutting them down a bit; they are very heavy to swing.
 
I have two 1-200 props one is a 84-44 and it will turn 2700 rpm as soon as I hit level flight it goes fast for a cub airplane does not slow down well but follows the throttle and pulls well max prop for o-360 like Sierra Bravo says for o-360. my other prop is a 1-200-90-33 and it will turn 2700 rpm on takeoff if you don't climb to steep it follows the throttle and the airplane also follows the thottle. It only goes 85 mph at 2550, it's my mountain flying prop. My point being is the motor is not putting out because you should see 2700 as soon as you hit level flight or down hill. The engine should be follow the throttle the whole way.What is your prop?235? or 1-200? Ron.
 
"What is your prop?235? or 1-200? "

I am not sure what these figures mean? I will go and look at the paperwork.

So I think you have described my delema. I am just assuming that my prop is over pitched. Maybe the motor just doesn't put out enough HP to spin it properly. It was the recommended for that engine.

maybe I should hunt up some one with the same configuration before I dive in and re-pitch!
 
Ok. Just looked up the prop log and it is not as I first described. It is a 43 that was re pitched to 44 before it was delivered!

So it is now IP235/AFA8444. So what does the 235 signify?
 
I was not trying to complicate things but your prop should be the right pitch for a cruise prop . Like Sierra Bravo said it should hit redline as soon as you hit level flight. I don't get that the thottle does not follow? That engine should pull that prop fine Is there any one you could borrow a prop from to try? Ron
 
Are you using a mechanical or digital tach? Are you sure you tach is accurate. I had a false reading on my C-85 J-3. Used a Tru-Tach meter to determine the problem. My tach was off by 200 RPM in the upper ranges.

I am embarrassed to say that shooter wins the prize. I borrowed a tru tack and my tach under reads by 150 RPM at the higher range. Lower range it over reads. So I don't have a problem, I can pull 2700 RPM and I wasted everyone's time. Thank you all for your advice and I apologise for being such a dick and not checking this first.
 
I am embarrassed to say that shooter wins the prize. I borrowed a tru tack and my tach under reads by 150 RPM at the higher range. Lower range it over reads. So I don't have a problem, I can pull 2700 RPM and I wasted everyone's time. Thank you all for your advice and I apologise for being such a dick and not checking this first.

I don't think it was a waste of time as the next guy that has a similar problem may read this and get some good suggestions of things to check.
 
told you so aussie plonker

Maybe I might believe you next time Ron. But we convicts have and ingrained mistrust of you guys over there. You steal our women and send us your disease ridden apples. What do you expect?

Goldy
 
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