Prop Re-pitch

idahoflier

idahoflier

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idahoflier
Hi,

Does anyone have any experience re-pitching a fixed pitch prop to more of a climb prop?

I have a C172N with an Air Plains O360-A4M conversion. Right now my prop is the stock prop for the conversion pitched at 60". Red line is 2700 rpm and I'm told if I re-pitch the prop to 58" I can expect another 100 rpm. Under most cruise conditions I can get 2600 rpm at full throttle so 58" is probably as far back as I would pitch the prop.

My question is how much will this additional 100 rpm affect my climb performance?

Thanks!
 
When I did it my prop shop told me to figure 30-40rpm change per inch of twist change. They were spot on as my level full power rpm changed by 80rpm after bending it 2" flatter. Coincidentally I lost 6-8mph total. Climb performance didn't improve that I could notice.

You'd benefit more by getting a seaplane prop installed.
 
Stewartb said:
When I did it my prop shop told me to figure 30-40rpm change per inch of twist change. They were spot on as my level full power rpm changed by 80rpm after bending it 2" flatter. Coincidentally I lost 6-8mph total. Climb performance didn't improve that I could notice.

You'd benefit more by getting a seaplane prop installed.
Click to expand...

That's great feedback, thanks! I was unfamiliar with a seaplane prop, thanks for the idea. It would probably be a bit more $ than I was looking to invest and a lot of regulatory hurdles to get it hung on the airframe...
 
Before you bend a prop be aware that a 172 has specific full power static rpm limits.
 
Stewartb said:
Before you bend a prop be aware that a 172 has specific full power static rpm limits.
Click to expand...

Thanks for the heads up, I'm aware of the static RPM limits. The Air Plains STC has static limits of 2250 - 2450 RPM. Right now I'm at 2275 @2,500 MSL +/- depending on DA so 2" should put me at 2330 - 2350. But based on your feedback it doesn't sound like it's worth it...
 
Don't forget if you repitch your POH/AFM will have to be updated to reflect the new numbers as well as instrument gages markings. Your IA should catch this on the annual inspection when they do the initial max power run up.
 
I personally am partial to a climb prop. Have flown identical aircraft with the only difference being the fixed pitch prop, Cessna 140. Climb rates varied from 600 fpm to 1,000 fpm. If you do the math 2” of pitch change at 2,500 rpm equates to a different airspeed of 4.7 mph. It will be a bit less of a change because the efficiencies at both pitch settings are not 100%. My guess is no more than 4 mph if that much, anymore change in the airspeed and something else is entering into the equation. Given a choice I will take the safety of higher climb rates and less load on the engine. On a 100 mile cross country the difference between 120 and 124 mph will be less than 1 1/2 minutes. What do they want to charge you for the pitch change?
 
My personal experience, do not do it.
I did a 172b
 
Stache said:
Don't forget if you repitch your POH/AFM will have to be updated to reflect the new numbers as well as instrument gages markings. Your IA should catch this on the annual inspection when they do the initial max power run up.
Click to expand...

I don't think any changes would be necessary or documented, other than the appropriate log entries. The Supplemental AFM lists the propeller pitch range as 56" - 62" with the static RPM between 2250 - 2450. I am considering a re-pitch from 60" to 58" which would put my static RPM somewhere around 2360. Thanks!
 
idahoflier said:
I don't think any changes would be necessary or documented, other than the appropriate log entries. The Supplemental AFM lists the propeller pitch range as 56" - 62" with the static RPM between 2250 - 2450. I am considering a re-pitch from 60" to 58" which would put my static RPM somewhere around 2360. Thanks!
Click to expand...
Just curious - if you are in level cruise flight and firewall your throttle how high does your rpm go for a brief short period? And what is red line on your tach?
 
Briar Rabbit said:
I personally am partial to a climb prop. Have flown identical aircraft with the only difference being the fixed pitch prop, Cessna 140. Climb rates varied from 600 fpm to 1,000 fpm. If you do the math 2” of pitch change at 2,500 rpm equates to a different airspeed of 4.7 mph. It will be a bit less of a change because the efficiencies at both pitch settings are not 100%. My guess is no more than 4 mph if that much, anymore change in the airspeed and something else is entering into the equation. Given a choice I will take the safety of higher climb rates and less load on the engine. On a 100 mile cross country the difference between 120 and 124 mph will be less than 1 1/2 minutes. What do they want to charge you for the pitch change?
Click to expand...

Agree, I'm not concerned with the loss in cruise performance, I was hoping to get better climb performance. The propeller shop I spoke with indicated the re-pitch would be about $200 which seemed reasonable.
 
Briar Rabbit said:
Just curious - if you are in level cruise flight and firewall your throttle how high does your rpm go for a brief short period? And what is red line on your tach?
Click to expand...

Depends on conditions, but most of the time I can get 2,600 in cruise, but that is usually above 5,000 MSL. Redline is 2,700. The engine is rated for 2,700 continuous but I prefer to stay around 2,500 in cruise.
 
idahoflier said:
Depends on conditions, but most of the time I can get 2,600 in cruise, but that is usually above 5,000 MSL. Redline is 2,700. The engine is rated for 2,700 continuous but I prefer to stay around 2,500 in cruise.
Click to expand...
I think I would do it also if that is the cost, might even go for 3 inches so you can reach red line in cruise. Not that I would ever fly at that rpm but it would give you more climb performance and less wear on the engine due to less load. You will likely never reach redline then during a climb at full throttle. Probably not a big difference on how long the engine will last but I think you will notice the climb performance change. And based on the Idaho handle guess you have a bit of altitude at some of the airports in your area?

Let us know your results if you proceed.
 
I had a Pacer with a “cruise” prop, was a real dog. Had the prop repitched to climb and it made a world of difference, Plus I could get max RPM
In level flight. Climbed better, cruised
Faster, all with a overhaul and repitch, about $1000 total.
Dave
 
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