Determining Pitch For A Fixed Pitch Prop

Stingray Don

Stingray Don

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Stingray Don
How does one determine the pitch of a fixed pitch prop? I have a 172N with a 180hp conversion, and the prop was replaced with the conversion. I suspect it may be more of a climb prop. I run 2400 RPM on the take off roll, which seems a little high. Also I can hit the 2700 redline in cruise and still have quite a bit of throttle left. I’m wondering if I could benefit from re-pitching the prop. I am aware of the trade off with climb performance.
 
First thing I’d look at is the engine conversion paperwork to see what they call out for a prop and see if yours matches. I’d also check to see if the paperwork calls out any static runup RPM range and see if your plane falls within it.

Without knowing anything more than what you describe it does sound a bit under pitched.
 
Check the calibration of your tachometer. I had exactly the same issue when I got my 180hp 172. Turned out the tachometer read about 5% high. Even with that, I could over-rev a little if not careful at times.
 
Stingray Don said:
How does one determine the pitch of a fixed pitch prop?
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It’s normally stamped near the hub, but you’ll probably need to take the spinner off to see it. Something like 72-64, which would be a 72-inch-long prop with a 64-inch pitch.
 
Stingray Don said:
How does one determine the pitch of a fixed pitch prop? I have a 172N with a 180hp conversion, and the prop was replaced with the conversion. I suspect it may be more of a climb prop. I run 2400 RPM on the take off roll, which seems a little high. Also I can hit the 2700 redline in cruise and still have quite a bit of throttle left. I’m wondering if I could benefit from re-pitching the prop. I am aware of the trade off with climb performance.
Click to expand...

Student here, just trying to understand. How does it work if you do have a climbing prop? My first (probably faulty) thought was that it wouldn't need as much rpm to do it's job on climbout, but obviously I'm missing something since you mention the RPM being high at 2400.

Jsut generally what is the relation to RPM and prop design for cruise or climb? I thought if it were pitched for climb it would be most effective and not need as much rpm to do the job.
Also when you mention in cruise 2700 red line but still able to increase throttle, I would have thought that meant it was a cruise propeller?

Can anyone explain to me?
 
LongRoadBob said:
Student here, just trying to understand. How does it work if you do have a climbing prop? My first (probably faulty) thought was that it wouldn't need as much rpm to do it's job on climbout, but obviously I'm missing something since you mention the RPM being high at 2400.

Jsut generally what is the relation to RPM and prop design for cruise or climb? I thought if it were pitched for climb it would be most effective and not need as much rpm to do the job.
Also when you mention in cruise 2700 red line but still able to increase throttle, I would have thought that meant it was a cruise propeller?

Can anyone explain to me?
Click to expand...
Think of it like car gears. To climb a hill in a car you downshift, increase the rpm, but decrease the gearing (pitch). For cruise, you upshift, decrease rpm and increase the gearing (pitch).

So, a climb prop will have less pitch and the engine will spin faster, but you won’t go as fast. A cruise prop will not have the higher rpm, climb slower, but will pull the plane faster once it’s up to speed in level flight.

A fixed pitch prop is like having a car with no transmission. You have to pick the “gear” you want to be in all the time.
 
LongRoadBob said:
Student here, just trying to understand. How does it work if you do have a climbing prop? My first (probably faulty) thought was that it wouldn't need as much rpm to do it's job on climbout, but obviously I'm missing something since you mention the RPM being high at 2400.

Jsut generally what is the relation to RPM and prop design for cruise or climb? I thought if it were pitched for climb it would be most effective and not need as much rpm to do the job.
Also when you mention in cruise 2700 red line but still able to increase throttle, I would have thought that meant it was a cruise propeller?

Can anyone explain to me?
Click to expand...
With a fixed pitch prop, the pitch is a compromise between climb performance and cruise performance. A climb prop will have a finer pitch, and a cruise prop will have a courser pitch. The climb prop (due to the finer pitch) will develop more RPM at full throttle, and climb faster as a result. However, once leveled off at cruise altitude, the throttle must be pulled back to avoid exceeding the red line. A cruise prop, with it's courser pitch, will take a bigger bite out of the air and allow more power to be applied to achieve a given RPM resulting in faster cruise speed. Thus it is a trade off between climb rate and cruise speed.
 
Here ya go:

% find the average pitch of a propeller from angles and radial location
% assumes angle of back of blade is the chord angle

% enter data here:
% Angle of hub. degrees (negative means that top of surface is aft of
% bottom)
ah = -8.75;
% Location of angle measurements on the prop (inches from center)
dim = [12 18 24 30 36];

% angle of blade back at each measurement (degrees) blade 1 and 2 - all
% measurements with blade to port side of the aircraft (leading edge up and level)
ab1m = [20.5 16 10 5 3];
ab2m = [20 16 10 5.5 3];

% end data entry

% correct angles for angle of hub
ab1a = ab1m - ah;
ab2a = ab2m - ah;

% circumference at each measurement
cir = 2*pi*dim;

% calculate pitch for each point.
p1 = cir.*tand(ab1a);
p2 = cir.*tand(ab2a);

% average pitch
Pitch = mean([p1 p2])

figure(1)
plot(dim,p1,'r',dim,p2,'k')
grid on
title('pitch distribution')
xlabel('Station, inches')
ylabel('pitch')
 
TCDS specs say static RPM minimum 2280, maximum 2400. With an 80” fixed pitch I expect 40-50 rpm static reduction per 1” of twist change. No idea how that converts to your shorter prop but your prop shop will know. Personally I try to target to get redline in level flight at wide open throttle.
 
Stingray Don said:
Thanks for the feedback. I may check into having it re-pitched at the next annual.
Click to expand...

You do realize that props are not "repitched," right? The prop is replaced with one of your desired pitch.

After reading this thread, I'm at a loss to understand why people think my constant speed prop is complex . . . .
 
The amount of repitching is limited, as is how many times in the prop’s life that it can be done. Again, talk to your prop shop.
 
LongRoadBob said:
Student here, just trying to understand. How does it work if you do have a climbing prop? My first (probably faulty) thought was that it wouldn't need as much rpm to do it's job on climbout, but obviously I'm missing something since you mention the RPM being high at 2400.

Jsut generally what is the relation to RPM and prop design for cruise or climb? I thought if it were pitched for climb it would be most effective and not need as much rpm to do the job.
Also when you mention in cruise 2700 red line but still able to increase throttle, I would have thought that meant it was a cruise propeller?

Can anyone explain to me?
Click to expand...
Horsepower is a result of RPM times torque. More RPM means more HP. A prop with a lower pitch allows the engine to spin faster and produce more HP. More HP means better climb. Constant-speed props do this and thereby maximize performance.
 
Hank S said:
You do realize that props are not "repitched," right? The prop is replaced with one of your desired pitch.
Click to expand...

As has already been mentioned they certainly can be. A friend has a repitched prop on one of his PA-18s and the hub was stamped with the new pitch.

Unfortunately, someone went too far with that prop. They took the standard landplane prop and added pitch to it in an attempt to increase cruise speed. It does absolutely nothing better than the other identically configured PA-18 he has with the standard prop on it. In fact it does worse; it burns more gas, has a longer takeoff distance, and cruises at the same speed as the other one. I can see taking pitch out of a prop to improve takeoff and climb performance but I'm not sure I'd ever add any pitch to try and improve cruise speed.
 
Dan Thomas said:
Horsepower is a result of RPM times torque. More RPM means more HP. A prop with a lower pitch allows the engine to spin faster and produce more HP. More HP means better climb. Constant-speed props do this and thereby maximize performance.
Click to expand...

This is a concept that I think eludes many pilots and instructors.

To go further with this, it is important to understand that with a constant speed prop the same horsepower setting may be able to be produced at several different manifold pressure and rpm settings. How can this happen? You're moving the same amount of air through the engine either wayAirplanes with fixed pitch props do not have this option.

I always feel that the old 10 speed bicycle or manual transmission car description of a constant speed prop is inadequate for properly describing its real value and function.
 
Our club towplane had a prop that had been repitched to make it a climb prop. Somewhere, over the decades, that was either never logged or logged incorrectly (I can't remember the details). I think we did the best we could with a tach to figure out the actual pitch, it just never seemed "right". I think we might have even had a prop shop measure it for us. The prop shop said it had reached its limits and couldn't be repitched any more, so we bought a new prop. This really was a climb prop. I flew it back from its first annual, right after the new prop was installed. That thing climbed like crazy and I was up around 7k agl in that 150/150 in a hurry. But getting back home at cruise was like driving in first gear the whole way.
 
If you have an AirPlains conversion the static RPM limits are 2250 - 2450 so if your tach is accurate it sounds like you're within spec. As others have mentioned you may want to check your tach. In my case I found my tach consistently reads 100 RPM less than actual. If you look at the AirPlanes performance data the highest cruise speed published is 122 KTS. If you're close to this then there is probably little to be gained by re-pitching your prop...
 
Globe Swift’s used to come from the factory with a ground-adjustable wooden prop. You could have a climber or cruiser by changing the pitch of the prop blades on the ground with a prop protractor (with the engine off, of course!). Not a real popular idea but it was simpler, weighed less and was cheaper than a constant speed prop. You might still see one if you attend a Vintage/Antique fly-in.
 
Stingray Don said:
How does one determine the pitch of a fixed pitch prop? I have a 172N with a 180hp conversion, and the prop was replaced with the conversion. I suspect it may be more of a climb prop. I run 2400 RPM on the take off roll, which seems a little high. Also I can hit the 2700 redline in cruise and still have quite a bit of throttle left. I’m wondering if I could benefit from re-pitching the prop. I am aware of the trade off with climb performance.
Click to expand...

The propeller specs should be in your maintenance logs. The STC for the conversion should specify the appropriate propeller and allowable pitch range. You would not be able to exceed the allowable pitch range and be in compliance with the STC.

The whole idea of a higher power engine is to provide more excess power on takeoff (or climb) which will result in more rpm under load and thus more robust climb rate. The high compression STC in my Traveler (aka "160 hp" STC) results in about 2400-2500 rpm at Vy rather than 2200-2300 rpm with the stock engine, even with a repitch from 59" to 61" with the STC Sensenich square tip propeller. Keep in mind that a couple inches of repitch will result in only about 4 kt in theoretical extra speed at full throttle. IMO, the extra climb performance is much more valuable than a 4 kt cruise increase at WOT. If you legally repitch for more cruise speed, you will lose some climb performance.

The "power upgrade" STCs are quite valuable in the small 4-seat light single category, transforming underpowered aircraft into respectable climbers with more efficient cruise, and ability to maintain high cruise power at altitude. I don't run out of throttle to achieve redline in my Traveler until well above 10,000 feet. I rarely beat up the engine that way in cruise, but rather throttle back to about 2550 rpm for a good 117 kt at 8 gph. The last engine went a long time treated that way. If the headwinds are fierce, and I have enough fuel, I can always call upon an additional 100 rpm to go a little faster and make me feel better when cars are passing me below.
 
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