whee wrote:
Maybe we should just take a vote. What say you guys: 86”, 85” 84” ? I’m getting the hack saw out tomorrow...

Easy to cut more off, hard to add it back on. I'd start at 86".

I currently operate as Tim suggests. Soon as I break ground flaps come off and I dial back the prop to 2600. Not too long after that I bring it back to 24 or 2500. If I’m leaving early in the morning I dial the prop back a few turns before shoving the throttle in.

I was leaning toward 86” but is 1” more clearance going to make any different in rock chips? Probly won’t see any speed increase either. But like ASA said, once we cut it we can’t put it back on.

I don’t have a need for big tires but have a set lined up but just to protect the prop. I think 4” extended gear may be a better long term solution. I’m about half way through specing them out but lost motivation cuz I wasn’t sure I’d actually build them. Also, I don’t need the 11” diameter 400x4 tailwheel tire I have on. I’m going to try a matco tire that is still 4” wide but only 8” tall.

Ya I dont think 1" will protect the prop much, but then again I dont think 2" will either. A lot of the prop damage I see, especially on 182s with long props, is further up the blade from rocks getting thrown into it. Where are the dings you see now?

hotrod180 wrote:

whee wrote:....The prop shop told me that a prop at the end of its service life, ie been overhauled enough times that it is barely inside service limits, has lost 40% of its performance compared to a new prop.....

I call bullshit on this one.
I removed an at-minimum length 82" C203 prop from my C180,
and installed a full-length 88" C201.
Some performance increase (not at much as I'd hoped for, of course),
but no way a 40% difference.

Sometimes "end of service life" and "being overhauled enough times that it is barely within service limits" can have a huge effect... I don't know about 40%, but here's a "single data point" for you:

A while back, I owned a Grumman Traveler that had a brand new prop. When I took it in for the annual inspection, there was an almost-identical Traveler that was also getting its annual done. The shop owner (a noted "Grumman Guru" in Texas) commented that he was really glad I got there that day, because the owner of the other plane was coming in to see my prop, side-by-side with his. He had been complaining that his airplane just wasn't performing up to book specs – climb performance was about 20-25% below book numbers, and cruise performance was about 10-15% off. He was just sure that his engine was "sick", but compression tests and borescope inspections both indicated nothing was amiss. The "guru" kept telling him that his worn-out prop was the cause of his performance issues, but the airplane owner kept insisting that a "prop shop" had inspected and overhauled his prop and told him it was airworthy, so that could NOT be the case.

But when you saw the props – side by side... Oh, my! They were – by the propeller specs – supposed to be identical. But his prop looked like a yardstick compared to mine – so skinny and so "flat". It had almost no airfoil shape to it at all. My prop was at least 1.5" wider in chord than his at the widest part, and my prop was also significantly more "humped" than his in the airfoil section. His prop really looked like a long skinny fish knife blade compared to mine. There had been so much material removed from that prop over the years to remove nicks and so forth, that it could NOT have been legally airworthy, in my view...

After some back-and-forth discussions between the three of us, I agreed to let the shop owner "borrow" my prop to put on the other guy's plane to answer the questions once and for all... Well, miraculously enough (NOT!), with my "good" prop on his plane, he made book climb and cruise speeds on the test flight. Upon their return from the test flight, he purchased a new prop, and was darn happy that it was the prop and not the engine!

(Oh yeah – since the "guru" was asking to remove and reinstall my prop, he sweetened the deal by offering to dynamically balance my prop when he was done. So I got a free prop balance out of the deal!)

I went flying on Saturday and a friend with a 180hp straight tail 172 followed along. Our cruise speeds matched almost exactly. That’s completely unacceptable so now I have to shorten the prop.

whee wrote: Here’s what I know. My out of the hole performance is great. However, when I roll the rpm back during takeoff I can feel a significant acceleration.

Don't let yourself get misled by this. That isn't an increase in prop efficiency you're feeling. That surge of acceleration you feel is the prop's kinetic energy being turned into thrust momentarily. A spinning prop has a lot of stored kinetic energy, just like a flywheel. The kinetic energy of a spinning mass is a function of the square of the rotational velocity (RPM) all else being equal a prop turning at 2800 RPM has about 16% more stored kinetic energy than the same prop spinning at 2600 RPM. So, when you dial the RPM back from 2800 to 2600, that stored kinetic energy goes someplace and it mostly gets turned into additional thrust through increased blade angle. Once that excess kinetic energy is used up, your momentary boost is gone. I don't know if you've ever flown floats in a plane with a CS prop, but you get this identical effect when you're doing your runup. When you cycle your prop from 1800 RPM, to 1400 rpm (for example) you feel that same momentary shove as the prop slows. It's also a lot more obvious that it's not a sustained increase in thrust.

Now, I'm not saying that your engine/prop combination doesn't produce more thrust at 2600 RPM than it does a 2800 RPM. Maybe it does, or maybe it doesn't, but dialing the prop RPM back in flight and feeling a little momentary boost doesn't really tell you much about it either way.

A1Skinner wrote:

whee wrote:
The two prop shops I've talked to agree with Halestorm on how a CS prop 'should' be set up; just touching the fine pitch stops at max static RPM. Once the plane starts rolling the governor takes over. This may not be reality because if the engine turns too high of static rpm mechanics will lower the rpm by adjusting the governor. How often that happens I don't know and neither did the prop shop I asked.

If they are adjusting the RPM by adjusting the governor, there's a good chance they are doing it wrong. For a hartzell top prop you adjust static rpm by moving the high rpm stop on the end of rhe prop hub. This ensures you are on the stops at max static rpm...

Sent from my SM-G870W using Tapatalk

It's probably worth noting that this is correct for Hartzell props. Hartzell props have an adjustable low pitch stop which can be adjusted without disassembling the prop, so it's possible to run it up and make field adjustments to dial in static RPM. The Hartzell manuals discuss this procedure. The McCauley props do not have an accessible low pitch stop adjustment. To change the low pitch stop, you have to open up the prop hub, and while I haven't done this myself, looking a the McCauley drawings, it looks like the low pitch stop setting is changed by swapping spacers (although I certainly may be mistaken about that) Dialing in static RPM on a McCauley prop by changing the low pitch stops would involve multiple repetitions of removing the prop, taking it to the prop shop and having them open up the prop and adjust 75 rpm up, then 25 rpm down, etc rinse lather repeat. Reading the McCauley manual, it sounds to me like there's a standard low pitch stop setting specified for each engine/airframe installation, and then Max RPM is set with governor adjustments. The OP has a MCCauley prop but he's kind of exploring unkown territory with an experimental engine/prop combination. The the low pitch stops set for an engine that produces 20 more horsepower at 200 RPM less than his engine, so not surprising that it doesn't hit 2800 rpm on a static run-up.

AAlexander wrote:

A1Skinner wrote:

whee wrote:
The two prop shops I've talked to agree with Halestorm on how a CS prop 'should' be set up; just touching the fine pitch stops at max static RPM. Once the plane starts rolling the governor takes over. This may not be reality because if the engine turns too high of static rpm mechanics will lower the rpm by adjusting the governor. How often that happens I don't know and neither did the prop shop I asked.

If they are adjusting the RPM by adjusting the governor, there's a good chance they are doing it wrong. For a hartzell top prop you adjust static rpm by moving the high rpm stop on the end of rhe prop hub. This ensures you are on the stops at max static rpm...

Sent from my SM-G870W using Tapatalk

It's probably worth noting that this is correct for Hartzell props. Hartzell props have an adjustable low pitch stop which can be adjusted without disassembling the prop, so it's possible to run it up and make field adjustments to dial in static RPM. The Hartzell manuals discuss this procedure. The McCauley props do not have an accessible low pitch stop adjustment. To change the low pitch stop, you have to open up the prop hub, and while I haven't done this myself, looking a the McCauley drawings, it looks like the low pitch stop setting is changed by swapping spacers (although I certainly may be mistaken about that) Dialing in static RPM on a McCauley prop by changing the low pitch stops would involve multiple repetitions of removing the prop, taking it to the prop shop and having them open up the prop and adjust 75 rpm up, then 25 rpm down, etc rinse lather repeat. Reading the McCauley manual, it sounds to me like there's a standard low pitch stop setting specified for each engine/airframe installation, and then Max RPM is set with governor adjustments. The OP has a MCCauley prop but he's kind of exploring unkown territory with an experimental engine/prop combination. The the low pitch stops set for an engine that produces 20 more horsepower at 200 RPM less than his engine, so not surprising that it doesn't hit 2800 rpm on a static run-up.

You you are correct. But if the low pitch stop is not set flat enough on a McCauley, you can adjust the governor all you want and you still wont get max RPM.

correct

A1Skinner wrote:But if the low pitch stop is not set flat enough on a McCauley, you can adjust the governor all you want and you still wont get max RPM.

Completely agree. When I was having a buyer's inspection done on my (now) 180, it wouldn't make 2600 RPM on a static runup. (O-470R and 88" C66, FWIW) Digging into it, it was discovered that the low pitch stop had been set to high. Nothing you can do with a governor to fix that

"The kinetic energy of a spinning mass is a function of the square of the rotational velocity (RPM) all else being equal a prop turning at 2800 RPM has about 16% more stored kinetic energy than the same prop spinning at 2600 RPM."

A more precise means of describing what we call dumping the clutch.

DeltaRomeo wrote:"The kinetic energy of a spinning mass is a function of the square of the rotational velocity (RPM) all else being equal a prop turning at 2800 RPM has about 16% more stored kinetic energy than the same prop spinning at 2600 RPM."

A more precise means of describing what we call dumping the clutch.

HAHA! I like it. AA's explanation is a good one though. An interesting note is that in cruise I don't see any increase in speed when going from 2600rpm to 2800rpm. Just more noise and more fuel.

On the fine pitch stops topic here are the specs for a C180:
An 82" C203 prop on an O470K the fine pitch stops should be set to 12.5 deg.
An 88" C203 prop on an O470K the fine pitch stops should be set to 10 deg.

Maybe noteworthy: An 80" C203 on an IO360D should have the fine pitch stops set at 9.8 deg.

My 88" C203 is off a C180G with no adjustments made. My engine turns it to 2570 static and pretty quickly turns it up to 2790 once I start rolling. I had arranged to meet with a prop shop to adjust my fine pitch stops because I figured the static rpm would be way low. 2600 is pretty good so I haven't had anything changed and I'm not sure I'd gain anything.

I believe that the low pitch stop blade angle are not measured at the same blade station on the 82 and 88 inch 203 prop. I could be wrong though.

Tim

DeltaRomeo wrote:
A more precise means of describing what we call dumping the clutch.

AAlexander wrote:.... I don't know if you've ever flown floats in a plane with a CS prop, but you get this identical effect when you're doing your runup. When you cycle your prop from 1800 RPM, to 1400 rpm (for example) you feel that same momentary shove as the prop slows. It's also a lot more obvious that it's not a sustained increase in thrust. ....

I've felt that doint a run-up on wheels,
never understood just why.
Thanks for the explanation.

bat443 wrote: I believe that the low pitch stop blade angle are not measured at the same blade station on the 82 and 88 inch 203 prop. I could be wrong though. Tim

The Cessna 180 TCDS spells out the low & high pitch settings for all the approved props for the various models.
FWIW the pitch settings for both the 82" & 88" 2A34C201 & 2A34C203 props are at 30" station.
Settings for 82" C203 are 12.5 & 25.0.degrees.
Settings for 88" C203 are 10.0 & 24.5 degrees.
Interestingly, the 2A36C, 2A34C, & 2A34C66 all use a 36" station for setting angles.

I stand corrected, thank you hotrod180.

Tim

whee wrote:Couple questions I've been thinking about:

1. If a constant speed propeller is removed from engine A, installed on engine B and the same max static rpm is achieved do engines A and B produce the same horsepower at that rpm?

2. Will a 88" long constant speed propeller turning 2600rpm produce more or less thrust than a 84" long constant speed propeller turning 2800rpm.

1: YES. If it is the same prop. Props react to HP and RPM. If you get equal thrust at same RPM on two different engines, they are making equal HP.

2: Many variables that are not defined or held constant in this question. We paid a bunch of money to various labs and computer analysis to demonstrate the MT 2-blade 58 series in 210cm and 205cm produce less than 2% difference in thrust at all power and pitch settings so that we could allow interchangeable use on our STC's.

Good questions that generated a lot of fun dialog.

John

john54724 wrote:

whee wrote:We paid a bunch of money to various labs and computer analysis to demonstrate the MT 2-blade 58 series in 210cm and 205cm produce less than 2% difference in thrust at all power and pitch settings so that we could allow interchangeable use on our STC's.

Propeller theory predicts a 1.6% to 1.8% difference between 205cm and 210cm, as long as the overall prop efficiency remains the same (~subsonic), and varies in this range depending on the spinner diameter.