propeller efficiency curves

[bagarre]

Does anyone have or know where to get the propeller efficiency curves for the various props approved for our airplanes?
I'm interested in knowing how much of an actual difference there is from say a wood prop to say a C170DM or C172MDM or EM at our RPMs and airspeeds.

I've been digging around on the internet for a while and I can't find anything that says the thin metal blades do anything until you exceed Mach .8 or so in tip speeds....something we never do.

So (from an efficiency standing) do we benefit from a metal blade and is a MDM/EM actually better than a DM?

There are quite a few threads on this forum that talk about propellers (I think I've read them all) but I can't find the actual charts that show blade profile X is n percent better than Y for a given set of parameters.

Or, are they all so close it doesn't matter in the slightest?

[Bruce Fenstermacher]

David, I have not charts to offer, but some first hand experience.

Wood props generally are not as efficient as metal because of their profile less of the blade is working. I've seen this first hand on Cubs.

Over the years there has just been two many folks with the thinner MDM/EM props flying agains the DM prop, and showing better performance to not believe there is. I've seen it first hand trying to keep up to a few of them.

My partner has the approved Sensenich prop for our 170s. Despite Cessna's chart showing the prop wasn't as good by the slimmest of margines over the DM, I just had to try it on my 170. There was just the slightest difference I thought, just like the charts said. Off it came.

So yes there is a difference. Just a little. Would I run out and buy a MDM, no. If I was in the market though that is what I'd be looking for.

[lowNslow]

Bruce, the MDM comes in a I believe is a 52, 54, and 56 inch pitch. Do these compare directly with the DM 51, 53 and 55 inch pitch? I am looking for something equivalent to the old "California Pitch" which I think was a DM7653.

[Bruce Fenstermacher]

Karl,

From what I gather the MDM 52, 54 and 56 pitch would compare to the DM 51, 53, 55 pitch in engine RPM. In other words the same engine in a given environment would be able to spin the MDM 54 at the same RPM it could spin the DM 53. The difference of course being the MDM with the courser pitch would pull the plane through the air faster.

[GAHorn]

Bruce, the MDM comes in a I believe is a 52, 54, and 56 inch pitch. Do these compare directly with the DM 51, 53 and 55 inch pitch? I am looking for something equivalent to the old "California Pitch" which I think was a DM7653.

While I've not seen the actual test data on he"California" pitched prop, I have visited with two owners who stated it was a progressively pitched prop. (It did not follow the same convention as standard props in that its pitch towards the tip flexed/progressed more aggressively coarse
than factory "normal". This required it to be thinner and more flexible forwards the tip in order for the tip to flex coarse when the aircraft reached cruising speed....yet retain finer pitch during takeoff. Like all things aviation, it was a compromise, the disadvantage being a loss of durability/longevity, and susceptibility to early condemnation from tip damage.)
Propeller design borders on"dark alchemy"and requires (expensive) testing because any change,
however minor, must be reevaluated for vibratory stress due to harmonics, and must be tested
with all combinations of approved engines. However certain axioms can be recognized, such as
metal blades can be made thinner than most other materials, which is more efficient (less
drag) and more durable, Their chief disadvantage is weight, which can be addressed with
composites...with added ease of repairability....but with diminished durability in certain
environments (rain, for example), and increased manufacturing costs.

[GAHorn]

Cont'.d: A new design is unlikely to produce miraculous improvements to the degree that
all of us are likely to run out and buy one, giving little incentive versus risk for mfrs, especially
considering the obsolescence (from their viewpoint) of our particular models.
As for existing props, the MDM was lighter/thinner/more efficient than the DM, and the EM
was the same (MDM) blade with a hub improvement to mount upon the later crankshaft flange
of the O-300-C/D engines. That last change allowed approx one inch increase in pitch efficiency.
The production of almost 12000 airplanes which could use that model propeller made it a profitable model for McCauley.

[bagarre]

The production of almost 12000 airplanes which could use that model propeller made it a profitable model for McCauley.

...and also made it a cost motivation to use less material in each of those blades.
So, it's possible that all the refinements had other motives and, when it's all said and done, they are all pretty much the same.

I was just curious how much of an improvement was possible from a wood prop all the way up thru a modern composite fixed pitch at the speeds we are turning.

[GAHorn]

...and also made it a cost motivation to use less material in each of those blades......

Actually, the machining necessary to remove the excess mat'l from the original forging represents an increase in mfr'g costs. The improvement can be observed in the increased performance of subsequent models of the aircraft as well as surmised through the increase in blade-pitch, although it's difficult to measure accurately due to other mods/upgrades performed simultaneously to the airframes. (They got heavier, yet kept the same or improved performance at least in-part due to the improved prop.)

[Bruce Fenstermacher]

I have the MDM7653 and had no idea it was specific to California...can anyone elaborate on why it's associated with CA?

It is not associated with California.

There was/is a prop shop somewhere that had an approval to take a DM prop and modify it to what was called a "California twist" or a "California prop". Presumably the prop shop was in California. In any case this "California twist" was a lighter thinner prop. And while probably not the same the MDM series mimicked this California prop at least in the area of improved performance over the stock DM prop.

So a MDM is not a California twist prop but there may be little difference.

[bagarre]

I just emailed McCauley to see if they can provide the actual curves.

It's more academic than anything.

Old wooden props could reach about 85% efficiency for tip speeds under Mach .8. The most efficient props today can make around 90% under Mach .8 (based on internet research) and that 5% doesn't equate directly to 5% more speed or climb (maybe 5% less fuel consumption at the peak of the curves in cruise but not an over all 5% difference.)

Because these are curves, it's possible that a propeller that is more efficient in cruise is less efficient and lower RPMs like take off than another propeller. I'm sure there are compromises all along the curve to make the best over all prop but I though it would be interesting to know the facts of the matter.

It reminds me of selecting a cam for my old Sportster. Everything was a compromise but it was fascinating to see all the different power curves for the different cam profiles available.

[Sixracer]

AH! Selecting cams.. done everyday in my shop!! ... since I'm real new to ownership I have a question about props/hubs. I have seen several prop hub spacers on E-bay.. what is that all about. What do they do and why would a guy need one? I'm not sure there is one on my 172B.
(Ebarf is not favored or promoted here so your link was deleted by the moderator. You do not
need spacers except for specific props none of which are applicable to 170s.)

[bagarre]

Spacers are parts very specific to the airplane they are intended for.

Don't know WHY you'd need to do that, other than to compensate for some other design consideration (Cowl shape?) for the prop to clear the nose bowl.

No, the 170 doesn't have one and doesn't need one.

[bagarre]

Good Morning David,
As far as production certificate dates the 1A170/DM CAR was dated April 23, 1947, and the other two models the 1C172/EM, and 1C172/MDM approved on December 14, 1956. So the “EM, and “MDM” were of a later development which you would suspect being a new airfoil design.

Popular airfoils for propellers of this era were almost all Double camber, RAF-6, and Clark Y design used in different combinations to try and achieve more performance.

When I'm asked this kind of question I have to step back and refer to some history that I was informed of when I first got into supporting propellers. This goes all the way back to the Wright brothers, and their airplane. It flew because of the aerodynamics work they done on the propeller/wing. It is said if they would have had 5 more horsepower their propeller would have been 81-82% efficient.

Today's propellers run 85-87% efficient, so in a hundred years we have gained 4%, that is not much gain at all, but where we did make large gains is in longevity, and safety. So, you can see when there is any gain in efficiency we our ecstatic, thus the reason for talking about instrumentation being needed to see the improvement. So the 1A170 vs. the 1C172 blade would be very hard to notice, but with every little improvement we at McCauley keep moving forward with a little better propeller.

Sorry about the long winded response, as this information is still considered proprietary and I just can’t say here is the facts, hope this helps a little with understanding the world of propellers thanks for the inquiry.

Regards
Gary Peak
McCauley Propeller Systems
Field Service Engineer

[Sixracer]

Bringing this thread back to life.
I found a listing for my 172:
Prop listing for 172 Climb Normal Cruise Max Min Static RPM
172B O-300-C (TCM) 1C172/EM7652 1C172/EM7653 1C172/EM7655 76.0 74.5 2350 2230
172B F O-300-C (TCM) 1A175/SFC8040 80.0 78.4 2480 2380
172B O-300-D (TCM) 1C172/EM7652 1C172/EM7653 1C172/EM7655 76.0 74.5 2350 2230 XXXXXXXX
172B F O-300-D (TCM) 1A175/SFC8040 80.0 78.4 2480 2380
F= foreign STC approval
Mine is marked with the XXX
What is or how is the static RPM tested?
Is the RPM range due to DA differences or engine efficency? Or is that the difference in Climb, Normal & Cruise props?
I have checked the prop flange on my engine is a 6 bolt. I guess the next step is to pull the spinner and see if I can determine what prop I have. Then I can do a static test, after someone tells me what it is>> :~)
I'm interested in setting up for some 400 mile trips. I have no need for STOL performance. Anyone have any ideas what mods & prop I need?

[Bruce Fenstermacher]

Four things determine the speed an engine will spin a prop. The atmospheric conditions at the time, engine performance or health, prop diameter and prop pitch.

172B O-300-C (TCM) 1C172/EM7652 1C172/EM7653 1C172/EM7655 76.0 74.5 2350 2230

You will note that in the above example static RPM is given for a range of prop pitch in this case from 52-55 and prop diameter 74.5"-76".

If you read the TCDS for your 172 you would probably find the pitch of the prop is not a limiting factor but the static RPM and diameter are. So as long as you have a 1C172/EM prop that is at least 74.5" but not more than 76" in diameter and the static RPM is within the range of 2230-2350, you have a legal prop. A 1C172/EM prop pitched between 52 and 55 should fall into the static rpm range available. A 52 pitch would be near or at 2350 and the 55 pitch would be near or at 2250 all depending on the diameter of the prop.

So what and how do you find your static RPM? Well remember the atmospheric conditions at the time not only determine drag on the spinning prop but engine performance. I don't know for sure but would bet the static RPM given is for a standard day and no wind. A standard day being sea level altitude, 29.92 on the barometer and 59 degrees. The important thing to keep in mind is any variance from that of the conditions for the limits set may render a different result. In reality unless you are extremely far away from the set conditions you aren't likely to see much variance, specially with our bouncing mechanical tachometers. Which brings up another point. No sense in doing this test unless you know your tach is accurate and since most mechanical tachs are not at some point in their range a digital tach is almost a must.

So how do you do a static test. Well static means not moving. So find the atmospheric test conditions or as close as you can and with the airplane not moving, run the throttle to full power and note the maximum rpm. That is basically it. Of course you may want to lean your engine to max rpm depending on the result and what your trying to accomplish.

Cautions are warranted when doing this test. One your creating a lot of thrust. Make sure your not blowing it on anything or that anything will be sucked into the prop. Second you will likely have to secure the aircraft to something because the brakes are not likely to hold it at full throttle or at least this is the case with lots of airplanes. If you secure it it should be in a open area, don't tie the tail to your hanger building directly behind the airplane.

Sixracer you say you are setting up for some 400 mile trips and don't need STOL performance. Well to be honest there probably isn't any economy changing what ever prop you have. If you happen to have a 52 pitch prop you can probably cruise at 100 mph at least making the trip 4 hours. If you repitch to 55 and you cruise at 115 mph you'll get there in 3.5 hours. Your potty breaks will make more difference in time of travel. Now if it was 4000 miles then I might tweek the prop with more pitch but only if it were a 52 pitch.

Of course no matter what it is always nice to know more about how your plane is equipped.