Comparing Props

[zero.one.victor]

There's been a little discussion lately about props,specifically for float plane use. I was wondering if there's any way to "quantify" props.
For lack of a better term,the "volume" (diameter times pitch) of a 76-51 prop is 3876. To obtain similar volume with an 80" diameter requires a pitch of about 48 (3840). Given the same blade design (both props being McCauley 1A170 DM models),how would the performance compare between a 76-51 (standard) & 80-48 (re-pitched borer)? Seems like they're both trying to move the same amount of air. Would the larger prop be more efficeint?
I know larger props seem to give better performance in the constant-speed variety,swapping an 88" for the stock 82" is a popular mod for 180's,especially for out-of-the-hole-use (float & bush flying).
Any thoughts?

Eric

[4-Shipp]

Eric,
The main flaw in your theory is the relationship between prop diameter and volume. I understand where you are going but the volume would be the product of the disc area and the pitch, not the diameter and pitch. The area is directly proportional to the square of the radius, not the diameter. When you add 10% to the diameter you add much more than 10% to the area of the disk. Add to this the fact that the outer portion of the prop blades do most of the work and are much more effecient than the rest of the blade, increased diameter should have much more effect than pitch change. I know there is a lot more science here than I have addressed, but I would have to dig out the old college texts to keep from embarassing myself. As an example, an aquaintance recently nosed his Citabria over slightly and managed to knock 3-4 inches off each tip. He cleaned them up and ferried the plane abotu 5 miles to get it fixed (No, I don't think it was a good idea,I wouldn't have done it, but that is not the point of this post). The point is with the same pitch and about 7 inches less diameter (115hp Lycoming) his cruise dropped from 110 to 70mph IAS! I would love to hear some opinions from folks with much more recent prop theory experience than I have on this whole "prop volume" idea.

[zero.one.victor]

I shoulda thought it out a little more,I guess. I do know that area is equal to pie times radius squared,and that pie times diameter equals circumference.,so prop "volume " isn't an accurate term when using diameter as one of the factors.
Still,anybody have any thoughts on some sort of formulae for comparing props,or is this one of those things that can't be calculated?

Eric

[N1478D]

I believe you would also have to account for blade area, the blades themselves.

[GAHorn]

"Blade area" makes an interesting point. Narrow blades are better for high-speed cruise, while broad blades make for short field performance in acceleration. Two-blades are more efficient than more, and constant speeds are more versatile performers at the expense of cost, complexity, maintenance, and weight. I think I can cut to the chase on this issue. The best, all-around prop for a 170 landplane, is the one that Cessna and McCauley spent a lot of money evaluating and developing and finally settling on,....the McCauley 7653. A 7651 however (according to them) is a good alternate for short field operations.
It all boils down to a simple fact that the Type Certificate specifies that a 7653 prop must turn not less than 2230 nor more than 2330 rpm in a static runup. The TCM C145/O300 manual states that this is a developed horsepower of 93-98. That's all. You don't get 145 horsepower unless you get 2700 rpm. The ideal prop in cruise, is the one that lets you develop recommended hp at the most economical rpm (for fuel consumption) at maximum throttle (manifold pressure), and that all boiled down to trial and error by Cessna and McCauley. Every other flight condition was a compromise. They did their homework and then published it for us. Special operation considerations would be the major reason to use a different prop than the optimum they've recommended.

[N1478D]

One of the explanations why GPH is not linear.

[zero.one.victor]

George,first of all,which 7653 prop are you referring to: DM,MDM,EM. or what? 1A170 or 1C172? The DM & MDM are different blade designs,with about a 2" difference in pitch for similar performance from what I hear. I don't know about the others.
"The best prop...." is a pretty ironclad statement. The best for what? Max cruise with min fuel burn is fine,IF you don't also have a need for short TO or steep climb. They're all a compromise....
That's like Joe saying his is faster just because it's an A model. There's probably a whole lot of different reasons why his is faster than yours

Eric

[GAHorn]

George,first of all,which 7653 prop are you referring to: DM,MDM,EM. or what? 1A170 or 1C172? The DM & MDM are different blade designs,with about a 2" difference in pitch for similar performance from what I hear. I don't know about the others.
"The best prop...." is a pretty ironclad statement. The best for what? Max cruise with min fuel burn is fine,IF you don't also have a need for short TO or steep climb. They're all a compromise....
That's like Joe saying his is faster just because it's an A model. There's probably a whole lot of different reasons why his is faster than yours

Eric

That's a good point, Eric. The DM prop is a 1A170 and the MDM and EM props are 1C172. The DM blade is slightly wider/thicker than the MDM/EM props. (In my previous post, I was actually intending that the standard prop as determined by Cessna/McCauley was the best all-around prop. I shouldn't have quantified it as "7653" without stating MDM or EM along with that number.) The 1C172 MDM is different from the EM only in the prop flange it bolts to, the MDM being an 8-bolt prop, the EM a 6-bolt. In theory both props are slightly more efficient (being thinner blades with less induced drag) than the DM series, and they also are lighter which is supposedly easier on the engine.
That's a good point about Joe's plane also. I wonder if his prop is flat too?

[N1478D]

George,first of all,which 7653 prop are you referring to: DM,MDM,EM. or what? 1A170 or 1C172? The DM & MDM are different blade designs,with about a 2" difference in pitch for similar performance from what I hear. I don't know about the others.
"The best prop...." is a pretty ironclad statement. The best for what? Max cruise with min fuel burn is fine,IF you don't also have a need for short TO or steep climb. They're all a compromise....
That's like Joe saying his is faster just because it's an A model. There's probably a whole lot of different reasons why his is faster than yours

Eric

That's a good point, Eric. The DM prop is a 1A170 and the MDM and EM props are 1C172. The DM blade is slightly wider/thicker than the MDM/EM props. (In my previous post, I was actually intending that the standard prop as determined by Cessna/McCauley was the best all-around prop. I shouldn't have quantified it as "7653" without stating MDM or EM along with that number.) The 1C172 MDM is different from the EM only in the prop flange it bolts to, the MDM being an 8-bolt prop, the EM a 6-bolt. In theory both props are slightly more efficient (being thinner blades with less induced drag) than the DM series, and they also are lighter which is supposedly easier on the engine.
That's a good point about Joe's plane also. I wonder if his prop is flat too?

"I don't mind saying it." Props NOT flat, wings NOT flat, tires NOT flat, and I don't spend much time looking at flat objects

[JDH]

Have you heard of the California prop?
A friend with a '53 B model has a C145-2 and has one of these props. I don't know who makes it (Sen or Mac) there is this shop in California that takes this "polished" prop and makes it thinner, I don't know if they repitch also, but they shave it down. According to my friend, it flexes much like a Borer prop, gives you better take off performance "and" better cruise. Snake oil? Hurban legend? Myth? Lie??? JD

[zero.one.victor]

JD, I assume you're referring to the "California Twister". I found a little info about it in an old Flypaper. It was a modification of a standard propeller (doesn't say which one) done by Aero-Propeller of Hemet,Calif,714-765-3178. Cost was $580 (exchange) back in 1993.
And George,quit pickin' on Joe,ain't nothing flat on his airplane but the tires!

Eric

[GAHorn]

JD, I assume you're referring to the "California Twister". I found a little info about it in an old Flypaper. It was a modification of a standard propeller (doesn't say which one) done by Aero-Propeller of Hemet,Calif,714-765-3178. Cost was $580 (exchange) back in 1993.
And George,quit pickin' on Joe,ain't nothing flat on his airplane but the tires!

Eric

I don't know, Eric. I heard his speed-curve was kinda flat.

[JDH]

Yes Eric, that is it. California Twister. Obviously, I don't know much abouut it, but my friend recommended that if I can find one that fits my engine, to jump on it. JD

[Collin Gyenes]

Hi,

I have the Cal twist prop on my '52 170 and my friend has one on his '55 170 also. The prop mod was recommend by his A.I. that had one on his 170 before he installed Lyc.180. Both my friend and I notice a climb improvement (about 200 fpm) and little improvement in cruse. I had a chance to fly formation with my friend with a '48 170 with a cruse prop. My climb was better. At Cruse side by side I was at 2400 rpm at 115 mph indicated and the '48 170 was at 2300 119 mph indicated. At full power top speeds were the same. I have been happy with the Cal twist prop and think it was a good improvement for the money. The cost was $700.

[N3243A]

Hi,

I have the Cal twist prop on my '52 170 and my friend has one on his '55 170 also. The prop mod was recommend by his A.I. that had one on his 170 before he installed Lyc.180. Both my friend and I notice a climb improvement (about 200 fpm) and little improvement in cruse. I had a chance to fly formation with my friend with a '48 170 with a cruse prop. My climb was better. At Cruse side by side I was at 2400 rpm at 115 mph indicated and the '48 170 was at 2300 119 mph indicated. At full power top speeds were the same. I have been happy with the Cal twist prop and think it was a good improvement for the money. The cost was $700.

Collin,

What is the pitch of your "Cal twist" prop?

Bruce Christie