Prop pitch, rpm, and performance

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blane.c

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There are a lot of techniques and formulas for propeller design, in fact I did my aerospace engineering senior design project on a propeller design using blade element analysis, but then there's the real world. The calculated design is just a starting point for testing.

The fact that my new propeller has less pitch than the old one, but turns slower, pulls worse at low speed, but goes faster in level flight (in short, acts like a higher pitch prop) is interesting. I suspect that the thicker section and higher camber of the wood prop gives it a higher effective angle of attack, I.e. more effective pitch.
Also it is lighter than a metal one so different inertia.
 

Vigilant1

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Yes, but I don't have a copy of Jan Carlson's like you do.
I wish I could understand better what goes on inside that black box. Like most folks, I wind up guessing/estimating a lot.
 

TFF

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This is not to down Sterba, but they are the budget prop makers. Value for money is their market. If you want performance you get a prop from Performance. It will cost you three times as much as a Sterba.
I know Dana has read the prop comparisons on the Biplane Forum but for everyone else not there, the metal Sensenitch ,performance wise, beat out all comers except in weight. That was the main reason to change props on aerobatics if the other losses were not to great. Now if you got the bucks Sensenitch has a composite version for homebuilts. To get like performance from a wood prop will require something like a Cato or Performance.
I can only afford the metal prop that came on my plane or a Sterba. Dealing in low budget or lower performance aircraft can be a little hard because there is going to be very little change for the amount available to spend.
 

Vigilant1

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I suspect that the thicker section and higher camber of the wood prop gives it a higher effective angle of attack, I.e. more effective pitch.
That sounds likely. Also, there could be an apples-to-oranges situation if the metal prop's pitch is measured along the chordline and the wood prop is measured along the flat at the bottom.
 

Dana

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Thanks Blane, that's one of the best prop articles I've seen. Back when Sport Aviation had real content...

One thing the article mentions, which of course should be obvious, is to look at similar performing certificated planes such the same engine and what they use. For me, that'd be the L-14 (Piper J-5D)... which uses a 74-44. But interestingly, one version of the PA-18 with the O-290 used an 80x32!

That sounds likely. Also, there could be an apples-to-oranges situation if the metal prop's pitch is measured along the chordline and the wood prop is measured along the flat at the bottom.
Sensenich uses the flat back to measure the pitch of their metal props, and my own measurement agreed. Sterba said the same, but the back of his prop isn't flat. But measuring 1/8" in from the leading and trailing edges is pretty close to the actual blade chord line, and agrees with what it's supposed to be.
 

mcrae0104

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It sure would be helpful if you could borrow a couple of props or three before modifying. Or maybe you could at least rent one or something.

Dia. = K (either 318 for metal or 285 for wood) multiplied by the [ fourth root of the { horsepower divided by ( RPM squared x the airspeed in mph ) } ]. This is in feet so multiply by 12 to get inches. Sorry I can't type a proper formula on this keyboard it lacks things like fourth root.

For pitch.
View attachment 90386

View attachment 90385

Having the propeller balanced while on the engine has proved worthwhile to me.
Blane, what's the source for this graph? I'd like to read up on it.
 

blane.c

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Thanks Blane, that's one of the best prop articles I've seen. Back when Sport Aviation had real content...

One thing the article mentions, which of course should be obvious, is to look at similar performing certificated planes such the same engine and what they use. For me, that'd be the L-14 (Piper J-5D)... which uses a 74-44. But interestingly, one version of the PA-18 with the O-290 used an 80x32!



Sensenich uses the flat back to measure the pitch of their metal props, and my own measurement agreed. Sterba said the same, but the back of his prop isn't flat. But measuring 1/8" in from the leading and trailing edges is pretty close to the actual blade chord line, and agrees with what it's supposed to be.
My O-290-D2 had a McCauley 82" by 42" pitch. I could of gone with a 39" pitch but at those speeds would of amounted to maybe 20 feet less take-off roll and I was already using less ground for take-off than landing so seemed moot and I chose a slightly better cruise speed. Cruise was an optimistic 90mph but I flight planned 75mph. That dia. McCauley for Cubs is called a "borer prop". http://store.cubcrafters.com/McCauley-Borer-Propeller_p_1586.html

The "borer" was originally developed for the O-290 125HP
 

plncraze

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Blane's reading list looks like he got bit hard by the propeller bug. Quentin Wald's article "Aerodynamics of the Propeller" and Neal Willford's spreadsheet that accompanied his propeller article in Sport Aviation are also good.
There are a lot of assumptions in designing props they seem to feed off each other.
 

Dana

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My O-290-D2 had a McCauley 82" by 42" pitch. I could of gone with a 39" pitch but at those speeds would of amounted to maybe 20 feet less take-off roll and I was already using less ground for take-off than landing so seemed moot and I chose a slightly better cruise speed. Cruise was an optimistic 90mph but I flight planned 75mph. That dia. McCauley for Cubs is called a "borer prop". http://store.cubcrafters.com/McCauley-Borer-Propeller_p_1586.html

The "borer" was originally developed for the O-290 125HP
On what plane, a Cub?
 

BBerson

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This is article, but he did the math to the wrong conclusion … he interpreted 5.36 feet to 54 inches instead of 64 inches. But otherwise good.
What is that 1 over the square root symbol mean? Can't make sense of that formula.
 

Dana

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Interesting.... just did some reading. I had heard of Borer props but always assumed it was a brand name. So the bush plane guys are going for large diameter flat pitch props for best takeoff and climb at the expense of cruise. Makes sense, if it's an acceptable compromise. But it might also make sense for a draggy biplane that doesn't go fast anyway...
 

Vigilant1

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What is that 1 over the square root symbol mean? Can't make sense of that formula.
That's a numeral "4", not a numeral "1". It denotes the fourth root (the square root of the square root)
 

TFF

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A Cub is a race plane compared to the Hatz. I believe someone on the biplane forum has an 80” prop on their Hatz.
 

blane.c

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On what plane, a Cub?
Yes. I forget the mans name who worked with McCauley originally to develop the propeller for the Super Cub, but they did it back when the Super Cub had 125hp if memory serves at all. It is considered necessary for off airport. Light my Cub would take off at 1700RPM if I didn't hold it on.
 

blane.c

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That sounds likely. Also, there could be an apples-to-oranges situation if the metal prop's pitch is measured along the chordline and the wood prop is measured along the flat at the bottom.
It seems that Sensenich uses the "Rose E" profile which does not have a flat bottom like other wood propellers. It is supposed to be more efficient nearly as efficient as metal without the stress of a metal prop.
 

Rockiedog2

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Interesting.... just did some reading. I had heard of Borer props but always assumed it was a brand name. So the bush plane guys are going for large diameter flat pitch props for best takeoff and climb at the expense of cruise. Makes sense, if it's an acceptable compromise. But it might also make sense for a draggy biplane that doesn't go fast anyway...
Also makes sense for a hi power UL we’re trying to keep legal. Like 58-18 on my LEU
 

blane.c

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Interesting.... just did some reading. I had heard of Borer props but always assumed it was a brand name. So the bush plane guys are going for large diameter flat pitch props for best takeoff and climb at the expense of cruise. Makes sense, if it's an acceptable compromise. But it might also make sense for a draggy biplane that doesn't go fast anyway...
One of the things that really slows down the Cubs is the large tires, you will lose 10mph or more with those 30" air streaks. So if your airspeed is already compromised you may as well go all in. 82"x2700RPM = 658.66mph which ain't bad. 82"x3400RPM = 829.42mph. Still under for a wood prop. Most say 90mph cruise for the Cub, I always planned 75mph and no issues with that, The actual cruise speed is more than 75mph but quite a bit less than 90mph that is for the bush outfitted O-290's and O-320 Cubs.
 

BJC

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82"x2700RPM = 658.66mph which ain't bad. 82"x3400RPM = 829.42mph. Still under for a wood prop. Most say 90mph cruise for the Cub, I always planned 75mph and no issues.
Even with rounding down to 658 and 829 MPH, that is impressive.


BJC
 
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