Prop pitch, rpm, and performance

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Pops

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The problem with props can be you get the RPM you are looking for and loose the performance you were trying to get at the same time.

Max horsepower is when the engine can’t spin the prop faster than max rated RPM. It has to be loaded. RPM and max manifold pressure is at that point. Of course at sea level.

Fixed pitch aerobatic airplane you would want 2700 rpm in vertical and you don’t care where it runs in level; you are not supposed to be level in aerobatics.
Top speed you would want 2700 in level.
Everything else is in between on how you load it.

My guess in the black magic is don’t go all the way to 3”. 1 1/2 or 2”. You don’t want to step over and turn it into wall art. It might be short on what you want but you also don’t want to go backwards. Different brand props do act different.

It’s good to see the data points with your findings.
That is the problem== Different brand props do act different. Darn.
 

Rockiedog2

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It’s like trying on shoes these days
The numbers are just a starting place
It’s experimental aviation especially with props, always has been
If it’s got an AD on it trim the tips that might fix it. Or make it worse. You do have a cable on the motor?
3/16 is better than 1/8
 

Pops

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It’s like trying on shoes these days
The numbers are just a starting place
It’s experimental aviation especially with props, always has been
If it’s got an AD on it trim the tips that might fix it. Or make it worse. You do have a cable on the motor?
3/16 is better than 1/8

IF you ever broke a crank or throw part of a prop blade, you will wish you had a cable on the motor. Been there and done that.
 

Dana

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Trimming the tips is an option, but I think I'd rather reduce the pitch and/or chord first, especially since three other prop makers suggested 44 or 45. Gonna talk with Ed Sterba first, see what his thoughts are.
 

BBerson

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Reducing both the pitch and chord is easy. Just sand off the trailing edge to some calculated thickness, such as 1/8". Then sand the flat bottom till the trailing edge is back to the original 1/16" or whatever it was.
 
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Dan Thomas

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Have you thought about trimming a half inch off of the length? (Each end?)Sounds like you got nothing much to lose?
I’ve trimmed a number of losers and sometimes the results are surprising.
Yeh I rather have the wood for the reasons you mentioned .
Interesting experiment?
I did that to my Jodel's wood prop in an effort to get some more RPM and therfore HP, and LOST performance when I got the few extra revs. That long prop (76-44) was so efficient that I was getting no indicated slip in cruise. The geometric pitch x RPM gave a speed that was the same as my TAS. Shortening it ruined that. I took an inch off each tip, then another inch, ending with a 72" prop. Should have left it alone.

When it was at 76" it had the strange habit of the RPM falling about 30 or 40 revs as the airplane accelerated on the runway before liftoff, and one could feel it "bite" more. A little bit more acceleration. After shortening, it didn't do that anymore.
 

Dan Thomas

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The metal prop also has a bit too much pitch, it's subject to an AD, and it weighs about 15# more; a wood prop runs smoother, puts less stress on the crankshaft during acro, and just looks right on a biplane.
And it saves the engine if you have a prop strike.
 

delta

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I had a friend get caught in a rainstorm with a culver prop on his Q2, and he picked up some performance out of the deal.
 

Marc Zeitlin

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Reducing both the pitch and chord is easy. Just sand off the trailing edge to some calculated thickness, such as 1/8". Then sand the flat bottom till the trailing edge is back to the original 1/16" or whatever it was.
While that certainly reduces the chord and prop area, won't that INCREASE the pitch, not DECREASE it? Take your procedure to a limit - if you sanded off 1/2 of the prop chord and then sanded the flat bottom to match the much larger TE thickness, that would substantially INCREASE the pitch, no?
 

BBerson

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While that certainly reduces the chord and prop area, won't that INCREASE the pitch, not DECREASE it? Take your procedure to a limit - if you sanded off 1/2 of the prop chord and then sanded the flat bottom to match the much larger TE thickness, that would substantially INCREASE the pitch, no?
Uh, no.
You only sand off the bottom at the trailing edge to lessen the pitch and the actual angle.
If you want more pitch then sand the top off the trailing edge instead, and a bit off the bottom of the leading edge. Doesn't take much, try just the outer 8" first. Make a new prop if you need a bunch of change.

Hope that makes sense.
 

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blane.c

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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.
upload_2019-11-16_8-36-18.png

upload_2019-11-16_8-35-37.png

Having the propeller balanced while on the engine has proved worthwhile to me.
 

blane.c

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yes, no.

After doing formula you will end up with a (usually) decimal fraction like .562 for instance. From the bottom using your number go up till it goes into an arc nearest the apex, so for .562 you would pass the .6 arc and go up to the .7 arc as it fits nearer the apex. Avoid being on the backside of an apex. Then .7 time the propeller diameter will = pitch … so for a 5.8 foot dia. then 4.06 foot pitch or 48.72 inches and that is at the 0.75 diameter. You can look to the left of the curve for propeller efficiency were your number intersects the arc so for .562 it will be around 78% or 79% efficient.
 

Vigilant1

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Got it. You enter the horiz axis with the result from working the formula.

The "black art" comes with figuring/estimating "N" (RPM). It is pretty important to this whole enterprise, but is a function of engine torque curves and the torque required curve for each prop. And at the end of this, you'd get a prop optimized for one point.
 

Dana

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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.
 

blane.c

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Got it. You enter the horiz axis with the result from working the formula.

The "black art" comes with figuring/estimating "N" (RPM). It is pretty important to this whole enterprise, but is a function of engine torque curves and the torque required curve for each prop. And at the end of this, you'd get a prop optimized for one point.
Yes, but I don't have a copy of Jan Carlson's like you do.
 
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