Could someone clear this one up for me??? I have seen the following formulas:

PLF=D X P

PLF= D Squared X P

PLF= D to the 4th power X P

I have heard some say that this is an arbitrary number, and just gives you a way to compare to props, and others treat it like it really means something. If it is an arbitrary number, allowing you to guesstimate how a given engine will turn a prop, it seems like any of the formulas's will work. If it is a hard and fast number that really means something, why are there so many versions of the formula, and which one is right??? The principal I understand, the reasoning behind all the number crunching, I don't. Any clarification will be appreciated!!![sm=confused.gif]

Andy

It is a semi-arbitrary number. You take a baseline prop, run it and call that number 1.0 or 100 or whatever. Then the rest of the props are compared to that baseline by using a number bigger or smaller than 1.0 or 100, depending on if they load the engine more or less than your baseline choice.

And you cannot use one load factor for all of the equations above, it is specific to the equation used.


Mark

I could not have said that better.
However, the true prop load factor is the number of watts needed to turn a prop at a specified speed (I.E. 1000 rpm) in normal air (1000 mBar, 50% RH) If prop manufacturers would provide that simple number, our prop simulation calculators would be much more accurate in the interrelationship between props.
Still wishing, I also could do with true pitch numbers, and not blade tangent pitch.

Hmmm! So, either one of the formula's will work, as long as we are consistent with the formula we use. Is there any reason we wouldn't want to just use the Diameter X Pitch???

Thanks for your responses!

Andy

Yeh- the advertised numbers are sometimes off a country mile -or more
If wishes were fishes -- we would have available - a certified series of props - sized to common engines
these would show power required to turn at given rpm under standard conditions .
A friend has a set of German made, aluminum clubs which do that . They are for small glo engines
On full scale aircraft engines - a "load prop" is used to test rebuilt engines . If the engine turns THAT prop at a specified rpm - it is consided to be delivering a correct power .

No, it won't.
To keep it simple:
The formula for power is thrust x speed per unit of time. This in turn equals absorbed prop power which is (D^4)xPx(n^3) + a few constants to account for a number of variables, amongst which is the prop load factor.
At pitch speed of the airflow at the prop disk, the P(pitch) factor becomes unity, and prop pitch looses it's importance.

power is simply an expression of work produced.
Watts old Horsepower formula still confuses many.
In a nutshell, you can't calculate prop efficiency with any real degree of accuracy.
you can only measure it
The interaction with the airframe - can easily reduce all the carefully developed data to garbage.
Or re stated -- the prop efficiency must be developed to suit the task.

Dick,
I won't burn my fingers on prop efficiency, because the thrust x speed relationship is a major factor, and like you said, the influence of the drag (airframe) behind the prop becomes quite large.
The Wright flyer prop was just shy of being as efficient as the modern Mejzlik, though on first sight it would not seem to be, because of it's shape.
You will never hear me say that one prop is more efficient than the other.

I recently saw an article where they took Pro Zinger, and cleaned up the design. The RPM it turned, dropped, but the thrust went up. Everyone puts so much emphasis on the RPM of a given pitch and diameter. The PLF really muddies these waters. I am guessing that prop composition is one of those variables Pe Reivers is talking about. It sounds like experimentation is still the best bet. That was easy, when I was running .40 -.91 sized engines. When I moved up to 40-50CC engines, experimentation got expensive. I can't imagine the cost with the real big props.

Thanks for all the input guy's! Your experience is priceless. I don't always understand everything I read, but as time goes on, more and more of it makes sense!

Maybe I repeat myself
Maybe I repeat myself
anyway - when I got into electric motor powered models -- I built a small stand -which measured thrust -in ounces and grams
(Someone now markets an exact copy of that stand!)
anyway - I used a WHattmeter in series with my motor /controller / battery- This setup gave me static thrust and power required to develop that thrust
Both measurements in very exacting numbers .
What was strange -- was that the prop used seemed very meaningless .
IF--the power consumed was 5 watts then the thrust measured was 10 ounces. (for example)
changing the prop to a different pitch or diameter or number of blades , gave almost the same results!
Power consumed and thrust produced were very consistant
As I tried other combos and power settings - the same results occured
Tho there is a difference in efficiency on all combos- --that difference was small - I tried the same thing on my gassers (50-80cc) and found - the results were somewhat the same in that as long as I propped for the same rpm results - on the ground and in the air ( my engines have stored rpm readouts) the speed and climb results were very similar.
I whack and reshape my props to check for best results and so far the results as described - carry through.

That observation is true and not recognised by many. There is a definite relationship between power and thrust. It only gets upset by prop disk load and scale factors, which allow larger props to operate at higher disk loads.
My first calculations did not account for the disk load yet, and I used 6 lbs specipic thrust for each hp. These absorbed hp needed of course different prop rpm with different prop makes. You might want to check that figure against your data.
My [link=http://www.mvvs.nl/prop-power-calculator.xls]current calculations [/link] show the specific thrust per unit of power obtained with the calculated disk loads.

Assuming no cavitation how much power increase is needed to turn a given prop at twice the speed?

How much power increase is needed to turn it 50% faster?

WRK

How long is a piece of string?
Assuming the prop load continues - the power requirement increases logarythmically.
Look at Pe's chart- that will give you a good idea . (it is free but needs EXCELL to load and use.)
as an example tho - 300-500 rpm may require 50% more power -on some props .

Though air can be considered a fluid for all practical purpose, there is no cavitation, becuase there is no fluid-gas transition.

It is not my intention to give a complete lecture on aerodynamics, one question at a time. Check out http://www.aerodyn.org/Propulsion/propeller.html and zillions of other publications where you will find your answers.
For the quick and dirty work, use [link=http://mvvs.nl/prop-power-calculator.xls]my thrust calculator [/link]
You will find most answers there, % relationships included if you program the two engines with different data.

Diameter(2) + Pitch = load factor.

22-10 ----------- (22x2) + 10 = 54

24-8 ------------ (24x2) + 8 = 56

18-10 ---------- (18x2) + 10 = 46

Do all 22-10 props load the engine the same amount? No. Same load factor, but 300RPM or even 500RPM difference. Do all props that turn the same RPM, regardless of load factor, pull the same? No.

If you want to know which prop performs the best on your particular airplane with your brand XYZ engine--then spend $500 on props and fly the plane on about 15 different brands.

I spent close to $800 on props last season. Now I know what brands I like and what brands I won't buy again. The load factor makes no difference. The pitch, diameter and thrust it puts out while moving dynamically through the air is what makes the difference. The RPM on the ground is an "okay" indication of RPM, but doesn't tell you squat about how it will pull in the air.

Perfect example:
APC 22-10 turns a lot of RPM and makes a bunch of noise--doesn't pull for squat. Unloads too much and rips the tips. Wasted energy.
Bambula 22-10 loads the engine too much and turns very low RPM. Doesn't pull worth squat because it doesn't unload in the air.
NX 22-10 turns respectable numbers on the ground. Pulls very well in the air. Excellent thrust and vertical performance. Doesn't rip the tips.
MSC 22-10 turns good numbers. Pulls very well in the air. Excellent thrust and downline braking. Doesn't rip the tips.
Pro Zinger 22-10 turns "okay" numbers and looks good on the ground. Pulls horribly in the air and rips the tips. Vertical performance is very poor.

You just have to buy a bunch of different props and see how they perform on your airplane. The NX 22-10 might totally suck on your 28% Edge 540, but you might love it on the 27% Extra 300L. The MSC 23-8 might be the cats meow on your 30% Yak and it might suck on the 35% Pitts. Different airplane--different prop.

And -if you change the exhaust system - all of your prop findings will change --as power levels will change.

further reading: see http://aircraft-world.com/prod_datas...p-proptalk.htm
The E-crowd has done a lot of work on propeller calculations and real-time measurements with surprising results. It is an advantage in this study, if the German language is mastered.