Propeller Worms
 

Hate to open this can of worms but here goes. :eek:
I have the PropPowerCalculator that has all the bells and whistles now. I notice the "Corrected Prop Constant" is different for different propellers and this makes perfect sense to me, but I notice the Master Airscrew black prop has a value of 1 while the Schimitar has a value of .95 and the APC 1.2.
It appears to me the schimitar just by it's shape would fall somewhere in between the APC and Master Airscrew black. Is the Schimitar a poor design? [:o]
As always I appreciate your enlightenment. ;)

RE: Propeller Worms
 

I can't help with the science of it. However I have run both the APC 16x6 and the MA Scimitar 16x6 on a Saito 1.25 mounted to a profile Fusion. On the ground the Scimitar gives an extra 900rpm. In the air the Scimitar jumps out of a hover. The APC is a steady pull.The Scimitar is supposed to make more thrust at a lower RPM. It does feel that way but the lighter wood prop also spools up much faster than the APC. So is the jump from a hover spool up or design related? I tend to believe it is some of each. Now if you sport fly with both props the APC makes the plane fly faster WOT straight and level. Even though the APC turns less RPM. At least when static. So I'm thinking the APC has to be more efficient at higher RPM. After all the spool up is a mute point running along at WOT. This could go back and forth all do I suppose.

David

RE: Propeller Worms
 

I don't think it's a good idea to apply a blanket factor across all props like the spreadsheet programs tend to.

I have noticed that loads can vary greatly between brands in some size ranges and the gap can close or the situation can reverse when the same brands are compared but in a different size range.

The Master Airscrew black props and APC props are a perfect case in point. Take an MA black 11 x 6 and an APC 11 x 6 and try them out. There's an enormous difference in load between these two - upwards of 2,000 rpm (the APC is a heavier load). Now, get 12 x 6 props in each brand and compare them. The MA black 12 x 6 and the APC 12 x 6 are to all intents and purposes the same static load as each other. Get a 12 x 8 in each style and the APC imposes a much greater load again. There's no consistency at all, it seems, when comparing MA blacks against APCs.

A blanket brand-specific coefficient does not apply to these prop load examples, hence my general cynicism relating to the use of them.

RE: Propeller Worms
 

David, the comparison you make between the two 16 x 6s tends to suggest that the true pitch of the APC 16 x 6 is greater than the Scimitar 16 x 6, hence the lower static rpm and sluggish pullout but greater flight speed of the APC.

It would be interesting to measure them.

RE: Propeller Worms
 

I have also noticed that APC props are not simply "scaled" versions of a single archetype. Sometimes two props that are only one inch different in diameter have a completely different hub mass and shape. This fact definitely creates a lot of variation in loads as HL pointed out.

David

RE: Propeller Worms
 

It has nothing to do with poor design. The correction factor is a prop constant that tells you how good an air brake that prop is for static thrust measurements, when efficiency is zero per difinition. The speed charts of the props are a good guess, which in most cases is better than a bad calculation.
ALL props that look anywhere near decent are about 82 to 85% efficient if allowed to fly at their design speed. Even so the Wright Brothers prop.
I know the APC prop makes more speed, but that is due to the larger true pitch than noted on the prop. The larger than noted pitch increases the prop constant in my spread sheet calculator, but does not increase flight speed, which is based on true pitch. I would need wind tunnel test results or fixed rpm caroussel tests with all the listed props in order to put in a pitch correction factor as well. It can be done, but not in my available time frame.

RE: Propeller Worms
 

OK that seems to help. Thanks Pe [8D]

RE: Propeller Worms
 

A differance in prop speed from one prop to another is not an indication of efficieny. It may be a differance in pitch and total area. At any rate if it were from a change in efficiency then the engine would slow down not speed up, because the prop is moving more air and doing more work.

RE: Propeller Worms
 

Sport Pilot, Notice the first sentence in my reply?:D I was just trying to give real world data from two props of the same length and pitch. If I understand some of the other posters the APC prop have more pitch then what is printed on the face. This would explain the lower RPM #'s in static and I guess the difference in speed @ WOT.

David

RE: Propeller Worms
 

The APC and MA-S props are made from 2 very different materials so I imagine that in itself makes a huge difference in actual thrust. APC props still scare me since that OS 46 LA "incident". :eek: Still hurts just thinking about it. [sm=lol.gif][sm=omg_smile.gif]

RE: Propeller Worms
 

The difference between any two props will (almost) always reflect a difference in efficiency. Pitch and total prop area affect how much air and how well the air is moved. One prop will always be more efficient than another, based on it's design, unless they are identical.

Each manufacturer strives to produce a prop that is the most efficient, i.e., moves the aircraft the most, with the least power consumed. RPM is one indicator; thrust is another, but with only a fixed amount of HP to play with, you have to play these two goals (RPM vs. Thrust) against each another. A flatter prop will spin faster, but generate less thrust wth every revolution. Whether this ends up generating more thrust overall depends on the torque curve of that particular engine. Also, a heavier prop will be slower to spool up, but similar in constant speed performance. Everything affects efficiency.

Jack

Small print - I'm a Mechanical Enginerd, relatively new to this sport and aircraft in general, possibly speaking out my butt on this issue, but I believe I'm correct.

RE: Propeller Worms
 

I thought you might be trying to say that the higher RPM meant it was more efficient. Sometimes people jump to the conclusion that because the prop turns more RPM and has more thrust it is more efficient, usually it means the prop has less pitch. When racers contour the square edges around the hub and improve the contor's of the prop the prop, this usually results in lower RPM and they may have to trim off the tips to get the RPM back to where they want it.

RE: Propeller Worms
 

I think you may be confusing effiiency with effectiveness. A dragster is not very efficient, but very effective. I was just trying to say if the only change is efficiency then the prop will load the engine down more than the less efficient prop. Of course you could try to prove that wrong by comparing it to a huge 0 pitch paddle bladed prop. Which will indeed load the engine down more, but thats comparing apples to oranges. If you take a basic design and improve efficiency it will do more work and work will load down the engine.

RE: Propeller Worms
 

I kind of agree. If you are getting the same amount of thrust, with a less efficient prop, then I agree, the engine is must be doing more work. However, if you improve the prop's efficiency, and it is the only change you make, you may get more thrust out, without adding much, or any load on the engine.

I do believe that different engines will find better or worse performance with the same prop though. This is due to the differences in the engine's torque curves; i.e., where they make their best power, because just like engines, different props will be more or less efficient at different RPM. This is why there is no "one right Prop" or engine for any plane.

Jack

RE: Propeller Worms
 

There are two thick branches on this tree...

A more efficient prop may have a higher load, because it transforms more of the engine's power into actual pulling power (thrust).
A less efficient prop may have a higher load, because it transforms more of the engine's power into turbulence...

The prop factor is supposed to represent the relative load of the prop, compared to other props of identical pitch and diameter.
...Not its efficiency...

The load could either increase, or decrease, from both higher efficiency and lower efficiency...


And as to the APC 11x6 prop... There has been a unanimous opinion on these pages that this specific prop exerts a MUCH higher load on any engine, than its prop factor suggests. It 'gulps' up more power, but gives good performance...

I suggest Pé Reivers look closely into this specific size... It may warrant a prop factor of its own.


Maybe categorizing only by brand and type is insufficient.

I found the Master Airscrew K Series props; although black, probably need a higher prop factor also.
They are spun more slowly than APC props of the same sizes.

RE: Propeller Worms
 

Want to hear an anecdote? OK:

When APC props first surfaced & info about them was chaotic, I decided to got to the mfr for the straight skinny. So I called Landing Products and the guy (one of the designers). I asked him about the design parameters. He said, " We carried the pitch closer to the hub than you can with other materials. That's where the increased loading and lower static RPM comes from." When I asked about the tapered tips, he said, "That was for noise reduction and looks." Honest, that's what he told me. Might get a different story now. :)

Folks, you may remember that APC props became known to the mainstream modeler through the AMA's noise source investigations. Pretty soon APC props were the THING!

CR

RE: Propeller Worms
 

Sorry about the dup.

RE: Propeller Worms
 

ORIGINAL: Sport_Pilot

No work is the actual movement of air, not turbulance, not the energy the engine is putting out. We are talking about prop efficiency, not engine's. I was not talking about more thrust with a less efficient prop, but more thrust with a more efficient prop, with pitch and size (diameter and area) the same then a more efficient prop with create more thrust and this will cause the engine to slow down. Even though you may streamline the prop the engine will slow down if you do it right but it will push more air.

RE: Propeller Worms
 

I suppose that could happen, but I understand a 12" square square stick can be spun at a higher rate than even the lowest pitched 12" prop. For a less efficient prop to to have the same load it is going to have to be bigger in diameter and area. But as I said that is apples and oranges.

RE: Propeller Worms
 

This may be true, Hugh, but a prop cannot be compared to a square profile stick...

Also, contrary to what most people think, a prop spinning at an RPM lower than it needs to develop pitch-speed, will make much more drag than a stationary prop... It is for this reason that a dead-stick landing will be longer and at a lesser slope than a powered landing.

RE: Propeller Worms
 

If you talk about efficiency this way, please rethink what efficiency means:
(Power-out + loss)/(power-in) = 1 If you know how to define power out and power in, you know how to define prop efficiency. Thrust is only part of it, but it means nothing without speed. Speed all by itself also means nothing. Enter time as well to transfer work in power, and you are getting nearer.
No more talk about efficiency please. A boomerang is more efficient than a wooden stick, but that is all that can be said here, and still it has no relation whatsoever to static thrust.
An air brake does not need to be efficient, yet it can prevent your engine from spooling up. A plain stick will do that. The best prop with the best foil for lift to drag ratio will also act as an air brake, but it will provide more thrust than the stick does. Enter airfoil here. All airfoils which have a depth greater than their thickness, represent something better than a stick. Not more efficient as a brake, but with ability to provide something more than drag. So there must be a shape that provides excellent thrust at static conditions. Not very effective in flight perhaps, but it will have best efficiency (efficiency!!!) at low speeds. Other foils will have less thrust and more drag at static tests, but will have the efficiency at higher speeds.
That is why static tests are called useless by some. I think that depends on what you want to know.
I use the static thrust because it allows me to define the engine, and evaluate all the changes I want to make in lieu of tuning etc. Flight characteristics come close if the flight prediction is anywhere near OK. Not good, but just OK.
It still needs not a fistfull of props, but props near the prediction to get best flight characteristis.

If you staid with me this far, I hope you will never talk about efficiency of a prop again. All props are efficient under the right circumstances. Performance yes! That is a different story.

RE: Propeller Worms
 

Your logic looks pretty good to me! :)

RE: Propeller Worms
 

I'm almost sorry I opened my mouth. [:o] Almost. ;):D
Thanks Pe.

RE: Propeller Worms
 

Trouble maker! <sarcasm>

Jack

RE: Propeller Worms
 

I gave plenty of warning in my initial post. [sm=angel_smile.gif]