Quote:

ORIGINAL: DarZeelon

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.

Pé,


Would you care to say a few words regarding this issue?

The higer flexibility that has MA (black props ) compared with APC mean something related with performance ???

I have seen that a 11x8 MA produce "similar" load to the engine that a 11x7 APC , I think the big flexibility of the MA may twist the blades remaining pitch . But what if the material were changed to the same material wich has APC props ??? I suppose that fact will load more the engine = less RPM rigth ???

Probably another factor that could modify the performance is the total mass produced by the weight of each prop .

I have reduced the airfoil of APC props and making it light and this mod. make fly faster the airplanes , what is this efficiency or efficient ??? [&:]

The only way to make sure that every prop in a prediction program is accurate is to test each one and define a loading constant and static thrust constant, corrected for atmosphere. Then, you have to decide if one sample is enough or if you will test more than one of each to take into account manufacturing tolerances. That is probably more of an issue with wood props. This would be thousands of propellers and thousands of hours. I've considered doing it for props I use on a regular basis, but have not found the time to do so yet.

That is not so easy Dar, unless I have single engine results with different well known props, and the power curve that goes with that engine/muffler combo.
I might however specify the 11x6 separately with i.e. a prop cinstant of 1.3 like most wood props, and wait for comments to trickle back to me.

@ speedboy,
It is like christal ball gazing what flexibility will do to a prop under load. Most cambered foils tend to increase their AoA (pitch nose-up, increase pitch) when under aerodynamic load, and tuck in (flatten) when plane speed exceeds pitch speed by a certain margin. There is no saying what will happen to the prop at which speed, as centrifugal forces enter the play as well.
In general, the more rigid a prop, the better performance predictions will be, but the worse it is for your engine if a tip touches terra firma.

Pe while I have your attention. Can you explain what is the use of the "disc load" calculation? Please. [8D]

The propeller disk theorem sais that thrust is caused by Newtonian principles. The action is to cause a pressure difference between front and back of the disk, the reaction is to accelerate the air passing through the disk. The pressure difference is what causes the thrust.

That is in a perfect world. We live in a non-perfect universe, so the pressure difference will cause air to flow from the high pressure region to the low pressure region, where the air flow path is around the outer disk rim, but also through the inner hole where the prop turns too slow to sustain any difference.
This pressure difference is the disk load. Flow around the disk (tip vortex) will increase with disk load, and decrease the specific thrust per amount of power applied to the prop.

Um ,,,,,,,,,OK. [sm=72_72.gif]
So how can I use this information in choosing my next prop? Thanks Pe. Jeff

Hi all this has been an interesting thread and has got me thinking. I am going to build a new test bench to run a multitude of tests running a spare YS91fz with a variety of props in the same size range. The fuel will all be the same batch, and I will test for rpm, thrust and also air speed behind the prop, which will be in a nacelle type set up with a windspeed indicator. This could be quite interesting me thinks[sm=confused.gif] I dont know if it will actually prove anything specific as such but will at least give me something to do

Very interesting test. I can't wait to see the results

You can use the power calculator to check out a range of props you think are suitable, then read the required rpm for engine power. You need to juggle the parameters a bit until the power matches your engine.
OR, you can test a prop, measure rpm, and read the power that is required to turn that prop. That way you can define and draw your engine's power curve. Having that, you can easily find any prop/rpm combo that matches the curve.

Hi!
I'm not going to be unpolite here ...but why on earth test props by bench running them ?? Bench running or static test ain't gonna tell nothing. It's what happens in the air that counts. The only thing you will get from this is how well a prop will turn a certain prop or how bad it will turn a certain prop. it doesn't tell you how well this prop will function in the air!
Being a pylon racer (and scale builder), !'m used to carving and modifing my props...but I test them in the air ...not using a calculator program or such and not bench running anything.
It's a well known fact by all competitors and experience fliers world wide that APC makes good props...better than most other propellers, but not perfect in any way (if there is such a prop ). But to have a better performing prop you can change it's shape. You can do this too all plastic and wood props. If you know what too do you can have them perform better than the standard variety. But you must try them in the air to really see if they work as predicted.

I too have raced pylon when I was younger, and one thing I do know is when racing you look for a prop than can deliver at ungodly revs, which usually meant shaving c/f props to near breaking point and balancing etc. I fly imac now and I am looking for a whole different set of criteria in a prop's performance, namely one that will deliver thrust more than speed but will still spool up at a reasonable and consistant rate, and the blades on these props are usually wider than speed props. We all know the bottom line is in the air, but without an accurate way to get readings in the air we can only test on the ground/bench. At least we will have some data to make a selection from then

Jan, See post #6.
The calculator is for engine testing and comparing static load.
Tests have been done to evaluate plane speed, and almost always showed the plane to fly at pitch speed, so the calculator can also be used to predict plane speed.
Now, a racing plane is outside of the normal window, but still you should be able to get guidelines for the direction you have to move when carving that perfect prop that lurks inside the not so perfect one that is in your hands.
Of course flight is the ultimate proof.
But suppose you want to rework or blueprint your engine. Then you need to know where your gain was, and where your loss.
props are perfect air brakes for the engine, so you do not need an expensive engine test stand with torque measurements, as long as you keep a set of props for reference during the time of your engine rework.

Then you go up in the air with your improved engine, and match the prop to engine and plane for best speed.

Why? Do the air molecules know the difference?

MJD

no the air only feels the difference in the way the prop flex's

The MA Shimitar is made of a soft material that I believe will plane out at high rpms while the APC is made of a hard stiff material that will no doubt flex less under load. They don't "flex" much at all when they hit the ground that is for certain, mostly, they just break. DOH!!!!!!!!

The MA props are a lot softer and if you look carefully side on you can see the disc change shape as the revs build up, thats why a MA will register higher revs compared to an APC of the same size. The Apc also has a thicker cross section and is a little heavier

" a MA will register higher revs compared to an APC of the same size"

The other day I tested back to back a Master Airscrew 11X6 black to an APC 11X5 (cause that is what I had) and the MA went 13,500 while the APC went 13,200 despite the lower pitch.

"""I have reduced the airfoil of APC props and making it light and this mod. make fly faster the airplanes , what is this efficiency or efficient ??? ""
Scary is the term I would use. APC props are very sharp and when they slice through your fingers it really hurts for a long time. I wouldn't want any blades thrown at more "sensitive"[X(] parts of my anatomy.

no, that could be excruciationg [&:] I had enough c/f props from my pylon days self destruct, not a good feeling having a blade go whizzing close by. I am at the point where all I do to a prop is balance and deburr it. I now use mostly Mejzlik or bolly c/f props and have to do little of either to them

Safety glasses!! always a great idea. [8D]

full wrap arounds snug fitting and polarised [8D]

I crunched the numbers, and they agree if I keep the prop constant the same, but correct for pitch.
That shows that the MA has less effective pitch than the APC.
MA will pull very slightly harder, but APC will fly a lot faster.

For some reason I just knew Pe and Dar couldn't resist this thread.[8D]

Well Pe knows what he's talking about............................................. ............