PiccoLino

I was in another room of this forum talking about missing RPM of my gas engine. Then the prop subject came up and I was asked which brand of prop I fly with.
I am not from Europe so that I had to mention a prop brand (TEAM MZ: woodden, large size) which is not available in the US.
Then I got stuck to this question and I internet checked the web pages of these woodden prop manufactures.
Where the difference is?
What can make a prop better than another?
Aren't all of them CNC machine manufactured?
If so, what does it take to check the blade airfoil of the best perfoming ones and copy them out like a xerox?

da Rock

One better than another?
It's better suited to the airplane and engine that you have bolted it on.

CNC machined?
Probably not. Wood has been shaped by tools that predate CNC by centuries. Wood props are cut out by tools that were designed to mass produce the rifle stocks for Napoleon's armies.
And plastic props are cast in molds.

If so, what does it take to check the blade airfoil of the best perfoming ones and copy them out like a xerox?
The airfoil isn't the only design feature that matters. And the planform isn't all of it. The pitch changes as you move toward the tip. A 6" pitch prop might be 6" pitch only a few stations along the span. The pitch near the tip might be greater or less than in the middle of the span. It takes a tremendous amount of checking to duplicate all the important features and have them in the correct locations.

rmh

The theories on blade shape --are carry overs from the old theories on wing shapes -and about as screwed up.
the idea that blade shapes "lift' like the old wing cross sections is . pure and simple - outdated n non relevant .
The high revving small props we use are curved - to add strength- the pitch -if all was by the book would, at any station- advance exactly 6"in one full revolution -
as noted --tain't so
The prop also has to match the task- - the best prop on a setup -say for a very small diameter fuselage on a light clean airframe -may be a dud on a large diameter cowled engine .

large slow moving props such as huge 14" fans -are likely more influenced by the curves.
For our uses - the thinnest blades which still remain rigid - work the best .
the advantage goes to a properly designed carbon fibre or filled Nylon blade.

drela

I've measured, analyzed, and tested many props. The thing which has by far the most impact on propeller efficiency is its blade airfoils, and the blade airfoils on the majority of props are simply awful. The most common flaw is that they are too thick, especially on the smaller props under 10", say. Another common flaw is simply a bad airfoil shape for the operating blade chord Reynolds number. I've found that the airfoils on APC props are the best, which is the main reason why APC props tend to be good. Even some fancy and expensive molded carbon props are inferior in this regard.

The attached drawing shows how I measure prop blade airfoils, using fine solder. I digitize the blown-up airfoil plots by hand, mainly because I find it convenient and adequately fast. However, there are automated methods for doing this, e.g. the bitmap analyzer in Profili2.

After the airfoils, the next most important feature of commercial props is the correct amount of blade area. Too little area will cause partial blade stall, and too much area will produce excessive blade profile drag --- the efficiency suffers either way. The thrust requirement also factors in, of course. However, all else being equal, more pitch requires more blade area, and vice versa. Most prop manufacturers do not heed this basic rule, and often only one prop in a range of pitches has the optimal blade area. The higher pitches have too little area, and the lower pitches have too much.

Finally, the next most important feature is the blade angle (e.g. pitch) distribution along the blade. This is important, but not as much as most people assume. Also, the "constant pitch" rule is just an approximation -- the best prop does not necessarily have constant pitch along the blade. This is especially true if the blade airfoil varies along the radius, as it does for most props. In this case, the geometric pitch, measured from the blade bottom surface, is not very relevant. What matters most is the aerodynamic pitch, which is measured from the airfoil zero-lift lines. Measuring the aerodynamic pitch requires measuring the blade airfoils, as in the attached GIF.

HighPlains

My Feedback: (1)

The thickness of props is due to the requirements of structure strength. The rest depend upon the application and model using the prop.

You might have really enjoyed the old Formula One pylon event Mark, as there was a lot of prop carving and experimentation. Thinning the blades gave more rpm, but too thin and it fluttered. Finding the right pitch distribution to match the airframe, and working out what pitch and diameter gave the best acceleration without killing the top end speed. Usually in that application with a very clean model having the hub about 1/2" lower pitch than the tips seemed to work best.

drela

Yes and no. The stresses rapidly go to zero at the tip, so the airfoil thickness can rapidly decrease.
But yet all the "bad" props have the crummy thick airfoils all the way to the tip.
Not only is this bad for aero, but the extra weight of the thick tips actually increases the stresses on the inner parts of the blade.

If flutter is a problem, the trick is to get the ideal distribution of airfoil thickness. This is typically very thin airfoils at the tip (say 5-6%), gradually increasing in % thickness towards the root to prevent flutter. Flutter is also suppressed by putting the max airfoil thickness well forward, like 20-25% chord, and making the TE as thin and light as possible. This incidentally is also the ideal low-Re airfoil shape.
Most commecial prop blade airfoils have excessively thick TEs, evidently for ease of manufacturing.

HighPlains

My Feedback: (1)

That's pretty much where everyone ended up. One of our problems was that we were using a natural material (maple), so repeatablitiy was always an issue.

Prop carving machines are limited to simple shapes, which creates a problem due to the variation in blade chord unless you have multipath and pass through the cutter. CNC equipment to carve wood, is too slow for the average application to be cost effective, and plastics have their own problems.

rmh

Look at APC- the thickness (% of chord ) ,make them almost flat - just thick enougn at high point to hold a rigid shape
I run/ran lots of these - in sizes from 8x4 to 20x8- also carbon fibre such as Bolly and now ZM / Mejzlic ans still some old wood paddles
The "airfoil" shape is pretty hard to equate with the pictures attached by Mark Drela.
I have modified lots of wood stuff - simply thinning to limits of integrity --swept edges on and on -
My tests -- were checked by using a thrust stand and an electric motor and measuring watts consumed vs thrust produced
The ONLY valid notation ?
thrust was always relative to watts consumed - minor variations but basically that's it -
blade thickness , high point, on an on, no noticable difference -
If any bowing or flexing was seen - the thrust relative to watts went down . I used stacked four blade setup wide n short and long and narrow - same results - thrust was aligned with power .
Sure - some small differences - but the airfoil? other than making the prop stiff - did not see where it was relevant.
This is similar to when I got into foamies - in the size n weight - the flat wing was as good as it got. There was a lot of theorizing that there must be an airfoil which was superior - turns out in the size and weight and speed the flat plate was the best -

allanflowers

Hanson>>My tests -- were checked by using a thrust stand and an electric motor and measuring watts consumed vs thrust produced
The ONLY valid notation ?
-------------------------------------------
The only valid notation? You've got to be kidding. Your tests of the various props - all running stalled - is not going to show up any differences in section at all. That is a unvalid testing procedure that you used.

rmh

OK - what have you tested and used?
How did you prove your results?
theory is one thing
results are quite another .

drela

That's because the airfoil on the drawing is a cartoon. It's there only to show the procedure.

allanflowers

Dick,
If I didn't have a proper test setup, I wouldn't even try because it would be meaningless. And for sure, I wouldn't present my bogus test results as being anything worth reading.
I have spent months of manhours in windtunnels on auto testing and am familiar with what procedures are scientifically valid and what aren't. That ain't THEORY, that's emperical testing based on valid aerodynamic PRINCIPLES, guided at every step by leading aero-engineers and scientists. That involves dozens of tests in tunnels all over the world, including some of the most advanced.
How much tunnel time do you have?

rmh

I don't have tunnel vision---- and your tunnel tests were totally unrelated to small propellers.
propellers are always somewhat stalled - or --they would not be performing any work.
I ran the props with a speed control and watched power consumprion from ZERO rpm as the thrust stand displayed the thrust applied
Quite frankly -being so well informed, why don't you test some props before you comment about on things with which you are not familiar

drela

Attached is some data for two props:
APC7x4 slowflyer
FK7x4 yellow nylon

The data was obtained in a thrust/torque test stand, operated in a wind tunnel.
The data is for the full range of airspeeds, from zero (static), to near pitch speed where
the thrust falls off to zero. The thrust for a given advance ratio is nearly the same, as
might be expected from the same pitch and blade area of the two props. But the FK
prop requires almost 50% more power, as can be also seen by looking at the efficiency
curves.

Because the FK's larger power persists all the way to zero thrust (pitch speed),
the difference must be due to the blade airfoils, i.e. the FK's larger blade profile drag
is chewing up the additional power. Comparing the blade airfoils at 80% radius,
we see that the FK's airfoil is way too thick for this Reynolds number, but the APC's
thin cambered airfoil is pretty good.

So it's pretty clear that blade airfoils do matter, at least in this example.

allanflowers

Mark,
You da man!
(Even though you also must have "tunnel vision" too )

Tall Paul

I sawed a Zinger 18x6 at one inch increments starting from the tip.
Looked like this..
tip, 7 inches, 6 inches, 5 inches, 4 inches from the hub

rmh

Drela - frankly I don't buy those tunnel results as meaningful Not calling you a liar - just don't see how this works unless the two props are wildly different
I use currently accepted blade shapes and a Astro Whatt meter and read output in grams
THRUST- was directly related to power- I never saw anything approaching 50% power difference -in power to thrust relationships a few % .
Do you guys fly these setups? I do and the flight tests were right in line with what my test stand showed - remember this is not a speed- in- flight type- test. These prove only the airframe/motor /prop combo as a unit.
Theory and wind tunnel stuf is great - then - you actually use the results and see if the info holds up -
As it stands - the thinnest blades which hold shape -are still th best blades -in actual use -I have seen. The "airfoil " is on the order of a few %- - go blow an hour in any good hobby shop and look at the APC electric prop selections also the GWS stuf
now measure the foils and draw a profile -
basically the curves and thickness are structural compromises .

Jim Thomerson

I did some prop testing on one of my airplane/engine combinations and had some surprises. One thing I did was take an old 11 x 5 Zinger "prop kit" and make a beautiful propeller out of it. It flew my airplane very well and I was most pleased. Then I made a mistake. I put on an untouched Zinger 11 x 5 prop kit and it preformed just as well as my special.[]

seanychen

My Feedback: (4)

Wow, it's an awesome thread, seeing how people from different fields bring their views to the table.

I do think airfoil shape matter, but I am not sure how much it matters, at which rpm range, and at what size range. My experience with RC glow engine sized props have been only with a tachometer & force gauge. I do have some wind tunnel and CFD experience from working at an Automotive OEM company (1 out of the 3 in Detroit). From that job I learned the single most culprit that robs power is the turbulence behind the vehicle. If you have ever looked at the vortex region behind a truck, you know why a truck's Coefficient of Drag is so poor.

I think most popular props on the market, airfoils themselves don't vary drastically to a point that by itself makes up the main difference in performance. Rather, 2 other factors matter more:

1) TE bluntness: wood props like Zinger have blunt trailing edge, which they advertise for finger safety. Theoretically, a blunt trailing edge in any airfoil causes turbulence and robs energy. This can arguably be an "airfoil" factor, but in a very remote sense.

2) Tip profile: swept tip (APC) versus square tip (MA black) cause some efficiency, because I think swept tip allows a smoother boundary layer from inside the prop wash perimeter to outside this perimeter. Therefore, less turbulence at this boundary layer results in lesser energy lost, so more of engine energy turns into thrust.

Then

3) Airfoil characteristics such as thickness to chord ratio can make some difference, but unless somebody makes a really thick prop (I don't know of any), minor thickness difference don't make up noticeable differences. This trait mainly trade RPM for lift. So while a thicker prop may swing slower peak rpm, the left it generates at that rpm may very well match the that of thinner prop at higher rpm

4) Airfoil size VS prop diameter (wide vs narrow) can make some difference, but is more subject to airplane type and engine power band. Sometimes it is better to use a narrow chord but increase the diameter to get greater thrust.


The most efficient prop I've ever come across is Bolly carbon fiber prop 19x6. My YS 140 DZ swings it at 10200 rpm, generating 21 lb. of static thrust. It has sharp trailing edge, swept tip, thin airfoil, and narrow chord. However, it only lasted until the first bad landing.



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In summary, I think prop airfoil does not affect performance as much as wing airfoil, because it mainly trades prop rpm with prop lift, which are inversely proportional with a given engine.

When I look at the prop efficiency from a total energy perspective, it helpfs to look at what energy is lost that's not translated to lift.

Energy In = Energy Work + Energy Lost

I don't think turbulence translates into lift. Thus, turbulence is a good judging factor for efficiency. That's what led me to intepret that TE bluntness and prop tip profile are the main contributing factors for prop efficiency.

BMatthews

A number of years back when a buddy and I got into the electrics deal we set up a current and thrust measuring setup to test for thrust to amps (basically watts since we were using a set battery pack). We tested a whole lotta props and modified a lot of them based on our rubber model free flight background and learned a lot.

First off other than lower pitches that applied to slower climbing flight I don't think static testing is really valid. But we were after powered gliders at the time so we felt that it was at least valid as a comparison. For the most part our later flight testing backed it up but some props that were on the edge proved to be better in the air.

Zingers looked like baseball bats....... but oddly enough after modifying about 10 of them to thin, undercamber or "APC" them in various combinations we found that the best we could improve them was about 5 to 8% in terms of upping the thrust/amp ratio. A finding that amazed us as we expected a lot more improvement.

We also found that thinning the airfoil generally resulted in the blades more easily stalling. But as long as it wasn't excessive these thinner profiles often turned out to be the ones that worked better in the air with motion to reduce the angle of attack. Not an unexpected finding at all.

We learned enough with our shots in the dark that I don't doubt that what Mark has said has some validity even in the real world. It explains why props with the same nominal diameter and pitch behave differently in use. And you must admit that there IS a difference in how one prop and the other works on any given model. It may be blade area or pitch distribution or shape but they all add up to give that last 5 to 10% that we see as the last "kick in the pants" to a model's performance.

For the most part I've seen the best performance out of APC's be it on gas models or electrics. But every so often some other brand just matches one of my models better so I don't discount any of them until I can fly them.

HighPlains

My Feedback: (1)

Mark,

Do you have any of the papers by Eugene Larabee on prop theory and design available?

rmh

I really don't hate wind tunnels - They are important and useful TOOLS.
The resultant info simply -is not the final answer - but tunnel info is a great starting point
Applied knowledge is the only knowledge which really matters .
Not trying to be nasty -
On cars - I have been a performance car guy since I could push the old 33 Dodge out at night and steal an hour of furtive fun -that mom did not discover. The aerodynamics of cars n trucks has been an ongoing thing - and tunnel workups are very helpful .
Much of what we now see is simply old hat to racers from 50 years ago - who looked for dust patterns on cars and tried setting bodies at different angles and rivited panels to underside and front of cars and taped off door seams..
Been there ----
Forgive me if I don't get too excited at the "tunnel results"

PiccoLino

"props with the same nominal diameter and pitch behave differently......".
That is precisely what I meant and I wondered why the props manufactures, who are surely aware of all the theories and calculations that some of you have mentioned in this thread, have not found out the best solution yet.
People at Zinger, Top Flite, Metz, Team MZ, ect... know very well how modelers are keen on the performance of their engines, however the ultimate prop is not on the shelves yet.
Why?
In spite of wind tunnel tests, mathematics, formulas, wattmeters and whatsoever, in designing props there must be a combination of practical factors that binds the manufactures to standard solutions, otherwise the matter would be simple.
In short it sounds like it is not possible to produce and sell, at a reasonable price, the ideal prop.
Am I right?

rmh

shorter yet - they do
(Ideal is a perception -not a finite quantity)

allanflowers

You really are out of date on car aerodynamics if you think that this stuff is "simply old hat to racers from 50 years ago". Having been involved with this area for many years and having worked with the best in the business, I've got to say your statement is so absurd as to totally disqualify you as having any credibility in that area. Which is fine for most people in the world although most people don't try to pose as experts.