I talked a buddy into testing one of my props on his plane the other day. We both have Tiger60s with OS75AXs on them. I tested a small mountain of props on mine and fly it with a 3 blade now. Best all round performance plus it's a bit quiter. His Tiger was distinctly loud for any model, and the goal was to actually see through a back to back test how much quieter the 3blade could make his plane.

After the two test flights, I was pulling my prop and he wanted to see the specs on it. When he saw what was written/molded on one blade he was really puzzled.

I started to explain, but as I usually do, I went a bit too, shall I say, wordy. Yeah, sometimes I carry on a bit too much...... When I got to the description covering the efficiency and performance of props as a function of their aspect ratio, I think I lost him. Or maybe it was that it'd gotten dark and we'd both missed supper......

What was hard to explain was why a prop with 8-pitch printed on it landed like a 4" pitch and had a top end like a 6" pitch and climbed like it had a constant speed hub (like fullscale's have).

Anybody care to try and put that into 25 words or less?

He saw and personally experienced what that 8" pitch prop did. It "flew in the face" of all the conventional wisdom he's absorbed from some of our field experts in the one summer he's been with us. And almost every bit of those "rules of thumb" were trashed by what he experienced. I tried to tell him that in theory, practice and theory are the same, but in practice usually aren't. But the problem with that is the 3blade in question actually DID perform within theory too.

25 words or less?????

The numbers mean the mfg had an ink stamp-really
blade width and tip shape are also arbitrary.

You must match engine/prop/ model
period

da Rock,

In days of old when knights were bold I flew EC-121's for the Air Force for several years. The airplanes were powered by four Pratt & Whitney 3350's equipped with Hamilton Standard constant speed props.

Your going to have to do some explaining to convince me how a fixed pitch prop with an approximate pitch of 8" is going to perform like a constant speed prop exhibiting a pitch of 4" and a 6" pitch and not like an 8" fixed pitch. HMMMMMMMMMMMMMM!

Sorry, but I'm throwing the hook on this one not biting, ain't possible.................... but I'm willing to listen. Please enlighten this crusty old fossil of an Air Force pilot how that works and how a 8" fixed pitched prop exhibits all those variable pitch charcteristics.

I think what DaRock was trying to explain was the concept of advance ratio. That is the ratio of RPM to forward speed.

There is an optimum ratio where the angle of attack is the same across the whole blade. At all other points, some parts of the propeller are less than optimum.
When the model flies at low airspeed, high RPM, you get thrust. When the model flies at high airspeed, low RPM, the prop makes drag. You can still get speed from a low pitch prop, provided the engine is capable of high RPM, which is I think what Dick is saying (match engine/prop/model).

The other myth that still propegates from the 1920s is that less blades is more efficient. The thing that determines the effective aspect ratio of a propeller is the diameter, not the physical dimensions of the blade itself. It was discovered in the 40s sometime that more blades is more efficient because the losses occur between the blades. Look at a jet engine compressor. The more blades you add, and/or the larger the diameter, the thinner the blades becomes and so the limiting factor is structural (and ground clearance). So, if a two blade diameter prop is more efficient than a 3 blade, it's probably because the diameter is larger.

What daRock was trying to do was see if anyone wanted to explain the observed performance of a prop that was labeled as an 8" pitch, but landed at a walking pace, and climbed like it was a 3D prop."

I know why. And know it happens all the time. But my explanations always go on for hours and I was hoping someone who had observed the same performance from model props would take a stab at explaining why model props don't always act like their labeling suggests they should.

Dick did way better than I do, but was a bit "concise" for something my buddy might understand. And wellss did really good. I was just hoping for a few more is all.

Anybody want to be closer to 25 words?

da Rock,

Right now I'm playing with pretty wide range of props on a OS .46AX hung on a Kaos 40 size ARF I intend on flying in a senior pattern event here the first of the month. I've tried everything from a 11X6, 11X7, 12X4, and now I'm trying a 12X5. I liked the climb thrust and slower speed of the 12X4, not that the 11X7 doesn't perform well, just a little quick between maneuvers and turnarounds w/still plenty of vertical. What I didn't like about the 12X4 was the braking when the power is reduced, the airplane just stops making landing approaches a bit more critical for good landing scores. The Kaos doesn't penetrate all that well with its wing design. We'll see if the 12X5 is a good compromise. Even with the lower pitches there is plenty of speed still keeping the rpm somewhere around 12K on the tach on a static runup.

wellss,

Wasn't sure what da Rock was looking to explain as there is a definite difference between fixed and constant speed performance. Nice explanation

wellss, I have to disagree with you on the more blades,the better. A lot of tests were done in the late 40's and early 50's on prop efficiency on a typical prop plane (before jets or ducted fans were available), primarily by the Canadians. All test showed that even a single bladed prop (counter balanced of course) was more efficient than a two bladed prop and that two bladed props were more efficient than three bladed props. The only reason that many of the military planes used 3 bladed (and some 5 bladed) props was to get more ground clearance without having exccesively long landing gear (a problem for retracts and strength) . The three bladed props could absorb more engine power with the alloted ground clearance problems than a two bladed prop could. The consensus was that the more blades, the more turbulent the air the following blades were in thus less efficient.

I thot 25 words were the limit for the daffynition.
But - to slightly expand-
the power curve of the engine/motor,will affect all of this.
drama tick ally.
My foray into electric power has shown this .
The props can be accelerated or "decellerated " -almost instantly . so take off "power is instant and braking power can be predicted.
I realize this throws more bull fodder into the question.
EXAMPLE- my ERATIX uses a 14x 7 APC weighs 3.75 lbs
take off thrust is akin to a slingshot
ditto for in air acceleration yet speed at full power is at structural "yellow line" in a heartbeat. for landings you can drop it in at 10 mph
obviously the total package mix is good.

Yeah, actually the efficiency trade off on number of blades really doesn't have any real value with most fullscales.

Those 2,000 hp fighter engines basically required pretty much a set load bearing capability from the prop, no matter how many blades on the sucker. And their clearance decided how that blade area was divided up.

In fact, if they had put all the blade area that could be swung into just two blades, the aspect ratio of those fat suckers would have hurt the efficiency of each blade measurably. So they played with airfoil a bit and went with more blades. When the power improved even more, they went with even more. And actually got better efficiency out of each blade but lost efficiency with the increased number of blade tips and airflow disruption.

With our model props, the 2blades we compare to the 3blades that are available differ so greatly in blade area and aspect ratio and a few others, that we actually aren't comparing apples to apples. Differences in efficiency that exist in theory are lost in the major practical differences in the props themselves with our models.

But why a prop that says it's an 8" pitch flies like a flatter one going slow and a variable one when going vertical is a different thing all together.

Heck, I'd say it actually points through a fair amount of the fodder.

One reason our models fly the way they do IS the engine's power curve for sure. I'm voting for at least one of your candidates.

Here is a site which will tell you all you want to know (well, maybe not).

http://www.supercoolprops.com/articles/pitchcompare.php

This is a case whre the POWERPLANT determines the prop specification
these setups are almost exclusively with engines which have a usable rpm range of 2000rpm.at best
My electric has a usable rpm range of 10000 rpm
most model IC setups have a usable rpm range of 50% of their rated max rpm .
The electric has max torque at zero rpm and if you ar a bit of a electric mods guy you can go from high amps to high volts to maximize power
The Killa Cycle motor cycle does just that
They start at max amps and once into higher speed shift from low voltage to high voltage
sorta like takeoff at low pitch and then shifting to high pitch on a prop.
In both cases the object is to keep the prop or motor working

A buddy of mine re-opened the old Y&O prop line some years back. As part of this he wanted to know why some props work and some are only good for stirring coffee. He found that much like Dick mentions in an early reply that most props are numbered because they are a little flatter than the big one but coarser than the flat one and I happen to be holding this number stamp so I think I'll use it.... OK, it's not THAT bad but from testing a lot of props he did find that the numbers stamped or molded in typically only describe the prop for a rather small portion of the blade. APC's were better than most on this account with most of the rapid reduction in true pitch only occuring in the first 1/4 out from the hub and a little reduction in pitch as washout in the last part of the tips.

Other than that all I see here is folks pretty much agreeing that it's the match to the engine and that electrics have changed the face of the issue pretty much completely.

One tool that will show the progression of "pitch" from hub to tip is a prop pitch gauge. All props are pretty much just wings. But the prop see air a bit differently than wings do. In fact, the prop sees airspeed and angle of attack differently from hub to tip because each portion sees different speeds from hub to tip. A wing sees much less speed change from root to tip, and much less AOA difference from root to tip than a prop sees. A pitch gauge shows you that the angle of the airfoil on a prop is extreme near the hub, and by comparison, almost flat at the tip. Why is that? Shouldn't a prop have the pitch that's written on the blade from hub to tip?

The speed the prop sees at the hub compared to the speed at the tip shows an awesome difference. And the speed at each stage on the prop is really the speed the airfoil sees at that stage. So prop designers figure out what speed the hub is going to see and using the expected rpm the blade will turn, they work out what speeds will be all the way to the tip. And then they set the angle for the airfoils all the way to the tip. The idea is to optimize those angles to the airfoil that's present at each stage to the airspeed each airfoil will see.

In fact each portion of the blade usually has a different airfoil, a different chord, a different AOA expected, a different airspeed expected. There really isn't a "pitch" angle. Each portion of the blade is expected to lift about what the pitch says it'll lift. Otherwise those portions hurt or help the efficiency of the prop to produce the advertised pitch.

The majority of thrust (lift) from a prop comes from the blade that's about half to 3/4 out the blade. The tip winds up giving less because it doesn't have more blade beyond to help keep it working. The hub end winds up doing less because it's usually got some airplane directly behind it and it's got to be a really thick profile and some other things. So if you think that looking at the angle of the blade shows you the pitch, then look about 2/3 to 3/4 out for that.

Props aren't actually screws. A screw's pitch can be measured almost exactly from the threads because the threads are going to cut their way and mechanically pull through a rigid medium. Props have to fly through their medium. And a screw's theads see the same "airspeed" (could we say, "woodspeed" or such? grin) from the point to the head lengthwise and from "root to tip" on the thread. A prop has to produce it's pitch with aerodynamics. And the angles we see from hub to tip on a prop might have a portion with an angle that matches up to something that if duplicated on a screw would match the printed pitch. But screws and props aren't the same animals.

Some mfg's actually use less angle and less airfoil toward their tips than their competitors do. It might give their props a couple hundred rpm better on the ground for example. And they know modelers think rpm on the ground means more than it does. So their props might have a competitive sales advantage. Props aren't simple sticks for sure.

BTW, some people call the angle on a prop the Angle of Incidence. It sort of suggests the true value/purpose of the twist you see change along the blade.

Some mfg's also offer combined pitch props. I think they actually call them "two pitch" or something like that, but truth is, any short name would be scientifically wrong. But the idea they're trying to convey, and to sell, is that there is more or less pitch to those props toward the tips. Truth is, there always is.

But the existence of those props with the funny names illustrates just how different design can produce better performance in specific performance envelopes. Those 12x6-8 sticks are supposed to do special in certain ways, aren't they. Do we see them landing at the speed we've come to expect from a 7" pitch (average of the example 6-8) or not. Heck, who has thought to look. (ok, I have for one)....... Nonetheless, they are gentle reminders that the pitch written on our props is pretty much just advertising and the mfg's guess.

Hey Bruce, would you say that electrics have actually made choosing the appropriate prop less easy? IC engines don't have the "design rpm" thing. So if you've got too little fan area, pitch, blade area, etc the IC engine will show you by turning too high an rpm. The engine screams along and the airplane flies like it's got less power. But with an electric motor, when it's prop hits the design speed, the motor sees no need to turn faster and won't. And the pilot doesn't hear anything that'd tell him he ought to swap on a bigger/pitchier/wider bladed prop?

My short time with electrics has shown me a couple of times that it's less easy to match a prop to the motor/airframe because half the clues don't exist. And more than one of my flying buddies who do mostly electrics use meters to fill in some of the missing clues.

And the motors ability to change their power with more cells (more voltage) messes up the prop choices even more. A buddy just ruined a motor because he saw that motor flying with a much different prop than he was using. The model he saw was bigtime faster and didn't have an anemic climb like his. Well, heck, same motor better performance with appreciably more prop, so his much be hitting that rpm thing. He swapped on the better prop and I don't remember if he burned up the motor or the ESC or both. But the other guy had been using a battery with a higher cell count and electric motors change their "size" when that happens. My buddy went around the rest of that day mumbling something about "our 46 engines don't become 60s when we put in a larger fuel tank"........ or somesuch....... (he used simpler, ruder terms

Dick, it looks like your Killa Cycle shows the "POWERPLANT determining the prop" AND shows that with our electrics, the battery/voltage also has a great deal to do with determining it too.

Our models don't have the chip controls the Killa's do, but if we swap a 4-cell battery into a model that'd been flying with a 2-cell, the prop sees the world as a new man..... uh, an entirely different motor.

Looks like we're agreeing on stuff again. I gotta quit doing that, it looks like collusion.

More fodder
The combo pitch approach,allowed the mfgr to start with a thinner blank of wood which saves munney.
From a strength standpoint it leaves a lot to be desired.
Being a snoopy person - I once tried to figure out the relationship of power to thrust and how prop types and pitch affected this relationship.
My thrust stand obviously only gives results of a non moving vehicle.
My fiddlin around showed that watts n thrust were directly related.
the prop shape and sizes meant very little- ok, nuthin.
To restate:
"if'n the motor/engine produces x power, the thrust will also be x
So 2x power should make 2x thrust . and it does
Don't throw any text books at me I am no good at math and can barely read.
try the experiment yourself.
Why should this work?
Because the prop is simply a load and the load and power to turn the load are directly related. again - don't believe it?
try the experiment
The test would work on a moving vehicle but my test bench won't fly.
The windy tunnel results will give results too.
obviously
the prop efficiencies will show up but basically to go faster -just stuff in more power.
On my electrics -this imakes it easy to figure power.
Just measure watts.
Mamma nature will give you the answers.
PS the test stand was my design from a few years back since then someone elsemade it commercially availble! It is a handy gadget and really quite accurate with electric motors.
It will not work with IC engines as power output is measured by prop load.
The cart n horsy thing.

.

I think Da Rock's explaination of pitch distribution along the blade is quite optomistic. The usual best is to make a helical pitch prop, where the pitch at any station gives the same forward "screw motion" as any other. And yes people put wash in or wash out to make things better, they hope.

hope in one hand and **** in the other

Having blown way too much cash and also doing prop evaluations for a few companies- The entire prop market is pretty much smoke n mirrors
Except
The carbon fibre props keep their shape at high power loads and therefor are more accurate and stable which allows the motor/engine to rev to full potential.
The APC's really are in a class by themselves when it comes to matching a particiular job. and a good price.
They make testing sooo much easier.

Put aside personal feelings about wide blade vs narrow blade - most of these old observations are invalid. It is all about load and the speed where the max possible motor/engine POWER is desired
fo example:
To go fast I prop up to where max watts hit at about the speed I am shooting for
then -ready for this?
I hope I guessed right.
Then I change props and try again
Anyone who thinks they can accurately compute the speed and load is kidding themselves
They may (read that MAYBE), get in the right ballpark but that's all.
You want to go fast ?
here is the secret formula -don't tell the aerodynamicists , some may feel left out
Get as much power as possible in the smallest lightest model possible
then shape it as well as you can to make it slippery
All in that order.

Right on Dick!

Watts n thrust are indeed related but prop shape and size mean a lot regarding Power/Thrust ratio.

A smaller prop requires more power to produce the same thrust as a larger one.

For example, a 12x7 prop takes about 85W to produce 27 oz of thrust at 6000 RPM.
To produce the same thrust a 6x5 prop needs about 195W at 18,000 RPM.

Thats the rub.. All the tweaking and tuning and taching doesn't mean squat as soon as the plane begins to roll down the runway. I'm frustrated that the IC engine manufacturers will not publish torque/power curves for their engines (I think Fuji does but thats it). We can't even iterate like Dick described because we don't know where to target for maximum power at target airspeed.

If we had a reliable engine dynamometer we could plot torque/power vs. rpm. With that information we might then "estimate" the rpm's difference between static and maximum airspeed. (On board rpm readings in-flight would make this a piece of cake and cheaper than renting windtunnel time!)

As it stands now, the glow guys have to iterate between at least 3 different unknowns (power curve, rpm, airspeed) to get at maximum speed in the air - which is darn near impossible.

da Rock's explanation was more an observation of what you'll see on any prop and restatement of how the disign is done. And the idea is that any station along the prop is often designed to give the same forward motion as any other. We both seem to be saying about the same thing. so???????????

What seems to be obvious from what everyone has said, including me, is that there is a tremendously complex set of variables concerning a props performance. And it seems to be the observation of quite a few that there really isn't any scientific way to apply what we can buy to what we have on our models.

And Cruncher gave a pretty decent explanation of why that is. Can you imagine what a dyno would cost, and what it'd be like if it was affordable.

OK, proove it-
I knew someone would drag in the theory on small n large in huge varuiations etc..
BUT I will stick to my original comment.
On my test stand which would swing up to 13" props only I could fus with all kinds of blades in roughly similiar diameters and pitch /etc., meant nuthin
In this case I saw a VERY similiar relationship of power input and the output in thrust.
Have you ever tested this?
Do you think I make this stuf up?
I actually TEST for it.
Since I can't read I have to rely on observed data.
Also- the differences in sizes you noted were huge -a completely different EFFICIENCY comparison.