When all is said and done, get various different propellers and give them a try. You may be surprised at which one turns out to work best in your particular application.

You know, some people seem baffled why our airfoils can be flat plates and work so well. And some seem to think that because so many almost flat airfoils work on our planes that the same would be true of our props.

It's actually very obvious why plate airfoils work. Same reason symmetrical ones with almost no max thickness (or WITH no thickness at all) work. The wing loadings our models produce is so far under what full scale sees, it isn't funny. Max coefficient of lift is a function of airfoil thickness. When you got virtually no payload, you don't need no stinking CL, much less what would be called "max". So our models don't need appreciable lift. And some don't need appreciable structural strength. Foamies weigh comparitively nothing and not virtually or comparitively but really don't need max CL or max strength.

But don't confuse how the wings can be with how the props can be.

Fact is, our engines produce finite power, gas, glow, or electric. And the airframes wind up producing drag that fits that power (or vice versa, depending on how you look at it). So we wind up with demands on our props that our wings will never have a hope of seeing. Unless we start building with wrought iron. Our structural materials also give our airframes some advantage in the payload area, btw.

And our props don't see the Reynolds our airplanes do. Airspeed is a major player in Reynolds effect. And our props can see all the way up to the speed of sound. So don't assume that the blade's chord or thickness is not doing something. Or it's width/chord. Want to see a prop that just might owe it's somewhat outstanding performance to either it's chord or wing area? Test a couple of Graupners if you can find them. Or just ask MinnFlyer what he thinks of 'em. They prove that chord and/or area means something with our models. And they might also prove that maybe some of the other prop mfgs have missed a trick or two. Or shaped their blades to catch this market's eye. But not seeing the same Reynolds isn't exactly a positive. Or really a negative. It just puts our props into a somewhat different world. For this discussion it really just backs up the idea that we can't cut out a foamboard prop and fly an IMAC whale with it.

We really shouldn't be to quick to blow off things we think aren't needed for model props. At least blow 'em off as a general rule for all model props. Chord doesn't matter if something else takes it's place. Truly thin blade thickness does work, but not over the whole span. And we don't have matching props with thicker blades to compare. Truth is, even the mfg's probably didn't.

The guys who "carved" their own back in the day really did learn a lot about props. Control Line in particular is one of the best, most accurate test setups anyone could hope for. Our speed planes used that to huge advantage, but thousands of guys racing rats did too. And the precision aerobatics boys weren't exactly wandering about in a daze. Both the rat and stunt guys discovered that "wing area" really did matter. But it had to be balanced with diameter. You USUALLY got your rpm range by adjusting diameter. And you didn't do it by buying the next size prop down at the LHS. (There were lots more LHSs back then too.) That next longer or shorter one was so far different it wasn't even funny. Why? Because the "wing area" was a different world as were so many other things. Experienced stunters could change their performance by changing radius by 1/8". And by thinning a half mm. And they could prove it. With a watch for one. Lap times and run time. (Are those affected by the "weather"? Yeah, and some of us recorded temp/pressure/humidity and worked from standard day.) They called Precision Aerobatics that for a reason. And it was more than just how the maneuvers were flown. And racers put them to shame. At least a few of the ones who won all the time did.

Lot's of guys took larger props and made smaller ones. You didn't just cut the diameter. You trued the bottom of the airfoil on a pitch gauge. Trued the top profile with a set of pattern forms that you'd made yourself, while at the same time setting whatever thicknesses you wanted. And there are a few other details..... Why did we do that? It was fun for one thing. And you learned who was BSing about his props and who wasn't. And you might win more often. But mostly it added to the fun. For them what did it.

Something that most learned along with all those details? Ain't no magic prop design or mfg.

I have used some props in 30 and 34" diameters on up to 20 hp motors ( ZDZ 160 and 210 to be specific)
the rated pitch on the best ones is 14". the 45 lb model on th 210 pulls vertically from a stop at 1/2 throttle and about 1/2 max rpm (well a bit more )
Watching the performance of the relatively narrow and almost symmetrical foil blades is an eye opener. the best ones - have extremely thick hub and a very pronounced twist right from the hub
These pull like a tractor even at low rpm ,yet provide excellent speed.
The material is one reason - all carbon fibre,
hollow for weight considerations and stiff as possible.
The "Clark Y" shape is really not used tho the backside is flatter than the face. These run in the 2000-6000rpm range
If you look at the blades (back a few posts) on my current selections of props for EFlite 15 thru 60 electric motors for 3d and some speed setups you see the extreme twist at base of many APC
These guys do have it figured out -based on results
A friend has some smaller ,excellent wooden props from Czech Republic which also work well The big props I mentioned are ZM which are also Czech prop designs
One thing we foundfor certain
the thin wooden props which flex when working, are for us-
worthless.
or worse

After Rock mentioned airspeed pitots I got to digging around and discovered a company called Eagle Tree. They have in-flight data recording or even real-time for such data as: airspeed, engine rpm, head temp, altitude, telemetry, and even acceleration. If ever you wanted to know what that static rpm translates to in the air this is the toy! I think you could also calculate prop efficiency, airframe drag, actual wingloads, etc......... I got to have one of these!

[link]http://www.eagletreesystems.com/[/link]

That's an interesting lineup of devices.

It looks like about $80 for the recorder and about $40 each for altitude and speed recorders. Setting up their software on you pc turns the stuff into decently useful data loggers.

I think it'd be worth the effort and for under $200, it looks like an entirely new dimension in prop testing. For years I've used a stopwatch to do "unlimited vertical performance" tests. (Never had it ever.) With this setup, those tests would be way more accurate. And if you were to add in the GPS locator stuff, and an accelerometer........ amazing............

That's one way
I will soon have my 526AFS Zlin ready -with a EFlite 60 (very soon just finishin it up) 7 lbs 750 squares here is a pic of customer's versions from years .
The $$$ will go into props and batteries . I will time it by flying it over our 600 ft runway.

If straight line speed is all that matters to ya' that's the cheapest way.

Neat thing about the devices are they'll give you the entire flight results. Wanna' know which prop was pulling best on the verticals without having to vote everybody's opinions? Wanna' know just how much each prop slows over the tops of loops?

Seems to me that the only limit on what it'll tell ya' is what you choose to ask it. For example, you could compare stall speeds. You'll have the speed graph right up to the point of the stall.... and beyond. Just one example there.

Will it be for everyone? Seems it oughta at least be for people who want to play with decently reliable data. Surely beats guesswork. But it's sorta like a tachometer. Or a wattmeter.

Sounds like a nice device .
I use wattmeters (my Whattmeter is extremely helpful.) and my ZDZ engines have on board recording tachs . both valuable devices. My little thrust stand also was a terrific help -My ability to quickly determine battery characterics originate with it. along with finding out how power in and thrust out relate.
Spektrum has a feedback system now for the race cars .

A properly designed and constructed prop will be effective through out the rpm range. APC is excellent for lower power (maybe up 5 HP). What these guys got right is blade area distribution and proper material choices for thin yet very stiff blades. They reduced area from the 70% station out to the tips, removing unecessary tip vortex amplitude which does nothing more than rob power to produce drag and noise.

I am doing the same thing with wood and refinishing with carbon cloth. Most wood props I have seen are just too thick for stiffness and have too large an area at the tips. I start with larger than needed and rework the blades to get better twist profile than the original, more closely resembling APCs. Depitching the tips of woodies is also desirable to unload the engine. A wood/carbon prop is a very good compromise and at a favorable weight to APC

Reccommended reading: Propellers, the first and final explanation by Jack Norris. At least it's reccommended if you are interested in theory, design and practice of why props do what they do. It's actually two books in one; the second he calls the Logic of flight, the thinking man's way to fly. The book is cheap considering what he gives you. We spend more on one prop. And BTW, the stuff I do to woodies I gleaned from Norris.

MattK