My plane is a lazy high wing floater with 5 foot wingspan. It has a brushless motor that can handle 35 amps continuous or 50 amps burst current. It has a 5-cell lipo. The largest diameter prop I can put on this plane is 13 inch. What I've been doing is to limit the current to 35 amps by setting the upper throttle endpoint lower than 100% so that I won't accidently over throttle it during flight. I've been very pleased with this setup, but I don't know if this is the most efficient combination.

I've heard to put the largest diameter prop the plane can handle considering ground clearance for the best efficiency but what about pitch?

If I change from a 13.6.5E to a 13x8E will the decrease in RPM while maintaning 35 amps be more efficient without loss of thrust for my lazy high wing floater? I don't fly this thing at full throttle all the time. All it takes is about half throttle to keep it flying comfortably.

I read some place that the most efficient prop was one which had a pitch of about half its diameter. I also have the general idea that large high pich props turned slower are more efficient than smaller low pitch props turned fast. I am presently flying an old style long-stroke 15 diesel on a control line airplane. I have tried 8 x 6, 9 x 6, 9 x 8 and 10 x 5 props. So far the 9 x 8 is giving the best performance. with the 9 x 6 not quite as good. Hopefully someone who actually knows something will answer your post.

Depends on the kind of efficiency you are after. If you are trying to get the most flying time, you will prop it one way, the most vertical or the most speed..you will use other props that meet those goals. There is no one size fits all prop........the best prop for whatever you are after will be the one that you haven't tried yet.

Some years back the results of a prop efficiency test were published. The efficiency they were testing for was pretty much "fuel economy" for prop airplanes that carry people. So it wasn't exactly all encompassing, something not possible in aeronautics anyway.

The blade area must be sufficient to handle the horsepower of the engine first. And then the tip speed must be kept under mach 1 to exclude that power robber. And then they found that the slower the rpm the more efficient the system across the board. And that when the pitch went below half the diameter, that killed the efficiency. And one of the most important details was that the prop had to operate in the power range of the engine. So...........

The most fan and pitch that the engine could turn in it's highest horsepower range won for that engine at whatever speed the airframe's drag allowed.

Ain't no three word soundbytes gonna work for this question. Everybody always wants a real simple answer for their aerodynamic questions. And they want that infinitely wise three word sentence to apply to everything from paragliders to jumbo jets.

Higher pitch and longer diameter usually give more efficiency. Mostly because those changes slow the prop, and slower is usually more efficient. Until you run out of engine, that is. And any pitch or diameter or combination that won't allow the engine to put out it's power is a loser. Yet that loser prop might be the most efficient prop for some other engine.

Most of the electric gurus claim that any prop with a diameter/pitch ratio of less than .5 is best used for stirring paint. There are two losses in props, one is slip, the other is form drag which is the wind resistance of the blades at a zero thrust advance ratio. The fact that large diameter flat pitch props brake so well when allowed to freewheel with the motor off is a good indication of just how much air resistance the blades encounter. A smaller diameter higher pitch prop will slip more but the reduced blade drag resulting from the lowered blade tip speeds more than offsets that loss.
On an electric, you can get good overall propulsion efficiency with a "climbing prop" by using the prop to climb and then shutting the motor off and gliding for a while and repeat. Make sure the motor brakes the prop and doesn't let it windmill.

A 3-D plane hovering on prop thrust is another matter entirely. Here, all props have an efficiency of zero because they are doing zero work. It's a matter of which prop consumes the least power.

Didn't know you could move masses of air with 0 work.

Down load "aero design propeller selector" on google. This will answer your questions. Safe download.

Zero work? Not really. The prop is still accerating the model to match the acceleration of gravity. Just the net speed is zero.

How about a short paper on Pitch effect on prop efficiency?

I dug out an engine test that included test results for 9 different props. And I had just found that the June issue of Model Aviation magazine has an excellent article about propellers and describes a proven prop thrust formula, and includes a speed estimation nomograph that has been accepted and proven for years. I pulled out three props from that old engine test and use them here in my "paper". I used the formula from the MA article and the old nomograph also.

prop___ MPH___thrust__ rpm
14x10___95___ 183___ 8,350
15x8____75___ 162___ 8,150
16x6____57___ 142___ 8,300

The three props were chosen because they show a progression of pitch and have basically identical rpm. The close rpm suggests that the engine was producing basically the same power and probably using fuel at close to the same rate. So if the engine is producing the same power, we can compare the speeds that would result and the thrust generated as an indication of the efficiency of the pitches.

The 10" pitch produced the highest thrust and that results in the highest estimated speed. I believe that the speed nomograph is empirical.
The 8" pitch was proportionally slower and less thrust was produced. The speed is actually almost exactly 80% which is the difference in pitch reduction, 80%.
The 6" pitch is 75% of an 8" pitch, and the speed is almost exactly 75% of the 8" pitch speed.

The comparison shows when an engine is run at a specific power, the higher pitch props provide more thrust and that gives higher speed. The higher the pitch, the higher the output. i.e. the higher the efficiency.

BTW, the engine test results covered nine props. There were two 10", three 8", and four 6".

The two 10" props showed the highest thrust and the two highest speeds.
The three 8" props showed the next highest thrusts.
The four 6" props showed the lowest four thrusts.

There was some overlapping of speeds with the 8" and 6" due to the fact there was way more range in diameters within those props. For example, the 14x8 prop would give 85mph as would the 13x6 prop. The 13x6 would have to spin over 30% faster to do it however. Overall, however, the speeds followed the pitch. For the most part, because the tester chose appropriate props for the engine.

Thanks everyone for your response. From what I gather then the 13x8 prop should be more efficient than the 13x6.5 prop at cruising speed. The prop selector program showed it was about 3% more efficient keeping speed and amp wattage the same. Anyway, I've ordered some of the 13x8 to try out.

BTW My prop collection is growing.

I never said that it took zero energy to spin the prop. A prop is doing zero work in a static thrust situation because the thrust is not moving the load. Work is force X distance. Power is force X speed. If the planes speed is zero, than the power delivered to the plane is zero. All the force in the world X zero equals zero.
It's like a car that is not moving at a red light, it is getting zero miles per gallon. A humongus Hum-Vee and a 50 cc motorcycle both get zero miles per gallon while stopped at a red light but one is still burning a lot less fuel than the other.

Not to throw a rock in the gears, but I question the idea that this will be the best selection. In general a larger diameter prop has the potential to operate more efficiently but in most cases the drag of the plane will keep you from reaching the speed necessary for the prop to reach this efficiency level. You have also said you do not fly for speed high pitch props are for speed. A large diameter prop with a lower pitch will generate more thrust at lower speeds and from what you are saying about the way you fly this plane the prop you are currently using or maybe even a prop with less pitch might be the way to go. You might even look at slightly smaller diameter prop with less pitch which might increase the RPM some, you may see a substantial increase in thrust at low speed which could help in getting airborne and recovering from trouble. If you are flying for endurance efficiency might come in to play if your flying for performance you might look at some other options.

From what I have researched, a prop has two different speeds where the propulsion efficiency is zero. The first and obvious one is zero speed. (hover and ground run up), the second and much less obvious one is the plane's speed is high enough for the prop to have zero slip. In that situation, the prop blade's airfoils have zero angle of attack to the airstream and so are making zero lift. At a zero lift angle of attack, an airfoil's lift to drag ratio is zero.
Maximum efficiency tends to occur when the advance ratio is around 80% of the pitch speed. (20% slip) At lower slips, the efficiency actually drops off because the skin friction and form drag of the blades still requires power to overcome.

Yes, often the best performance does not come with the most efficient prop but with a tradeoff between prop efficiency and the ability of the engine to make power. With electrics, the efficiency of the system is the prop efficiency X the motor's efficiency X the gear reduction efficiency. Don't obsess on one and overlook the other two.

You are confusing effective with efficiency the smaller higher pitched prop is always more efficient, howeve unless you have a sleek fast pland and want to go as fast as possible it is not more effective. Also the total efficiency of the system must be considered. Even though a small high pitched prop is more efficient, it will make a plane go faster which means more drag, this may mean more fuel burned for a given distance which is less efficient. But we as modelers are not concerned about that. So my recommendation is that untill you are working on full scale aircraft think more in terms of being more effective not more efficient.

That is totally wrong. The prop is moving air, and that is real work.

Not totally wrong, it depends on which part of the system you are referring to. The prop has done work on the air. W=FxD, F=MxA, the prop has accelerated a mass of air from one side of the prop to the other, but if you consider the airplane, which has mass, acted upon by an acceleration (gravity), then the prop has done no work (in this case) on the airplane cause the thing hasn't moved any distance.
Evan, WB#12.

F = MxA. If an airplane is hovering, isn't the F in the work equation = M airplane x A, where A = g?

Correct Jim, and if you use A as 1G then the mass and force are the same. Which is why it is hovering. Real work.

This might help.

I believe that electric motors don't work this way. You haven't limited the current to any specific amps by just limiting your TX. You have reduced what the TX will tell the RX/ESC, but electric motors simply draw according to their load. Load that model beyond the prop/motor's ability and that motor is still going to draw what amps it can get. And if the pack can supply more than 35amp, it will supply it and the motor will get it. Now, I'm not much of an Electric guru, but I help a guy fly his electrics and we have burned up one motor and ruined a couple of batteries flying around at half throttle. The guy simply doesn't want to pay for a wattmeter and has little idea what his prop/motor combinations are going to be pulling, and tries all manner of props.

Some things we hear are true. Some are not. The idea that efficiency is the function of diameter is not entirely true. Actually, it's not very true. OK, it's basically wrong. It leaves out so many things that affect efficiency that it's really just hot air. If you heard that from a good source, then I'd go ask them to go over the idea with you again, because you missed something he said. Honest, sound bytes like "largest diameter prop the plane can handle considering ground clearance for the best efficiency" are way too simple to have a hope of being true.

With an electric, about the only way to answer that question safely is with a watt meter. And remember, there really is nothing that "maintains 35amps" in that power system.

Your problem is really a lot more of an electric model power system problem than it is an aerodynamics or prop efficiency problem. The electrics forums should provide you with lots better help.

Keep in mind that maximum efficiency means going the furtherst distance on the same fuel. So going slow on a lot of thust is not the way to go, unless it match's up to a draggy airframe that needs that thrust to go a MPH or two faster. That is to say a plane built like a parachute will just go so fast with a small high pitched prop and will need the extra thrust of a larger lower pitched prop to drag it around.

How the prop interacts with the airframe can also skew the entire argument.
On my models -I have been playing with trying to get maximum prop efficiency by not restricting flow behind the prop
You don't need a Doctorate in Fluid Flow to see that any obstruction in front or behind a prop --will affect flow through the blades . On the big radial engines -this was a problem

That can be minimized with a good cowl design. A good radial cowl will allow the prop blast to go through the engine and out the back so that the prop will still provide good thrust, though slightly less than the outer blade.

Carson's speed gives the maximum range for fuel burn.

http://www.eaa1000.av.org/technicl/p...Cruising_Speed