Do yourself a favor and bookmark this for all your electric matching needs. An excellent article.

I watched that video, and the guy came across as an "expert". Who once said "Trust but verify?"

As fhhuber indicates, doing this sort of stuff without measuring results with a wattmeter is like hopping up and then driving a hot rod without a working speedometer. Eventually, it's going to cost you.

DennyV
Retired and the days are just too short, busier than ever!

What he stated was common sense. I am still learning about props, but it should be known right from the word go that you should always have a batt and esc that can do more than what a motor demands.

What he stated was common sense. I am still learning about props, but it should be known right from the word go that you should always have a batt and esc that can do more than what a motor demands.

Yup
If you ever have a desire to know far to much about those props, and how much horsepower, thrust, torque and one or two other things they have at a given RPM, APC Props has data on just about every propeller they make.

How do you figure out the info, seems quite complex to me. I'm guessing each section (1000 rpm, 2000 rpm) starts at zero rpm and goes up from there. How do I know how much horse power my, say P47, needs? And how do pitch and size change performance. From my understanding bigger props lower the total rpm of the prop and pitch increases speed of the plane, but slower takeoff.

How do you figure out the info, seems quite complex to me. I'm guessing each section (1000 rpm, 2000 rpm) starts at zero rpm and goes up from there. How do I know how much horse power my, say P47, needs? And how do pitch and size change performance. From my understanding bigger props lower the total rpm of the prop and pitch increases speed of the plane, but slower takeoff.

Lets take a look at the printout from the data, below:
PROP RPM = 6000

The V column is the forward speed of the model. I think the J column is the ratio of the forward speed versus the blade pitch speed. Pe, Ct, Cp,
But the PWR is horsepower input, Torque and Thrust in pounds.

Looking at the 13 MPH row, you get a thrust of 0.88 pounds while the motor is turning at 6000 RPM.

What we need is the thrust. Those values are fairly close to the real world. I've noted that those values are higher than measured on my models, but then there is drag on the fuselage from the airblast from the prop.

Do we need all this stuff. IMHO, no. Unless you're working on competition, where every last little bit of performance matters.

At any rate, there are computer programs that do all this stuff for you. One I use is www.motocalc.com, and its "Opinions" command. Motocalc is free for 30 days, then $39. Unfortunately, motocalc is only as accurate as the motor specs provided by the mfg. And some of those specs are way the heck off.

DennyV
Retired and the days are just too short, busier than ever!

I've used motocalc, input my weight without electronics (found the empty weight of the mini switch online), my motor, or something close, prop and battery, and airfoil and other data. Half the time I get a system can't be found, for what I need, and I am only asking for 6 minute flights

I've used motocalc, input my weight without electronics (found the empty weight of the mini switch online), my motor, or something close, prop and battery, and airfoil and other data. Half the time I get a system can't be found, for what I need, and I am only asking for 6 minute flights

Yup
There are so many news motors out there, I guess it's hard for any program to keep up. What I do is try to find the important one, the motor specs, and plug them in.

Problem with that, is some motor mfg's don't provide the motor winding resistance in their specs. That resistance has a major effect on the motors efficiency, and how much power can be run through it without getting smoke signals.

I've found some no-name (and even some brand name) motors have two or three times the winding resistance of a Hacker or Hyperion or similar type motor. That with the motor having the same KV rating, and motor weight.

Not hard to figure out why. Some of those cheap motors only fill the winding area of the motor half full. It's easier to wind that way.

Take a look at my Hacker A40-10L motor. The stator is completely full of copper wire. I had to do repair work on it after an aileron came off on one of my models. It was a design defect in the aileron mount.

This motor runs at over 1000 Watts, and runs reasonable temperatures.

DennyV
Retired and the days are just too short, busier than ever!

I've taken motors within motocalc and adjusted kv to give a rough idea of resistance. It's hard to use, I couldn't find anything for the p47 clearly, as I typed 10x6 and it said it would fly lol. Turnigy motors aren't easy to find in the program, guess its outdated slightly, needs more updates of brushless and less of the probably nonexistent brushed ones.

I've taken motors within motocalc and adjusted kv to give a rough idea of resistance. It's hard to use, I couldn't find anything for the p47 clearly, as I typed 10x6 and it said it would fly lol. Turnigy motors aren't easy to find in the program, guess its outdated slightly, needs more updates of brushless and less of the probably nonexistent brushed ones.

If you have a need to measure resistance, here is a way to do it. One of the meters can be your wattmeter. This procedure is as accurate as the meters you use, generally within a percent or three.

How much does the higher resistance in Turnigy motors hurt the performance over say a less resistance Hacker? Still trying to figure out what pitch is exactly, like how it's measured, and how to determine what is needed, assuming the motor is good for a particular prop

How much does the higher resistance in Turnigy motors hurt the performance over say a less resistance Hacker? Still trying to figure out what pitch is exactly, like how it's measured, and how to determine what is needed, assuming the motor is good for a particular prop

That is a very good question. This is one where a program like motocalc can provide some guidance on the subject.

Take my Hyperion AZ3015-10 motor. It's running a 13X6.5 prop, pulling 586 Watts out of a 5S A123 pack. Motocalc suggests the output is 525 watts, while turning the prop at 8500 RPM. Efficiency is 89%. This is close to what the real numbers are.

Some of those cheap motors on the same size as this Hyperion have four times the winding resistance. So, with the same prop and same battery, it will be pulling 440 watts out of the 5S A123 pack. Motocalc suggests the output is 315 watts. Efficiency has dropped to 71%.

So, to get back the same power and RPM on the prop, you could add more cells to the battery pack. But in doing so, that efficiency continues to drop like a rock. Motocalc suggests you can never reach that same 8500 RPM, even with six or seven cells in the battery pack. Add to that, motocalc suggests the windings will hit 500 degrees F.

This Turnigy motor has roughly the same winding resistance as the Hyperion motor at 0.02 Ohms. (VS the 0.018 ohms for the Hyperion) I can't vouch for the Turnigy motors though, since I've never used one. http://www.hobbyking.com/hobbyking/s..._46_Glow_.html

DennyV
Retired and the days are just too short, busier than ever!

That is a very good question. This is one where a program like motocalc can provide some guidance on the subject.

Take my Hyperion AZ3015-10 motor. It's running a 13X6.5 prop, pulling 586 Watts out of a 5S A123 pack. Motocalc suggests the output is 525 watts, while turning the prop at 8500 RPM. Efficiency is 89%. This is close to what the real numbers are.

Some of those cheap motors on the same size as this Hyperion have four times the winding resistance. So, with the same prop and same battery, it will be pulling 440 watts out of the 5S A123 pack. Motocalc suggests the output is 315 watts. Efficiency has dropped to 71%.

So, to get back the same power and RPM on the prop, you could add more cells to the battery pack. But in doing so, that efficiency continues to drop like a rock. Motocalc suggests you can never reach that same 8500 RPM, even with six or seven cells in the battery pack. Add to that, motocalc suggests the windings will hit 500 degrees F.

This Turnigy motor has roughly the same winding resistance as the Hyperion motor at 0.02 Ohms. (VS the 0.018 ohms for the Hyperion) I can't vouch for the Turnigy motors though, since I've never used one. http://www.hobbyking.com/hobbyking/s..._46_Glow_.html

Turnigy, you can't compare any of them. Some are great, some are not. So would a smaller prop, like a 9x6 or 9x8 increase my speed? Or will a 10x8 (assume my motor would burn up with 10x8) with say a 900kv motor increase my speed? I'm trying to figure out what settings increase speed. My motor turns the 10x6 fine, but I don't know if a lower kv with a 10x8 would do better, a smaller prop with a 8 pitch (still trying to figure out how pitch works based on prop size), or simply the same kv in a larger motor.

Turnigy, you can't compare any of them. Some are great, some are not. So would a smaller prop, like a 9x6 or 9x8 increase my speed? Or will a 10x8 (assume my motor would burn up with 10x8) with say a 900kv motor increase my speed? I'm trying to figure out what settings increase speed. My motor turns the 10x6 fine, but I don't know if a lower kv with a 10x8 would do better, a smaller prop with a 8 pitch (still trying to figure out how pitch works based on prop size), or simply the same kv in a larger motor.

All things being equal, the only way to increase speed is to increase pitch or kV.
Increasing prop size will increase the thrust produced by the prop.
Increasing the thrust means it moves more air allowing the prop to pull more weight.
A 10x5 and a 5x5 will theoretically have the same air speed at the same rpm. But the 10 inch prop will produce close to four times the thrust.
So a plane with 10x5 will get to max speed faster that the same plane with a 5x5. But in either case if the prop rpm speed is the same, the top speed remains the same.

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This is a fact, you have never been in an empty room.

I'm still learning about props myself. For the most part i stick with recommended props. But as I understand it, the pitch number is the theoretical distance in inches the the prop moves forward in one revolution. From that you can multiply by rpm and get your pitch speed.
The size of the prop can be interesting and when not careful can be the thing that causes magic smoke when not chosen carefully. A 12 inch prop creates a disc size in area nearly twice that of a 9 inch prop. That means at the same pitch the larger prop will have to work twice as hard to move the air at the same rpm. That means that in a perfect world the larger prop will draw twice the amps even though it is only 30% longer. Going to a larger prop is great if you need more thrust but you will have to pitch down, less pitch speed, to compensate for the larger prop.
Only a watt meter will tell you if the prop chosen will be within motor and ESC limits.
A larger prop will also require more torque to spin it, therefore you need a lower kV motor. This will again affect pitch speed as lower kV means lower rpms. To compensate that you will have to increase the cell count in your battery packs.
This is only an explanation as I know it and would welcome all input to correct anything I have misunderstood.

--------------------------------------------------------
This is a fact, you have never been in an empty room.

I'm still learning about props myself. For the most part i stick with recommended props. But as I understand it, the pitch number is the theoretical distance in inches the the prop moves forward in one revolution. From that you can multiply by rpm and get your pitch speed.
The size of the prop can be interesting and when not careful can be the thing that causes magic smoke when not chosen carefully. A 12 inch prop creates a disc size in area nearly twice that of a 9 inch prop. That means at the same pitch the larger prop will have to work twice as hard to move the air at the same rpm. That means that in a perfect world the larger prop will draw twice the amps even though it is only 30% longer. Going to a larger prop is great if you need more thrust but you will have to pitch down, less pitch speed, to compensate for the larger prop.
Only a watt meter will tell you if the prop chosen will be within motor and ESC limits.
A larger prop will also require more torque to spin it, therefore you need a lower kV motor. This will again affect pitch speed as lower kV means lower rpms. To compensate that you will have to increase the cell count in your battery packs.
This is only an explanation as I know it and would welcome all input to correct anything I have misunderstood.

That pretty much nails it on the head.

As far as the power input to a prop versus the prop diameter, the power input skyrockets with increasing the diameter.

For those with a scientific calculator (Or access to a 5th grader!) the prop formula is as follows.

Horsepower = PD^4*RPM^3/1.4E17.

So, for an 8 inch Pitch, 12 in Diameter prop, at 7500 RPM you get:

HP =8*12^4*7500*3/1.4E17 which calculates out to 0.4998 horses. Multiply times 746 to get watts, comes out to 372.9 Watts. Going from a 12 inch to a 14 inch diameter prop at the same RPM nearly doubles the watts input. This is why those wattmeters are so important. They will show the doubled current input, before you burn up the motor/esc/battery pack. These electric motors are DUMB. They will happily turn out double their rated watts, with no obvious problems. Until they start sending smoke signals.

DennyV
Retired and the days are just too short, busier than ever!

You can spin a 5X5 prop very fast and end up slower than using a 7X3 spinning slower for the same watts... on some models.

The power system needs to be matched to the airplane. The prop is part of that power system.
A draggy design needs more diameter for thrust to overcome the drag.
A slick pylon racer style plane of the same weight can potentially make use of the higher pitch prop at higher rpm.

Pitch speed is the rpm X pitch of the prop (+ conversion factors) which results in the speed the prop is trying to pull the airplane.

A draggy plane might fly at about 70% of pitch speed when you have the right prop. motor, ESC and battery.
A "slick" design might fly very near calculated pitch speed.

Thus if the 10x8 is too much, I need a 1000kv in a bigger motor, there is no other option really. One thing I don't get about pitch is 8x8 is high pitch on my mini switch, 8x6 is standard, why is 10x6 considered to have a low pitch? Now would a 11x8 help? I find thrust ample, but top speed isn't.

Thus if the 10x8 is too much, I need a 1000kv in a bigger motor, there is no other option really. One thing I don't get about pitch is 8x8 is high pitch on my mini switch, 8x6 is standard, why is 10x6 considered to have a low pitch? Now would a 11x8 help? I find thrust ample, but top speed isn't.

The blade pitch is important when considering flight speed. What might be a very good pitch selection for a Piper Cub would be lousy for a foamie jet capable of hitting over 100 MPH. Even when both power systems are running the same watts.

If you've used motocalc, one of its parameters is "Pspd MPH". That is the predicted velocity of the air going through the prop at the RPM on the same line. If you want 65 MPH flight speed, you won't get it with a propeller that has a blade Pitch Speed of 35 MPH.

On the other hand, if you have a Piper Cub with a flying speed of 35 MPH, and select a motor/battery/prop combination that has a blade pitch speed of 120 MPH, it may or may not fly. But if it did get off the ground, it would have lousy performance, as compared to how it would fly with a properly matched propeller diameter and blade pitch.

Do you have the model wingspan, wing area and weight? I could run some numbers through motocalc, and see what it shows.

As for me, I'm running high pitch values on my model airplanes. Those 1200 to 3000 watt motors on my models are low RPM, compared to a gasser. So, they can turn bigger diameter propellers than a glow/gasoline engine. My giant Big Stick model is powered by a Hacker A60-16M motor, turning a 19X12 APC-E Wide blade prop at 6800 RPM on the ground. Its been clocked at about 70 MPH at our club field. That 17 pound model can take off in 30 feet, and climb out vertically out of sight.

DennyV
Retired and the days are just too short, busier than ever!

Square props are for speed (10x10, 9x9, or faster, 4.5x5), half props are generally for slow speed/thrust (11x5.5, 10x5, 10x4.7)... generallly.

think of a 10x3.8 as first gear in a truck, and 10x10 as 5th gear.

your props are a way of "gearing" thrust. if you want a lot of low end and don't care about speed, get a slow fly style prop. you want fast, go for a square or over square prop. "Sport" style performance would be something inbetween.

slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).

Also, kv rating, cell count and prop are all directly related. lookup the 480 plus motor on heads up rc. its a 1000kv motor.

8x6 on 4 cells for "best" performance on 4 cells, 11x5? For 3 cells.

if the prop is spinning faster, you need a smaller prop or pitch to lower load on the motor. if you want the same results.

hence the different cell count gives you a higher/ lower rpm's. with 1000kv, should be about 14800rpm's with no load. on 3 cells, it would only be 12600rpm's. the slower speed needs a larger prop for the same amount of thrust at a lower speed.

slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).

Also, kv rating, cell count and prop are all directly related. lookup the 480 plus motor on heads up rc. its a 1000kv motor.

8x6 on 4 cells for "best" performance on 4 cells, 11x5? For 3 cells.

if the prop is spinning faster, you need a smaller prop or pitch to lower load on the motor. if you want the same results.

hence the different cell count gives you a higher/ lower rpm's. with 1000kv, should be about 14800rpm's with no load. on 3 cells, it would only be 12600rpm's. the slower speed needs a larger prop for the same amount of thrust at a lower speed.

This 480 would give the same kv, but allow for a greater load simply, allowing an increase in prop size or pitch. I will be using 3s.

Thank you, I intially thought it was some angle, and this 8 pitch is the same no matter the prop size. So a higher pitch takes longer to get up to speed, but more speed. Bigger size means more air, thus allows for bigger overall planes, plus more effiency, when spinning at a slower rpm, as it gets the same air over at a slower rpm as a higher rpm does with a smaller prop.