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Oct 26, 2014, 11:07 AM
Wisconsin
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Mini-HowTo

Newbie to Newbie: Propellers


Motors and propellers need to be matched to the purpose. Each motor is designed for a particular purpose for optimum efficiency. A high KV motor is designed to spin faster with small props and is used primarily for acrobatic aircraft. Low KV motors are designed for large props and heavy load applications.

http://www.pilotfriend.com/training/...wing/props.htm
Propeller specifications are noted as the diameter and the pitch. The pitch is the twist in the blades and is measured as the distance in inches the propeller would travel if it were like a screw being screwed into a wood block. Each turn of the screw would move the screw so far into the wood, depending on the twist. Same for a propeller; more twist and it goes further through the air. A propeller for a small mini-quad might be a 6 x 4, sometimes written 6040, which would be 6 inches in diameter with a pitch of 4 inches per rotation. A large prop for an aerial photography aircraft might be a 17 x 3.8, or 1738, which would be 17" in diameter with 3.8" of pitch.

Generally, high-pitch propellers are used for fast, quick low-torque applications like acrobatics. Low-pitch propellers are used for low speed, high torque, heavy-lifting applications. For acrobatics, you would most commonly choose a high KV motor with a high-pitch prop. For the other end of the spectrum, with a heavy aerial photography platform, you might choose a low KV motor and a large diameter low-pitch prop.
IT is a lot like the transmission in your car. First gear is for climbing hills. It won't go fast but it will carry a heavy load or climb a hill. Fifth gear is overdrive, fast on the freeway, it won't climb a hill or pull a trailer but it will move the car fast on the flats. First gear is like a large slow moving prop and 5th gear is like a small fast moving prop. In a car with a tachometer you can see that first gear has low rpm and 5th gear has a high rpm.

Most motors are very versatile and can be used with a variety of battery voltages. But there is a sweet spot that is the center point of the voltage range at which a motor can be used. For efficiency, a motor will be designed to work best at a particular speed. If you are running acrobatic aircraft, you might not care about efficiency and you may want to squeeze as much performance as you can out of the motor-prop combination. But if you want the maximum load and maximum flight time, you need to find the most efficient combination of motor size, KV, and prop for your airframe.

There is a lot of confusion about battery voltage and motors. Some people think that a 6S battery is somehow more powerful than a 3S battery. This is not fundamentally true. The mathematical relationship talked about before shows the truth: Voltage x Current = Power. However, there are advantages to both a 6S and a 3S system. If we talk about the same power demand from a motor, say 300 watts (w), we can work with our formula to show that 300w=12v(3S) x 25amps is the same as 300w=24v(6S) x 12.5amps. You can get the exact same power by using either a 3S or a 6S battery. But notice that the 3S system requires more current to do what the 6S does with less current, it shifts some of the work over to the voltage side. Same energy just more voltage means less current but the same energy used. This tells us that we could use a smaller-rated ESC for the same motor using 6S at 12.5 amps than we could with a 3S at 25amps. The lower current with the 6S will require a smaller wire than would be needed for the 3S system. The larger the current, the larger in diameter the wire must be to connect the battery to the ESC. The diameter of the wire is called the gauge or AWG. During a build, you would find a chart on the internet that shows the rated current for each size or gauge of wire. You then choose your wire based on the maximum current needed for your motors. The disadvantage of 6s is that most of the equipment, other than the motors, on a multirotor will not run at 6s so you must convert the 6s down to voltages the electronics need. This adds complexity, failure points and is an inefficient use of power. Also few motors are currently optimized for 6s. Most run at maximum efficiency at 3s.

To find an equivalent battery for comparison we use our formula (called Ohm's Law) and we find that a 1000ma battery x 6S(24v) = 24 watt-hours (wh) of energy stored. This means we can use 1 amp for 1 hour from a 6S 1000ma battery before it is completely drained. (But remember, we don't want to use more than 80% of the battery's capacity.) The equivalent 24wh 3S battery is 3S(12v) x 2000ma. Comparing the battery capacity for equivalent energy storage, the 6S 1000ma battery is equivalent to a 3S 2000ma battery. They both store the same amount of energy. Again a 6s1000mah battery is not equal to a 4s1000mah battery is not equal to a 3s1000mah battery.

The last point to understand regarding motors and batteries is that, for example, when we change the voltage from 3S to 6S, we are changing the speed of the motor, so the motor is being used in a very different configuration. When used with a 3S battery a 1000kv motor is spinning at 12,000 rpm, but with a 6S battery the same motor would spin at 24,000 rpm. You can't choose a battery voltage for your aircraft without taking into consideration what speed motor you need for your load and prop. If you have a design you assembled with a 3S battery, a 380kv motor, and a 15" prop, this configuration will change dramatically if you were to change the battery from 3S to 4S or to a 6S. Sometimes these changes work fine and sometimes they don't work. Note that it does not do any good to choose a low kv motor, for long flight times, and then power it with a high voltage battery. This is because for long flight times you need slow moving big props and lower kv motors run at a slower speed that higher k motors. But what we are really concerned about is the speed. The RPM and RPM=Voltage * kv. So if you pick a 580kv motor and run it with a 3s battery the speed of the props is 12v*580=6960 rpm but if you choose a 380kv motor and run it with a 6s battery your prop speed is 24*380=9120rpm. No longer a slow moving prop. You are defeating your purpose of long flight times. With the 6s example have a lower kv motor but the actual rpm is higher than the 3s example.





Good motor manufacturers provide charts where they have run their motors with different batteries and different props and measured the thrust and the current required. These charts give you an idea of the "sweet spot" for a motor. All motors are designed for best performance at one battery voltage and prop. In the charts you can calculate the efficiency of the motor for a particular battery voltage and prop by noting the current draw and thrust for a given combination. If they list a whole bunch of current readings from say .7amps to 17amps. Choose the middle which represents the 50% throttle point. This is where we want our multirotor to hover at, with the throttle half way up.

Some manufacturers calculate the efficiency for you and it will be listed as grams per watt or g/w. This is the amount of thrust you get for every watt of power supplied. So the bigger the number the more thrust you get for your battery power supplied. This translates into longer flight times. Most motors are more efficient at lower battery voltages like 3s and 4s. Take a look at the attached Sunnysky motor chart. Scroll down to see the tables. Notice the column labeled efficiency g/w. For this motor which combination of battery and prop yields the best efficiency? Do be wise though, many manufacturers publish bogus information whether due to a typo or to mislead. So if the specs look too good to be true they probably are not true.




Again, using a modeling program like eCalc can help you find the best combination for your needs. Also, remember that KV is not the only important specification for a motor. The power-handling capacity of the motor is very important, too. You can have a 2212 1000kv motor that might be good for 100 watts maximum and a 3510 1000kv motor that is good for 450 watts. They both spin the same speed but one can handle a much heavier load than the other. In motors power handling is roughly related to the simple mass of metal in the motor. If it is bigger and heavier it will probably handle more power.

Next: Flight Controllers
Last edited by mike_kelly; Mar 19, 2016 at 09:06 AM.
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May 17, 2016, 11:01 PM
Dextro_Sinister
First I want to thank you for all this great info! I still have a TON of learning to do, but you have at least given me a great foundation to start my question asking. I do have a question about this section though... You asked the question which is the best efficiency combination for this motor you listed. I am having a hard time figuring this out... Am I to be looking for the highest number on the efficiency? Sorry if this is obvious and I am just totally missing the point, and again thank you so much!
May 18, 2016, 09:31 AM
Wisconsin
Thread OP
Quote:
Originally Posted by Dextro_Sinister
First I want to thank you for all this great info! I still have a TON of learning to do, but you have at least given me a great foundation to start my question asking. I do have a question about this section though... You asked the question which is the best efficiency combination for this motor you listed. I am having a hard time figuring this out... Am I to be looking for the highest number on the efficiency? Sorry if this is obvious and I am just totally missing the point, and again thank you so much!
Yes. Efficiency is the grams of thrust divided by the energy needed to do it. So the more thrust for the least power is a larger number. In the example chart the efficiency is highest with the 11v battery (3s) with either a 11x4.7 APC prop or the APC10x4.7. It also shows that the motor is most efficient at low throttle and as you increase the throttle the efficiency goes down. The charts only give a rough indication of how the motor operates. They only tested with a couple of props and battery voltages but it does give you a clue that at least with this motor it is more efficient running at 3s (11v battery) that a 4s (14v battery). They will also usually give you a clue how big a prop the motor was designed to use. But just because the prop you want to use was not tested doesn't mean it won't work fine. Manufacturers only go to the trouble of testing a few combinations.

That is where eCalc (www.eCalc.ch) is so valuable. There are a lot of variables when designing a multirotor and they are not easy to design a combination that actually flies! So eCalc takes the details of the motor, the props you choose, the ESC and the battery and runs it through a formula to estimate how long it will fly.
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