We all want to be able to maximize the flight time that we get out of a given airframe, but how do we go about doing that? I wish I could answer that, but I simply don't know enough about the subtleties involved.
DJI has got their collective "stuff" together in the area with the ability to provide 20 minute flight times on their quads. Contrast that with my electrohub quad flying emax XA2212 980 kv motors with Emax 10x4.7 props, G-Force K series, 30 Amp escs, and a 4200 mah, 50C, 3 cell battery. I'm at 11.1 volts after about 7-8 minutes of flight! That's not so good.
Let's assume we have determine the "right" motor/battery size for our model. So, what does it take to "tune" a setup to provide the desired performance and max possible battery life? I suspect that the 2 most important factors are the motor/prop combo. But, the efficiency of the escs might be the most important factor. Let's start with the motor.
DISCLAIMER: The following is based on my understanding of the following devices. I could be wrong. In fact, my wife tells me frequently that I am. (But, she's wrong.) If I have anything wrong, please correct me! This post is about me learning more, not me professing knowledge!
Motor
Brushless motors are ridiculously simple things. They have a stator, which holds the windings. The "bell" (that's what I call the spin around thing on the outside) contains magnets. As electric current passes through the wires, elections whizzing around the wire coils, the change of direction and/or amount of electric current creates a magnetic field that interacts with the magnets on the bell to create a push or pull on any given magnet at any given time. The direction of current (forward or backward through the wire coils) is cleverly sequenced such that the motor spins in one direction. Just to state this, the permanent magnets on the bell are installed so that the polarity of every other magnet is reversed. So, Magnet "1" is installed with its north pole near the prop, Magnet "2" is installed with its north pole near the base of the motor, and so on. So, the ability to utilize the energy provided by the current generating the magnetic field must be a function of the winding of the stators, the number and position of stators, and the number and position of permanent magnets on the bell.
Electronic Speed Controller
These mysterious gizmos are the things that magically sequence the flow of current so that the prop spins and the airplane/multirotor flies. They also control the amount of current flowing so that we can control the speed of said prop. (Refer to disclaimer above to remind yourself that I could be horribly wrong before reading the next bit. Their MUST be a varying degree of efficiency among brands of escs. Brand A may be able to do its job with less energy loss (through heat, most likely) than brand B. That just makes sense to me.
I've heard that escs are generally more efficient when operating at full power and become less efficient as throttle is decreased. So, that means that the slower the prop is turning, the more energy is being lost. This might mean that grossly oversizing an esc for a given motor/prop will result in reduced flight times when compared to an "appropriately" sized one.
How does an esc reduce the power going to the motor so that we have throttle control? Does it increase the resistance to the current, thereby converting a portion of the electrical energy to heat? (This is wasteful!) Does it decrease the voltage applied to the motor, thereby reducing the current? How wasteful is this? Does it do both? Or, does it change the timing of the magnetic field generation such that there isn't as much of a push or pull on the permanent magnets? This seems unlikely to me for some reason I can't quite explain.
For max flight times, the esc needs to do its job with the least amount of energy lost to heat as possible. What makes one esc more efficient than another?
Propellers
These are the most straight-forward and understandable components of the power train for me to understand because they are purely mechanical devices! However, please remind yourself of the Disclaimer above. Propellers simply push air "backwards" relative to the flying device. We can argue over how much of this motion is the responsibility of Bernoulli's Principle and how much is due to Newton's 3rd law, but lets just say that the two phenomena work together to make flying magic. Let's look at 3 components of prop "efficiency." These are, pitch, diameter and prop design (taper, twist and airfoil shape).
Prop Pitch
We all know that some airfoils are "better" for a given plane's wing than others. But, choosing the correct airfoil is a compromise. Higher lift airfoils create higher drag. Low drag airfoils, in general, or more efficient at higher airspeeds because they need that airspeed to generate adequate lift. The same is true for a given prop.
I've read many a post where a person states that "the greater the prop pitch, the higher the forward speed." This is true only if we compare the two props operating at the same rpm (or kv in electric motor lingo). But, reality is not that simple! In fact, it is common that the exact opposite is true when we consider the capability of a given motor.
Here's an example of this. My Wing Surfer (same thing as a Sky Surfer, Bixler or any number of similar airframes) uses a 2200 kv motor with a 2200 mah, 3s batter and a 30 amp esc. It is a rocket with the correct prop! I can climb vertically until I can't see the plane. I try to avoid doing this as a general rule. The maximum diameter prop that I can use without slicing into the tail boom is 6 inches. And this is ok, because anything larger in diameter is too large for the motor. (Remember, we are talking about pitch, not diameter at the moment.)
Based on the general rule, "The greater the pitch, the faster the plane," this plane would be faster with a 6x6 prop than with a 6x4. But, its NOT! In fact, I get the best performance (defined by speed and rate of climb) with a 6x3 prop. Why is this? Its quite simple, actually.
Think of the pitch of a prop of a given diameter as the gears on a 10 speed bike for a given motor. The motor can only generate so much power. A higher pitched prop is like the higher gears on the bike. If you turn the pedals at the same speed as, say, 2nd gear, you go faster. But, you are limited by the power that you can generate. So, unless you are a beast, there is no way you can spin the peddles as fast in 10th gear as you can in 1st. The motor has the same problem.
My 2200 kv can produce more rpms on the 6x3 prop than it can with the 6x6. Its efficiency (again, defined as "go fast" or distance forward per rotation of prop) is obviously greater with the prop with the lower pitch.
Whew! That's a lot of words to just talk about prop pitch! What I'm getting at here is that there is an optimal prop pitch for a given motor.
Prop Diameter
(Disclaimer really applies here!) I think its easier for folks to understand the selection of a prop's diameter than pitch. An electric motor wants to draw a certain number of amps at a certain voltage, producing a certain power output at a certain rpm. As the load on the motor increases, it must draw more current to maintain this "happy spot." Eventually, the current will exceed the rating of the winds, they will heat up, melt the insulation and short out. This is when the magic smoke escapes.
I think it is as straightforward as, "the larger the diameter, the faster the plane will go." However, there is a limit to the diameter any given motor can turn without burning up. In general, for a given motor size, the lower the kv, the larger the prop the motor can use.
Prop Design
I read a study of the efficiency of RC props 2 or 3 years ago. I zoned at a few times while reading it, but the conclusions were interesting. I forget how many props and all of types tested. But, I do remember that Master Airscrew had the least efficient prop design and that APC was near the top in terms of efficiency (however they defined efficiency). I can't find the danged thing on the internet now, though. Bottom line is, the manufacturer and style of prop make a big difference.
So, those are the things that I think of when it comes to the efficiency of a power system. How in the heck do we, the lowly rc hobbyist select components that are as optimal as possible? Please, share your thoughts!!!
DJI has got their collective "stuff" together in the area with the ability to provide 20 minute flight times on their quads. Contrast that with my electrohub quad flying emax XA2212 980 kv motors with Emax 10x4.7 props, G-Force K series, 30 Amp escs, and a 4200 mah, 50C, 3 cell battery. I'm at 11.1 volts after about 7-8 minutes of flight! That's not so good.
Let's assume we have determine the "right" motor/battery size for our model. So, what does it take to "tune" a setup to provide the desired performance and max possible battery life? I suspect that the 2 most important factors are the motor/prop combo. But, the efficiency of the escs might be the most important factor. Let's start with the motor.
DISCLAIMER: The following is based on my understanding of the following devices. I could be wrong. In fact, my wife tells me frequently that I am. (But, she's wrong.) If I have anything wrong, please correct me! This post is about me learning more, not me professing knowledge!
Motor
Brushless motors are ridiculously simple things. They have a stator, which holds the windings. The "bell" (that's what I call the spin around thing on the outside) contains magnets. As electric current passes through the wires, elections whizzing around the wire coils, the change of direction and/or amount of electric current creates a magnetic field that interacts with the magnets on the bell to create a push or pull on any given magnet at any given time. The direction of current (forward or backward through the wire coils) is cleverly sequenced such that the motor spins in one direction. Just to state this, the permanent magnets on the bell are installed so that the polarity of every other magnet is reversed. So, Magnet "1" is installed with its north pole near the prop, Magnet "2" is installed with its north pole near the base of the motor, and so on. So, the ability to utilize the energy provided by the current generating the magnetic field must be a function of the winding of the stators, the number and position of stators, and the number and position of permanent magnets on the bell.
Electronic Speed Controller
These mysterious gizmos are the things that magically sequence the flow of current so that the prop spins and the airplane/multirotor flies. They also control the amount of current flowing so that we can control the speed of said prop. (Refer to disclaimer above to remind yourself that I could be horribly wrong before reading the next bit. Their MUST be a varying degree of efficiency among brands of escs. Brand A may be able to do its job with less energy loss (through heat, most likely) than brand B. That just makes sense to me.
I've heard that escs are generally more efficient when operating at full power and become less efficient as throttle is decreased. So, that means that the slower the prop is turning, the more energy is being lost. This might mean that grossly oversizing an esc for a given motor/prop will result in reduced flight times when compared to an "appropriately" sized one.
How does an esc reduce the power going to the motor so that we have throttle control? Does it increase the resistance to the current, thereby converting a portion of the electrical energy to heat? (This is wasteful!) Does it decrease the voltage applied to the motor, thereby reducing the current? How wasteful is this? Does it do both? Or, does it change the timing of the magnetic field generation such that there isn't as much of a push or pull on the permanent magnets? This seems unlikely to me for some reason I can't quite explain.
For max flight times, the esc needs to do its job with the least amount of energy lost to heat as possible. What makes one esc more efficient than another?
Propellers
These are the most straight-forward and understandable components of the power train for me to understand because they are purely mechanical devices! However, please remind yourself of the Disclaimer above. Propellers simply push air "backwards" relative to the flying device. We can argue over how much of this motion is the responsibility of Bernoulli's Principle and how much is due to Newton's 3rd law, but lets just say that the two phenomena work together to make flying magic. Let's look at 3 components of prop "efficiency." These are, pitch, diameter and prop design (taper, twist and airfoil shape).
Prop Pitch
We all know that some airfoils are "better" for a given plane's wing than others. But, choosing the correct airfoil is a compromise. Higher lift airfoils create higher drag. Low drag airfoils, in general, or more efficient at higher airspeeds because they need that airspeed to generate adequate lift. The same is true for a given prop.
I've read many a post where a person states that "the greater the prop pitch, the higher the forward speed." This is true only if we compare the two props operating at the same rpm (or kv in electric motor lingo). But, reality is not that simple! In fact, it is common that the exact opposite is true when we consider the capability of a given motor.
Here's an example of this. My Wing Surfer (same thing as a Sky Surfer, Bixler or any number of similar airframes) uses a 2200 kv motor with a 2200 mah, 3s batter and a 30 amp esc. It is a rocket with the correct prop! I can climb vertically until I can't see the plane. I try to avoid doing this as a general rule. The maximum diameter prop that I can use without slicing into the tail boom is 6 inches. And this is ok, because anything larger in diameter is too large for the motor. (Remember, we are talking about pitch, not diameter at the moment.)
Based on the general rule, "The greater the pitch, the faster the plane," this plane would be faster with a 6x6 prop than with a 6x4. But, its NOT! In fact, I get the best performance (defined by speed and rate of climb) with a 6x3 prop. Why is this? Its quite simple, actually.
Think of the pitch of a prop of a given diameter as the gears on a 10 speed bike for a given motor. The motor can only generate so much power. A higher pitched prop is like the higher gears on the bike. If you turn the pedals at the same speed as, say, 2nd gear, you go faster. But, you are limited by the power that you can generate. So, unless you are a beast, there is no way you can spin the peddles as fast in 10th gear as you can in 1st. The motor has the same problem.
My 2200 kv can produce more rpms on the 6x3 prop than it can with the 6x6. Its efficiency (again, defined as "go fast" or distance forward per rotation of prop) is obviously greater with the prop with the lower pitch.
Whew! That's a lot of words to just talk about prop pitch! What I'm getting at here is that there is an optimal prop pitch for a given motor.
Prop Diameter
(Disclaimer really applies here!) I think its easier for folks to understand the selection of a prop's diameter than pitch. An electric motor wants to draw a certain number of amps at a certain voltage, producing a certain power output at a certain rpm. As the load on the motor increases, it must draw more current to maintain this "happy spot." Eventually, the current will exceed the rating of the winds, they will heat up, melt the insulation and short out. This is when the magic smoke escapes.
I think it is as straightforward as, "the larger the diameter, the faster the plane will go." However, there is a limit to the diameter any given motor can turn without burning up. In general, for a given motor size, the lower the kv, the larger the prop the motor can use.
Prop Design
I read a study of the efficiency of RC props 2 or 3 years ago. I zoned at a few times while reading it, but the conclusions were interesting. I forget how many props and all of types tested. But, I do remember that Master Airscrew had the least efficient prop design and that APC was near the top in terms of efficiency (however they defined efficiency). I can't find the danged thing on the internet now, though. Bottom line is, the manufacturer and style of prop make a big difference.
So, those are the things that I think of when it comes to the efficiency of a power system. How in the heck do we, the lowly rc hobbyist select components that are as optimal as possible? Please, share your thoughts!!!
Last edited:
