How to choose a power system
This guide is intended to help give aeromodelers enough simplified knowledge to choose from the bewildering array of components available, and to assemble appropriate power systems for their model aeroplanes. This guide is not intended as an answer to everyone’s questions, but as a basic introduction so that we have a reference to help understand this subject.
Using this Guide
Choosing a power system is a more complicated procedure than I first thought when I started putting this guide together. Often choosing a power system includes a fair bit of educated guesswork. Although it is possible to accurately calculate every important bit of data to determine the optimum power system for a model, most of us will prefer the “educated guess” approach. Unfortunately there is some knowledge required to make an “educated guess”, and that is hopefully what this guide will provide.
For those completely new to the world of aeromodeling who also intend building a new model, I suggest starting from the beginning of this guide. Even if it doesn’t all make sense at first, it will eventually. Some trial and error might not seem like the most economical way of learning but it can sometimes be the most effective.
Many people will only want to access certain information, so each topic has a heading. Scroll down to see if you can find the information you need.
Wing Loading and Stall Speed
Wing loading is the loaded weight of an aircraft divided by the area of the wing. It is broadly reflective of the aircraft's lift-to-mass ratio, which affects its rate of climb, load-carrying ability, and turn performance.
Many aeromodelers try their hardest to make models as light as possible. This is because a model with a light wing loading is easier to fly as it has a slower stall speed. In a banked turn an aeroplane is subjected to a gravitational force (G) which increases its weight, the same as a weight on the end of some string gets heavier if you spin it around like a lasso. The heavier the wing loading in a banked turn, the higher the stall speed gets. Other factors that can affect the stall speed of a model aeroplane are aerofoil shape and aspect ratio. The only way of giving a heavy model a light wing loading is to increase the size of the wings. So a model with a high wing loading will have a high stall speed, which will become even higher in high G manoeuvres. Because we land our models at or slightly above their stall speed, a model with a high wing loading will have a high landing speed which can call for some very good piloting skills.
Wing loading is usually calculated in the form of oz/ft² or gr/dm²,1 dm² = 15.50003 inch². If you want to calculate the wing loading of your model, try this calculator http://www.csd.net/~cgadd/eflight/calcs_wingload.htm.
Volts, Amps and Watts
The most important terms you need to understand when choosing components for your electric model are Volts, Amps and Watts.
Here is the "Hydraulic analogy" from Wikipedia which explains these terms in a simple way.
"The hydraulic analogy is sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage is likened to water pressure - it determines how fast the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point, the rate of which is determined by the voltage, and the total output measured in watts. The equation that brings all three components together is: volts × amperes = watts".
As we will see, the size and efficiency of a motor and the load imposed on it by the propeller affects the Volts and Amps. The idea is to choose a motor, battery, esc and propeller combination that will fly your model in a desired manner within the specifications of the components, preferably at around the peak efficiency of the motor. This will be covered in more detail in the following sections.
Choosing a Motor
Weight Power and Dimensions
The most important thing to keep in mind before choosing a motor is its weight and dimensions. We would all agree that extra weight added to a model to achieve the correct centre of gravity is undesirable. I personally would prefer to have a larger, heavier and more powerful motor than a smaller less powerful motor and a lump of lead in my models. Sometimes there is no choice but to use lead, but just don’t forget about the relationship between the weight of your motor and the centre of gravity of your model. The dimensions of a motor are obviously important. Don’t buy it if it won’t fit in your model.
You will want a level of performance suitable for the type of model you are powering. A 3D model will need thrust greater than 1:1, and a scale WW1 biplane will need considerably less. Here is a table giving performance in Watts per pound. Remember that if you are running your motor above its maximum efficiency the Watts per pound rule won’t be accurate, as a higher percentage of the Watts going into the motor will be producing heat instead of power.
70-90 watts/lb. Trainer and slow flying aerobatic models.
90-110 watts/lb. Sport aerobatic and fast flying scale models.
110-130 watts/lb. advanced aerobatic and high speed models
130-150 watts/lb. Lightly loaded 3D models and ducted fans.
150-200+ watts/lb. Unlimited performance 3D models.
Inrunner or outrunner?
Now you have an idea of the weight and power you will need for your model, what sort of motor is best, an inrunner or outrunner?
Both have their pros and cons.
Inrunners are constructed with the magnets attached directly to the shaft, which is surrounded by the copper windings. Because the magnets are close to the shaft it spins very quickly. This means they produce high rpm but low torque. This high rpm can be converted into torque by using a gearbox (check the section on gearboxes).
Inrunners are more efficient and powerful, but need a gearbox to drive large propellers. They produce high revs per volt (Kv) compared to outrunners. For models requiring a small prop running at high speed like a Zagi (wing), pylon racers and ducted fans, inrunners without gearboxes are popular. Once a gearbox is used there are even more pros and cons. Gearboxes are an extra expense, require maintenance and can be noisy, but you will still get the best efficiency and power with a geared inrunner spinning a large prop. This is the reason why all competitive F5b models still use geared inrunners.
Outrunners are constructed with the copper windings on the inside. The shaft is attached to a “bell”, or casing that contains the magnets, which spin around the copper windings. Because the extra weight of the bell and magnets are further out from the shaft it acts like a flywheel. Generally outrunners produce lower RPM at higher torque than inrunners due to the way they are made. This enables an outrunner to spin a larger prop without a gearbox.
This means no maintenance, quieter operation and cheaper purchase price (no gearbox). These factors outweigh the higher efficiency and power of the inrunner for most sport flyer's.
kv is simply the revolutions per minute (rpm) an electric motor will spin at per per volt when under no load. You could think of high and low kv like the difference between a high performance 2 stroke racing motorcycle engine compared to a 4 stroke Harley Davidson motorcycle engine. Just say they put out approximately the same horsepower, but the 2 stroke does it at 11,000rpm and the 4 stroke does it at 3,000 rpm. The same can usually be said for high and low kv electric motors. Assuming the same voltage, a high kv inrunner with a small diameter propeller would be perfect for a high speed model like a pylon racer, and a low kv motor with a large diameter prop will be better for power i.e. getting a sailplane to altitude, or 3D manoeuvres like prop hanging. Kv is determined by the number of winds or turns. This is the amount of times the copper wire has been wound around each stator pole. More winds = low kv, less winds = high kv.
kv has two main implications.
A high kv motor will spin faster than a low kv motor at the same voltage. This means you may choose to use a high kv motor if you are limited to a lower voltage battery pack. An example of this would be in 7 cell glider competition (7 NiMh or NiCd cells at 1.2 volts a cell = 8.4 volts). A lower kv motor could not produce enough rpm at 8.4 volts to be competitive, so a higher kv motor is used.
If you are not limited to a particular voltage, a lower kv motor can be used at higher rpm by using a higher voltage. Large outrunners with a kv of 200 to 300 are a good example of low kv high voltage motors. Make sure you consider the voltage limit for any motor you are considering.
Gearboxes (By wolw)
Choosing an inrunner and a gearbox isn't as complicated as it sounds, it is basically the same as choosing an outrunner but you add the downshift of the gearbox to the calculation.
When choosing an inrunner you usually have two sizes to choose from. I'll use Feigao for this example, have their motors listed on their site Feigao.com with a complete set of numbers on every motor and they even have suggestions for different set-ups. Feigao call their smaller diameter motors (27.6mm) "380" and their larger ones (36mm) "540". Both come in three different sizes, Small, Large and XLarge. These are copies of Hacker inrunners, "380" for the B40 size and "540" for the B50 size which makes it very easy to find an successful set-up to copy if you search the net. The ratios are the same for both Hacker and Feigao gearboxes 4.1:1 for "380" size and 6.7:1 for "540" size.
But what does that 6.7:1 mean ? To get the kv of the prop shaft with a gearbox, simply divide the kv of the motor by the ratio of the gearbox. If we take the FG540-07S as an example, it's a 5070kv motor but using a gearbox with a ratio of 6.7:1 you get a kv of 5070/6.7 = 757. This would fit a 3S setup for a hotliner perfectly. If you want to use it to it's max amp rating (93A) check that your Lipos are up to it (Check C Rating in this Guide). Personally I go a little easier on the Feigao motor/gearbox combinations when copying a Hacker set-up, as Hacker are are a higher quality product.
Peak Efficiency is a very good site where you can see how your intended setup will perform. Nothing is as precise as your own measurements, but it gives you some idea of what to expect. The graph isn't very easy to read but you'll get the hang of it if you read the "how do I read this chart" first.
Electronic Speed Controls (ESC’s)
There are two main types of ESC, for brushed or brushless motors. You cannot use a brushed ESC with a brushless motor or vice versa. Think of the features you will need like a brake and soft start. You will need a brake if you are using a folding prop and a soft start if you are using a gearbox and an on/off switch for a throttle. These features can often be found on Radio Controlled Sailplanes. The most important thing to consider when choosing an ESC is matching the ESC to your motor. It is good to use an ESC rated at a higher amperage than you intend running your motor at as an insurance against over stressing your ESC causing failure and potential damage to your model. Often you will see a burst rating for an ESC, meaning you can run the ESC at a maximum Amperage for a limited time, and exceeding this limit is asking for trouble. Most sensible aeromodelers like to have an esc capable of 10 to 20% more Amps than they plan to use depending on its quality. You will need a meter to measure the Amps and Volts being generated by your power system to ensure you are not stressing the battery, ESC or motor.
What is a bec? Bec is an acronym for battery eliminating circuit. This device provides power for the servos in your model. Many ESC’s have a bec that can only handle a certain number of servos at a given voltage. The higher the voltage you use the less servos you can use. Using too many servos from the bec in your ESC will cause overheating and failure of the bec. This will be catastrophic if your bec fails in flight so how can you safely run more servos with your ESC? External bec’s, or Ubec’s use power from your flight battery pack and are a cheap way of safely using more servos than the bec in your ESC can handle. A receiver battery pack is another way of supplying reliable power to your servos without using the bec in your esc.
Cut off voltage
Set the cut off voltage on your ESC to 3 volts per cell to ensure you don’t over discharge and damage your lipo pack.
The propeller is the component that puts a load on a power system. With the wrong prop you can damage your battery, ESC and motor. Think of the prop like the gears in a car. Some props are like first gear and the motor will have to work at high rpm to go slowly. If you have driven a 4X4 you will know that this gives you power to climb steep hills at low speed without stalling the engine. You could compare this to prop hanging a 3D model where power is more important than speed. On the other hand you might want to go fast. This will require a prop that is more like the top gear in a car. It doesn’t have the power to take off and climb a steep hill at low speed, but once up to speed it can maintain that speed comfortably. The numbers on a prop, say 10X4, give you the diameter and pitch. In this case you would have a prop with a diameter of 10 inches and a pitch of 4. A 10X4 prop will give you more thrust at a lower speed like the 4X4 analogy above. If you swapped it for a 10X7 prop you would have a higher top speed, but your take off run would be longer. The extra load on the motor would also draw a higher Amperage.
Pitch and Pitch Speed
Pitch is the distance (normally expressed in inches) that the propeller "cuts" through the air in a single rotation assuming no slippage. To achieve pitch, the propeller blades are angled to move air to create thrust. The angle of the blade determines its pitch. Propeller blades are aerofoils, just like the flying surfaces on our models. When they have a higher angle of attack they create more lift. In the case of propellers, a higher angle of attack (pitch) at a given rpm will create greater thrust.
Pitch speed is the speed at which the propeller pulls through the air. It is calculated by looking at the pitch of the propeller, and the number of revolutions it performs in a unit of time. Pitch speed does not consider slippage, drag and other forces that may affect the aircraft.
With a high wing loading you need a higher air speed to stay in the air. A higher pitch speed means lower thrust > longer take off > high landing speed. You can get both thrust and high air speed but it will be at a weight penalty as the power needed to get thrust for a short take off will not be in proportion to the power needed to stay airborne.
Warbirds are an often examples of models with high power/high wingloading which are supposed to fly fast, and especially in glow to electric conversions you will need to take the wing loading into account.
Hotliners and F5b models are one of the most extreme examples of high power/high wingloading. The more extreme examples have such a high pitch speed a catapult is needed to get them airborne because of the square (16x16) or over square (16x17) props they use in order to get extreme high speed/climbs. In a perfect world (with zero airframe drag and 100% prop efficiency) you can calculate the speed of your model from RPM x pitch)/1056 = your speed in mph. For example 10000rpm x 7" pitch /1056 =66mph or 105.6 km/h.
Pitch speed isn’t only about wing loading it's also about what you want to do with your model, as I wrote above about hotliners and F5b. With an already light model or of moderate weight you can determine the behaviour from the choice of prop > pitch speed. Without the need of changing anything (keeping the same amps) you can take a GWS Formosa II with a 10x5 from being a sporty low wing aerobatic trainer to a fast aerobatic plane with a 9x6. As a general rule 1" pitch relates to 1" of diameter, if you step up 1" in pitch you need to step down 1" in diameter to keep the same amp draw.
With more normal kind of planes we usually use a prop with the proportion of 1:2 i.e. 10x5, 11x5.5, 12x6 and so on as it is most effective (from what I heard). A High wing trainer could very well use a more square prop like 9x7 instead of 11x5.5, it'll still have a high lift and once airborne you can throttle down, the higher pitch will give it airspeed and you'll get long flying times with low amps, perfect for photography or video.
The following is some extra information about prop selection kindly offered by brucea from RC Groups.
“As a rule of thumb, you want to have a static pitch speed within the 2.5 to 3 times the stall speed. So if your plane stalls at 15 mph in level flight you would like a static pitch speed between 37.5 to 45 mph.
For a particular motor, I know from testing that with a 12x6" propeller the motor is running at 7165 RPM. Each revolution pulls the plane forward 6". So my plane would be making 6" x 7165 RPM or 42,990 inches per minute. Dividing by 12" gives me 3,582.5 feet per minute. Multiplying my 60 minutes gives me 214,950 feet per hour. Dividing by 5280 feet gives me 40.7 miles per hour. The plane I has a calculated stall speed of 14 mph. 40.7 divided by 14 equals 2.9. This ratio falls within the desired 2.5 to 3 ratio of pitch speed to stall speed, which is good!
To select a motor you may have to work back-wards from prop diameter. The plane I have can take a 12" prop. I like to get the largest diameter prop that will fit.”
Always use your lipo packs safely.
See this link for safety issues concerning the safe use of Lithium Polymer batteries:
Broadly speaking, the "C" rating is a guide to how much current it is safe to draw from your battery. It's expressed in terms of the capacity or C. Beware that constant discharging of your lipo pack at its maximum C rating will almost definitely shorten its life. Depending on the quality of your pack, it is much wiser to keep your current draw to about 10 C with short bursts up to 20 C if you want your pack to last. The easier you are on your pack the longer it will last.
A 2200mAh 10C battery is rated to be discharge at up to 22A (10 x 2200mA/1000) and the same size 12C battery would be good for 26.4A (12 x 2200mA/1000).
The internal resistance in higher C rated packs is lower, meaning that the voltage drop found in higher C packs is not as pronounced giving higher voltage under load and slightly more power.
Here is Brucea’s experience testing two 2100mAh 11.1V lipoly packs which demonstrates higher voltage of a higher C pack under load.
“I tested a 20C versus 15C 2100mAh 11.1V lipoly batteries. Motor volts and thrust are as follows:
15C: 9.3 V motor voltage, 44.6 oz of thrust producing 271 Watts
20C: 10.1 V motor voltage, 48.9 oz of thrust producing 318 Watts
The motor draws 29 Amps from the 15C 30Amp discharge battery, and 31.5 Amps from the 20C 40 Amp battery. For this motor I am using two 2100 mAh 11.1v 15C batteries in parallel. This gives me twice the "C" rating or 60 Amps. This particular motor pulls 31 Amps with a 12x 6" prop and two 11.1V 15C 2100 mAh batteries.”
Also beware that lipo manufacturers often put an overly optimistic C rating on their packs. Unless you see independent test results you trust for lipo packs, use them at about half the stated C rating and you should get many more cycles from them.
mAh is an acronym for Milliamp Hour, which is how much current a battery will discharge over a period of one hour. Higher numbers here reflect a long battery runtime and or higher storage capacity. For example a 2000 mAh pack will sustain a 2000 milliamp (2 amp) draw for one hour before dropping to a voltage level that is considered discharged. A 1700 will sustain a 1700 mAh (1.7 amp) draw for one hour. 1000 mAh is equal to a 1 Amp Hour (AH) rating.
Like the C rating, the mAh rating also determines the maximum current that can be drawn from a pack as can bee seen in the calculation in the C Rating section above. For example if you have three 11.1 Volt 10C packs, one rated at 1000 mAh, one rated at 1700 mAh and the other at 2000 mAh, we can determine that it is safe to draw the following amperage from these packs. Multiply the C rating by the mAh rating and divide by 1000 to convert milliamps to Amps:
10 X 1000 mA/1000 = 10 Amps
10 X 1700 mA/1000 = 17 Amps
10 X 2000 mA/1000 = 20 Amps
Thanks to the following for their help and contribution to this guide.
Peter (wolw at RCG) for his contribution to the sections on pitch speed and gearboxes.
Bruce (brucea at RCG) for his contribution to the sections on wing loading, prop selection and C rating.
Gnascher, Biplane Murphy, The Bandit, jasmine 2501 and AEAJR (ED Anderson) and especially UncleBob, All from Wattflyer for helping me polish this guide enough to post.
If anyone has anything to add, finds fault with any of the content or any other constructive criticism that will improve this guide, please add your comments here. Hopefully together we can compile a really helpful guide that will help a lot of people. Any useful additions will be credited of course.
I am just learning this stuff, and have spent the past month finding this type of information from all different sources. I wish you had done this a month ago! It sums up nicely all the stuff I have had to look for myself. As a beginner, I would say it explained in simple enough terms to understand what people need to look for. The only thing you might want to add is a small section on gearboxes, explaining gear ratios. Good job, I am sure others will appreciate it.
Thank you for this. I know there are a lot of stickies already, but this should be up there with them.
I have not had a chance to read everything yet, but it looks to have all the info I am looking for all in one place, and explains a bunch of terms that I didn't understand
Thanks for the positive feedback.
Sorry it wasn't available when you needed it! I agree about gearboxes, I have never used one as they are a bit of a mystery to me too. I hope someone can help us out here. Also esc timing and many other things are not covered yet.
"Choosing an inrunner and a gearbox isn't all that complicated as it sounds, you basically do the same as when you choose an outrunner but you add the downshift in the gearbox into the calculation.
When choosing an inrunner you basically have two sizes to choose from (if you are going to use an gearbox) . I'll use Feigao for this example, Feigao have all their motors listed on their site www.feigao.com with a complete set of numbers on every motor, they even have suggestions for different setups. Feigao call the one with smaller diameter (27.6mm) "380" and the larger one (36mm) "540" both come in three different sizes, Small, Large and XLarge. These are copies of Hacker inrunners, "380" for the B40 size and "540" for the B50 size which makes it very easy to find an successful setup to copy if you search the net. Ratio is the same for both Hacker and Feigao gearboxes 4.1:1 for "380" size and 6.7:1 for "540" size.
But what does that 6.7:1 mean ? To get the kv you simply divide the kv of the motor with the ratio of the gearbox. If we take the FG540-07S as an example it's a 5070kv motor but with an added gearbox with a ratio of 6.7:1 you get a kv of 5070/6.7 = 757. This would fit a 3S setup for a hotliner just perfect, if you want to use it to it's max amp rating (93A) check that your LiPos are up to it. Personally I would (and do) go a little easier on the Feigao when copying a Hacker setup, after all Hacker are very good inrunners but at a price.
Peak Efficiency is a very good site where you can see what your intended setup will do, nothing is as exact as your own measurments but it's a good place to start. The graph isn't very easy to read but you'll get the hang of it if you read the "how do I read this chart" first"
I think a rough draft of this was included in an early version of the guide.
If you think it's understandable and usefull and should be included tell Chris and he'll put it in .
It was not until after 1AM I understood that there's more than one page over at WF and they was taking the article apart
It was written with the beginner in mind and as such a guide it works very well, it's supposed to be understanable to someone who is going to buy or build his first or second plane.
If I was to talk about bus speeds and memory bandwidth to an elderly couple buying a computer to email their grandson they'd loose interest pretty soon. K.I.S.S
Foam and tape,
You're not wrong. If this becomes a sticky there will be a better chance that people will read it.
I haven't spent much time over there but some of them are keen if not a bit pedantic. Anyway if I can keep them happy everyone should be happy.
Joined Dec 2006
Thank you Chris for a fantastic single source of so much information that I needed. I am just learning all of this so please help me to answer a question I had.
Under "Pitch and Speed" in the first paragraph you say "In the case of propellors, a higher angle of attack (Pitch) at a given RPM will create greater thrust".
Then in the third paragraph talking about high wing loaded planes you say "a higher pitch speed means lower thrust > longer take off > high landing speed".
If pitch speed is calculated as RPM x pitch these two statements seem to be in contradiction to each other, but I probably don't understand it yet.
Thanks for the help.
Joined Dec 2006
Here is a side question. Now that I am considering buying some test equipment so I can tell what kind of performance I am getting from a particular set-up, what do you use to measure the RPM of a motor on the bench?
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