Choosing the right power system for your plane

One of the most common questions we get asked is how to choose the proper brushless motor setup for a model. Modern technology has allowed for an abundance of data being produced for the various electronics used in our hobby. However, we understand sometimes this data can be difficult to translate into choosing the correct components for your model, especially if you’re just starting out in the hobby. We hope this blog post will help.

Motor sizing

Brushless outrunner motors are sized using one of a few primary methods. Many brands on the market use similar marketed sizes. You’ve probably seen a handful of brands advertising a “2814” motor. You’ve probably also seen one company selling a “2826” motor that is significantly larger than another brand’s “2826” motor. This is because some brands use the internal stator dimensions and some brands use the outer dimensions of the motor.

So, a 2826 motor that is labeled as such based on the outer dimensions of the motor casing is 28mm x 26mm in size. A 2826 stator motor has internal sizing of 28mm x 26mm, meaning the outer dimensions are actually much larger. This is why it is important to look at the specifications of the motor to determine if the outer dimensions of the motor match the model number being used. For example, our Suppo motor line uses the stator dimensions. Leopard and other brands we sell use the outer case dimensions which is generally more common.

Some common motor sizes and their equivalent counterparts are shown below:

Most modern airplane kits on the market today will generally include a motor recommendation from the manufacturer. Although the numbers between motors may be slightly different (calling for a 2312 versus a 2212 – only one millimeter different – for example), you will generally be able to narrow down your search for the proper size motor based on these manufacturer recommendations. It is just a matter of then deciding between brands. A 3536 1000kv motor in one brand will generally be close in output and specs to a 3536 1000kv motor in another brand. What will vary between brands is the quality of components used which can affect the efficiency and longevity of the motor. Brands with higher quality bearings and internal parts will require less energy to power and less maintenance over the long term. Brands will also have different bolt spacing/mounting dimensions on the back of the motor. Sometimes the decision on what motor to use will come down to which one lines up with the factory-installed firewall mount on your model. Often times, certain manufacturers will use proprietary spacing to make you have to use their brand motor on their airframe. In this case, some simple modifications in the form of drilling new holes may be required to make a different motor attach correctly to the firewall.

Motor Watts

One of the best determining specifications to look for when sizing a motor to a model is the motor’s rated watts. The motor will typically list a max wattage which is based on the max rated amp draw and max rated voltage the manufacturer says the motor can withstand. Volts x amps = watts. The max watt rating is typically the most the motor can generate, running the largest prop possible, on the highest cell count battery it can withstand without burning up. In this way, the max watt rating isn’t as important to look at it as most users won’t be running the motor to its absolute max under perfect conditions. It’s more important to look at the motor’s continuous watts which can generally be found on a propeller chart that the manufacturer has available for the motor. This will show specific watt ratings at a variety of different voltages with a variety of different props. To determine how many watts you need, multiple the following figures times the projected weight of your model.

• For trainer and slow flyer models: 100 watts per pound
• For general sport models: 125 watts per pound
• For 3D models: 150 watts per pound
• For ballistic 3D performance: 175-200 watts per pound

Motor Thrust

Most propeller charts will also give a thrust ratings in grams for the motor when used with the desired prop and voltage. A 3D model will need greater than 1:1 power-to-weight ratio, meaning the motor’s thrust must be higher than the model’s weight in order to get sufficient vertical performance. A scale model or slow flyer plane will be able to fly on a lower power-to-weight ratio.

Motor KV

Each motor is rated in kilovolts or KV for short. The KV factor determines the RPM of the motor. RPM = KV x voltage. You will see most motors being offered in several different KV options for the same size motor such as a 3536-960kv 3536-1100kv, 3536-1520kv, etc. This means the motor is identical in build and physical size but the internal windings of the motor have been changed to affect how slow or how fast the motor spins. A higher KV rating means the motor will spin faster. It will require a smaller prop for less load on the motor and it will produce more speed but less torque. Your model will go fast but the vertical performance will be more limited. A high KV motor is good for racers or faster planes. Conversely, a low KV motor will spin more slowly. It will need a larger, flatter prop to put more load on the motor and it will produce more torque but less speed. Lower KV motors are good for 3D planes that need to be slow and floaty but be able to have unlimited vertical performance or hang on the propeller. A KV option in the middle of the low and high range will generally be a good match for general sport performance.

Brushless motors are hand wound and, therefore, will always have slight variations between motors. These variations are fairly minor, averaging about plus or minus 30kv per motor.

Collet vs bolt-on prop adapter

Motors come in two main types of prop adapter configurations: collet and bolt-on style. We’ll explain the differences below and show a photo to illustrate.

• Collet version: this configuration is designed for the motor to mount in front of the firewall and includes a compression fit-style collet adapter which clamps to the shaft of the motor.
• Bolt-on prop version: this configuration can be mounted one of two ways. The first is a traditional style mounting method where the motor attaches to the front of the firewall. There is no front protruding shaft such as with the collet-style. Instead, the prop adapter bolts to the can of the motor. Alternatively, the motor can be mounted behind the firewall (as is common on most sailplane models) and the shaft of the motor will protrude forward past the firewall. For this type setup, you must use a collet-style adapter or spinner assembly (not included, available separately).

The style of motor you choose is important and will depend on your model. In general, bolt-on style adapters are more common as they are more secure. However, certain models (especially scale planes) need the forward protruding shaft in order to use a spinner assembly. The bolt-on style adapter is usually limited to the prop nut included with the motor. Whether your motor front mounts or rear mounts to the firewall will also dictate which style you go with.

Motor quality

We often get asked what the best quality motor line we sell is. Each brand of motor we sell in our shop is of proven quality and performance and we trust them all in our own models. Choosing between our brands really comes down to your budget. In terms of “tiers” of quality of components used in each motor, T-Motor AT-Series and Leopard can be considered the more premium of our motor lines. These two brands feature high end internal parts that will give you years of service-free, competition-level performance. T-Motor AS Series, HobbyWing Skywalker, and Flash Hobby are mid-tier motors, giving a good balance of price and dependable quality. DYS and Suppo are our budget series, offering a lower price point due to more economically-friendly components. Rest assured, even our budget motors are of great quality and we frequently get positive reviews about the performance and longevity of these brands. They will serve the average modeler just fine.

Choosing an ESC

As a general rule of thumb, it is best to pick an ESC that is rated for 20% higher than the max current rating of the motor. For example, if your motor is rated at a max current of 50A, pick at least a 60A ESC. This gives “cushion” should you accidentally pull more than the recommended current of the motor. Using an ESC that is “overkill” or significantly larger than what the motor needs won’t harm the motor but will result in unnecessary extra weight.

Most modern ESCs are compatible with most motors on the market. You will find a wide variance in the price of ESCs on the market. The price will typically reflect the number of programmable options of the ESC, the current capacity of the BEC inside the speed controller, and the quality of the internal components. Your project will dictate whether you need any advanced programming featured on your ESC beyond the typically features such as brake, soft start, timing options, etc. Most entry level speed controllers will have similar programmable features to higher end models but may have fewer possible selections for each parameter. However, this typically won’t be an issue for the average user. An example of an advanced programming feature that some users may need is a programmable BEC voltage. Less expensive ESCs will typically provide a fixed 5 or 6 volts to the receiver to power the servos. Whereas, more expensive ESCs may allow the user to select up to 8.4V to get the full benefit of high voltage (HV) servos. This is important for 3D planes requiring monster torque for their flight surfaces.

BECs

On the topic of BECs, lets explain what the BEC is. The BEC, or battery eliminator circuit, is a component within the ESC which takes power from the flight battery, regulates it down, and sends it through the signal cable to the ESC. The power is sent through the red wire of the signal cable. The signal cable is that servo-lead-looking wire that connects to the receiver. If you disable this red wire by cutting it or removing it from the plug, your ESC effectively becomes an “OPTO” ESC. An OPTO ESC means there is no internal BEC. If you purchase an OPTO ESC or disable the red wire on your ESC that is equipped with a BEC, you will need to provide an external power source to power your RX. It is best to find an ESC equipped with a switching BEC, or SBEC. Some less expensive or smaller ESCs on the market are equipped with linear BECs. Switching BECs are better capable of handling higher servo loads without shutting down. Linear BEC speed controllers are sufficient for smaller, lighter weight planes like slow flyers and foamies. Pick an ESC with a BEC having a sufficiently large BEC to handle the number of servos your model has. Most mid-range ESCs will have a 5-6A SBEC which will generally be enough to handle a standard 4 or 5 servo airplanes. Larger ESCs on the market often will feature 8-10A SBECs. In most cases, average size servos will typically only pull a few hundred milliamps of current each when under load. However, if the servo malfunctions or stalls at max travel, it could pull up to 1A or higher. If your giant scale model will be using a large number of very high current draw servos, you may want to consider using a larger external BEC or separate battery pack to power your receiver to avoid the receiver from shutting down.

It is important to remember in twin engine aircraft that you will need to disable the red wire on one ESC (when equipped with an internal BEC) to avoid sending double power to the RX.

ESC quality

Similar to motor quality, we often get asked what the best ESC is to buy. Many modelers are surprised to hear there are very few ESC factories in the world. Many of the same ESCs are sold under dozens of different brands, names, and labels on the market today, and they often come out of just a few factories in China. Many ESCs that come in popular ready-to-fly planes on the market are just lower-grade versions of common ESCs sold under the company’s own name. These RTF model producers sometimes request the ESC factory use lower grade components in order to keep costs low on large bulk purchases. This often causes the speed controller to fail faster than they should. Sometimes, lower end ESCs and motors may have similar quality components as higher end products but cut costs by employing lesser quality control. This can explain why a budget-grade electronic item may have a higher probability of not working right out of the package but give years of problem-free use if it does work on first start up.

The two most common ESC manufacturers in the world currently are HobbyWing and ZTW. We choose to sell their products under their own name as we don’t believe employing a lower grade manufacturing process to save a few dollars. You can be assured the ESCs we sell are genuine, full grade products direct from the factory. HobbyWing and ZTW ESCs are some of the highest quality and most dependable speed controllers you can find today. Both offer similar quality and performance, and each of their model lines are comparable across brands. Below is a chart explaining the different series and their equivalents.

All three tiers of ESCs feature each brand’s legendary quality and performance. “Budget” by both these brands doesn’t mean lower quality or less reliability, just fewer features. Higher tier ESCs by these brands will generally feature more programming options and larger internal BECs compared to their budget counterparts.

Propellers:

It is always best to choose a propeller within the manufacturer’s suggested size range for the motor. Using too small of a prop can cause the motor to spin at too high of RPM, generating excessive heat and putting more wear on the bearings. It will also cause the motor to have low output in terms of power. Using too large of a prop can put excessive load on the motor, resulting in a drastic spike in amps. This can cause the motor to overheat and fail. Too large of a prop can also cause the motor can to flex under load, potentially resulting in magnets coming delaminated from the casing. Select a propeller diameter within the manufacturer’s range and then use the pitch of the prop to adjust for the desired performance. A lower pitch of the same diameter will provide more torque/thrust and less speed. Lower pitch props are best for 3D planes needing slow and floaty performance or models where scale performance is desired. Higher pitch props will give more speed and less torque/thrust. These are best for racers or higher performance planes needing speed but less vertical performance.

Thanks for reading and shopping with us. As always, don't hesitate to contact us if you have further questions.