How to choose the right motor and propeller for your RC aircraft | Power to weight ratio

One of the most frequent questions of the hobby is the power/weight ratio.

There are several things to take into account regarding the propulsion system, the power/weight ratio, the wing loading and the propulsion system itself, which is composed of the engine or motor, the propeller, the speed controller, and the battery, the question basically refers to which motor/engine to use if, for example, the airplane model weighs 1 kg.

hélice y motor

The power/weight ratio.

There is a close relationship between the weight of the aircraft and the power or thrust that the model aircraft must have, also with the wing loading, the wing loading varies according to the type of aircraft, in gliders for example the wing loading is less than in aerobatic aircraft with shorter wings, then lower power motors can be used in gliders than in aerobatic planes which need to go fast to maintain flight. The power/weight ratio can be expressed as follows: 0.5: 1 means that the thrust is half the weight, with this in mind, for gliders or light and slow-flying model aircraft, a ratio of between 0.3: 1 to 0.4: 1 on trainer models from 0.5: 1 to 0.8: 1 and on fast models from 0.8: 1 onwards. To calculate this with the weight of your model plane, simply replace the 1 by the weight and multiply the number on the left side by the weight, so if your model plane is 1.2 Kg and it is a trainer type model plane, we are going to use 0.7 : 1 = 0.7: 1.2 by multiplying this we obtain 0.84 Kg (840g) this is the thrust we must have to fly comfortably.

How do I know the thrust that my motor can generate?

The manufacturer of each motor provides detailed information on the specifications of their motors, probably presented in a table that indicates test results with different propellers and voltages, it also shows the amperage drawn at a certain speed and the thrust that can be expected, Ideally, the motor have to offer the thrust we have calculated at 85% acceleration, it is not recommended to keep it at 100% constantly unless you use a battery with a lower voltage than it can withstand. This is so as not to keep the motor and ESC in constant fatigue unless your components can handle such power.

tabla de eficiencia hélice y motor brushlessThis table is an example of the information that a manufacturer must provide to be clear about what type of efficiency we can expect from the motor, some online stores will not show you a table like this, but they will give you the most basic information that is also of great help.

What propeller to use?

This is also something that the motor/engine manufacturer tells you, so it is important to read the specifications or the tables that show the corresponding information, if you use a propeller with a smaller size or lower pitch, it is possible that the motor/engine will not generate the expected thrust, if you use a bigger one then try to use less voltage or you run the risk of burning or damaging the motor.

And where is the KV number in all of this?

The KV is a constant of revolutions per minute per volt, this number has nothing to do with the power of the motor (it does indirectly), and only with the speed at which it turns, and this helps us to know which propeller is more convenient to use, so more KV means that you’ll use a smaller propeller or with less pitch, and less KV means that you can use a bigger propeller, we can treat this in another post with more detail.

Alternative technique to choose the motor to use.

Another way to calculate the power you need for your model aircraft is with the power generated by an engine or motor, power is measured in Watts, so let’s see which are the most common power settings according to the type of model aircraft.

  • Slow airplanes and gliders110W/Kg or 50W/Lb
  • Slow airplanes and vintage110W/Kg - 176W/Kg or 50W/Lb - 80W/Lb
  • General and sport176W/Kg - 264W/Kg or 80w/Lb - 120w/Lb
  • Acrobatic264W/Kg - 396W/Kg or 120W/Lg - 180W/Lb
  • Jets and powerful airplanes296W/Kg - 440W/Kg or 180W/Lb - 200W/Lb
How do I know the power that my motor generates?

Once again, check the data of the motor according to the manufacturer, the easiest way to know the power that the motor generates is knowing the amperage and voltage that it accepts (V.A) when multiplying both you get Watts (V.A = W) then a motor that supports 12v and consumes up to 30A generates 360W. These are the maximum values ​​that the motor should be at 100% constantly, and we are basing these calculations on what the manufacturer tells us but the amperage value is likely to be lower in real life, assuming the motor actually it consumes 15A with the propeller that we are going to use, then the calculation would result in 180W.

Combustion engines.

Among the combustion engines, we have those of explosion and turbines, those of explosion usually have measurements of the displacements and also the power they produce in horsepower (HP), taking into account that 1HP = 746W then we can calculate the engine we need with the data provided above, in the case of turbines, their thrust can be measured in Lb, Kg or N, making the calculation easier.

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