What motor do I use in my 40 size Plane?

 

This is the most common and difficult to answer question for those with an IC engine background.

The answer is of course, it Depends.

One of the reasons for this answer is that in the electric world there is not the "One size fits all" solution that exists for many IC powered kits on the market today. The theory behind the one size fits all is that if you overpower it, it will fly! This means motor choice is simple but is not always optimised to the model in question. For example the typical ‘40’ size glow trainer in my opinion flys too fast for many raw beginners, mostly because that’s the speed that 10x6 prop on a forty size motor wants to go to run reliably.

With electric power systems we need to be a bit more frugal on how we use the available power, as there is not so much in surplus. (With newer lithium based batteries now available the power to weight ratio’s available are steadily improving)

With an IC engine the number of variables that you can change are minimal. Generally the fuel is a constant, other than the amount you put in, and the mix will be recommended by the manufacturer. About the only thing you can change is the prop size. Even here you must stay within the recommended range that the manufacturer states or your engine may not run reliably.

With electric power you start with what is most important, and that is the batteries, for these determine the amount of power going in to the system ( number of cells and current capacity) and the duration of flight (Cell capacity). From here you move to the speed controller, which must be sized to handle the voltage and current expected to be used. Next up the chain is the electric motor itself, the size of motor will depend on how much power is going into it and how efficiently it can convert that into turning force for the prop. There are many variables in the motor its self to help match it to your system, but none of these are ‘user’ changeable after you have bought the motor, unless you’re a dab hand at rewinding motors! For some systems this may be the last step in the chain before the propeller. However in many electric systems you can add a gearbox to increase the torque and lower the revs to the prop. Finally with electric specific props there is a huge range of size and pitch combinations to choose from.

With either electric or IC power, perhaps the biggest variable of all is the plane itself.

The following two aircraft are designed for a 40-46 size glow engine.

 

Sig Rascal 40 ARF Kyosho Zero 40 SQS ARF
rascal40.gif (5K)

 

Although they recommend the same size glow engine when converted to electric power they can be powered quite differently.

The Rascal 40 is a high wing cabin plane with a large wing area and likes to cruise around at a mild pace. An example power system would be a Plettenberg Orbit 20-18 motor running direct drive to a 12x8 prop on 12xGP3300 NiMh cells. Power Input ~ 450W

The Zero on the other hand is a warbird which needs to have a presence in the air which requires a reasonable amount of power and speed. An example power system would be a Kontronik Fun 600-18 with 4.2:1 gearbox driving a 14x10 prop on 16xGP3300 NiMh cells. Power input ~ 700W

 

A standard 40 size glow motor produces approximately 1 Horsepower which equates to about 750W of electrical power. As you can see by the above examples there is a lot of scope when selecting a power system and you don't always need the same power.

Rather than asking "What electric motor is the same as a 40 size glow engine?" you should be asking "What motor system do I need to power this model"

How to choose a motor for that "40 size" model can be summarised into the following steps.

  • Is the "Empty" weight of the plane acceptable? Empty weight is the model with only the servo's and receiver installed ie with out motor/gearbox, flight battery and speed controller. This can be difficult to work out from an ARF kit as the weight quoted is usually ready to fly. Don't forget to subtract the throttle servo & fuel tank weight as well as the glow engine. If the model is heavy to begin with, things aren’t going to get any better!
  • What sort of speed range do I want? This will partly depend on wing area and aerofoil design of the model. Its relevant to determine how much pitch speed is required for optimum performance.
  • What sort of climb rate do I want? How much power do you really need? If a gentle flyer is all you want then don't go putting a V8 under the hood. More power also means more batteries, which in turn means more weight. The more power you put in, in general the physically larger motor you need.
  • How much prop clearance do I have? In general unless your making a pylon racer, the bigger the prop the better. It may even be worthwhile modifying the undercarriage of your proposed model to take a larger prop.

Once you have answered the above questions you can use various resources to narrow your selection down. One of the easiest ways is to find some one else who has already powered the same model. The internet is a useful tool for searching discussion forums and other R/C related sites.

Asking a more knowledgable colleague is another easy way out.

Some motor manufacturers have test data of there motors with various props and cell count combinations to assist your selection.

Lastly there are computer simulation tools such as "Motocalc" to help get you into the ball park, although these require some knowledge to get the most out of them. Simulation software suffers from the garbage in garbage out syndrome so this is were experience can help recognise when something doesn’t look quite right.

If all of this still seems too daunting it may mean that you need a bit of a refresher course in basic electric theory, depending on how much you paid attention to that high school science class. However we will save that for another edition.

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