Thursday, October 11th, 2012 10:33 am GMT -6 Thursday, October 11th, 2012 10:33 am GMT -6Thursday, October 11th, 2012 10:33 am GMT -6
Very large electric brushless outrunner motor

What was I thinking? Let me explain the logic behind my electric model airplane power system sizing rule.




RCadvisor’s Power System Calculator

I launched five years ago with my power system calculator. In fact, that is all that I had on the website. Other than an occasional technical article and an online glossary (which was very popular), I really did not have anything else on the website.

Writing the calculator forced me to look deep down into many technical issues that come up when assembling an electric power system for a model airplane. There are many gotchas in the process. The calculator works extra hard to hide the complexity behind what it does.

Something that the calculator does that may not be obvious is how hard it works to make the most of bad or incomplete input data. It has a lot of knowledge built-in on how real electric motors behave.

Electric Motor Efficiency

I did a lot of power system analyses while working on the calculator. I needed to test the algorithms that I had programmed. I also needed to make sure the user interface was as easy to use as possible.

Something I learned in the process of testing the calculator is that the efficiency of an electric motor can vary a lot. A range of 50% to 90% is very common. Where does this wasted energy go?

Motor Power Rating

The electric power going into a motor that is not turned into useful work on the output shaft is mostly turned into heat. The ability of an electric motor to get rid of this excess heat is what limits how much input power it can handle.

Learning this helped me see our electric motors in a new light. They were no longer black boxes. They were now devices which followed simple rules of physics.

Model Airplane Design Made Easy

I really enjoyed writing my first book, RCadvisor’s Model Airplane Design Made Easy. I wrote it about a year after I had created my power system calculator.

At the time I wrote the book, I was very familiar with the various rules of thumb on sizing an electric power system. The best known was the watts/pound rule, which gives guidelines on how much power you need depending on the airplane’s total flying weight. This power rule works reasonably well. But I thought it was more complicated than it needed to be.

The rule also looked very incomplete to me. It failed to answer the critical question of how many watts you can safely expect to get out of a given electric motor. It also said nothing about how big the battery should be, which is an equally important sizing decision.

Could I come up with a better rule and put it in the book?

Motor Sizing

The key to my power rule is the observation that our outrunner brushless motors can handle about 75 watts of input power per ounce of motor weight (2.65 watts/g). This of course depends on the shape of the motor and how much cooling air it gets. But as a rule of thumb, it was reasonably accurate.

Then I concluded that a sport electric model airplane needs about 75 watts per pound to have enough power to fly as it was intended. This was based on the old power system rule of thumb and my own observations of successful power systems.

From there, some simple calculations told me that the target weight of a brushless outrunner motor should be 10% of the total airplane weight.

I studied a lot of successful power systems to confirm the validity of the rule. The rule works well across a large number of different types of model airplanes. I noticed that very small model airplanes generally need bigger motors than predicted by the rule. Similarly, very large electric model airplanes can get away with smaller motors. But overall, the rule works extremely well.

I like this rule of thumb a lot. You can pretty much count on a product spec sheet telling you the weight of an electric motor. How much power it can produce is a lot less obvious.

Using this rule of thumb, sizing a motor up or down depending on the power requirements is easy. Do you want a fully aerobatic model? Make the motor 15% of the total weight.

Battery Sizing

Could I use similar reasoning to come up with a rule of thumb for sizing the battery, too?

Actually, coming up with a rule for batteries was easy. I already knew that most pilots fly for about six minutes. I knew the energy density of LiPo batteries, and I knew the power needs of the motor. Some more simple calculations later, I concluded that the battery should weigh about 15% of the total airplane’s flying weight.

Again, knowing how much a battery weighs, even before you buy it, is easy. It is also easy to scale it up or down depending on how long you want to fly.

Carlos’ Power Rule

Finally, here is Carlos‘ Power Rule is its full glory:

“For a sport electric model airplane, 10% of the total airplane weight should be the motor and 15% should be the battery. This is valid as long as you use a brushless outrunner motor, LiPo batteries, and fly for six minutes.”

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