rsuarez

Could someone give me a brief explanation on how motors are rated. What does 400 or 600 stand for? What procedure do you guys follow in order to select a motor? and the same for gearboxes?

In glow it is easy as engines are rated by cubic inches and HP, but I have been trying to understand electrics with no luck.

thanks


rsuarez

Matt Kirsch

My Feedback: (21)

400 and 600 are just model numbers. They have no meaning in any grand scheme. Each manufacturer has its own rating system and model number scheme. For example, 400 and 600 are specific to Graupner's Speed series of inexpensive ferrite magnet can motors.

With internal combustion engines, you can only pack so much air and fuel into the cylinder, so the difference between the best and worst engine of a particular displacement is relatively small. More displacement means you can pack more air and fuel in, which means more power.

In contrast, the physical size of an electric motor is completely meaningless. Several different motors of the same physical size will have completely different characteristics. Just by altering the number of times the wire is wound around the armature, you can increase a motor's top RPM or increase its low end torque. It's very complicated, to say the least.

However, the motor is the LAST thing you choose when powering an electric airplane. The first thing you choose is the battery. You see, on a glow plane the engine is the entire power system. On an electric, the motor is only one piece of the puzzle; the battery is the other. Batteries put out Volts and Amps. Multiply Volts and Amps together, and you get Watts. 746 Watts make one Horsepower, the same kind of Horsepower that your glow engine makes. In effect, the motor is equivalent to the crankshaft of an engine; it just transmits the energy released by "combustion" to the propeller.

First thing you do when choosing a powerplant is estimate how much the airplane will weigh when it's done and ready to fly. On a glow powered conversion, you can get a good estimate by adding about 20% to the manufacturer's highest recommended all-up weight.
Next, you figure out how you want it to fly and choose a Watts-per-pound figure that corresponds to that style of flying. For example, a converted sport plane like a Sig 4*40 might be powered with 80 Watts per pound to make it fly like it's glow-powered counterpart. 35 Watts per pound is about the minimum required to fly, and 100 Watts per pound gives you nearly unlimited vertical performance.
At this point, you design the battery. Even though NiCd and NiMH battery cells are rated at 1.2V per cell, assume 1V per cell. For conversions of glow planes between .15 and .60 size, you should choose a current level between 25A and 35A. The lower the current you choose, the longer the flight can be. Just be aware that lower current requires more cells, which means more weight and possibly shorter flight times. Lower current also gives you some room to grow, in that you can get more power by simply installing a larger propeller. Try that trick with a glow engine! Also keep in mind that with higher currents, the price of the equipment goes up, and the life expectancy of the equipment goes down. Finding out the number of cells for the battery is as simple as taking the Watts per pound, multiplying by the weight of the airplane, and dividing by the current.
Here's where things get a little tricky again. We're up to choosing the motor, gearbox, and propeller. I tend to do research and see what others are doing with similar airplanes, and simply copy their ideas. However, narrowing down your choice of motor is as simple as picking one or more brands, looking at their catalogs/websites, and picking out likely candidates based on whether or not they can handle the number of cells and the current you figured out earlier.
The figure you're intersted in at this point is the RPM/Volt of your list of potential motors. A motor with a low RPM/Volt rating is good for direct drive; it has lots of torque. A motor with a higher RPM/Volt rating is essential for gearboxes. The object is to turn the largest practical propeller at the lowest practical RPM to make the plane fly as it was designed to fly. Slow fliers benefit from a high gear ratio and a large, slow-turning propeller. Fast, sleek airplanes benefit from a direct drive motor with a small, fast-spinning propeller.

If this all overwhelms you, you can get a computer program to help out. Try out MotoCalc for a 30-day trial: http://www.motocalc.com

rsuarez

I really appreciate your help, you anserwed beyond what I was expecting, this is very usefull information.

I never thought that electric fly could get this complex...very nice challenge.

Now whats with brushless motors. Is the current beeing alternated by the ESC? are they better? (Performance vs. $$)

Thanks again

rsuarez