Motor Calc Program - find which motor, battery and prop is right for your EP airplane

Multiplex 2007 Catalogue in English

Brushless motors come in 2 different varieties: "Inrunner" and "Outrunner."
An inrunner is your standard motor shape/design. Usually in the higher kv rating of 2000-10000kv used with gearboxes for increased torque output.

Anything over ~5000kv is generally too fast / low torque for plane use and used mainly in ridiculously fast 1/18th scale cars and trucks with gear reduction of course.

"Outrunners" are higher torque/low kv motors, anywhere from 200 (big BIG power) - 2500 kv (ducted fan/high speed plane), and don't really use gearboxes (sometimes 2:1 reduction or similar for higher kv motors).

To make it easier to figure out your needs, some BRUSHLESS motor companies use the motor "Speed size" designation in the labels to help pick out your motor, such as the E-Flite Brushless motor series; Park 370 (in- and Outrunner), Park 480 Outrunner, though compared to their brushed motor counterparts these motors are well over powered (and goodly so). Then you have their larger motors named like so: Power 10, Power 25, which means they are about the same as a .10 sized, or .25 sized nitro/gas engine respectively.

To further complicate things, many companies name their Brushless motors according to their physical size, as in rotor or can/bell diameter/length, kv rating, # of stator poles (don't ask I'm no expert either) in Outrunner type motors.

But you are pretty smart it seems You already know the Kv rating under load means nothing (well it still is useful) and that watts is usually what you want, as I listed above. However, the lower Kv motors are good for larger props direct drive, and higher kv motors are good for smaller direct drive props, or use gearbox for larger ones. Also, voltage from the battery drops under load, too. Nimh/nicad batteries are worse at voltage drop under load than Lithium Polymers in that respect.

Also you want to keep amps down and volts up with your setup, in order to reduce heat and stress on the power system you use.

Work out your watts, then find a matching motor for your application, an ESC (electronic speed controller) that can handle the amp draw and voltage of your motor, battery that can deliver the calculated voltage/amp draw (very important), then figure out which prop works with your setup, i.e. if you have a sport plane, you want higher speeds and thus higher kv motors with smaller props, while for 3D or slow flyers, you want low kv with larger props for more thrust instead of speed. Though not always, as you'll see.

As you may know: Watts = Amps x Voltage.
E.g. you have a 1.5lb (24oz) ready to fly weight, sport plane. You want say 100w /lb for decent sport performance, probably a smaller higher pitch prop.

So 1.5lb x 100watts = 150 watts total power. Ok. Let’s see. The E-Flite Park 450 Brushless Outrunner (890 kv, by the way) direct drive, is rated for about 150-175 max watts @ 14 amps constant current, using a 11.1v 3-cell lithium pack. Since it is Brushless you can afford to fall short a few watts and not have any problems (better efficiency). 11.1v x 14 amps = 155.4 watts. Oh, and you'll want a prop that can put the right amount of load on the system and yet harness the power. I am guessing from my past experience, an APC 9" x 7.5 E prop will do just fine. (just in case, the 7.5" is the pitch of the prop). I recall good strong climbouts, nice top speed. ...And then the crash..... long story.

Now you'd want to make sure your lithium battery can handle the amps comfortably. So you look up, (off the top of my head) a Thunder Power 3-cell 11.1v 1320 mah 12C (someone correct me) battery . The 12C means it can provide a constant amp draw of 12 x it's capacity = 1.320 amps x 12 = ~15-16 amps.

BTW, I think it's a good idea for your own safety/accuracy that you use a power meter of some sort, like the Astro Watts Up meter, or E-flite's Power Meter so you can measure the amp draw/watts of your system so you don't over amp any part of your equipment.

Whatever I've missed someone else can assuredly step in and add!

I was in your shoes earlier this year. Practice and experience have brought me so far, and LOTS of MONEY (which I don't have). And of course, this very helpful forum! Thanks Al Gore for making the internet!

Ok bad joke.
Hope you found my thoroughly boring lesson somewhat helpful.

POWER = WATTS

I will be using the terms Volts, Amps and Watts throughout this discussion.
Let me define them.

Volts = the pressure at which the electric energy is being delivered - like pounds per square inch or PSI in a fuel system or water from a garden hose.
Volts is about pressure, it says nothing about flow. You will see volts abbreviated as V.

Amps = the quantity or flow of electricity being delivered, like gallons per minute in a fuel system or that same garden hose. Amps is about flow, it says nothing about pressure. You will see amps abbreviated as A.

Watts = V X A. This is a measure of the energy or power being delivered. This is how we measure the ability of that electricity to do work, in our case the work of turning a propeller to move our airplane through the air.
Watts is about both pressure and flow. This serves the same purpose as the horsepower rating of your car's engine. In fact 746 watts = 1 horsepower. So if you had an electric car, the strength of its motor could be reported in either watts or horsepower. You will see watts abbreviated as W.

If you want more depth on this, visit this thread.
http://www.wattflyer.com/forums/showthread.php?t=1933


MOTOR EFFICIENCY - Brushed vs Brushless
http://www.davison03.110mb.com/Peterlee%20Index.html
Whether brushed or Brushless, the motor's job is to convert electricity into mechanical motion to turn the propeller to move air. Efficiency is how we measure how much of the power, the watts, that our battery delivers to the motor is actually turned into useful work and how much is wasted as heat. A higher efficiency motor delivers more energy to the motor, and wastes less.

A typical brushed motor, say a speed 400, is only about 40-50% efficient. Only about half the watts delivered to the motor actually end up as useful work turning the propeller. The rest is wasted. Motors that have a "speed" designation, like speed 400, are brushed motors. There are other names for brushed motors but the "speed" term is a common one. They are inexpensive and they work. For example, you can buy a speed 400 motor and electronic speed control, ESC, for $35. A comparable Brushless motor/ESC combination would typically cost 3 to 4 times that much.

Brushless motors tend to be more efficient. They typically deliver 70-90% of that input power to the propeller, Thus you get better performance per watt with Brushless motors. Seen a different way, if you use abrushless motor , then, for the same flying performance you will use less energy which means your battery will last longer. Or you can use a similar size and weight Brushless motor and get much higher performance because the motor turns more of the watts from the battery into useful work of turning the propeller.

So, as with many decisions we make, this is a cost benefit decision. Am I willing to pay more to get more? That is up to you.

THE BATTERY IS MORE THAN JUST THE FUEL TANK

Think of the battery as the fuel tank plus the fuel pump and a supercharger all rolled into one. It feeds/pushes energy to the motor. So you have to look at the battery and the motor as one unit when you are sizing power systems for electric planes. In many cases we start with the battery when we size our systems because the motor can't deliver the power to the prop if the battery can't deliver the power to the motor.

The higher the voltage rating of the battery, the higher the pressure, like a supercharger on a car engine. More pressure delivers more air/fuel mixture to the engine, which allows the engine to produce more power to turn the wheels of the car. Higher voltage pushes more electricity into the motor to produce more power.

Using our electric motors, a given motor may take 10 amps ( the quantity of electricity flowing ) at 8.4 volts ( the pressure at which the electricity is being delivered) to spin a certain propeller. We would say that the battery is delivering, or that the motor is drawing 84 watts, ie: 8.4V x 10A. If you bump up the voltage to 9.6 volts, the battery can ram in more amps delivering more energy to the motor, which will produce more power to the propeller. In this example, if we move from an 8.4V battery pack to a 9.6V battery pack the motor may now take 12 amps. This will typically spin the motor faster with any given propeller or allow it to turn a larger propeller at the same speed.

However, if you bump up the pressure too much, you can break something. Putting a big supercharger on an engine that is not designed for it will break parts of the engine. Too much voltage can over power your electric motor and damage it. So there is a balance that has to be struck. Different motors can take different amounts of power, watts, volts X amps, without damage. For example, a speed 400 motor might be fine taking 10 amps at 9.6 volts or 96 watts. However a speed 280 motor will have a short life with the same combination of volts and amps.

If you match the right battery with the right motor, you get good performance without damage to the motor. In many cases airplane designers will design planes around a specific motor battery combination so that they match the size and weight of the plane to the power system for good performance.

PROPELLERS

Propellers are sized by diameter and pitch.

The diameter of the propeller determines the volume of air the propeller will move, producing thrust, or pushing force. Roughly speaking the diameter of the propeller will have the biggest impact on the size and weight of the plane that we can fly. Larger, heavier planes will typically fly better with larger diameter propellers.

Pitch refers to the angle of the propeller blade and refers to the distance the propeller would move forward if there were no slippage in the air. So a 7-inch pitch propeller would move forward 7 inches per rotation, if there were no slippage in the air. If we combine pitch with the rotational speed of the propeller we can calculate the pitch "speed" of the propeller. So, at 10000 revolutions per minute, that prop would move 7000 inches forward 70,000 inches per minute. If we do the math, that comes out to a little over 66 miles per hour.

By changing the diameter and the pitch of the propeller we can have a similar effect to changing the gears in your car or a bicycle. It will be harder for your motor to turn a 9X7 propeller than an 8X7 propeller. And it would be harder to turn a 9X7 propeller than a 9X6 propeller. The larger, steeper pitched propellers will require more energy, more watts, more horsepower, to turn them. Therefore we need to balance the diameter and pitch with the power or wattage of the motor/battery system. Fortunately we don't actually have to do this as motor manufacturers will often publish suggested propellers to use with a given motor/battery combination. We can use these as our starting point. If we want we can try different propellers that are near these specifications to see how they work with our airplane.

NOW WE CAN START TO MATCH UP THE PIECES!

The simplest approach I have seen to figuring power systems in electrics is input watts per pound of "all up" airplane weight. The following guidelines were developed before Brushless motors were common but it seems to hold pretty well so we will use it regardless of what kind of motor is being used.

50 watts per pound = Casual/scale flying

75 watts per pound = Sport flying and sport aerobatics

100 watts per pound = aggressive aerobatics and mild 3D

150 watts per pound = all out performance.

Remember that Watts = Volts X Amps. This is a power measurement.
In case you were wondering, 746 watts equals 1 horsepower.

AN EXAMPLE!

This should be fun. Let's see where these formulas take us! We will use a 24-ounce, 1.5 pound plane as our example. If we want basic flight you will need 50 watts per pound or about 75 watts input to your motor for this 1.5 pound plane. That is, 50 watts per pound X 1.5 pounds = 75 watts needed for basic flying performance. If you want a little more spirited plane, we could use 75 watts X 1.5 pounds which is about 112.5 watts.

Let’s use 100 watts as the total target, just to be simple, shall we? I am going to use a lot of round numbers here. I hope you can follow.

The Battery:

If we use an 8 cell NiMh battery pack at 9.6 V it will have to deliver 10.4 amps to hit our 100 watts input target ( 100/9.6 = 10.41amps) If my battery pack cells are NiMh cells that are rated at 10C then I need an 8 cell pack rated at 1100 mah to be able to deliver 11 amps. Sounds about right.

Now I select a motor that can handle 100 watts or about 10.4 amps at 9.6 Volts. From experience we know this could be a speed 400, a speed 480 or some kind of a Brushless motor.

We now need a propeller that will cause the motor to draw about 100 watts. I don't know off the top of my head what that would be. I would go to some mfg chart - GWS has good charts!

I see that if I use a direct drive speed 400 with a 5X4.3 prop at 9.6V then the motor will draw about 12.4 amps or about 119 watts. This would be a good candidate motor/prop for the plane using a 9.6V pack that can put out 12.4 or more amps. This would be a set-up for a fast plane as that motor will spin that small prop very fast.

However maybe I don't want such a fast plane but one with a really good climb and lots of low end pull to help out a new pilot who is in training.

I can also use a speed 400 with a 2.38 gearbox and run it at 9.6V spinning a 9X7 prop and run at about 12.8 amps for 120 watts. The larger prop will give this plane a strong climb, but since the prop speed has been reduced by 2.38 times, it won't be as fast. Spinning a bigger prop gives me more thrust but a lower top speed typically.

Back to battery packs and motors

So if I shop for a 9.6V pack to be able to handle about 15-20 amps, I should do just fine and not over stress the batteries. In NiMh that would probably be a 2/3 or 4/5 A pack of about 1100 -1500 mah capacity, depending on the quality of the cells.

We view the battery and motor as a linked unit with a target power profile, in this case about 100 watts. We use the prop and gearbox, if any, to produce the manner in which we want to deliver that power to the air to pull/push the plane.

If this is a pusher, I may not have clearance to spin that big prop so I have to go for the smaller but faster prop combo.

If this is a puller, then I can choose my prop by ground clearance or some other criteria and match a gearbox to it.

See, that was easy, right? But we are not done! Oh no!

I could try to do it with a 2-cell lithium pack rated 7.4V. To get 100 watts I now need a pack that can deliver 13.5 amps and a motor/prop combination that will draw that much. So if I have 10 C rated lithiums, then the pack better be at least 1350 mah. Probably use a 1500 mah pack to be safe.

Well, when I look at the chart for the geared speed 400 I see that, regardless of prop, at 7.4V I am not going to have enough voltage ( pressure) to push 13 amps into this motor. So the 2 cell lithium won't meet my performance
goal of 100 watts + per pound using this gearbox.

If I go back to the charts and look at a different gearboxes I can't hit my power goals using 7.4V. Maybe we go back to direct drive.

We see that the best I can get this speed 400 to do is a total of 70 watts at 7.2V ( close enough ) so I can't hit my power goals using a speed 400 at this voltage. but 70 watts would be about 48 watts per pound so I could have a flyable plane, but not an aerobatic plane using this two-cell pack.

The assembly was pretty basic and very enjoyable. My setup is Airtronics 94091Z Super Micro Servos, 92515Z Micro 5 Ch Receiver (although I would recommend a *full* range Receiver such as the Hitec 05S. It seems to be popular on the forums) and one 96334Z Super Micro Speed Control. I have a VG6000 Airtronics receiver and my charger is a Hobbico Quick Field DC Charger MKII 12 Volt. I really love the computer radio. Very easy to reprogram servos if they are reversed even for a rookie like me.

I bought all of my stuff from Tower Hobbies. A word of caution, don't listen to them on the battery pack for this plane. I wasted my money on a 7 cell 650AAA. It does not have enough power to fly the EZ. MY 7 cell 1100 mah and 8 cell 1800 mah work great. I also ordered the Multiplex ESC 400 Round 12A 400 Motor ESC and installed it in the beginning but it requires wiring changes when going from the 7 to 8 cell battery so I took it out and I installed the 96334Z Super Micro Speed Control that came with my VG6000 radio.

Well, let me know how it goes. I gotta go for now. The wind is perfect and I've got the bug! Good luck.