SIZING POWER SYSTEMS FOR ELECTRIC AIRPLANES

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Laureen Horton
5 years ago
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1 SIZING POWER SYSTEMS FOR ELECTRIC AIRPLANES 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. MOTOR EFFICIENCY - Brushed vs. Brushless 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 $30. A comparable brushless motor/esc combination would typically cost 2 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 a brushless 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. 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

2 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, IF AND ONLY IF, the battery has the ability to deliver more electricty. Again using the car analogy, if you put a big motor in a car and put a tiny fuel line and a weak fuel pump, the motor will never develop full power. In fact the motor might starve and stall once you got past idle. Such is the same with batteries. We need voltage, we need capacity, but we also need to know how many amps the battery is capable of delivering at peak. If we compare an 8 cell AAA battery pack to an 8 cell C battery pack we get 9.6 V for both packs. However the AAA pack may only be able to deliver 6 amps. After that the cells will heat up and either be damaged or the voltage will strart to drop fast. The C pack, also 9.6 V, might be able to deliver 60 amps without damage. So we have to size not only by voltage, but by the ability to deliver amps to the motor. Again, think of the fuel line and the fuel pump as your image of what I am trying to explain. If the the motor needs 12 ounces per minute to run but the fuel line can only deliver 8, the engine will starve and die. 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 into the motor, 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 bump it up to 12 volts and ram 15 amps down its throat and you will likely burn it out. Our goal is a balanced power system. 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

3 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 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 or 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 perhaps 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 watts. Lets 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

4 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 as a starting point. 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 or to do more low speed aerobatics 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. This is a common strategy for 3D planes. Back to battery packs and motors So if I shop for a 9.6V pack to be able to handle about 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 mah capacity. 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 may 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 gear box to it. See, that was easy, right? ( well sorta but.) 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 gear box. If I go back to the charts and look at a different gear boxes I can t hit my power goals using 7.4V. Maybe we go back to direct drive.

5 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 areality CHECK! Now, in fact that is NOT how I would do this. I would decide on the watt target, go to the chart, find a combo that meets my goals, then select a battery that will meet the demand and see if my weight comes up at the target I set. A little tuning and I come up with a workable combo. For those who like to be even more analytical about it, there are packages like MotoCalc that will allow me to play with all sorts of combineations and make suggestions on what I should use. There is a link for MotoCalc below. Summary So, in these few paragraphs have taken in a basic knowledge of how electric power systems are sized, the factors that are considered an how to predict the outcome. Simple, right? Of course there is a lot more to know and time and experience will teach you plenty, but with this basic understanding you are better prepared to begin playing with the power systems you put in your planes. flyable plane, but not an aerobatic plane using this two cell pack. BEC: means Battery elimination circuit and is part of the normal ESC. KV: The KV value is the speed the motor need to turn so it produce 1V of counter-electromotice force. Since the motor is not 100% efficient you can say that the 1000Kv motor will turn slightly slower than 12,000rpm when running on 12V with no load. Determine a Model s Power Requirement s 1. Power can be measured in watts. For example: 1 horsepower = 746 watts 2. You determine watts by multiplying volts times amps. Example: 10 volts x 10 amps = 100 watts Volts x Amps = Watts 3. You can determine the power requirements of a model based on the Input Watts Per Pound guidelines found below, using the flying weight of the model (with battery): watts per pound; Minimum level of power for decent performance, good for lightly loaded slow flyer and park flyer models watts per pound; Trainer and slow flying scale models watts per pound; Sport aerobatic and fast flying scale models

6 watts per pound; Advanced aerobatic and highspeed models watts per pound; Lightly loaded 3D models and ducted fans watts per pound; Unlimited performance 3D models NOTE: These guidelines were developed based upon the typical parameters of our E-flite motors. These guidelines may vary depending on other motors and factors such as efficiency and prop size. 4. Determine the Input Watts per Pound required to achieve the desired level of performance: Model: Hangar 9 P-51 Miss America Estimated Flying Weight w/battery: 9.0 lbs Desired Level of Performance: (100 average) watts per pound; Fast flying scale model 9.0 lbs x 100 watts = 900 Input Watts per Pound of power (minimum) required to achieve the desired performance 5. Determine a suitable motor based on the model s power requirements. The tips below can help you determine the power capabilities of a particular motor and if it can provide the power your model requires for the desired level of performance: Most manufacturers will rate their motors for a range of cell counts, continuous current and maximum burst current. In most cases, the input power a motor is capable of handling can be determined by: Average Voltage (depending on cell count) x Continuous Current = Continuous Input Watts Average Voltage (depending on cell count) x Max Burst Current = Burst Input Watts HINT: The typical average voltage under load of a Ni-Cd/Ni-MH cell is 1.0 volt. The typical average voltage under load of a Li-Po cell is 3.3 volts. This means the typical average voltage under load of a 10 cell Ni-MH pack is approximately 10 volts and a 3 cell Li-Po pack is approximately 9.9 volts. Due to variations in the performance of a given battery, the average voltage under load may be higher or lower. These however are good starting points for initial calculations. Model: Hangar 9 Miss America Estimated Flying Weight w/battery: 9.0 lbs Input Watts Per Pound Required for Desired Performance: 900 (minimum) Motor: Power 60 Max Continuous Current: 40A* Max Burst Current: 60A* Max Cells (Li-Po): Cells, Continuous Power Capability: 19.8 Volts (6 x 3.3) x 40 Amps = 792 Watts 6 Cells, Max Burst Power Capability: 19.8 Volts (6 x 3.3) x 60 Amps = 1188 Watts Per this example, the Power 60 motor (when using a 6S Li-Po pack) can handle up to 1188 watts of input power, readily capable of powering the P-51 Miss America with the desired level of performance (requiring 900 watts minimum). You must however be sure that the battery chosen for power can adequately supply the current

7 requirements of the system for the required performance. You must also use proper throttle management and provide adequate cooling for the motor, ESC and battery. Q) How do you tell which Electric Motor is equal to what Glow Engine? A) One of the biggest confusions for most people selecting an electric motor is, What is a watt? The glow guys are used to horsepower and electric power systems are measured in watts. (1 hp = 746 watts or about 750 watts) Don't go by the max rating for HP that engine manufacturers publish. That is a MAX figure and very seldom is an engine for sport use operated at that figure. The h.p. drops off quite a bit when the RPM is not at the rated figure which is usually around 16,000 RPM s or greater. Glow Engines vs. Electric Motors size glow engine / 300w electric motor 2. (OS Max 0.20 engine develops 0.4 hp = 300w electric motor (AXI 2820) ) size glow engine / 500w electric motor 4. (Fox 0.35 stunt engine develops 0.7 hp = 522w electric motor)(axi 2826) size glow engine develops 1.0 hp = 750w electric motor (AXI 2826 or 4120) size glow engine develops 1.3 hp = 975w electric motor (AXI 4120 or 4130) size glow engine develops 1.6 hp = 1200w electric motor (AXI 5320 or 4130) size glow engine develops 3.0 hp = 2250w electric motor (AXI 5330) 9. DA-50 develops 5.0 hp = 3750w electric motor (AXI 5330) 10. DA-100 develops 9.8 hp = 7311w electric motor (Double AXI 5330) See E-flite Park 400 and Power 60 series that mimic replacement sizes for old brushed motors and glow engines. Himax Brushless Motors HC50 Outrunner Watts HC50 - These motors are for large models weighing 6-8Lb for 3-D flight, 8-15Lb for aerobatic flight and 10-20Lb for leisure flight. Gas equivalent HC63 Outrunner Watts HC63 - These motors are for large models weighing 10-12Lb for 3-D flight, 17-22Lb for aerobatic flight and 22-30Lb for leisure flight.

8 Gas equivalent Himax Brushless Motors HC22 Outrunner Watts Wide selections - 4 motors in 2 frame sizes, Tiny in size but BIG in power high torque, direct drive, Dual ball bearings designed for durable and efficient operation, Three mounting options stick mount, nose mount, or firewall mount, Includes a prop adapter, mounting hardware, & connectors. HC2208 -This 30 watt motor is for very small models weight 6-8 oz. Replaces MPI EPU and GWS IPS gear mo For two-cell applications - use HC For three-cell applications - use HC Gas equivalent HC mm 6x4-9x HC mm 6x4-8x *Caution - Prop size is highly dependent on battery choice. Check the manual for full information. Always monitor current when HC This 50 watt motor is for very small models weight up to 12 oz. Replaces GWS IPSD gear motors, or 180 or 28 For two-cell applications - use HC For three-cell applications - use HC Gas equivalent HC mm 7x4-10x HC mm 6x4-9x

9 *Caution - Prop size is highly dependent on battery choice. Check the manual for full information. Always monitor current when Click here for spare parts for HC22 series motors HC28 Outrunner Watts Wide selections - 8 motors in 3 frame sizes, Replaces Axi-22 series motors, Dual ball bearings designed for durable and efficient operation, Three mounting options stick mount, nose mount, or firewall mount, Includes a prop adapter & two mounting brackets for stick mount & radial mount, The unique stick mount fits most GWS & E-Flight s planes. HC These 70 watt motors are for small models weighing 7-10 oz for 3-D flight, up to 8 oz for aerobatic flight and up to 10 Gas equivalent HC , 7, & 8 mm 6x4-9x *Caution - Prop size is highly dependent on battery choice. Check the manual for full information. Always monitor current when HC These 100 watt motors are for small models weighing 9-12 oz for 3-D flight, up to 16 oz for aerobatic flight and up to 2 Gas equivalent HC mm 9x5-12x HC mm 8x4-11x HC mm 7x4-10x

10 *Caution - Prop size is highly dependent on battery choice. Check the manual for full information. Always monitor current when HC These 150 watt motors are for small models weighing oz for 3-D flight, up to 24 oz for aerobatic flight and up flight. Gas equivalent -.10 HC mm 9x5-12x HC mm 9x5-12x HC mm 8x4-11x *Caution - Prop size is highly dependent on battery choice. Check the manual for full information. Always monitor current when HC These 200 watt motors are for small models weighing oz for 3-D flight, up to 32 oz for aerobatic flight and up flight. Gas equivalent -.15 HC mm 9x4.7-12x HC mm 8x4-11x *Caution - Prop size is highly dependent on battery choice. Check the manual for full information. Always monitor current when

The Magic of Electric Flying or. Volts and Amps for Dummies By John Wheater

The Magic of Electric Flying or Volts and Amps for Dummies By John Wheater IT SEEMS there are many who are confused with what goes where and why and what motor and prop should be used on what battery and