Building Tips

Picking a Power System
By: Pat Tritle

    Rarely a day goes by where someone doesn't write or call and ask, "how do I select the right power for a specific model". Do to the fact that there is so much information to be pass along it would be difficult, if not impossible for those attending to remember much of the pertinent information. For that reason, I decided to write up the basic guidelines so that those interested in getting into electrics would have the information available for reference at a later date. So here goes. I’ll keep the numbers as simple as possible to avoid unnecessary confusion.

    Meanwhile, in today's market place the opinion that "way too much power is almost enough" reins supreme. I am not a supporter of that theory in any way shape or form, and have seen so many otherwise terrific models fly poorly because they were simply over-powered. Power selection like anything else depends completely on the type model being built, and the desired performance expected from the model. If you're looking for 3D performance from a model designed for 3 D, then 50 watts / lb. won't do well, any more then a 16 oz. 50" Piper Cub would at 250 watts / lb. And since I'm not a seller of power systems, I have nothing to gain or loose by recommending larger (more expensive) motors, ESC's or batteries then are really needed to fly the model as it was intended to be flown. So let's get started.......

Here’s how it all shakes out. The basic power required to fly an electric model is as follows;

Direct Drive Systems- 60 Watts/lb.

Gear Drive Systems- 35 - 50 Watts/lb.

For mild aerobatic performance; 70 - 80 Watts/lb.

For all out aerobatics; 100 - 125 Watts/lb

3D performance; 175 Watts/lb. or more

    The above numbers are based on models with wing loadings from 8 – 16 oz/sq. ft. As with gas models, higher wing loadings require more power since they must fly faster to support the added weight. By the same token, a lightly loaded model with a wing loading in the 3 – 5 oz./sq. ft. range will fly very well at 25 -30 watts/lb.

So, what’s a “Watt”, and where can I get some?

    Wattage is the term used in electric flight to relate the level of power that an electric drive system will produce. To relate it to terms we’re familiar with, 746 watts = 1 horsepower. To calculate the wattage delivered by a given system looks like this;

Amps X Volts = Watts

Where do these numbers come from, and how do I know how many Volts and Amps are needed to fly a given model?

    Let’s say you want a mildly aerobatic sport model with a 14 oz./sq. ft. wing loading that will weigh in at 2 lb. We already know that the power requirement for a model like this is about 70 Watts/lb, so we’re going to need to generate about 140 Watts. Let’s assume that you are going to use an 8 cell NiCD battery. At 1.2 Volts per cell, 8 cells will deliver 9.6 volts. To arrive at the necessary current draw to achieve 140 watts, simply divide 140 (watts) by 9.6 (volts) and we arrive at 14.58 Amps.

    Now, let’s assume that you have a 3 cell Lithium Polymer battery for the model, which is rated at 11.1 Volts. The formula is the same; 140 (watts) divided by 11.1 (volts) = 12.6 Amps. As you can see, as the available voltage increases, the lower the current draw needs to be to deliver the necessary wattage.

    Here’s something to consider when selecting your system – The higher the current draw the shorter the flight duration will be on any given battery. So the ideal set-up would be to use a higher voltage battery, with lower current draw for maximum duration. On the downside, when using NiCD and NiMH batteries, as the cell count goes up, the weight will increase significantly as well. It works that way with Lithium too, but Lithium batteries are dramatically lighter then the old “round” cells.

    OK. Let’s say we’re going to use an 11.1 Volt Lipo battery. All we need to do now is select a motor that will swing enough prop at 12.6 Amps to fly the model at a top speed of around 40 – 45 mph and we’re in business. Now that you know the parameters, visit your LHS and select a motor that will fit that description. And here’s a rule of thumb for picking a brushless motor – the lower the KV (RPM per Volt) rating, the more prop the motor will turn, but at a lower RPM. A higher KV rating will swing a smaller prop, but at a much higher RPM.

    Gear Drive vs. Direct drive, and why is one better then the other? Well, it all depends on the kind of performance you’re looking for. If you’re looking to go fast, go with direct drive. Going fast requires a high pitch propeller turning high RPM. The formula to calculate propeller pitch speed is an easy one, it looks like this;

RPM X Pitch (in inches) divided by 1056 = MPH

Let’s say that you are turning a 7-6 prop at 14,000 RPM.

14,000 X 6 = 84,000 divided by 1056 = 79.55 MPH

    Let’s assume you are setting up a slow, relaxing Park Flyer with about a 5 oz./sq. ft. wing loading. If we swing a 9-7 prop at about 3500 rpm we’d be looking at a top speed of around 23 mph. To swing that much prop with a small light drive system we would use a gear drive unit at a very low current draw and a small light battery.

    Again to make a known comparison, we can relate all this to riding a 10 speed bicycle. A gear drive swinging a big prop is like riding your bike in low gear. You pedal like mad with little effort, you don’t go very fast, but you can climb steep hills with ease. The direct drive system could be compared to riding the bike in high gear. It’ll really go fast, and even though you’re pedaling slower, it requires considerably more effort.

Some Pertinent Information about Motors

    With the advent of brushless motors, we have some new choices to consider. To start, there are two types of brushless motors; In-runner, and Out-runner. In-runner motors are just like the old style brushed motors in that the “armature” turns inside the motor housing. Out-runner’s on the other hand work like the old rotary engines in WW-I, in that the “crankshaft” (Armature) is mounted to the firewall and the “engine” (Motor Housing) turns around it. The advantage is torque, and by nature, these motors replace gear drive systems because they will swing very large props.

    In-runner works basically the same as a direct drive brushed motors – high RPM for speed. Out-runner’s act more like Gear Drive systems in that they will swing a much larger prop and a lower RPM. There are gear drive units available for In-runner type motors, but because the Out-runners are available, there’s really no point.

    When selecting a drive system, the deciding factor will be the need for either speed, or torque. For speed, go with the In-runner, for torque go with the Out-runner. Here’s the reason why; each motor has a “KV” rating – KV is the RPM per Volt rating of the motor. The higher the KV rating, the faster the motor will run at any given voltage. Lower KV motors, typically Out-runner’s, will turn large props, but at much lower PRM.

    To sum it up, the higher the KV rating, the faster the motor will turn with any given battery, and the higher the current draw will be with any given prop. High KV In-runner motor’s swing smaller props at high RPM -- good for “direct drive” applications. Low KV Out-runners swing much larger props at lower RPM – good for slow flyer’s and models requiring more power at lower speeds, and are best suited where gear drives were once used.

    What all this boils down to is “propeller disc loading”. We all know what wing loading is; it’s the amount of the model’s weight that each sq. ft. of wing must carry. Prop disc loading works the same way. A large prop will be more lightly loaded, thus delivering more torque then a smaller prop turning high rpm. The trade-off, of course, will be speed.

    There’s one more thing to cover and we’ll give you a rest. Batteries are rated in “voltage” and “amperage”. Voltage dictates the amount of power the battery will deliver. The Amperage rating dictates for how long the battery will deliver that power. To relate that to glow fuel, consider the Voltage as Nitro content. High Voltage (Nitro) means more power. The Amperage is related to the quantity of fuel, or simply, the “size of the tank”.

    To figure the size battery needed, let’s go back to our 140-Watt sport plane. If we’re pulling 14 Amps from a 1400 MAH (1.4 Amp Hour) battery, we will have a full power duration of 5 – 6 minutes. In the real world, with proper throttle management, you’ll see flight times of around 7 - 8 minutes. Pretty common flight times, even with liquid fueled models.

    To arrive at that number, divide the battery amp rating by the current draw. 1.4 (Amp Hours) divided by 14 (amps) = .1. Then take 60 (minutes per Amp Hour) X .1 = 6 minutes. Now, to double the duration you must either cut the current draw in half (to 7 Amps), or double the battery size (to 2800 mah or 2.8 Amp Hours) – again we see trade off’s. To reduce the current draw we can use a gear drive with a larger, higher pitch prop turning slower with very little weight penalty. If we double the size of the battery capacity, the weight penalty is quite high, unless we go over to Lithium batteries, in which we will discover we have benefited from a tremendous weight reduction, but at a higher price then conventional batteries!

Some Pertinent Information about Batteries

    As long as we’ve mentioned Lithium batteries, here’s a bit of information that will come in very handy. Since this original writing, round cells have all but disappeared from the hobby shop shelves, and Li-Poly (Lithium Polymer) batteries have come on line to replace them. The principles for selecting a battery remain the same, but there are some aspects of Li-Poly batteries that are just a bit different. It goes something like this.

    The “C” Rating on Li-Poly batteries is very important. “C” means Capacity, and is rated in Amps. In other words, a 1320-mah battery has a Capacity of 1.3 amp hours. When charging a Li-Poly battery, the charge rate should never exceed “1-C” (unless specifically stated by the battery manufacturer) or in other words, the rated capacity of the battery. As an example, a 2100-mah battery will charge at 2.1 amps max, a 1320-mah battery at 1.3 amps max etc. Over charging a Li-Poly battery is at least dangerous, and at worst can be disastrous. And finally, a Li-Poly battery must be charged with a charger designed specifically for Lithium batteries – there are NO exceptions.

    When discharging a Li-Poly battery, the “C” rating again comes into play as well. On the battery label you’ll find the maximum recommended discharge rate. It will read something like “10 – 12 C”, or “20-C max”.

    In my experience, I’ve found that most Li-Poly batteries are over-rated, and won’t handle the maximum recommended discharge rates and survive for very long. When discharged too hard, you’ll see a dramatic loss of power, the batteries will come out hot, and in some cases will puff up, and in extreme case will actually burst and burn. The best bet is to live by the 80% rule – that is, take the minimum “C” number as in “20-C Continuous", multiply it X .8 (16-C) and use that as the maximum discharge level with your set up. Doing that will not only improve battery performance, but will greatly increase the overall life span of the battery as well.

    And finally, a word about Cell Balancing. Early on, Cell Balancing was not considered important – basically arrogance on the part of the battery manufacturers that cost modelers a lot of money! In reality, cell balancing is very important to both battery performance and battery life. There are several good cell-balancing tools available that in the long run will more then pay for themselves through the extended life of your batteries.

    OK, I promise I’ll quit, before we all end up in “system overload”. Once again, there’s a tremendous amount of information here for a newcomer to electrics to digest, so let’s do this. If you have specific questions about setting up an electric model, please feel free to drop me a line and I’ll do what I can to steer you in the right direction. But for now, I’ll offer up one last piece of advice. To get started, work with a known good design, and use the recommended equipment that has been proven to work. Talk to the guys who are successful and copy what they’re doing. The one thing I do know about modelers, though, is that they are always willing to share their knowledge with those interested in what they’re doing.

Pat Tritle

Building Tips