**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

505-296-4511

patscustommodels@aol.com