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To figure the size of battery needed, let's go back to our
140-watt sport airplane. If we're pulling 14 amps from a
1400 mAh (1.4 amp hour) battery, we will have full power
duration of five to six minutes. In the real world, with
proper throttle management, you'll see flight times of
approximately eight minutes. These are 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)/14 (amps) = 0.1. Then
take 60 (minutes per amp hour) x 0.1 = 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 tradeoffs. To
reduce the current draw, we can use a larger, higher-
pitch propeller 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 the new
Lithium batteries in which we will discover we have
benefited from a tremendous weight reduction, but at a
higher price then conventional batteries.
Okay, 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.
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 people who are successful and copy what
they're doing. The one thing I do know about modelers is
that they are always willing to share their knowledge with
those interested in what they are doing.
By Pat Tritle
From the Albuquerque Radio Control Club
Now here's something to consider when selecting your
system: the higher the current draw, the shorter the flight
duration on any given battery. Therefore, the ideal setup
would be to use a higher-voltage battery with lower
current draw for maximum duration. On the downside,
when using Ni-Cd 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.
Okay, let's say we're going to use an 11.1 volt Li-Poly
battery. All we need to do now is select a motor that will
swing enough propeller 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 local hobby
shop and select a motor that fits that description.
Gear Drive vs. Direct Drive: 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)/1056 = mph. Let's say that you are turning a 7-6
propeller at 14,000 rpm. 14,000 x 6 = 84,000/1056 =
79.55 mph
Now, let's assume you are setting up a slow, relaxing
park flyer with about a 5 oz/square foot wing loading. If
we swing a 9-7 propeller at about 3,500 rpm, we'd be
looking at a top speed of roughly 23 mph. To swing that
much propeller 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 propeller 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.
Discussion if we should have a landing fee at our club fun
fly events. Decided to leave that up to the event
organizers.
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 square foot of wing must carry.
Prop disc-loading works the same way. A large propeller
will be more lightly loaded, thus delivering more torque
then a smaller propeller turning high rpm. The tradeoff, of
course, will be speed.
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
.”
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