Old 02-23-2008, 05:31 AM
  #9  
AEAJR
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Join Date: Aug 2005
Location: NY, USA
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EXTENDING FLIGHT TIMES WHILE MAINTAINING BALANCE
by Ed Anderson
aeajr on the forums

Changing the type or capacity of your battery pack is typically done for one
of three reasons:

* You want longer flights
* You want to reduce weight.
* You want to do both

Here are some points to consider to get the most out of this change.

If you are currently flying NiCd packs, you can go to NiMh very easily. You
will gain about 40% in battery capacity at the same weight. The packs are
about the same size and shape so they fit easily and should not throw off
the plane's balance. NiMh and NiCd packs, NiXX for short, therefore can
typically be interchanged easily. I have eliminated virtually all my NiCd
motor packs and replaced them with NiMh packs.

If you go to lithium batteries you can either make your plane lighter or you
can maintain its weight but double, tripple or quadruple your battery
capacity. Lithium batteries have about 4 times the capacity per ounce as
compared to NiCd packs. Here are some steps to consider BEFORE you buy the
new pack:

Where is you battery pack located?

If your battery is forward of the CG, the balance point, then its weight is
helping to balance the plane. If you go to a pack of a different weight,
you MUST rebalance the plane or it won't fly well. For example, a lighter
pack will shift the CG toward the rear which may make the plane difficult or
impossible to fly. You must keep the plane in balance so that the CG,
center of gravity, the balance point, is in the right place.

This also applies to going to heavier packs as they will shift the CG
forward. A slight shift forward might not be a problem if you are adding
voltage as the more powerful pack will drive the motor faster which may mask
a slight change in balance and a more forward CG can make the plane more
stable. For Example I shift between 6 and 7 cell NiMh packs in my Aerobird.
The CG moves a little forward with the 7 cell pack but not enough to
seriously effect the way the plane handles. But optimally you want to keep
the CG in the same, the best location.

From here on I am going to assume you are going from NiXX packs to lithium
packs, as this is what many are doing and the one that takes the most
planning.

Before you buy that new pack:

* Weigh your current battery pack. A food scale or a postal scale is fine.
Many post offices in the US have self service scales. Great for weighing
stuff. Get it to the nearest .1 ounces. Write it on the pack so you won't
forget it.

* Now look at the space in the plane. Can the new pack go in the same or
almost the same place as your current pack? You can account for a location
shift by changing the amount of weight you add to the new pack.

Now decide on your goals based on what you can do in this plane and how much
money you want to spend.

1) Keep the weight the same and spend more money - Get a pack that fits in
the current space and weighs the same as your current pack - Now you can use
the new pack and your current packs interchangeably. Good deal! However
lithium packs are different sizes and shapes than NiXX packs so this might
be hard to do. If it is close, you might be able to modify the battery
space to allow the new pack to fit. A 600 MAh NiCd Pack weighs about the
same as a 2000 to 2400 MAh Lipoly pack, but the LiPoly may cost more.
Prices are dropping all the time and 4 times the flight time is definitely
cool!

2) Keep the weight the same and spend a bit less - Get a pack that is
lighter than your current pack and will fit in the same or close to the same
location, perhaps with minor mods to the plane. Maybe you go from a 600 MAh
NiXX pack to a 1300 MAh lithium pack rather than a 2400 MAh pack. This will
probably have a better chance of fitting where your NiXX pack fits. Great!
Add weight to the pack so it weighs the same as your NiXX pack. You can
still use both without serious modification to the plane. Good deal!

3) Make the plane lighter - If you can move stuff forward in your plane so t
hat a lighter battery can balance the plane, you can avoid the need to add
weight. Now you have a higher capacity battery pack AND your plane is
lighter. Lighter planes generally fly better. The only problem with this
approach is that your current "heavy packs" may not be able to be used
anymore unless you can leave space to adjust their position rearward.

If it won't fit, can you modify the space to make it fit?

If you remove foam, consider reinforcing the space with tape or glue and
light plywood as you have removed some of the structure of the plane. Can
you cut a hole in a former so the pack fits under it? Make sure you
reinforce to account for any cut away structure. By the way, tape, glue,
bals or plywood add weight so you so take these into account. Cut a little,
set some reinforcing in place but don't glue it. Position the pack and test
the balance of the plane; adjust accordingly. Be sure you pad the pack in
balsa or plastic planes so that a crash will not likely damage the pack.
Lithiums can not take the physical abuse that the NiXX packs tolerate.

If modifying the plane to move the pack forward won't get it done, then see
if you can move other things in the plane to shift their weight forward.
Some people have the receiver under the wings. Move it forward and it will
help to balance the plane and you won't have to add as much weight to the
lithium pack. Also see if you can move the ESC forward. Move any excess
wire that you have bundled to the forward area. Wire has weight.

If you have any components, like the receiver that sit behind the CG, moving
them forward will make a huge difference.

If you can move your electronics forward enough that you can balance the
plane without the battery pack, then you can set the battery directly over
the CG. Now it doesn't matter which battery pack you use as the weight of
the pack will not shift the balance of the plane. You can interchange packs
all you like.

When I rebuilt one of my sailplanes after a crash, I positioned my servos,
receiver and battery to more forward locations than the stock
recommendation. As a result I made the plane about 12% lighter with no
other modifications. That made a HUGE difference in how if flew.

I then made a removable motor for it and positioned it on a pod that sat
right over the CG so I could put it on or take it off without changing the
balance of the plane. Likewise I placed the battery right over the CG.
With the motor and battery mounted, the plane was much heavier, but it
stayed in perfect balance whether they were on or off the plane.

There are other considerations related to lithium batteries. You need a
special charger and charging procedures. You MUST protect them from damage
as they can not take the same abuse as NIXX packs. But these are covered in
other threads. This one is just about maintaining balance.

Clear Skies and Safe Flying!

* See if you can buy a lithium pack that is the same weight as your current
battery pack. If you can, and you can afford it you are all set and have
two to four times the flight capacity for longer flights.
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