SuperCub Lipo Mod (deluxe)

After reading many forums, I wanted to modify my Supercub to benefit from the power and capacity of Lipo batteries but keep the stock motor, radio and ESC (for ease and expense).

I also wanted to include 3 key features in this mod

- Minimal battery box changes (preferably none)

- Ability to switch between using Lipo and the NiMh battery types

- Ability to monitor Lipo usage

The pre-requisites for this mod are:

1. To convert the ESC, Charger and Batteries to Deans Ultra connectors (or some other
   type better than the stock connectors).
2. To mark on the fuselage (just under the wing) the centre of gravity (CoG) of the
   stock plane with stock battery. Using a finger either side of the plane
   under the wings and marking the balance point with a Sharpie.

The first objective, to avoid battery box changes is mainly achieved
through the selection of your Lipo battery. This limits you to a maximum length
of 67mm which are the ‘square’ shaped batteries usually between 1000mAh and
1300mAh capacity 3s (11.1v) batteries. While this is a capacity limitation from
the 1800 to 2200mAh 3s average, I felt it worth the compromise.

Many people on forums chose the Thunderpower which should be ‘drop-in’.
I chose the cheaper $21 hexTronic 1300mAh from Hobbycity which is supposed
to be 65mm long. In fact the hexTronic was about 10mm longer that the spec
which was very annoying.

Getting this slightly over-sized Lipo to fit required me to remove the
rear side of the battery box using a scalpel / craft knife. The white battery
box plastic carves quite easily with a sharp knife and this could be done without
removing the box from the plane. This allowed the battery to lay horizontally
in the box using the stock battery box door, the receiver could be kept in the
stock location above the battery box and velcro strap was still functional.

Next was adding a switch to select between Lipo and NiMh LVC (low
voltage cut-off). This was achieved by using a standard receiver switch to
control the LVC jumper on the receiver. I re-used the lead and connector from one
of the ACT sensors I had previously removed to extend the length of the switch
wires. I connected one side of the switch to the jumper header on the receiver
and ‘shorted’ the other side using the previously removed jumper. (Note, a
future mod would be to cut, join and solder the wires on this other side of the
switch to save a gram or two).

The switch now controls the LVC jumper. When in the ‘on’ position, the
jumper is closed, which is the NiMh position for a lower LVC. When the switch
is ‘off’, the jumper is open, which is the Lipo position for the 9v LVC. The switch can now be tested using a
multimeter circuit tester. It is then a good idea to test the LVC is working as
expected by connecting a charged stock 8.4v battery and switching to Lipo mode
(9v LVC). The propeller should turn as normal until the throttle reaches ~ ¾
and then cut out. Changing the jumper switch should allow the throttle to be
opened up to the max without anything cutting out.

For easy access (and easy removal if required) I located the switch at
the bottom of the plane in the middle of the holes already in the plane
fuselage. It was secured in place by the face plate compressing the foam and is
not going anywhere.

The final part of this mod was to utilize a $5 Lipo battery monitor
(Maxpro from Hobbycity). This monitor fits onto the battery balancing
connector, has a really small current
draw (~ 5mA) and automatically indicates the battery voltage with an LED and beeper
as follows:

-
above
11.0v it’s bright blue

-
between
10.0 and 11.0v it’s blinking blue

-
between
9.8 and 10.0v it’s bright red

-
under
9.8v it’s beeping and blinking red

I located the
battery monitor where the lower ACT sensor used to be, between the battery box
and the holes used to hold the LVC switch. I feed the monitor wires through the
sensor hole to the battery box area. I then marked around the sensor and then used
a hot soldering iron to increase the sensor recess to fit the monitor flush
with the bottom of the fuselage. The monitor was secured with a few drops of CA
glue and some temporary tape to hold it in place while the glue dried. Once
dry, the sensor was covered using clear and then nylon reinforced packing tape
(which helped hide the green circuit board)

I then changed the stock propeller (10×8) for a 10×7 one to reduce the
power draw on the ESC. Many forum entries and the propeller testing on this
site recommend the 10×6 size and being optimal but I was worried I wouldn’t
have enough power for the test flight and so decided to try the 10×7 as a
compromise first.

Finally I re-checked the centre of gravity and confirmed that it had not
measurably moved from the previously marked position.

The final Lipo ready setup looks like this:

Flight Testing with the Lipo …..

The blue light obviously comes on as soon as the balancer plug is
connected. The red light and beeping would continue until the balancer plug is
removed… seems a bit annoying, and I toyed with the idea of putting in another
switch, but when you think about if the voltage is below 10v, you really shouldn’t
just be getting the plane down, you need
to remove the battery to stop the battery drain of the receiver. So annoying is
a good thing. Actually I think the blue light when flying looks really cool,
especially at dusk.

Power. With the stock battery, the throttle started turning the prop at
about 1/3 of the way up and progressively increased the rpm to full throttle. With
the Lipo the propeller started turning earlier at ~1/4 of the way up and the
increasing rpms seemed to top out at about 70% of the way up. No noticeable
increase between 70% to 100% throttle. However there was definitely more
incremental control between 25% and 50%.

50% throttle gave significantly more power than my upgraded 8cell (9.6v)
NiMh battery at full power. I did most of my straight and level flying at ~35%
throttle. Brief wide open throttle got the SuperCub climbing steeply. Cruising
at 70% throttle needed work at keeping the plane level, but I didn’t spend much
time at this speed and didn’t bother trying to trim for it (on my first
flight). I couldn’t say if there was any
noticeable difference in the air between 70% and 100% and there was absolutely
no need to go above 70%.

Rolling and hand launched takeoffs were very easy with the SuperCub
leaping into the air at 70+% throttle. This was noticeably much easier and more
positive. Pulling loops was easy although with NiMh batteries I had never
bothered to reduce throttle after passing over the top. With Lipos, once over
the top with max throttle the plane accelerated like a rocket towards the
ground (luckily I had plenty of height), but this really showed me what the
plane was now capable of once I graduate from my current straight and level style
of flying.

General handling and landings were no difference to normal. The battery
monitor blue light was clearly visible whenever I flew near, even at ~500+
feet.

After two flights and a touch and go circuit for 15+ minutes the blue
light was still on, so I can’t comment on the red light or beeping.

Once home, I checked the battery and it was still at 11.4v, so the
monitor was correct (blue > 11v). Also after charging, the 15+ minute flight
had used only 850mAh of the battery, amazingly only 65% of the 1300mAh rated
capacity. This length of flight would have previously taken me about half way
into my second NiMh batteries. It made me smile as I had taken 2 x 1300 Lipos,
and both my old 7 and 8 cell NiMh batteries to the field, just in case !

This confirmed what I had read, that the Lipo conversion is relatively
easy to complete and pays dividends with additional power AND flight time. I
have already changed the prop down to the recommended 10×6 size for my next
flight which should lower the max power a little (there’s tons to spare so I
don’t see this as an issue) and further increase flight times.

I think the LVC switch of this mod may prove redundant as I can’t see
myself using NiMh out of preference and the 2 Lipos I purchased will be more
that enough for my flying.

PS – You may have noticed my wing strut fastener mod… a bent paperclip
which slots under the battery box door latch. One end is closed and the other
end slightly open like a hook. This makes removing the wings quick and easy
with no need for a screwdriver.