It’s
20th August. It’s been two days since ALP ended and I am back in the
land of tiger. One thing I realised was that I am sooooo lagging behind in
terms of putting up the posts. One reason is that I have been blogging in
Chinese and it takes two to three times more than if the posts were to be
written in English. So I will try to post in English until all the ALP posts
are up to date.
********************
What
came next after modelling the frame in Solidworks was to send the design to a
vendor so that he could cut the pieces out on a piece of carbon fibre. Carbon
fibre was used as the main frame because the material is so much lighter and
stronger as compared to other materials such as glass fibre, Aluminium alloys
and Nylon. At first we thought we could start assembling the frame right away
after the pieces were delivered, but we were so wrong! One possible reason that
we had to do extra work was that the blade that the vendor used to cut the
pieces was blunt, and hence certain holes on the carbon fibre pieces were
smaller than the intended dimension. It took us two days to sand the sides to
enlarge the holes. It was really a dirty (with all the carbon fibre powder
blackening wherever it landed) and painful experience! We felt so relieved when
all the holes were enlarged and all the pieces fell into places exactly how we
wanted them to (maybe except that the battery holder was 1-2mm smaller than the
battery zzz).
 |
| Busy sending the holes to enlarge them |
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| How it should look like |
Other
than the main frame itself, we also had to make a prop guard on our own out of
a special kind of Styrofoam called EPP, which is much stronger than the normal
Styrofoam that we used. We test its strength by bending it and it only broke
after it was bent ninety degrees. Sofia drew out the shape of the prop guard on
to this piece of Styrofoam and Sheen had to painstakingly cut it out with a
Styrofoam cutter. The outcome was not what we had expected because the prop
guard was too thin to be effective. While it is strong, it also bends easily.
As a result, in the many experiments that we had later on, the prop guard was
safe but the propellers all ended up destroyed.
 |
| How it should look like together with the frame |
With
all these in place, the quadcopter was still unable to fly without the electronics!
Well, I won’t be saying much about the electronics (GPS module, APM, ESCs, BEC,
etc) here because most of the soldering was done by Chenhui (a highly capable
Electrical Engineer). Reason being some of the soldering had to be very precise
and so precise that none of the three amateurs should attempt to do it else
risking malfunctioned modules. (But I did figure out the wire connections later
on! :D)
What
we managed to spend most of our time on were the motors and the propellers.
Before everything else, we had to check whether the blades on each propeller
had the same weight. Else, we had to use scotch tape to balance the weight on
both blades. This was to reduce the unwanted effect on the stability of the
quadcopter and the efficiency of the propellers.
 |
| Need to balance the weights on both sides |
Next,
we set up the apparatus as shown in the picture below (saved me some time to
explain in words :D):
The
idea was that the propeller was installed such that it would push the wind up
(hence forcing the whole set up down) instead of the other way round. The
thrust could be measured using a weighing scale that was below the whole set
up. At the same time, we would be varying the current across the motors (and
hence the angular velocity of the propeller) and recording down the voltage and
the corresponding thrust. With all this information, we were able to calculate
the power (IV) of the motor and the efficiency (g/W) of the propeller. With
other informations such as the weights of the quadcopter (including the frame,
electronics, props etc) without battery and battery specs (such as voltage,
discharge rate and capacity/weight ------ for a 3S battery, capacity/weight is
normally 0.0136Ah/g), we managed to obtain the battery weight and hence the
longest flight time.
 |
| Flight time VS Battery weight |
 |
| Efficiency VS Thrust |
It
was so scary while conducting the experiment because I had to keep increasing
the speed till the propellers gave a thrust of 280g. The speed was so fast and
the propellers were so noisy that Chenhui warned us that if the propellers were
lousy there might be a possibility that the blades would break and hurt whoever
that was near them. So throughout the whole experiment I was trying my best to
lower my head below the plan of the propellers.
It
was a rather boring job to take the readings for all 16 propellers. Glad that
it was over.
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