Page 35 - MFW Dec 2024

massive rolling circle, and now I’m at the downwind end of sure we measured something between 10 and 15G. So
the box. Climbing through a big half loop to the top of the 1.5Kg of fuel tank at 10G of force = 15Kg of push. And at
box, with a one and a half negative snaps to upright off the 15G your fuel tank is pushing 22Kgs of “I want out of here”
top. Easy. During the snap roll, my caller and I see force … ooooh. That’s a lot !! Where am I headed with this
something depart the airframe. Something big too. Oh sh!t ? Your avg 3m IMAC model will be running a 50oz tank,
what was that ???? throttle back and land (it wasn’t a wing and ready to fly (with fuel on board) it weighs about 20 Kg.
panel btw, they are made by me, all composite, and are About the same as the G force on the fuel tank when flying
PBG strong !!) Motor was still running, a big descending aerobatics, and its full. Do you think you could pickup your
360 and I line up for landing. No problem. On short finals model by the fuel tank, and not have the tank rip out of its
the motor stops (bit weird, maybe idle was too low ?) and I mounted location ? Now that’s a test for large model
roll in for a landing. We soon see the problem, a massive inspectors to think about, and maybe an easy one to do.
hole in the canopy. Huge piece of plastic missing. Enough Stand back mate, I’m about to try and lift your model up by
to get your head through. Wow … did we fly into a bird ? A the fuel tank …. wait are you crazy ??? …. no, it’s called
Hawk ? An eagle maybe ? … someone says, maybe it was physics pal. Either I do this test, or you are going to need a
a buzzard. Cheeky !! Soon enough it was time to pack up stronger canopy. If it were a 120cc model, then it will have
for the day, so its wings off, hook up to the fuel pump to a smaller tank (40oz), and the model is lighter (maybe
empty out the tank, and start to pull the canopy frame off. 14kg) … so the ratio is about the same. I bet it’s about the
Hey get a photo of me with my head sticking through the same on a 50cc too. Tank weight vs model weight. I’m sure
hole in the canopy. Gee that fuel tank emptied out quick ? my carbon tank tray setup is strong enough to suspend the
… oh hang on a minute .. take a closer look into the entire 20kg weight of the model from, but those velcro self
fuselage … no tank ? Dubro 60z fuel tank was nowhere to sticking straps I’ve always used ? Now I’m not so sure. In
be seen. Gone burger. If you haven’t seen a 60oz tank, it’s electric F3A we use the Velcro straps that wrap back on
pretty big, almost as big as a rugby ball. Only one way it themselves through a plastic buckle to hold 1kg of battery
got out, and that was through the canopy. That was what in place. Those models are easily doing 10g at times, and
we saw exit the model. No buzzards involved. Explains the straps seem to do the job well. It might be worth a test
why the motor stopped. It was running on what was left in to see just how much your avg 20mm Velcro strap will take
the 500mm of fuel line the tank had generously left behind. before it lets go. Huge cable ties maybe a better solution
Just enough to get me home. Lucky it didn’t quit on the yes, but at times its handy to drop your tank out quickly to
back of that rolling circle I had just done, might not have inspect it.
gotten back home at all. Phew. So how on earth has the That’s it till next time, I’ve got a canopy repair to do. I use a
tank gotten out ? The setup in this model, has the tank workshop vacuum clean set on blow, to push the canopy
slung under the wing tube. This is an all composite model. into place while the sealant dries. That’s a topic for next
There aren’t formers and ply bracing like your typical laser time.
cut ply and balsa “shake the box” and its ready to fly ARF.
The wing tube is a great anchor point, but there isn’t See you at the field, and roll on summer !!
enough room for the tank to sit on the top of the wing tube,
and its kind of in the way up there, so a tray the size of the Frazer Briggs
tank sits underneath the tube. It runs here because that’s
where the CG is, and we don’t want a trim change during
flight as the tank empties. The tray is a composite of 8mm
PVC foam and 200g carbon cloth. Its strong and its light.
It’s anchored to the wing tube with some supports that go
over the top of the tube. Like a bridge. The tray has some
bearers front and back to stop the tank from sliding back or
forth during flight. A pair of 20mm wide velcro strips go
around the tank, and through slots in the tray. The velcro is
the type sticks to itself when made in a loop. 1000’s of
flights with this arrangement, over many models for the
past 20+ years, and no problems … well until till now. It
looks like the tank has escaped the front velcro strapping,
and dropped down low enough, to then move foward
enough that it could then escape the back strapping too.
Then the entire tank has slid backwards (probably on an
upline) into the canopy area, and at the time of the big
down elevator pitch for the snap roll, all those G forces
said “see ya later Mr Tank” !! Wait … who pushed the eject
button ? Right through the canopy … as I said “Gone
Burger”. Now to make sense of all this, it’s math time. 60oz
of fuel = 1.7Kg give or take a few grams. It was a minute or
two into the flight, so let’s say 50oz of fuel in the tank at the
time. Handy … that’s about 1.5Kg. It’s well known we push
some pretty high G forces when flying aerobatics. We did
some testing on this 20+ years ago in a 11 pound / 2m
Laser, with a strain gauge and g meter in the model. I’m Tony Christiansen at Galatea IMAC
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