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Scratch
Built Collective Pitch - Hummingbird / Hornet Hybrid
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Part
1 - Chassis / Sideframes
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The
main chassis acts as a mount for the mainshaft and motor,
in addition the tail boom is fixed to the rear of the
chassis. To work effectively it needs to be lightweight
but strong and provide a rigid mounting point for the
mainshaft invariably at 90 degrees to the chassis itself.
I spotted some extruded
aluminium section in a local hardware store that fits
the bill nicely. One its lightweight, two its easily
cut and if need be bent, its easily drilled and tapped
where needed and its strong. For the main protion of
the chassis I am using 13.5x8mm 'C' section, the mainmast
that supports the mainshaft is made up of 6mm and 8mm
DIA tube.
To mount the chassis
to the landing gear I am using 1.5mm carbon fibre sheet.
This will also act as a mounting point for the battery
and give areas to locate and fix the electronics.
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Chassis
Making the chassis is quite simple. The
upper and lower portions are cut to length, I then printed
out full scale templates from the AutoCAD drawing, these
are shown above.
I simply glued these to the cut sections
using 3M Spraymount, centre punched all the holes then
drilled them all out.
I then filed to shape the mouning points
for the motor and tapped the vertical mounting holes
to accept M2.5 machine bolts that would hold both sections
together
The concept of printing templates out
from the AutoCAD drawing and temporarily glueing in
place is used throughout the construction of this hybrid
and works really well, no measuring!!!
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The next stage is to accurately mount
the mainmast. Two sections are cut to act as the vertical
spacers seen in the photograph, the upper and lower
sections can then be bolted together.
A single length of 8mm tube is then threaded
through both sections with a length of 10mm tube threaded
over this and in position on the lower section. I checked
for square and then clamped the assembly in a drill
vice on my drill press and drilled holes through both
the 'C' section and tube on top and bottom sections.
These where then tapped to M3 and the
top section countersunk to accept a countersunk bolt,
bottom is left to accept a pan head screw
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Four suitable bolts where cut to length
so as not to interfere with the mainmast, these were
then screwed in position and locked there with locktite.
The top and bottom tubes were then cut
to length.
The result is two perfectly aligned
tubes designed to accept the three bearings to be
used to support the mainshaft.
The next stage was to cut a length of
6mm tupe to act as a spacer for the two upper bearings,
the section drawing here shows these in position.
This was simply CA'ed in position.
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I then trial fitted the mainmast, maingear
and motor. As you can see the motor is undersluhg
as in the Hornet which has the advantage of a shorter
mainshaft and lower centre of gravity. I am using
a Hacker B20 22S BL motor, see the analysis page for
specs.
Locking collars are used top and bottom
to hold the mainshaft in place positioned to just
allow a slight degree of vertical movement.
The bottom bearing is free to move up
and down within its support shaft, support is only
provided to prevent lateral movement.
The maingear moves sweet as a nut, I
have used an auto-rotation hub to give the smoothest
disc possible.
The mainshaft is the titanium hummingbird
variety, the photograph shows how much protrudes below
the bottom bearing that is not now needed, after experimenting
with different setups I may trim this to length, then
again I may not!
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Sideframes
To make the sideframes I first printed
out templates from the 2D drawing as before, these
were then temporarily adhered to some 1.5mm thick
carbon sheet using spraymount.
I used a dremel fitted with a cut off
wheel to first cut out all the inner areas. When working
with carbon fibre in this way I recommend firstly
a well ventilated area and secondly position a vacuum
cleaner hose close to the working area, use duct tape
to fasten in place if needed. The idea of this is
to cut down on airborne dust which is nasty stuff.
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After all the cuts were completed I
used a set of needle files to trim the cuts to teh
template lines and generally tidy the frames up.
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Before removing the template and
cleaning I drilled out all the marked holes. To do this
accurately I taped both frames together with masking
tape and drilled through.Because
the templates are derived from the same drawing as the
chassis everything is completely accurate
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The side frames are fixed in position
with M3 hex head bolts.
The carbon fibre sheet works easily
and I am very pleased with the results, the helicopter
is starting to come together now
This picture shows the landing gear
in position
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To maintain accurate frame spacing I
have used 13.5mm standoffs as spacers, these also
act as braces to help support the battery
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This picture shows a close up of the
fixed sideframes plus the anti-rotation bracket in
position on the mainmast
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