Vertiflite Sep-Oct 2013 sample article - page 3

ground effect and agreed very well with
models from the literature and
experimental results. Another important
element was a parametric mass
estimation developed from the
Snowbird. Our design objective was
minimum power, for as much margin as
possible on the prize flight – with the
understanding that this would be
whittled away.
Many design decisions were based
on familiarity. Given a tight timeline that
aimed to win the prize by the end of
August 2012, we wanted to minimize
our learning curve. Plugged into the
human-powered community, we made
several human-engine decisions based
on expert advice rather than
independent study. For example, we
opted for an upright bike instead of a
recumbent one.
Building Atlas
I
n May 2012, we recruited the eight-
student summer team, made up
largely of undergraduate engineering
students from the University of Toronto.
Our workspace was an old barn at the
Great Lakes Gliding Club in Tottenham,
north of Toronto. Our team lived
together, biked to and from the barn,
and cooked meals together. This tight-
knit and focused approach was crucial
to our collaborative process and
passionate drive.
Light weight and creativity were
prioritized during the detailed design
stage. The rotor hubs were inspired by a
bicycle wheel and went through five
build/test cycles. The four-arm boom
structure was in fact a hybrid between a
truss and a wire-braced arch, designed
with a gradient-based FEA optimization.
A lift wire was also used for the main
rotor spar, saving weight over a
cantilevered spar at minimal drag
penalty.
We used axially wrapped, pre-preg
carbon-fiber tubes (a technique
developed for the Snowbird) for the
rotor spars and boom structure and a
built-up rib construction for the lifting
surfaces.We made few spares, reasoning
that parts would be fast and cheap to
repair on the field. Despite several
enormous crashes, the helicopter
weighed the same for the prize flight as
it did when first built.
For the drivetrain, we chose a system
that drove the rotors as string unwound
from spools on the rotor hubs and
wound up on spools connected to the
bike pedals.We used off-the-shelf or
modified components for the bike and
mechanical elements where possible.
The bike itself was suspended on lines.
We considered the control problem
from the outset, contrary to nearly all
other HPH designs. Our initial concept
was to tilt the entire helicopter by using
collective on each rotor. Each rotor’s lift
differential would be effected
aeroelastically, with a small all-flying
canard at the tip applying torque to
twist or untwist the blade. This
minimized the control force required
and profile/induced drag penalties.
Initial Testing
W
e transitioned to the Ontario
Soccer Centre in Vaughan,
Ontario in mid-August 2012,
with two weeks to do final integration
and flight test. Having determined that
flying outdoors would be prohibitively
weight-expensive for gust tolerance, we
searched for an indoor location. Of the
few facilities that could contain Atlas,
only the Soccer Centre was willing and
able to accommodate us. Our early
gambit had paid off.
Upon integrating the four boom
structures and tuning the bracing lines,
we found the booms were extremely
flexible in torsion. Under load, they
tended to twist and fall over.We
supplemented the existing bracing lines
with cross-bracing between the booms
and additional pre-tension in the truss
internal lines. These were the first of
many incremental stiffening steps.
Flight test was very slow going. For
one, the helicopter was stored in a 53 ft
(16.1 m) trailer. Typically, 2 hours of
assembly and rigging was required each
morning and 45 minutes of disassembly
needed in the afternoon. This left 6
Vol. 59, No. 5
9
A comparison of the original size of the Atlas versus prior flying HPHs. Atlas’ booms were later
reduced in length by 10%, resulting in rotor overlap. (AeroVelo graphic)
Altas blade configuration. (AeroVelo graphic)
1,2 4,5,6,7,8
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