Vertiflite Sep-Oct 2013 sample article - page 4

hours a day to fly.We began with
ground tests, with and without canards
attached, for rotor aerodynamic
balancing. (Weight balance was not an
issue.) Much time was spent
painstakingly trimming the blades and
canards. The rotors had been designed
with significant negative pre-cone and
minimal clearance to keep the tips in
ground effect. Imbalances often resulted
in tip strikes and broken blades. This
dominated testing during the last week
of August.We had six blade breaks.
During our final flight, a control
authority test resulted in dramatic
imbalance of the canards on several
rotors, breaking two blades upon
landing after the 17 sec
flight.
With the team back to
class after that, the two of
us focused on several
experimental studies and
modifications.We reduced
the negative pre-cone on
the rotors to improve
clearance. Using in-pedal
power measurement on the
drivetrain, we conducted
whirl-stand testing for the
optimal blade and canard
angles of attack as built.
Improvement of the truss
followed; we further tuned
the bracing lines and
modified attachment
points for better rotor clearance. There
was a fine balance between improved
stiffness and reduced failure margin. (By
the prize flight, several truss elements
would be buckled prior to loading the
pilot due to line pre-tension).
Testing resumed in January 2013; we
flew roughly one day every two weeks.
This model minimized disruptions of the
students’ academic work and allowed
time to analyze data and make
improvements between sessions.
During these tests, we found the
existing controls lacked authority, had
an extremely slow reaction time and
invariably caused dramatic and
destructive rotor blade imbalances.
Evaluating other options, we realized
the booms’ extreme flexibility could be
an advantage.We modified the bracing
lines under the truss to connect to the
bike frame, so when the pilot leaned
left, right, forward or back, the lines
would pull on the bottom of each rotor
axle and instantly tilt the lift vector. This
yielded intuitive control, instant reaction
and substantial authority.We removed
the canards for a 10% total weight
saving and 15% drag reduction.
Going for the Prize
I
n March, we conducted our first prize
attempt. Having made progressive
flights to 1 and 2 m (3.28 ft and 6.56
ft) heights, we saw that a flight to the
AHS Sikorsky Prize’s required 3 m was
possible, if fairly aggressive.We were
prone to taking risks, cutting corners or
rushing due to the race with Team
Gamera. Our helicopter climbed with
power margin and control, but flexibility
allowed a growing “wobble” in multiple
rotors. Upon starting the descent after
reaching nearly 3 m, the front rotor lost
lift and fell rapidly, breaking two booms.
The crash damaged much of the
helicopter, which required five weeks to
rebuild.We addressed all possible
causes of the crash, but couldn’t
establish definitive evidence of a root
cause.
In late April, we went from re-
trimming the rebuilt helicopter to
making a prize attempt in one day.
Again, a smooth climb to nearly 3 m
ended with the front rotor in a rapid
descent and a mid-air
breakup. During the six-
week rebuild, we re-
evaluated possible causes
and several outstanding
concerns with the truss
structure.We shortened the
boom arms by about 10%,
increasing their stiffness
and reducing weight (and
repair time). This resulted in
the rotors intermeshing, but
phase offset and slow rotor
rpm eliminated the
possibility of tip strikes.We
modified the lines under
the structure so the arch
loads were borne by lines
connecting the bottom of
10
VERTIFLITE September/October 2013
Robertson and team fairing the rotor spar mandrel in their barn workshop in May 2012. (AeroVelo
photo by Jake Read)
Atlas rotor spool on a test stand in the old barn in July 2012. (AeroVelo photo by
Todd Reichert)
1,2,3 5,6,7,8
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