HeliSASTM
Stability Augmentation System and Autopilot for Helicopters
HISTORY
HeliSAS was developed by Hoh Aeronautics, Inc. with the
objective of providing a low-cost, lightweight stability augmentation system
for helicopters. Hoh Aeronautics, Inc. accomplished
many years of helicopter research during the development of the U.S. Army
handling qualities specification for rotorcraft, ADS-33E-PRF. Many of the concepts developed therein were
used to create a low-cost, lightweight helicopter SAS/AP system for the civil
market. The original work to develop HeliSAS
was accomplished under Phase 1 and Phase 2 NASA Small Business Innovative
Research (SBIR) contracts.
The proof-of-concept test aircraft was a Robinson R44
helicopter that was purchased by HAI specifically to develop HeliSAS. A flying prototype was completed in 2005 and
the intellectual property licensed to Cobham in March 2006. Cobham provided the funding to achieve FAA
certification, which was achieved for the R44 in October of 2009.
SYSTEM OVERVIEW
The HeliSAS system weights only 9 lbs plus the weight of
the attitude gyro. The system will
accept any solid state attitude heading reference system (AHRS) that provides
standard ARINC 429 outputs.
Alternatively, an especially designed panel mounted gyro built by
Castleberry instruments can be used as the attitude source for HeliSAS.
The heart of the system is an
attitude-command-attitude-hold stability augmentation system (SAS). This means that the rotorcraft will always
return to a level attitude if disturbed, and will resist pilot inputs away from
a level attitude with approximately 3 lbs of force. HeliSAS is designed as a “fly through” system
so that the pilot can easily maneuver with the system engaged throughout the
flight envelope from liftoff to landing, including autorotation. There is significant confusion in the
rotorcraft industry regarding the definition of SAS and the difference between
an attitude SAS and a rate SAS. This are
discussed in an article that can be addressed by the following link – Rate vs. Attitude SAS.
Recoveries from bank angles up to 90 degrees have been
demonstrated in flight test. If faced
with the possibility of spatial disorientation, the pilot can simply let go of
the controls and be assured that the helicopter will maintain level
flight. In addition, the attitude
command attitude hold feature greatly simplifies the hover task, especially in
windy or gusty conditions.
Non-helicopter pilots have been able to hover with no training with
HeliSAS engaged.
The Autopilot functions are specifically designed to be
intuitive and many pilots report that the system is “easy to operate and does
what you expect it to do”. The autopilot
functions include:
·
Heading select and hold
·
Altitude hold
·
VOR/ILS tracking including all-angle
localizer intercepts
·
LNAV/VNAV approaches
PARALLEL SAS IMPLEMENTATION
HeliSAS has been designated by the FAA as a “new and
novel design”, and therefore required significant scrutiny for certification. That is because the SAS is based on a
parallel-servo mechanization. This means
that the servos are simply attached directly to the controls, and that all SAS
corrections are reflected in cyclic motion.
This was previously thought to be unacceptable because the pilot would
object to the cyclic moving in contradiction to his or her control inputs. Research conducted by HAI for the U.S. Army (using
the Canadian National Research Council variable stability Bell 205) showed that
if the control algorithms are phased properly, it is possible to use a
parallel-servo design that is not objectionable to the pilot because the cyclic
moves as expected. In fact most pilots
do not even notice the motion of the cyclic.
This differs from most other helicopter stability
augmentation systems which are based on a series design wherein the cyclic does
not move in response to SAS actuator inputs.
This involves significant complexity in that the series servo must be
limited (usually to less than 10% of travel), and an additional parallel servo
is required to keep the series servo centered.
This requires a total of 4 servo-actuators for a two axis system as well
as magnetic brakes and cyclic centering springs in each axis In addition, the FAA requires a control-position
instrument for each axis to advise the pilot where the swashplate is relative
to full travel, since its motions are not reflected in the cyclic.
Advantages of the parallel system are:
·
Installation is greatly simplified because
there is only one servo to connect to the flight controls in each axis, and it
is not necessary to cut into the flight control system. No calibration or adjustments are required
during installation or for the life of the system
·
There are significantly fewer components to fail
than with a series system.
·
When the SAS is disengaged, the pilot is back
to the basic helicopter. There is no
feel spring with potential mis-trim at disconnect to worry about.
·
The pilot always knows where the main rotor
actuator is because it is reflected directly in the cyclic. Control margins are just as obvious as
without a SAS.
·
Any system failure can be immediately
observed in erratic cyclic motion and easily overridden by the pilot.
·
There is no possibility of servo
saturation. Testing for the U.S. Army
showed that servo saturation was highly undesirable and can result in rapid and
unexpected rotorcraft motions (such as “digging in” during a quickstop).
·
Research conducted for the U.S. Army showed
that an attitude command attitude hold SAS is far more effective as a safety
enhancement than a Rate SAS. In fact the
Rate SAS was shown to be of no value in conditions of poor visibility.
The parallel SAS implementation was refined during the
extensive 5 year HeliSAS development and is protected by U.S. Patent 7108232.
Because HeliSAS was classified as new and novel, it was
subject to a “Special Condition” by the FAA.
This meant that the certification was accomplished under the very
stringent 14 CFR Part 29 regulations for heavy commercial helicopters. This included an extensive systems safety
analysis, rigorous environmental testing (DO 160F) including the new and very
stringent HIRF requirements. The Special
Condition also required that the system meet the same software testing as very
sophisticated fly-by-wire aircraft (DO178B Level A). These analysis and test results were reviewed
and approved by both the FAA Los Angeles Airplane Certification Office and the
FAA Rotorcraft Directorate in Ft. Worth.