Airworthiness Qualification Criteria for Rotorcraft with External Sling Loads

35 Pages·1.9 MB·English

by  Key, David L

NASA/TM-2002-211390
USAAMCOM AFDD/TR-02-A-002
Airworthiness Qualification Criteria for Rotorcraft
With External Sling Loads
David L. Key
Key Qualities, Oceanside, California
National Aeronautics and
Space Administration
Ames Research Center Prepared for NASA Ames Research Center
Moffett Field, California 94035-1000 under Contract NAS2-14360
IIIII [J l......I.........
...................................................... L ...................................
January 2002
The NASA STI Program Office.,. in Profile
Since its founding, NASA has been dedicated to the CONFERENCE PUBLICATION. Collected
advancement of aeronautics and space science. The papers from scientific and technical confer-
NASA Scientific and Technical Information (STI) ences, symposia, seminars, or other meetings
Program Office plays a key part in helping NASA sponsored or cosponsored by NASA.
maintain this important role.
SPECIAL PUBLICATION. Scientific, technical,
The NASA STI Program Office isoperated by or historical information from NASA programs,
Langley Research Center, the Lead Center for projects, and missions, often concerned with
NASA's scientific and technical information. The subjects having substantial public interest.
NASA STI Program Office provides access to the
NASA STI Database, the largest collection of TECHNICAL TRANSLATION. English-
aeronautical and space science STI in the world. language translations of foreign scientific and
The Program Office is also NASA's institutional technical material pertinent to NASA's mission.
mechanism for disseminating the results of its
research and development activities. These results Specialized services that complement the STI
are published by NASA in the NASA STI Report Program Office's diverse offerings include creating
Series, which includes the following report types: custom thesauri, building customized databases,
organizing and publishing research results.., even
• TECHNICAL PUBLICATION. Reports of providing videos.
completed research or a major significant phase
of research that present the results of NASA For more information about the NASA STI
programs and include extensive data or theoreti- Program Office, see the following:
cal analysis. Includes compilations of significant
scientific and technical data and information • Access the NASA STI Program Home Page at
deemed to be of continuing reference value. http://www.sti.nasa.gov
NASA's counterpart of peer-reviewed formal
professional papers but has less stringent • E-mail your question via the lntemet to
limitations on manuscript length and extent [email protected]
of graphic presentations.
• Fax your question to the NASA Access Help
• TECHNICAL MEMORANDUM. Scientific and Desk at [phone]
technical findings that are preliminary or of
specialized interest, e.g., quick release reports, • Telephone the NASA Access Help Desk at
working papers, and bibliographies that contain [phone]
minimal annotation. Does not contain extensive
analysis. Write to:
NASA Access Help Desk
• CONTRACTOR REPORT. Scientific and NASA Center for AeroSpace Information
technical findings by NASA-sponsored 7!21 Standard Drive
contractors and grantees. Hanover, MD 21076-1320
Available from:
NASA Center for AeroSpace Information National Technical Information Service
7121 Standard Drive 5285 Port Royal Road
Hanover, MD 21076-1320 Springfield, VA 22161
[phone] [phone]
TABLE OF CONTENTS •
Table of contents ............................................................................................................................... i
Summary .......................................................................................................................................... 1
Definitions ........................................................................................................................................ 1
Introduction ...................................................................................................................................... 2
Aeroservoelastic stability ................................................................................................................. 3
Definitions of ASE stability and Stability Margins ..................................................................... 4
Recommended ASE stability criteria ............................................................................................ 8
Pilot task performance - Handling Qualities .................................................................................... 9
ESL related paragraphs in ADS-33E-PRF ................................................................................... 9
Recommended HQ criteria ......................................................................................................... 10
Load suspension system failures ................................................................................................... 12
Load stability at all speeds ............................................................................................................. 13
External load flight envelopes ..................................................................................................... 14
Proposed criteria for airworthiness qualification with external sling loads .................................... 16
Rotorcraft airworthiness qualification ESL off .......................................................................... 16
Rotorcraft airworthiness qualification ESL on ........................................................................... 16
Guidance for application of ESL AWQ criteria ............................................................................. 17
Specific load airworthiness clearance and certification ............................................................... 18
References ...................................................................................................................................... 20
Appendix A. Flight test to establish load maneuvering envelope .................................................. 21
Appendix B: ADS-33E-PRF treatment of failures and reliability ................................................. 25
Appendix C: Recommended ESL Handling Qualities design Criteria for Low Speed and Hover in
the Degraded Visual Environment .................................................................................................. 27
SUMMARY
This report presents the results of a study to develop airworthiness requirements for rotorcraft
with external sling loads. The report starts with a review of the various phenomena that limit
external sling load operations. Specifically discussed are the rotorcraft-load aeroservoelastic
stability, load-on handling qualities, effects of automatic flight control system failure, load
suspension system failure, and load stability at speed. Based on past experience and treatment of
these phenomena, criteria are proposed to form a package for airworthiness qualification. The
desired end objective is a set of operational flight envelopes for the rotorcraft with intended loads
that can be provided to the user to guide operations in the field. The specific criteria proposed are
parts of ADS-33E-PRF, MIL-F-9490D, and MIL-STD-913A all applied in the context of
external sling loads. The study was performed for the Directorate of Engineering, US Army
Aviation and Missile Command (AMCOM), as part of a contract monitored by the
Aeroflightdynamics Directorate, US Army AMCOM.
DEFINITIONS
AFCS -Automatic Flight Control System
ASE -Aeroservoelastic
AWQ -Airworthiness qualification
db -Decibels [20 log)0 (output/input)]
DVE -Degraded Visual Environment as defined in ADS-33E-PRF (see ref 1)
ESL -External sling load
FCS -Flight Control System
FDCS -Flight Data Collection Sheet
Fe -Equivalent fiat plate area
e.g. -Center of gravity
GVE -Good Visual Environment as defined in ADS-33E-PRF (see ref 1)
GW -Gross weight of external load
HQ -Handling qualities
HQR -Handling qualities rating
MTE - Mission Task Element
OFE -Operational Flight Envelope as defined in ADS-33E-PRF (Ref 1) The envelopes
are defined in terms of airspeed, altitude, load factor, rate of climb, side velocity
and any other parameters required to define limits.
Q,q -Dynamic pressure
Se -Equivalent planform area
SFE -Service Flight Envelope, as defined in ADS-33E-PRF (Ref 1)
UCE -Usable Cue Environment, as defined in ADS-33E-PRF (Ref 1)
Configurations -A rotorcraft configuration is defined by the external geometry. This
includes the position of variable systems such as landing gear or flaps,
location of extemal stores, or carriage of sling loads (see Ref 1).
Loadings -Loadings refers to the mass properties of the rotorcraft configuration
and will be reflected in the total mass or weight, the center of gravity
location, and the various moments of inertia (see Ref 1).
LoadMass Ratio - the ratio of the mass of the load to the mass of the helicopter plus load
Settings -Settings refer to the selected functionality of rotorcraft components or
systems that affect rotorcraft response, or Usable Cue Environment
(UCE) that can be activated or deactivated by the pilot (see Ref 1).
States -Rotorcraft states are NormaI when the various systems are functioning
as selected. Failure states exist when the functionality is modified by one
or more malfunctions in rotorcraft components or systems that affect
rotorcraft response or UCE (see Ref 1).
ESL configuration -External sling load configuration includes all the parameters that affect
the load external geometry, and include at least the following:
Load shape
Sling set geometry and material
Vertical, longitudinal, and lateral hook location(s) relative to the
mtorcraft c.g.
ESL loadings -External sling load loadings refers to the mass properties of the external
sling load and include:
Load mass
Load c.g. location
Load inertia
Load distribution between multiple hooks
INTRODUCTION
The purpose of this report is to define a set of requirements and criteria that must be observed to
guide airworthiness certification of rotorcraft for carrying external loads. Many factors influence
the safety of flight with External Sling Loads (ESL). These safety factors need to be considered
during rotorcraft development. The desired end objective is a set of Operational Flight Envelopes
(OFEs) for the rotorcraft when carrying its intended loads that can be provided to the user to
guide operations in the field. These OFEs should be related to the internally loaded rotorcraft
OFE with addition of ESL parameters as necessary to define the appropriate limits.
Some of the limiting phenomena that will need to be considered when defining the rotorcraft ESL
OFE are as follows:
1. Interactions between the flight control system, the helicopter structure, the load, and the pilot,
can result in aeroelastic instabilities. The pilot-rotorcraft dynamics must be stable and
sufficiently well damped to resist turbulence and to permit accomplishment of tight tracking
tasks.
2
2.At lowairspeedsa,nexternalloadcanswingandcoupledhelicopter-loadresponsecsan
introducemotionsthatcausethepilotdifficulty ofcontrolespeciallywhileperformingprecision
tasks.Suchloadinteractionswill beworsewithhighloadmassratios.
3.Failuresintheautomaticflightcontrolsystem(AFCS)canresultinunacceptablteransientsor
degradedsteadystatehandlingqualitiesthatrequireloadjettisontoretaincontrolwithoutdamage
totherotorcratt.
4.At anyspeed, complete failure of one end of a dual point suspension can cause the load to
swing into the helicopter, introduce upsetting moments that may be uncontrollable, or induce
unsustainable loads on the remaining supports. Similarly, failure of one leg of a multi-leg
suspension can result in uncontrollable load motions.
5. As airspeed is increased, the aerodynamic characteristics of the specific load will become
important. The aerodynamics can cause the load to trail at an excessive angle, oscillate or rotate
about an axis that twists the sling(s), oscillate or swing as a pendulum in pitch and roll, or fly up
into the carrying helicopter. Some of these motions can occur suddenly with a violent motion.
With these phenomena in mind, the aeromechanics related considerations for airworthiness
qualification (AWQ) of a rotorcraft that has to carry externally slung loads would have to cover
the following aspects:
1. Aeroservoelastic stability
2. Pilot task performance- handling qualities
3. Flight control system failures
4. Load suspension system failures
5. Load stability at all speeds
These aspects will be discussed in turn in the following sections. In a subsequent section the
essence will be collected together to form a package for guiding AWQ of a rotorcraft to carry
externally slung loads, and provide procedures for specific load certification.
AEROSERVOELASTIC STABILITY
Stability analysis needs to deal with both aeroelasticity and aeroservoelasticity. The difference
between these two topics is the effect of the flight control system and the pilot closed loop
control of the helicopter. Aeroelasticity deals with the mutual interaction of elastic, inertial, and
aerodynamic forces. Helicopter aeroelastic stability analysis must be conducted to ensure that no
ground or air resonance conditions occur in the coupled rotor-fuselage-drive-landing gear system.
These analyses are open loop in the sense that control surfaces remain fixed throughout the
stability analyses. Aeroservoelastic (ASE) stability analysis examines the stability of the closed
loop system. In a closed loop system the control surfaces deflect in response to control
commands, elastic structural deformations at the rotor hub or sensors, and undesired or
involuntary pilot inputs (biodynamic feedback). It is very important that any analyses conducted
to predict stability include aeroservoelastic effects.
Moving a load from internal to an external sling can modify the ASE characteristics of a
rotorcraR, as well as introduce additional modes. On current US Army rotorcraft such as the UH-
60BlackHawk,andtheCH-47Chinookfamilies,theeffectsofexternallyslungloads(ESL)on
theASEcharacteristichsavenotbeensignificant.However,somelargemoreflexiblerotorcraft,
suchastheNavy/MarineCH-53family,or V-22Osprey, have required extensive tests and
analyses to define safe ESL OFE. Both of these rotorcraft have low frequency structural
flexibility modes which can interact with those of the external load suspension system. Since
these modes are structural, they typically display low damping characteristics. When the
frequencies of these modes overlap the active range of the pilot and the flight control system
(FCS), unstable coupling is possible and has been experienced operationally. At the other end of
the spectrum, small helicopters may encounter limits because of their flight control augmentation
or design objectives. For example, the RAH-66 Comanche uses a high gain full authority fly-by-
wire FCS to achieve the agility and handling qualities required to perform demanding mission
tasks and operations in a degraded visual environment. High FCS gains can lead to low damping
or even destabilization of rotor, or rotor-body modes. In addition, the Comanche program has
placed heavy emphasis on reducing structural weight. Consequently, the composite structure is
tailored for minimum stiffness, which increases the susceptibility of FCS to structural coupling
and biodynamic feedback. Upgrades to US Army helicopters usually involve significant increases
in gross weight and some re-tuning of the AFCS feedback gains, both of which have the potential
for ASE stability margin reductions. These factors suggest that during development of alI future
rotorcraft intended to carry ESL, whether major upgrades or entirely new systems, the ASE
stability margins determined with internal loading should be rechecked when carrying ESL.
Definitions of ASE stability and Stability Margins
There are at least two definitions of ASE stability requirements for flight control systems that
may be applied to ESL; these are MIL-C-18244A and MIL-F-9490D.
MIL-C-18244A
MIL-C-18244A (Ref. 2) is a military specification for automatic control and stabilization
systems for piloted aircraft. It was developed in 1955 and revised in 1962. The pertinent
requirement paragraphs are:
3.1.1.6.1 Stability Margins
The AFCS shall be demonstrated to be stable in all modes of operation in all flight
conditions as follows: All AFCS aerodynamic loops shall be flight demonstrated to be
stable for at least one and one half times the production gain. At the beginning of service
life and under standard conditions as specified in Specification MIL-E-5272, all AFCS
non-aerodynamic servo loops shall be demonstrated to be stable at three times the
production gain. All AFCS non-aerodynamic loops shall be demonstrated to be stable at
one and one-half times the production gain throughout all operating service conditions. At
the end of service life, and under standard conditions, all non-aerodynamic loops shall be
demonstrated to be stable at one and one half times the production gain. It shall also be
demonstrated that an additional lag of 45 degrees, when introduced into any loop with
production gains, shall not result in instability.
MIL-F-9490D
A more modem set of criteria for control system design is given in MIL-F-9490D, Ref 3. The
pertinent paragraphs in this document are:
3.1.3.6 Stability
For FCS using feedback systems, the stability as specified in 3.1.3.6.1 shall be provided.
Alternatively, when approved by the procuring activity, the stability defined by the
contractor through the sensitivity analyses of 3.1.3.6.2 shal 1be provided. Where analysis
is used to demonstrate compliance with these stability requirements, the effects of major
system nonlinearities shall be included.
3.1.3.6.1 Stability margins
Required gain and phase margins about nominal are defined in Table III for all
aerodynamically closed loop FCS. With these gain or phase variations included, no
oscillatory instabilities shall exist with amplitudes greater than those allowed for residual
oscillations in 3.1.3.8, and any nonoscillatory divergence of the aircraft shall remain
within the applicable limits of M1L-F-8785 or MIL-F-83300 or ADS-33E-PRF (added for
rotorcraft). AFCS loops shall be stable with these gain or phase variations included for
any amplitudes greater than those allowed for residual oscillations in 3.1.3.8. In multiple
loop systems, variations shall be made with all gain and phase values in the feedback
paths held at nominal values except for the path under investigation. A path is defined to
include those elements connecting a sensor to a force or moment producer. For both
aerodynamic and nonaerodynamic closed loops, at least 6 db gain margin shall exist at
zero airspeed. At the end of system wear tests, at least 4.5 db gain margin shall exist for
all loops at zero airspeed. The margins specified by Table III shall be maintained under
flight conditions of most adverse center-of-gravity, mass distribution, and external store
configuration throughout the operational envelope and during ground operations.
Table III: Gain and phase margin requirements (db, deg.)
Airspeed
Mode VoMIN to •_t 1.5
Below Vomm At Vr.
frequency Vo._ x VL
GM=+
GM = +4.5
fM >0.06 Hz PM = _+30 3.0 GM = 0
PM = + 20
GM = 6 db PM = 0
0.06< fM
No phase GM = + Stable at
<first GM = +6.0
aeroelastic requirement PM = +45 4.5 nominal
PM = + 30
mode below VoMIN phase
fM>first GM = + and gain
GM = _+8.0
aeroelastic 6.0
PM = _+60
mode PM = + 45
5
Ve = Limit Airspeed (MIL-A-8860).
VoM_ = Minimum Operational Airspeed (MIL-F-8785).
VoMAX = Maximum Operational Airspeed (MIL-F-8785).
Mode = A characteristic aeroelastic response of the aircraft as described by an aeroelastic
characteristic root of the coupled aircraPt/FCS dynamic equation-of-motion.
GM = Gain Margin (db). The minimum change in loop gain, at nominal phase, which
results in an instability beyond that allowed as a residual oscillation.
PM ---Phase Margin (deg). The minimum change in phase at nominal loop gain which
results in an instability.
fM = Mode frequency in Hz (FCS engaged).
Nominal Phase and Gain = The contractor's best estimate or measurement of FCS and aircraft
phase and gain characteristics available at the time of requirement
verification.
3.1.3.6.2 Sensitivity analysis
Tolerances on feedback gain and phase shall be established at the system level based on
the anticipated range of gain and phase errors which will exist between nominal test values
or predictions and in-service operation due to such factors as poorly defined nonlinear
and higher order dynamics, anticipated manufacturing tolerances, aging, wear,
maintenance, and noncritical material failures. Gain and phase margins shall be defined,
based on these tolerances, which will assure satisfactory operation in fleet usage. These
gain and phase tolerances shall be established based on variations in system characteristics
either anticipated or allowed by component or subsystem specification. The contractor
shall establish, with the approval of the procuring agency, the range of variation to be
considered based on a selected probability of exceedance for each type of variation. The
contractor shall select the exceedanee probability based on the criticality of the flight
control function being provided. The stability requirements established through this
sensitivity analysis shall not be less than 50 percent of the magnitude and phase
requirements of 3.1.3.6.1.
Current situation
Both of these criteria have been used to investigate ASE stability margins in helicopters. The
former criteria, MIL-E-18244A, was used by Sikorsky on a task for the Navy to develop MH-
53C safe operating envelopes for externally slung loads of 25,000 lb with dual suspension. The
latter, MIL-F-9490D, has been used by Bell-Boeing on the V-22, for both internal loadings and
for the ESL flight envelopes. MIL-F-9490D has also been used by Boeing-Sikorsky on the
Comanche, for flight envelope development with internal loading.
The Comanche originally had a requirement for sling load capability, and required clearance to
MIL-F-9490D with extemal loads. The ESL capability has since been deleted, but the standards
of MIL-F-9490D were applied to the internal loading. The pertinent analysis is described in Ref
4. MIL-F-9490D requires that the gain and phase stability margins be achieved at the most
6