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Handling qualities evaluation of an automatic slung load stabilization system for rescue hoist operations

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Abstract

The Handling Qualities (HQs) of a helicopter can be adversely affected through the presence of an externally slung load. Helicopter stability margins may be reduced, due to the additional dynamics of the load system, which can subsequently increase pilot workload, and reduce the operational envelope. An Automatic Load Damping System (ALDS) has been designed and has been successfully tested in flight. This system, alongside slung load scenarios, has been implemented within a piloted simulation in DLR’s Air Vehicle Simulator. In this article, the results from a simulated test campaign to observe the influence of the stabilization system on the vehicle HQs are presented. The system is assessed using three Mission Task Elements, modified for hoist operations. Results show that a conflict between pilot control and commanded inputs from the ALDS can cause unstable slung load oscillations and degradation in HQs in hover. However, it is shown that when the stabilization system is used only when required, both the HQs of the helicopter are conserved, and load oscillations are reduced. The results in this paper are intended to motivate future flight tests using DLR’s Active Control Technology/Flying Helicopter Simulator.

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Abbreviations

AC:

Attitude Command

ACT/FHS:

Active Control Technology/Flying Helicopter Simulator

ADS-33:

Aeronautical Design Standard 33

AGL:

Above Ground Level

ALDS:

Automatic Load Damping System

AVES:

Air Vehicle Simulator

BMWi:

German Federal Ministry of Economics and Energy

BWR:

Bedford Workload Rating

CONDUIT:

Control Designer’s Unified Interface

DLR:

The German Aerospace Center

GVE:

Good Visual Environment

HALAS:

Hubschrauber-Außenlast-Assistenzsystem

HH:

Height Hold

HQ:

Handling Qualities

HQR:

Handling Qualities Rating

LMR:

Load Mass Ratio

MTE:

Mission Task Element

OFE:

Operational Flight Envelope

RASCAL:

Rotorcraft Aircrew Systems Concepts Airborne Laboratory

RC:

Rate Command

SCAS:

Stability and Control Augmentation System

SISAL:

Sicherheitsrelevante Systeme und Ansätze in der Luftfahrt

TPD:

Task Performance Display

\(G(s)\) :

Transfer function

\({\text{GM}}\) :

Gain margin (dB)

\(J\) :

Non-dimensional characteristic value (-)

\(K\) :

Proportional gain

L:

Cable length (m)

\({\text{PM}}\) :

Phase margin (deg)

\(T_{1} ,T_{2}\), \(T_{3} ,T_{4}\) :

Time constants (s)

\(s\) :

Laplace parameter

\(\dot{\varphi }\) :

Lat. cable angular rate (rad/s)

\(\dot{\vartheta }\) :

Lon. cable angular rate (rad/s)

\(\Delta {\text{MAG}}\) :

Magnitude notch depth (dB)

\(\delta\) :

Control command (%)

\(\zeta\) :

Damping (-)

C:

Cable

lat:

Lateral

lon:

Longitudinal

max:

Maximum

min:

Minimum

w:

Worst case

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Acknowledgments

The project Sicherheitsrelevante Systeme und Ansätze in der Luftfahrt (SISAL) is part of the German Federal Aeronautical Research Program (LuFo V-1). German Aerospace Center (DLR), Airbus Helicopters Deutschland GmbH and iMAR Navigation GmbH would like to acknowledge the German Federal Ministry of Economics and Energy (BMWi) for funding the project SISAL. The authors wish to thank all persons who were involved and supported the presented study.

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Authors and Affiliations

  1. German Aerospace Center (DLR), Institute of Flight Systems, Lilienthalplatz 7, 38108, Braunschweig, Germany

    D. Nonnenmacher & M. Jones

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  1. D. Nonnenmacher
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  2. M. Jones
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Correspondence to D. Nonnenmacher.

Appendix

Appendix

In this Appendix, the MTEs and the task performance limits are described.

During completion of MTEs, the crew had the secondary task to maintain the cable angles within the limit of ±15° in all directions. This limit is extracted from the flight manual for the ACT/FHS with a rescue hoist installed.

All manoeuvres with the exception of the Load Placement task were flown with an external load clearance of 30 ft above ground level.

1.1 Load Placement

The Load Placement task was developed by Ivler et al. [3]. “The objectives of the Load Placement MTE are to check the ability to translate with, stabilize, and set down an external load at a specific location, within a reasonable time limit. In addition, this task checks the ability to set the load down without any residual motion of the load on the ground, such as dragging or swinging” [2].

Following changes to the task as proposed in Reference [3] have been made for the use with a rescue hoist:

  • Maintain altitude within defined limits during the whole task including load set-down (Table 6).

  • Use cable reel out for load set-down.

  • Keep load within performance limits during cable reel out for the last 10 ft above ground.

    Table 6 Used Load Placement task performance limits
    Full size table

The manoeuvre is initiated at a ground speed between 6 and 10 knots.

1.2 Depart/Abort

The Depart/Abort MTE is established and defined in the ADS-33 for externally slung load configuration [21]. The most important task objectives are:

“Check pitch axis and heave axis Handling Qualities during moderately aggressive manoeuvring. Check for undesirable coupling between the longitudinal- and lateral-directional axes. With an external load, check for dynamic problems resulting from the external load configuration” [21].

Following changes to the task as proposed in ADS-33 [21] have been made for the use with a rescue hoist:

  • Removal of the requirement: “For rotorcraft that use changes in pitch attitude for airspeed control, a target of approximately 20° of pitch attitude should be used for the acceleration and deceleration” [21].

  • Reduction of target groundspeed: 40–50 knots, new limit:15–25 knots.

  • Increase of time to complete the manoeuvre: ADS-33 desired: 30 s, new desired: 40 s ADS-33 adequate: 35 s, new adequate: 45 s (Table 7).

    Table 7 Used Depart/Abort performance limits
    Full size table

1.3 Slalom

The Slalom MTE is established and defined in the ADS33 [21] but not for the use with externally slung load configurations. This task was first used in Reference [13] for an evaluation with a slung load configuration. The most important task objectives are: With an external load, check for dynamic problems resulting from the external load configuration. Especially the lateral load oscillation can be exited using this forward flight manoeuvre. “Check turn coordination for moderately aggressive forward flight manoeuvring. Check for objectionable interaxis coupling during moderately aggressive forward flight manoeuvring” [21].

Following changes to the task proposed in ADS-33 [21] have been made for the use with a rescue hoist:

  • Reduction of airspeed:

    • ADS-33 desired: at least 60 knots, new desired: at least 20 knots.

    • ADS-33 adequate: at least 40 knots, new adequate: at least 15 knots (Table 8).

      Table 8 Used Slalom performance limits
      Full size table

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Nonnenmacher, D., Jones, M. Handling qualities evaluation of an automatic slung load stabilization system for rescue hoist operations. CEAS Aeronaut J 7, 587–606 (2016). https://doi.org/10.1007/s13272-016-0211-6

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  • DOI: https://doi.org/10.1007/s13272-016-0211-6

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