Research Papers

Rotorcraft Hard Landing Mitigation Using Robotic Landing Gear

[+] Author and Article Information
J. Kiefer, M. Ward

Guggenheim School of Aerospace Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332-0150

M. Costello

David S. Lewis Professor of Autonomy,
Guggenheim School of Aerospace Engineering,
Woodruff School of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332-0150
e-mail: mark.costello@gatech.edu

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT, AND CONTROL. Manuscript received February 2, 2015; final manuscript received December 3, 2015; published online January 12, 2016. Assoc. Editor: Yongchun Fang.

J. Dyn. Sys., Meas., Control 138(3), 031003 (Jan 12, 2016) (11 pages) Paper No: DS-15-1053; doi: 10.1115/1.4032286 History: Received February 02, 2015; Revised December 03, 2015

A unique, beneficial feature of rotorcraft is their flexibility in aircraft-to-ground interfacing. For a variety of reasons, hard landings can occur when the descent rate of the aircraft is larger than intended. The resulting impact can result in vehicle damage, structural failure, injuries, etc. To reduce these risks, an attractive solution is the implementation of a robotic legged landing gear (RLLG) system. The system softens a hard landing by acting as a shock absorber with a relatively large stroke, allowing the aircraft to decelerate over a much larger distance compared with a tradition landing gear system. This paper explores the mitigation of rotorcraft hard landings via RLLG through a comprehensive multibody dynamics simulation tool. The purpose of this study is to demonstrate the efficacy of the RLLG as a robust solution to reduce loads during hard landings for multiple landing configurations. The results show that when using RLLG in place of conventional landing gear, peak loads are reduced by approximately 70–90%, depending on the landing conditions. Through Monte Carlo simulation, robotic landing gear system performance is shown to be robust to uncertain conditions.

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Fig. 2

Exploded view of RLLG system consisting of nine rigid bodies

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Fig. 3

Joints connecting rigid bodies are modeled as pinned joints with spring and dampers

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Fig. 4

Snapshots illustrating the deflection of the RLLG during a simulation. The RLLG model was also used to represent conventional gear by stiffening the system so that the deflection in (b) is much less than shown. (a) Free fall from rest, (b) maximum deflection during impact, and (c) aircraft returned to static position.

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Fig. 5

Soft contact model

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Fig. 6

Controller state machine (a) and detailed decision tree (b)

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Fig. 7

Schematic of the example rotorcraft equipped with the RLLG

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Fig. 8

Schematic of an RLLG leg

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Fig. 9

Snapshots of a typical level hard landing simulation with the RLLG. The landing gear deflects during the impact, recoils, oscillates, and settles to its static, final position. (a) t = 0.000 (s) Phase 1, free fall with zero initial velocity; (b) t = 0.120 (s) Phase 1, free fall; (c) t = 0.370 (s) begin Phase 2, Maximum deceleration on initial contact; (d) t = 0.385 (s) RLLG acts as shock absorber; (e) t = 0.555 (s) Phase 3, aircraft is stopped at maximum displacement, begins to recoil; (f) t = 0.625 (s) recoil as aircraft ascends; (g) t = 0.800 (s) small lift off from ground during recoil; (h) t = 1.155 (s) small bounce and oscillations occur; and (i) t = 2.000 (s) aircraft is brought to rest at static position.

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Fig. 10

Time history of the vertical motion of the fuselage mass center for the hard landing simulation shown in Fig. 9

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Fig. 11

Time history of the total force transmitted to the fuselage through the RLLG for the simulation shown in Fig. 9

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Fig. 12

MIL-1290 Crashworthiness envelope [11]

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Fig. 13

The conventional landing gear model was refined until the fuselage mass center's vertical motion in the multibody simulation agreed reasonably well with the finite element model for the landing cases based on MIL-1290. The motion above is for the 10 deg Roll Impact.

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Fig. 14

Vertical motion of the fuselage mass center for level impact

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Fig. 15

Desired vertical, z-axis motion

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Fig. 16

Resulting vertical motion of the fuselage mass center during simulation with active control applied to the RLLG. Note that the active RLLG “flattens” the peak acceleration associated with the conventional gear.

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Fig. 17

Example of vertical acceleration versus time for closed-loop control for a nonlevel landing. The initial peak is due to the antibounce control operating during the parking brake mode, before all legs have touched the ground.

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Fig. 18

Max acceleration versus impact speed for the conventional and active models. The active model generally reduces the acceleration by approximately 70–90%.

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Fig. 19

A narrow stance (a) provides more distance through which the aircraft can decelerate compared to a wider stance (b)

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Fig. 20

Maximum acceleration versus stance width for the conventional landing gear and the active RLLG (right). Even at a large stance, the active model provides reduced acceleration. Note that opposite trends are seen in stance width, a larger stance width reduces acceleration for the conventional model and increases acceleration for the active model. The impact speed was varied from 33.3% to 100% of the critical speed.

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Fig. 21

Maximum acceleration with active RLLG as a percentage of the maximum acceleration with conventional gear for varying stance width. Note that even at wide stances, the RLLG still provides significantly smaller acceleration. The impact speed was varied from 33.3% to 100% of the critical speed.

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Fig. 22

Maximum acceleration versus maximum displacement of the RLLG during landing. Increasing the maximum deflection of the RLLG further improves the reduction in acceleration.

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Fig. 23

Results of a 1000 run Monte Carlo simulation for the maximum acceleration of the fuselage mass center. The active RLLG maintains a distribution near the nominal case, providing robust control to plant variations, and a much smaller peak acceleration compared to the conventional case. The mean acceleration for the active RLLG is between 111% and 113% of the nominal value at a 95% confidence level.

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Fig. 24

Results of a 1000 run Monte Carlo simulation for the maximum moment applied to the fuselage. Again, the Active RLLG maintains a distribution around a smaller level than the conventional gear system. The moment was normalized by the mean value for the active RLLG.

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Fig. 25

Results of a 1000 run Monte Carlo simulation for the maximum force on an individual leg segment of the landing gear. Again, the active RLLG provided a significant reduction in force.

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Fig. 26

Torque provided by the stiffness and damping in the rear legs for a level landing at the critical impact speed of 12 ft/s. The torque and angular rates are important design parameters for the actuation system. The simple profile shown above should be advantageous for different actuation schemes.

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Fig. 27

Work done on the rear legs of the active RLLG. The initial rise occurs as the RLLG absorbs energy during impact. Then, after the aircraft reaches the bottom of its descent, the RLLG uses a small amount of energy to return the aircraft to its static position. This causes the decrease in energy occurring shortly after 0.5 s. The final, steady value for each joint represents the net energy that could potentially be captured for each joint.



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