INTRODUCTION Ground resonance (GR) is an instability that can occur during landing or take-off conditions when a helicopter is in contact with the ground.
This is ground resonance. L Sanches, G Michon, A Berlioz, D Alazard.
Excellent example of *what to do* if entering into ground resonance. Just as well the internet is a recent thing and older skeletons remain in the closet? Ground resonance is an imbalance in the rotation of a helicopter rotor when the blades become bunched up on one side of their rotational plane and cause an oscillation in phase with the frequency of the rocking of the helicopter on its landing gear.
(B) a series of shocks causes the rotor system to become unbalanced. A dynamic model of a MR damper with bi-fold flow mode is built based on the hyperbolic tangent model and experimental data on mechanical properties; and its inverse model is derived for the control.
The helicopter sustained substantial damage.
dynamic characteristics of components (rotor lead-lag damper, landing gear wheel and. Helicopter Operations including hot weather operations, running takeoffs, slope operations, confined area operations, pinnacle operations, quick stops, settling with power, tail rotor failure, autorotation, and ground resonance
Found inside – Page 66The helicopter is descending in turbulent air that has just been accelerated downward by the rotor. ... In general, if ground resonance occurs, it will occur only in helicopters possessing three-bladed, fully articulated rotor systems ...
Ground resonance is mitigated in full size helicopters by employing the use of damping vibration isolators to attach the landing gear to the airframe. The rotorcraft may have no dangerous tendency to oscillate on the ground with the rotor turning.
The influence of stiffness dissimilarities between blades on the stability of the ground resonance phenomenon is determined through . Aeroelastic phenomena to be simulated are shown in the Fig.6 as ( 1-7 ) lines in the following way: (1) -system of equations of coaxial rotor blades motion ;
Maintenance records indicated that the front dampers had been replaced with overhauled units about 10 months and about 534 flight hours before the accident.
The effect is similar to the behavior of a washing machine when the clothes are concentrated in one place during the spin cycle. the lag motion) and the helicopter fuselage motion.
A design study has been conducted to define a landing gear concept that redistributes the impact energy of an autorotational landing in a manner that minimizes the occurrence of blade/tailboom strikes. Flying from a soft surface like a grass field can help.
Ground resonance has not been "solved", and is still a big concern for anyone who flies a helicopter with a fully-articulated rotor system. Post may be a little disparaging when you can see the pilot is still in the aircraft and incapacitated.
8: 2013: Response and instability prediction of helicopter dynamics on the ground. The influence of stiffness dissimilarities between blades on the stability of the ground resonance phenomenon was determined through numerical investigations . google_ad_slot = "7456372054";
1. (C) there is a combination of a decrease in the angle of attack on the advancing blade and an increase in the angle of attack on the retreating blade.
Dynamic rollover is a state where the helicopter tips onto one skid and despite full cyclic input to roll the helicopter level onto the ground, it continues to roll over onto its side.
This phenomenon is most evident when the helicopter is on hard surfaces that do not absorb vibration.
This paper presents a stability robustness analysis of the helicopter ground-resonance phenomenon. google_ad_client = "pub-7150318280681240";
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The helicopter must be throroughly inspected after such an occurance to preclude the possibility of damage.
A damping mechanism is disclosed for damping the oscillations of the rotor hub which are coupled with periodic motions of the fuselage when on the ground, known as ground resonance, and a waddle when in the air.
What exactly is the spin-up leve. Prediciting ground resonance is difficult but possible. Helicopter ground resonance. This is a destructive harmonic vibration caused by a dynamic reaction of the rotor blades to the lateral motion of the helicotper.
coupling of the blades motion in the rotational plane (i.e.
The Army has in recent years refined its methods of procuring aircraft. 392 Aeroelastic Rotorcraft-Pilot Couplings: Problems and Methods B. Flying a helicopter (full size or model) from a hard landing surface will aggravate the resonance because the harder surface tends to impart more energy to the helicopter.
Ground Resonance One of the disadvantages of a fully articulated rotor system, such as that found on the CH-47 Chinook helicopter, is the susceptibility to ground resonance.
Interestingly, the way to deal with ground resonance is just to take off.
Ground resonance is a vibration phenomenon that occurs at certain rotor speeds when the helicopter is on the ground. Ground resonance is a divergent oscillation of the helicopter usually in lateral motion, rolling left and right. It basically is an out-of-balance condition in the rotor system of a helicopter on the ground that rapidly increases in frequency until the helicopter shakes itself apart. This paper investigates the passive control of a rotor instability named helicopter Ground Resonance (GR).
At least a 300ft per/min descent rate or more.
Take a three bladed helicopter, the angle between each should be equal in this case 360/3=120. To get out of it simply lift the helicopter back in flight and centrifugal force will bring the blades back to 120, 240 and 360. Ground resonance can destroy a helicopter. Found inside – Page 683R ground resonance stability as high a lag frequency as possible is desired , but if ve is too close to 1 / rev the blade loads and vibration will be excessive . Thus even a hingeless rotor may require mechanical lag dampers for ...
The commercial pilot and the passenger were uninjured.
All items have natural vibration frequencies at which they resonate. How the helicopter flies! In plain English (as far as possible!)
A dynamic model reproducing helicopter GR instability is presented and transformed to a time-invariant nonlinear system using a multi-blade . I set down the left skid first and then the right, utilizing a two point touch down.
Answer (1 of 4): Indeed theory wise ground resonance should only be limited to fully articulated rotor system.
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Imagine if you cou.
For the modelers, the only remedies are to tighten the blade grips a bit, effectively creating more of a lead-lag dampening force, to encompass the helicopter skids with rubber tubing or foam padding, or to fly from a surface with more forgiving characteristics such as soft dirt.
It is divergent because when ground resonance occurs, the energy of the spinning rotor and the condition of imbalance aggravates the .
§ 27.241 Ground resonance. The purpose of this paper is to present a simulation method applied for investigation of helicopter ground resonance phenomenon.,The considered physical model of helicopter standing on ground with rotating rotor consists of fuselage and main transmission gear treated as stiff bodies connected by elastic elements. 0000044017 00000 n
I put slight left cyclic input in to keep the helicopter from drifting to the right as I lowered collective. google_ad_height = 90; 0000002222 00000 n
Helicopter ground resonance is a highly non-linear dynamic problem involving complex interactions between the various parts of a helicopter and constituting a major design consideration.
The tiltrotor offers the best mix of hovering and cruise flight of any of the current V/STOL configurations. Whilst the contents of the book are expressed in straightforward language there is no oversimplification and the content is based on established physics and accepted theory.
This dynamic instability results from lag motions of the rotor blades coupling with airframe degrees of freedom, while the helicopter is in ground contact.
A model of helicopter with a minimum number of degrees of .
How does "Settling with Power", occur? Remarkably, the 1940's gave birth to the study of ground resonance.
Note: There is no collision between the helicopter and the passing general aviation aircraft.On June 2, 2012, about 1135 mountain daylight time, a Schweizer . Ground esonancre monitoring 3 1 Introduction Ground resonance is an instability that may develop on helicopters when the rotor is spinning on or near the ground, and that may result in the destruction
In any case, model helicopter or full size, once resonance begins, the best way to get it to stop is to pull collective and break contact with the ground immediately. 0000042387 00000 n the lag motion) and the helicopter fuselage motion. 0000068426 00000 n
Ground resonance can destroy a helicopter.
ground resonance A resonance that manifests itself as a violent rocking of the helicopter when it is on the ground. (iii) The helicopter in-ground effect at a height consistent with normal takeoff procedures; and (2) The hovering ceiling determined under paragraph (a)(1) . (Ground resonance tore the helicopter above apart in just four seconds.) This phenomena only occurs in helicopters with fully articulated rotor systems, and more often those with 3 main rotor blades. 5.
A vital resource for pilots, instructors, and students, from the most trusted source of aeronautic information. The initiation of resonance usually occurs due to landing loads or other touchdown or liftoff condition that causes a shock loading to one of the helicopter's skids or landing wheels.
Within seconds, the helicopter had shaken itself apart.\"Postaccident testing revealed that all four landing gear dampers failed to meet the manufacturer's serviceable testing specifications, and two dampers were overcharged. The helicopter was registered to Top Flight Rotors LLC and operated by TYJ Global under the provisions of 14 Code of Federal Regulations Part 91 as an instructional flight.
Ground Resonance. The safety report, he said, seemed to show some indication of "ground resonance," a phenomenon caused in part by an imbalance within the rotor system that can cause helicopters to shudder . 0000065854 00000 n Because the rotor blades are gripped only loosely, or at best have a dampener that allows lead-lag motion, when the blades are perpendicular to this motion (i.e. 0000068974 00000 n Ground resonance is most likely to develop when (A) there is a sudden change in blade velocity in the plane of rotation.
Found inside – Page 285CONCLUSIONS The nonlinear characteristics of lag dampers have relatively great effect on helicopter ground resonance stability and responses. Because of Nonlinear characteristics of the hydraulic damper with a filling-oil device, ...
Helicopter ground resonance is a self exited dynamic instability, which may occur when helicopter is in contact with the ground. This manual has been produced for students undertaking their basic helicopter training.
So the angle between each blade is 120°. Resonance in flight is typically not encountered because the mass of the helicopter does not encounter external forces to excite the oscillation.
Found inside – Page 45Pendular Action It is normal for the fuselage of a helicopter to act like a pendulum , that is , to swing laterally and longitudinally . ... Ground resonance may develop whenever the center of mass of the rotor system becomes unseated . Ya.
We present a method to analyze the ground resonance phenomenon for a one-main rotor helicopter on the basis of a complete set of nonlinear differential equations, known in flight mechanics. Helicopter ground resonance phenomenon with blade stiffness dissimilarities: experimental and theoretical developments. By the time the helicopter has struck the other skid and started to right itself, the blades are causing an in-phase force to further exacerbate the motion.
L. N. Grodko, Ground Resonance of Helicopter on Its Run.
In this section, we look at what ground resonance is, how it is generated and how to prevent it. On the knowledge exam, there's a question: If the pilot experiences ground resonance during rotor spin-up, what action should the pilot take?
The ground resonant condition occurs because both blades are offset to the same side of the rotor mast, left or right, opposite the direction of initial lateral roll displacement.
Resonance in Vibration Testing.
It's caused by resonance between an oscillation in the main rotor and an oscillation of the helicopter on it's landing gear, typically in a poorly maintained helicopter.
This topic will be developed more in the future and include graphics to demonstrate.
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