Design accomplishment for unforeseen discrepancies

Design accomplishment for unforeseen discrepancies for better

structural strength and fatigue life of helicopter structures

Kalinga Gulbarga, Senior Manager(D), A.T.Rao , DGM(D) Stress Group, RWRDC,HAL

Abstract/Summary :

Airborne structural parts are designed for critical operating flight and landing load conditions

following the certification guide lines for enough strength and rigidity. But in the development test

environments they will be subjected to loads differently than expected in the operating life unexpectedly

leads to premature fatigue failures. Instances like fastener loosening due to loss of positive lock in

primary attachments will trigger vibratory dynamic loads and fail the structural attachments.

It is essential

to avoid these unsafe situations by considering such events in the design. This paper aims at dealing one

such instance practically observed during flight testing of helicopter. Redesign and analysis is carried out

that show simulation of critical stress levels for dynamic vibration loads for fastener loosing effects and

design accomplishment to avoid such premature structural failures and maintain original damage tolerant

behavior with better strength and fatigue life levels to meet operation life requirements.

key words/Tags : Airborne , positive lock, vibration, damage tolerance, strength

1.0 Introduction

Helicopter structures are designed and flight certified as per approved

international Federal Aviation Regulations (Ref.1). The design criteria is that structural

parts shall have ability to withstand severe flight and ground operation loads without

permanent deformation and last for its intended design life with no fatigue failure. During

the initial design and development phase the structural parts are analyzed for critical

loading conditions for the static and fatigue strength using aerodynamically estimated

loads. Also certification regulations calls for design of structural parts for ultimate loads

arrived from operation loads multiplied with factor of safety. The objective is that

structural parts shall not fail for specified design life and if failure happens it require to

survive until repair show causing damage tolerance without diminishing strength for

operation loads.

During development and flight testing phase unforeseen instances may happen

that have potential for structural damages and risk of catastrophic structural failures.

The failure type, location and size are neither genuine nor they are known from

manufacturing process defects. It is important to monitor such instances and consider

them in the initial design of primary structural parts of critical nature and eliminate risk of

catastrophic failures. One such practical event observed during helicopter flight testing

is taken up for study as a case of design accomplishment. It is found in the inspection

after helicopter flight testing, that tail end horizontal control surface structural bracket

near tail rotor got cracked and seen loosened bolt attachment. It could be due to loss of

positive locking at the bolt nut and subsequent high ‘g’ vibration loads at horizontal

control surface could leads to structural bracket failure. Once again the structural

bracket is redesigned with improved strength and attachment redundancy and verified

its ability to sustain vibration fatigue loads apart from its regular operation flight load

support capability.

2.0 Horizontal control surface attachment

Horizontal control surface is an inverted aerofoil which contributes helicopter

stability along the pitching axes. It’s attachment is designed to withstand for maximum

vertical load of Push forward flight maneuver. The details of the horizontal control

surface attachment is shown in the Fig.1.

The machined bracket is used to attach horizontal control surface laterally to the end

of helicopter tail boom using quarter inch bolts. This horizontal control surface is

assembled with vertical control surface for helicopter yawing stability. The total

assembly will act as cantilever and transfers the span wise and chord wise

aerodynamic pressure loads of horizontal and vertical surfaces to the helicopter tail

boom structure through machined bracket. The vibratory load magnitudes if any due to

stiffness loss will also transfer to the tail boom structure.

Fig.1 Horizontal control surface attachment details

Originally the machined bracket is stress analyzed for the critical Push forward flight

loads considering optimized attachments with quarter inch tension bolts. In the

helicopter ground run flight testing the bracket was subjected to vibratory dynamic loads

(Fx,Fy,FZ) which are entirely different from design analysis load of Push forward flight

load case. The load path and magnitudes got changed due to disengagement of tension

bolt and resulted in crack initiation on top flange corner stressed region of machined

bracket. The cracked location and the loosened bolt is shown in Fig.2.

The cracked bracket is replaced with newly redesigned bracket which is designed and

analyzed to sustain vibratory fatigue loads from unexpected instances like nut

disengagement in the operation .

Fig.2 Detailed view of machined bracket attachment showing failure

3.0 Vibratory fatigue loads :

As per regulations it is mandatory to monitor helicopter vibration during flight testing to

track the vibration ‘g’ levels are minimum . The vibration ‘g’ magnitudes in x, y and z

directions captured in the flight testing of bolt loosing instance is shown in the Fig.3

below. The peak vibratory load magnitudes are observed at horizontal and vertical

control surfaces in all x, y and z directions.

The estimated dynamic load magnitudes at horizontal control surface are Fx = 230 N,

Fy = 140N, Fz = 340 N and at vertical control surface are Fx = 260 N, Fy =280N, Fz

=252N. The bracket is redesigned considering these dynamic loads and verified for

fatigue life.

‘g’ Loads at Horizontal control surface ‘g’ Loads at Vertical control