He Whareleura-tin i Kaihautu 0 Aotearoa
THE OPE N P0|.YTE(HN|( OF NEW ZEALAND
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Vibration Effects and Control in Helicopters 555—3—8
CONTENTS Vibration
The Effects of Vibration Principles of Vibration Sources of Vibration Aerodynamic Sources Mechanical Sources Methods of Reducing Vibration Resonant Mass Nodal Beam Counterweights Types of Vibration Low-frequency Vibration Medium-frequency Vibration High-frequency Vibration Measurement of Vibration The Hand Vibrograph Electronic Vibration Measurement The Vibration Signature Analyser Analysis of Vibration Main and Tail Rotor Balancing Main Rotor Tracking and Balancing Main Rotor Blade Tracking
Main Rotor Balancing Tail Rotor Tracking and Balancing Tail Rotor Balancing Tail Rotor Blade Tracking Drive Shafts and Cooling Fans
555/3/8
AIRCRAFT
ENGINEERING
ASSIGNMENT 8
HELICQPTERS
VIBRATION g‘ N»
In this assignment, we shall discuss the various kinds of vibration that affect helicopters, together with their more common causes and remedies. A dictionary definition of vibration is To shake, to tremble, to oscillate, to swing, to change to and fro, especially rapidly, to resound or ring, to tingle
Chambers Twentieth Century Dictionary
With the possible exception of "to resound or ring", this definition fits the response of a helicopter to the various vibrations that can develop in its rotating components. Even a perfectly balanced shaft vibrates at all rev/min,
but at certain well—defined rev/min ~— for that particular shaft i =
—~ the vibration becomes severe. This is a material effect that cannot be avoided, but it can be minimised by avoiding those critical rev/min or by accelerating and decelerating through these rev/min without any delay. Most of the vibrations experienced in a helicopter are not natural ones but are the result of a rotating component or components becoming unbalanced The severity of the vibration will depend upon the amount of "out~of~balance" and the speed of rotation of the component.
5/8“/~15
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_ 2 _
The Effects of Vibration To the flight crew and passengers, the vibration of a helicopter can cause physical and mental effects, ranging from slight worry and annoyance to definite pain and distress. To the pilot, these effects can lead to an impaired ability causing, say, a poorly made landing at the end of a flight. To the ' passengers, these effects may cause such a deep distrust of the helicopter that they will refuse to fly in one again. Of equal, or greater, importance are the effects of all vibrations to the helicopter. These effects include 1.
Accelerated wear (a)
(b)
In the bearings, control rod ends, cables, pulleys and fairleads, and bellcrank attachments of flight control systems; In the bearings of all rotating parts; and
(c)
In all instruments.
2.
The cracking of fuselage skins, frames, and stringers (especially near the tail rotor).
3.
The loosening of rivets and of the attachments for component parts, which in turn leads to fretting and to corrosion.
H.
Internal damage to electronic equipment.
5.
The reduction in "life" of lifed components.
The "life" reduction of lifed components, which is especially dangerous, is brought about by the forces that cause the vibration changing the point of failure of the component on its S/N curve. Figure 1 (a) shows a stress cycle fatigue curve ~— S/N curve —— that does Q93 represent any particular component or material. On this curve, Eg is the endurance limit for failure after 108 stress cycles.
If, because of the forces causing the vibration,
S55/3/8
_ 3 l
the stress is raised to Eg+
[Fig. 1 (b)], then the failure will
occur earlier, at 207'? stress cycles. This is a considerable reduction in the number of stress cycles before failure. If this S/N curve were for a part of a main rotor head turning at
333 rev/min and each revolution gave one stress cycle, then failure in Fig. 1 (a) would occur at 108'? (333 X 60)
=
gggé hours of running
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Stress scoet
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or--
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logN » (s)
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S/N Curve
Failure in Fig. 3 (b) would occur at 107'? % (333 X 60)
=
£§Q§:hours of running
This is almost a 50% reduction in the time before failure. In practice5 a Factor of safety is used by the designer to allow for such factors as variations in the material used and overloading This allowance will cater for vibration in the short term. However, for safety of flight and the economics of helicopter operation, all vibration should be reduced to its lowest possible level as soon as Possible after it has started.
555/3/8
_ u _
Principles of Vibration The speed at which vibration takes place is known as freguencg and is expressed in hertz (Hz). One hertz is a frequency of 1 cycle per second, where a cycle consists of movement in one direction followed by movement in the opposite direction and then a return to the starting point. The amplitude (range of movement) of the displacement that takes place during vibration is usually measured from the mean or equilibrium position, but it may be measured on a peak-topeak basis. The units used for this measurement are inches, mils (1 mil =' 0.001 inch), or millimetres. Because it is difficult to measure amplitude directly, the related function of velocity expressed in inches per second or millimetres per second (in/s or IPS and mm/s) is often used. Because the velocity is not constant, the figure used is the velocity of the point being measured as it passes through the mid-position of its oscillatory motion. Figure 2 (a) shows the terms cycle and amplitude as they are used for vibration. ,
1 Cycle
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W ,;"‘: Time
D spacement
_____ \
____ __ Amplitude (A)
DAMPED VIBRATION
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D acspéinent
(B) mvsneem" v|an/mom FIG. 2
.Forms of vibration
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_ 5 _ The velocity of vibration may be related to actual displacement at a given frequency by using the formula D
=
_!l Zwf
where D is the displacement amplitude in inches o millimetres (i),
V is the velocity in inches per second or millimetres per second,
f is the circular frequency (Hz), and Y
TEV Hz is the *TX 60 (cycles/second) min
Example:
What is the displacement (D) of a tail rotor with
a vibration velocity (V) of H IPS at 3000 rev/min” severe imbalance) D2
(This 1S a
“X. Zflf
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0.0127
Qlgéiaa The displacement is 0.013" each side of the mean or a
peak-to-peak movement of 0.026" (2 X 0.01 Example: What is the displacement (D) of a main rotor with a vibration velocity (V) of 38 mm/s at 315 rev/min° D:
_Y_ Zflf
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555/3/8
_ 5 _ The displacement is 1.15 mm each side of the mean or a peak to peak movement of 2.30 mm (2 X 1.15 mm). Another term met when dealing with vibration is resonance.
Any object that is flexibly mounted has a natural vibration frequency. This is the frequency at which it will naturally vibrate when stimulated by an outside source. For example, if a thin _ wooden ruler overhanging a desk is struck at its end, it Will vibrate at a frequency that depends on how much of the ruler
overhangs the desk.
If a small weight is now fixed to the end
of the ruler, the natural frequency will be lowered;
the force
used to strike the ruler will not alter the frequency of vibration, but it will alter the amplitude; and the vibration of the ruler decays as the stimulation is removed [Fig. 2 (a)1. However, should the stimulation continue in phase (in time) with the natural vibration, the amplitude of the vibrations will increase Efig. 2 (b)] to the point where the strength of the ruler is exceeded and it breaks. Ground resonance is an example of this form of vibration, as we shall discuss later in this assignment. Rotor
Figure 3 shows the dynamic behaviour of a helicopter ~ structure in greatly simplified form. in Fig. 8 (a), a spring A and weight B form a system suspended from a rotor that excites the system. The main excitation frequency (Hz) will
-<—i— Spring (A)
t
(a) Rotor
be given by the number of rotor
blades multiplied by the rev/sec of the rotor. Weight B responds to this excitation in a way that
'-s-——-—-—-—-—-- Spring (A)
depends on the value of the L
-e---—— Weight (B)
weight and the natural resonant frequency of the weight/spring
as- Spring (0) + FIG.&§
4--i—*—--Weight (C)
(b)
Dynamic response to vibration (:1 (rs (J1
system. This response could be naturally damped (attenuated) or it could be divergent (amplified). If a weight C is
/3/8
_ 7 _
hung on weight B by another spring D [see Fig. 2 (b)], then the original response to vibration by weight B will be modified. Weight C will respond in opposition to the exciting force from weight 8 and tend to reduce it - or cancel it if the natural frequencies of the two weight/spring systems match each other. The structural response will now be zero. That is, the vibration has been absorbed. In practice, the absorption of vibration in a helicopter is a very complex matter. Because the structure is not a single mass of uniform material, because the rotor speed varies, and because the main rotor is only one of several sources of vibration, it is not possible to eliminate all vibration from a helicopter. However, careful maintenance with special attention to rotating components and accurate rigging and balancing will keep a helicopter to its design standard, and vibration levels will then be acceptable.
Sources of Vibration The two sources of vibration are 1.
Aerodynamic, and
2.
Mechanical.
Aerodynamic Sources The primary sources of aerodynamic vibrations are the main and tail rotor blades. Bach blade is of airfoil section and provides lift, or thrust, or both, and aerodynamic disturbances will cause vibrations of a frequency depending on the speed of rotation and a two—bladed vibration of vibration of
the number of blades in the rotor. For example, semi—rigid (teetering) rotor will have a normal 2/rev, and a five—bladed rotor will have a normal 5/rev. (These normal vibrations pass almost
unnoticed because of good design.) However, differences relating to one blade only, such as minor damage or natural ageing, will 555/3/8
- 3 _ cause the blade to develop more or less lift and drag and thus produce an additional 1/rev vibration. In an articulated rotor 7 variations in blade spacing (blade phase) will cause similar vibrations, and if only one blade is affected, the additional vibration will be a 1/rev. A variation of track of one blade, on any rotor, will also produce an additional 1/rev vibration. if two blades are out of track, there will be an additional 2/rev vibration. The same is true of a tail rotor, except that the vibration frequency will be higher because of the higher rotational speed of the tail rotor. Under some conditions, a disturbed airflow can cause the vibration of elevators, stabilisers, cowlings, and access doors, but more often these will vibrate in sympathy with some other source if their attachments are loose or worn.
Mechanical Sources Mechanical sources of vibration are, simply, rotating parts They are conveniently divided into transmission sources and powerplant sources. Transmi§siQn sources:
‘Sources in this group can be listed
ELSZ
1.
-Rotors: A rotor will vibrate as a result of any out-of—balance condition or because of wear in its control linkage.
2.
Drive shafts:
These shafts may have to be balanced
and=specially assembled on to the shaft couplings to keep the balance of the.whole assembly. Misalignment of a shaft can cause vibration as a result of the slight flexing motion produced. Wear in support bearings and coupling splines are further sources of vibration. 3.
Gearboxes:These devices have components running at.different speeds and supported on different kinds of bearings that provide sources of vibration at several frequencies. Also, due to the accumulation of manufacturing and assembly tolerances, gear teeth may vary slightly in position, shape, and mesh at different positions
555/3L8
_ 9 _ on the gear, which results in vibration. Varying loadings due to changes in helicopter attitude and airspeed are transmitted through the rotor mast, its support and thrust bearings, and the drive gear train bearings to give changes in vibration. ln addition, many main gearboxes provide drives for such accessories as generators, hydraulic pumps, and tachometers, and each of the components has its own characteristic vibration, which may vary with differing operating conditions.
4.
Ancillary equipment: Cooler fan_units that supply cooling air for the main gearbox oil cooler and, sometimes, for the engine oil cooler may be driven through a belt drive by the tail rotor drive shaft. This unit may vibrate due to defective bearings, damaged fan blades, or defective drive belts. A rotor brake device may act on the tail rotor drive shaft and can give rise to vibrations if its brake disc or pucks are damaged.
Powerplant sources:
Powerplants fall naturally into the two
divisions of piston engine and turbine engine, each having totally different vibration characteristics. 1.
Piston engine: The piston engine, with its one power stroke per cylinder for every two revolutions of the crankshaft, is a prime source of vibration, the severity of which depends on the number of cylinders ~— the greater the number of cylinders, the smoother the running of the engine. The vibration is transmitted not only through the engine mounts but also as a torsional vibration through the crankshaft into the helicopter transmission. For this reason, many piston-engined installations use a flexible coupling between the crankshaft and the transmission, and the_crankshaft itself usually embodies counterweights to absorb torsional vibration. This last reason is why only the speeified engine model may be fitted to a helicopter, as seemingly similar engines may be very different in their vibration characteristics.
2.
Turbine engine: Although the turbine engine is naturally smooth running, its higher rotational speeds mean that the effects of any imbalance are magnified as much as ten times compared with a piston engine. Ingestion damage to compressor blades, creep damage to turbine blades, and wear on bearings and seals are all causes of abnormal vibration. As with the helicopter main gearbox, all engine—driven accessories can produce noticeable characteristic vibrations, which, on the piston engine, are hidden by its usual clatter.
555/3/8
_ 19 _
Methods of Reducing Vibration If good maintenance practices and attention to detail are followed during all maintenance and repair of a helicopter, then vibrations will be kept.to an acceptable level and the helicopter will fly safely and well. .However, there will always be a certain degree of vibration natural to the design of a helicopter We shall discuss below some of the ways that manufacturers dampen these natural vibrations.
Resonant Mass The principle behind this vibration damping method is shown in Fig. 3 and explained on page 6. One type of helicopter uses its 2H d.c. battery and mount assembly supported by three cantilever springs, as the resonant mass. [This is weight c in Pig. 3 (b).] The spring length, which is adjustable, is set during construction using special equipment. The assembly is tuned to allow for any variation in battery weight by adding or subtracting weights, weight being added to lower the resonant frequency.
Another application of this principle is used in the cyclic controls of a helicopter to prevent vibration being transmitted through the control runs.
See Fig. M.
555/3/8
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FIG. 4
Vibration dampers in a control system
The damping is adjusted by repositioning the weights on their slotted arms.
A further application of the same principle is in the vibration damper installed on the main rotor head of a light helicopter. See Fig.
5.
Being mounted on the rotor head, the vibration damper
absorbs most of the excitation at its source before it can be
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transmitted to the rest of the WEIGHT
helicopter.
In this design, a
weight is supported in the rotor head by a ball-joint that lets
i
MAIN
ROTOR HEAD
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plane parallel to the rotor head. The weight is restrained by three
SUPPORT TUBE
sown wxsr
equally spaced springs, which BALL JOINT Id-LL’
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it move in any direction in a
control its movement and form a
U
Spring~loaded main rotor damper
585/3/8
_ 12 _ tuned system. This system is excited by the vibratory loads developed in the rotor head and responds in opposition, thus reducing these vibratory loads. -
Nodal Beam The principle of a nodal beam can be demonstrated with a long, thin piece of wood. If you hold the wood at its midpoint and shake it up and down, the ends, because of the flexibility of the wood, will move in opposite directions to the mid-point. That is, they will be out of phase. This will occur when the induced vibration is near the natural vibration frequency of the piece of wood. The positions on the wood where the motion changes from one direction to another are known as nodal Qoints. Here, there is no movement. If a beam arrangement is built up, see Fig. 6, and a helicopter fuselage is attached at these points, with the gearbox and rotor attached to the centre of the beam, then the fuselage will not be subjected to the vertical vibration induced by the rotor. vnamvrme component moron AND ssaneoxl /~ ,__/ ‘‘I '~J\
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FIG. 6
Principle of nodal beam damping
In a helicopter, the beam consists of flexible members with inertia weights attached at their ends. These flexible members may be formed from glass fibre with a low Young‘s modulus of elasticity (it is quite an elastic material) and high allowable stress that is fairly easy to form into complicated shapes.
555/3/8
M 13 _ The mounting to the airframe is through elastomeric bearings, and the response of the system is adjusted by tuning weights mounted OI1
arms
Elastomer:
A rubber—like substance
Figure V shows an inservice version of a nodal beam damping assembly.
Compare this system with the one in Fig. 6. a-ww*\m
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Counterweights One type of vibration damper that does not have springs, used in at least one model of helicopter, is generally known as a pendulum dynamic vibra t ion
damper . A vibration absorber assembly is attached to each main rotor blade so as to rotate in the same
555/3/8
....j_l.§._
plane as the rotor.
It consists of a support bracket that carries
two pendulums loosely mounted and free to move within the
restriction imposed by the support bracket. vibration absorber installation. BRACKEI
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Figure 8 shows a
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3'PER REVOLUTION /PENDULUM
This vibration damper uses the inertia forces generated by
the in~p1ane pendular dynamic weights to oppose the forces generated by the rotor. Because the weights are subject to centrifugal force, which varies with rotor speed, the damping provided by this type of absorber is effective throughout the operating speed range.
Types of Vibration Vibrations in<1he1iccpter can, for practical reasons, be conveniently put into three ranges:
1.
LOW
frequency
——
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2.
Medium frequency —w 16 to 30 hurts, and
3.
High frequency —— 30 hart; and upward.
S55/3/8
_ 15 _ This classification is rough and ready, but its use is a help when diagnosing the cause and source of a vibration.
Low-frequency Vibrations Abnormal vibrations in this category are nearly always associated with the main rotor. The vibration will be of some frequency related to the rev/min and the number of blades of the main rotor. It is very often referred to as a "1 per rev", "2 per rev", and so on. A "1 per rev" is usually slow enough to be counted without any help,but a "2 per rev" can be impossible to count without resorting to a vibration measuring instrument. For example, a helicopter with a rotor speed of M20 rev/min and a "2 per rev" vibration will have a vibration frequency of is hertz. You calculate the frequency by finding the product of the rev/sec and the vibration per rev. Thus: 420 (rem) X Z_(vibrations), 60 (second) 1 (féfil
=
14 (vibrations) 1 (second)
= _.L.%..5s A 1s—hertz vibration is too fast to count unaided. The frequency and strength of a low frequency vibration
will cause the pilot and passengers to be noticeably bounced or shaken laterally or vertically or in a combination of both
movements.
'
A vertical vibration is usually caused by a main rotor blade(s) being out of track, and a lateral vibration by an out~of-balance condition of some kind in the main rotor head and blades assembly. These vibrations may change in strength with rotor rev/min, with airspeed, and with power-on and power~off flight. Specific instructions for these vibrations will be found in the maintenance manual for the helicopter. However, in using these instructions, you must be prepared to find and rectify a cause that they may not mention.
555/3/8
-16-‘ Ground resonance: This is a form of low-frequency vibration found only in a helicopter on the ground that has 1.
An articulated main rotor head (whose blades are controlled in their movement about vertical hinges by lead-lag blade dampers), and 0
2.
An undercarriage system with a sprung undercarriage.
If either of these two conditions are removed, then ground resonance cannot occur. For example, neither a helicopter with an articulated main rotor head, mounted on floats or skids, nor a helicopter with a semi~rigid main rotor head, mounted on a sprung undercarriage can get ground resonance. The vibration is started when one or more blades move on their vertical hinges to become unevenly spaced in the rotor disc. See Fig. 9. Ollploced blade
Olaplncod CMG
\
‘
Contra of mum
¢ 1’
§E§L_g
This uneven
spacing means that the centre of gravity of the assembly does not coincide with the centre of rotation of the head. The head then tries to rotate about ‘
the centre of gravity and, ‘~
as a result, the fuselage rocks, a low—frequency vibration, from side to side.
Displaced blade
when the period of the fuselage vibration coincides with the natural vibration frequency of the undercarriage, the rotorinduced vibration amplifies the undercarriage vibration and vice versa. The result is a very quickly growing vibration amplitude, that, if unchecked, will cause the helicopter to roll far enough for the blades to touch the ground. The helicopter usually ends up on its side on the ground, destroyed. Figure 10 shows, simplified, the development of ground resonance.
555/3/8
_ 17 _ Ground resonance can occur ~7L\
1.
a--~_¢$'
2.
As you are bringing the main rotor up to operating rev/min from a standstill; During taxying over rough or uneven ground;
“ I5
:ii_
- o
3.
‘ y rial __ U-
struts and tyres are incorrectly inflated;
_ H.
\
if -,/,¢¢¢aI§r¢,',,,, “
If the undercarriage oleo
or
If, during landing, the cyclic control is moved suddenly as a main wheel touches the ground.
The'immediate for ground resonance is cure to lift the helicopter off the ground,
1
pi
@_
T
whereupon the rocking motion swiftly dies down.
if liftoff
is not possible, then the rotor must be slowed down and . §%5x:x\ vg»
stopped as quickly as possible. Use the rotor brake, if one is fitted, to do this.
§§5$ Ground resonance
The rocking movement of the helicopter in ground resonance is violent and develops very rapidly. The pilot must be secured by lap straps and a shoulder harness in his seat so that he can regain control of the helicopter before it destroys itself.
555/3/8
....j_8...
NOTE Because of possible ground resonance, it is usual for a helicopter with an articulated main rotor head to be ground run only by a qualified pilot.
Medium~frequency Vibration These vibrations are too fast to be counted unless an instrument is used. In some helicopters, their source will be the tail rotor and its associated drive—shaft.
High-frequency Vibration These vibrations are too fast to be counted unless an instrument is used. Their usual source will be the engine, but on some helicopters in which the tail rotor rev/min is nearly equal to or greater than that of the engine rev/min, the source could be the tail rotor. In this case, a means of positively identifying the source will have to be devised.
SUMMARY Vibration causes accelerated wear of components, cracking of skins, and frames, loosening of rivets, and internal damage to electronic equipment. It also reduces the "life" of lifed components.
\
Careful preventative maintenance will keep vibration levels to an acceptable standard. Sources of vibration are either aerodynamic or mechanical. \ \
The sources of aerodynamic vibration are the main
rotor blades and the tail rotor blades. Mechanical vibration is caused by any rotating part that is in an out—of—balance condition.
555/3/8
T
_ 19 _
Three design methods used to reduce natural vibrations are 1.
Resonant mass,
2.
Nodal beam, and
3.
Counterweights.
Vibration frequency is measured in hertz (Hz).
Vibration amplitude can be given in inches, mils, or millimetres, but it is usually given as a velocity in inches per second (IPS) or millimetres per second (mm/sec)
PRACT ICE EXERCISE A In each of the following, choose the option that correctly completes the statement, writing A, B, C, or D as your answer. 1.
The natural vibration from a three~bladed main
rotor would be a low frequency:
D '
2.
3.
A.
1 per rev
B.
3 per rev
C.
4 per rev
D.
6 per rev
The probable cause of a low frequency 1 per rev vertical vibration is A.
A main rotor blade out of balance
B.
A tail rotor blade out of track
C.
Nothing unusual
D.
A main rotor blade out of track
A tail rotor turning at 3000 rev/min has one blade out of track (to give an impossible to count 1 per rev vibration). This vibration will have a frequency of
555/3/8
_ 29 _
4.
A.
3000 Hz
B.
60 Hz
C.
50 Hz
D.
30 Hz
T
The peak-to~peak displacement of a main rotor turning at 420 rev/min with a vibration velocity of 6 IPS is: A.
136.4 mils
B.
68.2 mils
C.
273 mils
D‘.
546 mils
NOTE: "“*“*
6 IPS gives an unacceptable level of vibration.
(Answers on page 50)
Measurement of Vibration An assessment of the level of vibration is often all that is needed to know that something is wrong, or going wrong, with a helicopter. However, knowing that a level of vibration is unacceptable does not tell you its cause. Some faults have obvious and well defined symptoms, but many can be hard to locate and careful and systematic thought and actions are necessary to identify them. To do this efficiently, you must measure the frequency and velocity of the vibration. Three instruments that may be used to measure vibration are: 1.
The hand vibrograph,
2.
The electronic vibration measurer, and
3,
The vibration signature analyser.
555/3/8
_.2j_...
The Hand Vibrograph This instrument consists of a steel reed with a weight at one end. Varying the length of the reed will vary its natural vibration frequency, the length being calibrated in hertz. To pick up the maximum amplitude, vibration measurements should be made in the flight conditions where the vibration is most noticeable —— but only if it is safe to do so - with the hand vibrograph placed against the aircraft structure as close as possible to the suspected source of vibration. Remember that vibration may be found in both vertical and lateral places, and so readings should be taken in both positions. From the vibration frequency given by the instrument, the source of the vibration can, with some detective work, be found. A different form of this instrument gives a readout in the form of a graph. Figure 11 shows a trace, with diagnostic additions, from a hand-held vibrograph.
i
um time
1<——1 Rev of rnoin rotor
—-|
+
3
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Vibrograph trace
555/3/8
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_ 22 ,
Electronic Vibration Measurement In this system, an accelerometer is mounted in the helicopter in either a lateral or vertical position, and its signals are fed to an instrument that electronically processes them and then displays them on a meter. Some forms of this system use two accelerometers, both reading at the same time, and whose signals can be selected as needed. Besides the accelerometer(s) a ' magnetic pickup is fitted to the main rotor to provide a phase reference of 1 signal per revolution of the rotor. This signal is fed.into a display that shows the phase relationship between the rotor and the vibratory motion and is used to determine the location of any balance weights that may be needed. In the case of the main rotor, this display ~— the .__._-_.--____..................._..... clock angle We is in the form of a circle of 2% lights giving such positions as "ten o'clock", "half past two", and so on. For a tail rotor, the clock angle is shown by a reflective target or by special paint fixed to a blade that is lit up by a strobe light slaved to flash in time with the vibrationMany helicopters have magnetic switch actuators permanently fitted to each push—pull rod attachment on the rotating star. All that needs to be added to make a vibration check is the magnetiC piCkuR from the instrument kit. This pickup is installed, as a temporary fitment, on the fixed star. See Pig. 12
555/3/8
-23..
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Magnetic switch indicators and magnetic pickup installation
when carrying out a fault diagnosis of a main rotor, first check the blade track and, if necessary, correct it before you do any rotor balancing. Blade tracking involves 1.
The adjustment of individual blade angles to ensure that all blades "fly" in the same plane, and
2.
Where applicable, rectification to ensure that all the blades "fly" equally spaced within the rotor disc.
Blade tracking is discussed later on. For main rotor balancing, install the accelerometer(s) and magnetic pickup in their correct positions in the helicopter and head ground. above 15 rotor on
the helicopter into the wind on flat, level, firm Avoid operation in gusty conditions or in windspeeds knots as these conditions may mask faults. Run the the ground at its nominal speed and rough—tune the
555/3/8
6
_ gu _ electronic equipment to a frequency corresponding to a 1/rev vibration of the rotor being checked.
.I
For example:
Main rotor rev/min
=
450
Frequency of a 1/rev vibration
=
YET
=
Zaifié
450
Hover the helicopter, and fine tune and check the equipment, using its own internal test facility. If the rotor is correctly tracked, two readings may be taken:
1.
Phase.
Note which clock angle light is on.
2.
Amplitude.
Note the reading on the meter scale.
After the helicopter has landed and been shut down, these values are plotted on the correct ggyogram for the helicopter. Prom the nomogram, you can find the number and size of balance _ weights and the position where to fit them to achieve balance. See Fig. 13. c a In this example: Meter reading
=
0.15 IPS
Clock angle
e
5.30
These twp readings are plotted as position M on the nomogram Two lines drawn from M at right angles to the nearest scales give the blade location and amount of weights to be added to achieve balance. when corrections have been made, repeat the balance check until you obtain good balance. Acceptable balance limits are given by the helicopter manufacturer, but, the closer the balance gets to being perfect, the better it is for the helicopter.
If you can read a definite clock angle, then the
555/3/8
balance can still be improved As the balance approaches perfection, the clock angle does not remain constant but moves around at random, becoming jlttérg I
K
BALANCE MEASUHEM
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at5psjnAcmNs Z a CLOCKANGLE § E
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FIG
13
apt 0 Q1;
NOTES
\
Nomogram for a 4 bladed rotor
W
"' wn In Table Balnnc1ng to
-26.-
This electronic equipment may be used for tail rotor balancing and for locating the unknown sources of vibration. To locate a source,mount the accelerometer in the area where the vibration is most noticeable and tune the equipment slowly through its ranges until the meter reading peaks. This gives the frequency of the vibration. By reference to drive speeds in the helicopter, you should then be able to pinpoint the source of the vibration.
The ¥fibration Signature Analyser This is a-very useful diagnostic tool, especially when used at regular intervals, for example, before a 100-hour inspection. The analyser is a self-contained piece of equipment that measures vibration frequency against peak velocity and automatically makes a record of the test in the form of a graph. The output from the accelerometer is processed electronically by a tuneable band pass filter that separates vibrations on the basis of their frequency. The filter automatically scans through the selected frequency range, and a pen records the result on a card. This card should be kept as a part of the helicopter maintenance records. This analyser is not an alternative to the electronic vibration measurement equipment but is a supplement to it. Figure 1H shows a card from this type of instrument.
555/3/8
_ 27 _
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Vibration signature record card
' Use the same location each time you use the accelerometer, so as to keep the readings comparable. Normally, both lateral and vertical, and sometimes fore and aft, signatures would be taken in each frequency range. Any significant peaks are then analysed and the faults, if any, corrected. In this way, a complete vibration history can be built up for a particular helicopter and the information can also be used to form the basis of a data bank for that type of helicopter. ' Calibration: As with all measuring tools, the value of the information given by vibration measuring equipment and the analyser depends upon the instruments being properly used and their being calibrated at regular intervals. You should record the calibration results so as to build up a history of the instrument and adjust the calibration period to suit the conditions of that instrument.
555/3/8
- 23 ,
Analysis of Vibration Measurement of a vibration will give you the two values of freguencg and amplitude (Or velocity).
Knowing these tW@ Va1ue5
allows you to locate the source and determine the severity of the vibration. »
Frequency and Speed Relationships To be able to relate the frequency of the measured vibration to the speed of different rotating components, you need an intimate knowledge of a particular helicopter. You can relate a vibration of 1/rev to the speed of rotation by multiplying the measured frequency (F) in hertz, by 60, to give the speed in revolutions per minute. Thus: ‘ rev/min
=
F(Hz) X 60
A 1/rev vibration is typical of an out—of-balance condition affecting a rotating component. For example, one blade out of balance on a main rotor turning at 203 rev/min gives a vibration frequency as follows. _
rev/min
F "
so
_ 203 T
Q?
so
3@f.?.4
In a case affecting, say, five blades in a rotor, the frequency would be F
=
rev/min No. of, blades ,_ .,. X">> . . . _. 60 203 X 5
= “_YiT' =
16.9 Hz ‘H
555/3/8
2*50 ‘m N
—-- ~==
_ 39 _
In the same way, you can find the frequency produced by the meshing of gear teeth in a gearbox if you know the number of teeth on the gear and its nominal speed. In this way, you can make up a table of vibration orders for a specific helicopter model. Some manufacturers publish such a table in the maintenance manual. Table 1 is a list of vibration orders from which, if you know the vibration frequency, you can find the component producing it. TABLE 1
ical Vibiation Order Table "
\
Rev/Min
Component
l Frequency (Hz)
i
g
3.4 15.7
?; 7;
203
Main rotor (1/rev)
939
Main bevel gear (MG B)
17.0 20.7 50.5 53.2
l I
203
Main rotor (5/rev)
1243
Tail rotor (1/rev)
3031
Tail rotor drive sh aft
3195
Input bevel gear (MGB)
\ ~
.1
.
or Rotor brake disc
; A i
101.0 140.0
\
T
8410
Tail take off free wh 881 unit
\
1‘
:1
6060
Oil cooler fan (MGB )
\
316 .0
1;
J
\
V 1
Nos 1 and 2 input Pinion gears
325.0
j
Engine
832.0
1!
Meshing, tail-rotor gearbox (M.TRG)
To identify the different orders, abbreviations are often used.
Some examples are
555/3/8
- 39 _
1R
=
1/rev main rotor
5R
=
5/rev main rotor i
1T
=
1/rev tail rotor
1E
=' 1/rev engine output shaft
MGB
=
Main gearbox
TRG
=
Tail rotor gearbox
The letter M, used as a prefix, shows that the vibration comes from the meshing of the gear identified. In Table 1, the last item gives the frequency generated by the meshing of the gears in the tail rotor gearbox. It is usual to quote vibration orders based on speeds related to 100% main rotor speed.
Vibration Levels Once you find the source of a vibration, you then have to decide whether the vibration is normal or excessive. In the cases of the main and tail rotors, a maximum acceptable vibration amplitude gig be specified by the manufacturer, but if nothing is specified, then the opinion of the pilot must be seriously considered. In general, a gentle and almost unnoticeable vertical or lateral 1/rev is acceptable for flight, although if the helicopter is to be used for photography, then even this may not be acceptable. If possible, a component should not be rejected solely on the result of one set of vibration measurement readings. If you make a series of vibration measurements on a regular basis, you should see a trend in the amplitude of the various frequencies. A steadily worsening trend would show the need for careful monitoring but would not mean the need for an immediate component change. However, a sudden and large increase in the vibration level from, say, a tail rotor gearbox would call for an immediate investigation. In practical terms, this would mean replacement of that gearbox.
555/3/8
_ 31 _ Remember, all vibrations have different sources, and a combination of sources can amplify a specific reading. Remember, too, that a particular vibration may only be felt in a certain stage of flight or when certain equipment is fitted.
Correction of Excessive Vibration Medium- and high-frequency vibrations are usually corrected by replacement of the component concerned. This means of dealing with the problem returns the helicopter to service in the shortest time. You can then investigate the component and repair it in a workshop. As well as replacing or repairing the component, you must inspect the structure and fitments nearby to ensure that there is no cracking or looseness of bolts and rivets. If you have traced a vibration to the tail rotor, inspect all parts for wear and damage. Items requiring special attention are the pitch change bearings, the pitch change links, the tail rotor control cables, the tail rotor hub, and the gearbox attachments. If you find that mount bolts are slack, you will need to decide whether the slackness is the cause or an effect of the vibration. The rectification work will involve inspecting the mounting faces for fretting and replacing the attachment bolts and nuts, and checking holes and locking devices for wear and damage. Make sure that your inspection covers a wide enough area to discover any symptoms of secondary damage, such as cracking of the tail pylon structure.
-
Don‘t try to correct any vibration coming from the main rotor head unless you are certain that the blade track is correct and, with an articulated head, that the lead~lag dampers are operating properly. As with the tail rotor, inspect the pitch change bearings, the pitch change links, and the security of attachment of the hub to the'drive shaft and rectify all faults before balancing.
555/3/8
- 32' _...
\
\
SUMMARY The electronic vibration measurement equipment, together with a strobe light and blade reflector
kit, will help you to rapidly pinpoint the source of a vibration and to accurately balance and track rotors. ~ ' Any equipment fitted.for the flight test of a
helicopter must be installed to an airworthy standard. i
Vibration signature analyser cards should be kept as
,
part of the maintenance history of the helicopter 0
PRACTICE EXERCISE B
State whether each of the following is true or false. 1.
An accelerometer must be fitted in a vertical
position.
‘
2.
A magnetic pickup provides a phase reference for main rotor balancing and blade tracking.
3.
The frequency of a 2 per rev vibration from a 5~bladed rotor turning at 360 rev/min is 6 Hz.
4.
Different vibration frequencies occurring together will not affect the amplitude of any one vibration.
5.
Replacement of a defective comonent that is a source of vibration is all that is needed to return the helicopter to service.
6.
The main rotor blades must be in track before any rotor head balancing is done.
7.
Magnetic pickup actuators may be permanently fitted to each rotor blade.
8..
Main rotor balancing should not be done when the
windspeed is above 25 knots. 9. 10.
A pinion with 19 teeth turning at 6000 rev/min will have a vibration frequency of 1900 Hz. Acceptable vibration levels are always specified by the helicopter manufacturer.
(Answers on page 50) 555/3/8
\
0
_.f-13..
MAIN AND TAIL ROTOR BALANCING Main rotor balancing is usually taken to include 1.
The adjustment to the angles of individual blades to bring them into track;
2.
The adjustment to the timing rate of the lead/lag dampers (if fitted); and
3.
The addition/subtraction of balance weights to individual blades.
Tail rotor balancing includes 1.
The addition/subtraction of balance weights.
2.
The adjustment —— on some types —~ of individual blade angles to bring them into track.
Main Rotor Tracking and Balancing The main rotor hub assembly is symmetrical, and if it is assembled properly, it will be very nearly in perfect balance. Any imbalance in the hub assembly, because it is small and acts at a small radius and at a fairly low rev/min, will not be a cause for worry as it will be corrected during the subsequent balancing of the hub and blades assembly. After manufacture, a main rotor blade is balanced to a master blade and is then given its own serial number, and often a type number. when fitting main rotor blades, you must be careful to use blades that are compatible with one another. Reference to the manufacturer's manual will often tell you which blades may fly together, the blades being identified by serial number or blade type. If some old but serviceable blades of the correct type are to be used, better balancing results will be had if the flying hour ages of the blades are nearly the same. That is, do not fit a blade, say, 608 flying hours old into a rotor whose three other blades are each around the 2000
555/8/8
_ 3u _ flying hour mark. You will not have an unserviceable rotor if you do this, but you may not be able to balance the assembly to as high a standard as you wish.
Main Rotor Blade Tracking when the rotor has just been installed, the blade trim tabs will have been set to their neutral (trail) position and the rotor head will have been rigged. If you are about to undertake corrective blade tracking, then first inspect the rotor and blades assembly for any defect that could cause the blades to go out of track and rectify that defect. Typical defects are i.
Slightly damaged blades, and
2.
Pitch control arm push—pul1 rod—end wear.
The track of rotor blades can be checked by 1.
The stroboscopic (strobe) light/reflected image method,
2.
The flag method,
3.
The tracking stick method, or
H.
The reflector method (rarely used now).
Of these methods, the most satisfactory is the first. However, if the right equipment is not available, then either the second or the third method can be used, depending upon the shape of the blade tips. Whichever method is used, the helicopter must be on flat, level ground headed into wind that must not be gusting or have a speed greater than S knots (15 knots for the strobe method). An experienced helicopter pilot should be at the controls because, during the tracking procedure, the helicopter is very close to becoming airborne (in the strobe method, it will be airborne), and unforseen events could require the helicopter to become airborne.
555/3/8
_ 35 _ The strobe lightjreflected ifiage method:
jg ,-
@~¢,
1 a“\
a special tracking reflector is attached to the tip of each blade. Each reflector is different, either in colour or in markings, so that it can be easily identified when the.strobe light flashes as the rotor is turning.
1
\
See Fig. 15.
A magnetic pickup is
installed under the rotating
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FIG. 15
In this method,
;
III‘II III III
III‘IIII‘III
“R!
Blade tip reflectors
actuators installation.
star, which has a magnetic actuator installed at each push-pull rod position. Figure 12 shows a_magnetic pickup and
The actuators may be bent a little to
provide a spread of the reflected image. The strobe light is connected to the helicopter d.c. power supply and to the magnetic pickup.
CAUTION Install this equipment securely, and leave adequate clearances between moving parts.
To track, ground run the helicopter at a low power setting (flat
pitch) and note the track of the blades. To make changes to the blade track, shut down the helicopter engine, stop the rotor, and alter the lengths of the blade pitch change (push~pull) rods. When the low-power track is satisfactory, make a high power track. Adjust the track at this condition by bending the blade tabs —— a tab is bent downgtg_ lpweréa blade and up to raise it.
See Fig. 16
ass/3/a
....36..
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Strobe tracking
with both the low power and the high power tracks satisfactory, the manufacturer may recommend that the track be checked in flight at various speeds and flight manoeuvres.
WARNING When a pitch change rod is adjusted, its safety locking must be restored to its original condition before the track is re—checked. Failure to do this can result in a wrecked helicopter.
Finally, you mgst check the autorotation rev/min of the helicopter. Changes to blade angles during blade tracking can, if superimposed on previous blade angle changes, lead to an unacceptable change in the autorotation rev/min. If these rev/min are too high or too low, then the collective pitch controls may have to be re-rigged. when you are strobe light blade tracking a helicopter with an articulated rotor head, you can also see how each blade lead/lag damper is performing. Any change in the spread of the . 555/3/8
_ 37 _ reflected images will be caused by a change in a blade position about its vertical (drag) hinge. Figure 17 shows a typical strobe track result for a four bladed rotor. REFLECTORS AS VIEWED
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Reflectors as viewed by strobe light and spread for identification
You can either replace a defective damper with a serviceable one or adjust it. In each case, re—check the track to ensure that the blades are behaving properly. The flag method:
This tracking method can be used only if
the blades have smoothly contoured or rounded tips and if any optional balance weights, washers, or screws are removed from the tips for the tracking procedure. For blades with square-cut tips the tracking stick method should be used. The "flag" used may be either bought from the helicopter manufacturer or made locally from thin—walled steel tubing (automotive exhaust tubing is suitable), bungee rubber, and a lightweight canvas. The base of the flag should be weighted, which makes it easier to handle. The crayons used to mark the blade tips should be good quality, fairly soft, wax crayons, and the colours used should be red, yellow, blue, black and, if needed, white. To give accurate results, the crayons should be applied to the blade tips in as thin a line as possible. The tracking procedure is different for each type of helicopter, and so you must refer to the maintenance manual of the helicopter type concerned before you do any tracking. However, a general procedure is as follows: 555/3/8
_ 33 _
Mark each blade tip with a wax crayon whose colour corresponds with that on the blade pitch housing. Place a strip of 25 mm (1") wide masking tape or white surgical tape along the forward edge of the tracking flag to record tracking marks. Identify the top of the tape, and make sure that the flag is taut on the_j@3nmg. Head the helicopter into the wind. A light, non—gusting wind is acceptable —~ if it does not exceed 5 knots. Start and warm up the helicopter. With the collective pitch stick fully down and with the cyclic pitch control column "leaning" into wind, carry out a low rev/min (2000 engine rev/min) l0w~power blade track. At a signal from the pilot, lift the flag up in a counterclockwise direction —— see Fig. 18 —--and move it very slightly into the rotor disc until each blade tip touches the flag, leaving a coloured mark on the canvas. The sound made by each blade tip touching the flag is distincitve, and with a little practice, just one set of contacts can be had. Shut down the engine, stop the rotor, and then adjust the blade pitch change rod lengths to reduce the blade track spread (see Pig. 18) to less than 6 mm (%"). Clean and re-mark the blade tips,'and check the track again. When the track at low pitch and 2000 engine rev/min is satisfactory, make a track at low pitch and take-off rev/min using the same technique. Make any correctionto the blade track at this stage by bending the blade trailing edgegtabs (UP to raise a blade and DOWN to lower a Blade). A blade spread of not more than Q mm (%") is acceptable. Make a power track at take-off rev7min and with enough power to make the helicopter "light on its skids". You may need to add ballast to enable a high power setting to be used without the helicopter becoming airborne. The blade track spread should not be more than 9 mm (%"). Adjust it by altering the lengths of the blade pitch~change rods. when the power track is acceptable, test fly the helicopter to check the vibration level and the autorotation rev/min.
555/3/8
_ 39 -
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Flag tracking
The tracking stick method:
In this method, a-tracking
stick is carefully lifted up to make a light contact with the underside of the rotor disc fairly close to the blade tips. Marks left on the underside of each blade show which is the low blade. The tracking stick consists of a light wooden pole“with a piece of 6 mm (%") thick by 25 mm <1") wide sheet rubber bound firmly on one end with a tongue about 50 mm (2") long extending beyond the end of the pole. The end of the tongue is coated with a white spirit soluble and nonedrying paint. A suitable paint is red lead or enamel paint, of any colour, mixed with lubricating oil to stop it drying.
555/3/8
...L|.[]...
Tracking requires a pilot to operate the helicopter and one man t 0 use the tracking stick. 1'
Find out the power settings to be used for the tracking from the maintenance manual for the helicopter type.
2.
The tracker must stand under the tip path plane in clear view of the pilot and face in the direction that the rotor turns while holding the tracking stick slanted upward in the same direction.
3
Head the helicopter into wind (not more than 5 knots) and warm it up.
H
n
At a signal from the pilot, he should raise the tracking stick carefully until the rubber tongue just contacts the blades. A further track or two, may be made immediately inboard of i the first track as checks on the first trackl
5Q
After the engine has been shut—down and the rotor stopped, examine the blades for the track marks and adjust them to suit.
6
Make another track to check the adjustments made and then test fly the helicopter to check for vibration level and the autorotation rev/min.
I
Figure 19 shows this trackihg procedure and the blade track marks
0
555/3/8
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The rgflegtor method:
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Low blade (furthest from trolling edge)
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Tracking stick tracking
This method uses the principle of
"persistency of vision“, which occurs when you look along a beam of light that is being intercepted by two light reflectors. One reflector is installed at the tip of each blade. The surface of one reflector is plain white and the surface of the other is white with a horizontal black stripe painted across its centre. The reflectors are illuminated by a hand~held powerful light from the cockpit of the helicopter. A perfect track will be seen as two equal»width white bands separated by a black band. A track with a thin white band on top will mean that the blade with the all~white reflector is flying low. The actual tracking procedure used is similar to the tracking flag method, except that the final tracking may be checked while the helicopter is airborne. . The object of blade tracking is to get a vibrationless rotor. Some combinations of rotor blades may give higher levels of V vibration as they are brought into a close track. In this case, the lowest level of vibration should be accepted even though the blade track may be outside its specified tolerances. 555/3/8
....L[.2_-
As we have seen, the strobe light tracking method will pinpoint an unserviceable lead-lag damper, which is the usual source of low~frequency lateral vibrations of an articulated rotor head. The flag method cannot do this, and so a process of inspection for defects and elimination of causes will have to be used. The rotor head and blades assembly-should be inspected for 1.
Damaged blade tips and missing tip fittings.
2.
Such damage to lead»lag dampers as fluid leaks, low reservoir levels, and insecure attachment to blade or rotor hub. '
If no defects can be seen then the dampers must have their timing rate checked and, if necessary, adjusted. This is followed by a hovering and then a flight check. If the lateral vibration persists, then the rotor blades are out of balance and need to be replaced. Strobe light tracking, with its electronic vibration measurement counterpart, can, in a few minutes, tell you how to get the blades in to track, which damper is unserviceable, and where and how much balancing weight is needed.
Main Rotor Balancing We discussed the balancing of a main rotor using electronic vibration measuring equipment on page 24. The rotors of all types of helicopter can be balanced (as can propellers) using this equipment, providing the right nomogram is used. If this equipment is not available, a persistent lateral low~frequency vibration from an articulated rotor will show that the rotor blades need to be removed and rebalanced. returning the blades to the manufacturer.
This usually means
The semi—rigid, two—bladed rotor is balanced after its assembly by being placed on a special balancing mandrel set, that permits the rotor to pivot about its centre point and, usually, hold each blade grip in a definite position. The rotor is balanced: 555/3/8
....L1,3....
1-
Chordwise;
2.
Spanwise.
and
Figure 20 shows the directions for these two balances.
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Spanwise and chordwisebalance
Chordwise balaqge is made by firstly aligning the blades in their grips and then moving a blade aft in its grip until the assembly lies level in a chordwise direction. sganwise balance is made by adding or subtracting balance weights from the blades until the assembly lies level in a spanwise direction. If a lateral vibration develops in a helicopter with this type of rotor head and an inspection has determined that the rotor head is serviceable, then the rotor can be balanced without removing it from the helicopter by 1.
Adding a small known weight to any blade and hovering the helicopter to see whether the vibration is better or worse.
2.
If the vibration is better, add more weight until the vibration has gone. 555/3/8
_ an 3.
If it is worse, take away the added weight, install it on the other blade, and check the result. Add more weight as needed. -
If adjusting the spanwise balance does not affect the vibration or only makes it worse, then H.
Move one blade aft in its grip by a small measured amount and hover the helicopter.
5.
If the vibration is less, move the same blade a little more aft until the vibration has gone.
6.
If the vibration is worse, return the blade to its original position and move the other blade aft by a small measured amount until the vibration has gone.
This sequence of balancing is only a general guide. You must always read and follow the directions given for balancing in the maintenance manual for the helicopter. It is sound practice to write down the detail of each adjustment that you make as you make it. Do not rely upon your memory.‘ You must always make good all locking devices and torque all nuts/bolts after making each adjustment.
Tail Rotor Tracking and Balancing Even a small degree of imbalance in a tail rotor, either of weight or of blade track, is unacceptable because of the highfrequency vibration it can generate. Before you begin any tail rotor balancing or tracking, you must closely examine the complete installation for damage and wear. (See page 31,)
555/3/8
_ 45 _
Tail Rotor Balancing Tail rotor blades may be supplied as a matched set, as individual blades balanced to a master blade, or as individual blades that you will have to balance to the existing blades. How they are supplied varies from manufacturer to manufacturer, although the matched set seems to be the most popular way. After you have assembled the tail rotor, you can balance it on a balancing fixture. The two—bladed rotor is usually balanced, like the semi~rigid main rotor, in both chordwise and spanwise directions. However, some tail rotors are balanced on the helicopter, in which case, great care must be taken during the first run—up that an excessive vibration does not develop and cause damage. Tail rotors may be balanced using the electronic vibration measurement equipment. See page 22 and Fig. 21, where the angle read from the protractor assembly and the vibration level in mils from the vibration analyser dial are put into a balance weight location chart supplied by the helicopter manufacturer. You then fit the balance weights indicated and check the result. Zero angle Ilne ix
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555/3/8
Protroctor assembly and strobe "QM
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_ 45 i
As with main rotor balancing, you must always follow the directions given for tail rotor balancing in the maintenance manual for the helicopter.
Tail Rotor Blade Tracking. Tracking of tail rotor blades is now almost a thing of the past. At one time, the blades were tracked by the "tracking stick" method, where the tracking stick was brought to the side of-the blade disc nearest to the tail boom. This prevented the operator from falling into the disc, and the tail boom made a convenient rest for the tracking stick. However, unless this tracking was done with the greatest of care, you were likely to get damaged tail rotor blades or tracking marks that were of no use at all. when a tail rotor is out of track, you will see a "fuzzy" tip when viewing the turning rotor from directly in front or from behind. You will also feel a high-frequency vibration in the airframe. If you are unsure of the vibration, you can observe, or feel, the trailing edge of the nearby horizontal stabiliser, or else you can increase the tension of the tail rotor control cables for the duration of a test hover and feel the tail rotor pedals for abnormal vibration. If the tail rotor has only two blades and adjustable pitch change links, then altering the length of one link will move that blade either further out of track or else back into track. Should the fuzzy tip appear thicker, then the blade is further out of track, and the pitch change link should be reset to its original length, plus a little extra in the opposite direction. Adjustable pitch change links are usually altered in "half turns" of a rod end, making it easy to keep a check of the exact adjustment made. when you have completed tracking, test fly the helicopter and check the vibration level and the response of the tail rotor. Many tail rotors do not have adjustable pitch change rods, so an out-of-track condition will be due either to an accumulation of tolerances in the control linkages to the blades or to the blades themselves. 553/3/8
_ u7 _ Finally, tail rotor blade tracking is done at the same time as tail rotor balancing, as both are closely related, and the job must be done strictly according to the instructions in the maintenance manual for the helicopter type concerned.
DRIVE SHAFTS AND COOLING FANS
All drive shafts and cooling fans are potential sources of high-frequency vibration. The causes of the vibration can be 1.
An out-of-balance condition of a drive shaft or a cooling fan,
2.
Defective drive shaft bearings,
3.
Misalignment of a drive shaft,
H.
Looseness of attachment, or
5.
Rarely, a foreign object, such as a piece of rag rotating,with the fan or drive shaft.
An outeof-balance condition: This can be found by using electronic vibration measuring equipment, although you must be careful not to confuse a high-frequency vibration with one from the tail rotor. Defectjygfdriye shaft bearings:
A defective bearing may
run hot and may be noisy. Carefully feeling each bearing for heat and listening to each one, with a sounding rod, while the helicopter is running will often pinpoint a poor bearing. The condition of the grease in a greased or grease—packed bearing will often give an indication of the condition of a bearing. -
Misalignmentiofma drive shaft:
Misalignment can often be
seen quite easily-by looking along the shaft. Structural damage of the airframe can cause misalignment, and a thorough visual inspection of the airframe should be made if this defect is suspected.
555/3/8
- we -Looseness of attachment and the presence Qfjforeign objects
can be found by visual inspection and by checking the torque of securing bolts and nuts. Components that are out of balance will have to be either rebalanced or replaced.‘ Defective bearings and » misaligned drive shafts will have to be aligned using the methods specified the helicopter maintenance manual. If a component has worked loose, you must look for any damage caused by its looseness and determine whether the looseness caused the vibration or vice versa. Remove foreign objects and check for damage. For example, a piece of rag can score a drive shaft
‘
SUMMARY
Do not use together rotor blades with greatly different flying hour ages, even though they may be of the correct type.
Always follow the manufacturer's instructions for rotor tracking and balancing. An experienced pilot should be at the controls during bladeitracking and balancing. Strobe light/reflected image is the preferred method of'blade.tracking. Autorotation rev/min must be checked after blade tracking is finished. Control rods must be safetied after they-have been adjusted and*before a blade track is made.
PRACT ICE EXERCISE C In each of the.following, choose the option that correctly completes the statement; writing A, B, C or D as your answer.
555/3/8
_L|.Q...
Flag tracking should not be done when the wind speed is in excess of A.
5 knots
B.
10 knots
C.
15 knots
D.
20 knots
The object of tracking a main rotor blade is to give A.
All blades flying in the same path
B.
The smoothest running rotor
C.
The correct autorotation rev/min
D.
The best performance from the rotor
The tracking method that will show up improperly timed blade dampers is
A.
The tracking stick method
B.
The reflector method
C.
The strobe light method
D.
All of A, B, and C
A semi-rigid rotor is balanced:
5.
A.
Spanwise
B,
Chordwise
C.
Spanwise and tracked
D.
Spanwise and chordwise
An out-of—balance tail rotor can be indicated by:
A.
A high frequency vibration
B.
A fuzzy appearance of the blade's tip paths
C.
A high frequency vibration felt in the tail rotor control pedals
D.
All of A, B, and C (Answers on page 50)
555/3/8
_ 50 _
ANSWERS TO PRACTICE EXERCISES EXERCISE A 1.
Answer B is correct .
See page 7.
2.
Answer D is correct .
See page 15.
3.
Answer C is correct .
4.
Answer c is correct .
See page 15 for method See page 5 for method.
EXERCiSE B
Statements 2, 5, 8, and 9 are True. 1 is False, Page 22 refers. 8 is False, page 28 refers. H is False, Page 31 refers. 5 is False, Page 81 refers. 7 is False
>
fig 12 refers.
10 is False, page 30 refers.
EXERCISE C 1.
Answer 5 is correct .
See page 3%.
2.
Answer § is correct .
See page H1.
3.
Answer Q is correct .
See page H2.
H.
Answer Q is correct .
See page H3.
5.
Answer Q is correct .
See page H6.
555/3/8
-
_ 51 _
TEST NOTE:
PAPER
8
Questions 1 to 10 in this test paper can be answered in no more than 50 words for each answer.
What damage does vibration do to a tail rotor control operated by a cable and push rod system?
What single word is used for "the speed of vibration", and what SE unit is used to represent it?
Name three different units of measurement used to give the amplitude of a vibration.
Give the two general sources of vibration in a helicopter.
Why does misalignment of a drive shaft cause_vibration?
What are the three ranges of vibration of a helicopter called? Give one cause of vibration in each range.
What are the two possible choices of action available when a helicopter enters ground resonance? State when each choice should be made. '
The main rotor head lead~lag dampers are serviceable, and yet the helicopter gets into ground resonance easily. What components would you suspect as being unserviceable, and what would you expect to find wrong with them?
Explain why autorotation rev/min should be checked after adjustments have been made to the track of the main rotor blades.
558/3/8
_ 52 _ During a blade tracking procedure, why must a rotor blade pitch change be safetied after it Eas been adjusted and before any test flight?
Refer to Fig. 13. >The clock angle light is on at "quarter past nine" on the 0.25 IPS circle. What balance weights should be used, and where should they be fitted to achieve balance? i
Make a freehand sketch, showing clearly the directions for spanwise and chordwise balance of a two—blade rotor
Make a freehand sketch showing a three—bladed rotor strobe track with one blade low in track and another blade
lagging.
Refer to Fig. 1H.
(a)
What is the frequency of the O.H IPS vibration?
(b)
If the rotor has two blades, what are its rev/min if the vibration in (a) is a "1 per rev"?
TABLE 2
4%‘
Sp§eds_(rev/min)
Main Rotoi
Tail Rotor
Engine
24
1654
6600
_ T/R Drive Shaft
4300
From the drive speeds given in Table 2 for a helicopter with a semi-rigid, two-bladed main rotor and two~bladed tail rotor, calculate the vibration frequency of Ga)
A main rotor "1 per rev"
€b)
An out-of~balance tail rotor
555/3/8
_ 53 i (c)
The normal engine vibration
(d)
A vibration from the tail rotor gear—box input pinion that has 5 damaged teeth.
-&""‘3-?"@'A~ 555/3/a
‘_‘i_’§_; _4‘fl____
Thanks to The Open Polytechnic of New Zealand
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