Main Rotor Aerodynamics
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Transcript of Main Rotor Aerodynamics
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CONTENTS
PART I
1. BASIC AERODYNAMICS
PART II
2. METEOROLOGY
PART III
3. NAVIGATION
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PART I
BASIC AERODYNAMICS
1. DEFINITIONS
2. FLAPPING TO EQUALITY
3. HOOKS JOINT EFFECT
4. DISSYMMETRY OF LIFT
5. TAIL ROTOR DRIFT
6. TAIL ROTOR ROLL
7. LOSS OF TAIL ROTOR EFFECTIVENESS
8. GROUND CUSHION
9. RECIRCULATION
10. FLARE AND ITS EFFECTS
11. DANGEROUS CURVES
12. BLADE SAILING
13. LIMITS OF RPM
14. ADVANCE ANGLE
15. POWER REQUIREMENT
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DEFINITIONS
1. Maximum All Up Weight. The maximum weight
at which the aircraft is permitted to fly within normal
design restrictions. Found in the LIMITATIONS
section of the Aircrew Manual.
2. All Up Weight (auw). The actual weight of the
aircraft at any given time. AUW must not be greater than
MAUW.
3. Basic Weight. The actual weight of the
aircraft with its basic equipment including oil, hydraulic
fluid, fire extinguishers, first aid pack, etc, but excluding
specific role equipment, fuel and crew. Found in thedocuments of the aircraft.
4. Variable Load. Items, which may vary from
sortie to sortie but are not expendable on flight, e.g.: crew
and role equipment (winch).
5. Expendable Load. Items such as fuel, oil,
weapons and other cargo/stores which may be
airdropped, including parachutists.
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6. Payload. The total load of passengers/cargo
actually carried in the aircraft.
7. Operating Weight (Role operating weights
excluding fuel). The sum of basic weight and variable
load which when subtracted from MAUW provides your
LIFTING CAPACITY. Lifting Capacity determines the
compromise between expendable load and payload.
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FLAPPING TO EQUALITY
1. Definition. Moving the cyc stick does not alter
the total rotor thrust but simply changes the disc att. This
is achieved by the blades flapping to equality when the
cyc pitch is applied the flapping to blades can be defined
as angular movement of blades above and below the plan
of hub.
2. Suppose a hel is hovering in ideal wind conditions
and one of its blade is having an angle of attk 6.
3. Now we mov the cyc stick to fwd posn. This mov
of cyc will decrease the blade pitch and this decrease in
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AOA of blade is due to mechanical linkages or due to
con orbit. Here we consider that RAF remain unchanged
this reduction in pitch will reduce both the blades AOA
and rotor thrust, and when the rotor thrust is reduce
therefore lift of the blade will reduce so the blade will not
be able to maint its original horizontal flight and will
definitely begin to flap down.
4. Now when the blade is falling down there will be
some flow which is coming up we call it up flow. Now
this up flow will be resisting the induced flow and
causing its reduction till its AOA is reached to 6, and
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blade thrust will return to its original value and the blade
will continue to follow the new path required to keep the
AOA constant.
Thus cyc pitch will alter the plane in which theblade is rotating but AOA remains unchanged.
5. The reverse takes place when a blade experiences
an increase in AOA when the cyc stick is mov aft.
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HOOKS JOINT EFFECT
1. Definition. Hooks joint effect is defined as the
mov of blade to reposition itself relative to the other
blades when the cyclic stick is applied.
2. Explanation
a. GM, today we will be studying a very
important cause of dragging of main
rotor blade that is Hook Joint Effect.
Before we see what is Hook Joint
Effect. Let us see what is dragging.
Dragging is the ability or freedom
given to each blade to lead or lagindependently of the other blade. The
causes of dragging are periodic drag
changes, changing posn of CG
relative to the hub and Hook Joint
Effect.
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b. Now let us consider a four bladed
rotor system is rotating in POR. If the
cyclic stick is maint at neutral posn,
the blades will maint track as shown
in fig if viewed from above.
B
Figure 36 Hooks Joint Effect
AA
B
C C
DD
Shaft
AxisShaft
Axis
Tip Path Plane
Original Tip Path
Plane
New Tip Path Pla
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3. Now if the cyc is moved fwd the disc will tilt in
the fwd dir and if still viewed from above. Now the blade
A would have increased its radius from the center of the
hub where as the blade C would have decreased it radius
relative to the center of hub.
4. Consider the law of conservation of angular
momentum that is M = m Vr where m is the mass of the
blade V is rotational velocity and r is the radius of the
blade. Now when ever the blade is having closer dist i.e.
(radius) to maint the momentum constant velocity V has
to be increased.
5. As the blade C increases its Vr and blade Adecreases its Vr the blade B, D would try to gain their
original path and while doing so blade B would try to
speed up as the blade C has an increased Vr and blade D
would try to slow down as blade a has low velocity.
It can be seen, that the blade which is moving with
high V, has a tendency to lead and the blade moving with
less V has a tendency to lag in their planes of rotation.
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DISSYMMETRY OF LIFT
SYMMETRY OF LIFT
1. Definition. A structure that allows an object
being divided by a point or line or plane into two or more
parts exactly similar in size and shape and in position
relatively to the divided point.
2. Still Air Condition. Let us consider for a
moment that the helicopter is hovering in still air
condition, the rotor thrust produced by each blade will be
uniform. The speed of RAF over each blade will be equal
to the speed of rotation of each blade irrespective of its
position throughout the blades 360 degree of travel.Therefore it can be said that the RAF for each blade is
Vr, that is rotational velocity of the blade. In this
condition where all the factors affecting the lift are
constant, the blades will experience same lift throughout
their 360 degree of travel.
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DISSYMMETRY OF LIFT
3. Suppose the condition has changed and the
helicopter is now facing into the wind, with a velocity of
Vw, during hover. Half of the time the blade will be
moving into the wind and for the remainder time it will
be moving along the wind. The disc can therefore be
divided into two halves, one half being the advancing
side, and the other retreating side.
4. When the rotor blade starts moving from point A
towards advancing side, as it moves the RAF will start
Vr
V
r
V
r
V
rVw = Wind Velocity
Vw
Vw RAF
Wind
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increasing and a component of wind velocity will also
start supplementing the rotational velocity Vr. The RAF
will attain its maximum value of (Vr+VW) when the
blade reaches point B. As the blade continues to rotate
the value of Vw will start decreasing. Once the blade
reaches point C the value of Vw will again be Vr.
5. From point C as the blade moves forward it enters
the retreating side where the wind Vw is acting along
rotational velocity Vr, and partially canceling the effect
of Vr. As the blade is advancing on the retreating side the
value of RAF will keep on reducing by an amount Vw
and will reach its lowest value of (Vr-VW)once it
reaches 90 degrees on the retreating side that is at
point D.
6. If no change takes place in blades attitude than the
advancing blade at point B will produce lift L = CL 1/2
(Vr+Vw)2 S and the retreating blade at point D will
produce L = CL (Vr-Vw)2 S. The value of lift on
advancing side being more than the retreating side. This
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condition where one side of the disc produces more lift
than the other is known as dissymmetry of lift.
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TAIL ROTOR DRIFT
1. If a fuselage is being turned by a couple yy about a
point x the rotation will stop if a couple zz of equal value
pulls it in the opposite direction.
2. The rotation will also stop if a single force zz was
used to produce a moment equal to the couple yy but
there would now be a side force on the pivot point x.
3. The tail rotor of a helicopter produces a moment to
overcome the couple arising from torque reaction which
in turn causes a side pull on the pivot point or axis of
rotation of the main rotor. This sideward force produces a
movement known as tail rotor drift and unless corrected,it would result in the helicopter moving sideways over
the ground.
YY
X XX
Z Y
ZZZ Z
YY
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4. Correction for Tail Rotor Drift. Main rotor
thrust is offset to produce a side thrust to correct for tail
rotor drift. This is achieved either automatically through
design or by the pilot tilting the disc in required direction.
5. Implication. Implication of TR drift can be
well understood if we carefully monitor the flying
techniques of Schweizer helicopter during under
mentioned phases of its operation :-
a. During normal hover the cyclic stick has to
be placed slightly left, it is because of the
drift corrective force.b. Once carrying out hovering auto we have to
ease the cyclic towards right to avoid hel
drift towards left, it is because the engine
torque during auto is no more there and
correspondingly the drift corrective force
will finish.
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TAIL ROTOR ROLL
1. As the tail rotor roll is by-product of tail rotor drift,
the drift corrective force acts at the main rotor hub center,
in this situation tail rotor thrust is in the opposite
direction. If the tail rotor is mounted below the horizontal
level of the main rotor hub, a couple is formed between
tail rotor thrust and tail rotor drift correcting force.
2. This rolling couple causes the helicopter to hover
one wheel low.
3. Compensation. The tail rotor roll can be
compensated by mounting it in level with main rotor hub.
This is achieved by :-a. Cranking the fuselage to the level of main
rotor hub.
b. Fitting tail rotor to a pylon to raise it to the
level of main rotor hub.
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Rolling Couple
Tail Rotor
Thrust
Total
RotorThrust
Tail Rotor Drift
Correcting Force
Weight
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LOSS OF TAIL ROTOR EFFECTIVENESS
1. Unanticipated right yaw or loss of tail rotoreffectiveness (LTE) has been determined to be a
contributory factor in a number of accidents. In most
cases inappropriate or late corrective actions may have
resulted in the development of uncontrollable yaw. These
mishaps have occurred at low altitude, low airspeed flight
region while manoeuvring, on final approach to a
landing, or during nap of the earth tactical terrain flying.
Severity of this problem depends upon the characteristics
of tail rotor or simply the resultant airflow pattern on tail
because of various in-flight manoeuvres. In this article
we will restrict ourselves to anti-clockwise rotating main
rotor helicopters, in which tail rotor thrust is towards the
right side.
2. Effects on Anti Torque System. Some
important effects on anti-torque system :-
a. Tail rotor thrust is the result of application
of anti-torque pedal. If the thrust is more
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than required to counter main rotor, required
to counter main rotor, required to counter
main rotor, the helicopter will yaw or turn to
the left about the vertical axis and vice
versa. The environments in which
helicopters operate vary the thrust
requirement, because of constantly changing
wind direction and velocity due to main
rotor vortices.
b. Certain relative wind directions are more
likely to cause tail rotor.
c. Manoeuvring helicopter at low altitude andhigh power setting, needs a greater tail rotor
thrust as measured by its cross-wind /
sideward flight capability. Higher sideward
flight capability translates directly into
greater protection from LTE.
3. Conditions / Manoeuvres Conducive to LTE
a. Low Airspeed.
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b. Cross weight and density altitude.
c. Power droop.
d. Right sideward taxi.
e. Right hover turn.
f. Left sideward taxi.
g. Preventive Measures
(1) Plan approaches to avoid high rates of
descent that in turn, will require sharp
power pulls to stop.
(2) When manoeuvring between hover
and 30 knots :-
(a) Avoid tailwinds. If loss of translational lift occurs, it will
result in an increased high
power demand and an
additional anti-torque
requirement.
(b) Avoid out of ground effect
(OGE) hover and high power
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demand situations, such as low
speed downwind turns.
(c) Be aware that if a considerable
amount of left pedal is being
maintained a sufficient amount
of left pedal may not be
available to counteract an
unanticipated right yaw.
(d) Stay vigilant to power and wind
conditions.
(e) Avoid right pedal turns at low
altitudes and high powersettings.
h. Recommended Recovery Techniques
(1) If a sudden unanticipated right yaw
occurs, pilot should perform the
following :-
(a) Apply full left pedal and
simultaneously move cyclic
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forward to increase speed. If
altitude permits, reduce power.
(b) As recovery is effected, adjust
controls for normal forward
flight.
(c) If the rotation can not be
stopped and ground contact is
imminent, an autorotation may
be the best course of action.
The pilot should maintain full
left pedal until rotation stops,
then adjust to maintain heading.
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GROUND CUSHION
1. Gen. GM in free air the resistance opposing the
mov of air being induced to flow down ward from the
MR disc is simply resisted by the surrounding air.
2. However in hover close to gr, the gr will also resistthe induced flow this addl resistance being max when
hovering just above the surface. The down wash from
rotor is deflected by the gr into a flow radiating out word
from the hel and is dissipated against the surrounding air.
3. Now at the same time same down wash is deflect
inward underneath the hel belly. This is brought to rest
forming a dome of stagnate air, or slow moving air. This
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done of stagnate air dead air or gr cushion slightly inc pr
and causes a reduction in induced flow.
4. Comparison between Hovering in and Out of
Ground. The dir of flow relative to blade changes,
increasing AOA thereby the same AOA can be maint in
gr eff (IGE) with less coll pitch and power than req for
OGE. This reduction is power is possible because of
reduction in rotor drag.
a. Factors Eff the Gr Cushion
a. Ht of Hel. Hovering above the gr (the eff
disappears at a ht equal to approx there
quarter of disc).b. The Nature of Gr. Rough gr
dissipates the cushion.
c. The slop of the Gr. Produces an
uneven gr cushion.
d. Wind. The cushion is displaced down
wind.
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GROUND RESONANCE
1. Ground resonance can be defined as being a
vibration of large amplitude resulting from a forced or
self-induced vibration of a helicopter in contact with or
resting upon the ground. The pilot will recognize ground
resonance from a rocking motion or oscillation of the
fuselage and, if early corrective action is not taken, the
amplitude can increase to the point where it will be
uncontrollable and the helicopter will roll over.
2. Causes of Ground Resonance. The initial
vibration which causes ground resonance can already is
present in the rotor head being fore the helicopter comesinto contact with the ground. Ideally the disc should have
its centre of gravity over the centre of rotation, but it for
any reason its position is displaced, a wobble will
develop, the effect being similar to an unbalanced fly-
wheel rotating at high speed. Ground resonance can also
be induced by the undercarriage being in light contact
with the ground, particularly if the frequency of
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oscillation of the oleos and/or tyres is in sympathy with
the rotor head vibration.
a. Rotor Head Vibration. Rotor head
vibration can be caused by:
(1) Blades of Unequal Weight or
Balance. Blades should be
correctly weighed and balanced
during manufacture, but flight in icing
conditions can cause imbalance due to
the uneven accumulation of ice on the
rotor blades. Moisture absorption or
blade damage can also be a caused ofimbalance.
(2) Faulty Drag Dampers. With a three
bladed rotor system the blades should
be equally spaced 120 apart. If a
damper is sticking or is allowing
uneven spacing of the blades, the
centre of gravity of the rotor will be
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displaced away from the axis of
rotation.
(3) Faulty Tracking. A rotor, which is
greatly out of track, may set up an
unbalance condition, which will be
transmitted through the helicopter.
This type of imbalance usually results
in nothing more than a rough
helicopter and a beat in the cyclic
stick. However, if enough track
imbalance exists, it is possible that a
combination of factors may beencountered that would result in
ground resonance being induced.
b. Fuselage Vibration. Fuselage vibration
can be caused by:
(1) Mislanding aggravated by continuous
lateral movement of the cyclic stick.
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(2) A taxying take-off, or run-on landing,
over rough or uneven ground.
(3) Incorrect or unequal tyre pressures.
3. Recovery Action. The more appropriate of the
following actions must be taken:
a. Take-off immediately if take-off rotor rpm
are available. Rotor rpm should always be
maintained in the operating range until the
final landing has been completed.
b. Shut down immediately if take-off Rrpm are
not available, or if take-off is not
practicable; i.e. lower pitch lever, reducepower, apply rotor brake and wheel brakes
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RECIRCULATION
1. Introduction. Not necessary that whenever
helicopter is hovering near the ground that you can get
ground effect. As we know that some of the factors
which restrict in forming the component of dead air
underneath the aircraft. So instead of assisting the
helicopter to hover, whenever such situation arises more
power is required to hover IGE than OGE.
2. Recirculation. Whenever a helicopter is
hovering near the ground, some of the air passing
through the disc is recirculated and it would appear that
the recirculated air increases speed as it passes throughthe disc a second time. The local increase of induced
flow near the tips give rise to a loss of RT. Some
recirculation is always taking place but over a flat, even
surface the loss of rotor thrust due to recirculation is
more than compensated for by the ground cushion effect.
If a helicopter is hovering over tall grass or a similar
surface the loss of lift due to recirculation will increase
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and, in some cases, the effect will be greater than the
ground cushion.
3. When this situation arises, more power and
collective pitch is required to hover near the ground than
to hover in free air. Recirculation will increase when anyobstruction on the surface, or near where the helicopters
is hovering, prevents the air from flowing evenly away.
4. Correction for Tail Rotor Drift. Main rotor
thrust is offset to produce a side thrust to correct for tail
rotor drift. This is achieved either automatically through
design or by the pilot tilting the disc in required direction.
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5. Implication. Implication of TR drift can be
well understood if we carefully monitor the flying
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FLARE AND ITS EFFECTS
1. Definition. Tilting the disc in the opposite
direction of helicopter flight.
2. Thrust Reversal. Figure 1a represents a
helicopter in normal forward flight and figure 1brepresents a helicopter in forward flight with the pilot
executing a flare. By tilting the disc away form the
direction in which helicopter is traveling the thrust
component of the TRT will now act in the same direction
as the fuselage parasite drag, causing the helicopter to
slow down very rapidly. The fuselage will respond to this
rapid deceleration by pitching up, because reverse thrust
Parasite
Drag
Total Rotor
Thrust
Thrust
Total Rotor
Thrust
Thrust
Parasite
Drag
Pitch
Up
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is now being applied while parasite drag decreases. If the
pilot takes no corrective action, the disc will tilt back
further still, causing an even greater deceleration.
3. Increase in TRT. Another effect of tilting the disc
while the helicopter is moving forward is to change the
airflow relative to the disc. As we know, a component of
the horizontal airflow, due to the helicopter forward
movement, is passing through the disc at right angles to
the POR in the same dir as induced flow. When the disc
is flared, a component of the horizontal airflow will
oppose the induced flow and the changed direction of the
airflow relative to the blade will cause an increase inangle of attack and therefore an increase in TRT (Figure
2a, 2b).
Rotor Thrust
RA
FTip Path
Plane
b Flare
Induced
Flow
Component
of Horizontal
Airflow
Vr
Vr
RAFTip
PathPlane
Rotor
Thrust
a Forward Flight
Component
of Horizontal
Airflow
Induced
Flow
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If no corrective action is taken, the hel will climb.
Collective pitch must therefore be reduced if constant
height is to be maintained.
4. Increase in Rotor Rrpm. Unless power is
reduced when collective pitch is reduced to maintain
height, the Rrpm will rise. They will also increase rapidly
in the flare for two other reasons :-
a. Conservation of Angular Momentum. The
increase in TRT will cause the blades to
cone up. The radius of the blades CG from
the Axis of rotation (AOR) decreases andthe blades rotational velocity will
automatically rise. Power must therefore be
reduced to keep Rrpm constant.
b. Reduction in Rotor Drag. Rotor drag
is reduced in the flare because the total
reaction moves closer, towards AOR as a
result of the changed direction of the RAF.
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In figures 3a and 3b, lift and drag vectors
have been used to position the total reaction
and to show that in the flare, even allowing
for a work lift / induced drag ratio as a result
of a greater angle of attack, forward so
reducing the rotor drag. Since engine power
is being used to match the rotor drag for a
given Rrpm, any decrease in the drag will
require a reduction in power to maintain
constant rotor rpm.
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Axis of Rotation
DragLift
Total
Reaction
Rotor Drag
Rotor Thrust
Tip Path Plane
RAF
a. Forward Flightb. Flare
Axis of Rotation
Drag
LiftTotal
Reaction
Rotor Drag
Rotor Thrust
Tip Path Plane
RAF
Axis of Rotation
Drag
Lift
Total
Reaction
Rotor Drag
Rotor Thrust
Tip Path Plane
RAF
a. Forward Flightb. Flare
Axis of Rotation
Drag
LiftTotal
Reaction
Rotor Drag
Rotor Thrust
p Path Plane
RAF
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DANGEROUS CURVES
1. The establishment of fully developed autorotation,following an engine failure, will involve a loss of height.
This loss of height will vary, depending upon the air
speed at the time of the engine failure. In the hover, or at
low forward speed, the loss of height necessary to
establish full autorotation will be considerable as it will
be necessary to lower the lever fully to restore rotor rpm.
At high forward speed it may be possible to flare the
aircraft before lowering the lever, which will help to
restore the rotor rpm and may even result in a gain of
height. However, as speed is reduced it will be necessary
to lower the lever to prevent the rotor rpm from falling
again. If the engine failure occurs at or about the
minimum power speed some height will be lost in
establishing full autorotation, but it will be much less
than that lost when in the hover. For these reasons the
helicopter should not be kept in the hover between
approximately 10 ft and 400 ft AGL for any period
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longer than is absolutely necessary, and flight within the
avoid area should be kept to a minimum. The relevant
aircraft Aircrew Manual should be consulted for specific
techniques. Fig 1 shows a typical helicopter avoid area
diagram.
Air Speed (knots)
10
20 50
400
200
300
100
500
10 40 60
Avoid
AreaHeight
(Feet)
30
Figure 1 Typical Autorotation Avoid Area
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BLADE SAILING
1. A condition known as blade sailing can occur
when the rotor is starting up or slowing down in strong
wind conditions, particularly if the wind is strong and
gusting. With hel facing into wind, the adv blade
experiences an inc in lift and will flap up excessively due
to the low centrifugal force, reaching its max retreating
side it experiences a sudden loss of lift and will flap
down rapidly, flex and reach its lowest position to the
rear of hel i.e. over the tail cone. There is a danger of the
blade striking the tail cone. Now due to poor stick
response and low Rrpm, it is almost impossible to controlblade sailing. The effects can be minimized by following
methods :-
a. Displace the stick fwd and slightly into
wind.
b. Face the hel slightly out of the wind so that
lowest pt of blade passes sideways.
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c. Slow down rotor quickly by applying brakes
on shut down.
d. During start up engage rotor at a faster rate
than normal.
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LIMITS OF RPM
1. Gen. We know that max Rrpm are ultimately
governed by design considerations, but in prac may be
restricted by such factors as max eng rpm in the piston
engine or Txmn limitation in gas turbine eng.
2. Before discussing the limits of Rrpm let us revise
certain terms :-
a. Lift. It is the force produced by an aerofoil
that is perpendicular to RAF.
b. Centrifugal Force (CFF). It is the
force which tends to pull a rotating body
away from the AOR.c. Centripetal Force. It is the force that
counter acts centrifugal force by keeping an
object a certain radius from the axis of
rotation.
3. Consider when the rotor blades are at rest they
drop due to their weight and span. When the rotor system
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begins to turn, the blade starts to rise from the static posn
because of the centrifugal force.
4. As the hel develops lift during take off and flight
the blades rise above the straight out posn and assume a
coned posn amount of conning depends on Rrpm, gross
wt and G forces experienced during flt.
5. Excessive conning can cause undesirable stresses
on the blade and a decrease of total lift because of a
decrease in effective disc area. Let us consider the
resultant of lift and centrifugal force.
6. The vertical force is lift produce when the bladeassume positive angle of attack. The horizontal force is
caused by the centrifugal force due to rotation. Since one
end of blade is att to the rotor shaft it is not free to move.
The other end can mov and will assume a posn that is the
resultant of forces acting on it, and the blade posn is
coned as a resultant.
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7. Limits of Rrpm. As the centrifugal action
through Rrpm gives a measure of control of the conning
angle, provided the Rrpm are kept above the specified
minimum, the conning angle will always be within the
safe operating limit. There will always be upper limits to
the Rrpm due to engine or transmission consideration and
end loading stresses where the blade is att to the rotor
head. Rrpm limitation will be found in the relevant Air
crew manual.
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ADVANCE ANGLE
1. As we have seen that blade flapped position will
always be 90 degree out of phase with control orbit and
blade reaches its highest and lowest point 90 degree later
than where it experiences the maximum and minimum
increase and decrease of cyclic pitch so if the control
orbit tilts in the same direction as cyclic stick is being
moved and as a result of change in cyclic pitch the rotor
disc tilts 90 degree out of phase with the control orbit,
then the disc will always be tilting 90 degree ahead of the
cyclic stick application. Unless compensated by some
way, moving the cyclic stick forward will cause thehelicopter to move sideways. This can be over come by
following.
a. To arrange the blade to receive the max
alteration in cyclic pitch change 90 degree
before the blade is over highest and lowest
point over the control orbit.
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b. The control orbit tilts 45 degree out of phase
with stick movement so 45 degree advance
angle is needed only to compensate for
phase lag.
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+2 -2Blade
High
Blade
Low
AdvanceAngle
90
O
O
Stick Right
Figure 45 90 Advance Angle
Control Orbit
Tilt Axis
+2
-2
Blade
High
Blade
Low
Advance
Angle
O
Stick Right
Figure 46 45 Advance Angle
Control Orbit
Tilt Axis
+2
O
O
O
45 45
Control Orbit
Tilt Axis 45
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POWER REQUIRED
Parasite Power
1. Parasite power is the power required overcoming
the drag of the fuselage when the helicopter is in straight
and level flight. If the drag is calculated for a given speed
and that speed is doubled, the drag will increase four
times but the power required to overcome this rise in
drag will increase eight times. The curve produced by
plotting parasite power against forward speed will have a
zero value when the helicopter is in the still-air hover but
will rise progressively steeply as speed increases (see
Fig 1).
= TotalPower
Required
Parasite
Power
Induced
Power
RotorProfile
Power
Power
(Drag xVelocity)
TAS
Figure 1 Power Curves
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Rotor Power
2. If a helicopter is hovering in still air, the total rotor
thrust being produced will be equal to the weight. Within
limits, depending upon the type of helicopter, this total
rotor thrust can be produced from a wide variation of
collective pitch settings and rotor rpm (Rrpm), but the
drag, and therefore the power, will vary with each
combination, and only one combination will minimum
rotor drag. The power needed to drive the rotor can
therefore be considered from two aspects:
a. The power related to a variation in the value
of pitch or drag coefficient (CD0: this isrotor profile power.
b. The power related to a change in pitch or Cd
for a constant rotational velocity; this is
induced power.
Rotor Profile Power
3. The Cd of a rotor blade will vary with collective
pitch setting, but will remain constant and have its
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minimum value when the collective pitch is minimum.
For a given will vary only with the changes in Rrpm and
air density. However, whenever the main rotor is turning,
ancillary equipment, associated drive shafts and the tail
rotor will also be absorbing power, the power absorption
varying mainly with the velocity of the main rotor. All
these power requirements are included in calculating
rotor profile power so that rotor profile power can be
defined as being the power required to maintain a given
Rrpm when the collective pitch is minimum and to
overcome the drag of ancillary equipment, associated
drive shafts and the tail rotor the rotor profile powercurve will start at a position on the vertical axis o the
graph at Fig 1 depending upon the Rrpm selected and the
air density. Assuming a constant value of CD, as forward
speed increases the power required maintaining this
Rrpm will increase. This increased power requirement is
because in forward flight the increase in drag of the
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advancing blade will be greater than the decrease in drag
of the re-treating blade.
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PART II
METEOROLOGY
1. MET ORG / OBSERVATORIES IN
PAKISTAN
2. LAYERS OF ATMOSPHERE
3. WINDS AND ITS CAUSES
4. LOCAL WINDS
5. VISIBILITY
6. TURBULENCE
7. PRECIPATION8. THUNDER STORMS
9. FOG FORMATION
10. ICE FORMATION
11. WEATHER IN THE MOUNTAINS
12. EFFECT OF FLYING CONDITIONS IN LOW
PRESSURE AND HIGH PRESSURE
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MET ORG / OBSERVATORIES IN PAKISTAN
1. Introduction. Met is the branch of science
which deals with the earths atmosphere and physical
process. Imagine that even the most modern ac today are
dependent on this branch of Aviation. Every military
mission planning includes deliberate MET planning
including minutest details to ensure success of mission.
2. Function of MET Organizations. These are
designed to fulfill 2 main functions :-
a. Record and report information of past and
present weather.
b. To forecast future development of weatherthrough :-
(1) Network of weather observations.
(2) System of rapid communication for
collection and dissemination of
information.
(3) A central forecast office.
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(4) System of local Met office in assisting
provision of information.
3. Main Types Observatories in Pakistan. There
are 6 different types of main observatories available in
Pakistan:-
a. Surface observatory.
b. Pilots balloon observatory.
c. Radar wind observatory.
d. Radio sonde.
e. Radar station.
f. Satellite weather picture.
4. Categories of Met Offices. There are 3categories, details as follows :-
a. Cat I
(1) Established at main operational
airfield.
(2) Maintain a constant forecasting
watch.
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(3) Supply of Met information, briefing
to Avn personnel.
(4) Provides guidance to Cat II & III
offices of Met.
b. Cat II
(1) Situated close to main operational
airfd.
(2) Maintain restd watch. (Upto 100
units).
(3) Prepare enroute wx forecast under
guidance of Cat I.
c. Cat III(1) Situated at a base which does not have
routine operational commitment.
(2) Supply Met info on request basis
received from cat I/II.
(3) Brief / Debrief aircrew on operational
occasions.
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(4) Maintains record of local as well as
out station weather information.
5. Conclusion.The weather info is provided to help
air crew to plan and operate with max efficiency. To
extract the fullest benefit from met service one must
acquire a good working knowledge of the all physical
activities atmosphere.
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LAYERS OF ATMOSPHERE
1. Gen. The atmosphere in which we are living
consists of various gases, i.e., 78% Nitrogen, 21%
Oxygen and 1% other gases. Besides these gases some
other items like water vapors are also present in the
atmosphere and play a significant role in formation and
development of various weather phenomena. Particles of
dust smoke etc. are also available in the atmosphere.
2. Let us see that how far particles can go, what is
temperature and height and how the weather in the upper
layer would be.
3. Division of Atmosphere. Atmosphere isdivided into following layers:-
a. Troposphere. It extends from ground to
6 miles where temperature is constantly
decreasing with the increase in height.
b. Stratosphere. It extends from 6 miles
to 22 miles where temperature is initially
decreasing and then increase with height.
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c. Mesosphere. Extends from 22 miles to
50 miles. Temperature initially increases
with height and then reduces to 130C.
d. Thermosphere. Height is 50 to 70 miles
where temperature constantly increases with
increase in height.
4. Conclusion.Besides gases atmosphere also
contains a small but very variable amount of invisible
water vapor which causes many weather changes like
formation of clouds fog, rain and snow.
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WINDS AND ITS CAUSES
1. Gen. We in Pakistan have a variety of terrain,
starting from snow covered peaks to the barren deserts
and long coastal areas. We being army pilots have to
operate over every type of terrain. The Wx phenomenon
are going to be different at different types of terrain.
Every terrain has its own peculiarities. So we can not
generalize the weather throughout the country.
2. There are certain local Wx phenomenon in certain
parts of the country e.g. if you fly in the Northern Area in
the morning the winds are going to be different than if
you plan in the afternoon. The same way there are certainvalleys where you experience generally isolated build up
and strong wind through out the year. The same way if
you are close to the coastal areas the wx phenomenon is
going to change in the morning and in the afternoon.
3. Causes of Wind
a. Pressure variation.
b. Rotation of the earth.
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c. Diurnal variation of surface wind.
d. Different land surfaces.
4. Pressure Variation. At any given level in the
atmosphere the barometric pressure is everywhere the
same, there will be no isobars, no pressure gradient, and
consequently no wind. When there is a pressure gradient,
however, there will be an initial tendency for the air to
flow across the isobars in the direction from high to low
pressure, but the resultant flow will be almost parallel to
the isobars.
5. Rotation of the Earth. We have seen that when
air begins to move horizontally it blows from high to lowpressure, but that after the motion has continued for some
time the flow tends to be along the isobars, with low
pressure on the left in the northern hemisphere. This
apparent deviation to the right of the direction we should
expect the air to follow is a relative effect, being due to
the fact that we measure the motion relative to the
rotating earths surface.
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6. Diurnal Variation of Surface Wind. There is a
fairly regular change of wind in each 24 hours: it veers
and increases by day, reaching its greatest strength in the
afternoon, and then backs and decreases, with a
minimum strength about dawn. This sequence is known
as the diurnal variations of wind. The normal diurnal
variation of wind over land in the surface frictional layer
in the northern hemisphere may be summarized as
follows :-
a. Surface winds normally veer and increase by
day, but back and decrease at night.
b. Above the surface, at say 2,000 ft., the windnormally backs and decreases by day, but
veers and increases at night.
7. The diurnal variation of surface wind over land is
especially significant to aircrew because of the important
part it can play in helping the formation of very low
cloud or fog at night and leading to its dispersal by day.
The diurnal effect over the sea is very small.
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8. Different Land Surfaces. layer owing
mainly to the nature of the local topography, abnormal
surface winds are regularly observed in particular
localities. Land and sea breezes are a case in point: other
important examples are :-
a. Valley winds.
b. Katabatic winds.
c. Anabatic winds.
9. Conclusion. Wind are of major importance to
aircrew. At every take off and landing a pilot must take
in considerations the direction, speed and its gustiness for
safe operations.
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LOCAL WINDS
1. Gen. Winds at the surface is normally closely
related to the geostrophic wind in the free atmosphere
above friction later owing mainly to the nature of the
local topography, abnormal surface winds are regularly
observed in particular localities.
2. Types of Local Winds
a. Land and Sea Breeze. Consider two
columns of air initially of the same
temperature and with same atmospheric
pressure at pt C and D. Imagine that column
AC is over land and BD on Sea and the Sunjust arisen. Soon the air over the land will
become warmer than that over the sea. The
air in column AC will expand and so the qty
of air above A will increase and hence the
pressure at A will become greater than B.
Air will start to blow from A to B. As soon
as this happens, the amount of air above C
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will be depleted and above D augmented, so
the surface air pressure at C will begin to
fall, and that at D to rise. With lower
pressure now at C than at D, air will begin to
flow from sea to land, a sea breeze will set
in.
b. Valley Winds. A wind blowing against a
mountain has more speed as in a river large
rock blocking the stream. If a barrier is
broken by a narrow gap the stream flows at
increased speed through the channel.
Similarly if a mountain barrier is broken bya valley, the wind tends to blow along the
direction of valley at a speed greater than
neighboring region on other side. In valley
the strongest winds are likely to be along the
general dir of valley.
c. Katabatic Winds. When land is cooled by
radiation during a clear night, air in contact
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with the ground is also cooled and in
consequence its density increases. If the land
is sloping, the denser air tends to flow down
the slope and the air movement is known as
a katabatic wind.
d. Anabatic Winds. The opposite
phenomenon of air moving up the slopes of
valley when the land is warmed on a sunny
day is known as an anabatic but effect is
generally slight.
3. Conclusion.The local winds phenomenon is
significant mostly in mountainous terrain and coastalareas. So when planning in to a mountainous area, you
must consider the prevailing phenomena and must
consult the pilots who have operated in that area.
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VISIBILITY
1. Definition. Met visibility is defined as the
greatest horizontal distance at which an objects can be
seen and recognized by an observer with normal sight
and under conditions of ordinary daylight.
2. Factors Effecting Visibility. The distance at
which one can see the objects by day, or lighted objects
at night depends on many factors, of which the most
important are :-
a. Geography i.e. presence of surrounding
objects.
b. Amount of smoke mist or haze in the air.c. Color, brightness and size of the object.
d. Color and brightness of the background.
e. Sensitivity of the observers vision.
f. Transparency of any window, windscreen,
etc, through which the observer looks.
3. Measurement of Visibility by Day. A met
observer estimates horizontal visibility with the help of
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suitable objs at known distances. Visibility objs should
be dark in color with a lt back gr. For distances over 10
miles, hills, are usually the only suitable objs.
4. Causes of Poor Visibility. It is due to :-
a. Solid particles such as dust, sand, smoke or
soot.
b. Visible moisture in the form of cloud,
precipitation, spray, fog or mist, consisting
of water droplets or ice crystal.
5. Effect of Sun or Moon on Visibility
a. The distance at which objects can be
recognized may also vary with the directionof viewing in relation to the position of the
sun or moon.
b. Observe having his back towards the sun
would be able to see at longer dist then
another observer who is looking towards the
sun.
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c. Opposite happens in case of moon light as
the visibility increases towards the
moonlight and away from it.
6. Oblique Visibility is Effected By. There are
few limiting factors for the observer sitting in the ac or in
other words visibility from the ac is hampered due to
some factors which are :-
a. Curvature of the earth.
b. Mist / Fog above the surface of the earth.
c. Height of the observes.
7. Descriptive Terms. Terms like good,
moderate and poor are usually applied to definite rangesof visibility, which are :-
Visibility Description
Less than 44 Yards Dense Fog
44-220 Yards Thick Fog
220-440 Yards Fog
440-1100 Yards Moderate Fog
1100-2200 Yards Mist
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1.0 5.0 NM Poor Visibility
2.0 5.0 NM Moderate
5-11 NM Good Visibility
11-22 NM Very Good
Over 22 NM Excellent
Visibility
8. Conclusion. Despite improvements in blind
flying equipment, visibility remains one of the most
important weather items affecting air operations. While
planning air operation all the factors effecting visibility
must be catered for to ensure safety of men and materiel.
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TURBULENCE
1. Gen. It has assumed increasing importance to all
pilots during the last few years because of high speed
aircraft. Previously it had meant no more than physical
discomfort. Now a days the pilot must bear in mind the
structural stresses placed on the aircraft by the
turbulence.
2. Types of Turbulence
a. Clear Turbulence. This type of
turbulence is not necessarily associated with
clouds and is often difficult to forecast. It
occurs at high altitude and its height band isvery shallow.
b. Thermal Turbulence. The steep lapse
rate result from thermal effect. These are
found in unstable air. These are usually most
noticeable at low altitude.
c. Ground Turbulence. These type are
specially noticeable in the leeward of hills,
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woods and large building. These are likely
to be severe, and strong downdraft may
endanger ac flying very close to ground.
These types may produces a thin layer of
clouds.
d. Turbulence due to Mountains. In
addition to ground turbulence and irregular
vertical convection current over mountains,
the disturbance of the airflow by a mountain
range may upset a regular flow pattern
waves. These may be experienced to a
considerable height above the crest of amountain and may persist for many miles
down wind. Pilot will experience this most
when he is flying leeward side of mountain.
Pilot should be more careful while flying in
these areas.
3. Effect of Speed. Speed is the most important
factor when flying in turbulence. As an ac enters updraft
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the loading on the wings suddenly increases until upward
motion of the ac is adjusted to that of surrounding
updrafts. If airspeed is low the effect of turbulence will
be less.
4. Flying Technique Turbulent Weather. The
main req is to keep the ac on a fairly even keel by the
moderate use of cons. Aim should be to allow the ac to
ride the bumps rather than to fight them. Rough handling
only aggravate the stresses already imposed by
turbulence.
5. Conclusion.Being an aviator one should
understand enough about the weather to unable air crewto asses the restrictions imposed by it.
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PRECIPATION AND ITS FORMS
1. Gen. The onset of precipitation is usually
accompanied by rapid lowering of the cloud base and by
reduced visibility from the cockpit. The weather
deterioration of this kind poses greater threat to avn
operation.
2. Types of Precipitation
a. Rain. Rain consists of water drops of
appreciable size up to about half an inch in
diameter. In temperate latitudes rain usually
originates in cloud as aggregates of ice
crystals, which melt on falling below thefreezing level and turn into raindrops. Hence
it is quite usual of aircraft to encounter snow
when flying in temperate regions at an
altitude where the temperature is near or
below 0C., although the precipitation
reaching the ground may be in the form of
rain.
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b. Hail. Hail consists of small lumps of ice
and is too well known to need further
description. Hailstones smaller than golf
balls have fallen in Pakistan on rare
occasions.
c. Snow. The ice crystals in cloud above
the freezing level may grow and interlock
until they become too large to be supported
by the ground without melting if the air
temperature below the cloud is sufficiently
low.
d. Sleet. Sleet is defined as rain and snowfalling together, or snow melting as it falls.
e. Drizzle. Drizzle consists of water
droplets so small that their individual impact
on a water surface is imperceptible. It is
often associated with mist or fog, and
usually falls from thin layer clouds.
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3. Effect of Precipitation on an Ops
a. Effect on Rain
(1) Reduction in fwd visibility.
(2) Tendency of skidding on corners or
turns.
(3) In T/O, fwd flight, reduced fwd
visibility.
(4) Risk of icing.
(5) In landing heavy rain will affect radio
nav aids i.e. GCA Eqpt.
(6) Damage to horizontal stabilizers / tail
rotor during landing when tyres throwup water.
(7) Efficiency of brake.
b. Effect of Hail
(1) Damage to airframe of engine
(canopy) fwd contours.
(2) Greater speed greater is damage.
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(3) By flying above or below freezing
level chances are minimized.
c. Effect of Snow. Generally create
problems from reduce visibility and
difficulties in landing, T/O, and taxiing.
(1) Before T/O remove moisture and
snow.
(2) During taxing brakes less effective.
(3) May blow up in hover and whiteout.
(4) Helicopter may slip on slope.
(5) In landing hel may sink in and
resonance is experienced.4. Conclusion. Precipitation produces lot of
flying hazards. All pilots must know their types and
condition surrounding them.
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THUNDER STORMS
1. Gen. Thunder storms produce the most severe
type of weather known to mankind. Tornadoes with
winds reaching 350 mph, hailstones the size of baseballs,
and extreme turbulence can result from thunder storm
formation. A thunderstorm is invariably produced by a
cumulonimbus cloud and is always is accompanied by
lighting and thunder. Thunderstorms are a hazard for all
types of flight operations since top of cumulonimbus
clouds may reach as high as 75000 feet.
2. Factor Effecting Formation of Thunder Storm.
Several factors must exist for the formation of athunder storm. First, for the formation of the cumulus
cloud and for its continuing build-up, some sort of lifting
action may be orographic, conventional, or frontal.
3. Stage of Thunderstorm
a. First Stage. There are three stages to the
development of a thunderstorm. The first
stage is known as the cumulus stage. The
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main feature of this stage is the cumulus
could and the updraft which may extend
from near the earths surface to several
thousand feet above visible cloud tops.
Water droplets are vary small but grow into
raindrops as the clouds build upward. Many
times the raindrops remain in the liquid state
even above the freezing level. These rain
droplets are suspended by the currents
within the clouds.
b. Second Stage. The second, or mature
stage is the most intense phase. It begins asrain begins to fall at the earths surface.
Raindrops and ice particles, by this point,
have grown to such a size that they no
longer can be supported by the updrafts. The
mature stage occurs approximately 10 to 15
minutes after the cloud has been built
beyond the freezing level in the atmosphere.
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Occasionally, during the mature stage, a
cloud may build as high as 50000 to 60000
feet, but 25000 to 30000 feet is the norm.
Severe up and downdrafts occur in the
mature stage. As the raindrops fall, they pull
air with them and create downdrafts that
may exceed 2500 feet per minute. This
causes gusty winds at the surface as the
downdrafts strike the earth and spread out.
c. Third Stage. The dissipating stage is
characterized by the collapse of the
cumulonimbus cloud. Downdrafts continueto develop and spread vertically and
horizontally while updrafts weaken and
finally dissipate completely. Soon the entire
thunderstorm becomes an area of
downdrafts. Rain decrease, then ceases, and
the thunderstorm begins to dissipate. The
top of the thunderstorm, at this stage, begins
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to develop the characteristic anvil
appearance with the point of the anvil in the
direction of the prevailing winds.
4. Conclusion.The hazards that exist with thunder
storm activity are not confirmed just to the storm area
itself. But pilots flying near the storm area can also
encounter its advance effects.
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FOG FORMATION
1. Gen. Fog is basically a cloud which is very near
or touching the Earths surface. It consists of small water
droplets or ice crystals suspended in the atmosphere.
They are very small (droplets) to see with the naked eyes.
But they are so numerous that visibility is reduced. In
short we can say it is minute droplets of water or ice
suspended in the air with no visible downward motion
and visibility less then 1100 yards. Various forms of fog
are :-
2. Formation of Fog Depends Upon. For the fog
formation following are the pre-requisites:-a. High Relative Humidity. The high
humidity necessary, for fog formation can
occur in distinct way:-
(1) When air is cooled to its dew
point.
(2) When the moisture is added to
its dew point.
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b. Wind. Light surface wind is also
necessary wind provides a mixing action. If
no wind is present the fog will likely be
shallow and also to the ground, strong winds
are not conducive to fog formation. It tend
to break up the fog layer.
c. Condensation Nuclei. Condensation
nuclei, such as smoke dust and salt particles
suspended in the air, provide a base around
which moisture condenses. Our country is
quite rich in this regard and sufficient nuclei
would be present to permit fog formation.The amount of smoke particles and sulfur
compound in the vicinity of industrial areas
is pronounced. In such area persistent fog
may occur.
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3. Types of Fog
a. Radiation Fog. It forms as the earth
rapidly loses is heat on clear nights.
b. Advection Fog. Is formed when warm
moist air flows over a cold surface this type
is found mostly along the coastal regions,
where temperature of land and water differs
widely.
c. Up Slope Fog. When moist air flows up
hill. As it rises the temperature. Drops
through adiabatic cooling and by
evaporation.d. Steam Fog. When cold air flows over water
which is much warmer than air.
e. Sea Fog. It happens when moist air mass
usually of tropical region moves slowly over
cooler seas.
f. Valley Fog. During evening, cold dense air
will drain down into low areas or valleys.
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g. Ice Fog. When air near surface becomes
saturated in extremely cold region fog will
form.
h. Smoke Fog. Around industrial city.
4. Fog Dissipation. Fog would tend to dissipate
under following conditions :-
a. Dec in Relative Humidity. It tends to
dissipate when relative humidity decreases.
During process water droplets evaporates or
ice crystal sublimate, and moisture is no
longer visible.
b. Strong Wind. As it is stated above thatstrong wind mix the cool saturated air at the
surface and warms air of the atmosphere.
c. Heating Up of Atmosphere. Air which is
heated as it flows down slope or by day time
solar radiation evaporates fog. Most of it
dissipate after sunrise.
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5. Conclusion.Fog though is one of the cause of poor
visibility but posses greater damages since it hides the
mother earth completely leaving air crew with no contact
with it. Knowledge about various types will help air crew
in negotiating it safely.
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ICE FORMATION
1. Gen. Ice would be a serious hazard if it forms on
an aircraft in flight. With modern aircraft, icing problems
rarely arise. If icing regions are encountered, they can
often be crossed in few minutes or can be avoided by
changing altitude, while on different aircraft deicing
equipment are fitted. Nevertheless, ice formation may
still sometimes cause a serious loss of aircraft
performance. A knowledge of the types of ice and their
modes of formation is necessary if aircrew are to take
most effective option in these circumstances.
2. Types of Ice Formationa. Hoar Frost. It occurs in clear air and is
easily recognized as light crystal deposits. It
occurs below 0,C temperature.
b. Rime Ice. It is rough white ice which
forms on the windward side of trees and
other exposed objects. It also occurs below
0,C temp.
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c. Clear Ice. It is most dangerous ice and
consists of a transparent ice with a glassy
surface.
d. Pack Snow. It occurs on aircraft surface due
to super cooled water droplets.
3. Dedication of Icing. Early detection of icing
its necessary, as it may cause serious difficulties if timely
action is not taken. By day it is easy to detect ice by
observation of windows, LE, propellers, and aerial masts.
4. Anti Icing Eqpt
a. Thermal. Hot air from engine is led to the
surface to be protected, or the surface isheated electrically.
b. Chemical. Alcohol spray, other deicing
fluids or special non freezing oil or grease is
applied to the surface of an aircraft.
c. Mechanical. A pulsating rubber boot
may be fixed to the surface such as LE, and
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employs the mechanical mean to break the
ice by intermittent inflation and deflation.
5. Conclusion. Forecasting of icing condition
is very difficult, however endeavour should be made to a
certain its presence and take timely counter measures.
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WEATHER IN THE MOUNTAINS
1. Convection and Air Mass Stability. Stable air
mass conditions with negligible surface heating will
temper the wind deviation effect but in conditions of
unstable air mass, with strong surface heating, the
vertical air currents produced will accentuate vertical
deviation thus increasing up and down drafts. In addition,
even when a slack wind gradient persists, a strong
surface heating during the day will produce quite fresh
anabatic winds, e.g. in the Aden Protectorate mountains a
40F temperature difference between valley floor and
mountain top is quite common producingg slope winds of
15 knots in mid afternoon. This surface heating is often
not even and results in strong thermal up currents and
accompanying turbulence.
2. Cloud. Clouds of orographic origin are often
found in the mountains and, dependent on temperature,
may give conditions particularly dangerous to flight, e.g.
frost, icing etc.
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3. Fohn Effect. In stable conditions another
effect of orographic cloud is to produce Fohn effect. This
may mean that a landing platform upwind of the ridge
would not be suitable for use.
4. Always keep in mind the possibility that through
down draft or turbulence the pilot may have to break off
an approach or break away from a chosen flight path and
position the aircraft so that an escape route is always
available.
5. Whenever possible when flying near the ground
reduce speed to climbing speed. Avoid areas where down
drafts are likely to be found. Should the helicopter enter adown draft, maintain climbing speed and try to counter
the loss of height with power. If unable to do so then go
with the down draft but maintain climbing speed.
6. Anticipate the increased wind strength that obtains
near cols, crests, valleys and peaks. Where possible fly at
a safe height, within reach of a reasonable landing area,
and avoid crossing close to sheer faces. In the event of an
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engine failure make the best use of any available landing
site to ensure that, whatever else happens, the helicopter
does not roll down the mountain side.
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EFFECT OF FLYING CONDITIONS IN LOW
PRESSURE AND HIGH PRESSURE
1. General. Numerous aircraft accidents have
been caused directly due to failure to read the altimeter
correctly. Proper use of this instruments is therefore
mandatory. In flight, altimeter is one of the most
important instruments which provides information for
clearing obstructions, making low approaches, avoiding
other traffic, and hazards.
2. Height / Pressure.Pressure decreases with
increase in height. Air has weight and therefore exerts
pressure known as atmospheric pressure. At any point onthe surface of the earth, the atmospheric pressure is
equivalent to the pressure exerted by a column of air
approx. 50 miles high, and can be expressed as millibars,
pounds per square inch or as the height of a column of
mercury which that pressure would support.
3. Error. Altimeter has the following errors:-.
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a. Pressure Error. This is caused due to the
position of the static vents. Because of
disturbance in the vicinity of the aircraft
false or extra pressure is fed into the system
and as a result the altimeter gives erroneous
indications. This error is negligible except
when flying through severe turbulence.
b. Barometric Error. Suppose an
aircraft is flying at an altitude of 5000 and
heading from an area of high pressure to low
pressure. Once the aircraft is over low
pressure area it would still indicate 5000but would be actually lower, say 4000 only.
This is so because in a low pressure area the
equivalent pressure i.e. 30 in this case, will
be present at a comparatively lower height.
In this instance then, the altimeter would
over read.
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d. Temperature Error. Atmospheric
temperature and pressure vary continuously.
Rarely is the pressure at sea level exactly
29.92 or the temperature +15 deg C.
furthermore the lapse rate, for both pre-
pressure and temperature, deviates from the
standard. For instance on a warm day the air,
having expanded, is lighter in weightt per
unit volume than on a colder day, and the
pressure levels are raised. Therefore, the
pressure level where the altimeter will
indicate 10000 will be HIGHER on a warmday than it would be under standard
conditions (diagram). On a cold day the
reverse would be true, and the 10000 level
would be lower.
e. Lag Error. The altimeter may tend to lag
particularly when rapid and large changes
in altitude are made. This error called
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hysteresis or after effect will vary with the
climb or descent.
f. Instrument Error. Since the
expansion and contraction of the wafer stack
are greatly magnified, it is impossible to
avoid magnifying minute irregularities.
g. Blockage Error. Should the static tube or
vents become blocked, pressure within the
case will remain constant and the altimeter
will continue to indicate the height at which
it was blocked. After breaking the glass of
the VVI it will give indications but with a 6-9 seconds lag.
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PART III
NAVIGATION
1. DEFINITIONS
2. CHOICE OF HEIGHT TO FLY
3. MAP PREPARATION
4. NAVIGATION IN MOUNTAINOUS
TERRAIN POINTS TO REMEMBER
5. PRE-REQUISITES FOR AERIAL
NAVIGATION
6. PRE-FLIGHT PREPARATION FOR LOW
LEVEL NAVIGATION7. GPS
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DEFINITIONS
1. Definitions
a. Altitude. The vertical distance of a level,
a pt, or and object considered as a pt,
measured from mean sea level.
b. Height. The vertical distance of a fixed
pt above ground level or some specified
datum other than mean sea level.
c. Elevation. The vertical distance of a fixed
pt above or below men sea level.
d. Flight Level. Flt levels are surfaces of
constant atmospheric pressure related to thestandard pressure setting of 1013.2 Mb and
separated by specific pressure intervals.
e. Transition Altitude. The altitude in the
vicinity of an airfield above which 1013.2
Mb is to be set on the altimeter. At or below
the transition altitude the vertical position of
an aircraft is determined with reference to
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altitude above means sea level or height
above an airfd.
f. Transition Level. The lowest flt level
available for use above the transition
altitude. Descent must not be made below
the transition level without setting the
appropriate QNH or QFF on the altimeter.
g. QFF. QFF is the observed pressure
corrected for temperature at an airfd
elevation. With QFF set an altimeter will
read zero height at the airfd datum.
h. QNH.QNH is the observed pressure at aselected datum, corrected for temp and
reduced to a mean sea level assuming that
the atmosphere conforms to the ICAO
standards. When set to QNH, altimeter will
indicate altitude AMSL.
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CHOICE OF HEIGHT TO FLY
1. Gen. The most important pt in the technique of
Nav is the choice of height to fly. Keeping apart the rules
of semicircular height there are certain other
considerations which must be taken is account.
2. Imp considerations for Ht to Flt
a. High Gr. Ht flown above should be safe
or above safety ht specially in the inst met
conditions (IMC).
b. AC Performance. The ht selected should be
that where best performance of the ac is
aval.c. Quadrant Ht. Always plan to fly
quadrantal heights regardless of weather
conditions.
d. Wind and Weather Forecast. It is possible
to take advantage of winds to avoid adverse
winds by careful selection of ht.
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e. Oxygen Condition. Flt above 10,000 ft
requries use of oxygen an prolonged flt
above 10,000 ft is dangerous due to hypoxic
conditions. Do not fly in these conditions for
more than 30 minutes.
f. Msn Requirement. Msn requirement
may dictate to fly as low as tree top. In these
conditions fly as per proper low flying
technique.
3. Conclusion.The factors mentioned above besides
allowing air crew with the choice of height to fly ensure
safe and efficient operation, hence must be given primaryimportance.
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MAP PREPARATION
1. Gen. When a msn is assigned carryout the pre flt
procedure and select the desired map. The carryout map
prep as follow :-
2. Imp Consideration for Map Prep
a. Draw the tack.
b. Calculate the bearing and distance and
record these things on map.
c. Mark the time markers after 10 interval.
d. Mark the imp check point along the route
and measure the distance from the main tack
either side of the track..e. Calculate time between each check pt.
f. Mark the danger and prohibited areas.
g. Mark the position of radio aids and mark the
freq.
h. Check the air fd and ATC frequency of each
air fd.
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j. Check pt and time of entry in any con area,
trg area, air fd.
k. Study the map in detail.
l. Fold the map properly so that the complete
coverage of track is achieved.
3. Conclusion.The increasing complexity of air
operation using multi-navigational aids to accomplish a
mission still desire the basic navigation aid Map for
successful, preparation / playing of a sorties.
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NAV IN MOUNTAINOUS TERRAIN POINTS TO
REMEMBER
1. Gen. Navigation in mountains is comparatively
easier than that of plains due to good visibility condition
and aval of prominent land marks, important features
and ref pts. However there are chances when the
perspective is changed and features along a designated
route appear to be different and difficult to be
recognized. To over come this sit fol pts must be kept in
mind while flying in mountainous terrain.
2. Points to Remember
a. Loss of Attitude. Since helicopter inmountains will fly oftenly in valley so
normal horizon will be deprived to the pilot,
so must cross check to fly instrument
specially to A/H.
b. Sun into Eyes. While flying in valleys
there will be a number of occasions when
sun will be into your eyes which will lose
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your perception for the depth and width of
valley also denying you the visual contacts
of turns and bends. In such condition if the
height of the crest permits then climb and
clear crest line or descend down enough and
stay on the side of valley which gives you
best vision.
c. Hazards in Narrow Valley. While flying
low in valleys must be watchful for wires
and trolley cables which are normally
crossing valleys. As a generally rule never
fly below the road level. If possible flydown the valley than up a valley. If flying
up a valley do not cross a point beyond
which a 180 turn is not possible.
d. Winds. Wind condition is much more
severe and unpredictable than that of plains.
In mountains specially in valleys stay on the
lifting side i.e. In case of cross wind stay on
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the lifting / climbing wind side of the valley.
As a general rule stay on the sunny side of
the valley to get up draft.
e. Turbulence. Avoid turbulence zones
if possible otherwise try to get out as early
as possible. Once in turbulence do not flight
the controls, reduce the pitch slightly to
minimize stresses on transmission.
f. Icing. Do not fly in icing conditions if
possible avoid situations where visible icing
conditions are present.
g. White Out. Avoid entering in suchcondition. Keep looking around for different
references.
h. Low Air Density. With increased altitude
density of air decreases so the engine power
is reduced specially it has more effects on
piston engines thus giving less reserve
power. So avoid maneuvers which involve
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reserve power e.g. quick stop, steep tuns or
abrupt pulling of collective pitch./ the rotor
remains turning at same RPM so with
increased altitude, higher pitch setting is
required, which needs increased pitch setting
of tail rotor as well, limiting pedal control
and also bring main rotor retreating blade
close to stalling angle.
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PRE-REQUISITES FOR AERIAL NAVIGATION
1. Gen. An air navigational map may be defined as a
small scale representation of the earth and its culture. It
depicts the land marks and other information useful for
pilots during aerial navigation. The ability of pilots to
comprehend all the details help them to accomplish the
sorties with success.
2. Basic Principle of Aerial Navigation. While
flying four basic principles for aerial map reading should
be followed :-
a. Orientation. While reading the map,
orientate the map in a way that the north ofthe map is towards the north. Only then the
course lines on the map parallel the intended
course lines of aircraft and objects on right
and left of the course appear to the right and
left of the aircraft if it is on course.
b. Appreciation of Rate of Travel Over
Map. For appreciation of rate of travel on
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map, the speed and scale of the map are
necessary. It is worked out in pre flight
planning and subsequently in the air by
noting the elapsed time between two check
points.
c. Anticipation. Appearance of terrain
varies at different altitudes. In low flying a
checkpoint appears for a very short time. the
vision is also restricted but because of
oblique angle of sight, depth increase and
detail is blurred and landmark present
different appearance to that shown on map.From high altitude ground seen to appear to
move very slowly. In good visibility large
area can be seen and distances appear small.
With sun low position, shadows are long
causing strong contrast and emphasizing
difference in elevation. At moon when there
are no shadow, terrain appears to be flat
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broken clouds may block the view
completely thus hiding a check point.
d. Appreciation of Perspective.
3. Pre Flight Procedure
a. Review all relevant information documents.
b. Collect the meteorological forecast from
meteorological office and study the forecast.
c. Select the planning charts and maps to be
used on selected route.
d. Draw tracks and measure distances. Study
the safety heights and decide the height to
fly.e. Complete the map preparation.
f. Plan flight for alternate airfield.
4. Conclusion.Principles of aerial navigation have
been worked out over a period of time to ensure
accomplishment of successful mission in different terrain
/ type of country.
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PRE-FLIGHT PREPARATION FOR LOW LEVEL
NAVIGATION
1. Gen. Some hazards to flight, even if not entirely
new, assume a much greater importance in low level
flight, and the crew planning such a flight must take into
consideration factors which can normally be assumed
negligible otherwise. Sound and deliberate planning can
reduce the low level flight hazards to a great extent, some
tips for pre-flight preparation are discussed in succeeding
paragraphs.
2. Mission Briefing. Before a low level flight
begins, and before flight planning is started, the crewmust be given a clear brief on the mission, any restriction
on the routing, and the flying limitations to be observed.
In particular, they must be aware of the minimum height
above ground they are to observe. The brief should be
clear and holding no ambiguities.
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3. Map Selection. The routine million map should
be supplemented by a map showing larger details,
map is recommended.
4. Map Study and Preparation. A thorough map
study of intended area to be covered is vital. In addition,
all the hazards to low level flight such as airspace
reservation, airfields, relevant information should be
gathered.
5. Route Selection. Within the limits imposed by
mission briefing, a route should be selected which :-
a. Is favorable as regards terrain and weather.
b. Takes the full advantage of available mapreading features.
c. Avoids airspace reservations, other hazards.
6. Flight Plan
a. Plan as much as possible, to fly at a constant
ground speed for ease in navigational
calculation.
b. Ensure calculating safety altitude.
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c. Obtain flight / route clearance if required.
d. Mark and study diversions.
e. Carryout fuel planning.
f. Workout emergency procedures.
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GPS
1. GPS can operate in part of the World during day ornight in all weather conditions. It takes its data from
satellites revolving around Earth. There are 21 satellites
out of which 18 are active and 3 are reserve. Each
satellite makes 2 revolutions around the earth in one day.
The orbital distance over the Earth is 2000 KM.
2. Basic Components
a. Satellite receiver.
b. Computer.
c. Display and control panel.
d. Power supply.
e. Antennas and cables.
f. Rechargeable batteries with battery charge.
g. Internal antenna.
h. Data transfer cables.
3. Principle of Operation. The receiver gets
information from the satellites and give three
dimensional co-ordinates of the position of the ac. These
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three co-ordinates are longitude, latitude and altitude
(AMSL). This information can only be given if the GPS
is receiving min of four satellites. If GPS is covering
three satellites then it will not give altitude information.
GPS requires minimum of 3 satellites for its operation,
otherwise it gives POOR GPS COVERAGE message.
The satellites are synchronized among themselves and
are transmitting coded pulses. The difference in time of
reception from different satellites will determine the
coordinates. There are two types of codes which are
being sent by the satellites. P-codes. These are only used
by US ARMY are not available to others, C/A codes.These codes are being used by the rest of the
organizations of the World. C/A codes are precise to
hundreds of meters and contain following information :-
a. Position in three dimension.
b. Velocity information.
c. Time information.
4. Features
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a. Feeding of way points by three different
methods.
(1) Lat / Long.
(2) Bearing Distance.
(3) By autostoring.
b. Can set a route comprising of maximum 9
way points.
c. It displays fol navigational information :-
(1) Bearing flown.
(2) Bearing to be flown.
(3) Elapsed time.
(4) ETA.(5) Time to be flown to the destination.
(6) Distance covered.
d. It can autostore any position as a way point.
e. It allows to plan a trip including the fuel
calculations / stops.
f. It can plan vertical navigation.
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