For, flying the helicopter we call it collective
pitch, tail rotor pitch.
And we call them as cyclic, because then,
it is split into longitudinal cyclic, lateral
cyclic. But, then when you give that these
commands, basically what pilot does is? He
is just moves the stick and the effect of
that motion of the stick to the rotor system
is basically initially the rotor disk may
be horizontal. And when he gives longitudinal,
the disk may tilt with respect to the shaft.
So, initially the disk is like this, then
it tills; as a result, it develops a forward
component of the rotor lift force and that
will move the helicopter forward. But, that
will also give a movement about the CG therefore,
it will pitch. So, these two, you we cannot
decouple. Similarly, you can do lateral that
means the tilting it this way, which implies
he can fly in any 360 degree. This is very
primitive way of explaining, the helicopter
controls very simple way.
But, in reality, all of them are coupled.
Because, if you do one in the sense, if I
want to climb up very simple; but, because
of the increased lift or we call it thrust,
there will be a torque will increase; as a
result, the helicopter will start turning
that means yaw has to, he has to control.
Similarly, when you want to do longitudinal,
because of the coriolis, centrifugal, all
those affect that gyroscopic effect of the
rotor, he will get the role which is undesirable,
but you cannot help it that means he has to
keep on adjusting. If he wants one, he will
get an undesirable response, and he has to
compensate for that. That is why, helicopter
flying is more difficult.
So, first the pilot gets trained in fixed
wing flying, all the helicopter pilots. They
first go through flying in the fixed wing
then they come to helicopter training. But,
here the flying is different. So, they go
through another training and these pilots
are then helicopter pilots that is all. And
they have to get the hang of how to fly, because
his hands, his legs everything is engaged
in the vehicle dynamics.
Now, you see basic and another thing is the
helicopters are unstable, both statically
as well as dynamically. So, the pilot has
to keep on, he has to fly it is not that,
he can take his hands and legs off from the
system and the vehicle will beautifully fly,
no it does not happen that way. So, he is
in the loop to fly the helicopter.
Suppose, you say you do not want the pilot,
like now you says autonomous. That means some
other system should be inside to take care
of the control, that is where automatic flight
control systems; basically that is to relieve
the load that is given to the pilot, but those
are expensive then you increase the complexity.
That is why in the design of helicopter usually,
if you design the vehicle, the basic vehicle
without any of this close loop controls, if
the system behavior is very good, then the
effort required to make it better is less.
But, if the basic system is not very good,
then the effort required to fly by the pilot
is more. That is why when I spoke to some
pilots he said, our job is not just to fly
the vehicle. But, even though a pilot has
to fly the vehicle, it is not that he has
to only fly, because he has to do other tasks;
like other tasks means, may be taken a if
it is a attack helicopter, he has to operate
certain other things. He will look through
various other environment that is why his
work load is more; whereas, if he is always
concentrating on flying, then he cannot do
the other work.
So, the developments of see them, I would
say the in the helicopter field is towards
these directions; how do you reduce the pilot
work load? And then, how do you reduce noise?
How do you reduce vibration? How do you increase
the capability of the helicopter? There are
several directions.
And the gap between theory and experiment,
that is still there is a big gap. What do
you measure? And what you theoretically calculate?
There is a gap; and how do you fill it up?
That is were the research, all the research
is going towards this direction and with the
technology improvement. Can I replace some
rotor blades with metal rotor blades with
composite materials, and the cockpits you
will have all sorts of meters. Now, remove
all those meter, have a glass carpet they
say just a simple display, pilot can push
whatever he want, it will be displayed.
So, the technology improvement is in those
directions. But, still where these scientist,
there is a lot of problems enormous number
of problems exist in helicopters, even from
a research point of view. So, you have learnt
about these basic controls.
Now, I will give you a schematic of how these
controls are this requires a little explanation
more. See, this this is for to this particular
lever, we call it the collective, that is
when he moves what is he really do it, because
you say we have given the pilot a stick on
the left hand and a stick in the front, and
pedals for the leg. But, when he moves what
really happens? What is that he is controlling?
So, that is by this diagram, I will briefly
explain because this is important, when we
go for the next stage. When the pilot moves
this stick up, what he does is? Through a
mechanical please understand, mechanical and
hydraulic system today, there is something
called a swash plate; please note, this is
a new terminology which you do not see in
air craft.
Why what is this, I will show a component
which we have. Because, the pilot is sitting
in the non-rotating frame he is in the non-rotating
frame, the blades are in the rotating frame,
you have to transmit, the control from non-rotating
frame to the rotating frame right. Now, the
control to the blade is simple, because we
all know we are all from aeronautics, we have
only aerofoil; what you can do at the most
to an aerofoil? You change the this angle,
which angle of the blade you change that is
it, you have no other control.
Now, you may ask, whether RPM is controlled?
In big helicopters, RPM is regulated you do
not vary the RPM for flying. But, pilot has
some control over the changing the RPM little
bit, but he always keeps the RPM at a fixed
value, that is by like your motor cycle through
this he holds it and he can turn it.
That means, engine throttle increases a little
bit, but he cannot change it to any value
he likes, it only with in a zone, because
he will have a meter that tells you what is
the rotor RPM and what is the engine RPM,
please understand. Now, you see in helicopter
whatever we are going to develop from now
on, in terms of the equations of motion or
thrust power anything, rotor RPM is a fixed
quantity; it is not a variable quantity that
is in helicopters.
Now, you imagine if you want to make a small,
all these autonomous that microware vehicle
everything, there you may vary the RPM; see
the control may be through RPM control, but
here RPM is fixed. What you have control is
only the pitch, pitch input. Now, what that
pitch input? Because, we have several blades
on the rotor from two and more, we will come
to the rotor system later. When the pilot
pulls up, what he does is? This is the swash
plate mechanism; it has two components, one
is a non-rotating, another one is a rotating
with some Baring in between that is all.
And that system I do not know whether if I,
I will show another picture. But, it is like
this, this is actually the swash plate, I
do not know whether you can see I do not know
how do you put it, this is the swash plate;
you see, I can move the inner one you see
the inner one can move, and the outer one
is fixed. From the outer, the control rods
from the outer will go to the pilot that is
non-rotating; and from the inner, it will
go to the, I do not know whether you can what
inner one goes to the blade. So, depending
on how many blades you have, you will have
a non-rotating non rotating and rotating;
this is to pilot, this is to blade. So, the
swash plate mechanism has two parts.
And schematically there is another another
picture, which I can show here it will be
like this that is very simplistic, because
please understand; this is a see as a mechanical
system, this is a very beautiful thing. Because
people have thought about, how to really design
a mechanism through which you control from
a non-rotating to a rotating system.
Now, if you want to do some this is a very
heavy system, heavy mechanical weight as a
heavy weight; suppose you say, by electrical
some wireless beams I control, if you do that,
while may be that will become after 50 years
that may become retrained. Because mechanical
systems still are reliable you follow. So,
they still use this, swash plate because,
if it goes that is all, the vehicle vehicle
is gone.
So, these are all very critical systems of
the helicopter flight control system it should
never fail that is it. So, you see this is
the non-rotating, and this goes to pilot control
and above this is the rotating swash plate,
rotating ring and this is the control rod.
Now, if I move this entire unit up or down,
what I am doing is I move this control rod
up or down, not only this control rod all
the; if I have four blades, every blade will
have its own control rod, I move everything
simultaneously up or down.
When I do simultaneously up or down, I change
my there is a pitch bearing. So, you see what
happens? This, if I move it is going to rotate
about the pitch axis. So, all the blades will
experience same change in pitch angle, I am
not using the word angle about act, I am only
using pitch angle; please understand, because
angle about act we will learn later, because
it is a too complicated, pilot gives only
pitch input.
So, now let us write the pitch to the blade
as data 0, this is to any blade K represents
k th blade, we always used a subscript K to
represent k th blade. You can say, k th blade,
I put a 0 to indicate that this is a collective
pitch input, this is called collective; collective
pitch input pilot gives, which means he moves
the stick up, because it is collective lever
is up, this lever this swash plate will go
nicely up or down, up normally it goes up,
depending on where this control rod this it
is in front or back please understand.
I can have this rod in front of the blade
or back of the blade, depending on I move
it this way or I pull it. Basically, he moves
up and down is collective; if he moves it
up, which angle up all the blades in the rotor
system change simultaneously; there is no
difference, all the blades will experience
same pitch change.
Now, let us go to the this stick, this is
this control stick for cyclic, this is the
cyclic stick, I would not if he moves forward,
there is a lot more inside of this, vehicle
will fly forward please understand that is
all. How the swash plate will move, I will
say there is some ringing done will at present
we will not bother about the ringing; what
it does is? Swash plate is instead up moving
up, it will tilt; it will tilt in the sense,
the swash plate instead of up it will tilt
like this. You can now tilt it any plane please
understands right.
Let us assume, it is tilted about some axis
and it is tilted like this. Now, as the blade
goes round and round, what happens? This control
rod keeps going up and down; and when it goes
up and down, it goes up and down in one cycle.
So, we call it one per revolution. What happens?
The pitch angle changes once in a revolution.
Now, you can tilt it in this about this axis
or you can tilt it about this axis, swash
plate. So, these are the two axis tilt, you
can look.
Now, if you tilt in these two planes independently,
which mean by suitable adjustment I can tilt
along any axis you follow; because, if I can
tilt in this axis, if I can tilt about this
axis that means I can suitably combine I can
tilt like this, I can tilt like this; that
means this is I can go in this fashion, in
this fashion any fashion. So, that you called
cyclic pitch input. And it varies once in
a revolution depending on where the blade
is. Because, if you say I tilt like this,
if the control rod is here then no change,
because whereas, if my control rod is here,
if I tilt it goes down and the control rod
at the back goes up. So, depending on the
location of the basically control rod, but
we do not write control rod in pitch angle,
we write it like this.
Now, I will represent what is cycle? You take
this as your rotor disk and this is my, this
is the standard reference, this is to the
tail you are viewing the rotor disk from top.
This is my rotor omega and the rotor blade
can be in 
this is called psi, but if it is a k th first
blade which is psi 1; if it is a second blade,
you will have psi 2 like that.
So, k th blade it will be psi k. That is why
I put psi k, the azimuth location of the blade.
Now, please understand we do not represent
the azimuth location of the control rod, control
rod may be in some other location, because
of this tomorrow I will, when you come to
the lab, I will show you. But, we represent
mathematically a, my pitch angle varies cyclically
this is these two are cyclic inputs.
So, you have collective, I call them cyclic,
I am not now assigning whether it is a longitudinal
cyclic or lateral cyclic please understand;
then that requires dynamics, what do you call
it as longitudinal, what you call it as lateral?
You have to know a little bit more about the
dynamics right now they are called cyclic
inputs. And k represents the azimuth of the
k th blade.
So, now, if you have that is why this is written,
psi this is the first plate you say, 2 pi
over N, N is the number of blades in the rotor
system, number of blades. Because, if it is
the first plate this is 0 like a psi 1, psi
1 is psi that is this is; if there are two
blades, this will be pi; so, you add 90; if
three blades, it will be 120; four blades,
90 degree. So, this is the general expression
for for the location of the k th blade, azimuth.
And this is, the psi is what? That is the
angle of the first plate, but that is nothing
but, omega t right; omega is the rotor angular
velocity got it. Now, you have only these
three inputs pilot gives. Now, please understand
this is the input to main rotor, sorry because
I am now restricting this to one main rotor,
one tail rotor system.
Now, you please imagine, if you have a two
contra rotating rotors, there is no tail rotor,
if you have one rotor in front, one at the
back that is tandem; there is no tail rotor,
only thing is one is the main rotor, another
one is also look like a main rotor both are
horizontal rotors. Now, the controls as well
as the aircraft is concerned, in the helicopter
field, this is what, it is not that when you
have two rotors, pilot may give collective
pilot may give cyclic to both, but he will
have only this control.
Now, the you imagine for a, that is why mechanically
simple is the single main rotor, single tail
rotor. Because, tail rotor the pilot puts
the pedal, you will have a pedal here, he
will somewhere and he will act and that goes
to the tail rotor and tail rotor you have
only collective. So, please note, tail rotor
only collective. So, we call it theta 0 tail
rotor. Now, you see four controls, this is
for a single main rotor, single tail rotor,
three for main rotor one for tail rotor.
Now, if you have a tail I am not going to
be like this, I want to build two two rotors
let us say contra rotating one above the other.
One above the other, if I do then makes a
little tricky, you cannot represent it in
this, because there is no tail rotor. Then
you always talk about, what motion you want?
If you want to climb up, you give collective;
if you want to fly forward or sideward, you
give cyclic; but, if you want to turn your
heading, then you give yaw that is all, that
you may do by a differential mechanism between
the two rotors.
Because, if one rotor produces a little less
torque than the other rotor, because they
are all contra rotating, please understand
both do not rotate in the same direction,
when you have two rotors; whether it is a
contra rotating, one below the other or it
is side by side, or one behind the other.
Now, you imagine I say hey, what are the controls
I have to give to generate motion of the helicopter?
Collective you give then, when you give collective,
both the rotors collective pitch will increase
simultaneously, so you will go up; when you
want forward you say cyclic, you can generate
by differential collective, because one goes
up another comes down, then you will tilt
you will go. That is were the control rigging,
all the control mechanism becomes different;
whereas, if you want to yaw, yaw means I change
my heading. In the case of single tail rotor
very simple thing, change the pitch angle
of the tail rotor. So that, I increase or
decrease my force; so, as a result I will
yaw very simplistically.
Now, imagine, if you have different different
types of rotor system, in the sense rotor
configurations, helicopter configurations
helicopter configurations not rotor, whether
it is a contra rotating, one main rotor one
tail rotor or whether you have tandem configuration
or side by side configuration; please understand,
the mechanical linkages in those cases, all
the transmission becomes complex that becomes
a little difficult.
I want you to read that how they control those
helicopters configurations; ultimately you
have to do this you find they are more complex
systems. So, that is why mechanically simple
is single main rotor, single tail rotor. Now,
you understand why most of the people, those
who have the expertise in manufacturing a
particular helicopter configuration they stick
to that. Because, today of course Boeing makes
tandem that is C H 40, C H Chinook; and only
they make, it is did not that every other
fellows starts making it. Similarly that contra
rotating, the Russians Kamov helicopter there
experience and they make.
Of course, US now wants to call the same thing
by a different concept; you know this is just
is all you know they call it ABC, Advancing
Blade Concept. There is nothing fancy except
the ABC you call it, Advancing Blade. Because,
just last week I think I saw, because this
was this particular thing, it is a contra
rotating rotor system that is all; there is
nothing, only thing is you know sometimes
it is fancy to call the new name. So, that
people oh what is that, otherwise there is
no nothing special.
US started this long time back Sikorsky, but
of course they had lot of problems. Because,
they thought as we go along a program you
will learn, the advantage of something they
thought I can get the advantage from both
the rotors; but actually, the disadvantage
also comes from both the rotors. So, what
it they thought they could achieve, they could
not achieve and the project was abundant.
But, again I saw just about last week in newspaper,
oh they made this helicopter again; they are
again trying to do it, but they call it advancing
blade concept, but it is nothing but, contra
rotating rotors which the Russian Kamov has.
But, only the see that is why the technology
and the knowledge, once you gain in a particular
configuration of a helicopter, please understand
particular configuration of a helicopter,
the industry does not want to loose it. Because,
you have an advantage in manufacturing and
you know what problems you will face; and
you have address the problem, you have solve
the problems.
Even I give you just little story, but it
is not a story it is a reality. There was
a contract for building that, I think it was
apache at that time if I am right. So, that
was initially bell helicopter also tried for
that, but bell has been manufacturing two
bladed rotors initially. So, when the demand
for increased weight and other things started
coming up, there is that they again gave two
bladed. But then, when the attack version
there was some advantages, so it went to four
blades, because they did not have, then only
they started making four bladed helicopters.
So, you loose a big contract itself, if you
are not experienced in a particular configuration.
Of course, each industry has it is own specialty.
Because, apart from theoretical knowledge,
the practical knowledge of implementing, manufacturing,
proving, is much more tougher; because, any
new rotor configuration please understand
it takes more than 10 years; when they wanted
to change a metal blade to composite blade,
metal blade to only blade change, every else
is same.
They said, I will replace a in the helicopter,
current helicopter whatever is a metal blade,
I will remove it put a composite blade, that
alone took 10 years please understand just
replacement of a blade. Because, the complexities
the whole dynamic, the structure, the aero
elastic, all the problems you have to, now
it is a new set of problems.
So, even today, there is one small article
says, still people do not know they changed
some metal to composite identical, dynamically
it is a dynamically means frequencies everything
is same. But, then when they put that rotor,
you have a higher vibration, they said, still
we do not know that is all the question is;
finally we do not know why? ok. So, these
things happen I change something, you think
everything is identical; but then, when you
put there, know the measurement shows it has
a higher vibratory levels.
And that because he was an industry person,
who was Prouty which I showed one of the books
he writes small articles on it; what we call
that? That is a magazine, which is called
Vertiflite. It is an American Helicopter Society
that is a journal; this is a magazine. In
that you know small without equation, one
article will always come. So, very interesting
articles, there is a still we do not know
why.
So, there are many unknown things. So, in
this course you will stick to one main rotor,
one tail rotor configuration; and now you
know the input input is collective and cyclic.
So, three inputs go to the pilot and sorry
to the main rotor, one input goes to the tail
rotor. That is all for helicopter controls
there is nothing more, pilot gives only these
four.
And now, I will briefly go to the rotor systems.
It is a rotor system, what do we want? We
have a blade, we need to attach it to the
hub and then the blade should rotate that
is all. How do I attach that is a very simple
question; and the type of attachment determines
the type of rotor configuration. When I was
telling you the history last class, we said
that Cierva put a flap hinge that means he
deliver, it was a engineering solution for
a problem. So, he introduced a physical flap
hinge.
And what we need is, now you know that there
is a hinge, we also need to change the pitch
angle. That means, if I rigidly attach to
the hub I cannot change the pitch angle that
means I should have a pitch bearing. Now,
you see I should have a pitch bearing which
will change the pitch angle of the blade.
Now, you need these two, you must attach it
pitch bearing should be there, but you can
have a hinge you may say.
So, now let us look at the now you know what
can what you require. This is called a see-saw
or teetering this is always for a two bladed
rotor system. But, tomorrow what we are going
to see, which is a two bladed helicopter model
that is not this system that is a little different.
This is two bladed is see-saw, what is happening
is one blade here, another blade here, it
is one unit you come and hinge it at the center.
So, hinge is it is like a see-saw. If one
blade goes up, other blade goes down do not
think about operating another just normal;
I have put a hinge here, you see this is the
attachment, this is the rotor shaft I put
the hub and then attached it and I have a
pitch bearing, please understand I have a
pitch bearing. Pitch bearing is always required
yeah otherwise I cannot change the pitch angle
of the blade, is it clear.
Now, this is called the see-saw or teetering
rotor. They have only the this flap hinge,
because the blade can go like this that is
all. So, I have only a flap hinge, which is
located at the center of the hub please understand
you look at it; the flap hinge is right at
the center. This is the see-saw two bladed;
this is only for two bladed please understand
this is two bladed, four bladed they do not
make like this.
Now, you go this is another rotor system,
which is called the fully articulated rotor
system. In this, you need to have a pitch
bearing so that is here, that is what this
arrow that semicircular arrow represent I
can change the pitch angle. Then I have two
physical hinges, it has a two physical hinges,
one hinge is this that is flap, another physical
hinge is here that is lead lag. So, you have
two hinges, one bearing. Then, you may ask,
what happen to the lead lag here? There is
no, no hinge for that, two bladed rotor systems
they do not put it.
And you will say, what they will do that design
part later, because it is little bit more
you have to know to see why they make it like
that. Because, it is there is a lot of dynamics
involved in that; why you make certain decisions
the way. This you will understand as will
go along you will say oh this is the reason,
this is the reason, this is the reason this
is it.
Now, the interesting part is these hinges
are not at the hub center. Please understand,
they are not at the hub center, they are away
from the center of the hub. So, there is a
offset, so you call it a hinge offset.
That is you have a, this is like this, you
have you put a hinge and the blade goes. So,
this distance we call it the symbol I am using
is the same throughout the course I will not
change, today I write one symbol and tomorrow
another symbol for the same quantity, e is
hinge offset. Now, this is for flap motion,
flap is that is why I plotted, this is the
plane of rotation and the blade is can go
like this.
And I have put deliberately the offset here,
because physically I cannot put when I have
four blades I cannot do it or four or three
blades; because, you have to attach them,
you needs some geometric you have to have
some space. So, it is tilted, this is called
fully articulated rotor system. All the blades,
all the helicopters were built like this;
even the attack helicopters you go to Russian
helicopters everything till 80’s. But of
course, some slight change will be there are
like this.
But, now you know that, when you have a physical
hinge, the blade is going up and down, what
will happen you have a problems, maintenance,
number of parts everything is high. So, now
you see it is from a operational point of
a view. You find that articulated rotor systems
a good I can attach to everything, but maintenance
is is you have to replace parts, replace parts
means you have to bring down the whole unit
and then remove it, put it annually make it
operational.
So, from a design point of view, number of
parts you have to lubricate several places.
I will show later one picture, then you will
get an idea that I we should remove the hinges
that was everybody will say, why you put a
hinge, I remove a hinge. That means I take
the blade simply bolt it, it is good. But
then, that is what the hingeless that means
there is no hinge right.
I said it easily, just take the blade bolt
it, it is easily said then done. Because,
if I attach it I must allow flat motion of
the blade for flying, because what do I do?
When I have a rotor like this, if I want to
fly forward I am tilting the rotor disc. How
do I tilt the rotor disc? The shaft is straight,
please understand really I showed that the
shaft is always straight, but the rotor disc
is tilted; that means, when the blade comes
here it goes up; when it comes to the front,
it is down. That means I want the blade to
go up and down in a prescribed way; that means
I want flap motion.
Flap is completely restricted like a propeller,
you have a propeller blade is there in aircraft
it is very rigid you attach it, bolt it. Then
you have this is rotating, if you want to
fly forward what? It will not do this, you
have to tilt the entire shaft; in helicopter,
the shaft is not tilted only the rotor disc
is tilted; that means rotor disc tilt happens,
because of flapping motion of the blade. So,
the flap dynamics, now you understand flap
dynamics means the motion of the blade in
out of plane of rotation is important, only
by controlling that, the helicopter is flying.
Now, you say I make get hingeless. Hinge is
kept to relive the moment, because he said
that, if I have a movement I do not want moment,
but I want the motion; moment should be that
is a good solution, but then this has it problem.
Now, you come here you do not have anything,
but of course pitch is there, pitch bearing
is required, but how you design? That is why
I said, when the technology for manufacturing
change from metal to composite. Please understand
metal to composite, when we changed, then
only the realization of a hingeless blade
became possible; because by putting layers
of the composite cloth you may call it or
prepaid, you can adjust the stiffness of this
particular region, I call it this region please
understand this region.
What I do is? I do not put a hinge, but I
will deliberately make it 
is flexible there, the blade is made to be
flexible at some zone, and it is a virtual
hinge. This is like a, you want to be the
blade to behave like this, but there is no
hinge, you follow; that mean the manufacturing
you have to, how do you design the blade such
that there is a flexibility at the near the
root, because this is a root section near
the root, but it would have sufficient strength
also, because centrifugal force is pulling.
So, the strength and the flexibility the stiffness,
so by tailoring by adjusting the layer, they
manufacture. Today of course, this is quite
common very common; hingeless rotor plates
main rotor please understand, now I am saying
main rotor blades; even Indian that advanced
like helicopter that is this composite.
Now, you see once this was first started in
Germany, they made this helicopter with a
composite. Actually, no company would like
to be left behind in technology. So, because
they see the potential of course, composite
has its own problems its own advantages, we
will not get into all those issue. Because,
if we go that will be another story of its
own. What we say is, with the technology development
in composites, the realization of a hingeless
blade was made possible.
And today companies make hingeless blades.
ALH blade is a hingeless that is advanced
Indian helicopter blade that is a hingeless
composite rotor. That means the manufacturing
technology you have to know, the design technology,
the design everything you must know. That
is why, today this common, composite rotor
blade. But, still you have a now you see you
have a simplified the rotor system by removing
the hinges, but still you have a pitch bearing.
Pitch bearing is also a bearing, now can we
remove that. That is the next level of rotor
system, which is called the bearingless rotor
system; I remove that bearing also you got
it. Now, when I remove the bearing then how
do I change the pitch? This is a very very
primitive schematic; I have a, this is called
a flex beam. So, this is called a flex beam,
which is a flexible beam in torsion it should
be flexible in bending also. Bending means
please understand this blade can bend this
way, this blade can bend this way lead lag
and it should be flexible in torsion; and
that was made 80’s.
Of course, the concept people at even 1960’s
please understand. I can have a concept, but
then to make it a reality, it may take 50
years, may be you would have written something
and that is all. But, somebody else will keep
on working working working, now this is the
flex beam. And then I will show another picture
which is today you they manufacture this blades,
but only tail rotor blades.
Tail rotor blades are this; main rotor blade
still this is what is used, only one helicopters
tested, only one helicopter, lot of complexities.
Is a German I think, what is a D G 135 or
some helicopter I do not know that numberively;
they manufactured, they tested flight tested
few years back earlier, they were flying they.
Because, now you see I have made the rotor
system simple, but the dynamics the manufacturing
all other problems.
So, maintenance people it is easy, but they
say it is a nightmare for the designer; because,
designers do not understand many things. Please
understand you may say, I want to do it, I
will put the new things and then it will work;
industry has to mature to take that concept
and then say, they will not immediately buy
it. Because in practical implementation, some
concepts may not even realizable with today’s
technology, and the knowledge.
So, one may have to wait for another 50 years
to again start. But, these are the rotor configure.
Sometimes, people may call it this rotor as
a rigid rotor rigid, but it is not really
rigid it just for some reference. But, I would
not call any rotor as a rigid rotor, because
all rotors are flexible. So, for clarity of
distinction, you call them as articulated,
hingeless, bearing less is it clear. Now,
I will just show the real helicopter hub to
the schematic; so that, you will have an idea.
This is the articulated rotor hub, actual
helicopter more than 450 parts and about 20
lubrication points; this is the schematic
for that. Then hingeless rotor, much much
it looks cleaner and simpler, it look simpler.
But, the design designer say nightmare it
is much much difficult, because later will
come to that it much simpler.
And this has 150 parts and may be 1 lubrication;
lubrication is basically they put some damper
we will go to that later. To this, this is
the hingeless, I will explain briefly. See
there is a caller, the blade is here, this
is the flex beam which is a composite material;
what you do is? You twist this beam by pull
the lever up and down this is connected to
this outer sleeve, so there is a sleeve. What
do you do is, this is the inner flex beam
which is bolted to the hub; this is like a
sleeve, the sleeve goes and joins.
So, it will be like this, if you look at it
and here you have that beam will be going,
this will come and join. This is the outer,
this is called the torque tube, this is called
the flex beam and this is the blade. Now,
what you do is, you have a lever attached
to this, you move the lever up and down; that
means this will twist, when it twist? It will
twist at this end of this flex beam and then
the blade which angle will change.
But, now this is the several path load path,
the blade as one control rod, this is the
path that what will always be there, and this
is the flex beam. And this should be it should
take the centrifugal load it is flexible enough
to flap, lead lag then twist. So, manufacturing
is much much tougher, because you have to
know the dynamics and then properly design
the entire section.
Now, you see this has the number of parts,
it is less than 50 and there is no lubrication
for this. So, you see the complexity of articulated
in the mechanical side to the very compact
clean system of the bearing less. Now, all
this things took several years, actually several
decades.
Now, you will find, if a company is experienced
in manufacturing this, they will stick to
it, because it is not easy from here to here.
This may again require 10-15 years of developmental
work and testing work to replace. That is
why they will not everybody is making let
us also make, no it is not like that. That
means it is your life time, please understand
in aerospace technology it is a life time
of an individual to concentrate working on
that to achieve.
When you start at 25, you will say, this is
what I want; may be when you are 50 you may
realize that is what now I have made. It is
the that period is very long, it is not like
every 6 months I will have a new design coming
up new product, no. It may be a generation
before a new product comes or there may not
be any new product, only improvements in the
technology that changes. So, this is how the
whole thing went yeah.
No, there is a fatigue damage see fatigue
that is always there yeah. Suppose, you say
hingeless blade you wanted to flap up and
down right; that means, what? You are actually,
suppose you take a pin you want to break it,
what you do? You keep doing like this and
then it will break that is the fatigue. Here
fatigue is very important, but you do not
want the blade to break, which means what?
You have to design such that, the fatigue
life is what is man dated, what is the required;
because, I do not know 5000 hours the blade
requirement 5000 hours, although blade life
after that through it.
Every component may have some life through
it out, because I was reading some material
now that is very interesting thing. The pitch
link rod, what we showed that pitch link,
this rod the rod which goes from blade to
the hub; that gets all the load, the pitch
pitching movement comes as a axial load on
that member. Now, you can make it very bulky
heavy, so that you will give infinite life,
but they do not want infinite life. So, what
you do is? You reduce the weight, but life
also goes down, you say so many after that
remove it.
Now, in the Indian air force this is one of
the main things. You know we operate a Russian
helicopters plenty air force operates, they
all have metal blades and there is this life
after that, the blade has to be removed. The
blades looks beautiful, there is no visible
damage nothing everything is wonderful, but
the blade has to be removed; because, the
two new blades have to be procured. I think
the two new blades whether it is 75 lakhs,
now I do not know blade each blade is 1 crore
or pair is 1 crore it is about a crore.
Now, aircraft was we have to keep on import,
they have large of blades. So, the whole question
is, can you tell us that I can operate at
least another 10 percent of that life, life
which was given. The manufacturer of the blade
will say no, because he can sell more blades
right. But, the operator will always say 8,
you know I want to extract as much as possible,
but without endangering. Because, if anything
happened, he will only blame the operator,
he will say that, say I told you do not operate
but you operated.
Now, what is the scientific way of improving?
Because, those blades we have done manufactured
by us. Now, there is a program, which is called
a life extension. So, they did lot of test,
they have given some extension of 10 percent
life or something like that. So, these are
problems. That is why all this things, there
is a 5000 hours say I will not give some of
things which are actually you know whatever
little I know from the industry thing. Something
may be its good, actually as a student is
fantastic to know, hey man this is all really
amazing, there are problems lot of problems.
So, on one hand somebody will say, I buy is
like your bicycle you buy your bicycle you
know my father had even I came to me. But,
aircraft is not like that, they will say you
use it after that through 5000 hours; 5000
hours means whether it is a 5000 hours of
operation or they will also specify calendar
year after that you through. Suppose, if you
do not use, it also you throw it these are
these are the money.
Now, you know that, if you make a flying vehicle
actually you dictate the economy to a large
extend in that sector. Because, it is all
high cost items nobody will tell you, how
they make it? They will say, we have put all
our money, effort, everything to come to this.
That is why I said, to come to this blade
composite blade, the company took 10 years;
10 years mean, they are not going to come
and tell you easily. They will if they tell
they will charge you lot of money, enormous
amount of money they will charge.
So, these are reality. But, what we learn
in this course is of course, some practical
thing interesting aspects, because that is
to give some, what is that? Interest to the
course; otherwise the mathematics involved
is really trouble in helicopters. Nobody would
like to work in helicopters, because equations
are very very long; very long means one term
may run to 30 pages.
Then you will say, what are you sure you were
deriving it right? You know, because that
is the first question will come you write
keep on writing; what is that, is it right
or wrong? Right or wrong determine only finally,
when somebody does an experiment, you say
is it close that is what. And that is the
reality you will learn you will see. But,
what we I will introduce here is very very
at a low level.
Now, there are some interesting points in
the helicopter thing. One one into there are
several interesting things, one of them I
just want to show this is something which
normally does not happen in aircraft, that
is why I am putting this. This is the rotor
disk, if it is in hover, hover means the helicopter
with stationary, wind is not blowing you are
just above.
Omega r is the velocity of the blade and that
is equivalent to relative air is omega r at
any section; it is coming radially that is
normal to the leading edge you can say. But,
the movement helicopter starts flying forward
that is here, what happened is? This is the
forward speed of the helicopter please understand
that is nothing to do with the rotational
speed of the rotor rotational speed of the
rotor.
So, we divide this whole region into two parts.
We will get to the math later, right now only
explanation I am giving just. What happens
is, on the this half where it is rotating
it is going into the air, the relative air
is actually increased; but, please understand
it is not that, because it is going at different
different azimuth locations. This V has a
fixed re direction; omega r has depending
on the azimuth location, velocity is different.
But, when you go to the this half of this,
rotor disk actually this is called; you divide
the disk into two parts, if this is omega,
this is V you call it advancing side. We we
will do this again when we do, this is called
retreating side; advancing side, retreating
side, it is two half. But, in the retreating
side what happens? The blade looks like this
please understand. This is the leading edge,
this is the trailing edge, this is the velocity
V coming and this is my omega r.
Now, if V is greater than the omega r that
means the flow is coming from trailing edge
to the leading edge. And this is this happens
in a region and that region depends on what
speed you are flying, we will derive that
in the course later, how to get this. So,
it is very simple geometry, there is nothing
more.
And this region is called the reverse flow
region that means the flow is coming from
the trailing edge to the leading edge. Whereas,
in aircraft that there is nothing like a flow
is from the trailing edge to the leading edge,
always it is from leading edge, trailing edge.
Now, that is why in aerofoil’s for helicopters,
when they get the C l, C d, C m, actually
they go through full. They plot that, C l
verses alpha, alpha going to 360 degrees that
means you do one full loop that means flow
can come in any direction. And of course,
earlier you can neglect it in a sense ok and
there is because there is a hub, you have
to know, what is the size of the region. These
things we will learn later.
And some they may not be very effective sometimes
and you have to know the aerofoil characteristic
in the sense in terms of C l, C d etcetera,
when the flow is coming from trailing edge
to the leading edge; that means you have to
do an internal test. But, these are all lot
of aerodynamic characteristics of the aerofoil
they are required, when you are actually make
into.
But, when we use it when we actually design
most of our analysis many things, we still
take the 2 d leading edge to trailing edge
only. But, the approximation we may make we
may neglect that region, sometimes you may
take the only the drag effect or lift view
put it opposite you do anything; these are
left your own ingenuity. But, they restrict
the maximum speed the helicopter can fly;
but, this is not the only region please understand
this is one of the reasons, because there
are several reasons which restrict the maximum
speed a helicopter can achieve.
Because, there are only one helicopter which
I think 400 kilometers are something like
that that is all that is the maximum speed
it has gone. That is only as a record; otherwise,
most of them fly around 250, 280, 280 is a
very high speed 280 kilometers per hour that
is the high speed all right. But, one went
around 400 something; because what will happen
is this this is this region? If you keep on
increasing V what will happen this omega,
this region is going to be become bigger and
bigger and bigger and this side will become
in effective.
That is why when now I am going a little of,
when you study helicopters there is you cannot
say I fly at any speed. There is a non dimensional
number you will learn it is only below this
you can fly. But, if you want to analyze theoretically
you can take that non dimensional any value,
but it has no meaning as were as the helicopter,
as a vehicle is concerned. That is why sometimes,
even your research as a parametric study you
can always say I will vary this parameter
from 1 to 2, 2 to 3, 3 to 4 anything; but
in reality, in the helicopters even, because
there are some equations which I will introduce
you later. It is an interesting equation,
because which you will come across normally
is in your course, just the flap dynamics
and forward flight that is the equation.
And you will find for curiosity, you can change
the forward speed to change to any value just
to see how the solution looks; how the flap,
not the how the helicopter blade flaps, how
the equation gives the solution. That is interesting,
but in reality you do not go to that region.
Because, if you say when you write an article
or anything you say oh this is useful for
helicopters, no it is not useful for helicopters.
I saw I know some professor says, what he
is talking about, he is talking some rubbish;
what value he is taking? Because, that value
is meaningless as far as the helicopters are
concerned. So, we will also in this course,
you find some and they will say, what range
there are real helicopters operate, because
that is very very important; because, it is
not that any value you can take and then starting
putting it.
Aircraft you can say subsonic, transonic,
supersonic or hypersonic, you can take anything,
here no. There is no supersonic, but transonic
some regions are there that is in the blade,
may be some sonic will come. But, the helicopter
will not fly at supersonic speed or anything
like that. If you want to make then we have
to think how to make that is a different issue.
So, our speeds are highly restricted right.
So, please note that, we do not fly at any
speed, there is a restriction.
