good morning friends the last lecture we were
discussing about gross weight and then importance
of aerofoil in a generic manner we also discussed
about the speeds required for lets say ensuring
maximum rate of climb or under fly at a minimum
power and we have trying to discuss in a generic
manner how the aerofoil will play an important
role
we need not debate much on this point regarding
the importance of aerofoil because after all
lift which has to be equal to weight for the
cruise is given as half rho v square s c l
this is a dynamic pressure this is the wing
area and this is the c l what we are talking
about also we know as a designer we should
not only thing in terms of c l it should thing
in terms of c l by c d because c l will also
induce drag which we know is called induced
drag so when i am selecting an aerofoil i
should not only take in terms of c l but also
that you see what is the c l by c d maximum
i get and under what conditions and at under
what speed
so if i try to summarize as far as airfoil
selection is concerned loosely i can say i
need to know what is the cruise speed i am
going to fly the machine i need to know takeoff
and landing distance required and also stall
speed i will explain all these term then handling
qualities and aerodynamic efficiency 
why cruise speed should be back of our mind
in selecting the aerofoil because finally
the designer i want that should fly with drag
minimum also i should know that if the cruise
speed a low subsonic then there is a drag
characteristics which is differently when
i try to design an airplane for high subsonic
speeds on a near transonic speed or near supersonic
speed that is how the aerofoil shape is likely
to be different
for example for a supersonic speed you want
the nose part to as pointed as possible provided
you can manage it structurally right takeoff
and landing why this is important because
you know takeoff and landing that speed takeoff
or landing roughly is one point two times
v stall ok and v stall is two w by s by rho
c l max if i want v takeoff and v landing
to be lower then i should ensure the v stall
to be lower during takeoff and landing what
indirectly means is for other thing will constant
c l max should be high the c l max could be
high could be possible if you select any aerofoil
instead of this if you select an aerofoil
your c l max is high
so for the beginning you have a aerofoil selected
which you will have c l max high but you understand
the moment i look for c l max high i am mentally
prepared to be penalty in terms of drag even
the stall angle may reduce so to overcome
that you will find people use flaps we look
we will come to that so this is basic aerofoil
and then you put the flap flap down that is
if this is a basic aerofoil i put flap down
some degrees ten twenty thirty degrees and
ensure that the c l max is enhanced so that
is where when we have selecting an aerofoil
i also keep in mind takeoff a landing distance
because if v takeoff when v landing distance
ah speeds are less and for a given thrust
to weight ratio the takeoff and landing distance
also will be less right stall speed you understand
when i am selecting an aerofoil i would like
to know what is the v stall i will always
like v stall to be as low as possible but
it should not be lesser than the amount it
should not be so less that it becomes too
sensitive to wind and and the environment
conditions but yes stall speed depends on
c l max inversely so when i select an aerofoil
i try to see c l max to be as high as possible
handling qualities via handling qualities
because the performance of the airplane near
the stall is a very very important parameter
for the pilot to have in smooth handling or
better a good handling qualities
so again in handling qualities you find somehow
the stall angle this angle plays an important
role so i need to look for an aerofoil which
generally has higher stall angle and of course
aerodynamic efficiency which we have talked
about i will look for in aerofoil where c
l by c d max is what i look for it should
be higher one maybe thirteen fourteen fifteen
within stall much much higher right if this
is from the aerofoil side what i look for
in a design when i translate this aerofoil
into wing right there is a another important
parameter which you should ah clearly understand
with a direct implication
if i draw an aerofoil this distance you could
find thickness to chord ratio this also plays
an important role if you see that if t by
c is large then the flow will accelerate here
faster than aerofoil having t by c less the
t by c large gives me an advantage for a given
chord i got larger volume in the wing where
i can store fuel i can store other components
but the point is as i increase t by c the
drag may also increase so i have to be careful
if i increase t by c the end critical may
reduce so if i increase t by c these are all
generic statement if the if the t by c m critical
may reduce so t by c will play the important
role
and as a designer without thinking of big
big aerodynamics if you see if this t by c
is large and the flow accelerate faster than
decelerates then there could be a region where
there is a high adverse pressure gradient
and flow may separate in a manner which you
dont like the flow may not exist as a laminar
flow if you are designing a laminar flow aerofoil
the flow will become turbulent sooner than
you are desiring so t by c also will play
an important role now you can understand when
i am generating data for different types of
aerofoil which you have to use the database
we used to call naca aerofoil nowadays of
course you have a customized aerofoil
but if you see when the data were generated
data will be generated in such a manner that
it will give you thickness to chord distribution
the leading edge radius it will tell what
is the ah speed for which c l by c d is maximum
what will be c l max i can achieve all those
parameters will be given as a parameters to
select which aerofoil you want so i will just
go back to the naca aerofoil ah era and explain
you some nomenclature to understand ah what
is the aerofoil and what are its special features
so that we can select aerofoil based on those
nomenclature
before i go for the nomenclature which is
purely mechanical thing but where the designers
should know how important it is for us to
understand the database via those nomenclature
before i go to that let me also explain you
if you recall most of the drag polar c l and
c d of an aerofoil take bits out of a shape
and we say this is the minimum c d now the
point is we will find latest aerofoil which
have been used their shape not only looks
like this it may be something like this then
rows like this and there is a bucket oh sorry
there is a bucket this is a bucket formation
typically this sort of aerofoil where c l
by c d shows a bucket recall it laminar bucket
then the importance is this if you see in
this region even if there the increase in
c l there is hardly an increase in c d but
if you see the other type of aerofoil where
it is something like this and this is c l
and this is c d if there is increase in c
l the c d will go on increasing right here
it is this this this so the designer i will
prefer a laminar bucket type aerofoil so that
even if i am changing c l because of different
different operational requirement i do not
give too much of drag penalty but please understand
this is typically a laminar aerofoil and nomenclature
wise will find there are six c d aerofoil
mostly
but there is an issue with laminar bucket
type of aerofoil is that if there is a slight
imperfection in the construction even if i
you find some insect material dead material
is there the flow immediately becomes turbulent
right but there is a trend that mostly people
will be using six series or derivative of
six series or customized six series aerofoil
which for seven series x series all those
things the idea would be what is the most
radius i am looking for what is the point
where i am finding out that the c p pressure
coefficient is becoming zero it is not necessary
that pressure coefficient is zero only at
t by c maximum location because c p is zero
ah pressure coefficient zero is the important
parameter because then i know where from the
adverse pressure gradient will start acting
or pressure gradient variation how it will
act which will be responsible for ah flow
separation right
with this background let us come back to that
nomenclature part i will not talk about all
the nomenclatures available but few one and
it expected that you read books and really
ah understand why these nomenclatures and
how much a designer can take information from
those nomenclature i am adding few more designers
perspective before you go for the nomenclature
especially ah to get a feel how thickness
ratio will implicate in terms of flow separation
on an aerofoil in a very generic manner and
i am referring a text from ah ramos book you
can read ramos book
so i give a heading as stall and i say important
role you will find some aerofoil will show
gradual reduction in lift and some aerofoil
will show violent loss of lift and rapid change
in pitching moment 
for example if you take cambered aerofoil
wings this is a cambered aerofoil wing if
you see this diagram if c l max will be more
but it will have large c m a c negative that
is if i just throw it that will turns in to
go down like this right so there in ramos
perception he has explained three types of
stalls very wonderful summary of he explains
and i thought it must be shared
one he talks about fat aerofoil that is rounded
what is the characteristics rounded leading
edge that is if i draw an aerofoil this leading
edge is rounded i get and t by c thickness
to chord ratio is greater than equal to fourteen
percent it may be cursing me why suddenly
he is talking about t by c what is t by c
ratio so i have not mentioned anything about
how aerofoil looks like what are their chambered
line how to select how to choose and how to
see when a cambered line so let me do that
for you which i presume you are all knowing
this because you have done already two process
this is a leading edge and the trailing edge
ok and this distance (Refer Time: 18:00 is
called chord right and if i take every section
like this in the middle part and this will
give me the camber line
this is the camber line 
and this is the thickness of that i say t
by c ratio or thickness with chord so what
is camber then if i for completion i will
write then explain you what is camber camber
i am in particular to write exactly what is
we accepted by everyone but you could see
that so if you have commonsense you can find
out what is camber i need not all these follow
this definition so is the maximum distance
between the mean camber line and the chord
line this is important measured perpendicular
to the chord line this is important the here
which are the chord line is horizontal here
so we are talking about the maximum distance
between the main camber line civil camber
line and the chord line chord lined measured
perpendicular to the chord line so that is
what is the camber that is this is the chord
line this is the camber this is the camber
this is the camber this is the camber ok it
is possible that the chord line is something
like this and mean camber line is something
like this and i take the mean position so
that time i will say the camber i have to
draw a perpendicular perpendicular the chord
line so that will become the camber right
this would be clear in your mind ok
then leading edge radius so i was talking
here mostly we will find this is point zero
times c bar or c bar r this is a c bar chord
two percent of chord around that so now you
know this nomenclature now i feel ok to talk
about a mechanical name that nomenclature
and start with four digit nomenclature this
is naca lets say naca two four one two 
what is the information you will get from
here is that is maximum camber maximum camber
is point zero two c 
this is this one
what is this four signifies that location
of location of maximum camber is point four
c from leading edge right c is the chord and
this twelve that is t by c maximum is zero
point one two c [o/or] or t by c is twelve
percent t by c max is twelve percent so once
you see a naca four digit nomenclature immediately
from this number you could easily find out
what are the information you are getting after
four digit nomenclature now we are discussing
about five digit nomenclature you will find
naca two three zero one two how to read that
the guideline or the direction is how to read
this is the three by two multiply by first
digits will give you design c l in tenth
three by two first digit is two so three it
will be point three what is the next two digit
how do i take information from that you say
next two digits divided by two is that gives
me the location of maximum camber along the
chord line from leading edge in hundredth
of the chord right is not a big philosophical
statement right if you just follow it let
us see if i try to understand this naca two
three zero one two what other information
we get lets say five digit is naca two three
zero one two and for the first statement three
by two into two because it is two is the first
digit this gives me three but in tenth it
is telling so point three is the design c
l right
second one is next two digit divided by two
next two digit is thirty so thirty divided
by two will be fifteen but it [sta/says] maximum
camber along chord line from leading it in
hundredth of the chord so fifteen divided
by hundred is point one five c so [is/it]
tells me that if i move from the leading edge
here and at point one five c the camber will
be maximum and of course last two digit tells
me t by c maximum which is twelve percent
right which is also we say twelve percent
divide sequence so this is what an information
we get from a nomenclature which is five digit
nomenclature after five digit nomenclature
or five series nomenclature we are talking
about six series nomenclature the six series
when you talk about you should be mentally
prepared that we are now going into a laminar
type of aerofoil ok i just give an example
naca six four one two one two at a equal to
point six what does it mean the six is a series
designation for the nomenclature and if you
understand that for a six aerofoil has been
designed to maximize the region over which
the flow is laminar advantage the drag over
small region of lift coefficient can be substantially
reduced because you know laminar will give
in this drag but i i have been telling you
uh the word of caution the manufacture will
becomes a challenge on a very particular and
maintenance of the surface of the wing is
also very very important in fact if i take
cleanliness of the surface that is also a
challenge on one has to a particular about
it the slight ah impurities asks foreign limit
sitting on the wing may turn the flowing to
a turbulent flowing right so that was all
about six
now the next one is four four tells you location
of minimum pressure in tenth of chord that
is if this is the wing airfoil and as you
know there will be accelerations of the low
pressure drop and then will be a point where
which correspond to the minimum pressure point
and that is being explained here that at point
four c from the leading edge will have a point
where the pressure coefficient will be minimum
what is that one indicates one indicates that
low drag is maintained at a lift coefficient
equal to point one above and below that design
lift coefficient point two where from this
point two came this is from here the next
is two so point two is the design lift coefficient
you know from here and this one tells you
that point one or above by point one below
point two we will have the drag coefficient
or the drag lower correct
so we say indicates that low drag is maintained
as well as i was mentioning for a six series
or a laminate of aerofoil there is a laminar
bucket right that is being talked about this
will if i further clarify this ok and this
is your laminar bucket ok if this is the design
c l with point one i hope of point one down
will have low drag configuration right so
this is also important because you will as
a designing to know all the time you are not
going to fly a design c l so what i goes of
conditions and whether your [a/aerofoil] is
optimally selected or not so that helps you
in this t by c is twelve here is the max is
twelve percent and also a equal to point six
this is also additional information you get
from a six series nomenclature
it tells you sixty percent of the chord over
sixty percent of the cord this is over sixty
percent of the chord the pressure distribution
is uniform goes like is not that sharply is
not falling ok so it will be over sixty percent
of the chord the pressure distribution will
be uniform that is also important when you
talk about minimizing drag or flow separation
stall and all those things we will be very
clear when i talk about supercritical airfoil
these are the steps towards ah you know defining
a supercritical airfoil as well you see that
this is very important a equal to point six
sixty percent of chord distribution is uniform
i started with ramons perception about aerofoil
as a designer i started some point then i
went back to remembering this ah i am again
coming back and please appreciate what a designer
should have
all these nomenclatures are fine theory is
fine but as a designer how do you perceive
things he characterized the aerofoil as fat
aerofoil thin aerofoil very thin aerofoil
so fat aerofoil what is his message to us
is typically aerofoil rounded leading edge
and t by c greater than equal to fourteen
percent and what we will see such aerofoil
near the stall it is important we are discussing
about stall and how an aerofoil should have
ah characteristics near the stall because
that is going to decide the handling qualities
which is very important design parameter says
for a fat aerofoil so your rounded leading
edge t by c greater than fourteen percent
such aerofoil stalls from the trailing edge
then turbulent boundary layer falls
boundary becomes very turbulent as angle of
attack increases and the flow separation starts
from trailing edge and moves towards leading
it as alpha is increased the loss of lift
is gradual this is very important and pitching
moment changes are small understand this loss
of lift is gradual and pitching moment changes
is small changes it starts stalling from the
trailing edge ok and boundary layer becomes
turbulent as you increase the angle of attack
which has direct implication on the drag point
right
if i try to draw this if this is an aerofoil
to a t by c is greater than fourteen percent
if i translate those observation it says stalls
from the trailing edge so separation is stalls
from the trailing edge as i increase angle
of attack this stalling or the separation
moves towards the leading edge and finally
hold flow separates and in that process loss
of lift will be gradual and pitching moment
changes will be smaller this is the perception
of ramon when he talks about fact which are
rounded in the leading in aerofoil with t
by c greater than fourteen percent
if you are designer you will find how useful
are these observation similarly he has talked
about thinner aerofoil let me talk about thinner
aerofoil after that fat aerofoil let us also
see the thinner aerofoil as far as ramons
perception is typically the t by c between
six to fourteen percent around nine ten will
be ok we talked about thinner airfoil and
those are as a design ah the designers language
we call it moderate thickness what is a moderate
thickness means that to nine percent ten percent
like that the moment that going be on [towel/twelve]
to a caution is no more a moderate fitness
what is the beauty of those aerofoil the flow
separates near the nose at a very small angle
of attack very small angle of attack flow
separate but at small angle we have separates
but reattaches but as we will increase the
angle of attack there is no reattachment and
suddenly the flow stalls or the flow separates
and there is abrupt change in c l and pitching
moment thats what one designer should be very
very clear if we have working with this type
of aerofoil depending upon what flight regime
you are flying what angle of attack you are
flying so typically if i repeat that small
angle of attack then we separation but it
reattaches but as angle of attack is increased
there is no need of attachment and suddenly
you will find the hole flow will separate
very abruptly the flow separates but this
happens from leading edge to trailing edge
this is important the flow separation starts
from leading edge to trailing edge unlike
for fat aerofoil it was from trailing edge
to leading edge so the lift loss was gradual
but here it starts from leading edge so naturally
the leading edge portion is the portion where
the leave generation is most ah efficient
if that part stalls the naturally the lift
stall will be very very abrupt and naturally
when the abrupt change in lift coefficient
and pitching movement coefficient
so that is the story when you are working
for a moderate thickness aerofoil which your
nomenclature as thinner aerofoil then last
one which i want to share with you is a very
thin aerofoil very thin aerofoil again it
is ramons perception so i am says flow separates
from the nose at small angle at small alpha
and reattaches immediately this is fine if
i say separation bubble 
stretches towards trailing edge as alpha increased
if i try to um see the similarity between
what we talking about thinner and very thin
airfoil here you will absorb loss of lift
is smooth but large changes in pitching moment
that is the difference is very thin aerofoil
three percent four percent two by c if i close
separate from nose at small angle of attack
and reattaches same separation bubble stretches
so it is something like this if this is thin
there is a separation bubble now as i am give
angle of attack the separation bubble goes
over this and then further if i increase an
[an/angle] of attack this whole supreme bubble
abreacts
movement is from leading edge the trailing
edge so there is a separation bubble stretches
towards the trailing edge as alpha will increased
and there is a separation here observation
is loss of lift it smooth but large change
in pitching moment so this is what the perception
of our knowledge base shared by ramon as a
designer you need to see this re service the
aerodynamic characteristics of different aerofoil
right and you sit within aero dynamists and
talk to him what does it mean how do i customize
my aerofoil so that this thing doesnt happen
or these things should happen right so that
is why there is a constant introduction with
aero dynamists where a designer looks for
the particular type of performance
for example if he is designing an airplane
or the unmanned aerial vehicle of around class
six seven hundred kg wait speed is not high
so it is proud to experience large angle of
attack if there is a vertical aboard guest
so designer will ask the aero dynamists can
you give me an aerofoil which has a very good
stall characteristics designer may not be
satisfied with whatever aerofoil are available
even customized aerofoil ah characteristics
the designers may not be happy about it there
is a limit to which an aerofoil shape can
be configured
but then designer will not sit idle that is
what i am telling you what a designer how
a designer at different from and persons will
do analysis even designer what it will do
ok this is the maximum stall angle you are
able to give to this aerofoil so i will put
a split aerofoil here the split aerofoil that
here on as use that so what will happen if
is this was not there this was having stall
angle lets say twelve thirteen degrees the
moment you put a split aerofoil then if you
see that the air goes from here and comes
out like this and it pushes the stall the
stall angle increases right
the important thing is how do you contour
this part so that the air coming we should
not go like it should come and follow the
path that is where the million dollar design
winter testing c f d s is set a play ground
right so whatever i am trying to give you
i am sharing few knowledge base of designers
few from analysts who are analytical person
and also trying to give you example ah how
you are not an aero dynamist you are not a
structural man you are a designer right and
you have to be smart just being bright will
not make a good designer a designer has to
be sufficiently bright and more importantly
he should be smart as well are you prepared
for that or not we will learn from this courses
and i can guarantee you that we will become
all smarter
thank you very much
