Lecture 08    Basic Aircraft Design
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Lecture 08 Basic Aircraft Design

August 28, 2019

good morning friends ah im really happy that
this course is being offered primarily for my friends who are involved in maintenance
you all will understand that ah how important it is to keep an aircraft air worthy you must
be knowing me if you are attending performance and stability control course offered by britain
moocs i am professor ghosh i am heading aerospace department iit kanpur and i have a long association
with flight laboratory where we have got so many aircraft i will
do experiments for teaching and research purpose the basic question is when my student or me
ah sits in the aircraft they need to feel assured that best has been done as far as
maintenance is concerned so that airplane is not only air worthy it is safe and comfortable
so who is the task force it is the maintenance team my chief engineer mister vipul mathur
is taking primarily this course with an aim that when you go for license exams you are
well prepared on the seriousness of the maintenance protocols why i am here i understand maintenance becomes
a manual driven activity although i do not agree completely but its a fun its highly
enjoyable if you understand why we are doing why they are following those instruction given
in the manual there is a purpose to that and it has been evolved after serious considerations
where a large time period lot of testings in doing that there is a possibility that
we forget or we do not pay attention on the basic question how does an aircraft fly so to bridge that gap i am taking module which
will ah give some sort of an understanding for you how does an aircraft fly if we know
how does an aircraft fly is the next question comes how do we ensure that i should be able
to maintain that status of an airplane where it can fly safely i can maintain the status
where the passenger the pilot feels comfortable thats biggest role of an maintenance engineer so if i ask this question to myself how does
an aircraft fly i will simply start thinking if i throw a stone like this because of gravity
it has a natural tendency to come down but when i am talking about any aircraft i want
it should go up and go certain distance and when i desire it should come down so this
is different from just throwing a stone and if you see now here if i want this stone to
go go up once i have given a velocity v if i want that it should not just come like
this that means the moment it tries to come down i need to have a force upward so if i
draw it like this this is the gravity mg gravity force i need to generate a force f here we
should cancel this pull of the earth so that it can go straight this simple thinking the
question is how can i generate this force and that is what the concept of a fixed wing
aircraft and you all know who generates that force
and that is wing if you see a wing it will be something like this this is aerofoil and
if i draw it like this this is typically cro a wing where at each
cross section you have an aerofoil typical shape which is primarily to accelerate the
lift generation what is lift let us see if i take a plate like this and if i somehow
move it with speed v how can i move it with speed v lets say i
put an engine engine gives a thrust so i can always go the forward direction if i do this
like this then because of reactions offered by the resistance by air it will have a normal
force n right now if i resolve this normal force one in the opposite direction of v and
one perpendicular to the v with the component which is perpendicular to the v or the speed
is called lift because it is lifting the body and this opposite
which is trying to reduce the speed we call it drag as and lift and drag are two important
things the primary lift i get from the wing larger the area more the lift larger the speed
more the lift better this contours more the lift so if it is better this contour that
means once you fix a contour of an aerofoil a maintenance engineer will always inspect
that the contour is maintained or not over a time is there any possibility that
contour has been deformed then you lose the lift if it is the area is looking after you
will see because there is a large span whether wherever the joints are there the rigidity
is there or not over a period of time whether there are some corrosion or some small failure
has started occurring so that is again maintenance man looks for it then you have seen there
are surfaces called aileron which moves up and down like this so maintenance
man we look for whether as per the manual how much deflection you should have for a
particular stick movement whether there have having or not or there are some misalignment
right because if there are some misalignment then the pilot will have a different feel
okay so once we have a wing and we know that i need to generate a lift we should balance
weight so if i if i see like this i somehow should generate lift we should balance the
weight and whatever drag it is experiencing i should
have enough thrust to counter it once i ensure thrust and drag are equal a lift and weight
are equal that means net force acting on this airplane is zero so as per newtons law it
will maintain whatever state it has so it was moving with lets say hundred meter per
second and if you somehow can manage lifting all
the weight and thrust equal to drag then it will keep on moving at hundred meter per second
right is the inertia okay but once i write like this what happens i i get more information
the thrust will be equal to i can use these two equation i can thrust equal to weight
by l by d that is weight by lift to drag ratio that is if i am a designer i am a designer
of an airplane i should always want thrust required should be minimum which means for a given weight l by d lift
to drag ratio should be maximum simple to keep lift to drag ratio maximum i need to
ensure that the contour of the wing is maintained and thoroughly checked that there are no deformations
okay something which you know that it comes under your subject of rigging ah okay this is one you need to understand and second
thing what is more important that when i thinking of an airplane how does it fly one thing i
have realized that if this is my first lodge this is my wing right lets say this is somewhere
cg is here center of gravity of the airplane and this is your horizontal tail this is a
vertical tail and lets say this is the aerodynamic center of
the wing now what will happen if i want to fly i need to have some angle between the
velocity vector and the wing right which we call angle of attack if i typically draw if
this is my aerofoil cross section of the wing and if this airplane is going with speed let
us say climb being with a speed v and lets say this is the horizontal then the angle between the chord line what
is chord line if this is my aerofoil this is the leading edge this is the trailing edge
if i join them by straight line this becomes the chord of the wing c h o r d chord of the
wing ah the angle between the chord line and the velocity vector is what we talked about
alpha the angle of attack the angle between the velocity vector and
the horizontal we call it gamma which is flight path angle okay now you understand the lift
on the wing depends upon this alpha angle of tag and the speed velocity who decides
their speed for a given configuration how much thrust you are generating ah that is
from a mentor this point of view the thrust who produces thrust it is the engine right
so for a particularity v i need a particular power or thrust power
or thrust from the engine so as i a maintenance man you will need to ensure that your engine
is in the perfect condition so that that when the pilot flies he gets exactly that amount
of thrust or power what is expected in the design or given by the manufacturer it is
possible initially it is giving same and if you are not doing proper maintenance some
time you find the thrust expected its not happening and then there is a problem so who are you sure that everything is what
the is being prescribed is a maintenance engineer once you understand the importance of a maintenance
engineer for maintaining the engine in the perfect condition you also should realize
the lift on the wing depends upon the angle of attack what is angle of attack it is a
angle between chord line and the velocity vector so as a maintenace man you need to
check whatever way you have installed the wing whether it is maintaining that position
or not also you physically check is there any deformation
on the wing surfaces right was that the chord line generally will not change because originally
fixed but you never know so there are drills you have to check and those things come under
the topic of rigging right now the question is lets say the lift force is acting somewhere
here right so drag is also acting opposite what is moving this is a thermodynamic center
ah where i can represent the forces acting on the wing now see if this is the lift and cg is a head
so this will give a nose down tendency it will try to push down the airplane now attach
the push down the airplane that is not the way to fly it will never fly it will always
try to come like this so somehow you have to ensure that this nose down pitching moment
is not there or even if it is there it is nullified who can nullify this you see this
is a horizontal tail right if this is seeing some angle alpha which is
giving the lift it will also see some alpha which gives you a lift upward like this lift
content ah this also will give a noise down moment about cg this although they will give
lot of lift upward for same time it will try to take the aircraft down like this so you
have to nullify that how do you do that if you see our until tail that is elevator so you put the elevator up
the moment you put the elevator up it generates the force downward and this downward force
will give a moment in the opposite direction so this elevator deflection should be sufficient
enough to generate a downward force which will marginalize or neutralize the nose down
moment because of lift on the wing and lift on the tail for these configuration so now
you understand isn’t it just not wing it is the elevator you have to check from maintenance point of
view you have to check when the pilot is giving particular pull or push whether that is correctly
translated into deflection by dastardly into deflection the elevator or not suppose in
the design or during the days a couple flying high you find for a particular force deflection
is two degrees or three degrees over a period of your negligence you find same force deflection
is only one degree so the pilot will be confused he wont be able to fly that is where the maintenance
people put their best effort to see all the control deflections are okay or not and if
you recall whenever you go for any any flight initially pilot will do checks he will move
all the converse surface and he knows okay how many did we supposed to come like this
everything is fine then only flies as a pilot does that check so then again i see the maintenance
becomes important in general please understand whenever something
is moving moving parts the maintenance become very critical honestly very very cautions
right for a fixed the level of maintenance is effort wise is not that much generally
but rotary component anything moving we see up to very very careful right since we are
talking about elevator you see this is our tail horizontal tail and part of it when it
moves up and down we call it this is elevator moving up and
down how does it move pilot is here somewhere pilot is sitting somewhere else some stick
is there you pulling it or pushing it and there are in a a smaller aircraft this pull
or push motion gets translated into deflection through a cable and a pulley system right
so if it is a simple cable and pulley system then the main test engine it has to check
what is the status of cable whether they are overhang or not sometimes what happens once you put a cable
initially it may be taught but with the time there may be sag so have to ensure that there
is no sagging so again the maintenance engineer checks those thing in a proper manner which
is defeated by the manual then there are hired in aircraft where there could be actuators
okay which could be electric actuators sometimes there woud be electro hydraulic actuators
so again you have to ensure that the maintenance of elect electro hydraulic actuators or the
actuators are proper so that finally you are able to get what you desired from the deflection
point of view right another important thing you understand that as i am flying initially
i have a landing here right okay here somewhere lets say but if i fly like this in the air
this will offer a lot of drag correct lot of resistance so what is the better way of
doing is as soon as you take off you take them inside you may
take them just inside in belly so we call it retractable landing area that is landing
here was like this now it comes back to the belly and this motion is achieved by hydraulic
electra hydraulics all those combinations imagine if this system fails i am going to
land now right the landing gear instead of coming full it came half how difficult it
is to land or sometime landing gear may not come out then you have to belly landing so
risky everything is fine but if landing gear is misbehaving you are
in a very serious accident situation so who ensures that the possibility of this is minimal
it is only maintenance engineer they do thorough checkup and then we feel very assured in airplane
i know whenever i press a button or press ah stick that surface or the landing it will
come out so gear maintenance becomes very important and for that you need to know the
principles of actuators understand the systems hydraulic systems some part of avionics and
those little bit of understanding will be required
so that you appreciate overall ah maintenance philosophy why we are doing all these things
there is another very important thing which should also understand when a pilot is flying
one of the most important input he gets is air speed right he to know what speed is flying
right like your car when the action is more critical due to the air speed how a speed
is measured it is pitots tube you all must be knowing
pitots tube and simply if you see the diagram is schematically there is something like this
if i join something like this just to make for understanding i see i have put a gap here
small gap here this is called static port so from here this part of this chamber will
see p static this okay but when the air goes from this here i get p stagnation right and
you know p stagnation or p naught is p static plus half rho v square so i know v equal to
two p naught minus p s by rho under root this understanding we convert in calibrating
the airspeed right now imagine suppose you have a poor maintenance and some deformation
has come here or some insect may be lying here right so it will not give you correct
speed at which you are flying so say there are some obstruction and you are flying at
actually hundred meter per second or fifty meter per second it is showing you are flying
at thirty meter per second what pilot do look pilot will immediate try to increase angle
so that he gets more lift and either process it may lead us to a stall
situation so so small item we just do so important from maintenance point of view you have to
be very very thorough in seeing that which has to be clean and functional right very
important aspects if you’re a maintenance engineer do not think is a small thing small
hole what is the problem hole problem is here and if there is a problem is a serious problem
right so ah what i thought of as this course progresses i will come intermittently and we will stress
on few things which will have more understanding about the physics of the situation so that
you can connect yourself why we are doing all these things right and for more details
those who are interested they can always do my course on airplane performance well in
exhaustive manner you will get an understanding all a as well as there is a course on stability
and control you can do that but you will find me coming intimately in this course wherever
i think some elements of physics has to be added which strengthens your maintenance understanding thank you very much

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