Yeah today we will be discussing about a stick free neutral point you have already done how
to find a stick fixed neutral point through a limited deflection right we'll be talking
about stick free neutral point and before we talk about the experiment they test recapitulate
what is a stick free neutral point and how it is different from a stick fish neutral
point and what is the relevance of it okay. Please understand if IC c inverse s alpha
or fear graph it is something like this right and this point is a stream point.
. Rochas now suppose you want to fly at this
CL that CL 1 you want to fly here. .
So what is the action the pilot should do now you know from the static stability of
the airplane the moment you try to change the sea and that means you want to change
the angle of attack then it will automatically generate a negative moment because it is statically
stable right. .
So it will have a tendency to come back to this original CL at which it was flying just
to give you a scenario suppose I am flying at an altitude let us say one kilometre flying
at a CL point to the speed v1 0 SE CL point2 and we let us say he's 100 meters per second
list at that point is this, this corresponds to CL equal 2.2 and reach 1 200 meters per
second. .
When I am saying is a pilot desires to fly at a new CL with a CL 1 let us say now pilot
wants to fly at CL equal to 0 point 3 and if we have to maintain a level flight the
same altitude so he has to do what reduce the speed number one and also the L&O deflection
he has to do something right because why he has to reduce the speed because he'd be flying
at a multitude Now the moment that increased CL 2.3 the lift
will become more than the weight because left is equal to half Rho v square HCL if I am
increasing the CL and this is equal to W if I am increasing the sphere then this half
Rho v square SPL will increase unless and until you reduce the speed.
So he has to reduce the speed that means what is happening is going on some angle which
corresponds to scalar point to you know increasing the sea at the point 3 by raising the angle
of attack but then he has to also throttle that it was the speed otherwise we'll start
climbing okay and you also know that if I have to fly at this pl 1 it will generate
a negative moment. So I have to either generate a positive moment
by giving elevator deflection right that is why if you see this graph Delta ie., watch
the CL trim for a given cg location X is e 1 you have definite value of a limited deflection
required if you want to fly at the CL ad vcl address field or a GCS of different value
of Delta required. .
So the pilot if you want to fly the airplane at different feel how should I develop field
for you in eastern / field house looks like so one ways you make a chart Delta e + CL
or CL means you can always relate to speed by giving density,
. And he sees that chart or this much of VI
put this letter Delta this much as we pull this much of Delta or sometime you know that
for the given Delta there may be a rotating shaft I will you cannot rotate this many combinations
were there the basic understanding is you are developing field of the airplane for the
pilot through delta e and the speed 3 develops will this much altitude this much speed this
much all over deflection I will get but that may not be very good way of giving him a field
right. For example if you are driving a motorcycle
if for every speed there is a marking put this March it will take this much as quick
put this mark this will you give this much of speed do think that is a correct way of
getting a feel how do you get a feel when you are driving a scooter as you accelerate
you get that tall right and then you know it as getting more and more talk you are getting
is a feel for the airplane a sudden further sorry feel for the motorcycle and then reduce
that is how you get the field okay for airplane also what is done.
If I somehow can give the field to the pilot in terms of stick force right if you have
to reduce the speed it pulls it like this it is going to increase the speedy pushes
like this right and this feel of the force or the gradient will help him in getting the
feel for the airplane right and that is where you need to define something called stick
force and you need to use that concept of a stick force to define a stick free neutral
point okay but be very clear if you seem y last the module the airplane has the only
neutral point right it is stick free neutral point we are defining.
So as to develop a mathematical model which will help to give a feel for the pilot that's
all okay so now the question is we want to find out the streak-free neutral point through
the experiment if you see the expression for a strike force you know C regan refer my lectures
or in a textbook from there you can easily see that these excess by q VAR q bar the pressure
dcl well 20 and sticks free neutral point is defined at x PG location at which the DFCS
by Q Bar a DC l is equal to 0 this is the definition or this is the mathematical back
up to determine the NM bar on n-not bar sorry repeat n-not bar stick for a neutral point
and generally for speak free we put an ash here for the sticks fries you do not put it
here right. .
How do I find 60 neutral point? I need to find that CG location at which DFS by Q Bar
by DC l is equal to 0 right if I understand this then let us see how do i do experiment
first question comes how do I measure stick force.
. You know this is basically based on strain
gauge the of the stick let's just stick like this the strain gauges are on the stick so
as I pull it so there will be a bending there is the stress developed and from that so the
strain gauge measurement I can find out how much the forces have been applied in terms
of pool or a push right so use strain gauge principle you also use wisdom brief.
But that is what through strain gauge we make sure the thick force and now if I want to
conduct an experiment to find out strictly neutral point is what I do I go for a cruise
right and again I look for altitude I note down the altitude I note down the outside
air temperature alright, then four different we listen we I am choosing you note down what
is the stick force and again for v2 v3 v4 you note down all those stick force whatever
being applied so you just need this two measurement. .
So you know what is the stick force for dynamic pressures how you calculate dynamic pressure
what information is required for this half Rho v square all right.
. How will you get Rho density through outside
air temperature and the standard table how do you get V we as we indicated then you correct
it for true air speed right so all the information available but we need to also find out CL
because I am which see the change this gradient with respect to CL, so far CL what additional
information I should have? the weight of their in fact so you will also make sure which you
are doing anywhere weight of the airplane. You can take the average weight during the
cruise you get an initial weight final weight average or takeoff weight, then landing where
take the average or the carrier stop watch see how much time it was not cruising how
much time it was cruising, so you can you know the full consumption rate so you can
find out the average weight that is your choice. So once you have this V and Fs and you can
easily calculate what is CL CL will be 2W by L by Rho v square correct luscious we are
closing but this experiment will be done responding to it xpg location that is there will be fewer
student and the pilot fly which corresponds to a given cg location so this should be repeated
for different cg location. .
So you can always manipulate the weight of the passenger and swell wait we cannot go
on changing so I can generate this sort of a data for number of CG location let's play
from XCg 12 x cg 556 points we generate that means.
. I have this measurement corresponding to x
PG configuration when either batch goes it generate this experiment again that data corresponds
to x PG to XC g 3 x 3 4 x 55 so what we have now? What will have FS in all this process
is by Q Bar. So you can put as a column here and you can calculate what is the value of
Q Bar go to half Rho v square. .
So this information is available to you when you plot SS by Q versus CL let's say dependable
speed let's say they look like this is for XC b 1similarly there could be another 4 x
cb to solar look for another x cd3 like that you can have n number of points right.
The upper this point is clear, now what is our aim our aim is to see what is that CG
location for which the offense by Q Bar by bcl equal to 0 that's digit affectionate with
DFS x cube IDC le 0 it is called a stick free neutral point right.
. Now what I should do I should plot from this
data I put it corrects seedy bar and here i put DFS by Cuba by dcl which essentially
means the slope of this line isn't it right. Now you have to plot DFS by Q by dcl versus
x eg and you take one graph x is e 1 take the slope ,slope is negative at this point
the second one you take this third one you take this and then you joined by a straight
line and this point is what? this is the ecstasy location at which the offense by 2 by dcl
is 0 so this becomes you know a stick free Neutron was yes because Ross check with 66
neutral point earlier we did that was d delta e by d CL vs XE g and the XC location at which
d delta e by d silver 0 right. So this is the way to do stick free experiment
to find out a strictly neutral point so i will just write the device table but this
is you are having an altitude to note down outside air temperature you note down you
know down the weight, then fly at different V v1 v2v3 v4 like that and measure stick force1
2 3 4 5 you also find q bar equal to half Rho v square you calculate CL 2 W by L always
wear then you find out DFS Mike you buy dcl and this should be four different city locations
right xdd one two three cross plot and get difference.
. So this table will correspond to one deliberation
xcg one similar terminal be there for others configuration x32 it was a devil over there
for EXO III so you have to pick up the slopes and cross blood respect to TV and see where
after accelerating this point where this line cuts that is your streak free neutral what
as simple as that okay here now to end this part we will also see how to find a stick
free manual in point? This whole point is over like you have a stick fix to turn point.
You have a stick free to third point or a strictly neutral bind you are talking in dance
or stick force right similarly a stick free man moaning point we'll be talking in Tulsa
again stick force what mastic six neutral point? what was the condition for stick? fixed
which we did earlier six fixed neutral point NM bar was defined as xcg location at which
d delta e by dn equal to 0 isn't that is how it is now you should look for tix free manoeuvring
point it's free NM bar prime that is the exes II location at which DFS by d n equal to 0
what is in here is a load factor. .
So what sort of an experiment we do we do not go for a pull-up manoeuvre what do we?
do we go for a turn steady coordinator turn like this and you turn like this and same
time we maintain that it doesn't lose the height that is if I draw the diagram here
if you see when the airplane was not having any bank and if this was a lift then the lift
was balancing the weight that is how it is not losing the height now suppose if I blanket
like this what will happen? .
Now the component of lift will try to balance the weight say L cos Phi will try to balance
the weight so if the lift was same then it would not be able to balance the works it
will start sinking right okay but we when we do this manoeuvre we ensure that altitude
is same it is steady co-ordinated turn that mean for that to happen you can see I have
to ensure that L cos Phi is able to balance the weight component of lift should go to
balance the weight. So here I could see l by w equal to 1 by cos
phi and what is l by w load factor so n so I know n equal to 1 x plus 5 so if the pilot
is flying the Machine and taking a bank of 45 degree and making a steady colony return
so what is the load factor you're experiencing? let's say 30 degrees then one by cos 30 all
right okay if it is 60 degrees in let's factor off not for us to say very large number right
okay do not understand my load factor of two. .
So this is to tell you how we generate this n because we have to find out a condition
where from I can extrapolate to get the city location for which DSS by DN becomes 0 then
it could be a stick free manoeuvring point right so what do we do again using the experiment
we go and note down altitude outside air temperature and then wait in the pilot goes for it steady
call in a turn a different speed right. .
So he does that one, two, three, four, five, six, seven, eight each time we measure what
is the stick force applied? This is all the measurement required Bank angle you get from
the ton bank indicator a stick force will get from the strain gauge based sticks we
from the air speed indicator and it goes without saying you know that that speed is equivalent
air speed we have to correct it for a true air speed by using the relationship.
. So once I have this now I see what I will
do I will simply again conduct this experiment for a given x PG x XE g one configuration
so then I will plot FS versus end and how will I get the value of n and I will get from
here n equal to1 by cost five right so i get the value of n I plot FS by n and let's say
this values for different speed again this is shay xpd-154 another xpg to where Ican
get for another xcg 3 like this and go on generating once I do this then what is the
next step it should be smart enough to not tell me how do I get a stick free manoeuvring
point. I will do cross plot between XCG and here
DFL by n what is the SS by n for a given X 2 g's occasion let us say X eg1 what is the
SS by n there is the slope of this line right, so I pick x cg 1 and hen whatever slope I
note it down here early for XC g 2 I again do this XC g 3 like there are few points I
will get I will try to fit it to the best fit and what is this point this would be that'll
be stick free manoeuvring point it is understanding clear any doubt anybody have it okay. Thank
you.