this episode of real engineering is
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flying on a plane through turbulence can
be a bit of a nerve-wracking experience
for some hearing that announcement bell
ring followed by the pilot calmly saying
that you are approaching rough air you
fasten your seat belt the plane starts
shaking and to those who have a fear of
flying this can be terrifying knowing
that the plane is specifically designed
to deal with these disturbances without
any input from the pilot may or may not
ease your fears but that is exactly what
they do every passenger plane is
designed with something called
static stability static stability
essentially means that an aircraft left
to its own devices flying in a straight
level flight will return to straight
level flight even when it is knocked off
course this makes them much easier and
safer to fly without having to
constantly adjust the control surfaces
to balance the plane yet the exact
opposite is true for fighter aircraft
any aircraft designed for air-to-air
battles are designed to be capable of
out maneuvering their opponent and one
of the factors that affects a plane's
ease of movement is how stable it is or
in other words how unstable it is and
thus how ready it is to deviate from a
straight and level flight with minimal
force I noticed with my recent video
detailing the physics behind the forward
swept wing that there is a general
misconception floating around the
internet surrounding the notion of
instability in fighter aircraft in my
description of the x-29 I mentioned that
the plane was too unstable which was
meshed by a mountain of comments saying
that this was a good thing the more
unstable the aircraft is the more
maneuverable it is surely that is a hot
take from people who have not studied
stability and control of aircraft in
depth so let's see why this is a
nonsensical approach to aircraft design
let's think about this with a simple
analogy here we have two situations a
ball placed on top of a hill and a ball
placed in a valley if we push the ball
on top of the hill even a tiny bit it
will begin to accelerate down the hill
and will not stop until we put energy in
to slow it down and we will need to put
even more energy into it
to the top of the hill this is an
unstable system the opposite is true for
the ball in the valley apply a force and
the ball will roll uphill and gravity
will now provide a restoring force to
bring it back it may all eight back and
forth a few times before coming to a
stop but it will eventually return to
its original position this is a stable
system so how does this apply to planes
planes have three rotational degrees of
freedom pitch roll and yaw let's first
approach this problem with regards to
pitch stability which is where the x-29
was extremely unstable small general
aviation planes like Cessnas are
designed to be very stable and so return
to level flight automatically after they
are knocked up or down by a gust of wind
but how do they manage this pitch
stability is determined by three primary
factors the location of the center of
gravity the location and design of the
wing and the location and design of the
horizontal stabilizer the center of
gravity for our cessna is located about
here this is the point which all lift
will act around it's like the fulcrum on
a seesaw our wing is located slightly
behind us and thus the lift it generates
is slightly aft of the center of gravity
this would force the plane to pitch
downwards without a counter acting
downwards force further back on the
plane
which is exactly what the horizontal
stabilizer provides a downwards force
this force does not need to be the same
magnitude as it has a greater control
authority as a result of its greater
distance from the center of gravity once
again just like a seesaw this is how a
plane maintains pitch stability without
any outside influences but what happens
if turbulence knocks our plane off
balance if the force has remained the
same the plane would continue on in
whatever orientation the cost knocked it
into this is not what happens just like
a ball in a valley example we have a
restoring force to bring the plane back
to its original position this is a
result of how our horizontal stabilizers
down force changes with the pitch of the
plane if there are two primary factors
that influence this the first is
downwash when air passes over the wings
it is deflected downwards this creates a
Down
of air behind the wing this downwash
strikes the top of the horizontal
stabilizer and this produces a downward
pressure the magnitude of the downforce
on this surface is dependent on downwash
and the magnitude of the downwash is
dependent on the speed of the aircraft
the faster we go the more air is
deflected downwards the slower we go the
less air is deflected luckily our speed
is also dependent on our pitch if the
plane pitches upwards it will lose air
speed and the downforce on the
horizontal stabilizer decreases as a
result the weight of the plane acting
through the center of gravity forward of
the center of lift now wants to move the
nose down again the opposite happens
when we pitched the nose down here we
gain airspeed and the downwash on the
horizontal stabilizer increases causing
the downforce to increase forcing the
plane to nose up again this explanation
is often provided as a complete
explanation but it falls apart when you
consider a t-tail configured plane where
the horizontal stabilizer is lifted out
of the downstream airflow of the wing
here our other factor comes into play as
a result of the angle of attack on the
horizontal stabilizer here the
horizontal stabilizer has a negative
angle of attack this angle of attack
changes as the plane pitches up or down
if we pitch it up the angle of attack
decreases and thus the downforce
decreases allowing the weight of the
nose to pull it back down if the plane
pitches down the angle of attack
increases and increases the downforce
which forces the tail of the plane back
down this is an elegant solution to the
problem which is thrown out the window
for planes like the X 29 here we have
the forces acting upon the X 29 in the
long eternal plane the center of gravity
and the center of lift have shifted
backwards as a result of the Ford swept
design and thus in order to stabilize
the plane the canards need to produce
lift ahead of the center of gravity this
is fine and level flight with no
disturbances but what happens if we
pitch upwards here there is no downwash
that shifts to increase downforce on the
stabilizer the angle of attack on the
canards also increases with increased
pitch which increases the lift forward
of the center of gravity which pitch
the nose of even more and thus for a
very small energy input we could pitch
the plane a tremendous amount just like
giving that ball on the hill a little
nudge this is great when you want to
pitch the plane wildly but that's not
always the case to achieve level flight
the x-29 control computers had to be
constantly adjusting to compensate for
little disturbances up to 40 times a
second this isn't a huge deal especially
with today's computers that is not why
the x29 was too unstable let's take it
back to our ball and hill analogy and
think about this as an energy problem if
we push the ball and it begins to fall
the steepness of the hill will determine
not only how quickly it deviates away
from its original position which is our
analogous for maneuverability but it
also affects how much energy we have to
put in to roll it back up the hill to
return it to its original position this
is a problem because our original
position is a straight and level flight
and we are going to want to return to it
at some point so if we make this hill
too steep we have to apply an excessive
amount of force to get back to our
original position exactly the problem we
are trying to solve with introducing
instability where we have to apply
energy to push the ball up the valley
walls we introduce instability to reduce
the energy and time required to maneuver
not increase it and in a worst-case
scenario we won't have the energy
required to return to a straight and
level flight and end up in an
unrecoverable situation in an air-to-air
battle energy isn't just a fuel burning
problem it's a speed problem our energy
source for maneuvering is our kinetic
energy our speed to change our
orientation we have to extend control
surfaces into the free stream which
creates drag which saps our speed
fighter pilots have a saying speed is
life speed and maneuverability is what
wins a dogfight in reality we want to
achieve something between a nice level
field where the energy to shift the ball
is the same in all directions and the
ball on the hill scenario where we don't
have to apply a huge amount of energy to
get the ball to move
visualizing that with a plane would look
something like this this line would be
statically stable where the plane
naturally wants to return to its level
flight this is statically neutral and
this is statically unstable the f16 was
the first plane to enter white service
that was deliberately designed to be
unstable departing from many of the
design principles that influence planes
like the f4 phantom it's older brother
the f4 was found to have role
instability during Windtunnel testing so
the engineers added a 12 degree dihedral
to the wing tips to increase its role
stability and the f4 horizontal
stabilizer generate downforce in a
similar way to our example earlier
creating a long incidentally stable
plane the f16 in comparison had a
noticeably straight wing making it more
or less statically neutral in roll like
our ball in the flat field the f16 also
shifted its center of gravity rearward
behind the center of lift and
necessitated a horizontal stabilizer
that produced lift making it unstable in
pitch albeit nowhere near as unstable as
the x-29 producing a plane that is
unstable enough to allow for energy
efficient maneuvering this is a
complicated topic with a huge number of
variables that I haven't mentioned here
for example the center of lift per wing
tends to shift forward with an increased
angle of attack and with supersonic
planes the center of lift shifts as it
goes from subsonic to supersonic flight
there is a lot more to this problem than
you are going to get from a YouTube
video and many of my viewers are
actually students and practicing
engineers who are likely to use this
video as inspiration to go and learn
more about the subject and to keep track
of all the variables you will probably
need a notebook while I was studying and
working as a research and development
engineer I always wanted nice moleskin
style notebooks that had graph paper
instead of lined pages I could never
find ones I liked so I decided to just
make my own the only way I could do this
without spending an absurd amount of
money was to buy in bulk I've already
sold about half of them from a community
post so if you act quickly you can buy
some of these limited availability
notebooks for yourself
I wanted to keep these a reasonable
price in relation to normal moleskin
notebooks so the margin on these are
small so we may or may not do another
printing run but if people like them and
we can afford to print a couple of
thousand of them off we can look into it
and just let me know on Twitter or
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to below
[Music]
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