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since its inception in 1958 nasa has
been involved in
countless strange and bizarre
experiments in the name of advancing the
united states understanding of
aeronautics
after all nasa stands for the national
aeronautics and space association
their mission is to pioneer the future
in space exploration
scientific discovery and aeronautic
research to do this
they have built and tested some of the
weirdest planes the world
has ever seen each designed to push the
limits of our understanding
and evaluate the value of new innovative
designs
the x5 featured variable sweep wings
that allowed the plane to alter its
sweep angle
during flight the x-15 a sleek
rocket-powered plane designed to push
for the record of the fastest plane in
existence
smashing into the hypersonic regime at
mach 6.7
and the x-29 with its odd forward-swept
wings
gave us valuable data on the flight
characteristics
of this interesting wing configuration
these are all weird and wonderful planes
but one stands out to me as the weirdest
a plane that looks like the weird
lovechild of the x5
and the x29 a variable sweep wing
but with a single wing pivoting about a
single point
resulting in the plane having this
bizarre asymmetric wing configuration
with one wing pointing forward and one
pointing backwards
what could the designers of this bizarre
plane have been thinking
what advantages did they possibly
foresee
the primary advantage of variable sweep
wings was well established before the
ad1
with the x5 experimental aircraft which
first flew in 1951
when nasa was called naka when the
organization was focused purely on
aeronautics
variable sweep wings have a couple of
advantages
during takeoff and landing you want to
maximize lift
so the plane can minimize its speed
making it easier and safer to take off
and land
particularly on short runways like
aircraft carriers
so a variable sweep wing can take off
with its wings set to its lowest sweep
angle
maximizing lift then as it speeds up and
generates more lift
it can increase its sweep angle which
lowers the dry the wing generates
as a result of the lower frontal area
increased sweep angle also increases the
critical mach number of the plane
the critical mach number is the speed at
which supersonic flow
first appears at some point along the
plane's surface
this happens before the plane itself
reaches supersonic speeds
because air speeds up as it travels
across the wing's upper surface
this can cause some funky aerodynamics
as flow separates
from the wing's surface when supersonic
speeds are reached
which can interfere with controlled
surfaces and drastically increases drag
this prevents some planes from flying
faster for example
airliners cruise just below critical
mach number
so increasing the critical mach number
is advantageous
to understand this let's look at the
flow components over the bell x5's wing
at its lowest sweep angle 20 degrees
here we can separate the airflow into
two components
spanwise flow which flows along the wing
and cordwise flow
which flows along the cord of the wing
the spanwise flow
runs along the length of the wing and
does not accelerate and thus
does not lower the critical mach number
at lower speeds
when supersonic airflow is not a worry
you want as much of that airflow to be
cord wise
which is the flow that generates lift
however
as the speed of the plane begins to rise
we are generating more than enough lift
thanks to the increased airspeed and
thus can afford to convert some of that
airflow into spanwise flow
we do this by increasing the sweep angle
which the bell x5 could do
in flight to a maximum sweep angle of 60
degrees
as the sweep angle increases a larger
portion of that airflow is converted to
span wise flow
which increases the critical mach number
however it can cause some troublesome
stall characteristics
because a large volume of air is now
originating at the root of the wing
and traveling to the tip of the wing a
stall will begin
at the tip of the wing and move towards
the roof
this is the problem because our ailerons
the control surfaces that allow us to
roll the plane
are located on the outer wing if a stall
occurs
on the outer wing we lose roll control a
major problem for say
a fighter jet attempting a high angle of
attack maneuver
while maintaining full control and this
is one of the problems the forward swept
wing
was trying to fix like the x-29s which
we have explored in a previous video
this reverses the direction of the span
wise flow
so it originates at the wing tips and
travels to the root of the wing
meaning stall occurs at the root of the
wing first allowing us to maintain
control of the plane for much longer
not only that but it reduces induced
drag as a result of the wingtip vortices
where high pressure from the lower wing
travels and mixes with the low pressure
air
on top of the wing at the wingtips so
the advantages of variable sweep and
forward swept wings are established
but this plane is some weird indecisive
frankenstein's monster blend of both
we have one wing forward swept and one
wing backward
as you have probably guessed this
bizarre asymmetry results in some
strange control issues
side force on the plane the force that
attempts to push it to the side
increases with the sweep angle as more
of that span wise flow is redirected
down each wing in a single direction to
compensate for this
the plane has to increase its bank and
side slip angle
flight at the maximum sweep angle of 60
degrees
at 260 kilometers per hour required one
degree of nose right side slip
and seven degrees of right side down
bank to remain
in straight and level flight as we saw
the sweep direction impacts where the
stall occurs
the forward pointing wing will first
stall at the root
while the rear facing wing will first
stall on the wingtip
on top of this the rear-facing wing
stalled earlier than the forward facing
wing
meaning we lose lift on one wing first
causing the plane to roll
and yaw to the left while also having
decreased ability to control roll
as we lose aileron control on that wing
if not recovered this would result in a
deep spinning dive
there are more stability and control
issues with this plane
that i can explain in this video but i
have linked an excellent nasa report in
the references for the aviation nerds
that want to really dive deep into this
complicated subject
the weird aerodynamics were obviously
predictable
so what were the anticipated advantages
of this bizarre configuration
the first and most obvious is the
simplification of the sweep mechanism
a single pivot point for a sweep
mechanism is simpler in construction
than a twin pivot but it also
drastically reduces
the stress being placed on the mechanism
to understand this
let's first see how the forces on the
mechanism changes
as the sweep angle increases starting
with the oblique wing
let's imagine this as a simple beam that
is being pushed upwards
at its end point at zero degree sweep
the beam is simply under
symmetric bending which means the center
pivot point is
only being pulled upwards creating
tension in the linkage
attaching it to the plane as the sweep
increases
in level flight the forces should remain
the same
meaning that linkage is only ever in
tension in level flight
now let's imagine a single wing as it
rotates around this pivot point
once again we will imagine it as a beam
being lifted upwards
here there is no equal lift on the other
side of the pivot point so
this manifests in the mechanism as
bending not tension
and as the sweep increases we begin to
incorporate a torque
around the y-axis of the plane as the
force lifting this beam gets further
from the center of mass
these more complicated forces result in
a pivot
that needs to be stronger and we need
two of them so
a single pivot point reduces the
complexity
and structural weight needed to
facilitate a variable sweep wing
next we reduce wave drag as a result of
conforming
more closely to the area rule this
simple rule tells us the distribution of
the cross-sectional area
along the length of a transonic or
supersonic plane should look something
like this
that includes the wings engines
everything
this gradual increase and decrease of
cross-sectional area
results in a reduction in wave drag
which is the drag that arises from shock
waves
forming around the plane as it
approaches the speed of sound
so let's take a cross-sectional scan of
the early version of the f-102 delta
dagger
which was designed with a straight
fuselage it looked like this
we can see sudden bumps where the engine
the cells protrude from the fuselage
and a steep rise in cross-sectional area
as the large delta wing
increases in width before a sudden drop
along the straight
trailing edge of the wing this plane
would have had quite a lot of wave drag
because it deviates from the ideal
distribution so much
so the engineers later redesigned the
f-102 so the fuselage
tapered in along the length of the wing
thus decreasing the rapid rise in
cross-sectional area
as the wing widened this decreased fuel
consumption
and increased the plane's top speed now
you probably see where this is going
by having one wing pointing forward and
one pointing backwards
it naturally distributes the
cross-sectional area of the wing
over the entire length of the plane
allowing it to conform
more closely to the area rule and thus
decreases the effects of wave drag
finally
traditional variable sweep wings have
one last disadvantage
as the sweep of the wing increases the
center of lift of the plane
shifts backwards which if not
compensated for with pitch controls
will cause the plane to nose down using
the control surfaces in this way
will increase drag to keep the plane in
straight and level flight
by having one wing shift forward and one
wing shift backward
the center of lift remains relatively
static and pitch trim does not need to
compensate
decreasing the fuel lost to drag the
designers of this plane
envisioned it as a plane that could take
off from short runways
and be capable of loitering in the air
for extended periods of time
making it suitable as an air-to-air
refueling tanker or
submarine hunter in the end any
advantages the designer sought using
this flight configuration
did not overcome the extreme
disadvantages it experienced in
stability and control
it incorporated all the structural
issues associated with ford swept wings
which we have explored in a previous
video it required extreme trim
alterations in straight and level flight
and became near impossible for a human
to control while performing extreme
maneuvers
even its more traditional brother in
symmetric variable wings
fell out of favor in the 70s due to its
own disadvantages in heavy mechanisms
and trim drag
and advances in flight controls and
materials made delta wings the new norm
but that's a video for another day nasa
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