if you are flying at hypersonic speed
the familiar aerodynamics governing the
flight of planes doesn't apply straight
away lift is generated by a different
mechanism, control requires special
aerodynamic surfaces, the flow itself
doesn't behave as you would expect these
are crucial differences that require the
unmistakable design of hypersonic cruise
missiles to make everything work properly
I am the Crow, welcome to millennium seven star
the channel that helps you make sense of
military history and military technology
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the lift of an airfoil at subsonic or
transonic speed is generated by the fact
that the flow speed above the airfoil
is higher than below, the pressure
above the airfoil is smaller than the
pressure below and the wing is sucked
upward producing lift. This starts to
change at hypersonic speed and
surprisingly the lift generation mechanism
is even easier than that. At hypersonic
speed the so called Newtonian hypothesis
is a very good approximation. Newton
actually believed that the source of
aerodynamic forces was due to particles
of fluid hitting a body and transferring
the momentum perpendicular to the
surface to the body. In practice every
air "particle2 was supposed to bump into a
body and the total effect of all
combined bumps was the aerodynamic force.
As if it was a sort of horizontal rain.
At slow speed this is not true,
but surprisingly at hypersonic speed
this is a very good approximation. The
stream of particles impacts the portion
of the body facing the flow and we may
assume that the rest of the body is in a
sort of (aerodynamic) shadow, and pressure is near zero.
The efficiency at producing lift is
characterized by the use of a lift
coefficient. At low speeds the lift
coefficient depends only from the angle
of attack of the body; at Super Sonic and
hypersonic speed depends also from the
Mach number and it tends to become
smaller for the same angle of attack.
If we consider the contribution of the
lower and upper surfaces to the lift
coefficient of the whole body, we can see
that the lowest surface dominates at
hypersonic speed which is actually in
good agreement with the Newtonian
hypothesis. You can immediately see that
in this case a flat body works very
well at generating the lift
with no use of wings.
The total lift being generated depends
obviously from the speed and density of
the air the weapon is traveling into but
the efficiency in generating lift
depends roughly from the square of the
sine of the angle between the flow and
the local surface but this is not the
only element in play there is also the
pressure coefficient that measures how
efficient is the fluid in applying
pressure in a given point at a specific
speed. It is high and the pressure is
high where the fluid is slow, like right
after the bow shock and near the nose
and it decreases quickly while the fluid
accelerates towards the back of the
vehicle. This needs to be taken into
consideration to design a missile that
is stable in pitch. Obviously, more stubby
vehicles like hypersonic gliders
or space vehicles are less influenced by
this problem even if NASA actually got
close to losing the first space shuttle
during the re-entry for a similar
problem.
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So, is there a way to understand why this
simplified lift generation mechanism
happens at hypersonic speed but not at
the lower speed? To explain this
intuitively we can use an analogy. If
someone is actually lobbing a pebble at
you the pebble is relatively slow and
you have the time to react and get out
of the way safely. If someone is firing
a bullet at you,
it is so fast that you
can't react in time. You are hit and knocked on the
ground because all the bullet momentum
has been dissipated inside you.
No, sorry, this is creepy, this is really creepy who
did write this? Me? Oh, I suppose it's okay then.
At hypersonic speed the lumps of air
are so fast that they go through the bow
shock and impact the surface without
changing direction too much. They're too
fast for the rest of the aerodynamic
field that aim in large influence them
So they don't move in the relatively
complex way which is typical of the low
speeds, where all the lumps of air have
the time to heavily affect each other.
heavily affect each other.
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While the lift generation is
simplified, at hypersonic speed also exists
what is called the hypersonic
directional stability problem. Now this
is really hard to explain without the
equations because the equations show a
coefficient that becomes zero when Mach
increases and stays zero if the
Newtonian hypothesis is true. However I
try anyway. I am sure that many of you know
that the stability of planes is assured by
the tail aerodynamic surfaces. For
example the stability in yaw is assured
by the vertical empennage. Wow this
happens: well suppose to give a little
bump to the plane such that it goes to
starboard. The flow on the vertical
fin, which moves with the whole plane, changes.
The angle of attack increases and with
the angle attack the fin starts generating
lift. Since the fin is vertical the
lift is directed sideways in a way that
produces a torque that puts the plane
straight again. At hypersonic speed we already
know that the lift coefficient that
measures how good is anything in a flow
at generating lift, decreases with the
Mach number so a small deviation will
change the flow on the vertical fin but
the lateral lift produced will be very
very small. A small force might easily
end up not being enough to put the plane
straight. At that speed the (uncorrected) deviation may
be enough to create other issues like
erodynamic shadows on the horizontal
tail or part of the plane
intersecting the bow shock which is as
hot as a blowtorch and can damage the
plane structure. This is the reason why
hypersonic planes, but also
space rockets, often have fins with a
triangular section.
This generates more drag but also
generates a lateral lift. As long as the
flow is parallel to the axis of symmetry . 0:07:59.740,0:08:05.260 The lift is the same on both sides
they balance each other. Once the flow
deviates right, the tail fin is already
producing a lift which is high enough to
set the planes right again.
For example the vertical tail of the
x-15 is actually a massive wedge; if you
ever asked yourself why, well this is the
reason. The design of an hypersonic in
Sall is a sort of minefield but usual
concepts of lower speed flow dynamics
may be subverted and all sort of this
strange and counterintuitive situations
like the one just described for the
vertical fin may happen so there would
be plenty of other considerations to do
but I think that for today I have
overheated your brain enough so if you
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bye
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you