hello everyone and welcome back to
military aviation history I'm standing
in front of a mig-21 this is a Soviet
jet fighter from the Cold War era
however this one has finished markings
because in the 1960 only nineteen
sixties these aircraft were sold to
Finland now the question I often get
with this aircraft is what is that
enormous Lance kind-looking device up
front this is a pitot and it essentially
allows the pilot to read the aircraft
speed so the pitot tube might need a bit
more explanation and yes it is
pronounced pitot after French engineer
Jorge pitot who invented the tube in the
18th century and not pitted like a
collet for some reason now there are
different models of pitot tubes for
different users but the basic function
is always the same
imagine a tube of a hole pointing in the
flow direction of a fluid this being air
in our case on the other side of the
tube a chamber is blocked off by a
pressure transducer this chamber holds
the total pressure then imagine a second
hole perpendicular to the fluid flow
with a respective chamber separated from
the first chamber via the aforementioned
pressure transducer the pressure
transducer measures the pressure
differential between the two taking that
measurement and taking into account some
additional figures such as air density
the velocity can be measured with
Bernoulli's equation and some fancy
number crunching now because you want
undisturbed airflow to do this the pitot
has to be beyond the pressure field that
is built up when you're going subsonic
when going supersonic correctional
values are taken into account to provide
accuracy as the total pressure changes
PTO's are not without measurement arrows
but with time we got better at them but
the question is why is it so far out and
why do you see this in quite a few jets
that they have these pedals that are
just enormous this is because at the
speed that these guys are going you
really need to have the the pitot as far
away from the actual aircraft fuselage
what happens if you if you're going
subsonic yeah is you build up a pressure
field that is around and especially in
front of your aircraft
and this pressure field causes all kind
of problems to accurate speed reading
especially at high angles of attack and
with aircraft that have a high wing
loading and yes the big 21 has a high
wing loading with the pit of having to
go into a into the flow Direction you
can see how how that is a problem
especially the closer it is to the
aircraft the higher the angle of attack
the more bleak the angle it will be so
was a long pitot you can get around this
because it reaches relatively afar
outside this kind of pressure field SB
that is being built up and this allows
the pilot to get a more accurate speed
reading and also M that the aircraft has
to get less compensation factors in has
to take less compensation factors into
account so there you go that's why you
sometimes see this kind of Lance looking
device on fighter jets and I hope you
guys learned something here at this
aircraft you can visit it at the finish
Aviation Museum thanks very much for
them for letting me film this and a
couple of other episodes if you enjoy
this kind of content please consider
supporting it so I can make more of this
this kind of stuff do you find all the
links in the description below to my
patreon for that anyway I will leave you
with a nice little look of a finish like
21 it's quite a late ball oh one one
more thing I want to tell you guys when
the mix or anyone started out the lands
was below the engine coming out on the
bottom and then they moved it up top and
one or a in the later models anyway just
a little bit of trivia for you so yeah
as always guys have a great day good
hunting and see on the sky
[Music]
you