Hi, everyone, and welcome to this new class in aerodynamics, in this class, we will be discussing
the theoretical part of what is turbulence and stall.
So in terms of fluid dynamics, we need to take into account that there's a difference between laminar
and turbulent flows.
Laminar flow is the preferred type of flow, which is fully attached to the wing.
It is ideal if we want to maximize the lift capacity of the wing.
As you can see in this picture over here, laminar and turbulent flows look completely different.
The turbulent flow, as you probably know, already is chaotic, seemingly random, and it touches quickly
from the surface below.
The laminar flow, on the other hand, is attached to the surface and is characterized by being smooth.
That transition point is defined as this point or transition line, right at which the change between
laminar to turbulent occurs.
Note that the transition is always from laminar to turbulent.
At this point, the flow becomes detached.
You will also find this point in the literature as a separation point or the point at which there is
a boundary layer separation.
So this transition is due to a prolonged interaction between particles, which ends up creating a chaotic
behavior of the flow.
It can also be derived from entropy and the tendency for systems to move from ordered to chaotic the
more they are exposed to an external condition.
In this case, the rough connection between the surface and the flow.
So we must keep the flow around the wing lamina to maximize lift.
A turbulent flow does not create lift since it is not attached to the wing and depending on the angle
of attack, turbulent flow can take over the flow over the wind, right?
The study of fluid dynamics and particularly the transition from laminar to turbulent flow is common
not only in aerospace, the main applications, but also in many industrial processes.
Here are shown the coefficients of lift curve with respect to the angle of attack and two experimental
simulations of the flow moving around an airfoil.
Before looking at the simulations on the right, let us just take the coefficient of lift curve again,
as we know the high angle of attack rate, the curve picks and moves downwards with the angle of attack.
It is also true that the curve actually disappears.
It does not actually go the whole way back to zero lift, right?
This is due to the fact that lift at these high angles of attack is simply distorted and cannot be represented
easily in a curve like this.
This, of course, is because the flow becomes turbulent, right?
So notice on the left animation that a slight increase in the angle of attack draws more pronounced
streamlines of the flow around the wing.
The higher the angle of attack, the more difference in velocity and pressure, therefore the higher
lift, as we mentioned before.
On the animation over the ride, however, the turbulent flow takes over the upper surface of the wing,
completely destroying the flow of air.
This produces the so-called stall condition, a condition at which the aircraft loses its capacity to
produce lift.
At this point, the aircraft will stall, which means that it will plummet, it will fall without being
able to generate lift.
This, of course, is very dangerous.
Another way of describing this phenomenon is to say that the transition point between laminar to turbulent
flow moves forward towards the leading edge of the airport.
Of course, this means that the turbulent flow will have more presence around the wing.
So to summarize this, at high angles of attack, the flow detaches from the wing, so the flow becomes
increasingly turbulent at very high angles of attack.
The flow is completely taken over by turbulence and becomes unstable and detached.
This is a well researched topic, which is fundamental to aerodynamic performance and win design.
Of course, a way to avoid stall conditions is to put slats on the wing, which, combined with flaps,
make an optimized way of flying at high angles of attack with an increased amount of flight of lift.
Right.
Flying at stall conditions is never an option for commercial aircraft, meaning the pilots will always
fly and reasonable angles of attack if lives could be put in danger.
Unfortunately, though, many accidents in the history of aircraft could have been avoided in the case
that stall had been better understood and safety measures taken into account.
So we already discussed what the stall condition is and how it is related to the emergence of a turbulent
flow around the wind.
Let me ask you a question, though, if we were to fly at very high angles of attack.
Would it matter what type of wind configuration we had?
Well, actually, yes, it does matter.
The transmission line between laminar to turbulent flow, which is the essence of stall, as discussed,
can appear in different regions of the wing.
Four rectangular wings, for instance, the turbulent flow will appear close to the fuselage.
And if we continue to increase the angle of attack, it will move further towards the side.
And towards the leading edge of the UN.
In the case of a high, tapered wing, on the other hand, Stone will begin at the tips of the wing.
So as you can see, all configurations have particular distinctive patterns in terms of the transition
line moving from the trailing edge of the wing right towards the leading edge of the wing.
However, in all cases, we if we increase the angle of attack, the turbulent flow will move towards
the front of the wing, which is clearly not desirable.
As mentioned.
So this suggests then when designing an aircraft, we also have to take into account this effect note,
for instance, that elliptical wings and moderate tapered wings are optimal in terms of homogeneously
distributing the turbulent flow.
This can be a better solution to avoid the turbulent flow to move too quickly towards the front of the
wing.
Another thing to take into account is the effect on aliens, for instance.
So elements are control surfaces, and they are located at the tips of the wing.
Right.
And they are capable of controlling the rotation of the aircraft.
So if the tip of the wing becomes turbulent, as we can see, then the pilot will not be able to control
the tip right, meaning that they won't be able to control the ignorance.
So, in other words, if turbulent flow appears over strategic control surfaces of the aircraft, we
could lose control of the aircraft.
But more this whole 'nother crisis.
So as always, it comes down to trade offs and, you know, especially to avoid flying its stall conditions.
Note, however, that technology today is sufficiently advanced so that these are not realistic problems
if you are to design an aircraft.
However, this you should really take into account, of course.
Even in the case of an emergency, as we will see in the last slide, still conditions can be counteracted
with some clever maneuvers, so not to worry plenty of safety systems installed on aircraft.
So to summarize, depending on the wing configuration, stall may appear at specific locations of the
wing.
Stall means that losing control of a surface, and of course, if we lose control of the surface of
the clearance right, we can lose control of the aircraft, so we must take into account stall conditions.
So I just wanted to show a couple of examples in this transition between laminar to turbulent flows.
In this image, a few truckers are located on top of the wing to illustrate how the wind is moving above
it.
Note the laminar flow means that the trackers are attached to the wing, whereas the turbulent flow
distorts the wind around the wing and pulls out the truck, causing chaotic waves.
So you can also see it with region of the wing, the turbulence appears first, right?
In this animation at the right, this is a Boeing 747, and it is actually performing a stall test in
which the pilot must safely guarantee the stability of the aircraft even under the utmost adverse wind
conditions.
So notice how the angle of attack is increased initially, as you will see?
No.
Until a noticeable sudden loss of lift is perceived at that point, the pilot points the nose of the
aircraft down, preventing the aircraft from stalling more.
I'm actually not sure if this is typically performed by the pilot or you simply automatically corrected
by the systems of the aircraft.
I guess I should check that out.
And finally, above you can see a typical maneuver that pilots have to perform when training to get
the pilot license.
As you can see, the pilot is asked to enter stall conditions by increasing the angle of the deck of
the aircraft with respect to the wind.
So at a certain point, stall appears and the aircraft starts pulling down due to a lack of lift.
It is here that the pilot must quickly act to push down the nose of the aircraft using the elevator
of the tail to ensure a safe recovery from stall.
So this would be all, as always, if you like the content provided, I would appreciate it if you could
leave a five star review of the course, everything that this course is worth it.
Thank you.
And now we can move to the next class.