## What is the Magnus effect?

What is the Magnus effect?
 They see me spinnin', they hatin'.

### Spinning Cylinders

In 1930, Plymouth A A 2004 flettner airplane used a bunch of spinning cylinders (flettner rotors) to achieve flight. This puts forth the question: CAN SPINNING CYLINDERS FLY?

By the Magnus effect, that is a possibility.

### Magnus Effect

Almost a hundred years before this flettner aircraft took flight, in 1852, Heinrich Gustav Magnus was working on projectiles from firearms. The bullets seemed to take a deflected path rather than the destined path. Pissed off, he grabbed the bullet by the collar and asked it what the hell it was doing. “It’s the pressure difference man! Look I’ve got a family to feed ” said the bullet. Thus he proposed the Magnus effect. True story.

Fast forward to the 20th century, there came a revolutionary theorem barging into the abode of aerodynamics and that was the Kutta Jowkowski theorem. Kutta and Jowkowski sat together and whipped up some dank mathematics to prove our man Magnus’s effect.
As opposed to going into the mathematics of the Magnus effect, and analytically proving its existence, we’ll look into the physics of it and see what sort of sorcery it is.
 Spinning cylinder
Suppose a cylinder is spinning in the clockwise direction. There’s going to be attached flow right above and below the cylinder creating a boundary layer. (layer where friction effects are felt). After this attached flow, there’s going to be flow separation in the aft of the cylinder (One could say after but Aircraft Nerds like cool words). This flow separation creates low pressure vortices. (The physics behind this attachment and separation has been elaborated in the article on Coanda effect linked below) Let’s step outside the boundary layer for a bit and examine the velocity above the cylinder. It’s going to be higher than the velocity below the cylinder. Why? Because it spins. Any spinning object rotates about it's axis by an angular velocity. This angular velocity is induced by the velocity acting tangent to the circle at every point. Over the top of the cylinder, this clockwise spin or rotation creates a tangential component of velocity which is added to the free stream velocity (velocity of the flow V which is a constant at a distance from the cylinder) creating a velocity Vtop. Why is it added? because the flow velocity and the spin velocity are in the same direction. And in the bottom of the cylinder the velocity is reduced from the free stream velocity (V) creating a lower velocity Vbottom. Why is it subtracted? Because the flow velocity and the spin velocity are acting in opposite directions. So the velocity over the top is greater than the velocity below.

 Tangential velocity over top > Tangential velocity below

Bernoulli: “That makes the pressure over the top lesser than the pressure below the cylinder!”

What he said. So as mentioned in the linked article on Coanda effect, flow moves from high pressure to low pressure. This difference in pressure creates a force which pushes the flow in that direction. In our spinning cylinder, there’s high pressure below the cylinder and so the force acts upwards. This upward force is, drumroll, please…
Lift.
Or Magnus force.
Now look at the aircraft again, see those spinning cylinders separated by discs? They're flettner rotors. These flettner rotors take advantage of the Magnus effect to generate lift.
Random cricket fanboy: Curiouser and Curiouser..
what?
 Topspin on a cricket ball
Random cricket fanboy: This sounds an awful lot like topspin and backspin in cricket.
Right you are random fanboy, right you are.The Magnus effect explains the deviations of spinning balls in ball sports like golf, tennis, baseball and ofcourse cricket. In top spin the ball goes down because the Magnus force acts downwards. In backspin the ball rises.

Now you know how to throw a killer serve with physics.

## Why are fluids clingy? Coanda Effect?

Lift on an airfoil is a by-product of the attached flow over the airfoil. (this is explained in the article on lift linked below). But how do the air molecules know that they have to climb up the curvature and follow the contour of the airfoil. Is there a rule in nature which says that being clingy is socially acceptable behavior?
Yes, in fact, there is, and it's called the Coanda effect.
 fluids- where an attachment is not a disorder

### What is the Coanda effect?

It is the tendency of a jet of fluid (gas/liquid) to curve towards a surface. In other words, it's the attachment complex of a fluid.

Why are fluids clingy? Let's delve into physics behind the Coanda effect.

Setting: a stream of water flows over a convex object (works on concave objects as well)

Cast:
Fluid: anything that has molecules not too closely packed to be considered a solid.

Bernouilli equation: energy equation that says if pressure increases (pressure energy), velocity decreases (kinetic energy) and vice versa. So as to maintain constant energy.

 Pressure on the walls of a container
Hydrostatic pressure: The pressure that comes out of particles in the fluid bombarding against one another. The pressure at a point in a non-moving fluid is directly proportional to the height of the fluid above it. It can be thought of as the pressure exerted by the fluid on the walls that restrain it.
Continuity equation- the equation that says an area is inversely proportional to velocity. (If you wanna remember this: picture a fat man run.
 Viscosity prevails only in the boundary layer
Viscosity: the friction that exists between the layers of the liquid. When the fluid flows over boundary the boundary acts like it’s deadbeat husband, holding it back, not giving it the permission to go as fast as it can. So the layer above it has significant velocity decrease(for subsonic flow). The layer over that is a little faster, and so on, until it’s out of the boundary layer, and the fluid layer moves as fast as it can. Viscosity tells you all that you need to know about a dead weight, the further you move away from them, the better your life can be.

Entrainment: The movement of one fluid by another. (can exist between two layers of the same fluid at different velocities). You can visualize this concept as how an audience sways to a musician’s tunes. standing awkwardly not knowing what to do one minute, to rhythmically shaking what your mother gave you the next. It’s not magic, it’s just emergent behaviour….but yeah it’s pretty close to magic.

Movie:  Tiny little fluid particles are on their way over the curved surface. So at time t=0 they listen to inertia and they don’t curve downwards around the curve, they move on down the same line (they don’t get far but wait for it). This moving layer entrains the layers right below it (past the curve). This entrainment causes the layers to leave that region leading to a reduced pressure there. Why reduced pressure? where there is a lack of fluid particles (owing to the fluid layer eloping with it’s second cousin), there is a lack of pressure being exerted on the walls of the surface. (Like when you’re finally done with high school ,but you feel a pang of loss for some reason). At this point in time, there’s a higher pressure on top and lower pressure below, this pressure difference creates a force which acts downwards. Force gives you displacement so this facilitates the movement of the fluid to move from high P to low P curving downwards.

Crazed fan’s plot interpretation on the pretentious blog: All of this happens at t=0.0000000000001, and so you don’t really see them trying to move down the same line. They get curved by the fluid military (force arising from pressure difference), leaving the law of inertia in a blubbering mess. Valiant attempt, my little-wet friend.

Click over here, go on, I dare you. How do wings generate lift?

## Do aircrafts have horns?

In a machine which has evolved over time to be as sophisticated as the modern day aircraft, you’d expect there not to be annoying horns like the ones that exist on its ground cousins. (no disrespect to cars but it had to be said). But they do exist.

### Why? Why do they exist? – a sincere question from a concerned migraine

For one, It’s to communicate to the ground crew. An alert like a horn could get them to put on their headset.
Also, an aircraft is pretty big. It can be as long as 60 m which are 60 adult human legs put together.

 F-16 looking fly

The cockpit needs to communicate the essential information received from the various parts of the airplane. Contrary to popular belief, it can’t be achieved by having monitoring minions in the engine yelling THE PLANE’S GOING DOWN. That’s not going to sit well with the anxious aunty who brought her own snacks in the economy cabin. So a series of sensors are set up at various critical parts of the aircraft and they transmit information via cables to light and sound signals on a panel in the cockpit called the master annunciator panel.

 master annunciator panel
This panel has a set of lights – red, amber and blue. When a system has a fault, depending on the magnitude of the problem, the appropriate lights illuminate.

Red- Caution, needs immediate action (YOU.)

Amber- Warning, needs crew’s immediate awareness

Blue- advisory, to let crew know that something is working as it is supposed to

But it’s not just visual cues, the (CAS) crew alerting system sets up auditory and tactile cues as well just for good measure. Hold up, what cues?

Auditory cues or in simpleton's tongue: Horns. And Other stuff.
"Delta four five one, winds two eight zero at eleven, cleared for takeoff." Young pilot grant just having been given permission, takes off from JFK and heads east towards Heathrow. After cruising at 14,000 ft, he climbs to 15,000 ft and is given a single chime of validation. Grant sighs taking in a deep breath. Working on little sleep and overwhelmed with exhaustion he switches over to autopilot. Maybe a little shuteye? PULL UP! PULL UP! Woken with a jolt, he gains control of his senses enough to know that Delta 451 was fast descending to a nearing hill. Just as he begins taking action, CLIMB! CLIMB! As if a series of unfortunate events were to follow, a timed beep beep beep beep and a series of horns successively bombard his auditory cortex. Hanging on to the single thread which holds his sanity, Grant realizes he can’t do it alone. As the co-pilot, you need to step up to the occasion (considering you were watching in awe as almost every system was gloriously failing before your eyes). But how can you help, if you don’t know what the sounds stand for?
TCAS and GPWS- voice warnings like pull up! Climb! For traffic or impending collision, or warning the closeness to ground.

 goddammit grant, you had one job

(Cool fact: TCAS and GPWS have been nicknamed bitching betty and the first ever voice to be digitized for this system was Kim Crow)

Pressurization: A continuous horn is noised when there are cabin pressure issues.

Configuration: A horn at intervals or a beeping sound when the control surfaces like flaps, slats, stabilizer trim, or speed brakes are not properly configured before takeoff.

Landing gear: A horn sounds and appropriate gear position indicator lights illuminate when an unsafe gear configuration exists.

Altitude: A single chime, along with a visual cue, alerts pilots when they are leaving the current altitude or approaching a new one.

Autopilot disconnect: Various kinds of siren, klaxon, or chime sounds, accompanied by red warning lights, signal that the autopilot has disconnected.

Engine or APU fire: FARs require that engine and APU fires be indicated by a bell accompanied by red fire warning lights.

Overspeed: An over speed "clacker" sounds when a limiting mach or airspeed is exceeded.
There you have it. No time to waste, young pilot grant needs you. Godspeed.