If you take a feather and a ball, and drop them simultaneously from your hand or from the top of a building what would you observe? Obviously the ball drops faster than the feather. But why?
Air resistance is the result of air molecules bombarding onto the object as it moves through the layer of air. The feather offers more air resistance and hence it falls slower.
Now you can up the ante and ask what if you remove the air resistance?
If you remove all the air molecules from the air,you would just get vacuum, a space devoid of any matter. With no molecules to bombard the object,
The feather and the ball would fall at the same rate as you can see in the animation. The demonstration was carried in the world’s biggest vacuum chamber.
( Extra: The same demonstration, but this time done on the moon :
Potential Energy of jumping dude — > Kinetic energy upon jumping — > hits the blob( some energy lost here ) — > Remaining energy propels the launching dude.
What i am trying to say is :
m1 = combined mass of the jumping dudes,
m2 = mass of the launched dude
Projectile is not exactly a parabola
If you look at the projectile, you would notice that it is not exactly a true parabola. This is because the projectile resembles a parabola without the effect of air resistance.
But under its influence, the shape of the projectile is a bit different.
The main thing to know is that a difference in pressure across the
wing–low pressure over the top and higher pressure below–creates the net
upward force we call lift.
Upon reaching a certain velocity, the aircraft’s lift is more than its weight and as a result, the aircraft takes off .
The Concept of a Boundary Layer (BL)
There is a high chance that you might have heard this word even in a casual conversation about wings and that’s because its an important concept in the context of aerodynamics and associated fields.
To understand the physics of a stall, lets consider the interaction of a moving air on a flat plate.
The nature of airflow over a wing/plate is the result of stickiness or viscosity of air.
The first layer sticks to the wing/plate not moving at all.
The second layer in frictional contact with the first moves slowly over it.
And the third layer moves somewhat faster than the second
Thus layer by later the flow builds up to the free stream velocity or airspeed. These layers of flow are known as boundary layers.
It takes a pressure difference between the top and bottom parts of the wing in order to produce lift. But when the flow of air becomes turbulent ( i.e during a stall ), this pressure difference is no longer established.
As a result of which, the lift drastically decreases and the aircraft starts dropping to the ground.
How to get out of a stall ?
Stalls can cause problems only when the pilot is not aware that the aircraft is stalling. ( Unlikely but has caused accidents in yester times )
As the airplane loses altitude, its nose dips down and airspeed picks up quickly. This restores the lift and the pilot would be able to regain control and bring the aero-plane into level flight.
How are stalls detected ?
On light aircraft there is a reed (much like used on a musical wind
instrument) mounted on one wing root, which is angled such that at the
Angle of Attack which would cause a stall, the reed “plays” which can be
heard in the cockpit.
Here is a view of where this system is mounted on a Cessna
On some aircrafts, it is a similar principal, however instead of a
reed, it uses a fin which at critical AoA pushes a micro-switch which
activates a buzzer/horn inside the cockpit.
Here is the assembly on a Beech 18
Large commercial aircraft typically rely on either Angle of Attack (AoA) Vanes or Differential Pitot Tubes to supply input to flight computers for the purpose of calculating AoA.
Source
Review:
A lot of important stuff regarding aerodynamics in this post. Here’s a summary of the post:
Boundary Layer concept — > Why do aircrafts stall ? — > How to get out of one — > How are stalls detected ?
That’s all folks!
Hope you enjoyed today’s post and learnt something new.
I am almost every time put in a trance whilst spectating an aircraft/jet takeoff . There is always something interesting in the occurrence that makes me go nuts!
And this time around, there are these series of rings that one can see in the exhaust plume of a jet engine when it takes off ( usually when the afterburner is on).
I had no clue about the phenomenon nor did I know how to express it in ‘search engine’ terms to find a match.
But upon discussion with some of friends, I was shown this video of a space shuttle launch that seemed to produce a similar pattern.
Hmm.. Interesting
Shock Diamonds
These set of rings/disks that are formed in the exhaust plume are known as Shock Diamonds or Mach discs ( and by many more names ).
And usually occurs at low altitudes when the pressure of the exhaust plume is lower than the atmospheric pressure.
How does it form ?
Since the atmospheric pressure is higher than the exhaust, it will squeeze it inward. This compresses the exhaust increasing its pressure.
The increased pressure also instills a increased temperature.
As a result, ignites any excess fuel present in the exhaust making it burn. It is this burning that makes the shock diamond glow.
The pressure is now more than the atmospheric pressure, and the exhaust gases start to expand out.
Over time,
the process of compression and expansion repeats itself until the exhaust pressure becomes the same as the ambient
atmospheric pressure.
In other words, the flow will repeatedly contract and expand while gradually equalizing the
pressure difference between the exhaust and the atmosphere.
The same occurs in rocket engines as well.
What if?
What if the atmospheric pressure is less than the exhaust plume ( like at higher altitudes ), would we still see shock diamonds ?
Yup, we would! And here’s a picture of it too ( The Bell X-1 at speeds close to Mach 1 )
The same phenomenon as discussed above occurs except that the cycle starts with the exhaust gases expanding to atmospheric pressure first.
Sulphur Hexafluoride is denser than air, hence accumulates in the bottom of the container. ( if you find it weird you can think of Sulphur hexaflouride analogous to water ). The boat filled with air, as a result floats!
The tendency for an object to follow a curved path instead of a straight one is known as the magnus effect.
This is due to the generation of a side wards force on a spinning spherical or cylindrical object known as the Magnus force.
The mechanism.
The mechanism is rather a simple affair – As the rotating object moves through the air, it pushes the air on side of the ball whilst disturbing and slowing the flow on the other side down.
What this does is, it creates a pressure differential. Faster moving air exerts lesser pressure and slower moving turbulent air exerts a higher pressure. And as a result, the ball experiences a force ( known as the magnus force ).
This force vector points from the higher pressure region to the lower pressure one.
The Aerofoil similarity
If you think about it, it is the same principle as a lift in an aircraft. ( the difference off course lies in the fact that for the Magnus effect to take place, the object must spin )
More the pressure differential, more the lift.
Now an obvious question might strike your mind – What if the wings rotated? I present to you the Flettner Rotor Aircraft which whose wings rotate in order to generate lift.
But sadly, the drag that these things produced were also considerably high rendering them impractical! Ahh..
( If you are interested, check out the No sail boat that Flettner built that actually works and uses the magnus effect )
Magnus effect in sports
Magnus effect underlies various sporting action. For instance that spectacular Ronaldo’s free-kick was a consequence of the magnus effect.
Cricket – Spin Bowling.
Notice the change in direction of the ball during flight.
Tennis – The dreaded Topspin
The ball experiences a downward push as a result of magnus effect.
Baseball – A pitcher’s delight
This pitcher just knows how to rule the game.
You get the idea, right ? Basically, any sport that involves a rotating body, you are most likely to find the magnus effect coming into play.
Have fun exploring this effect in your own unique way and as always have a good one! Cheers!
*** The phenomenon simplified for the sake of explanation. For those who are seeking out answers from a fluid mechanics and mathematical perspective ( boundary layers, flow stagnation,etc ) check out any standard Fluid mechanics Text.
This is known as wingtip vortex. It is a ramification of the design of the wing and how it works.
How does an aircraft fly? Think of it like this, due to the design of the wing, larger number of air molecules are hitting the bottom portion of the aircraft than the top.
As a result, a upward force acts on the wing, hence the wing lifts!
This works fine till we get to the wing tips.
In the wingtip, the air from a higher pressure wants to move to the region of lower pressure. And as a result, this forms vortices ( fancy name for the swirling motion of air ) known as Wingtip Vortex. ( because its formed in the wing tips!!! )
Why do birds fly in a V formation?
Migratory birds take advantage of each other’s wingtip vortices by flying in a V formation so that all but the leader are flying in the upwash from the wing of the bird ahead. ( Look at the image, each one is exactly out of phase in its wing motion ).
This upwash makes it easier for the bird to support its own weight, reducing fatigue on migration flight.
And somehow birds know about this and recalibrate themselves in flight?
Wow! There is so much more to a bird’s flight that that meets the eye. I will take up the same sometime down the line. But, If you are really curious to find out why, read this nature article.
On November 7 the Tacoma Narrows was seen dancing happily to the song of the winds. But unfortunately, the bridge collapsed only after an hour of swaying.
The reason for collapse of the Tacoma Narrows bridge is often attributed by many physics textbooks as Resonance. But it is incorrect. Atleast, the way in which it is explained in textbooks.
This real reason for it’s collapse is what engineer’s call as Flutter.
The collapse Mechanism.
I’ll start explaining this by highlighting some key points in the bridges design.
The bridge itself had a span of 4944 ft (1506.9 m) and connected the city of Tacoma to the Kitsap Peninsula. The bridge consisted of two pillars which suspended the central span which itself was 2800 (852.4 m) feet long and 39 ft (11.9 m) wide. During the construction of the bridge itself, it was reported that some transverse (vertical) oscillations occurred across all three segments of the bridge with the two pillars and the connection to the shore acting as nodes (areas of no oscillation).
To counteract this, the left and right sections of the bridge were reinforced by diagonal ties and hydraulic buffers that damped the oscillations but the center of the bridge was still free to vertically oscillate.
During the short commercial lifetime of the bridge (between it’s completion on 1st July, 1940, until 7th November 1940), it earned the affectionate name of “Gallopin’ Gertie” as it frequently oscillated with a range of 0 – 8 nodes between the two pillars. The maximum amplitude before the gale that caused the collapse of the bridge was recorded to be 5 ft (1.52 m) from crest to trough at a frequency of 0.13 Hz and well within the range of maximum stress the bridge was designed to withstand.
Several days before the 7th November, it is believed that that K-bracing under the deck and diagonal ties at the support pillars had been weakened during a storm; with one witness reporting to have seen the bridge behaving differently (this is later interpreted to mean that the bridge had been displaying larger than normal transverse oscillations).
On the morning of 7th November, the Tacoma Narrow bridge was buffeted by wind velocity of 42 miles per hour. The high winds, suspected structural damage and the loosening of diagonal ties combined to cause a fatal combination of transverse and torsional (twisting) oscillations.
But what caused the eventual collapse of the bridge?
Most A level text books will attribute the collapse of the bridge to resonance caused by the frequency of the driving force (supplied by the wind) being similar to the natural frequency of the bridge causing a drastic increase in the amplitude of oscillation (see resonance curve below) and the eventual collapse of the bridge due to the stress exerted by the increased amplitude.
This has several fatal assumptions, the most obvious of which is that the gusts of wind would occur with any defined or regimented period (which is clearly ridiculous).
[Though if you want an example of a bridge collapsing due to resonance, England’s Broughton Suspension Bridge is a fun one or the Millennium bridge is enticing as well ].
A more credible explanation is that that collapse was triggered by a phenomenon called Vortex Shedding.
A vortex is a region in fluid medium that flows around an axis. Vortex shedding is where a fluid flows past a buff (non streamlined) object and results in an oscillating flow of vortexes that detach periodically from either side of the body. It is believed that these periodic vortexes exerted a periodic force alternately on the top and bottom causing the torsional oscillation around the central line of the bridge.
It can also be inferred that the frequency at which vortexes were produced must have been similar to the natural frequency of the bridge. This would have caused resonance and thus would explain why the amplitude of the torsional oscillation was large enough (and able to overcome frictionaltension forces that would have reduced the amplitude) to exert enough strain on the bridge to cause it to collapse.
However, there is more to this argument than originally appears.
For the mathematically inclined, the frequency of shredding vortexes is defined by,
Where St is a constant called the Strouhal number (for the Tacoma Bridge this constant is 0.11), f is the frequency of the detachment of vortexes, D is frontal dimension (in this case, the depth of the bridge platform was 8 ft (2.44 m)) and V is the Velocity of the wind.
From this we can calculate the the frequency of the vortex shredding is close to 1 Hz (1 vortex being produced per second). The observed frequency of the bridges oscillations was close to 0.2 Hz.
This means that the Shredding Vortexes cannot have caused resonance and there must have been another phenomenon in action.
Whilst the shredding vortexes may have caused the initial torsional oscillation, the increasing amplitude was a self induced.
When an object changes direction in a fluid stream, it causes new vortexes to form behind it. This is known as a Flutter Wake. This is caused when the air flow is disconnected from the surface and vortexes flow into the newly formed low pressure region. This can be seen with a schematic of a plane wing changing direction.
Similar to the shredding vortexes, the flutter wake exerted a force on the bridge increasing the amplitude of oscillations. As the amplitude of the oscillations increased, so did the difference in air pressure between the surface of the bridge and the undisturbed air flow causing more vorticity. This in turn increased the force exerted by the flutter wake and as a result increased the amplitude until the bridge reached breaking point.
The mean net force experienced by the bridge can be described as negative damping, rather than the amplitude decreasing over time, the amplitude increases until breaking point.
This is a perhaps similar to “Which came first, the chicken or the egg?”.
The (shredding) vortexes causes motion, and the motion causes more (flutter) vortexes.
The wind supplies the power, and the motion supplies the power tapping mechanism.
A big shout out to Sophie Meredith who painstakingly drafted the collapse mechanism. This post wouldn’t have been possible if not for her. Thanks a lot !
Total internal reflection is a phenomenon which occurs when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface.
This occurs when the incident angle is more than the critical angle and the refractive index is lower on the other side of the boundary ( like from water to air ).
Pardon me for being melodramatic. Think about it, rain drops fall from thousands of feet in the air and yet we hardly twitch when one falls on us.This defies all logic, if you drop a penny from the top of a building and let it fall on you, we know that it hurts!
Have the angels cast a magical spell on the rain drops to spare us from the pain?
Terminal Velocity.
When you drop something in air, it does not accelerate forever. Molecules in air constantly bombard with the object, exerting an upward force. This is known as air resistance or drag.
As the object gains velocity there comes a time when the force of the air resistance is enough to balance the force of gravity, so the acceleration stops and the raindrop attains terminal velocity.
Terminal Velocity is the maximum velocity an object can travel in air!
The Angel’s mystical spell.
Rain drops are only 0.5 mm-4 mm in diameter. Their terminal velocity is only about 10 m/s ( 20 mph ) That’s the maximum speed that they can travel in air irrespective of their initial height. Also the mass of the rain drop is about a few milligrams.
Hence, the force that it exerts on the body is really small, small enough that we find the experience pleasurable and soothing.
Ecstasy Shots 