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A Crash Course in Aerodynamics and Physics


So for my 60th topic I decided to make this. I chose this topic because on the forums you will see a lot of speculation and such as to planes performance IRL I figured it would be a good idea to make a thread explaining what in the world we are talking about to newcomers. For starters let me explain how a plane is even able fly.
Bernoulli's principle
the actual equation to calculate the effects of this principle is the following
307ff04dd5267d3c26bab53b54ed3637.png
now for those of us who don't have a clue to what half those symbols even mean, let me give the definition of the principle

"an increase in the speed of the fluid occurs simultaneously with a decrease inpressure or a decrease in the fluid's potential energy

-wikipedia

now how does this apply to flight you say? Simple, lets look at the crosssection of the wing like the one displayed below,
right2.gif
As you can see the bottom section of the wing is much straighter then the curved upper pressure, this causes the air on the top to move faster then the air on the bottom causing it to have a lower pressure on the bottom, this lower pressure generates lift due to the slight vacuum effect created.
 
This also explains why flaps make the aircraft land slower, basically as the flaps are extended they slow down the are on the bottom surface of the wing even more. This slower are causes the pressure differential between the lower and bottom part of the wing to be even greater. Because of this more lift is generated
 
Angle of Attack (AoA)
Angle of attack is the angle of a wing relative to the oncoming wind, the higher the angle of attack the closer the aircraft gets to stalling. The reason this happens is because at the critical angle of attack the air over the wing will no longer have a laminar flow to it. This loss of laminar air flow will cause the pressure differential between the top and bottom part of the wing to disappear. This disappearance of a pressure differential causes the wing to no longer produce lift. This lack of lift then produces a stall.

The interesting part about AoA is the fact that as you approach the critical angle of attack the wing is actually producing more lift. The reason this happens is due to the same reason that flaps create more lift. Basically the greater the AoA with a laminar air flow the faster the air flows over the top of the wing and the slower over the bottom. This causes a even lower pressure on the top and a even higher pressure on the bottom. This ultimately creates more lift.

incline.gif

376px-Aircraft_flaps.svg.png
 
Compressibility/trans-sonic aerodynamics

First off I need to explain how going supersonic works. Going supersonic means breaching the sound barrier, or going faster then a sound wave. Now a sound wave travels at the speed of sound (who woulda thought?) and that happens to be 1,236km/h or 767.7mp/h. The speed of sound is also a speed measurement known as MACH. So if someone says they are going Mach 1 that means they are traveling at the speed of sound. It should be noted though that a Mach number is not always listed as the same way as most measurements, instead of saying "I am going 1 mach" you say "I am going mach 1" 

Simply put as a aircraft reaches the sound barrier the air around it becomes turbulent and causes the aircraft to shake. This turbulence can and has shaken aircraft apart before. Now let me explain how the air becomes turbulent. Basically as a aircraft passes through the air it creates shock waves, typically these shock waves all travel at the same speed, however as a aircraft approaches the speed of sound these shock waves become uneven with some of them being super sonic and some of them being sub sonic, this difference in pressure causes the wing to shake back and forth creating the turbulence created at these speeds. however as Soon™™™™™™™™™™™™™™™™™™™™™™™ as the aircraft passes over the sound barrier these effects dispensary due to the fact that the wing's shock waves are traveling at the same speed again.
transflow.jpg
It should also be noted that the speed of sound decreases with altitude making these affects appear sooner
speedsoundaltitude.jpg
 
 
Swept Wings
Swept wings help a aircraft achieve higher speeds in a similar way angling armor helps give tanks more protection. This is because every single wing has a critical mach number This number is the point at which the wing will begin approach speeds where it will break apart due to drag and the effects of compressibility as it reaches the speed of sound. Now the reason the wing can go faster when it is swept is because critical mach number is effected largely by the length of the wing. This is due to the fact that the larger the surface area a force is applied to the amount of force needed to push it increases. Therefore when on makes a wing swept he is making the surface area of the front of the wing to the back of the wing seem longer to the air coming at it from a forward direction therefore making it able to achieve a higher critical mach number.
f0491-02.gif
 

Newtons laws

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IAS VS TAS 

Ok, for starters let me explain what those two acronyms mean, IAS means indicated air speed and TAS means true air speed. Now at first glance you would probably say "Ok say we only use TAS right?" the answer is actually no. The reason for this is that TAS is your speed relative to the air at sea level which is good for navigation but not much else. However TAS varies with altitude and is not relative to your speed on the ground. Let me explain what I mean by this.

 

Aircraft measure speed by measuring the air pressure exerted on a plate in a device called a pito tube. This also means that any wind coming at the plane will make you airspeed indicator say you are moving  even though you are completely still. now here is why you should use IAS more then TAS, despite IAS usually not conforming to how fast you are moving across the ground it is still more useful then TAS since it indicates how close your plane is too stalling. The reason for this is the fact that the Pito Tube measures off air pressure (as explained before) and your wings work due to air pressure. Therefore when the Airspeed indicator says you are near stall speed you really are near stall speed. Meanwhile your TAS could be saying you are no where near stall speed.

 

pitot.jpg

 

For example lets say you are in a Spitfire MK VB traveling at 150kmh IAS well at 6,000ft. Now knowing your IAS you would realize you are near your stall speed and would try to get some more speed built up. However if you were using TAS you would see that you are traveling at 209kmh and not think to much about your planes stall speed.

 

IasTasChart.jpg

 

Diving

Ok this might sound a bit simple but in practice it is not. When a plane dives there is more then just one factor in play, in fact there are many, here are just a few.

  • Air Resistance
  • Engine Power
  • Weight
  • Type of fuel system
  • Length of the dive
  • start speed

There are a few more factors then the ones I just listed but these are the big ones.

The air resistance is important is because if take a cube with a volume of 1m3 and a sphere with a volume of 1m3 that both have the same material, hence the same weight, and drop them off a 10m high building the sphere will hit the ground first. Why? simple a sphere has less air resistance then a cube which means that it is able to fall at a higher speed. The shape factor favors the sphere.

 

The engine power is fairly self explanatory. The more powerful the engine the easier it is for the object to push through the air in front of it due to the added force of the engine. For example lets say you have two identical cubes that both weigh the same (even with the engines attached) however one has a 10HP motor and a propeller with a radius of 1cm  and the other having a 20hp engine and a propeller with the same radius. As you would expect the second cube would hit the ground first due to its more powerful engine.

 

The weight is important due to the same reason a bigger engine is important. The heavier the weight the easier the object can force its way through the air (at least when falling). For example lets say you have two identical cubes with a area of 1m3 however the first cube weighs 1 kilogram and the second cube weighs 2 kilograms.If the distance is big enough, the heavier cube will hit the ground before the second one. Both have the same acceleration (gravity), but the heavier one will have a higher terminal speed (max speed before friction force compensate gravity force)

 

The type of fuel system is important but a bit trickier to explain. Basically in an aircraft's fuel tank there is a device called a fuel pump which pumps fuel to the engine. Typically this fuel pump is located at the bottom of the tank because of gravity (fuel moves to bottom of tank), however lets say you nose down into a dive. Where does all that fuel go? Due to inertia it actually goes to the top of the fuel tank. Know what happens when your engine has no fuel? Simple, it stops working. So until that fuel travels back down to the bottom of the fuel tank you have no engine which means that you will not be diving as fast. Know at this point you are probably thinking "Wait wouldn't this last for just a few seconds and make it not that big of an issue?". In fact, the location of the pump is also a problem: if the pump is at the "back", when you dive, the whole fuel is at the "front", so even though you pump is at the bottom, the fuel won't feed the engine.

 

The length of the dive is important because some aircraft (Usually lighter ones) will accelerate faster at the beginning of a dive but Soon™™™™™™™™™™ loos their acceleration advantage to heavier aircraft a bit later into the dive. So this means in short dives a BF-109 can actually out dive a P47 despite the P47 being considerably heavier.

 

Start speed is important because if you have two planes (both the same model) diving from the same alt but the first planes starts at 200kmh and the second one starts a 400kmh,  the second one will be diving better until it hits its break up speed (it will be even faster, but uncontrolable). This is despite the fact that the two planes could be exactly the same.

 

Mass VS Weight

Ok if someone here were to mention the mass of an object would that mass directly correlate to the weight of an object? The answer is both yes an no. if you are referring to the weight of the object on earth then yes mass directly correlates to the weight of an object. However lets say we are on the moon, in that case then no mass does not directly correlate to weight.

 

The reason mass and weight are not always the same is due to the meaning of the two words.

  • Mass, the amount of matter contained in an object
  • Weight, The amount of the force of gravity acting upon an object

What this means is that an objects mass is defined as how many atoms are contained within the object. No matter where you go this amount will always be true unless you physically go up and take so of the mass away from it by breaking it off. Weight on the overhand is how much force gravity exerts on an object. Due to this, if you have an object on earth and an object on the moon that both have exactly the same mass the one on earth will weigh more then the one on the moon.

 

Now if you are still confused let me explain further, lets say you have 10 pins, no matter where you are you always have 10 pins unless you choose to get rid of one. This is similar to the way mass works, an object always has the same amount of atoms in it no matter what unless one is removed (be it by a collision with another object or some other mean) this means that even if the force of gravity changes mass will always be the same. 

 

Weight on the other hand is not a number but rather a value. Lets say you have the same 10 pins falling at 10m/s but then you suddenly turn off the earths gravitational field and place a piece of wood under the pins to make them stop. now, has the mass of the 10 pins changed? no. Has the weight of the pins changed? yes. why? because there is no longer any gravitational force from the earth acting upon them (though there would be from the sun the moon and possible mars).

 

Now here is where things get confusing. Lets say you see a plane take off, based on what I say you would think the weight of the plane has changed. Though in actuality it has not. Confused? let me explain. Weight is the amount of the force of gravity acting upon an object, when a plane takes off it is due to its wings generating lift due to low pressure areas caused by them. This means that there is still the same force of gravity acting upon the plane meaning that its weight is exactly the same.  

 

G-Forces

Simply put G-Forces are simply an effect of inertia. In fact it is nothing more then a measurement of it really. This is because 1 G-force represents the earths gravity in its normal state. Now lets say you are in a car that is accelerating really fast. Due to your inertia you will be pushed back into your seat. This pushing effect is likely 2-3 G's or 2-3x the force of earths Gravity.

 

Now hears where G-Forces get interesting, what happens if you get negative G-forces? The answer is you take off into the air. It can also create simulated weightlessness when aircraft go into dives and such. This is because the plane is falling faster then the body can so it seems as if your body is just floating when it is in fact falling. 

Here's a good example

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​Wake Turbulence (By the Pandanator2)

F0703001.gif

 

So, what is wake turbulence?

 

 

Wake turbulence is what you get when you fly behind an aircraft. What is causing it is the rotating vortices of air coming off the wing tips of any plane. This is shown in the picture above. So, when these little vortices hit your plane, it causes buffeting, or shaking. It's quite a simple idea, but with far deeper things that I won't get into. 

 

 

Further Reading

Aerodynamics for Naval Aviators

http://www.avia-it.com/act/biblioteca/libri/PDF_Libri_By_Federal_Aviation_Agency/Aerodynamics%20for%20naval%20Aviators.pdf

Aircraft Weight and Balance Handbook

http://www.avia-it.com/act/biblioteca/libri/PDF_Libri_By_Federal_Aviation_Agency/Aircraft%20Weight%20and%20Balance%20Handbook.pdf

Aeronautical Information Manual

http://www.avia-it.com/act/biblioteca/libri/PDF_Libri_By_Federal_Aviation_Agency/Aeronautical%20Information%20Manual.pdf

Aircraft Maintenance Technician General Handbook, Physics

http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_handbook/media/FAA-8083-30_Ch03.pdf

Aeronautical Information Manual

http://www.faa.gov/air_traffic/publications/ATPubs/AIM/aim.pdf

Pilot's Handbook of Aeronautical Knowledge

http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/FAA-H-8083-25A.pdf

 

Did this make no sense to you?

Read this then try again.

http://forum.warthunder.com/index.php?/topic/114673-aerodynamics-for-dummies/#entry2206540

Edited by Flying_pig2
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Great idea, but no Sir Isaac Newton? Currently the FAA dictates that we teach Bernoulli and Newton but modern aerodynamics seem to be leaning on it mostly being newton and only a little Bernoulli that makes airplanes fly.

However I only skimmed I did not have time to read the hole thing so I likely missed something. Edited by Aoe
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Great idea, but no Sir Isaac Newton? Currently the FAA dictated that we teach Bernoulli and Newton but modern aerodynamics seem to be leaning on it mostly being newton and only a little Bernoulli that makes airplanes fly.

However I only skimmed I did not have time to read the hole thing so I likely missed something.

Am working on getting my AMP certificate right now, was taught the Bernoulli's principle more then newtons 2nd law. (and this was about a week ago...)

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Great idea, but no Sir Isaac Newton? Currently the FAA dictated that we teach Bernoulli and Newton but modern aerodynamics seem to be leaning on it mostly being newton and only a little Bernoulli that makes airplanes fly.

However I only skimmed I did not have time to read the hole thing so I likely missed something.

 

F*ck Isaac Newton. Invents calculus to torment four centuries worth of students, yet it takes a German to invent a legible notation.

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Inb4 Nabutso says FAA is and 18th century physicist are biased.

+1 would laugh again. Wouldn't be surprised if this thread gets pinned

Edited by Bug4799

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I'm a cfi-i and advanced ground instructor. There are holes in your education.

a very very good place to start reading and learning it on your own so you can prove me wrong is " aerodynamics for naval aviators"

Or if you don't mind very old but still usefully books in plane English " stick and rudder" each easly found on amazon but when I have questions I find it best to read from people far smarter than me.
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That is the point of this thread, just to give newcomers a quick 'crash course' into what we are all talking about, this is not perfect but was more meant to just explain the concepts.

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P-51 won the war cuz US rocks, laminar flow only helped. Though really it was the uber powerful freedom cals that saved the day as the 30mm shells bounced off the fuselage like no big deal  :crazy:

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You must remember that I just got 2 weeks of this stuff 3 hours a day 5 days a week... this is a crash course compared to that... (will add newton eventually)

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learning to repair the plane is considerably harder to teach... (you do it wrong and you could kill hundreds of people...) it will take me 2 years to complete the course I am in...

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