Rapitor

Aerodynamics for dummies

Hello everybody.

I read carefully this thread http://forum.warthunder.com/index.php?/topic/109074-a-crash-course-in-aerodynamics-and-physics/ from Flying_Pig2. This is a crash course more technical than mine. I wanted a thread about concept, not equations. I wanted for a "dummy course" about aerodynamic.

 

So, today I want to explain, for those who don't know, with light concepts and few simplifications, the basis of aerodynamics.
 

Here is a little intro (from Sshadow2015 suggestion):

https://youtu.be/Gg0TXNXgz-w

 

My plan will be the following:

- Lift
- Basis of a wing
- Steady flight

- Drag
- Stall

- Role of flaps / slats
- Climb speed

- Dive mechanic
- Speed lock

- Ailerons reversal
- Compressibility

- Divergence
- Flutter

 

But first, a few glossary:
Leading edge: LE
Trailing edge: TE
symmetrical: sym.
Angle of attack: AoA

Lift
Lift is the vertical component of the force that allows your plane to take-off and fly. A wing generate a disturbance in the flow, and a pressure differential. This differential will create a force than will lift your plane. This force is not  always vertical, as below:
resolut.jpg

There is lift because there is flow. A static plane cannot fly. Lift is a difference of pressure. Due to fluid mechanics, the upper part pressure is low, the lower part is high. Low pressure creates "suction", high pressure "presses" your wing, so you have lift from down to above.

 

I said that the pressure is not equivalent at the upper and lower part. Basically, a Swiss Scientist (Swiss rox!) named Bernouilli showed that for a fluid, neglecting high speed effect and loss, there is a relation between speed, pressure and altitude. A wing is thin, so basically all the points are at the same altitude. We have, for a wing, a relation between speed and pressure.

If speed increases, pressure decreases. Due to the wing shape, the speed on the upper part is higher than the speed on the lower part. Here come our pressure differential, thus our lift. Adding to that Newton 2nd law (you deflect air to the ground, so have deflect you to the sky), and you have all the reasons why a plane can fly.

 

Your wing creates lift, but also drag.

Basis of a wing
There is two important points on a wing, if you want it to generate lift:
1: the shape
2: the angle of attack

1: the shape can be symmetric or cambered. With 0 angle of attack, a sym. wing will not generate lift, while an cambered wing will generate lift. However, due to it's shape, the sym. wing will generate less drag. We can summarize the situation with the following picture:
fig18.gif
Thick and/or cambered = high lift, low speed, thin and /or symmetrical = low lift, high speed.
There is no perfect wing. It depends upon the use of it.

2: The angle of attack, is the angle between the flow and the chord. The chord is the line from the Leading Edge to the Trailing Edge. As explained below:
angle_of_attack.jpg
The chord can be outside the wing for a good portion if the wing has a huge camber.

chord.gif
The bigger the AoA, the higher the lift. Well, it is true until a certain point.

However, the higher the angle of attack, the more drag you produce. This drag will slow your plane down. Hence, you will be too slow to generate enough flow, so you will eventually have no lift at all and stall, then crash.

One great (still not very known) thing about AoA: for a given wing, at a given speed (IAS), there is a given AoA to get a given lift.

Steady flight
The steady flight is a flight when everything is balanced: thrust compensates drag, lift compensates gravity.
670px-0,978,0,463-Steadyflight.png
The max speed of a plane is when you can't compensate more drag with your engine at full throttle. Drag depends on the speed, so the faster you go, the more drag you will suffer. Engine power and plane shape influence the max speed.

 

Drag

Drag is the unwanted component when it comes to fly. really useful when you want to brake, really annoying to reach high speed. There is mostly two types of drag: induced drag, due to the way the lift is generated at low speed and parasite drag. Parasite drag is totally dependent on the shape of your flying object. Both depends on speed, but not on the same way, as you can see below:

drag-curve.png

Induced drag tends to disappear with speed, while parasite drag grows. Induced drag is the consequence of the pressure differencial between lower and upper part of the wing. On the tip of the wing, pressure is trying to balance, hence there is an airflow coming from below to the top. This creates a vortex. Vortices disspate energy, and in this case, your plane energy. Parasite drag is a form factor. You experienced that your hand in the wind is pushed back by the airflow in a stronger way when your palm is perpendicular to the flow. When you are "slicing" the air with the side of your hand, the resistance is lower.

You can see that the faster you go, the more drag you create. This is why you cannot go infinitely fast with a 200HP engine.

There is a minimum drag point. Keep it in mind, it will be useful later on.

 

Stall

A stall is the action to lose a significant part of the lift, hence not flying but falling.
There is two type of stall: leading edge LE stall (thin wings) and trailing edge stall (thick wings). The first one is "instantaneous", while the second one is "progressive", starting on the trailing edge, slowly progressing toward the leading edge.
StallFormation.gif
TE stall will reduce progressively the lift, while LE stall with reduce all the lift in an instant.
This is why biplanes, in general, recover faster from a stall than a fighter or a jet: slower plane with

thicker wings.

 

Slats / flaps
The slats (LE) / flaps (TE): they are mobile parts, that can deploy or retract, and change the effective chord of the plane. You can go slower, and/or with a higher angle of attack, without any stall. You are delaying the critical point of stall, in exchange of a lot of drag. Still, while landing or taking-off, you want lift, altitude, not speed.

2.gif

Why using them ? in "retracted" configuration, you have a nice thin wing, to go fast with few drag, to land you can go slow and nice, less stressful for the pilot and for take-off, you can climb fast and reach ASAP cruise altitude, without being a pain in the neck for the nieghbour.

Slats/flaps are a cure against stall, and allow a plane to have both "Thick" and "Thin" wings advantages when it needs to. The system is generally heavy and complex, so you need to justify such device on a small plane.

 

On the same idea "combat" flaps increase a little the lift, and a little the drag. More lift = smaller turn radius = better maneuver

 

Climb speed

Remember when I told you to keep the drag minimum in mind? It is for now !

Basically, every plane has a preferred climb speed. This is basically when you are racing the minimum of the drag curve. For example, a BF 109 E has a climb speed of 250 kmh. Going slower will raise your AoA, with your drag too. The nose is higher, but the real fly path of you plane is below the previous one at 250kmh. On the other way, if you go too fast, you have not a good AoA, and you are not climbing efficiently.

Using climbing speed = best altitude in a given time. Really usefull to be on the top of the map before the other in RB / SB.

You have 2 typical climb speeds. Vx, which will allow you to be as high as possible over a distance as little as possible, and Vy, which allow you to be has high as possible, without any consideration for the distance flown to be up high. This sum up on this picture:

vxvy-1.jpg

As you can see, Vx < Vy, but the distance flown is also smaller. It might be interesting to use Vx when you don't want to go too deep within enemy line while climbing, even though I would recommand to simply use Vy and do not aim straight at the enemy airfield while climbing.

Being slower than Vx will make you be on a slope below the steeper one, even though your speed is lower (bad) and being faster than Vy will also put you a slope lower than the bigger one, which in both case results in you wasting time and altitude.

 

Dive mechanic

There is little tricky thing here: you have two opposite phenomena. First, a lighter plane will be able to accelerate faster than an heavier one thanks to its engine. On the opposite, a heavier plane can rely more on gravity than a light plane to counter the parasite (speed) drag.

In general, (i won't show you the equations and stuff), the critical point is the max level speed of both plane. Hence, the beginning of a plane favor the light one, while the middle / end of the dive favors the heavier plane.

Another problem to take into consideration is the maximum speed that the plane can sustain. Obviously, if your plane break at 500kmh, you will not dive again a plane that can sustain 800kmh, since you won't be able to lose him in pure speed.

 

Speed lock

Dive mechanic brought us speed. Speed is actually dangerous at some point. To maneuver an airplane, you have control surfaces. These control surfaces are disturbing the flow.

differential_aileron.gif

However, to divert the flow, you need to apply a force to it. 2nd Newton law, the flow will apply a force to the control surface, hence your stick. Air is light, but when you are going at 700kmh, the mass flow is enough to prevent you to do so.

This is why, at high speed, a plane seems slow to obey the input command: the pilot lacks of strength, and cannot use the stick at full potential, hence the plane don't react at full speed.

 

Ailerons reversal

Basically, this is a consequence of speed lock. Let's say you can use you still, because a device allow you to exceed the flow's force applied on your control surfaces. The force, even if it is exceeded, is still acting on the surface. And the surface is bound to an hinge, which is not on the structural beam of the wing. This create a lever effect. If your wing is not stiff enough, you will twist it.

964189.jpg?528

This will nullifiy your input, or worse, make your plane behave the opposite way !! Spitfires (before model MkV), or even Mig-15 and Mig-17 (early models) suffered from that. Nowaydays, Airliners can suffer from this problem, and the onboard electronic prevent the pilot to do so by limiting the input.

 

Speed regime

-Subsonic: you are slow (below Mach 0.3) everywhere. Everything is fine

 

-Transsonic: remember when I said that upper part airflow is faster than lower part ? Well, above Mach 0.3, there is a risk that your airflow on the upper part is so fast, that your are locally supersonic, and creating a local shockwave. The theory stated above is generaly true, in exchange of a few considerations / correction factors. This local shockwave will strongly modify the flow, and sometime you will have a separation after the shock. So, even though you are fast, you are like in a stall: uncontrolable.

 

-Supersonic: the incoming airflow is at Mach 1 or above. Nothing from the theory stated above is true. Supersonic mechanic is way different from subsonic mechanic. The shockwave creates a huge increase of pressure after the shock, and your airframe, as well as the airflow are affected by that. The airframe might break ("Sonic wall"), and the airflow behave differently from subsonic case. This will affect your plane behavior.

 

Compressibility

Compressibility is a property of gas, but also, in some extent, liquids. When you reach a critical speed on the upper wing, you will have some air going at the speed of sound, even though the whole plane is still below the speed of sound. This local area will suffer from a local shock wave.

4-56.jpg

The flow goes smoothly supersonic, but creates a sockwave to slow down. This shock wave will disturb the flow, creating a (possible) separation of the flow. Separated flow implies that your control surfaces might not respond at all. The is why your plane can, at high speed, not respond at all.

 

However, people tends to confuse compressibility and speed lock. Mostly because they don't know a single principle of aerodynamics.

 

Divergence

While flutter (next section) is a good sign to tell you that you are going too fast for your plane, divergence is a deadly consequence.

Your wing is a beam. A shaped beam, but a beam. It has a certain rigidity.

When you are going really fast, you have a lot of lift and drag on your wing. This is compensated in some extent by the rigidity of your wing. However, at a certain point, the wing is not stiff enough, deforms too much, increase then even more the AoA, thus is under even more constrain, and finally break (everything in like half a second). This is why you can break your wings in a straight line.

To prevent this, we can have sweep wings, like most of the planes nowadays. Swept wing, due to the geometric shape, does not suffer from divergence past a certain sweep angle.

 

Flutter

Flutter is when various phenomena happen on your wing, and excite it round the eigenvalue of the beam. You saw a flag being blown by a strong wing ? It is flapping like hell. This is flutter. So, you have an excitation from the flow around your wing, and this might be at the same frequency that the material natural frequency. If it is, you wing will oscillate more and more.

Flutter can happen before or after the divergence. However, "after" is purely theoretical, since you have no more wings.

 

If you have any questions / comment, I would be glad to have you shearing them with me. I would love to answer any unclear point.

 

Rapitor.

 

Updated 18th February 2016

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Does this apply to AB?

 

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Just messing with you! This was informative and well structured. +1

Edited by Spookas
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Does this apply to AB?

Like all law of physics: it does not apply to AB

 

Joke aside, it does apply to AB, but eveything is boosted. While prop-hanging is not possible for a long time in RB, AB might allow you twice as much given the same condition.

 

If you can pull RB energy-maneuvers in AB, you might be an good E-fighter in RB.

Edited by Rapitor
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I don't know why the hell I can't give you thanks for the first post, but atleast I can upvote you.

This is really good written and I really appricieate your work with this.

Thank you for that Rapitor !  :salute:

 

I would love to Pin your post here, but I think it deserve better place than Academy.

Sadly, I have no power outside Academy, so let's wait for Moderators...

because this really deserve to be elsewhere.

Edited by Carrier_
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I'll reread my typos (English is not my native tongue though) and explain more clearly. See flying_pig2 thread (link in the OP) for now
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Lol, no worries mate. It was just a tongue in cheek post really. I'm still in the "I just wanna fly around and kill people!" stage of the game. "Technical" hurts my head. Both of you write amazingly good and helpful threads.   :salute:

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Sorry Rapitor, aeroelastic flutter is NOT resonance. Otherwise a well structured post.

The difference is resonance is created by harmonic effect, while flutter is by constant aerodyamic force.

The Tacoma narrows bridge was a classic aeroelastic flutter event that is mistaken for resonance some times.
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Sorry Rapitor, aeroelastic flutter is NOT resonance. Otherwise a well structured post.

I'll make the flutter part more clear, just in case.

The difference is resonance is created by harmonic effect, while flutter is by constant aerodyamic force.

No, flutter is not constant at all. Wing oscillates -> angle of attack changes -> aerodynamic forces vary.

The Tacoma narrows bridge was a classic aeroelastic flutter event that is mistaken for resonance some times.

flutter.

If the fluid-structure coupling frequency is not close to the natural frequency of the wing, then the flutter is not really perceptible. Flutter, like divergence or galloping, is a fluid-structure interaction. there is fluid, but also structure. When a structure oscillates, there is resonance, in order to make it go past the breaking point. Flutter is not resonance, but without resonance, flutter is not critical and will self sustain and lead to destruction.

FE computing showed that the oscillation of Tacoma's bridge where almost perfectly at resonance frequency.

Edited by Rapitor

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For that, you'll first need to gimme one more subject, I'm out of idea =)

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well done, this is good information not overly technical that you should see in a beginning lesson.  that other thread you've mentioned ...LOL you don't talk about what swept wings do for you in a beginning lesson.

 

 

also your 4 forces diagram is what the faa say's to use...but this one seems to be more accurate.

091101_4_forces_new.jpg

 

you would have a very good, well rounded basic aerodynamics starting place if you were to add a section about sir isaac newton and daniel bernoull that would be sweet.

Edited by Aoe
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I'll try to add them when I have time.

 

And I did mention, shortly, the sewpt wings in the divergence part, since the main reason to use them is to prevent divergence.

 

However, I don't really see what bringing Newton and Bernouilli in my thread would add, but if you can explain me, it might motivate me to add those.

Edited by Rapitor
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Uh that's what makes a wing generate lift?

Pressure differential + 2nd Law + Kutta-Joukowsky.  Bernouilli itself is not sufficient, and the whole package is hard to explain.

 

I suppose that's Probably not important. Best to stick to divergence in an aerodynamics for dummy's thread.

And divergence IS important, since the 2 main causes of crash in RB are not landing and low altitude maneuvers, but wing clipping at high speed and / or high G turn. At least, someone wondering why his wings are ripping off when flying too fast can have a clue in this thread.

 

Nonetheless, I gotcha.

 

Gonna rewrite e little bit about Newton, and mostly Bernouilli

Edited by Rapitor

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That's not true. It's mostly Kutta-Joukovsky.
Newton gives wrong result for a plane intrados and a curved extrados, and Bernouilli works poorly with plate shaped wings. K-J works for all.

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Raptor loved your article about flight characteristics. One problem in the game I encounter over and over is during the straifing as I begin my pull up from a dive, I am anywhere from 100 to 150 m above ground, traveling at 400 to 500km/h. For some reason this is when the game decides to slam my plane into the ground!

Belly bashing is not fun. It is most preveilant in my ME 109s, IMO the best ground attack aircraft made and general use fighter ever built. A pure joy to fly  :Ds!

Do you think the game developers built this feature into the game on purpose  :lol:

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Well, At this speed, your Bf will suffer from speed stiffening. Reduce speed or have a lower angle with the ground when straffing, and you should be fine.

Don't straff with a nose lower than 40° below the horizon, especially at high speed.

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Thanks Rapitor.

 

I have 2000+ free fall jumps logged where most time I tried my body to do some of that... fun. lol.png

 

Good writeup.

Edited by Skyzulu

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Great Job and very comprehensive for dummies like me  :crazy:  and your other posts show that you are well educated about the generation of lift. (I am writing  a master thesis about this subjekt currently)

 

For anyone who cant wait till the OP has written about the fight of the Bernoullians against the Newtonians (since neither B. nor N. wanted to describe the behaviour of flow around an airfoil) the following link might be of interest.

 

http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

 

Concerning the validity of this source: I suppose there are one or two employees at the NASA who roughly understand how lift is generated ;)s

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I still feel sad that they don't speak about Kutta-Jukowsky theorem.

 

Actually, as I said before, both Newton and Bernouill have exemples where they do not work, but the other theory does.

K-J is always true. But that is way harder to explain, since it is a mathematical concept related to the two previous mentioned theorem.

 

Interesting article anyways.

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