Aerodynamics – How airplanes fly, maneuver, and land
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Aerodynamics – How airplanes fly, maneuver, and land


For an airplane to fly, the wings need to create an upward force which is called “lift.” It is also necessary for the airplane to be able to maneuver. The engine of the aircraft provides a forward force that is called “thrust”, which counteracts the force from air resistance, which is called “drag.” Unlike airplanes, birds generate thrust by pushing their wings against the air molecules. But, airplanes and birds maneuver and generate lift the same way. More lift is achieved when air molecules flow faster across the wings. Suppose that the air around a wing is completely still. The air pressure from above and below the wing will cancel out, and there will be no net force. Now suppose the wing is moving relative to the air. As air flows around the wing, the wing’s shape forces the air above the wing to flow faster than the air below. The air below the wing is now creating a greater pressure than the air above, which creates a net upward force on the wing. This is one of the two reasons that lift is created. The second reason is that if we tilt the wings, the air molecules will generate an upward force when they bounce off the bottom part of the wing. The fact that air molecules generate a force when they bounce off a surface is also what allows an airplane to maneuver by using movable surfaces controlled by the pilot. These movable surfaces are what is called the rudder, the elevator, and the ailerons. The rudder controls what is called “Yaw.” The elevator controls what we call “Pitch.” And the ailerons control what is termed “Roll.” When the aileron on one wing goes up, the aileron on the other wing goes down. When an airplane rolls, the force of lift created by the wings changes direction. A component of the lifting force is now pointed sideways, and this allows the airplane to turn and change direction. But, when the direction of lift changes, this also decreases the component of the lifting force pointing upward. We need to compensate for this loss of lift in the upward direction during the airplane’s roll, and we can do this by simultaneously controlling the airplane’s pitch with the elevator. Changing the airplane’s pitch with the elevator allows the pilot to change the strength of the lift that is produced. Changing the airplane’s pitch changes the angle between the airplane’s wings and the direction of the incoming air molecules. The angle between the wings and the direction of the incoming air molecules determines how much lift is created. If the force of lift is stronger than the force of gravity, the airplane’s elevation increases. If the force of lift is weaker than the force of gravity, the airplane’s elevation decreases. As we increase the angle of the wings relative to the direction of the incoming air molecules, the lift increases. However, there is a certain angle which will give the maximum lift possible, and if we increase the angle even further, the lift will decrease. If this happens, the airplane is in a stall, and the airplane will end up losing altitude. To recover from a stall, the nose of the airplane should be pointed downward, so as to decrease the angle between the wings and the incoming air flow. The amount of incoming air flow can be altered by changing the airplane’s speed. The faster an airplane is moving relative to the surrounding air, the more lift it generates. When it is time to land, the airplane’s speed needs to be significantly reduced, and this is done by reducing the amount of thrust generated by the engine. This, then also reduces the amount of lift generated, causing the airplane to lower altitude in preparation for the landing. However, right before the airplane is about to land, the speed may be so slow that the wings are not generating enough lift. We can increase the amount of lift being generated during landing by extending the wing flaps. Extending the wing flaps also significantly increase the amount drag from the air resistance, causing the airplane to slow down more quickly. It is always desirable to land into the wind, so as to shorten the amount of runway necessary to stop the plane. It is also always desirable to take off into the wind, so that the incoming wind increases the airflow over the wings, thereby increasing the lift and shortening the amount of runway necessary to take off. Once airborne, the velocity of the airplane relative to the ground is determined by the velocity of the airplane relative to the surrounding air plus the velocity of the wind relative to the ground.

100 Comments

  • FartingNinjaFrog

    Very good! As a pilot I felt that some aspects were a bit too oversimplified, but it's a very informative video regardless. Keep up the good work, Eugene! 🙂

  • Pär Johansson

    Erm… Perhaps You should check
    https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
    and
    https://www.grc.nasa.gov/www/k-12/airplane/right2.html

  • Shirshak Bajgain

    Yes finally waiting for it 🙂 Now waiting for refrigerator working etc 🙂

    Future generation will imagine how we people learnt physics without eugene 🙂

  • MrD

    I love your videos, they teach me so much. Appreciate the effort you go to with all the animation as well, must take you a long time!

  • SteichenFamily

    Good elementary visualization! Not sure if you can edit these after publication, but a quick explanation of why faster moving air exerts less pressure would be useful. Your graphics are great, so you could use arrows to visualize the vectored force exerted by fast moving molecules vs slow.

  • grywacz

    What about flying upside down? 😉
    Also, I think it requires a leap of faith to accept that "still air moves faster over the wing" and "air moving faster generates lower pressure". 😉

  • Philip Y

    Thank You, Eugene!!.. Great Video….. as always!!… I appreciate the time and effort you put into each and every one of your videos. Keep up the great work!!… 🙂

  • Anders Andersen

    Great video, as usual, Eugene!
    I don't understand why the air molecules have to arrive at the end of the wing simultaneously (as shown in the animation). Why can't they move in the same relative speed over the wing, and arrive "out of sync" with the colour-matching molecule pair.
    I also don't get why faster moving air molecules generate a lower pressure (over the wing) than the slower moving molecules (under the wing).
    Hope I'm not asking too dumb questions, and thanks if anyone can clarify.

  • Sydnius Alminia

    Your videos are always a great help, Eugene! The feeling of elation I get from understanding something new makes me a happier person and your videos always facilitate that. You are a excellent person. Thank you!

  • blake301987

    Great video Eugene.
    It would be great if you could make a video on why faster moving air has lower pressure than slower moving air. Thanks

  • Kevin Ellis

    Great video. Wish it would have have gone into some detail about how the rudder needs to be applied to counteract opposite yaw that results from aileron input in most aircraft. The illustrations are done well enough to explore into these more technical aspects intuitively and that's what I like about your videos.

  • BState

    From 1:41 to 1:56 you state that the air "below" the wing is creating a greater pressure, which creates a net upward force on the wing… but isn't it the opposite? The air "above" the wing is the one that creates greater pressure (therefore "drag"), for it moves faster, resulting in the net upward force.

  • Im Genius

    Amazing video! Btw, where did you get these information? Are you a real genius who just understand everything without seeing animation? Or are you a scientist?

  • Andres Franco Valiente

    I haven't really taken an aerodynamics course yet but referring to the first reason why a plane generates an upwards lift. Why are the air molecules producing a lower pressure above the wind? Does this have to do with the fact that fluids decrease their pressure when they move faster? (I haven't really heard anything like this since 9th grade, sorry).

  • Pushkar Soni

    what if put a roller in the upper part of wing instead of a simple curved surface. so that we can spin the roller in the desired direction to create more lift/ drag.
    your videos are great. i am 15 and i can understand your videos well.they are simple, informative and interactive.

  • Patrick Houlihan

    Finally I know I always had a gut feeling that air caused a force when it hit an angled wing but everywhere I looked it always said the lift was only due to the air pressure from the air foil and I just thought I was wrong without looking too deeply into it. Great Video!

  • John Studer

    Love your animation – what software do you use?
    Would be great to see more videos on modeling, simulation, linear algebra applications, how google and watson work,
    how computers work.
    You and KhanAcademy are changing the world – tremendous work!

  • idioy no gi

    Eugene how can we be sure that the air molecules bounce off the bottom part of the wing when it's tilted? Do the air molecules have to be moving fast enough relative to the air plane to bounce off the wing? Would the molecules simply slide on the bottom side if they would be moving to slowly?

  • Alex I

    A common misconception is that the air over the top of the wing meets up with the air on the bottom of the wing. This is actually blatently wrong, the air over the top of the wing is long gone by the time the air under the wing passes the trailing edge. This is because the downwash pulls the air above the wing, and the greater downwash at a higher angle of attack pulls the air over the top even more. If they met up, the upper surface would need to be over twice the length of the lower one, but airfoils don't have that kind of difference in length, and if they did they would have too much drag to be efficient.

  • Sean Wiesen

    Great video Eugene. I just have a couple questions. At 6:52, you mention that more lift can be generated during landing by extending the wing flaps. However, the wing flaps animation at 7:00 suggests that extending the wing flaps would also cause the nose of the airplane to tilt downward to some degree (like when the elevator is used to achieve the same effect), which would decrease the amount of lift generated, I think. Does this downward tilt actually happen? If so, is it accounted for in some way or is it negligible?

  • Observ45er

    Ah!  Another amateur Lift video.

    Eugene,
    You are, unfortunately, repeating some common misconceptions about lift that have been repeated so many times that it is almost impossible to get the true story. …
    Your animation shows the "Equal-Transit-Theory", long known to be false.  The air above the wing, in reality, beats the lower air to the trailing edge by a considerable amount.  See Professor Babinsky's video of an actual wing: https://www.youtube.com/watch?v=UqBmdZ-BNig

    Pressure difference in a fluid is what accelerates the fluid. Speed does not cause a lowered pressure.

    A wing (actually any object) moving through the atmosphere (or any fluid) creates pressure differences in order to make the air (fluid) flow around it.

    There are three main parts to the complete explanation of lift.  The upper pressure reduction and the lower pressure increase which is the total lift; then there is the downwash resulting from these pressure changes that satisfies Newton's Third Law.

     The pressure difference (top to bottom) on a wing is the COMPLETE lift force, but it has two parts for the total pressure difference.  By the very movement of the wing through the air, the underside of the wing increases the pressure there and this also results in the downward acceleration of that air. This increased pressure is part of the total pressure difference top-to-bottom.  While a fair analogy, the molecules don't bounce. This is the "Hail of bullets that Newton postulated, and is poor science. They are pushed down by the increased pressure at the lower surface. This is easier to see if you view the wing at a somewhat increased angle of attack.

    The curved path that the wing causes the air above the wing to take is what causes the lower pressure at that surface, not "speed".  This is the second part to the total pressure difference. When a mass (even air) flows in a curved path, this is an acceleration caused by lower pressure on the inside of the curve – at the wing's upper surface in this case.   The lower pressure thus created is also responsible for the acceleration of a considerable amount of air that is above the wing to be accelerated downward.
    … If you understand this, you will see that under the wing there is also a lower pressure toward the inside of that curved path.

     The upper air and lower air combine to form the total downwash.  
    See this animated gif of a very large test model and mote how much of the downward thrust air starts above the wing:
    http://amasci.com/wing/lasrWing.gif

    The reality is that the wing's movement through the air causes a pressure reduction above it and an increase below it. 

    The speed of the air above a wing is almost unchanged as the wing passes by.   It is only accelerated downward.  Look closely again at Prof Babinsky's video linked above.  Believe it or not, the air below the wing is pushed and sped up as the wing passes and is therefore faster, from the still air's frame of reference.  See Complete Aerogeek's video (time 4:00) of three frames from Professor Babinsky's video.  Note the streamlines #8 & #9 as the wing passes the STATIONARY vertical line.  The one that moves the most is the fastest. [#8]
     https://www.youtube.com/watch?v=MlK4ZL5N7MA 

    It is the total pressure difference which allows lift and control, not just the lower surface phenomenon.  Control surfaces have an effect on BOTH the air above and below the wing, just as the whole wing does.

    ALL WINGS produce lift in this very same way…inverted, symmetrical, flat.  More pressure below than above, curved paths below and above and with the resulting downwash.

    The angle of attack changed by the elevator, changes the pressure differences created an,d therefore, the lift….For a good and authoritative source to support what I say here, see:erodynamics Krzysztof Fidkowski associate professor, Aerospace Engineering Department at the University of Michigan.Krzysztof Fidkowski How Planes Fly.  https://www.youtube.com/watch?v=aa2kBZAoXg0 –
    Cheers, ScienceAdvisorSteve
    www.challengerillinois.org

  • Observ45er

    Per my recent comments you should review your Newton video.  Air has mass and requires a force to accelerate it.  This applies to air, since it has mass.–      Cheers, ScienceAdvisorSteve

  • John Shearing

    Hi Eugene, I have also used computer animation to make difficult aviation concepts accessible. The following is about avoiding stall and spin The computer helps because you can't see air nor can you see the flight path that generates relative wind without it.
    https://www.youtube.com/watch?v=UJQsAxB7E4Q
    Anyway, your videos are an amazing contribution toward the spread of understanding. Thank you so much for doing this good work.

  • samuel kazimierczak

    i hope this person is proud of them self because i am in one of my last years of school and the videos this person has made have helped me allot in my understanding of aeronautical engineering and opening up more career doors for me. thank you again

  • Jay Smith

    The spheres flowing around the wings should have shown acceleration, i.e. the top surface spheres should have been leading the bottom surface spheres by the time they reached the trailing edge.

    As it stands, it looks like this video shows the 'equal transit fallacy', or even worse, it shows that the bottom surface air is leading! In reality, at positive angle of attach (or zero with a cambered wing like you have) the top surface molecules should be going faster and actually be in the lead!

  • carultch

    If you wake up in an airplane, with all the window shades closed, that is in the middle of executing a banked turn, can you perceive which direction it is turning by feeling the "g-forces" inside?

    You can tell that it is either turning or climbing because you feel slightly heavier. But can you tell which direction?

  • Jaideep singh

    This is not the actual phenomena of generating lift in airplanes. Studies shows that the result will.not agree with this principle of generating lift.
    video on learn engineering Youtube channel and nasa research experiment proved it.

  • Captain Styles

    This was a very informative video and very detailed and I personally found it very enjoyable to watch. The only thing that I didn't find quite right is the wing shape generating lift due to shape causing the air molecules to move faster. this does happen but it is due to the already present pressure differential over the wings surface due to the angle of wing to the relative wind or angle of attack. When the angle of attack increases it causes the molecules to be deflected downwards slightly and a pressure differential on either side of the wing thus the wing experiences and opposite force upwards but often the pressure on top of the wing doesn't change all that much it stays very close to that of atmospheric pressure and it's actually the slowing down of the air on the bottom of the wing that generates at higher pressure on the bottom so although the lowering of pressure does occur and a small amount of lift is produced it is a fraction of total lift. As well as this the equal time theory relies on two molecules of air starting at the same point but one going up over the wing and one going down under the wing and then rejoining at the trailing edge of the wing this is not going to happen because they are part of two different stream lines and are dynamic equations and not applicable to two separate molecules on two different streamlined as well as this in theory a wing that is flat bottomed as very wavy on top should according to this theory generate masses of lift but it doesn't in reality and it also suggests that a symmetrical wing should only produce a tiny amount of lift because of tiny difference in path length between the top surface and bottom surface when even at a high angle of attack but this is not the case. In reality most light sport aircraft and even going up to the extra 300 have symmetrical wings and they generate enough lift to produce up to 12 g manoeuvres at just 200 kts which should be impossible for a symmetrical wing and the last piece of my evidence is paper aeroplanes. They are able to generate enough lift to support themselves for sometimes quite a distance yet their wings are the thickness of a sheet of paper or card and are usually flat and not curved in any way even under aerodynamic loading so although air does change speed over wing it's generally the speed of the air under the wing changing more causing lift

  • sule shangodoyin

    Showing repeat of Aerodynamics theory for learning purposes will be highly appreciated!
    It's me,
    Sule Shangodoyin.

  • weltensegLA

    I flight school they said the lower pressure above the wing is actually the greater force over the pushing force below.
    And the aileron is increasing and decreasing the curvature and hence creating more and less lift while altering the lift vector due to changing the angle of the cord line.

    Your videos are awesome especially quantum ones. Good visualizations!

  • Bob Gilchrist

    This theory of lift has successfully been challenged. There is no force which constrains the air molecules above the wing to stay in the same relative vertical position as the air molecules below the wing. Thus it is not true that the air above the wing has to travel faster.

  • MrTiti

    3:38 this is what i asked myself some 15 years ago. when you tilt the lift vector, why does the plane turn? the plane will move with a parallel component to the ground. NOT turn.
    So this is wrong.

  • Florin

    Extremely intuitive! Nice and I can understand every single point!  Can you sldo explain the role of the spoilers and why a plane with flat wings can still fly? Thanks!

  • daffidavit

    Imagine a side cut section of a curved upper surface wing. Now place another similar wing inverted above it. What is in the space in between? A Venturi.
    Now, imagine just the bottom have of the wing again. Imagine the curved air flowing above the wing. At some point, that curved airflow meets a smooth uninterrupted air boundary above it. What is in between the space above the top surface of the wing and the smooth boundary layer above it? A Venturi. Like the inside of a carburetor. Thus the lower pressure on top and higher pressure on the bottom.
    NACA proved you don't even need a curved top portion to create lift. In a smoke chamber, NACA (predecessor to NASA) used a flat board tilted on an angle to create lift. However this type of wing was not as efficient because the "burble point" was closer to the leading edge of the board.

  • wombat

    The background music is totally unnecessary, too noisy and distracting. @3:44 need more details of how the sideways force makes the airplane turn. The video is otherwise very nice and clear. (So, I see, the flaps are not only for slowing down, but also for more lift)

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  • Steve Canham

    Great video, I am showing this to my F-15 avionics student as in introduction to our flight control systems block of instruction. Be aware that the portion of the video that shows flap operation, at 7:00, the outboard flight control surfaces of the F-15 are labeled as flaps when, in fact they are actually ailerons. The inboard surfaces are the only ones that operate as flaps. The F-15's ailerons only move asymmetrically for roll control and always move independently of the flaps.

  • Manulal M Inasu

    Thank you for the video. Reached here after watching your General relativity videos. If lift is created by the first reason you mentioned, I mean wing's shape causing the air above the wing to move faster and the pressure difference thus generated and all, then how is it possible for a fighter plane to fly upside down?

  • Jolo

    I was playing with my paper airplane with the fan and wondering how jets maneuver which was answered by the second lift factor. The notion that air molecules bump the plane kinda blew me away.

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