What You Learned in School About How Planes Fly Was Probably Wrong

By Daniel Miessler on July 27th, 2009: Tagged as Physics | Science

So we all know how planes fly, right? The top of the wing is rounded and the bottom of the wing is more straight. Air takes longer to travel over the top of the wing than the bottom, which results in more pressure on the bottom, hence the lift. Right?

As it turns out, no.

This is what I was taught, and it’s what I’ve always believed (it’s even in most lower-level text books), but it’s simply not true. This concept, called the Equal Transit Time Theory does not generate nearly enough lift to keep a plane in the air.

The main reason planes fly is far simpler: wings force air downward, which in turn force the wing (and therefore the plane) upward.

The primary actor at play here is called the Angle of Attack (AoA), and it’s easy to conceptualize. We’ve all put our flattened hand outside a car window while the car was in motion. You noticed that if you angled it straight on you could hold it steady, but if you angled the front edge upward you created massive lift that forced your arm up in the air.

The same concept works for kites, helicopter blades, sailboats, ceiling fans, and planes. In fact, winglike surfaces can generate lift almost regardless of the shape of their top and bottom surfaces. Notice that ceiling fan and helicopter blades are basically symmetrical, yet they create a downward columns of air just fine. You can actually use a barn door to generate lift…if you were so inclined.

In other words, lift simplifies nicely to Newton’s third law of equal and opposite reactions: air goes down, wing goes up.

Finally, if you’re still harboring any fond feelings for the equal transit time theory, ask yourself a simple question:

If the top vs. bottom wing shape is so important, how can planes fly upside down?

Alas, there are actually multiple ways of describing, and calculating with varying degrees of precision, the way in which lift is generated. There is the mathematical/engineering approach, and there is the physical approach, which is what I’ve described above. John D. Anderson said it best:

It is amazing that today, almost 100 years after the first flight of the Wright Flyer, groups of engineers, scientists, pilots, and others can gather together and have a spirited debate on how an airplane wing generates lift. Various explanations are put forth, and the debate centers on which explanation is the most fundamental.

The one thing the experts agree on, however, is that the way most have been taught about how planes fly (the Equal Transit Theory) is absolutely incorrect. ::

[ There are, of course, other factors at play other than angle of attack, e.g. the Coandă Effect. The point of this piece is to illustrate that the primary factor with flight is the forcing of air downward (to which Coandă contributes) rather than equal-transit. ]

References and Notes

  • Pingback: How Planes Fly: What They Taught You In School Was Wrong | danielmiessler.com

  • Pingback: How Planes Fly: What You Learned Was Wrong | danielmiessler.com

  • CarlM

    I'm not sure that there is a misunderstanding of the Bernoulli Principle .. merely a misunderstanding of what is going on in the airflow surrounding a wing. An easy way to see that the Bernoulli Principle CAN'T (completely) explain the lift is to consider that (some) planes can fly upside down. (Stunt pilots have been flying upside down for a long time.)

  • http://danielmiessler.com/ Daniel Miessler

    Yes, exactly. I just added that piece to the notes as well. Upside-down flight is explained by Angle of Attack, not wing shape (Equal Transit Time).

  • CarlM

    I'm not sure that there is a misunderstanding of the Bernoulli Principle .. merely a misunderstanding of what is going on in the airflow surrounding a wing. An easy way to see that the Bernoulli Principle CAN'T (completely) explain the lift is to consider that (some) planes can fly upside down. (Stunt pilots have been flying upside down for a long time.)

  • http://danielmiessler.com/ Daniel Miessler

    Yes, exactly. I just added that piece to the notes as well. Upside-down flight is explained by Angle of Attack, not wing shape (Equal Transit Time).

  • Cyclothymia

    What is the purpose of the distinctive curvature of the wing cross-section then? What would happen if I install the wings on upside down and fly the plane right-side up?

  • GarrettZilla

    It will still fly, but you'll need to fly at a higher angle of attack. It will be less efficient, and the stall characteristics will be less pleasant.

    Serious airshow planes have symmetrical curvature, and usually the wings are in the middle, so that the inverted flight characteristics are comparable to upright flight.

    Look up the “Coanda Effect”, or just turn on the faucet and slowly move your extended finger into the falling stream. You'll see the water pulled toward your finger and deflected in that direction. For every action, there's an equal and opposite reaction, so there's a force on your finger equivalent to the force deflecting the water. Replace water with air and finger with curved airfoil – so the Coanda and Bernoulli effects make the wing more efficient, and make the flying characteristics better when you're at high angles of attack.

  • Clem Kadiddlehopper

    If this were true then there'd be no reason to curve the top of the wing. Obviously, it's a combination of the two reasons.

  • Pingback: uberVU - social comments

  • Ben Smith

    Obviously, angle-of-attack isn't irrelevant, why else would a plane have flaps? Angle of attack DOES provide lots of lift at slow-ish speeds.

    But not as the plane speeds up.

    As the plane speeds up, the difference between the angle of attack and the direction of the plane drops. At cruise speeds, most smaller planes have a wing angle of attack that's almost exactly horizontal – if you were to compare the chord of your plane's wing against a level horizon, you could see the two are parallel.

    This is particularly visible in a high-wing Cessna, especially a faster 182. At cruise, the wing chord is dead level against the horizon.

  • Chris Kirby

    A higher angle of attack provides more lift at all speeds. The reason an aircraft is level at cruise conditions is because the aircraft is designed for that. The cruise angle of attack is near zero in order to minimize the amount of drag on the aircraft while still maintaining enough lift to stay airborne. It is still producing lift due to the use of a cambered airfoil which has in effect an induced angle of attack.

    • Pvt. Pilot1

      You’re ignoring Critical Angle of Attack. At slow speeds, you can reach it with a 20 degree Angle of Attack. AoA may create lift at all speeds, but certainly not enough to overcome weight.

  • zmitrok

    Somewhat relevant to this topic is the concept of countersteering in single track vehicles. Although it is taught, many of us refuse to believe in it.

  • http://www.backlinkintelligence.com Oes Tsetnoc

    Good thing my Science teachers didn't know much, thus they didn't know Equal Transit Time Theory. So my common sense made more sense with the Angle of Attack Theory.

  • Jonas

    If your explanations was the simple truth, why aren't we all flying delta wings? If your hand in the wind is a resonable approximation of a aeroplane, is there not a direct correlation between wing area and lift? A larger area would require a smaller angle of attack and therefore save fuel.

  • john n

    I'm pretty sure it's now widely accepted in Aero Engineering that it is a combination of several factors. If AoA were the dominating effect, the optimal angle for lift production would be 45 degrees. We know that to be false, of course, due to the separation of the boundary layer from the leading edge of the foil (causing a stall). The reason so many texts may present differing information is because of the difficulty of modelling air travel around an airfoil in flows other than laminar. In fact, the Navier-Stokes Equations, which are fundamental in fluid mechanics and flight, still have not been fully solved (they must be simplified first).

  • ngp

    I think the most fundamental explaination is that the wing disrupts the air flow in such a way that the fluid over the surface of the wing moves faster, due to conservation of mass, and therefore has lower pressure, Bernoullis Principle, which can be derived form Newotns laws. The air speed is clearly shown in that nice animation. I don't really see the air moving down

  • IPcamper

    If you really want to know how lift is generated, then you should study “Theory of Wing Sections: Including a Summary of Airfoil Data “
    ~ Ira H. Abbott (Author), A. E. von Doenhoff (Author)
    This cover a great deal of math covering lift and performance of airfoils. Angle of attack is only one aspect lift, and in many conditions, not the most important.

  • IPcamper

    If you really want to know how lift is generated, then you should study “Theory of Wing Sections: Including a Summary of Airfoil Data “
    ~ Ira H. Abbott (Author), A. E. von Doenhoff (Author)
    This cover a great deal of math covering lift and performance of airfoils. Angle of attack is only one aspect lift, and in many conditions, not the most important.

  • guest

    frisbees seem to belie this.

  • Jason

    Author is retarded. The basic concept, as IS included in most early text books, is not as he says. What they say is that the air must travel FASTER over the greater distance of the top wing in order to maintain an equilibrium at the end of the wing. This causes low pressure on top and high pressure on the bottom, causing lift. Do your homework before trying to look like a genius online.

  • GarrettZilla

    You may wish to revisit your comments. For a nice little book which examines and debunks everything you've said, read “Stop Abusing Bernioulli: How Airplanes Really Fly”, by Gale Craig.

    http://www.regenpress.com/

    Or, you can purchase an audio copy of the forum he presented at this year's AirVenture convention. Then rethink your throwing around of the “retarded” insult.

  • GarrettZilla

    How so?

  • marcsmith1234

    planes can fly with a simple wedge design these days simply because we can build planes with great trust to weight and strength to weight ratio. Original by planes could not take inverted flight because they would loose to much altitude to fast. The redesigned bipod of today can sustain inverted flight at a stable altitude because of the strength and power of the aircraft

  • marcsmith1234

    angle of attack is to increase altitude, if a wing was designed with angle of attack alone it would be drastically inefficient. In time the stress on the wing will overcome the structure and fail. The wing has to be designed with aerodynamic lift. Without the BP flight is impossible.

  • marcsmith1234

    higher angle of attack does not create lift, but increases drag. A plane will increase altitude with drag alone but is to inefficient.

  • marcsmith1234

    THIS PERSONS THOUGHT PROCESS IS PROOF THAT THE AMERICAN EDUCATION SYSTEM IS FAILING.

  • GarrettZilla

    Paper airplanes? Simple balsa gliders? Have you ever even watched an airplane fly? Tell you what – go to the airport, take a flying lesson, and tell the instructor you want to do an hour of slow flight. Let me know what happens during that hour.

    At this point, most of your comments are so ignorant and ill-informed that I don't think you actually believe them, but are getting some sort of fun from trolling.

  • GarrettZilla

    Efficiency and flying characteristics are two reasons we're not all flying delta wings.

    And you're right, there is a direct correlation between wing area and lift (find the equation for lift – velocity squared, coefficient of lift, air density, and wing area). Don't forget that drag goes up similarly – you're not getting all that lift with a bigger wing for free. Also don't forget that it weighs a lot more. Investigate the relations between lift, induced drag, and parasitic drag.

  • GarrettZilla

    I agree. “THIS PERSON” refers to yourself, right?

  • http://www.funnelbrain.com/ Laura Zaillian

    A great post! I am a freshman @ the UCLA. I am planning to major in Physics. As marc said, planes can fly with a simple wedge design these days simply because we can build planes with great trust to weight and strength to weight ratio. Thus, Upside-down flight is explained by Angle of Attack, not wing shape (Equal Transit Time). By the way, i came across these excellent physics flash cards. Its also a great initiative by the FunnelBrain team. Amazing!!!

  • BriGuy

    Uh, no offense, but the equal transit theory, as you state, is a misunderstanding of the Bernoulli Principle. As for the foil-helmet-types that can't fathom how a fast moving aircraft can fly upside down using a lift-generating wing, did you consider the other control surfaces on the aircraft and the speed involved? Certainly the Wright Flyer would have had a tough time flying upside down, but it didn't approach anything near the speeds of todays (even light, recreational) aircraft. Once a plane has sufficient speed, flying upside down is achieved only at a considerable loss of lift efficiency, due mostly to the angle of attack required to fly “level” while upside down

  • PaperAirplanes

    I'm not that into physics. I just love folding and flying paper airplanes. Nice discussion though.


    ==========
    Learn how to fold 50 cool paper airplanes at http://www.paperairplaneshq.com

  • http://zerowon.blogspot.com/ Bucky

    lets talk pulse jets… specifically, can it be methane powered and used to create AC power?

  • Pingback: What I Didn’t Know About How Planes Fly « Paige’s Pages

  • http://quidnuncsimcha.gather.com/ Chris Harding

    I disagree. Conservation of mass and how pressure relates to velocity of molecular, bulk, flow is what affects the lift.

  • tayyab

    well if you are right there are few plane who take off with almost zero angle of attack can you please explain them

  • Ace

    I thought planes flew by the floating properties of multiple $100 dollar bills.

    • http://www.resumewritingservice.biz//home/premium-resume-writers/ resume writers

      Haha! You should share your theory with the world!

  • tayyab

    well if you are right there are few plane who take off with almost zero angle of attack can you please explain them

  • Ace

    I thought planes flew by the floating properties of multiple $100 dollar bills.

  • Jeffreywwilson

    welcome to the bad side of the internet

  • Smithdave467

    World School

    World school concept has been started with a vision to put emphasis on providing happly learning environment, where child’s talent can be nurtured to fulfillment.

  • Hi

    Hi Guys :P

  • Uforainier

    who says!! I have been reading these know-it-all sites about equal transit time fallacies and everyone writes  like they understand it. your explanation is crap. all it is is a parroting of what you heard someone else say. which means you didn’t understand the equal transit time theory in the first place. you are just jumping from following one religion to following another.


Top

Popular

Information Security / Technology

Politics

Philosophy & Religion

Technology & Science

Culture & Society

Miscellaneous

Arguments

Projects

Collections

Twitter

What I'm Reading

Favorite Books and Essays

Top Blog Categories

Inputs