Q: How can planes fly upside-down?

Physicist: The narrative that usually leads to this question is something like: “It was the Wright brother’s brilliant wing shape, among other design innovations, that first made manned flight possible”.  So if the wing shape is so important, why does it still work if it’s flipped upside-down?

A wing (or airfoil or whatever) creates lift by taking advantage of a combination of the Bernoulli force and “angle of attack”.  By increasing angle of attack (tilting the nose up) the oncoming wind hits the bottom of the wing more, and pushes the plane up.  However, this force also increases drag substantially.  The Bernoulli force shows up when the air over the top of the wing is faster than the bottom, but it requires a bit of cleverness to get it to work.  Cleverness like the Kutta-Joukowski condition.

The Kutta-Joukowski condition: If air didn't flow faster over the top of the wing, then the air from the bottom would have to whip around the trailing edge with a very high acceleration. Too high, in fact. K-J assures that this singularity at the trailing edge doesn't show up.

When the Wright brothers built “the Flyer” (they were smart, not particularly creative) the engines available at the time were not powerful enough to lift themselves using only an angle of attack approach, so using a slick airfoil shape to take advantage of Bernoulli forces was essential to get off the ground.  Using the engines we have today (jets and whatnot) you could fly a brick, so long as the nose is pointed up.

So to actually answer the question; back in the day planes couldn’t fly upside-down.  But since then engines have become powerful enough to keep them in the air, despite the fact that by flying upside-down they’re being pushed toward the ground.  All they have to do is increase their angle of attack by pointing their nose up (or down, if you ask the pilot).

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16 Responses to Q: How can planes fly upside-down?

  1. Jeroen says:

    You didn’t really answer the question! So how _do_ jet planes fly upside down? Do they tilt their nose up enough so the upward force cancels the downward force from the wings? Does that mean that no aircraft can fly level to the ground (perfectly horizontal) upside-down?

  2. The Physicist Physicist says:

    That’s exactly right. Somewhere in the middle of writing a post there comes a choice between including stuff I think is interesting, and staying on topic. It doesn’t seem like much of a choice though.

  3. Scott says:

    Oh snap!

  4. taylor says:

    Holy S*it

  5. Christopher Miller says:

    So if aircraft are using their engines to fly upside down why can’t more aircraft just fly straight up and not stall? I’m a pilot and in my experiance aircraft will stall in all sorts of attitudes at any speed.

  6. The Physicist The Physicist says:

    When flying straight up you lose not just the “Bernoulli advantage”, but the advantage of the angle-of-attack, which means that the engine has to hold up the plane on its own.
    Unless you’re plane’s engine is also a rocket, there’s a good chance it won’t be able to handle that (and the plane will stall).

  7. Rob M says:

    How do they fly on their side?

  8. The Physicist The Physicist says:

    Not real well. You don’t have to worry about the Bernoulli force pulling downward of the plane (just sideways), but you also don’t get to take advantage of the angle of attack, because there’s no flat surface available to deflect the incoming air down.
    If you watch stun flyers you’ll notice that the nose of the plane is always pointing up when the plane is sideways, and they don’t do it for long (because they’re dropping).

  9. Eid Al-Ameri says:

    do you have any references to back up what you are saying. i hope you dont take offense to the question i would just like to read up further on what you have said

  10. The Physicist The Physicist says:

    No offense at all!
    I double check every scientific paper I read, but never double check articles about “Nazi moon bases” or “flat Earth theories”.
    This post was answering a question that’s rooted in common knowledge to most people (wing shape pulls a plane up) with what’s common knowledge to aeronautic folks. There should be a discussion of this stuff (angle of attack) in any first-year textbook you can find on aeronautical engineering or fluid dynamics, but unfortunately I don’t know of a specific reference off-hand.

  11. Pedro Isaacs says:

    Thank the Gods! A rational explanation at last!

    Please tell me if I’ve got it wrong:

    The Angle of attack is the critical component, however, the engine of the aircraft provides a linear force that can counteract the pull of gravity for a brief period.

    If the aircraft is upside down, the pilot angles the nose of the plane up so that thrust from the engine is pulling the aircraft “up”. At the same time, by angling the the plane up, the pilot maintains a positive angle off attack but looses lift substantially to increased drag on the wings. The inverted wings angle of attack is much greater than if it was upright and as such the drag increases.

    It’s the aircraft engine that compromises for the drag and loss of lift, providing the forward and therefore downwards thrust which “defies” gravity. It makes sense that the mechanics need to be reliable in order to keep the engine running in this position and thereby providing the counter force too gravity.

    I have only recently discovered for myself that lift is really generated by angle of attack and not wing shape so much (Bernoulli forces).

    Thanks for the epiphany!

  12. roy sigman says:

    If F=ma It seems to me an object moving at a constant velocity and hitting you should exert no force. How can this be?

  13. The Physicist The Physicist says:

    An object moving at a constant velocity merely has zero net force acting on it. In the case of airplanes, the drag force is balanced by the propulsive force.

  14. Raymond says:

    the whole idea is newton’s 3rd law; end

  15. Devin says:

    I dont understand how this article answered the question at all. Bernoulli’s principle explains how they fly straight and level, not upside down.

  16. Stacy Elliott says:

    Devin,

    If the plane is flying upside down the pilot forces the plane Upward by forcing the nose toward the sky slightly to keep it flying level.

    Remember that the Thrust vector is adding to the lift because the thrust vector is now aiming skyward slightly.

    Just like if the pilot wants to dive when upright, he forces the plane nose down toward the earth.

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