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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19 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.

  17. Buck Rogers says:

    Any airplane can fly inverted for a limited time; however, not all airplanes are designed to do so safely. General aviation aircraft are divided into three categories: Normal Category, Utility Category, and Aerobatic Category. It would be against FAA regulations to roll a normal or utility category aircraft upside down, but in the hands of the right pilot a roll could be executed without incident in any category. A roll can be induced with little G-force. The categories basically determine the stress an aircraft can resist while maneuvering.

    Aircraft in a non-aerobatic category can suffer damage to the gyros because the gyros can’t be caged, the fuel tanks do not have inverted fuel pick-ups, and the engine oil systems can’t be inverted. If all maneuvers are done with positive G-force, as such as a roll or inside loop, the fuel and oil systems are of little concern for continued engine operation. If a pilot wanted to fly inverted straight and level for a period of time, the pilot would want to be sure the aircraft had inverted systems. Without such systems the aircraft engine would suffer oil starvation most likely before fuel starvation.

    As for the aerodynamics, yes angle of attack is about 75% of what generates lift the rest is in the profile of the wings chord curvature. Some aerobatic aircraft have conventional wings (non-symmetrical), curved on top, flat on bottom. Theoretically air passing over the top of the wing will arrive at the trailing edge at the same time as the air passing under the wing. The air on top the wing has to travel a further distance because of the curvature, this causes an increase in velocity of the air on top the wing, resulting in a lower pressure than the air pressure under the wing; look-up Bernoulli’s principle if you don’t understand this concept.

    Anyhow, if a wing is not symmetrical, when inverted the curvature is now on bottom and the flat side is on top, with this type wing an increased angle of attack is necessary to maintain the same amount of lift as when the aircraft was right-side-up, increased angle of attack results in increased drag, so to have the same performance more power would be necessary as well. With too much angle of attack into the relative wind, the air will burble while it tries to get over the top of the wing and the wing will stall (no longer capable of producing lift); this principle is amplified while inverted with a non-symmetrical wing.

    A symmetrical wing is curved the same on top and bottom, so regardless the attitude of the aircraft (inverted or in normal flight) the wing gives the same performance. This type wing produces less lift, but the aircraft is usually compensated with a higher performance engine and a faster rated maneuvering speed, which results in more air pressure under the wing; remember angle of attack is 75% of lift; more speed, more lift as well. It only makes sense to have symmetrical wings on aerobatic aircraft. An aircraft that hasn’t an inverted fuel and oil system, has no need for a symmetrical wing, because the aircraft will only be doing positive maneuvers.

    An aircraft flying inverted straight and level or in a turn while inverted is a negative maneuver (you’re hanging in the harness by your shoulders, rather than being pinned to your seat). When performing an inside loop, that is a positive maneuver, as where an outside loop is a negative maneuver.

    Yet planes rarely fly upside down, it’s the pilots that do, it just depends on your systems (inverted or not), gyros, and structural category.

    As flying on the side of the fuselage banked over 90 degrees in a low pass down the runway at an airshow, this requires the fuselage shape to have the same type profile as a symmetrical wing, an awesome amount of power to hang the airplane on it’s prop, and a great deal of airspeed.

    Actually, flying straight and level while inverted is somewhat boring after the novelty wears-off, but g-forces from loops, snap rows, and hammerheads are the way to go.

  18. Buck Rogers says:

    Mr. Physicist, most airplanes flying with wings 90 degrees to the horizon and at ground level are not dropping, A Thunderbolt bi-plane can sustain a 1.3 g maneuver when flying on its fuselage; this means it can climb on it’s side. The reason the nose is pointed upward, is because there is very little surface area on the fuselage to produce lift, compared to that of the wings; the aircraft is literally hanging on the prop, and it can sustain this maneuver as long as the engine is producing power and the airplane has ample airspeed; I know for a fact :)

    If you’re going to play the part of Mr. Physicist, then why throw us your assumptions?

    Aren’t you supposed to know what you’re talking about.

    A man who admits he doesn’t know the answer when asked will gain more respect than the one who pretends he knows.

  19. Evelyn says:

    Buck Rogers, you are awesome!

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