30

As an engineer I can explain in very technical terms exactly what makes an airplane fly, however, it isn't easily understood by non-technical people. How can I explain it to a non-technical person, or a nine year old, in a way that isn't really incorrect but is far less technical than what we learn as pilots?

Daniele Procida
  • 12,858
  • 2
  • 51
  • 94
Lnafziger
  • 58,886
  • 39
  • 242
  • 423
  • I don't really think these qualify as an answer, but http://youtu.be/bv3m57u6ViE & http://youtu.be/_LXW3pHNn_U might help (the latter, a delightful 1960s video from the FAA, is also a FDA approved treatment for insomnia -- oh that voice!) – voretaq7 Dec 28 '13 at 00:00
  • 10
    As long as you don't say that a plane flies because of the shape of its wing. I'm still furious at my high school teacher for telling me that was the one and only reason a plane could fly. Too bad I wasn't clever enough to ask him how a plane can fly upside down. – Philippe Leybaert Dec 28 '13 at 02:58
  • 4
    According to Mary Shafer, retired NASA engineer, it's all about Lift Demons and Thrust Pixies. – Gerry Apr 26 '17 at 15:55
  • 2
    Big Bank Accounts... – Dave Apr 26 '17 at 17:01
  • Lift is 100% from air-deflection by tilted wings, but confusingly, this then causes 100% of the pressure-difference blamed on "Bernoulli Effect." There's no 60%/40% here, instead it's 100% Newton's wing-tilt versus 100% Bernoulli's velocity-difference. Lift requires both. They're just two equal viewpoints. Choose whichever one is easier to explain. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/bernoulli-and-newton/ – wbeaty Nov 03 '23 at 08:04

4 Answers4

52

The simplest explanation is the "hand out of the window" one:

Hold your hand out of the window while in a moving car and hold your hand at an angle. It gets pushed upwards. Your hand represents the wing of an airplane. The plane's engine simply makes sure the "hand" keeps moving forward against the force of the wind.

Even a 4-year old understands the concept using this explanation.

Community
  • 1
Philippe Leybaert
  • 7,523
  • 9
  • 42
  • 72
  • 1
    Only "wind" is somewhat misleading - do you need wind to fly? – yankeekilo Dec 28 '13 at 09:56
  • 21
    You don't need wind to fly, but you shouldn't use the word "airflow" when explaining flight to your Grandma. If people stick their head out of the window of a moving car, 99% of people will say they feel the wind in their face, not "airflow" :-) – Philippe Leybaert Dec 28 '13 at 15:26
  • 2
    Oh, dear, all that sticking out of limbs calls for a security briefing ;-) My gripe with wind is that fact that people often mistake "wind" for thermals when talking about gliding: "Oh, a sailplane - do you need wind for that?" – yankeekilo Dec 28 '13 at 15:38
  • 2
    Oddly enough, this is how I explain it now (along with the requisite "you can make a plate fly this way if it is moving fast enough"), but unfortunately the lift generated by the impact of the relative wind on the body of an airplane is a very small part of the total lift generated, and I am trying to be a little more correct in my explanation (and at least cover where the majority of the lift comes from). Great answer though, and you still get a +1 from me! – Lnafziger Dec 29 '13 at 17:00
  • 9
    Consequently, this is from where the term "airplane" comes from. The wings are the *plane* that deflects the *air*. – dotancohen Feb 05 '15 at 15:19
  • 2
    I only wish I had more upvotes to give... I understood this as a 4 year old, because I would stick my hand out the window and do this. I can now understand and appreciate the engineering explanations too, but it isn't as complicated as we often make it. – Michael Hall Dec 19 '18 at 22:46
  • @dotancohen so, strictly speaking, does the word "airplane" refer only to the wings? – Someone Mar 31 '23 at 21:32
  • 1
    @Someone: Only if you consider the word parkway to, strictly speaking, refer only to the place where one parks. – dotancohen Apr 01 '23 at 19:31
  • @MichaelHall "isn't as complicated" It is if we explain 3D wings, which are examples of propulsion. Wings are pumps; reaction-motors. To use Bernoulli, we make our wings infinitely wide, so "Ground Effect" is 100%, and no energy is needed for flight. Instead, real wings are like helo rotors, using net energy to create a down-moving plume, injecting net energy into the atmosphere, and requiring full-blown Navier-Stokes equations. Airplanes and hummingbirds and helicopters all fly by reaction, by vortex-shedding, which is just what we feel when sticking tilted hands out the car window. – wbeaty Nov 03 '23 at 08:23
  • @wbeaty, are you furthering my point, or refuting it? Because I think we agree, but you are coming across as if we don't... – Michael Hall Nov 03 '23 at 15:50
  • @MichaelHall BOTH! It's easy to explain verbally. But the lift calculations even defeated Ludwig Prandtl! So he cheated in 1924. We've been confused ever since. Wings employ propulsion, act as reaction-motors. This involves shed vortices, where Euler/Bernoulli eqns fail, and instead require (unsolvable) full Navier-Stokes equations. Prandtl is the one who "simplified" it by removing the Newtonian net downwash. How? Just make wings be infinitely wide. Or, fly infinitely fast. So, he's only explaining UNPHYSICAL aircraft. It doesn't explain 3D wings, even the ideal versions. – wbeaty Nov 07 '23 at 04:17
  • ...cont. Prandtl insists that airplanes fly by pure pressure-diff alone, where the air far behind the wing is not left moving downwards. In reality this is impossible, a direct violation of Newton's 2nd and 3rd laws. But that's OK? Because it makes calculations easy? (No, that's not how physics works. Prandtl is "searching for lost car keys across the street, because it's dark & the light is brighter over there.") Here are two animations which depict the "downwash" produced by a wing. Remove it, and lift will vanish. http://amasci.com/wing/lasrWing.gif http://amasci.com/wing/rotgif1.gif – wbeaty Nov 07 '23 at 04:37
18

Here's one way to do it: airplanes work by pushing air. The wings push air down, and the airplane turns by pushing air in different ways. Engines push air to the back, pulling the plane forward. Wings work by turning the air downwards--the airflow on the top of the wing follows the curve of the wing, which points it downward.

My apologies if this is too low-level--is this what you had in mind?

cmn.jcs
  • 608
  • 4
  • 10
6

Every pilot knows the first thing that makes an airplane fly is money.

as I tried to explain the 4 forces to my 6 year old daughter and trying to coach her I asked, what makes it fall out of the sky. I expected weight or gravity and she said lack of money. She was right.

I try to explain it like a straw. You have four straws, one in front, one in back, one above and one below. The one below the aircraft sucks at a constant rate and never stops (gravity/weight). The one in front is the prop generating thrust. It is essentially sucking its way forward. The one in back is drag and it varies by situation but it's always trying to keep you from going forward. The final one is the one on top and that is lift. Lift is generated by the wings and sucks the airplane up. The wing creates lift by moving forward. If any of the straws win other than the front one and the top one, you crash.

As for wind, no you can fly through no wind but the addition of wind coming at you effectively helps fill that forward straw so you can essentially take off at a lower ground speed. This is called relative wind. If your plane flies at 54 knots and you could essentially fly into a 34 knot headwind at 20 knots and achieve backwards flight at 14 knots.

cberthold
  • 406
  • 3
  • 3
1

Blow over a sheet of paper that is lying on the table, and watch it take off. Then explain of course that there is a lot more air involved in keeping a 747 in the air, but you could make it plausible by mentioning how strong tornadoes and hurricanes are.

Koyovis
  • 61,680
  • 11
  • 169
  • 289
  • 3
    Planes don't fly because of fast-moving air above them. In particular, there is just as much fast-moving air below a plane as there is above it. – David Richerby Apr 26 '17 at 07:17
  • I know that, but I'm not grandma. It just shows what air pressure can do. – Koyovis Apr 26 '17 at 09:56
  • 5
    OK but planes don't fly because of air pressure, and planes are much bigger than a piece of paper, so what does this analogy help to explain? – David Richerby Apr 26 '17 at 11:15
  • 7
    @DavidRicherby why you say that planes don't fly because of air presure? This answer is based on the Bernoulli's principle. Wikipedia "From Bernoulli's principle, the pressure on the upper surface where the flow is moving faster is lower than the pressure on the lower surface where it is moving slower. This pressure difference creates a net aerodynamic force, pointing upward." – Robert Apr 26 '17 at 15:00
  • 2
    You're intentions are right but you're explication is lacking. – Notts90 Apr 26 '17 at 16:15
  • Exactly right. Tiny pressures add up fast. People don't know this? A yard-square plate, with only 1psi less pressure above, will lift 1300lbs of load. (Or, a 200lbs person, standing on a yard-square plate, can be lifted by a pressure-difference of only 0.15psi.) Yet the surfaces involved are normally WAY bigger than a square yard. Air is far from weightless, and even slightly-deflected air produces the small pressures needed to lift tons of aircraft+passengers. A big trash bag contains a couple lbs of air, yet it feels weightless, because it's immersed in a "sea of air." – wbeaty Nov 03 '23 at 07:55