# How would you explain Bernoulli to a nine year old?

This is a aviation principles question coming from my nine year old daughter that I need help with. I was explaining to her the principle of flight and the fact that you need airflow under the wing in order to create lift and that this is why an airplane in a steep climb will stall etc. So she said, why not have two large fans that blow air across the wings (we are not talking about the propellers here, but rather an external fan to create air-flow) so there is always un-interrupted airflow!!!! I am not sure exactly how to answer this. The best explanation I could think to give her was just as a sail boat cannot sail simply by placing a big fan on the boat.....but I don't think she was convinced.

• Actually an airboat is kind of a sailboat with a big fan. (Well, technicall no sail either, but hey.) An aircraft can fly with just a big fan too, but it's called a helicopter. May 9, 2014 at 3:51
• Something very similar to this is actually used in real life: check out blown flaps. May 9, 2014 at 3:53
• May 9, 2014 at 3:59
• While the question is good, it's got very little to do with Bernoulli's principle. May 9, 2014 at 7:47
• Also note that a sailboat can move by placing a big fan on it and blowing at the sail. The airflow from the fan bounces off the sail and creates thrust, just as if you'd have turned the fan backwards. It's not very efficient, but it works. (Mythbusters have an episode on it too, if you want to see it in action) May 9, 2014 at 7:52

Bernoulli's principle is probably not the best starting point to explain lift generation, especially if you do not first explain the origin of the Bernoulli effect! Naïve invocations of Bernoulli have led to a number of incorrect 'explanations' that are quite widely circulated, such as the idea that air flows faster over the upper surface of a wing because it is more curved that the lower, and thus creates a longer path. If this explanation were correct, paper airplanes and inverted flight would be impossible.

Bernoulli's principle is a consequence of Newton's laws of motion applied to incompressible fluid flow. To explain lift, you can start by invoking Newton directly, by observing that lift is the reaction to the wing's downwards acceleration of the airflow around it. Bernoulli enters the picture when you get to calculating the changes in pressure and velocity within that airflow.

From this perspective, we can see that while blowing air over a wing would create some lift, as the wing will deflect at least some of that airflow downwards, you would get more lift by pointing the fan directly downwards. This is, of course, what a helicopter does, and it takes a lot of power, so wing-blowing, being a less-effective way of generating lift, would require even more. If, instead, you use the fan as a propeller, you only have to produce enough thrust to counter the drag, which may be as little as 1/50 of the lift.

• Great answer, but I don't think a 9 year old would understand it... May 10, 2014 at 23:34
• @Danny : You could go with "yes, it would create lift, but it is not the most efficient way of doing so, and would take a lot of power to get airborne - more than a helicopter of the same weight." May 11, 2014 at 14:22
• One point that I didn't include originally is that this mechanism would produce a lot of thrust as a side-effect. That would make landing difficult. May 11, 2014 at 14:27
• @Danny : I should have added that this particular 9 year old seems pretty sharp. May 11, 2014 at 15:13

Blow over the top of a piece of paper and it will rise up. It rises up for the same reasons as a wing does.

• ...which has no downward acceleration of air at all...?!? Jan 27, 2015 at 14:12
• Of course the paper causes downward acceleration of air. Like germs, just because you can't see it doesn't mean it isn't there. Jun 6, 2016 at 18:11
• Not that it is the cause. The downward acceleration is the consequence of the aerodynamic forces involved, not the cause. How could it be the cause? It happens after the air leaves the paper. Sep 26, 2021 at 13:37

It works, but it is just the wings. For a controlled flight, we'd need two big fans for the wings (one for each), two smaller fans for the elevators, and one more for the rudder.

So that's 5 fans for each airplane.

The concept of controlled flight can be demonstrated by hanging two strings to any model airplane (plastic ones recommended, model airplanes stall just like real planes...). If we want to pitch up or down, we would need at least 3 strings.

Another consequence of having the fans blow air backwards across the wing is, due to action and reaction, there is a thrust pushing the plane forward. That's not a bad thing, planes are designed to move forward to get from A to B, not create lift vertically then settle down exactly where it started.

So the 5-fan plane will move forward anyway. Engineers figured out it is better to have one fan, called the "propeller", at the nose of the plane instead of 5.

• Not "just for the wings". If you move the air sufficiently fast enough over the top surface of any object, it will reduce the static normal (Perpendicular) pressure and cause Lift. Indeed, see links in my comment above, even a rotating cylinder causes such a [pressure differential. Sep 26, 2021 at 13:35

Drive on the freeway. Tell her to stick her arm out the window and pretend it's an airplane wing. That will be a great jumping off point for explaining how when the air flows over her arm, it creates areas of high and low pressure, which causes lift.

To explain high and low pressure to her, (which is a whole other can of worms entirely) bleed air from a bike tire. Also opening a soda can would help.

Also it sounds like you need to explain Newton's Laws (or at least the third one). There are countless examples of this in every day life. You walk on the ground, the ground supports you, etc, etc...

Everyone learns differently. Personally I'm an advocate of learning by experimentation, trial, and error. Try to find new ways to let her figure out the physics concepts on her own through experimentation, and then provide the academic understanding.

Wind, lift, flow, and fluid mechanics are very broad concepts which require simplification for many years into the learning process. Which is what learning is, a process. Start small, build gradually, and you will see results faster than you think.

Experience: I've been a student for most of my life, so I know a lot about not knowing much.

• As far as airfoils go, hands/arms are pretty much symmetric, so while there will be changes in pressure due to Bernoulli's principle, it'll be symmetric and no lift is generated (due to Bernoulli's principle, angle your hand and there will still be lift though). While sticking your hand out of a moving car is excellent for explaining stalls, it's not so great for explaining Bernoulli's principle. May 9, 2014 at 7:42
• Oh, my bad. The question isn't really about Bernoulli. May 9, 2014 at 7:46
• with a bit of origami skills and some glue you can make a small airfoil you can use for demonstration May 9, 2014 at 7:54
• Eeek! I would not advise sticking any limbs out of the window. May 9, 2014 at 11:20
• Well, technically it is still Bernoullis principle, it's just not an efficient airfoil May 9, 2014 at 14:26

Though rather belated, FOR THE OP's Question: The wing's downward push on the air under it is a good start. Pushing down on some bunch of air causes the wing to be pushed up. This is Newton #3 and what I call The Natural Phenomenon of "Paired Forces".

I use the example of pushing off from a boat to jump to the dock. The boat then moves away. You can even talk about loading the boat with more mass to reduce its movement to avoid falling in (as shown on many "funny" videos). The more mass (of boat, or air) that is pushed, the less it has to be accelerated. Watching tests and doing some calculations you can see this mass of air accelerated to about 30 ft per second for a large airliner. (F=MA)

You may also mention helicopter downwash, an ordinary propeller propwash, or a jet engine jet blast for examples of the "push on something and get pushed the other way" of Newton's #3. The explanation of pressures and pressure gradients around the wing to accomplish this gets longer than a 9 year old probably needs.

However, scientists discovered that the air moving over the top must curve around and follow that upper surface and this (details omitted) also causes more air to be pushed down and adds to the lift. Detail explaining why the upper surface effect occurs gets more complex and it must be well understood before attempting to explain it and only if necessary. (The "longer path" does not cause it to speed up. It has to do with accelerating air around a curve) The student can accept that there is more that they won't understand and can be satisfied by a partial, but correct, explanation.

I'm not sure where that climbing part is going, but it is always the relative motion of the wing through the air (called "Relative Wind") that is important, not the climb angle. I believe you refer to the "Angle of Attack" (AOA). This is the angle that the wing makes as it meets the relative wind. If the nose is pointed up, but it is flying more or less level, this is because more lift is needed, USUALLY at low speed, when airspeed, and therefore lift is lower. At some point, as the angle of the wing points up too far, the smooth flow of air OVER THE TOP turns turbulent. It swirls around instead of following the wing smoothly and this destroys that upper flow and a large part of the lift. This is called "stall". This is proof that it is not juswt the lower surface that produces lift, otherwise the loss of smooth flow over the top would be a 'don't care'.

Also, the fan over the wing is actually a GOOD idea. This uses what is called the Coanda Effect and has been used on aircraft. The first I was aware of, as a youth, was the F-104 with its tiny wings. In order to reduce the landing speed (and actually make it landable -- no engine, no land---bail out) they directed some engine blast-air through carefully designed slots over the wing. It was called "Boundary Layer Control".

PLEASE, all readers, see these authoritative sources. They discuss lift and you should be able to adapt parts of them and simplify as needed.

Peter Eastwell Bernoulli?
http://www.scienceeducationreview.com/open_access/eastwell-bernoulli.pdf As a teacher, he didn't like what he read and researched this to find the truth.

Weltner in PDF - "Misinterpretations of Bernoulli's Law"

Prof Babibski @ Cambridge has a youtube video, but you must get his slides from links under the video. XWdNEGr53Gw

His missing slides are at docs.google.com
/file/d/0B0JABuFvb_G_MkpBZHJmRGo3UkU/edit?usp=sharing

He also has a comparable paper on ww3.eng called "Senior-glider/howwingswork.pdf"

Anderson & Eberhardt AAPT paper: The "Newtonian Description of Lift of a Wing"-Revised 2009: on comcast.net/~clipper-108/Lift_AAPT.pdf

gifday.com has a great animated GIF of an A380 test allowing you to actually see the downwash: 2012/02/a380.gif

-- Regards, Challenger Learning Center Science/Technical Advisor Steve N.

# You can... and some do... but you will not go fast that way

Well actually, she is right: you can use an engine to blow at the wings and create lift that way. And that principle is used on some aircraft, like on the Lun-class ekranoplan.

...but only when taking off.

The problem is that a huge portion of the engine trust hits the wings, which then brakes the plane, because of the drag. The thrust you gain from the engine is therefore lost back on the wings. The plane is pushing against itself, so it does not get up to the speed that we want to.

So you can tell her: she is right, you can do it that way.

But you do not want to do it that way because once the plane goes fast enough that it does not need the "help" from the engines to stay aloft... then we cannot turn that off. And then her scheme just becomes a big brake instead, so the plane cannot go very fast.