# 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. – Lnafziger May 9 '14 at 3:51
• Something very similar to this is actually used in real life: check out blown flaps. – TypeIA May 9 '14 at 3:53
• – Lnafziger May 9 '14 at 3:59
• While the question is good, it's got very little to do with Bernoulli's principle. – falstro May 9 '14 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) – falstro May 9 '14 at 7:52

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.

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... – Danny Beckett May 10 '14 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." – sdenham May 11 '14 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. – sdenham May 11 '14 at 14:27
• @Danny : I should have added that this particular 9 year old seems pretty sharp. – sdenham May 11 '14 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...?!? – DevSolar Jan 27 '15 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. – Ryan Mortensen Jun 6 '16 at 18:11

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. – falstro May 9 '14 at 7:42
• Oh, my bad. The question isn't really about Bernoulli. – falstro May 9 '14 at 7:46
• with a bit of origami skills and some glue you can make a small airfoil you can use for demonstration – ratchet freak May 9 '14 at 7:54
• Eeek! I would not advise sticking any limbs out of the window. – yankeekilo May 9 '14 at 11:20
• Well, technically it is still Bernoullis principle, it's just not an efficient airfoil – OneChillDude May 9 '14 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.

How would you explain Bernoulli to a nine year old? [...] I was explaining to her the principle of flight and the fact that you need airflow under the wing in order to create lift

Hum, there are two aspects in this question:

1. How to explain Bernoulli's principle and its low pressure area.
2. How to explain lift.

However, while the pressure differential from both wing sides and lift are indeed related, lift created by this effect is not as large as lift created by the reaction to the downwash created by airspeed and viscosity. For those who doubt it, then just read what Nasa says about it:

The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction

There are simple experiences to demonstrate Bernoulli's principle and the pressure differential aspect. Explaining viscosity and downwash to a little girl might seem more challenging at first sight, but there is also a simple experience to do with a fork under a faucet (even if it is based on tension surface rather than viscosity). This experience also demonstrates stall and vortexes.

Bernoulli principle

On its simpler form, Bernoulli principle talks about static and dynamic pressure. Static pressure is the one exerted by a fluid in all directions, dynamic pressure is the one exerted by a moving fluid put to a stop by an obstacle:

Static vs dynamic pressure

The theorem states a fluid total pressure (static pressure + dynamic pressure) is constant along a streamline. Dynamic pressure being proportional to the square of airspeed, it means when airspeed changes in a way (e.g. is increased), static pressure changes the other way (e.g. is decreased), by a larger amount.

As air is incompressible at subsonic speed, when air is forced to flow in a tube which section is reduced, speed must be increased to maintain the same throughput (Venturi tube). By Bernoulli's principle, pressure in the smaller section is therefore reduced:

Let's use this in a simple experience:

By blowing in the funnel, the ping-pong ball is lifted by a small amount by any instability. From this stage, the ball must then move in the direction air is blown from, contrary to what is anticipated.

Bernoulli is behind: As soon as the ball enters the funnel, and the space between the ball and the funnel inner wall is reduced, pressure decreases and the ball is maintained by atmospheric pressure pushing on the surface not exposed to accelerated air.

Does Bernoulli/Venturi explain the suction effect of the top side of a wing?

Similarly to the funnel low pressure area, there is also a low pressure area on the top of the wing and the pressure differential indeed creates a part of the total lift.

It seems this low pressure area is created by Venturi shape of the wing top side. Well, this would be only the bottom half of a Venturi to start with. And if we needed a Ventury to explain the lower pressure, then how would we explain this lower pressure also exists in a flat plate, where no Venturi shape can be seen?

I'll leave this discussion to experts.

Downwash, reaction force

The greatest part of the lift is actually generated by the downwash and explained by Newton's third law: The wing rotates air downwards and accelerates it. The acceleration downwards creates a reaction force which pushes the wing up.

A part of the lift is created by reaction

Reaction contributes to the greatest part of the lift, but we need to explain why air is rotated. There are multiple ways to do that, one is viscosity effect.

Viscosity is the source of the downwash

Viscosity is very effective in the boundary layer. Air behavior is affected by its propensity to not shear (viscosity) and its propensity to remain unchanged (inertia). Reynolds number expresses how much a given object is subject to inertia vs viscosity for given speed, size and fluid.

Viscosity tends to maintain streamlines close to the wing surface and slow them down to the wing velocity (that is to zero relative velocity). Both mechanisms are more effective on air close to the wing, their effects are smoothed out as the distance from the wing increases, they disappear when leaving the boundary layer (by definition). This progressivity tends to turn the flow downwards.

Here is the experiment with another fluid, water (the mechanisms at work here are tension surface and viscosity, but the result is closely related to the one of pure viscosity):

• Put a fork (or another curved object) under the water coming vertically from a faucet.

• Adjust the angle of the object and the distance to the flow so that the flow stick to the object and follows its curve.

Here I used two stream speeds by opening more or less the faucet.

Water viscosity (and tension surface) at work on a fork

What to observe:

• Water follows the curve.

• The situation is similar to a wing, except the flow is vertical. The direction is not so important, except gravity accelerates water (the section of the jet decreases --top arrow,-- meaning its velocity increases), this has some impact on flow behavior.

• On the second fork, the flow detaches before reattaching (second arrow). This is a separation bubble, if we increase the angle of attack, the flow detaches, the fork is stalled. More viscosity allows for a larger stall angle.

• After leaving the fork teeth the flow creates a vortex because teeth alignment is such the different sub-flows are accelerated in different directions and are not immediately realigned by gravity when they are free from viscosity.

• Regardless of the vortex, which could be eliminated, the flow continues for some time in the direction it was accelerated along the teeth. At some distance gravity takes over and the flow is vertical again.

• As soon has there is a rotation of the flow, the fork is pulled in the direction of the flow by reaction. This is lift.

• Comments are not for extended discussion; this conversation has been moved to chat. – Federico Feb 22 '20 at 21:37

# 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.