How to explain bernoullis principle to a new student
Posted by Intelligent-Pin1843@reddit | flying | View on Reddit | 147 comments
Studying for Cfi checkride, trying to understand a simple yet effective way to teach lift without overcomplicating it
i think most of us learned that the air has to go faster over the top to reach the air on the bottom at the same time, but that isn’t a great way to teach this topic.
which happens as a result of the other? does fast moving air cause low pressure or does low pressure cause fast moving air?
and what causes the air on top to move faster than below
videopro10@reddit
Yes, not a great way since it's totally wrong...
Intelligent-Pin1843@reddit (OP)
what is correct then?
Practical-Mix-5465@reddit
There’s a concept in fluid mechanics called the virtual Venturi. Think of the top of the wing as half of a Venturi. We know that a half Venturi will behave the same as a full Venturi. The maximum camber point of the airfoil is the narrowest point of this Venturi. The continuity equation tells us that a reduction of area must increase the flow velocity since volumetric flow rate cannot change. Bernoullis equation then tells us that an increase in velocity will cause a decrease in pressure since total energy cannot change (you trade one form of energy for another). Hope this helps. Feel free to message me if you have any questions.
Bergasms@reddit
The air does go faster over the top, but it's under no obligation to reach the air on the bottom, in fact it doesn't care one iota about the air on the bottom.
Here, this is great
https://www.av8n.com/
jaylw314@reddit
That's a brilliant book I always come back to every few years
Criminy2@reddit
Even bigger “in fact,” the air on top is going many times faster than air underneath. If two molecules of air meet the leading edge at the same time, the one that went over the wing will be at the trailing edge long before the one that went underneath.
hate737@reddit
…how are tou a CFI candidate
Spiritual_Ad8882@reddit
Hey no need to be mean! Training for CFI is where you see the gaps in your knowledge. A good pilot is always learning.
cazzipropri@reddit
His instructors also didn't understand it.
One_Event1734@reddit
The chance the DPE actually understands aerodynamics is slim.
Longwaytofall@reddit
Chances are high that the DPE has a poor understanding of lift passed down from generations of instructors who didn’t understand it either.
cazzipropri@reddit
I agree completely.
shadowalker125@reddit
CFI taught wrong before them teaches wrong thing to new students. Welcome to CFI inbreeding.
Embarrassed-Row6801@reddit
I feel like if you are going to he a decent CFI, you won’t settle with what you are being taught and will go deep into more advanced on your own to really understand some concepts. It’s surprising how many CFIs just continue with what they were taught and never really question anything
hondaridr58@reddit
😂
PopPleasant8983@reddit
It took dating an engineer for me to finally discover that this was not how it worked. CFI inbreeding indeed
videopro10@reddit
that's a big question that has been discussed to death on here. I suggest you start reading some of NASA's pages.
https://www.grc.nasa.gov/WWW/k-12/VirtualAero/BottleRocket/airplane/wrong1.html
https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong3.html
jet-setting@reddit
This is a great resource, I just had copied it to post here too.
https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/foilshape/
This section has a good interactive airfoil and AoA tool but it’s not optimized for mobile. You’ll probably need a computer I couldn’t change any values on my phone.
burnheartmusic@reddit
Ya, most of us did not learn that because it is incorrect
Flyinghud@reddit
"Airplane wings are shaped to make air move faster over the top of the wing." From fucking NASA.
srkjb@reddit
https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong1.html
Peep the article title
Al-tahoe@reddit
It’s the part about meeting together that is wrong. Also quoted from NASA
Flyinghud@reddit
Clearly my reading comprehension is struggling this morning
jemenake@reddit
My current qualitative explanation of how BP works is this: at a given temperature, the molecules collectively have a certain average kinetic energy (this is the definition of temperature) and a certain average velocity (given by the Boltzman distribution). Under normal circumstances, these molecules are going in _all_ directions equally, on average (which is to say that, for every molecule going directly upward, there will be either one going directly downward or a few going diagonally downward, etc). The point is, if you grabbed any one of those molecules, you'd find it to be equally likely to be going sideways as up or down. To use a crosswind analogy, we expect an equal sideways component as the vertical component.
Now, when air "speeds up over the top of the wing", the molecules haven't sped up (since we didn't change their temperature, so the molecules have the same velocity as before) but rather their collective direction changed; more their velocities are going to be in the "sideways" component (along the chord line of the wing) which is going to leave less velocity for the vertical component (which would be the part of its momentum striking the top surface of the wing).
What I'm still wrestling with is how, when a wing comes cutting through an innocent section of air that was minding its own business, how does the wing cause that imbalance in the horizontal and vertical components of the average velocities in the molecules?
FrankenberryPi@reddit
Aerodynamics engineer here. When someone brings up wings and Bernoulli I can pretty safely assume they don't know what they are talking about. Wings produce lift by pushing air down. That's primarily accomplished via the angle of attack. Any answer more detailed than that requires caveats, nuance, and boatloads of math.
To actually explain where lift comes from is extremely complicated. You need to account for conservation of mass, momentum, and energy, and you probably need to explain turbulence, boundary layers, and the thermodynamics of gasses. The Bernoulli equation can fall out of some of that stuff, but it's not generally a useful equation for explaining anything about a wing.
A useful thing to know: the slope of the lift coefficient vs. AoA curve is surprisingly linear. So if you double the angle of attack you double the lift. This works until you get close to stall.
A less useful thing to know: the slope of the lift coefficient vs. AoA curve is 2piAoA. Pretty neat, not all that useful to most pilots.
Aerodynamics is in a weird spot where the basics are very simple, but the details are insanely complex. Any kid who has stuck their hand outside a car window has an intuitive sense of how lift relates to angle of attack, but they probably don't have the language to explain it. But Richard Feynman called turbulence the biggest unsolved problem in classical physics. That was in the '70s, and we haven't made all that much progress since then.
"It's easy to explain how a rocket works, but explaining how a wing works takes a rocket scientist." -Philippe Spalart
adiabaticgas@reddit
Easy example: symmetrical wings fly, some aircraft can maintain level flight both right side up and upside down.
NolanGrayson711@reddit
It's better to just accept it's a combination of multiple things, including bernoulli. A wing wouldn't take off when the angle of attack is 0° if it was only Newton's law.
primalbluewolf@reddit
They don't. By definition at 0 AoA your wing makes zero lift.
St-JohnMosesBrowning@reddit
That’s only true for symmetric airfoils. Check out a plot of lift coefficient (CL) vs AoA for any nonsymmetric airfoil - at zero alpha, CL is nonzero and positive. https://images.app.goo.gl/mJgkPkXpLckkqJUz9
primalbluewolf@reddit
This is an artefact of there being two ways to define AoA, and your plot is using the other one.
Your statement is correct for geometric AoA, and against the definition for absolute AoA.
In comparing designs absolute AoA is much more useful. Geometric AoA being the angle measured between the chord line and the free stream. This is convenient for measurement, and intuitive to explain - and leads to unintuitive outcomes such as lift at zero degrees AoA being non-zero.
Absolute AoA defines a zero lift direction, and measures the angle the free stream makes against that as the AoA. This has the effect that 0 AoA is zero Cl, for all wings, regardless of camber.
Either approach is fine, but it does mean when comparing multiple sources talking about "the" AoA, that you confirm which definition is in use.
FrankenberryPi@reddit
It is many different effects, but it is not Bernoulli. Bernoulli does not tell us anything useful about how wings work. It only applies along a streamline, and looking at pressures along a streamline is not helpful for evaluating wings.
primalbluewolf@reddit
Is it not? Here I thought this was how the various NACA sections were evaluated?
FrankenberryPi@reddit
It is not. We usually only look at streamlines when a customer wants some pretty pictures. I did used to use them when I did thrust reversers to evaluate reingestion. Streamlines are pretty difficult to calculate and require more or less complete knowledge of the pressure and velocity fields.
My history is a little fuzzy, but I think the early NACA airfoils were just about describing the geometry and they got performance with testing, not theory. When the 6 digit NACA stuff came along they probably used potential flow or vortex lattice methods. But the problem was the same, you need to know the full flow field before you can find a streamline.
primalbluewolf@reddit
You seem to be working on the "calculate a streamline" approach - I had thought when this was an area of active research, it was the other way around - start from the film slides of the streamlines from the wind tunnel, which were directly photographed, not calculated - and then use those to find the pressure and velocity fields around the airfoil section.
Obviously today with CFD tools this is not required.
squawkingdirty@reddit
Take a piece of paper and hold each corner with your fingers
Blow on the top and watch Bernoulli do the rest
Accomplished-Ice-604@reddit
Not a physicist, so I’m still figuring some stuff out, but doesn’t this better demonstrate Coandă effect? The tendency of a fluid to stay attached to a surface plus the momentum of the air lifts the paper. It may be a strong force, weak force situation, to be fair. I believe Coandă contributes(?) to lift and to induced drag but still isn’t Bernoulli.
You can less ambiguously demonstrate Bernoulli with liquid rising in a straw when you blow over the top of the straw.
primalbluewolf@reddit
That isn't the Coanda effect. The Coanda effect specifically is about entrainment of additional airflow at the blown section.
Accomplished-Ice-604@reddit
Nah. The Coandă Effect is ALSO about the entrainment.
Coandă Effect - Merriam-Webster describes it as "the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surroundings so that a region of lower pressure develops."
primalbluewolf@reddit
http://www.av8n.com/how/htm/spins.html#sec-coanda
Some longer discussion on Coanda effect and some reasons it is not generally worth discussing in the context of introducing new students to aerodynamics.
Accomplished-Ice-604@reddit
Sorry about using the dictionary.
“Coandǎ effect is properly applied to any situation where a thin, high-speed jet of fluid meets a solid surface and follows the surface around a curve.” - the webpage you just linked. I brought up Coandă concerning a piece of paper, also mentioned in your reference.
primalbluewolf@reddit
So then - walk me through how exactly that relates to a wing? Where's the jet?
How does it apply to a flat wing, with no curve?
Accomplished-Ice-604@reddit
Again, piece of paper.
primalbluewolf@reddit
Merriam Webster also defines "literally" as meaning "figuratively" and its the descendant of Noah Webster's abomination that lead to the spelling difference between English (Traditional) and English (Simplified).
I don't give them a lot of credit on physics.
All that said: they're correct here, because they mention the entrainment specifically.
Note that their definition also specifically excludes the airflow around a typical wing from having anything to do with the effect: their definition applies specifically to a jet of fluid emanating from an orifice.
BattleAnus@reddit
That's not the Bernoulli effect, that's the Coanda effect
squawkingdirty@reddit
I’ve heard it both ways
WhiteoutDota@reddit
I've actually heard that it's newton's third law rather than bernoulli. In fact, most scholars to my kbowledgd think newton's third law is the primary reason lift is created. MIT doesn't even mention bernoulli when they explained lift in their pilot ground school.
primalbluewolf@reddit
Unless the question was "is Bernoulli's Principle the best way to explain aerodynamic lift to non-aviators", this is probably not the ideal answer.
The laws of physics are consistent, not cumulative. You don't get a bit of lift from applying Bernoulli's Principle, then a bit more from Newton's laws, then a bit more from Coanda etc. You get 100% of the lift from applying Newton's laws, and you get 100% of the lift from applying Bernoulli's Principle. Its not a second 100%, its the same lift force being looked at in a different way. (Typically the Coanda effect has no applicability to most aircraft wings).
MIT is probably on the right track, pedagogically speaking. Bernoulli's is very useful to folks studying aircraft design. About the only useful thing it explains for the average pilot is that a venturi works - and not too many aircraft use a venturi vacuum pump these days.
yellowstone10@reddit
Arguing about whether Bernoulli or Newton is the "primary reason lift is created" is like arguing whether conservation of energy or conservation of momentum is the "primary reason" for some physical phenomenon. They're both right, it's just that depending on the circumstances, one or the other approach can make the math easier.
WhiteoutDota@reddit
I mean it is interesting that some people don't even mention bernoulli as even a component of it, but I'm just a simple CFI who teaches his students what the FAA wants me to
nascent_aviator@reddit
Newton's third law is a much easier way to explain lift without getting into the weeds of fluid mechanics. If you mention Bernoulli you can't really avoid at least some of the weeds
yellowstone10@reddit
This. If you're willing not to dig too deep, it's entirely accurate to say "an airfoil like a wing is a special shape that deflects oncoming air downwards - and by Newton's Third Law, if the wing puts a downward force on the air, the air puts an upward force on the wing."
"But why is a wing so good at deflecting air downwards?" is a much harder question that requires fluid mechanics and quite a lot of complicated math to answer.
nascent_aviator@reddit
For sure. I think it's fine to get into a little bit of it- basically the air doesn't like to change directions suddenly so a smooth contour makes it follow the contour better. Basically just enough to convince them that a flat sheet of metal would work as a wing, but that an airfoil shape works better.
tdscanuck@reddit
Both…Coanda is why the flow “sticks” to the paper. Bernoulli is why the pressure is lower, and why the paper rises.
yellowstone10@reddit
Ehh... be cautious there, because a lot of people will hear that and think "oh, I get it, you're blowing fast air out of your mouth, and Bernoulli says fast = low pressure". But try setting a piece of paper on a table in front of you and blow over the top of it. It doesn't go anywhere (well, it might blow away from you due to friction, but it certainly doesn't lift up). You have to blow over a bent piece of paper (that will deflect the airstream) in order to see the effect.
TxAggieMike@reddit
This.
But have the student hold the paper and blow.
Flight instructors are about providing experiences so students learn.
After they successfully get the paper to rise, ask them to explain why and see how far they get. Then fill in blanks.
CavalrySavagery@reddit
If i suck the tighter it gets the more speed you'll get in exchange of pressure.
Don't tell this to a girl. Proly you'll be a sex offender afterwards.
MEINSHNAKE@reddit
Blowing over a strip of paper is my go-to… high pressure wants to fill low pressure areas.
makgross@reddit
Bernoulli’s Principle is an expression of energy conservation in a parcel of fluid. One factor in that product does not “cause” the other.
Start with understanding the principle yourself before asking how to explain it.
Does loss of speed cause a climb, or does a climb cause loss of speed? The correct answer is that increased angle of attack causes both.
Accomplished-Ice-604@reddit
An airplane in steady-state climb has a LOWER angle of attack, relative to airspeed. It’s gravity that slows you down.
primalbluewolf@reddit
I believe u/makgross is referring to the initiation of a climb, and trying to draw a parallel between the physics in initiating a climb, and the physics under discussion regarding production of lift: that in both cases, its not that one aspect of the physical situation causes the other, its that both happen as part of the situation.
I.e. loss of speed doesnt cause a climb, climb doesnt cause loss of speed, increased angle of attack causes both - and therefore, bernoullis: fast airflow doesnt cause pressure drop, pressure drop doesnt cause fast airflow, both are products of the situation.
I'm not so sure I agree with the argument as presented, but I agree with the conclusion drawn.
Accomplished-Ice-604@reddit
I see the case you’re making, but this is a discussion about how to instruct, so accuracy of language matters. The statement that AOA causes a climb is still inaccurate. Pitch angle or deck angle causes the climb, but AOA decreases in climbs and descents. This is counterintuitive, so it’s worth discussing. Helps to solidify the understanding that we climb on excess thrust or power. AOA does change with load factor, so pitching will change the load factor and AOA, but pitching for a climb is not a climb.
primalbluewolf@reddit
Agreed that word selection is crucial in instruction, but disagree on the statement being incorrect in this case. You are correct that in a typical climb the AoA is lower, but it is not always as a rule. Cruise at 110 and climb at 50, the AoA at 50 will be higher than the cruise AoA.
You seem to be distinguishing between the initial entry to the climb and the sustained, in-equilibrium climb, which is fine - but not a contradiction of u/makgross' comment. Hard to initiate a climb without an initial increase in AoA, no?
cazzipropri@reddit
Here's a deep dive lesson plan only for that part of Task II.D.
https://docs.google.com/document/d/15BmsDr0RvDATuJTDy5MywBcqtX-wJweKfZo5PxzvEa0/pub
primalbluewolf@reddit
I love it. Why isn't this further up?
I might suggest some more diagrams might be helpful - I really like these ones, and the accompanying explanations are useful IMO also.
https://www.av8n.com/irro/lecture_e.html
cazzipropri@reddit
Thanks for the feedback - I'll work on it. I have to have all my lessons plans ready in a couple weeks!
hakrsakr@reddit
Are the explanations in the beginning of Stick and Rudder incorrect? If I remember Wolfgang says something to the effect that it's the angle of attack pushing/sucking the air down that gives lift. Never mind the complicated physics of why most of the lift is generated by the top of the wing. I've never liked the Bernoulli explanation because how is it possible for some planes to fly inverted? They don't all have more thrust than weight, they need the wings to be doing something.
majesticjg@reddit
How about: Air moves over the wing. Because of the shape of the wing, lift is created. If there's not enough air moving over it, it doesn't make lift anymore.
I'm not sure you need more detail than that to fly an airplane. I can't describe the spark timing and firing order, but I can still start the engine.
primalbluewolf@reddit
If you could, there might be more of a push for modern engines!
littlespeck@reddit
They're studying for a CFI checkride. I don't think that answer is sufficient.
majesticjg@reddit
Sadly, it isn't. It's frustrating when what you need to know to safely fly the plane doesn't line up with what you need to know to pass the test.
Privileged_Pear@reddit
I may be wrong, but the way I always taught it was by explaining that the top of the wing has relatively excess curvature. There’s more surface area at the top of the wing which acts as the “restriction” in the Bernoulli principle causing the air to speed up and the pressure to decrease.
primalbluewolf@reddit
So when you examine a venturi, the fastest air is in the centre. Air next to the restriction is slowest, and as you get further away from the walls of the restriction, towards the centre of the tube, the airflow increases in velocity.
Based on the above, I would reason that in this case, the air above a wing would be slow, and increase in speed with increasing distance above the wing - but this is not what occurs.
Instead, the fastest airflow is found directly above the wing, immediately adjacent the boundary layer, and the airspeed decreases with increasing distance above the wing.
I would suggest this makes any claim that a wing acts "just like a venturi" automatically suspect.
Boring_Concentrate74@reddit
Wait until you have to talk about the Coandă effect which is something I never see anyone discuss but plays into lift
primalbluewolf@reddit
Not on most aircraft, it doesn't.
We've built a handful of experimental aircraft that relied on the Coanda effect, but for the most part its a lot of hassle for not enough benefit, and not very resilient in the case of failure.
The avrocar is one of my favourite aircraft, and is one of a very small number of aircraft you can (truthfully) explain the Coanda effect with.
TheGreenicus@reddit
The real question is why teach it at all. Bernoulli's principle would rely on 2 air molecules that diverged at the leading edge rejoining at the trailing edge. They generally do not. Wings are shaped the way they are for reduced drag. Lift is almost entirely newtonian - lift is produced as a reaction pushing air "downward" (relative to the airplane's frame of reference).
primalbluewolf@reddit
You were so close to being correct, and then you snuck an "almost" in there.
The laws of physics are consistent, not cumulative. You don't get a bit of lift from Newton, more from Zhukovsky, more from Kutta, more from Coanda, more from Bernoulli - Newton's laws describe 100% of the lift, and so does Bernoulli.
Lift is entirely Newtonian. Its also valid to describe it using Bernoulli's Principle instead, although for student pilots this is of limited applicability.
NolanGrayson711@reddit
Nah you are wrong, it's a combination of both
Flyinghud@reddit
Fast air causes low pressure. The wing is designed so the air has to move faster on top and slower on the bottom and since air likes to move from high to low pressure, it causes a lifting force.
GreenNeonCactus@reddit
Why does the air on the upper surface have to do anything? Why can’t it just travel slower since it has a longer distance to cover?
primalbluewolf@reddit
Excellent question.
Its not about the distance to cover - not directly. You can fly an airplane upside down, with the "longer distance" on the bottom, and aerobatics planes typically have a symmetrical airfoil, so there is no "longer distance". Some aircraft are even designed with a reverse camber, so that the longer distance is on the bottom.
With a surface inclined against a flow of air, whether its a wing, or a door, or a sail - we see a variation in air speed, and pressure. In the case of a wing, the pressure on the top decreases, and the pressure on the bottom increases - and the air underneath slows down in the face of the increase in pressure, and the air on top speeds up into the low pressure region. The slowing down of air contributes to the high pressure though, just as the accelerating of the air on top contributes to the low pressure region. Its not so much "cause and effect" as it is "collection of effects".
The pressure changes depend on the shape of the surface and the angle of attack it has, and ultimately the pressure field at the surface determines the aerodynamic lift and moment.
tdscanuck@reddit
Because if it didn’t the flow would be unstable and change velocity until it was. The entire point of airfoils is that the only stable flow pattern around them has lower pressure on top than on the bottom.
Longjumping_Dog3019@reddit
Does fast air cause low pressure or does low pressure cause fast air? I don’t think there’s a 1 way causation that can be blamed here but the relationship can give intuitive understanding.
Also air linking to move from high to low pressure can be explained with Newton easily, high pressure side is acting as a net force accelerating the flow (F=ma)
lil_layne@reddit
To apply both concepts of lift and Bernoulli’s principle I really think just drawing a diagram that you would see in a textbook is the best way by explaining what an airfoil is and why it’s shaped like it is and how that airfoil shape causes higher pressure air to be pushed underneath the wing to generate lift.
For intuitively understanding each concept separately, I would say for Bernoulli’s principle I like the putting a thumb over a water hose analogy. When you do that the pressure of the water decreases (not as much water is coming out) but the speed/velocity at which the water is coming out is faster. And for lift the good ole stick your arm out of the window of a fast moving car explanation is something everyone is familiar with and is pretty easy to understand how air is a fluid and how it can produce lift at high speeds.
cazzipropri@reddit
I would avoid the water analogy because it's an incompressible fluid.
nascent_aviator@reddit
Subsonic air is also an incompressible fluid.
primalbluewolf@reddit
Are you certain? I compress subsonic air routinely. My air compressor suggests air is highly compressible.
nascent_aviator@reddit
Very different conditions than exist around an airfoil. I'm talking about the stagnation density that exists when free flowing air impinges on an object. This density is nearly unchanged for speeds much less than the speed of sound.
primalbluewolf@reddit
Also - sorry for linking a lengthy piece :/
primalbluewolf@reddit
I don't disagree that the density is nearly unchanged, but I do disagree that this makes the air incompressible.
As usual, Denker has commented on the subject: https://www.av8n.com/physics/bernoulli.htm#sec-gamma-games
cazzipropri@reddit
If you mean that we like to use Bernoulli's equation for incompressible fluids because it's much simpler to use and it's not terribly off, that's one story.
If you believe that air is incompressible in subsonic conditions, then we don't agree...
nascent_aviator@reddit
Air is not completely incompressible (but then, neither is water!) But correction terms due to compressibility are proportional to the mach number squared. For low mach numbers it is a very small correction- certainly not something important enough to kill a good analogy.
primalbluewolf@reddit
For OP: drawing a diagram is a great way to teach. Drawing a diagram that is wrong is a great way to teach something that is wrong.
If you do this, please take care with drawing your diagrams. Drawing a freehand aerofoil can be done, with practice. Drawing a freehand streamline diagram is almost always going to lead to teaching misconceptions, and is very time consuming. Here I would strongly suggest relying on existing resources such as can be found here : https://www.av8n.com/irro/lecture_e.html
The animations at the above link are not as high resolution as Id like, but a careful study will allow you to produce your own if need be.
tdscanuck@reddit
No, the pressure of the water doesn’t decrease. This is part of why using Bernoulli to explain lift is really dangerous, it goes unintuitive really quickly.
Any (non-supersonic) free stream like what’s coming out of a hose is at ambient pressure. When you put your thumb over it you restrict the flow so the pressure drop in the hose is lower, so more pressure makes it to the outlet, so it can accelerate more before it drops down to ambient.
VileInventor@reddit
https://www.grc.nasa.gov/WWW/k-12/VirtualAero/BottleRocket/airplane/wrong1.html
lil_layne@reddit
I’m not gonna claim to be an expert in physics but the water hose example is the way I was taught first in physics (not being related to aviation) and it stuck with me because I initially had a hard timing grasping the concept of how velocity increasing correlates to pressure decreasing. Another good example I was taught was with how fluid flows faster through a narrower pipe where pressure is decreased.
Isn’t the concept the same due to the conservation of energy that when a fluid narrows through a pipe, the fluid must speed up, and as the speed of a fluid increases in a narrower section, the pressure decreases? Isn’t this called the Venturi Effect which is an example of Bernoulli’s principle? Genuinely curious, does the constricted pipe example not apply with the ambient pressure, but the thumb over hose does? I’ve just looked up other people use the same example to explain Bernoulli’s principle and I don’t know enough about physics to understand how it actually isn’t correct.
tdscanuck@reddit
Kinda. This gets tricky because Bernoulli only applies when you have no losses and the “thumb on a hose” situation involves tons of losses.
A venturi is, absolutely, a Bernoulli situation. And your thumb on the hose is a decent approximation of a venturi. The part that gets misleading is assuming that the pressure of the water coming out past your thumb is lower than it was when the hose was open…that’s the part that’s not correct.
In a closed tube the fluid can’t “feel” ambient pressure. The fluid can be higher or lower than ambient, no problem, the tube protects it.
In a subsonic free stream, like water exiting a hose, that isn’t true. There’s no pipe wall. The water has to be at ambient (static) pressure. Whether you put your thumb over the hose or it, the pressure of the exiting water is the same.
What’s changed is the amount of pressure drop in the venturi. Not because it’s lower pressure at the outlet, that’s fixed at ambient, but because you’ve got higher pressure at the “inlet” to the Venturi, where the hose meets your thumb. Since the hose supply pressure is also fixed, the only way that can happen is if there’s less pressure drop in the hose between the supply and your thumb, which means to the flow velocity in the hose must be lower, which means the flow rate must have dropped. And, indeed, if you measure to flow rate with and without your thumb it does fall.
But a Bernoulli explanation won’t “see” the pressure drop in the hose because Bernoulli assumes to losses to start with.
lil_layne@reddit
Thats a good explanation. Thank you. Im not afraid to be wrong here in front of everyone, maybe someone else learned about this too. I know I won’t use that example for my future students and just use the constricted fluid in a pipe example now.
tdscanuck@reddit
I'm wrong on Reddit constantly, it's all good.
Don't necessarily throw out the entire example...just don't get into the details of what the water pressure is at any particular point. The thumb-on-hose is a *great* example of general conservation of energy (which Bernoulli is just a simplification)...you can turn higher pressure/lower speed flow (upstream in the hose) into lower pressure/higher speed flow (the spray). That fundamental interchange is still valid, with or without losses, with or without referencing particular pressure values. It shows up in the carburetor (carb icing), it shows up in the fuel injectors (the actual injection), it shows up on the wings (low pressure/higher speed on top), the pitot tube (ram pressure), the air vents (if you have ram air like a Cessna), etc.
In case it comes up with future students who wonder why the pressure isn't ambient all around a wing, keep in your back pocket that the flow above/below a wing *isn't* a freestream...the wing is there. That's a solid surface, like one side of a pipe or venturi.
primalbluewolf@reddit
Do you need to explain Bernoulli's Principle at all? Are you specifically tasked with that, or with explaining Lift?
Simple, effective way to teach lift without overcomplicating it - sounds like Newton's laws of motion to me. The wing interacts with air flowing past it, such that air is accelerated downwards behind the wing. To do that, the air pushes up on the wing, providing aerodynamic lift.
Regarding the "equal transit time" you mentioned, please don't use this idea with your students. How will they understand reverse camber wings, where the longer distance is on the bottom?
Neither. The presence of the wing and the other air causes the pressure and momentum changes.
Do you actually need to explain "why"? "Why" requires a great deal of math your students will almost never be equipped to deal with. Most instructors will hear "how" and think they heard "why" anyway, so why not explain what physically happens and rely on that?
My suggestion - start with reading chapter 3 of See How It Flies. https://www.av8n.com/how/htm/airfoils.html
If you really feel you need to bring up Bernoulli's Principle, the math behind it is explained thoroughly here: https://www.av8n.com/physics/bernoulli.htm
I would recommend instead looking at these nice airfoil diagrams instead: https://www.av8n.com/irro/lecture_e.html
siegward_with_boof@reddit
Take a piece of paper, and hold it on the bottom edge with both hands in a way that the paper flops away from you and makes a vague airfoil shape. Blow on the leading edge of the "airfoil".
The paper goes up: lift. Ta-da.
Fast air makes lift, airfoils make fast air on top.
Quiet-Recover-4859@reddit
That’s called Coanda effect.
primalbluewolf@reddit
Strictly speaking, its a demonstration of the kutta condition and that air has mass.
Puckdropper@reddit
It's been said that Nature abhors a vacuum. Would it be too wrong to think about it like you're blowing the air particles away and nature is filling the vacuum with whatever it can? More air, the wing?
Turkstache@reddit
Listen to all the corrections below... but the honest answer is "just barely enough that a pilot can answer the question correctly in an oral with confidence, and maybe a common follow-up question."
Unless you're an engineer designing a wing or working on certification and envelope expansion, there is almost no knowledge of aerodynamics that can help you fly the plane in a way that learning maneuvers and procedures and feel and parameters couldn't teach you 10000000x better.
Don't send your student to a checkride with misconceptions about it, but Bernoulli's principle is just trivia.
Comere@reddit
I have taken a piece of paper, held 2 corners up to my mouth. Blow hard and steady over the top of the paper and it will lift up towards the low pressure.
Use that practical knowledge demonstration with the shape of an airfoil inducing low pressure and you can translate the theoretical to the practical and back
Al-tahoe@reddit
It can quickly get outside the scope of what’s required to be known for any rating through CFI when you start getting into the why.
Air over the top of the wing is accelerated by pressure gradients generated by curving an airstream. This is due to the conservation of momentum.
You can SAY since we know air over the top is faster relative to below, that the pressure on top is lower. This is an application of Bernoulli’s principle.
Bernoulli’s principle actually just says the energy of a fluid along a streamline is constant. The fluid’s energy is made up of its velocity and pressure. If one changes, the other changes simultaneously. That’s why it’s confusing as to which happens first, because it’s neither.
tomdarch@reddit
Yep. I love the nitty gritty - conservation of mass and energy, etc.
But I know that when I do the oral part of my checkride, I need to explain things in the way that the FAA describes it and be able to think about aerodynamics in a way that helps me actually fly a 172, not design one or do a PhD in aerodynamics.
i_use_this_for_work@reddit
Storybots does a great job at this.
Thrust drag weight lift.
photobusta@reddit
Two ping pong balls taped to a pencil, have the student blow in between them, and see what happens. You’d think they separate but because of Bernoulli’s principle, they come together.
T-Hangarz@reddit
The way you explained it sounds like “Equal Transit Theory” and is incorrect. Please re-read the PHAK on this, as it specifically says to NOT say this.
I usually just explain that fast moving air has lower pressure. Then just say the top of the wing causes the air to speed up. Then I draw it and draw an arrow from the high to low pressure, which is a force by a difference in pressure. They’ll need to get this to understand how wind works, so spend some time on it.
nissbd@reddit
Under my administration, teaching the Equal Transit Theory will be a class A felony
csl512@reddit
You teach equal transit theory they put you in jail. Right away. No trial, no nothing.
ap0r@reddit
Bernoulli's principle. Imagine you are walking along and you enter a crowd. You will slow down. Now you exit the crowd. Since there's only a few people, you can speed up to your normal walking speed. You are an air particle. It takes work to enter a high-pressure region, and you will slow down. Conversely when pressure drops you speed up. Now that you know why lower pressure means higher velocity, imagine you walk into a square room blindfolded. You start walking in any random direction. You are equally as likely to hit any wall. Repeat the experiment a thousand times, you will hit each wall about the same number of times. Now enter the room running and upon entering the room turn in any random direction. You are more likely to hit the wall opposite the door, and furthermore you are likely to hit the walls at an angle. On a static wing, air particles sometimes hit the top straight on. On a moving wing, it is rare for a particle to hit the top straight on, hence higher speed means lower pressure.
nascent_aviator@reddit
Wings work by making air go down. The bottom of the wing pushes it down and the top of the wing basically pulls or "guides" it down since air is a fluid and wants to follow the contour if it's smooth enough.
By Newton's third law, if the wing accelerates the air down, it feels an upwards force.
If you want to use Bernoulli's principle, you can, but it makes things more complicated. You need to be able to explain why the air velocity over the wing is higher than that below it. The short of that is that at subsonic speeds air is incompressible. If you pretend that the air above and below the wings is going the same speed then you inevitably end up with the air density above the wing going down. (Incidentally, the density decreasing would also make the pressure go down and cause the air to speed up. But at subsonic speeds the air density doesn't actually change appreciably to begin with).
Mental_Director_2852@reddit
my god i love how much is being argued over a concept that has no practical application in flying lol. Sure its interesting and not a bad thing to know but it is never actually used by a pilot to fly a plane
Creative-Grocery2581@reddit
P1V1 = P2V2. If V1 over the wing increases, it lowers P1 which causes wing to be pushed up. And why does V1 increase in the first place. Because of the curve on the upper side of the wing, air has to travel more distance in the same time compared to the bottom side. Hence V1 of air on upper side is higher than V2 of air on the lower side of the wing.
propell0r@reddit
Shape of the airfoil causes a local speed change in the air. That’s it. That’s all. You can speed up/slow down the air by changing the airfoil shape. Air speeds up, pressure drops. Air slows down, pressure rises. That’s Bernoulli. Pressure gradient force (weather crossover here) from below to above the wing is lift.
If you took a leaf blower and blew it against a wall, there’d be high air pressure against the wall as the air stops and has nowhere to go (momentarily before spreading out), this is the bottom of the wing, and “slow air causes high pressure”. If your leaf blower blows away from the wall and sucks air in, the air against the wall goes from stationary to moving due to the leaf blower sucking it in. Air moving from slow to fast = low pressure.
centexAwesome@reddit
If Bernoulli was anywhere near as important as it is often taught no plane with anything but a fully symmetrical wing would be able to generate lift while inverted. The shape of the wing is optimized to efficiently deflect air downward at the expected speed and angle of attack for that aircraft.
EnvironmentCrafty710@reddit
What causes the air on top to move faster?
Nature abhors a vacuum.
The air under the wing isn't moving. The wing is moving above it. It's not touching the wing.
The air "on top" however, it isn't "on top" to start with.. it's "in the way".
That wing smashes into it like a plow. It pushes it backwards a little actually... It compresses it. And up due to the shape of the wing.
As the wing moves forward, where that air was is now occupied by the hump of the wing.
Then, as the hump passes, then what? That's where the wing gets thinner... So there's no "wing" there anymore.
That. Creates a vacuum.
And nature freaking hates a vacuum. So all the air around it wants to be there. Badly. So all that air in front, that got all smashed up and pushed up over the hump wants to be in that vacuum more than any of the other air around (cuz it's compressed), and it absolutely races to fill that gap.
That is why the air "on top" moves faster.
Quiet-Recover-4859@reddit
Just parrot the Phak. I doubt the DPE is an expert in fluid dynamics.
dakota137@reddit
This one took me a while conceptually.
But if you think of air pressure being caused by the sum of all these little molecules moving around and hitting a surface it starts to make more sense.
The faster these molecules are moving across a surface they, on average, hit the wing fewer times in transit. Therefore less pressure.
NolanGrayson711@reddit
Use a picture
AOA001@reddit
I like Newtons 3rd Law, personally.
SifuT@reddit
Maybe a contrary opinion: don't. Just explain lift via Newtonian mechanics. I think it's cleaner, easier to grasp, and can be easily correlated with the aerodynamic behavior of the plane. Bernoullian fluid dynamics leads to a lot of confusion in many cases.
burnerquester@reddit
Just memorize the answers tbh.
red_0ctober@reddit
I kind of wish explaining lift wasn't taught as part of the pilot curriculum. It matters not one bit whether you believe lift is caused by bernoulli, deflection, or sprites. Last I checked it's still an active research problem at NASA.
What matters is whether you know how to harness it and how it behaves in different regimes of flight.
One_Event1734@reddit
dmspilot00@reddit
Don't lose sight of the reason why we teach this in the first place. The moral of the story is that airflow across the top of the wing is crucial for the production of lift. A stall will make much more sense in that context.
SemperScrotus@reddit
I asked Gemini to explain Bernoulli's Principle like I'm five. This is what it came up with:
Imagine a slide! * When you go down the slide FAST, you feel lighter, right? * That's like air - when it moves FAST, it has LESS pressure. Airplanes use this! * The top of the wing is curved, so air moves FASTER there. * Less pressure on top means the wing gets pushed UP! * That's how they fly!
mickcham362@reddit
Here's how I explain it to people.
A wing produces lift in 3 ways.
1, the hand out the window of a moving car way, air his the bottom of the wing, deflects the air down and pushes the wing up.
The movement of the air interacting with the wing causes high pressure below the wing and low pressure above, pushing the wing up.
The low pressure above the wing causes air to accelerate as it follows the shape of the wing, accelerating the air down.
Realistically Newton's 3rd law explains it better, accelerating air down causes an equal and opposite reaction pushing the wing up.
A plank of plywood could be used given enough power, the shape of the wing maximises efficiency for typical weight at cruise speed.
sgund008@reddit
Whatever you say should also explain why paper airplane wings also create lift. This is to say the curved shape of the airfoil is not the reason for the pressure difference
cuttawhiske@reddit
Do the hand out the window wavey thing that's how bernolleh makes airplanes fly
VileInventor@reddit
As velocity of a fluid increases pressure decreases.
Bastards_Sword@reddit
Here you go: https://youtu.be/NBsvzMi9-f8?si=EXswpGwjJv35yQQT
Regular-Schedule-168@reddit
When you turn a shower on, and it sucks the curtain into the water.
Almost pinch two fingers together under a stream of running water. You feel your fingers get pulled together.
Blow over the top of a piece of paper.
WhiteoutDota@reddit
The shower example is poor because that phenomenon is related to the temperature. Hot air is lower pressure. Try seeing it if does the same thing with cold water lol
Regular-Schedule-168@reddit
https://www.scientificamerican.com/article/why-does-the-shower-curta/
SSMDive@reddit
The air DOES NOT meet at the back. This is a bad explanation. Called the "Equal transit theory" and for that to be valid the particles of air above the wing would have to somehow be magically tied to a particle under the wing and that simply does not happen. The idea that the particles of air try to race to the back "so the top has to go faster to meet" is just a horribly wrong explanation.
The air going over the top goes faster and this creates a low pressure area. It goes faster because of the shape of the wing. The normal pressure under the wing tries to fill that void, but there is a wing in the way and the wing ends up moving up to fill that low pressure area. This also explains vortices as the air on the bottom slips around the end of the wing and rolls into the LP area.
To show lift, take a dollar bill and hold it between your index and middle finger on the edge. Then bring your index finger to your bottom lip and rotate your hand so your palm is down. Then blow across the top of your fingers and when you get the angle right, the dollar bill will lift up. This is Burnoulli.
Then blow on the bottom of the dollar under your fingers and the dollar will again go up. This is is Newton's third law.
It gets a hell of a lot more complicated including the Coanda effect... But the dollar bill and the explanations above are sufficient for a CFI ride. And if you don't TOTALLY understand the idea, I'd simply not try to get more complex because you will screw it up.
MammothAd7334@reddit
Grab a sheet of paper and blow along the top of it.
Longjumping_Dog3019@reddit
Your explanation is just wrong. Unfortunately there isn’t really an easy way to explain lift as it’s just a really complicated phenomenon. While others have addressed this, they also are slightly wrong in simply saying the increase in speed causes decrease in pressure because there is no simple 1 way causation. It could also be said that the decrease in pressure causes the increase in speed. In reality, both of these factors are both at play and it’s not a 1 way street. Bernoulli also doesn’t completely explain this as conservation of momentum is also important.
Why does air speed up? A force analysis shows the high pressure in front of top side of wing would cause a net force accelerating the flow. So Newton is quite important here also. But that pressure differential is helped to be maintained by the increased inertia of the now faster moving flow. So it’s just a really complex phenomenon and there is no easy this causes that but lots of factors are all in play. It definitely needs both Bernoulli and Newton though.
This is a great video debunking some of the common misconceptions by a retired Boeing fellow. I have read some of his textbook and need to finish it up still but it’s probably only interesting to engineers. The video he gives may be of some interest though. https://m.youtube.com/watch?v=QKCK4lJLQHU
NorthCheap932@reddit
Wings are designed in such a way that air on top of the wing moves faster than the bottom. Increase in velocity will decrease in pressure so the pressure on the top will be less than the bottom hence the pressure of the bottom of the wing will push the lower pressure on top (lift)
davidjtodd@reddit
I always liked blowing across a piece of paper to illustrate.
cazzipropri@reddit
I have a bit more of a background in physics and I think I can make a lesson... I need to make a video about this lesson plan.
theogmichaelscott@reddit
Keep it simple. Everyone has stuck their hand out a car window and felt the forces of that and noticed that having your hand like a knife edge vs open creates 2 different effects. Start there as a common ground. Then explain the 2 principles at play. You've got bernoulli and newton's 3rd law both working together to create lift in this scenario. That's why when you face your hand towards the wind your arm gets dragged back (newtons 3rd law), whereas when you have your hand flat and thin, there isn't much force. Now if you change the angle of your knife edge hand, you notice your hand wants to move up or down relative to that (bernoullis principle in theory but is still more newtons third law in this scenario) Once you establish that baseline understanding then dive deeper. Why does my hand move up or down when I change it's angle? How does relative wind effect lift. Etc. The hardest part in teaching is to create an understanding and not just a rote memorization of fact. Another example to explain bernoulli's principle is to draw a water hose that gets narrower at one end. Then ask what the speed of the water will be at the narrower end. Think of putting your thumb over the end of a water house to get it to shoot further. Why does the water shoot farther? The flow rate doesn't change but the size of the area it's flowing through is smaller, meaning speed has to increase to "keep up" with the flow rate. A great visual demonstration to show students bernoullis principle is to take a sheet of printer paper, put it up to your chin and let it drape down, then blow air straight out your mouth. The paper will lift itself up in the air. This is the principle of lift. Higher speed=lower pressure. Areas of high pressure(air below wing) want to move to areas of low pressure(above wing). The force created by this pressure differential is lift.
iwannaSprintmx2@reddit
1/. Turn on tap on sink. 2/. Place back of spoon into edge of water flow. 3/. Watch/Feel how spoon ‘drawn into water flow.
Science yo! You’re all Welcome. y@
TxAggieMike@reddit
This suggestion from earlier in the thread…
https://www.reddit.com/r/flying/s/oAlARQIMKE
These videos may also help:
https://youtu.be/eKEorBipbO8?si=CENcEF639Hj4M0sO
https://youtu.be/YrSUxgiwoFk?si=Tn2cQ6fRE3T9_IqN
https://youtu.be/PPweiE9Z568?si=b74TNNEz6NzGJr5n
https://youtu.be/aFO4PBolwFg?si=p_OvZGcODWo8zL9_
rFlyingTower@reddit
This is a copy of the original post body for posterity:
Studying for Cfi checkride, trying to understand a simple yet effective way to teach lift without overcomplicating it
i think most of us learned that the air has to go faster over the top to reach the air on the bottom at the same time, but that isn’t a great way to teach this topic.
which happens as a result of the other? does fast moving air cause low pressure or does low pressure cause fast moving air?
and what causes the air on top to move faster than below
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