Forget Bernoulli and Newton | The easy way to explain lift

It can be a challenge to explain lift in an easy way without getting the realities wrong. Here’s how experts in aerodynamics prefer to explain lift for laymen.

Note: The first video I made is removed because it contains some errors.

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[notjackschannel]

It’s such a pleasure to see someone admit that their previous knowledge was incorrect and unabashedly correct themselves when presented with better information. And THAT is what makes you a true scholar. Well done, please keep these videos coming 👍🏼

[arnobozo]

The first video was good. :)
This one is poor ! :(
I am surprised with the Cambridge professor :(

The explanation with stream curvature is wrong.

@5:50 note that the cross section between A and B has narrowed since the front of the stream. Because the mass of the stream is constant, the speed through the cross section must increase. That is why the stream accelerate when you have this curved shape.

[michaelhoffmann]

I was fortunate to have flight instructors who gave me excellent rules of thumb with a certain amount of humour: "take 2 parts of Bernoulli, 1 part of Newton. Some times it's the other way around" - followed by going inverted in an aerobatics plane. 😆 That has always worked for me. Once I stopped screaming. 🤣 (no, I didn't really scream, I loved it!)

[JeanLucCoulon]

A flat plate in a stream, if it presents an angle with the stream, also generates lift. This explanation, which relies on a curvature, does not allow to explain that.

[josteineide]

short add.
We must consider what comes first, pressure changes or mass deflection?. Our first action is initiated by moving / displacing the mass package. Than by the inertia, the compressibility and viscosity, i.e. stretching of the rubber band comes the changes in pressure and the adiabatic temperatur changes. A continuous process.

[josteineide]

Hei Magnar,
Quite an improvement which I can follow nearly to the end. There is still a confusion re Bernoulli's. We must not forget about viscosity, bounder layer, energy, Joule Thomsen, and more
Bernoulli's is valid along a single streamline. Many professors over the years have differences in understanding and explanations. The sum of it all is quite up to what I learned taking my engineering degree in 1979. We must integrate the work done (Newton) by deflecting a mass of air, a fluid, The equilibrium sum equals that halves of the lift comes from the upward deflection up front of the wing, the other halve from the deflected air leaving the trailing edge, i.e. the "vast" the energy is picked out, no more to gain from it. The pressure and temperature changes involved in the process can not pr Bernoulli's definition directly be linked to the process. No fluid likes to be disturbed, moving it is a mass that are to be moved and needs energy. The viscosity forces keep the fluid together which also needs energy to overcome. This in total can be linked to Newton. The rest of what happens are in many ways side effects. Stretching and compressing the fluid while we are moving the mass around. Air has mass as everything else trying to move it around, accelerate it demands energy / forces. 1 kg of air or 1 kg of steel is no difference F=m*a. Except viscosity compressibility etc. Air will by this be more "soft" to move around, like suspended by a rubber band, similar as if the 1 kg of steel was suspended by a rubber band.

You can not simply always say Ps1+Pd1= Ps2+Pd2. You can have different velocities but the pressure remains the same. As you demonstrate by blowing down your strait henging papir sheet, flowing air at one side, no flow at the other side of the papir sheet.
Pretty presise metering instrument are in use which utilize the effect of Bernoulli's. Venturi Orifice plate, V-cone. Static pressure drop while the flow velocity increase. Rule of tumb pressure drop equals the differences i pipe diameters d/D which are named the beta value, +discharge coefficient adjustments. There is no beta value in free space around an airplane it becomes infinite. A curiosity from the old faction explanation of lift and Bernoulli's you can not split a venturi i halve and say you have "the upper surface" of a wing.

[dronemonkey]

What about angle of attack….What about wings that have no camber? Perhaps a partial explanation in a very good video.

[go]

Magmar i ask you :::victor schauberger implosion technology wat it is ???

[jimbo]

You say the air speeds up over the convex surface, but why. What is providing the force? Granted if it does speed up, which it does, then the pressure drops. But where is the force causing this acceleration. Your explanation is incomplete.

[dhy]

The confusion in the minds of many is in the word LIFT. Showing the high-speed/low-pressure airflow above the wing makes one think that the low pressure is lifting the airfoil when in fact, it's the higher pressure below the wing that is pushing the airfoil upward. Low pressure doesn't pull; high pressure pushes and symmetrical airfoils work just fine as long as the angle of attack is positive.

[Hopeless_and_Forlorn]

Look at the larger picture and lessen the confusion: a heavier than air aircraft can sustain itself in flight ONLY by accelerating a mass of air downward with sufficient force to counter the force of gravity. Regardless of what goes on on the top or bottom of the wing, or on any other aerodynamic surface, the net effect of the interaction must be that air is accelerated downward, or the aircraft will fall toward the ground. Hope this helps.

[steveststst]

A water ski produces lift by deflecting water downward. No water flowing over top.

[charleswesley]

Air has weight ! The Centrifugal force exerted on the air following the top curvature of the wing at high velocity supports the negative air pressure that creates lift on the top of the wing . That boundary layer across the wing doesn't lift by magic . It has to be anchored to something . High velocity air has weight . A simple leaf blower should demonstrate that. A wing does have down wash, but that is not where lift comes from. How could you calculate your CG if that were true ? Vacuum caused by the high velocity air molecules trying to adhere to the top curved surface of the wing along with any planning effect from the bottom of the wing is the source of lift . When the angle of attack increases to far the flow of high velocity air across the wing detaches .
Not one explanation discusses that High Velocity Air Molecules have a density and weight and deflecting air around an airfoil top surface creates kinetic energy for example Centrifugal Force . That force is where the most of the lift comes from and is transmitted to the top surface of the wing through the iconic Boundary Layer creating a vacuum on the top surface of the wing .

[bobparis]

No mention of angle of attack? No consideration of non cambered wings working in simple toy gliders?
If all the lift is low pressure above the wings, what keeps the lift required to carry a 400K pound B747 from ripping the thin skinning off the upper wing surface? There has to be more.

[msd]

A plausible EXPLAINATI9ON but not the PROOF.

[grahamj]

As I've stated elsewhere, It's not a question of "either/or": Newton's Third Law is fundamental to the creation of a lift force. Air must be directed downwards to produce a force in the opposite direction: if it's not, there can be no lift force. Don't be misled by what I characterise as 'The Wind Tunnel Syndrome': think instead of a wing moving through still air.
A bumble bee demonstrated this for me years ago in the Model Shop at Rolls-Royce, Bristol. It was flying low over the floor, which had a covering of fine sawdust. The downwash from the bee's wings cleared a little pathway of sawdust beneath its flight path.
An aircraft can do the same thing to the cloud tops. There are numerous photos on the internet, which show an aircraft producing a 'trough' in the cloud tops, with the inward/downward curl of the wingtip vortices also clearly visible.

[shapegroove]

Just a note, "deceleration" is incorrect. It is always called acceleration, simply because acceleration is a vector, so it has magnitude and direction. Like the difference in speed (scalar only) vs velocity (movement with direction)... The same goes for electric circuits, there is NO CURRENT FLOW, only directional movement or jostling (oscillation) of charge (DC vs AC). Hence current NEVER FLOWS, current is a form of flow AND thus, CHARGE is what moves. Current flow is nonsense, can a flow flow? (second derivative of what??) In the same sense, acceleration is the ONLY term and when a car, for instance hits the breaks, it accelerates to a HALT. Because the acceleration vector changes direction to oppose the motion, via the breaks and friction exerted on the road.

[shyam]

Can u also explain then why water rises in Venturi effect? When we blow air over the top of U-tube containing water then the pressure at narrow region would be smaller I know but it would be smaller than the pressure compared to WODER REGION OF TUBE ABOVE. Why do we compare it to the air pressure inside the U Tube which I static and say that water will rise up?

[davetime]

I'm confused. At 3:38 you say "this is called the Coanda effect", talking about the curved paper. But hasn't it been often suggested elsewhere the Coanda effect in "contaminating" the upper flow with an external source of dynamic pressure relative to the bottom pressure's starting conditions, means this cannot explain lift in the situation of a wing where the air split between top and bottom starts off with the same homogenous conditions (same static and dynamic pressure and temperature).

Furthermore, you seem to be following Babinsky in saying the Coanda effect needs to be included in the explanation of lift.

What am I missing here?: The Coanda effect, due to the mixing in of an external source of dynamic pressure unequally to one side of the wing than the other, is said to not apply to the case of lift on a wing.

Yet both you and Babinsky are saying that the Coanda effect is needed to explain lift? Something that doesn't occur in the generation of lift (without a differential external augmenting source), is used to explain lift?

And how does this at all replace Bernoulli, in the sense of conservation of energy not needing to be invoked?

[SaeedAcronia]

1:41 You just used Newton's first law of motion right there. So, yeah. There's no escaping Newton's laws when it comes to macroscale motion.