What causes lift?

Question

Now this seems to be the question of the ages, and various questions on this site regards it.

The often told description of lift is that it is due to air traveling faster over the wing than under it, creating pressure differences according to the Bernoulli equation.

This however, makes little sense to me and as I've read it is entirely false. Particularly interesting is this article where the author writes that

Those with this view must also believe that one can pinch with one finger and clap with one hand.

I had settled with the description that lift is caused by the downwash of air which imparts an equal but opposite force on the wing itself. As noted in the linked article, the downwash is attributed to the impact of air on the lower side of the wing constituting some, albeit little of the lift, and the air on top of the wing being curved downwards following the coanda effect.

Now, my confusion was re-initiated as I am currently reading "Aircraft Design: A Conceptual Approach" 6th edition, by Daniel P. Raymer - a highly regarded aerospace engineer.

In the 4th chapter on airfoil and wings, he writes:

An airfoil generates lift by changing the velocity of the air passing over and under itself. The airfoil angle of attack and/or camber causes the air over the top of the wing to travel faster than the air beneath the wing. Bernoulli's equation shows that the higher velocities produce lower pressures, so that the upper surface of the airfoil tends to be pulled upward by the lower-than-ambient-pressures while the lower surface of the airfoil tends to be pushed upward by the higher-than-ambient pressures.

Now I cannot imagine Raymer to have any misunderstanding on what causes lift, which makes me very curious about this statement.

The pressure difference found in calculations by using the Bernoulli equation and an equal transit time is much insufficient to lift an aircraft and windtunnel experiments show that the air on the top of the wing reaches the trailing edge much sooner than the air traveling below the wing.

Is it a simplification to avoid becoming too technical and into the "physics" for the average reader - or is something else going on? It is very difficult to become confident in what actually causes lift with a seemingly endless number of descriptions.

As a side note: on the bottom of the page in the book, this common dispute is mentioned and told that this view of lift is completely correct as is the Newtonian downwash description.

Answer 1

Just because someone has access to an editor and a webpage does not mean that they understand the topic they write about.

Short answer: Raymer and Bernoulli are right. The finger-pinching / hand-clapping comparison is silly.

When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it. See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area. Due to the acceleration, the packet will be stretched lengthwise and its pressure drops in sync with it picking up speed. Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. This contraction, by the way, is necessary so the air can move out of the way of the wing. The thickness of the wing alone already causes this acceleration and stretching on both sides of a symmetric airfoil at zero angle of attack. With increasing angle of attack and/or airfoil camber, the flow around the wing becomes asymmetric and more acceleration will happen on the suction side of the wing.

Once there, the air molecules will "see" that the wing below them curves away from their path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Reluctantly (because it has mass and, therefore, inertia), the packet will change course and follow the wing's contour. This requires even lower pressure, to make the molecules overcome their inertia and change direction. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction. You see, as Bernoulli found out, faster air has less static pressure and the speed difference between both sides creates a pressure difference. Integrated over the wing surface this is lift.

Note that lift can only happen if the upper contour of the wing will slope downwards and away from the initial path of the air flowing around the wing's leading edge. This could either be camber or angle of attack - both will have the same effect. Since camber allows for a gradual change of the contour, it is more efficient than angle of attack.

With the changed direction of air flow the air is leaving the wing at an angle compared to its initial direction. This change of direction imparts a momentum on the air which, according to Newton's third law, needs an opposite force, which is lift. You see, lift can be explained in different ways and all of them describe in the end the same process.

Answer 2

Both Bernouilli and Newton indeed happen in subsonic airflow over wing profiles, but Newton provides a much better picture:

• Bernouilli describes incompressible flow, yet wings still provide lift in high subsonic and supersonic circumstances.
• A flat plate still provides lift when at an angle of attack other than zero, despite Bernouilli not being present.

In the flat plate case there is higher pressure at the bottom (at AoA > 0) and lower on top - due to the sharp nose the air cannot follow the surface immediately, yet there is still some underpressure in that region.

My choice: Newton.

Answer 3

Welcome to the club. What happens on the bottom of the wing is fairly straight-forward action reaction, also seen with water skis or sticking your hand out the window of a moving car.

The "top lift" is a little tougher to explain and actually only happens under certain conditions pertaining to Reynolds number. Top lift is desirable because it comes with very little drag expense. Effects of Reynolds number with various airfoils can be studied at www.airfoiltools.com.

Just remember, the wing hits the air, but is explained in a relative sense as an "airstream" (also true in wind tunnels). Coanda may be a better explanation of top lift than Bernoulli. Using the "airstream" model, consider that air molecules have mass, and if the flow is fast enough, their momentum cannot quite follow the curve of the wing, causing pressure to drop near the upper wing surface. This is Coanda!. A little like a bunch of race cars approaching a turn. There will be a void of cars near the inside of the turn.

Remember, that "vacuum bubble" on top is only possible with adequate chord and airspeed, and will collapse (with massive increase in drag) if proper Angle of Attack is not maintained. This is the "stall" that early aviators, in their slow, light aircraft, experienced.

Answer 4

Air hits the wing. Air hits every square inch of the surface of the aircraft. And it is hitting every square inch of the surface all the time, even when the aircraft is not moving, when there is no relative wind. Air molecules are always moving (Brownian Motion). And because they are always moving, they are always hitting the surface of the aircraft. Every time an air molecule hits and bounces off the surface of the airframe, there is an exchange of momentum (a.k.a. a FORCE) and this force is applied to the surface, perpendicular to (or Normal) to the surface. The total aerodynamic force on the airframe is just the vectorial sum of all these forces caused by all the molecules hitting and bouncing off the airframe.

Lift, is just the component of this total aerodynamic force that lies normal (or perpendicular) to the flight path of the aircraft through the air. Why is the distribution of Lift uneven? because the momentum exchanged when an air molecule hits the wing is dependent on the velocity of the air molecule and the angle at which it is moving locally relative to that surface.

Drag is just the component that lies parallel to the flight path.

An air molecule hitting the very back tip of a ICBM reentering the atmosphere at Mach 20 is still pushing the missile forward.... But the air molecule at the front of the ICBM is hitting the nose with a much greater velocity - More velocity, bigger momentum change, greater force.

All the other explanations are just aggregate simplifications used to make calculations and help explain complex side effects of aerodynamic forces on specific airfoil shapes, (or observations of aggregate effects like wingtip vortices, or downwash, etc. etc. etc., which are necessary to conform to principles of conservation of Energy or Momentum).

The Bernoulli explanation is especially silly. How then do Symmetrical airfoils create lift? Golly, How do aircraft generate negative Lift and fly upside down? The increase in speed on the top of the wing is a side effect of the lift, not the cause. The underlying justification for the Bernoulli explanation is called the "equal Transit time" theory, that any air molecule traveling over the top of the wing must transit a longer distance in the same amount of time as a molecule transiting the bottom of the wing. This has been shown to be false in wind tunnel tests.

If you're interested, you can watch a more detailed academic presentation of why both Bernoulli and Newtons "downwash" explanation are factually incomplete, and only represent an explanation of the consequences of LIFT, not an explanation of the causes of lift. It's available on youTube at: Common Misconceptions in Aerodynamics

NOTE. I use the word "hitting" with some literary license. The molecules never actually "hit" anything, they just get close enough for the electromagnetic forces exerted by their electron clouds to repel one another.