How is lift REALLY created?

11 Answers

1. 
   

   
   

   Bellanca Decathlon on Dec 29, 2010

   This argument has been going on for years. I hate to say this, but I teach what is in FAA publications (Bernoulli). The finer point’s of lift creation is great for “stump-the-dummy” type questions, but is does nothing for your “go / no-go” descisions, division of attention, ect. that really matter.

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2. 
   

   
   

   Brian on Dec 29, 2010

   Science has defined lift force, for many years, as the summation of static pressures across an airfoil. In fact, you’re first link includes this sentence:
    
   “The difference in pressure across the airfoil produces the lift.”
    
   That said, I typically just cover Bernoulli while highlighting pressure being the direct cause of lift. I also like to make clear that Bernoulli only helped us understand pressure; he didn’t discover, nor is his theory the cause of, lift, as I believe one might infer from reading FAA texts on the subject.
    
   If the student is interested in delving into the topic, however, then I spend my time explaining the incorrect theories. The truth is this tunnel goes miles beyond where my knowledge currently peaks. So, instead of chance screwing them up down the road unknowingly, I stick to articles like the ones you provided. Here is another article I use to help disprove downwash causing lift (which it doesn’t):
    
   Note On Vortices On Their Relations to the Lift of Airfoils Pages 10-12 are of significant to the disproval of deflected air, downwash, causing lift.

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3. 
   

   
   

   Steve Pomroy on Dec 30, 2010

   Hi Victor.
    
   Unfortunately, the production of lift is a much more complicated subject than we’d like it to be.  But here’s a shortened version:
    
   1)  The curvature and AOA of the airfoil changes the velocity distribution of the airflow via continuity (or, if you prefer, circulation, whcih is based on continuity anyway).
   2) The velocity changes result in pressure changes via Bernoulli’s equation (velocity goes up, pressure goes down and vice versa).
   3) The pressure difference between the upper and lower surface of the wing results in a net upward force that we call lift.
    
   There are several correct and valid ways to describe lift production (continuity/Bernoulli, circulation/Bernoulli, and momentum-change – AKA Newton).  So the real question to ask here isn’t “How is lift produced?”, but “Which (correct) description is useful to us as pilots?”.  Newton (momentum change) is conceptually correct, but not very useful in some ways.  Circulation is correct, but in the realm of engineers doing sophisticated calculations.  So my preferred approach for pilots is continuity/Bernoulli.  I’m not familiar with the FAA’s doctrine (I’m Canadian), but from the sounds of the comments here, continuity/Bernoulli is their preferred approach, too.
   For a lengthy, but perhaps useful discussion of the do’s and don’t’s of lift, check out these links:
    
   http://www.flightwriter.com/2010/12/straw-man-of-equal-transit-time-lift.html
   http://www.flightwriter.com/2010/12/wherefore-art-thou-lift-lift-part-2.html
   http://www.flightwriter.com/2010/12/going-in-circles-lift-part-3.html
    
   Cheers,
   Steve
   http://www.flightwriter.com

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4. 
   

   
   

   Thomas Vaillencourt on Jan 02, 2011

   LOL! I posted a very similar question a couple of months ago and it seems there is always been an ongoing debate and many theories about the true origin and production of lifting force, it is in fact a culmination of many many factors, and I have decided to settle my restless mind and get sleep at night by keeping this one thought in mind:
   “As a pilot you ultimately do not really need to know how lift is produced. Aeronautical engineers need to know this. All we really need to know is that lift is produced and that the airfoils the engineers have designed for our aircraft’s are making efficient use of that force.”
   No one can completely explain the law of gravity, we all know it exists, we know that objects with mass are attracted to one another, but we don’t truly know why. Yet, here we all are stuck to the earth, and getting along just fine not knowing why.

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5. 
   

   
   

   Ben Taves on Jan 07, 2011

   When a wing is not moving through the air, the pressure (atmospheric) on the bottom of the wing is the same as on the top and you have no lift.  We have no good way of increasing the pressure on the bottom but we can reduce the pressure on the top. If we reduce the pressure on the top of the wing below atmospheric and keep the pressure on the bottom the same it will have lift and be pushed up.  Bernoulli explains how the pressure on the top goes down as a result of air traveling over the top. That’s how we get the pressure differential that creates lift.
    

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6. 
   

   
   

   SkyBoy98046 on Jan 15, 2011

   The wing is pushed up by the fluid wind in a balanced ballet. Wing shape assists by reducing pressure against this push making the push more effective. But this wing shaping above is not necessary as symmetrical wings also fly. I my 20 years flying, learning and teaching, the taught therory has changed. I think it’s still just magic ;-).

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7. 
   

   
   

   Jordan on Jan 20, 2011

   The answer is… no one knows. 
   The reason that this question keeps coming up is because no one has a completely solid and correct understanding of how lift works like we know that 2+2=4. It doesn’t matter if its night or if we’re on the moon or if we’re diving in the ocean… 2+2=4. Not the case with all the theories. One theory can’t explain exactly what’s happening with lift in all conditions. There are many theories and many principles that have been addressed with aerodynamics. The reason for so many is because the study of fluid dynamics is extremely complicated.
   The easiest way to describe how lift works is to start with “every force has an equal and opposite reaction” –> that leads to the downwash part of the description. I also go into explaining Bernoulli’s theory. That’s usually good enough for most people to understand how lift works. There is also the circulation theory as well as a few more.

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8. 
   

   
   

   Steve Pomroy on Jan 24, 2011

   QT: “The answer is… no one knows.  The reason that this question keeps coming up is because no one has a completely solid and correct understanding of how lift works …”
    
   Yikes.  I’ve often wondered how people who believe this can work up the nerve to get in an airplane at all!  But it does seem to be a very common belief in the aviation inustry.
    
   The fact is that the production of lift is very well understood, and has been for many decades.  Thin Airfoil Theory was developed by Ludwig Prandtl during WWI, he first published Lifting Line Theory in 1919, and later published compressibility corrections for the production of lift at high speeds.  These developments are still used today, at least for preliminary design work.
    
   Unfortunately, there is more than one correct way to describe the process of producing lift.  This multiplicity of descriptions results in people arguing over which one is right.  The argument ultimately stems from a lack of undersanding.
    
   I don’t really see a solution to this, since developing a deeper understanding is so involved.  The time and energy required to learn the physics and math of lift is a waste of resources for pilots who are not also engineers or test pilots.  That deeper understanding just isn’t necessary. But I would suggest that understanding enough to know that the three common explainations (continuity/Bernoulli, Newton, and circulation) are equivalent would be nice.  Further, I would think it would be nice to acknowledge that someone out there knows the nitty-gritty of lift production, even if pilots generally don’t.
    
   Cheers,
   Steve
   http://www.flightwriter.com

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9. 
   

   
   

   Jordan on Jan 24, 2011

    
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   Hey Steve,
   I think you need to consider my whole comment as a whole and not in a snippet. I’ve learned and believe the three common explanations that you stated and I have enough knowledge about aerodynamics that I’m pretty comfortable explaining the dynamics of stalls, spins, lift vs AoA graphs, power required vs power available, thrust available vs thrust required, lift vs drag, the effects of flap and leading edge devices, etc. We do know a significant amount about how lift is formed, however, calculating lift based on one theory such as Bernoulli would give results that were extremely far out from actual performance figures on an aircraft. Those “theories” should really be considered factors when explaining lift because there are a multitude of different “factors” that affect lift production and drag reduction. Engineers have a pretty good understanding of how lift works, a lot more than the general pilot, and with complex computer simulations they can predict, based on programmed fluid dynamics, the effects of an airfoil on air to a relatively decent degree of accuracy. The most windtunnel tests for example only show flow over what’s called an infinite wing and does not accurately show flow around a finite wing which is what the real world is. NASA and other agencies have massive wind tunnels that are able to fit large scale mockups of the aircraft to analyze the aircraft as a whole. The body of the aircraft actually produces lift in most cases, there are things such as area rule that engineers have to consider especially for transonic and supersonic aircraft.
   If we really did understand exactly what lift is and how it’s formed, why are doctorate students still firing out more theories and reports on lift and how it’s created? I just read one a few months ago by a doctorate student from one of the big university aeronautics programs, I believe MIT… I can’t find the PDF anymore but I’ll have a look around. 
   Cheers!

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10. 
    

    
    

    Steve Pomroy on Jan 26, 2011

    Hi Jordan.
     
    You’re right about the snippet of course. It’s always a balancing act when quoting.  Too much quote is just noise and clutter, not enought and you run the risk of quoting out of context.  I do try to get the balance right.  If I failed, my bad.
     
    Having said that, whether or not it’s your belief, it is indeed a common belief in aviation that we don’t really undersand the origins of lift.  This belief is false.
     
    Running the risk of misquoting again, allow me to look at a couple of your points:
     
    QT:  ” … calculating lift based on one theory such as Bernoulli would give results that were extremely far out from actual performance figures on an aircraft.”
     
    Actually, that’s not right.  Bernoulli’s equation is premised on incompressible flow, so when you operate at Mach numbers above 0.2 or 0.3, it may well be discarded.  But at lower speeds, it’s quite accurate.  The problem we run into is determining the velocities of the flow at various points.  Once velocities are known, Bernoulli’s equation is quite reliable.  Velocities are determined using potential flow theory, which is a mathematical description of the flow built largely on the principle of continuity (which is the conservation of mass applied to a flowing fluid).  Unfortunately, potential flow theory is built on the assumption of zero friction, so it falls apart in the boundary layer.  There are fairly reliable ways to account for erros introduced by the boundary layer, but the fall apart near the stall becasue of the nature of separation and separated airflow (see my comments below about non-linearity).  So, yes, predicting the stall is one place where we need wind-tunnel data in place of calculations, although Computationa Fluid Dynamics is quickly becoming the cheaper way to tget the same results (see my comment below about research on numerical simulation).  But the actual lift under normal operating conditions can be predicted quite accurately.
     
    Incidentally, at high speeds, when we discard Bernoulli’s equations, we replace it with the more general energy equation, which allows for temperature and density changes.  So lift can still be calculated, it’s just much more complicated.
     
    QT:  “most windtunnel tests for example only show flow over what’s called an infinite wing and does not accurately show flow around a finite wing which is what the real world is.”
     
    That’s true.  But infinite wings data is often more useful because it enables us to consider different planforms, different washout profiles, and different variable-airfoil profiles along the span.  All of these changes can be accounted for with calculations based on the 2-D wind-tunnel data corrected to account for 3-D effects (such as wingtip vortices).  This reduces the amount of experimental data required by filling in the blanks with calculations — which is the whole point of calculations in the first place.  The full-aircraft testing (which you mentioned in your post) takes place later in the design process.  If every 3-D full-aircraft concept was to be tested, test costs would make all but the simplest of projects cost-prohibitive.
     
    QT:  “Those “theories” should really be considered factors when explaining lift because there are a multitude of different “factors” that affect lift production and drag reduction.”
     
    OK, I don’t understand this comment.  So rather than responding to it, I’ll ask for some clarification.  Are you saying, for example, that Bernoulli’s equation is a “factor”?  If that’s right, what exactly does it mean?  If that’s not right, then, well, clarification please.
     
    QT:  The body of the aircraft actually produces lift in most cases, …”
     
    That’s true, and is perfect example of a difference between what pilots need to know and what engineers need to know.  The interference between the fuselage and wing, along with the lift and drag of the fuselage itself are important contributors to aircraft behavior.  But engineers need to know a lot more about this than pilots do.  Further, this information doesn’t do anything to change the basic process of lift production.  Local velocity of the air changes, pressure changes as a result, and we get lift.
     
    QT:  ” … there are things such as area rule that engineers have to consider especially for transonic and supersonic aircraft.”
     
    The area rule and supercritical airfoils, both of which are important for transonic aircraft, were developed in the 1940’s.  Sweepback, which is a related development, came even sooner.  As I said in my previous post, we’ve known a lot about lift production for a long time.  And yes, even though each of these 3 developments apply mainly to drag reduction, they do influence lift production in known and predictable ways as well.
     
    QT:  “If we really did understand exactly what lift is and how it’s formed, why are doctorate students still firing out more theories and reports on lift and how it’s created?”
     
    Indeed, fluid dynamics is a great area for research.  However, I think you’ll find that the vast majority of research is focussed on either turbulence modelling, numerical simulations, or both.  There are many areas in aerodynamics where we understand the physical mechanism of the phenomenon, but we can’t do reliable calculations because of the extreme non-linearitly of the math problem.  This is especially problematic when we start to account for viscosity (friction) and/or compressibility.  So, much of the research is focussed on reliable ways to crunch numbers.  Important stuff for designers and engineers.  Not so for pilots.  We can get aways with a solid understanding of the physical mechanism and let the designers worry about the rest.  The most complicated math we need to deal with is the lift equation.
     
    Cheers,
    Steve
    http://www.flightwriter.com

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11. 
    

    
    

    Jordan on Feb 11, 2011

    Hi Steve,
    I’m starting to see your point of view. 
    Re: my quote — “Those “theories” should really be considered factors when explaining lift because there are a multitude of different “factors” that affect lift production and drag reduction.”
    I mean that there are a few “theories” required to give a really good picture of lift. I haven’t found one cohesive theory that explains lift entirely. It seems Bernoulli, Newton, area rule, etc all come together to give the explaination of lift, and one on its own doesn’t entirely explain lift. Does that make sense?
    Jordan
     

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