Only One Form of Lift

I was reviewing a presentation on aerodynamics for a great aviation educational non-profit (FlyQuest in Huntsville, AL) when I came across a slide using a page out of Rod Machos’ Private Pilot Handbook that discusses lift generation. It said there were two forms of lift. One was due to impact pressure and one was due to low pressure over the top of the wing. Since I have been recommending Machado’s Light Sport Handbook to several of my students, I thought I better check to see if that explanation had been carried across there, and I discovered it had. I totally disagree with this description. What he calls “impact lift” is really drag acting in a vertical direction. There is only one form of lift,and it’s created by the Bernoulli effect and a little ditty called “circulation”.

Let’s start at the beginning.

Newton and Bernoulli can be called the kings of aerodynamics, though there are other scientists whose discoveries also play a very important role. I’m going to center in on the biggest areas where each makes a contribution, i.e., the conservation of momentum and Bernoulli’s’ principle. I’m only going to to address subsonic aerodynamics since that’s where most educational efforts lie and the issue of discussing supersonic lift becomes much more complicated. In the subsonic regime, I maintain there is only one form of lift; further, I think it’s extremely problematic to teach beginning pilots that “impact pressure” is a form of lift since, in the worst case, it can cause a pilot to do the wrong thing. The importance of understanding lift in terms of its relationship to the wing’s critical angle of attack cannot be understated; if you’re stalled, you need to reduce the critical angle of attack to get the wing flying again, not increase it in the hope of using “impact pressure” to keep you airborne. Further, Machado’s reference to low pressure lift allowing flow greater than the actual engineered circumference of a wing is a reference to the effect of circulation, which is in play anytime a wing is producing lift. More on that in a minute.

Every text I used when learning aerodynamics (as an aerospace engineer) starts off with an explanation of the aerodynamic flow fields using Bernoulli’s principle. One of the first things you do as an early aero student is put a positively cambered wing section in a wind tunnel and then plot the pressure distribution around the wing. You wind up with a plot that looks like this:

When you add up the positive pressure and negative pressure contributions, you find the the negative pressure contributions far exceed the positive; in other words, the wing is more accurately described as being sucked up from above rather than pushed up from below. This is very easy to see and explain when using Bernoulli’s principle and yields a practical and calculable result. The resultant force is what we call lift, the force acts through a point called the center of Pressure or CP. (This is opposed to Cp which is the coefficient of pressure, a dimensionless number engineers can plug into an equation and calculate the pressure at any point if they also have the air density and velocity. In the same manner, engineers define Cl (C sub L—can’t write that properly using this) as the coefficient of lift, a dimensionless number that describes the lifting force an airfoil section can generate that can be calculated in the same manner, using the airstream velocity and density).

Admittedly, when you look at the above diagram, you can see the free stream pressure at the nose of the airfoil has positive pressure. That can lead you to call that “impact pressure” and it is, but I can assure you it is never separated out and treated as a separate entity when calculating lift. In addition to the forces explained by using Bernoulli’s the movement of the airflow over the airfoil is influenced by a phenomena called “circulation”, which naturally occurs due to the presence of the airfoil in the flowfield. A good model of its effect can be made by spinning a cylinder in the direction of the airflow past the airfoil, and the velocities it induces (called the Magnus effect) along with the freestream velocities produces the overall flow field. (This helps explain why the airflow over the top of an airfoil moves faster than that underneath.)

And, so now, you’ve got the story about how lift is generated for a 2 dimensional wing, i.e., a wing of infinite length. That is how engineers explain, understand, and examine the generation of lift due to an airfoil. But real airplanes have finite wings with a length and width. So what happens on them?

Well, we already saw that there is a high pressure area under the wing and a low pressure area on top. That high pressure air is going to move toward the low pressure; and at the wingtips, it then curls around over the top of the wing creating vortices. These vortices affect the flow field over the wing and create “downwash”. In the drawing below, the vector labeled “effective lift” is the lift before taking downwash into account; the lift vector after taking downwash into account instated slightly aft, and the force vector between the two shows the horizontal component of the real lift vector acting in a rearward direction, i.e., creating drag. That is induced drag. Induced drag is often termed the “drag due to lift”, and you can see why.

With this, you’ve got a pretty complete picture of how a wing generates lift. There is a lot more to it than I’ve put in this blog; but the reason I’ve done this is to try to capture a technically correct explanation of how an airfoil works. I’ve used “the Bernoulli” approach here because I think it’s the easiest to understand, and I could have used “the Newton” approach but I don’t think it’s as relevant to pilots. There is a lot of misconception and disinformation out there about how a wing works; and I’m seeing too many oversimplifications that are simply flat out wrong. (More than I’ve ever seen in my long flying and engineering career. I’ve discussed this more in two others blogs, i.e, “Fuzzy Aerodynamics” and “Flying a CTSW with Flat Plate Wings: Analyzing “Impact Lift””) I will caveat this discussion by saying that I have limited it to subsonic aerodynamics at low altitudes where most of us fly; all bets are off when talking high altitude and high Mach flight, which is a different horse indeed.