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. Additionally, the aerodynamics we have been discussing so far is actually 2 dimensional aerodynamic flow, i.e., something good for discussing mathematical and engineering modeling and understanding the basic mechanics behind how a wing works. In the 3 dimensional world we actually live and fly in, there is another piece of this we must understand, and that is the phenomena of “circulation”.

A real wing has both a finite length and width. As the pressure distribution above shows, there is a high pressure area under the wing and a low pressure area on top. In addition to the flow we can understand using Bernoulli’s, the high pressure air from underneath flows around the wing leading edge and the wingtips, causing a circulation to develop around the wing in the direction of the free stream velocity on the top of the wing (clockwise in this illustration). The overall airflow field acts like a combination of the effect we see due to Bernoulli’s and the Magnus effect, the same thing you could get by putting a rotating cylinder in a moving airstream. (The FAA’s Handbook of Aeronautical Knowledge puts this together and explains it on p 3-4 through 3-10).
The circulation is what causes downwash, and the overall effect is to cant the lift vector slightly aft. The horizontal component of the resultant lift vector is induced drag.) 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.)

This is a technically correct explanation for the creation of lift and is not so hard to understand that anyone can’t grasp it or any good instructor should be able to explain it. This is also inline with what’s already in the FAA handbooks. I don’t think I need to teach anything else.