The Mystery of Lift

In science writer-editor Jules Bergman’s book, Anyone Can Fly, he talks about asking a little boy why airplanes fly.  The child explains, in essence, that airplanes float on the air.

I like that—because it is so simple—and almost true.

When I ask the same question of older students, some reply with a somewhat similar answer, only with a little more sophistication.

Then I slam them with the truth. Sometimes they have a little trouble handling the truth, like the famous line delivered by Colonel Nathan R. Jessep played by Jack Nicholson in the movie, A Few Good Men. “The truth? You can’t handle the truth!”

The truth, in this case, involves a little math. Basically, lift equals 1/2 rho times velocity squared times the coefficient of lift times the wing area (L=1/2 r V² Cl S).

In math, the equation (in parentheses, above) is somewhat intimidating. Less so written in English, as in the first part of the paragraph. However, it remains intimidating to the majority of flight students.

But the words and the equation should not be scary in the least. It is simpler than one can imagine. The equation can be reduced down to a very simple explanation.

First, there is nothing that can be done about rho, air density. So let’s throw that out of the equation. (If you’re really curious, rho is .002377 slugs per cubic foot at sea level in standard conditions.)

The other factor over which a pilot has no control is the wing area. We will assume for cruise, we cannot adjust the wing area in flight. For landing, once the flaps are full down, again, wing area remains constant.

So, what this boils our equation down to is this: L = V x Cl. In English, this means Lift equals Speed times Angle of Attack (AoA).

Now, to what kind of practical application can we put this information? 

Well, if we are cruising, we know that if we speed up, the airplane will climb. If we slow down, we are going to lose altitude. Another thing we know is that if we do keep the airspeed constant by not varying the power, if we increase the AoA by pulling back on the yoke, we will climb. Conversely, pushing the yoke forward and decreasing our AoA will cause us to lose altitude.

These four things, altitude, power, pitch, and airspeed, are interrelated and pilots must understand the relationships very well. You must also understand and realize where you are on the power curve. Either you are on the “front side” or on the “back side” and your position on the curve has everything to do with how you operate the flight controls.

Understanding the lift equation is particularly important for executing short field landings. Maintaining the correct glide path and airspeed for touching down precisely is extremely difficult if you do not understand the equation. It is much easier if you do.

More on the short field landing later.

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© 2011 J. Clark

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