Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.") It's part of what makes aviation so exciting for all of us... just when you think you've seen it all, along comes a scenario you've never imagined.

Aero-News has called upon the expertise of Thomas P. Turner, master CFI and all-around-good-guy, to bring our readers -- and us -- daily tips to improve our skills as aviators, and as representatives of the flying community. Some of them, you may have heard before... but for each of us, there will also be something we might never have considered before, or something that didn't "stick" the way it should have the first time we memorized it for the practical test.

It is our unabashed goal that "Aero-Tips" will help our readers become better, safer pilots -- as well as introducing our ground-bound readers to the concepts and principles that keep those strange aluminum-and-composite contraptions in the air... and allow them to soar magnificently through it.

Look for our daily Aero-Tips segments, coming each day to you through the Aero-News Network. Suggestions for future Aero-Tips are always welcome, as are additions or discussion of each day's tips. Remember... when it comes to being better pilots, we're all in this together.

Aero-Tips 06.06.06

We're all taught about Bernoulli effect and lift generation as the primary means by which an airfoil generates lift. But we often hear of another mechanism relating to the reaction of air molecules striking the underside of an airfoil with a positive angle of attack -- action and reaction, often called the Newtonian theory of lift.

(Note: There's another theory called the Magnus effect we've already discussed in Aero-Tips).

NASA (the National Aeronautics and Space Administration) tells us, however, that as a lift-generating theory, Newton is wrong. More correctly, the idea that air hits the underside of the wing and rebounds away, forcing the wing upward like a stone skipping on water, is mistaken. Here's why:

• This theory concerns only the interaction of the lower surface of the wing and the air. It assumes that all the wind-flow turning (and therefore all the lift) is produced by the lower surface. But the upper surface also turns the flow. In fact, when one considers downwash produced by a lifting airfoil, the upper surface contributes more flow turning than the lower surface. Newtonian theory does not predict or explain this effect.
• Because this theory neglects molecules striking the upper wing surface, it does not predict the negative lift present when the angle of attack is negative. On the top of the airfoil, no vacuum exists. Molecules are still in constant, random motion on the upper surface (as well as the lower surface), and these molecules strike the surface and impart momentum to the airfoil as well.
• The upper airfoil surface doesn't enter into Newtonian theory at all. Using this theory, we would expect two airfoils with the same lower surface but very different upper surfaces to give the same lift. We know this doesn't occur in reality. In fact, spoilers on the upper wing surface are used to change lift by disrupting flow over the upper surface. Newtonian theory does not predict or explain this effect.
• If we make lift predictions based on Newtonian theory, using knowledge of air density and the number of molecules in a given volume of air, the predictions are totally inaccurate when compared to actual measurements. The chief problem with the theory is that it neglects the physical properties of the fluid. Lift is created by turning a moving fluid, the air, and all parts of the solid object can deflect the fluid.

So is Newton wrong? No... but using Newton's laws of motion to explain lift is. The interaction of air flow across the bottom and the top of an airfoil, according to NASA, is the prime generator of lift. Sounds like we're back to good ol' Bernoulli.

Aero-tips of the day: Understand basic aerodynamics, so you can better understand the effect of changing angle of attack.

FMI: Aero-Tips