Thanks!
Patrick
Greetings Patrick,
It's part of the human condition to debate ideas. Everyone does it. If you could put Mahatma Ghandi and Mother Theresa in a room, they'd surely debate each other about who ate less. It seems that flight instructors, engineers, and physicists are no different when it comes to debating the process by which a wing produces lift. Experience tells me, however, that when arguments like this have been around for a long time and continue to sustain themselves, then both sides are probably speaking the truth, but doing so in different ways.
Let's begin with this question, "What is the fundamental reason behind a wing's ability to keep the airplane airborne?"
Of all the physical laws we can choose, it seems that Newton's laws of motion answer this question best. In particular, Newton's third law states: For every action, there is an equal and opposite reaction. Therefore, if a wing produces an upward-acting force, it does so because it pushed something downward first. That something is air. Remember, this is a law and, by definition, the explanation that uses it doesn't get more fundamental than that. Fundamental explanations, however, may not always be the most effective means of helping people understand lift.
When teaching, it appears that many instructors find it easier to use Bernoulli's principle to explain how lift is produced. They have a lot of support in their choice considering that five of aviation's most respected educational texts describe lift in terms of Bernoulli's principle without any mention of Newton (three of these texts were written by aeronautical engineers, two with a Ph.D.).
I suspect that Bernoulli's explanation is often favored because it's easier for most people to grasp. Anyone who's sailed, stood between two buildings in a strong wind, or watched a roof disappear in a hurricane probably has an intuitive sense of what high- velocity, low-pressure air can do. Bernoulli's principle provides an explanation that uses the basic experience possessed by most individuals.
Bernoulli's principle also helps easily and clearly explain the single most important principle that all pilots must understand: how a wing stalls. I suspect that many aviation authors avoid using Newton's laws of motion to explain lift because it's simply a less-efficient means of imparting the lift-stall concept. This seems even more likely when you consider that additional concepts such as the Coanda effect must be introduced to make a Newton-based explanation of stall complete. (Those who may disagree with this last sentence must then explain why three aeronautical-engineer educators select Bernoulli in lieu of Newton to explain lift).
Yes, Bernoulli's principle as a means of explaining lift does have its limitations. Sometimes science is oversimplified when discussing how this principle aids in the development of lift. For example, consider two air molecules separated by the wing so that one goes above and the other below. The molecule taking the high road speeds up in relation to the bottom-traveling molecule. That's a fact.
It's often said, however, that both molecules meet at precisely the same time at the trailing edge. Not quite. To be precise, the top-traveling molecule actually arrives at the trailing edge before the bottom-traveling molecule as long as the wing is developing lift. That's a fact, too.
It's apparent, however, that many aviation educators don't believe that it's unreasonable to use the meeting-at-the-same-time explanation when teaching private pilots. After all, many prominent aviation texts use precisely the same explanation. While this is an oversimplification, it nevertheless aids students in understanding a complex subject.
Don't get me wrong here. Scientific accuracy is always desired, but not at the cost of a student's ability to function safely in an airplane. An overly complex or math-intensive explanation of lift might result in a student dismissing the entire subject as unknowable. Remember, flight instructors teach people to fly airplanes, not design them. So, while Newton's laws may provide the fundamental explanation of lift, the evidence indicates that Bernoulli's principle is the explanation of choice in helping others understand how lift is produced.
Here's something else to consider. The genesis of most FAA exam questions is aviation accidents. The FAA creates specific questions to educate a pilot in those areas where a knowledge deficiency might lead to an accident. Yet, there are no questions on the FAA's private or commercial pilot knowledge exams that ask about how lift is produced. Instead, the FAA asks about the factors controlling or affecting lift. I'm speaking of such things as density altitude, ground effect, critical angle of attack, region of reversed command, etc. That must tell you something. It's doubtful that anyone has ever crashed an airplane because of a sole reliance on Bernoulli's principle to explain lift. It's more likely that someone has crashed because he thought the whole of aerodynamics to be overly complex and undeserving of study.
Patrick, since you asked about how a wing creates lift, consider this. The wing's development of lift is a process that involves several factors. For example, the wing bends or adds a curve to the air; it accelerates the air that flows above it, producing lower pressure on its upper surface; and, ultimately, it deflects air downward. It seems to me that neither Newton's laws nor Bernoulli's principle alone are sufficient to provide a complete and thorough explanation of the lift-creating process.
That's why, if you feel it's necessary to provide a student with a complete understanding of the process by which lift is created, I recommend the explanation given by John S. Denker, as presented at his Web site titled "See How It Flies" ( www.monmouth.com/~jsd/how/).
Denker says, "There is only one lift-producing process. Each of the explanations itemized above concentrates on a different aspect of this one process. The wing produces circulation in proportion to its angle of attack (and its airspeed). This circulation means the air above the wing is moving faster. This in turn produces low pressure in accordance with Bernoulli's principle. The low pressure pulls up on the wing and pulls down on the air in accordance with all of Newton's laws."
Taxi Into Position And Hold
The following question was posed to me at a recent aviation seminar by a student pilot named Kevin. Here are the question and the answer I provided to the folks at the seminar:
Rod, I was cleared to taxi into position and hold behind a jet that had just begun its takeoff roll. Immediately thereafter, the controller cleared me for takeoff. During the takeoff roll I became concerned about wake turbulence and asked for an immediate right turn. The controller didn't acknowledge my call. I called again, and the controller still didn't respond. Finally he gave me permission to turn right, which I did with great enthusiasm. Did I need to obtain the controller's permission before making a turn to avoid wake turbulence?
Dear Kevin,
The answer is "No." Now for the details.
Aviation priorities can be cast in three simple words: aviate, navigate, and communicate. Fly the airplane first, navigate second, and talk last. If you need to turn to avoid a wake turbulence encounter, then turn (as long as the turn can be done safely), even if the controller isn't talking to you. Resume communicating with the controller as soon as possible but only when the wake turbulence is no longer a threat. Here are a few additional things you should consider.
Depending on the size and the location of the aircraft that departed, the controller may only be required to provide a wake turbulence warning without a mandatory hold time. This doesn't mean, however, that you can't encounter wake turbulence. You can. In these situations, it might be better to take a delay on the ground if you suspect that wake turbulence might become a problem.
When the controller clears you to taxi into position and hold, this is the time to express your concerns, not while you're sitting in position on the runway or while beginning the takeoff roll. In a situation similar to what you described, you might say the following: "Tower, this is 2132 Bravo, we'd like an immediate right (or left) turn after departure for wake turbulence avoidance or we can take a delay of (minutes) on the ground." This way you'll have a chance to negotiate with the controller before taxiing onto the active runway. At least the controller knows what you want to do, and he or she can more easily arrange traffic to accommodate your request.
Finally, please keep in mind that an immediate turn after departure doesn't absolve you of any noise abatement procedures that happen to exist at the airport. This is one more reason why taking the delay on the ground is often prudent.
Cross-Country Signoff
Dear Mr. Machado,
I'm a student pilot, and I'm confused about something. Am I required to have someone sign my logbook at the destination airport on my solo cross-country flight? My instructor isn't sure if this is required.
John
Greetings John,
We'd all have to take lethal doses of Ginko biloba not to be confused about something. Let me help you here. No, you are not required to have your logbook signed at each destination airport on solo cross-country flights. You are, however, required to carry your logbook with you on all solo cross-country flights as a means of proving that you have all the required endorsements. The reason an instructor may request that you obtain a signature is to prove you actually landed at that airport. It seems that some students would rather fly around in the practice area for a couple hours in lieu of doing the cross-country flight. Keep in mind that flight instructors are going to be a tad suspicious if each destination has the signature "Uncle Bob."
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