Instructor's
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We have set aside this area of AlphaTrainer.Com specifically to provide information for you--the instructor. Feel free to submit any experiences you have had, or knowledge you have gained, while instructing.
Sink and the Power Curve
I studied this "phenomenon" back in Aero 401 .. many years ago. The explanation can get pretty complex, because it involves all of the factors that affect lift and drag (speed, angle of attack, wing shape), and couples the lift and drag. If you consider the lift equation in constant, low-speed flight, you vary the lift by changing the angle of attack...more lift requires a greater Cl if you hold speed constant. But the induced drag of the aircraft depends on the square of the Cl. So pulling a little higher Cl to increase lift, makes much more drag (double Cl can nearly quadruple Cd).
The higher drag causes the aircraft to slow down and the lift actually decreases because lift depends on the square of the velocity. So you need even more Cl to hold altitude .. which produces even more Cd...and you are caught in a dangerous circle. The effect is that as you pull higher angle of attack the plane sinks because the velocity effect is going down faster than the Cl effect is going up. You aren't stalled until the angle of attack exceeds the stall limit. To maintain lift (altitude) on the back side of the power curve, you have to apply power (go faster)...not pull AOA, that just makes matters worse.
Tom Benson
Engine Systems Technology Branch
NASA Glenn Research Center, MS 5-11
Cleveland, OH, 44135
Lift and Downwash
I posed the following question to Tom Benson of NASA's Glenn Research Center (Thanks, Tom!):
"Is lift created by downwash?"
I usually try to stay out of the middle of "lift generation" arguments. It's easy to debunk the very obvious incorrect theories, but the real explanation of lift is quite complex. The lift on the body is simple...it's the re-action of the solid body to the turning of a moving fluid. The "turning" implies an acceleration (change in vector velocity) of the fluid so a force must be applied to the fluid (Newton's first law). In response, a force is applied to the body by the fluid (Newton's third law) and that force is resolved into a force in the direction of the initial fluid motion (drag) and a force perpendicular to the fluid motion (lift).
Now why does the fluid turn the way that it does? That's where the complexity enters in because we are dealing with a fluid. Here we get into "cause" and "effects" arguments. The "chicken or the egg" argument is a cause and effect problem: chickens cause eggs/eggs cause chickens. So it's circular, and there are some systems (like chickens) which work that way. But in physics there are other systems that don't work that way. Take gravity, for example. Mass causes gravity, but gravity does not cause mass--gravity is an effect of mass. The cause for the flow turning is the simultaneous conservation of mass, momentum (both linear and angular), and energy by the fluid. And it's confusing for a fluid because the mass can move and redistribute itself (unlike a solid), but can only do so in ways that conserve momentum (mass times velocity) and energy (mass times velocity squared). Velocity and momentum are vector quantities, so there are actually three spatial momentums that must be conserved; and they (and the mass) are all interdependent. A change in velocity in one direction can cause a change in velocity in a perpendicular direction in a fluid, which doesn't occur in solid mechanics.
So exactly describing how the flow turns is a complex problem; too complex for most people to visualize. So we make up simplified "models". And when we simplify, we leave something out. So the model is flawed. Most of the arguments about lift generation come down to people finding the flaws in the various models, and so the arguments are usually very legitimate. The article which you referred me to is very good, and has an interesting "model", but even that model has its holes. A wing is not an air scoop. Why did they choose an elliptic scoop? What happens to the scoop if I change planform? Those kinds of arguments can be raised. Downwash: Simple, observable, related (or can be correlated) to the lift. But the flow aft of trailing edge is not "causing" the lift. It's not even in physical contact with the body; and lift is mechanical force (not a field force like gravity). So the body has to be in contact with the "cause". All kinds of silly arguments and counter-arguments about the specifics of the model.
I prefer, when discussing lift with students, to just stop at the Newtonian 3rd law, lift is the re-action to the turning of the flow. No turning, no lift. The causes for the flow turning are the conservation of mass, momentum and energy of the fluid; and that's complex. But it happens, and we can observe the effects of the flow turning-like downwash, like shed vorticity, like the pressure variation around the object, and like the velocity variation around the object.
Center of Lift
Another question posed to Tom Benson of NASA's Glenn Research Center by an AlphaTrainer.com visitor:
"How is Center of Lift calculated? Will FOILSIM calculate Center of Lift for a given airfoil? Or where would you suggest I look for this info. I've done a lengthy search of the web for this info."
In general, you calculate the center of lift (or more correctly, the center of pressure) by integrating the pressure times the distance times a differential area around the airfoil and dividing by the integral of the pressure times the differential area. This gives you a distance from a reference point and locates the Cp. For most thin airfoils, this is near the quarter chord. FoilSim is doing a very specialized analysis, called an ideal flow analysis, of a special class of airfoils, called Joukowsky airfoils and named for the Russian mathematician who developed the analytical geometry of the foils. For these foils, the center of pressure is exactly at the quarter chord (1/4 of the way from the leading edge to the training edge). But this a very special case.
Lift and its Misconceptions
The Internet has made the world a lot smaller and much more knowledgeable. As a flight instructor, I admit that I squirmed in my chair as I read NASA's article, What is Lift? In this piece, NASA offers three incorrect theories of lift: Longer Path or Equal Transit Time, Skipping Stone, and Venturi. After accepting NASA's view, my quest was not to recognize different effects of lift, but to understand the unseen cause of lift. After many months of intense study, I presented the following to Tom Benson at NASA's web site, "Beginner's Guide to Aerodynamics". He said, "Yep .. that's it!" to this simpler language explanation of "creation of lift":
"The key to understanding creation of lift is that it is a mechanical force. To be a mechanical force, there must be interaction and contact of a solid body (airplane or wing) with a fluid (air). 'Contact' is the keyword because that is were the air molecules crash [collide] into the wing/airplane, transferring their momentum to the surface. Similarly, the effects of lift are also present; like the pressure variation around the object, velocity variation around the object, downwash, and shed vorticity."
So what do we now teach? I believe we must teach the facts. If there is an inconsistency with the FAA's books and written test because of oversimplification or dated material, we must be well prepared to avoid confusion in our teachings.
Lift and Weight
In slow speed and a high angle of attack, does weight equal lift? Is it that weight is greater than lift, which causes many pilots to sink at an unnoticed rate? I posed this question to NASA's Tom Benson, and he offers an interesting solution to this perplexing question.
"On aircraft, weight is pretty much constant assuming you aren't dropping bombs, paratroopers, or something else. The only change is for fuel usage. So weight is pretty constant, but lift can change with all kinds of factors; speed, altitude, shape of the wing (flaps, slats, spoilers), and angle of attack. With real airfoils, the angle of attack dependence gets real complex, because it affects both the amount of lift and the amount of drag. So, lift could be going up because of increased angle of attack, but the speed could be decreasing because of increased drag (and lift would be decreasing with the square of the velocity.). So exactly what angle of attack does to aircraft performance depends on some other variables, including the speed when the maneuver is initiated, and the power setting of the engine. At altitude, at high speed, increasing angle of attack increases lift and the aircraft moves up. At low speed, (like during landing) increasing angle of attack decreases speed and the aircraft drops. I understand that this "reversal" causes a lot of problems for new pilots. At low speeds, you use the throttle to go up and down, and angle of attack to go faster and slower; exactly the opposite of high speed flight."
The Three Incorrect Theories of Lift
NASA offers three incorrect theories of lift that are commonplace in our teachings: