Instructor's Corner

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.

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'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.

Lift and its Misconceptions

-Tom Shefchunas

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.

Local Angles of Attack

-Tom Shefchunas

We’ve all heard that an airplane can stall at any airspeed and any attitude but it will only stall at one angle of attack (AOA).  “A stall occurs when the smooth airflow over the airplane’s wing is disrupted, and the lift degenerates rapidly. This is caused when the wing exceeds its critical angle of attack. This can occur at any airspeed, in any attitude, with any power setting.”Airplane Flying Handbook (FAA-H-8083-3A 4-3).  The one key item that needs to be understood is that an AOA indicator is measuring one location or an average AOA of the whole airplane.

We are very fortunate that X-Plane flight simulator exclusively uses “blade element” technology.  The aerodynamic forces are calculated on hundreds of small polygons along the entire aircraft, allowing physics calculations for movement and rotation on each polygon.  This method allows for calculating AOA at a single point, averaging AOA for the whole airplane or averaging AOA for specific areas of the airplane, such as each side of the wing.

With the AlphaTrainer 3D plug-in for X-Plane, our programmers separated the areas of AOA for the left and right side of the wing to show that each side can have its own local AOA.  These two different angles of attack are exposed when unwanted yaw is introduced as uncoordinated flight or what is witnessed as the “ball” being out of center.  (An inclinometer ((“ball”)) is used to depict airplane yaw, which is the side-to-side movement of the airplane’s nose.)  Correction is made by proper rudder usage or keeping the “ball” centered in “coordinated” flight.  Centering of the “ball” is very important, and must be fully understood because a stall in uncoordinated flight forces one wing to drop before the other.  If this situation is aggravated, the one side of the wing will further drop, placing the airplane into a nose low twisting turn (autorotation), or what is commonly called a spin.  Only AlphaTrainer 3D depicts these local angles of attack in such a clear and simple manner.

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

Center of Lift

Another question posed to Tom Benson of NASA's Glenn Research Center by an 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 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:

Incorrect Theory #1

Incorrect Theory #2

Incorrect Theory #3

How an Airplane Enters a Spin

From Clark "Otter" McNeace, VP - Flight Operations, APS Emergency Maneuver Training:

In order to visualize the principal effects of an airplane entering a spin, suppose the airplane is in uncoordinated flight (e.g. the ball is out of center to the right, in other words, some yaw to the left is being generated) at the moment of stall. This uncoordinated flight could be due to any number of reasons: P-factor, slipstream, excessive rudder application, or asymmetric thrust. The secondary effect of yaw is always roll because the left yaw tends to produce higher local velocities on the right wing than on the left wing. The higher local velocities on the right wing tend to increase local lift resulting in a left roll even though both wings are still stalled (e.g. beyond critical AOA).

(Note: It’s important to understand that both wings must be stalled to enter a fully-developed spin (e.g. autorotation). Two critical aerodynamic factors are required for an aircraft to enter autorotation: a) continuous stall, b) continuous yaw. )

The left rolling velocity generated by the yaw tends to increase the angle of attack for the downgoing left wing and decrease the angle of attack for the upgoing right wing. At angles of attack above the stall, important changes take place in the aerodynamic characteristics.

If the airplane has a rolling displacement while at some angle of attack above the stall (beyond critical AOA), the downgoing wing experiences an increase in angle of attack with the corresponding decrease in the coefficient of lift (Remember, we are on the back side of the coefficient lift curve) but increase in coefficient of drag. The upgoing wing experiences a decrease in angle of attack with the corresponding increase in coefficient of lift but decrease in coefficient of drag. In other words, the upgoing wing becomes less stalled than the downgoing wing. The rolling motion is aided or increased rather than resisted and the left yawing moment is increased in the direction of the left roll as well. As the yaw increases, the roll will increase. This is called negative roll damping. An airplane has negative roll damping when both wings have angles of attack higher than the critical AOA. A pro-spin couple (i.e. yaw-roll couple) is spawned by uncoordinated stalled flight. If this yaw-roll couple is allowed to generate sufficient rotational energy, the airplane will enter the fully-developed spin (autorotation).

Incipient spins are a transitional phase during which the airplane progresses from an aggravated stall to a pure Autorotation. This phase may only last two turns, during which the rate of rotation (i.e. yawing and rolling) tends to accelerate en route to the developed phase. Incipient spins are typically pilot-driven, especially in the early stages. The forces of autorotation alone usually cannot sustain the incipient spin, so pro-spin inputs must be held for it to continue. Fully developed spins represent a state of equilibrium between aerodynamic and inertia forces and moments acting on the airplane. Unlike incipient spins, developed spins are aerodynamically-driven.

Stickiness of the air changes with altitude

Now let’s investigate the notion that an airplane will stall at any airspeed and any attitude--but only at one AOA. Now when we say this, we are speaking in basic low altitude conditions. Because of all of the new private jets flying at high altitude, it is important to understand that there is an exception to this “stall-at-one-AOA rule”. Remember when I said in Part 1, that some air molecules stick to the wing and create a Boundary Layer? Well, the problem with being at high altitude is that this adhesiveness of the gas (or air) gets less sticky with altitude and is a part of the reason why the airplane stalls at a lower AOA at these altitudes.  Tom's research:

 As an object moves through the atmosphere, the gas molecules of the atmosphere near the object are disturbed and move around the object. Aerodynamic forces are generated between the gas and the object. The magnitude of these forces depend on the shape of the object, the speed of the object, the mass of the gas going by the object and on two other important properties of the gas; the viscosity, or stickiness, of the gas and the compressibility, or springiness, of the gas. To properly model these effects, aerodynamicists use similarity parameters, which are ratios of these effects to other forces present in the problem. If two experiments have the same values for the similarity parameters, then the relative importance of the forces are being correctly modeled. Representative values for the properties of air are given on another page, but the actual value of the parameter depends on the state of the gas and on the altitude.

Aerodynamic forces depend in a complex way on the viscosity of the gas. As an object moves through a gas, the gas molecules stick to the surface. This creates a layer of air near the surface, called a boundary layer, which, in effect, changes the shape of the object. The flow of gas reacts to the edge of the boundary layer as if it was the physical surface of the object. To make things more confusing, the boundary layer may separate from the body and create an effective shape much different from the physical shape. And to make it even more confusing, the flow conditions in and near the boundary layer are often unsteady (changing in time). The boundary layer is very important in determining the drag of an object. To determine and predict these conditions, aerodynamicists rely on wind tunnel testing and very sophisticated computer analysis.

Copyrighted, Thomas Shefchunas.