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,
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
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
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
"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
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
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
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 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 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
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.