WHAT PRODUCES THE “LIFT&rdquo: THAT MAKES AN AIRPLANE FLY?
ã Jim Trusty 2005
Some insight into the theories we now teach and believe with explanations that come as close to being correct as anything else you may have been told.
It has always amazed me, both as a pilot and a flight instructor, that we could, in this technological age, have so many theories or answers to a question that has gone unanswered since the legendary Daedalus and Icarus went flying many years ago, if they really did. First, let’s define the question we are really asking: “What makes an airplane fly?” or “What keeps us in the air?” or “How come we aren’t falling down?” or “What is producing the lift that is keeping us airborne?” or probably some combination of all the above. Way back in 1807, Sir George Cayley stated, “All that must be done to achieve flight is to make a surface support a given weight by the application of power to the resistance of the air.” Sure sounds simple, doesn’t it?
The question, as we are about to discover, can be answered in several ways, and all of them have some modicum of truth involved and also some theory. Some of the answers have grown considerably longer over the years, and others have simply been added when the credibility of the older ones grew thin and harder to sell or prove correct. Actually, no one completely agrees with absolutely only one way to answer the question. That’s why I feel so comfortable introducing yet another opinion and feel that a few aviation enthusiasts out there might even agree. As long as I class it as a theory, it allows you as the reader and me as the writer a lot of leeway in making a final decision, right or wrong.
Before we go much further, I am going to add some definitions just to bring you back up to speed on the subjects we are going to include in this article.
THRUST: The force exerted by the engine and its propeller(s), which pushes air backward with the object of causing a reaction, or thrust, of the airplane in the forward direction.
DRAG: The resistance of the airplane to forward motion directly opposed to thrust.
LIFT: The upward force created by the wings moving through the air, which sustains the airplane in flight.
WEIGHT: The downward force due to the weight (gravity) of the airplane and its load, directly opposed to lift.
ANGLE OF ATTACK: The angle between the chord of the wing and the relative wind.
ANGLE OF INCIDENCE: The fixed angle at which the wing is attached to the fuselage; the angle between the chord line and the longitudinal axis.
CHORD LINE: An imaginary line from the leading edge to the trailing edge of an airfoil.
RELATIVE WIND: Movement-generated wind that’s equal and opposite to the motion of the airplane.
And now the definition’s we will use to get you through this class on lift—
THEORY: An unproven assumption. A position assumed for the sake of an argument. A hypothetical set of facts, principles or circumstances. Pure
speculation. Example: The best defense is a good offense or the best offense is a good defense.
Now let’s discuss the theories we have and use on a daily basis to explain to the non-aviation community exactly why the airplane doesn’t fall down when we reduce the power. The theories listed are in no particular order and all are still in use. This list is not all inclusive, so I am hoping that after reading this article you will get in touch with me and tell me of others you have heard of and probably used. I promise to pass them on in some future scribbling of mine. Here we go.
The most often repeated include:
LANGEWIESCHE , of Stick & Rudder fame, believes that the “Angle Of Attack” is everything. The wing keeps the airplane up by pushing the air down. I sincerely subscribe to his thoughts and ideas simply because I can demonstrate them in the air and on the ground in any airplane and on any given day . . . and have.
BERNOULLI’S PRINCIPLE has been proven mathematically several times and disproven over and over in wind tunnel tests. The question still exists as to how the theory was adopted at all since Bernoulli himself died 123 years before the Wright brothers took the first powered flight in 1903. Most experts concur that this principle has the most flaws and the most followers. Go figure.
NEWTON’S 1st LAW states that every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it, followed closely by NEWTON’S 2nd LAW which states if force remains constant, acceleration is constant. Then NEWTON’S 3rd LAW states that for every action, there is an equal and opposite reaction.
VENTURI THEORY proposes that the upper surface of an airfoil behaves like a venturi nozzle constricting the flow. LONGER PATH states that the top of the airfoil is shaped to provide a longer path than the bottom. EQUAL TRANSIT asserts that air molecules must move faster over the top in order to meet the slower molecules at the trailing edge that have gone under the airfoil. SKIPPING STONE declares that lift is the result of a simple action/reaction as air strikes the airfoil and imparts momentum.
BULLETEFFECT states that air bullets hit the bottom of the airfoil transferring upward momentum to it. CIRCULATION says that a circulatory flow will partially cancel the wind below the wing, thus producing low velocity and high pressure, thus lift. KUTTA CONDITION states that air hates to turn a corner at any sharp edge because of friction. JACK & JILL gives names to the air molecules passing over and under the airfoil and states that they are trying to meet each other at the trailing edge. (My guess is to miss the evil witch?)
BARN DOOR THEORY, which can apply to any flying machine no matter the shape or configuration, proposes there is way too much engine and the object flies simply because we have propelled it off the ground with power and angle.
Rather quickly we have discussed over a dozen ideas or theories, as we have decided to call them today, and have not begun to reach the bottom of the sack of explanations that we use to answer that not so simple question, “What produces the lift that makes an airplane fly?” I want to introduce you to one last idea that I have done a lot of research on, and believe that it is the true secret answer to our question. Just so we don’t get this last idea mixed up with someone else’s, we will call it THE TRUSTY THEORY. (That ought to get me some nasty letters from a brainiac or three.)
I think that the impact of the wind hitting the bottom of the airfoil as we speed down the runway pushes the wing off the ground just enough to create a positive angle of attack and then the excess thrust kicks in to get us to our desired altitude. Fact: The angle of incidence is where the manufacturer decides that the wing will be attached to the fuselage so that in straight and level flight there is just a slight turn up and the wind hitting the leading edge is at an angle that pushes up rather than try to follow the outline of the wing. I watch so many airplanes take off everyday at my airport, including the Mitsubishis that I fly, the C-130’s from the local Air Guard, and several other monsters from the sky. What you see is an intentional lifting of the nose wheel by the pilot so that the wind can get under that airfoil. If they don’t get that nose wheel off in a certain distance, they are going to have to abort and get more distance or power or speed.
This is my theory and I’m sticking to it . . . at least until they come and get me in the white coats and tell me it is time for me to buy a Harley and park the MU2. I realize that some tiny part of every other theory that we talked about probably plays a role in the lift factor needed to get airborne and then to stay there, but I am completely sold on this impact idea. I must admit that a lot of my ideas as an FAA Certificated Flight Instructor came from reading Stick and Rudder at a very early age and the writer’s ability to explain very complex thoughts in a manner that could be easily understood to others that fly. He made it easy for me to make a provable point no matter its time in aviation. For this, I will always thank Wolfgang. I remember as a kid riding in the backseat of my Dad’s Hupmobile and sticking my hand out the window and watching it fly. Until I changed the angle of attack and raised the front of my fingers just a little bit, the hand would stay straight and level, but with a tiny change in attitude, up came the hand in almost a straight to the moon climb.
By the time you have finished reading this article, you might agree with the FAA’s thought that we should also be tested mentally on a regular basis. Or, careful now, you might agree that it is possible that some of the ideas that have been passed down to you over the years concerning the Easter Bunny, Santa Claus, and the theories on lift might need to be reworked just a little. I want each and every one of you, as a joint project, to do some research. Get on the Web and punch in Aviation and go from there. As a group, let’s find out who, what, where, why and when and most assuredly how these airplanes work.
There is no limit to the information that is available to you, textbooks, library, and computer. It is time that you formed an educated opinion, something that you completely understand and are able to transmit to others. Something that at least sounds plausible. I hope that you will give “THE TRUSTY THEORY” more than just a passing glance and then take the time to write or call me and voice your thoughts. I really look forward to hearing from you.
In closing, I am going to list some of the conclusions you will have arrived at by now, a list of proven facts, if you will. #1: Lift is produced when a solid moves through a fluid. #2: Even when flying upside down, you can make the airplane produce lift by establishing an “angle of attack.” #3: The amount of air diverted by the wing is proportional to the speed of the wing and the angle of attack. #4: The lift is proportional to the amount of air diverted times the vertical velocity of the air. #5: The power needed for lift is proportional to the lift times the vertical velocity of the air. #6: In all probability, in theory and reality, there is more than one way to solve this problem of lift, and we may not be able do it completely simply by writing a tongue-in-cheek article. But if it makes you do just the tiniest bit of research and question any of the methods known to man just a little, then I have succeeded.
Have fun with all this and then let me know what you have decided. I sincerely await your input.
Permission required to reprint this copyrighted article. (2005)
Jim Trusty, ATP/CFI, was named the FAA/Aviation Industry National Flight Instructor of the Year for 1997, and the FAA Southern Region Aviation Safety Counselor of the Year for 1995 & 2005. He still works full-time as a Corporate Pilot/ “Gold Seal” Flight & Ground Instructor/ FAA Aviation Safety Counselor/ National Aviation Magazine Writer. You have been enjoying his work since 1973 in publications worldwide. He welcomes your comments and e-mail works best (Lrn2Fly@bellsouth.net).
