The Gee-Wiz Factor: Life of Air

Imagine having a front-row seat at a debate on some hot-button political issue. Words are flowing and emotions are getting heated. Interruptions abound and tempers flare. You are trying to keep up with the conversation but it's all just happening too fast! What can you do to get a grasp of the crux of the argument? Unfortunately, my brain just isn't quick enough to keep up with this kind of discussion. I like to go back and rewatch the debate several times if I can. 

Take for example a debate on kitchen cleanliness. The first time I go back to watch it, I might focus on one person and ask, ”What is the pro-dish-soap advocate thinking and saying?” Then I go examine what the anti-dish-soap advocate is thinking and saying. Finally, I see how these two are interacting with each other. 

This is what we have done so far in examining our airplane in-flight. We first looked only at how the airplane bent and groaned when forces acted on it. We then looked at how air and airplanes interact with each other through Angle of Attack. Today, let's just look at how air responds to the airplane acting on it.

You might be surprised to know that as far back as Archimedes in 250 BC, we have known that fluids are made up of individual particles. The size and interaction of these particles, however, remained a mystery until much more recent times. Today we know that fluids are made up of an ever changing conglomeration of either atoms, molecules, or a mixture of the two. These particles are in constant motion relative to each other and are not stuck together like solids are. In fact, they spend their days playing a constant game of bumper cars.

Let’s follow along with Ollie the Oxygen Atom as he carries out his normal existence up at eight-thousand feet. The clock strikes midnight and Ollie is currently zooming straight up at 1000 mph. In only 50 nanometers he collides head-on with another oxygen atom travelling straight down at 500 mph. Now Ollie starts travelling straight down at 500 mph and his collision buddie is travelling straight up at 1000 mph. What just happened is known as an Elastic Collision and since it was head-on and both colliders weighed the same, they exactly traded speeds with each other. This is, however, relatively rare. As Ollie starts travelling straight down at 500 mph he makes it about 60 nanometers before a nitrogen atom only going 50 mph collides with him at an angle. They both glance off at angles and at speeds only a nerdy physics student could calculate. 

All of this collision business has been going on for as long as Ollie has existed. He has been over the north pole at 60,000 feet, bounced off the wings of butterflies deep within the Amazon rainforest, and even had a fly-on-the-wall spot for the signing of the Magna Carta. Millions of creatures have breathed him in and exhaled him out. He has collided off of his fellow oxygen and nitrogen atoms more times than you could tally using all the pieces of paper on earth. At Ollie’s scale though, temperature, sound, and friction don't really exist, among many other properties we are familiar with at our scale of observation. In fact, between Ollie and his nearest neighbors vast areas of empty space exist relative to their size. In fact, Ollie is only 346 trillionths of a meter in diameter. To put that in perspective, you would need almost 500 million oxygen atoms lined up side by side to make it across the dot of an i on this page. All Ollie has ever known is this constant game of collisions. At this scale, what seems to only be chaos rules.

Today Ollie is in for a treat. He has stood nearly still for some moments and has also gone well over 1500 mph during others. As the dawn breaks, Ollie notes a slight increase in the umph of the collisions he's experienced. He still slows down some, but on average he noted he's been moving faster. To us humans, we would sense this increase in atomic motion to be temperature; the air hitting our skin would be moving faster and we would recognize it as heat. 

At 7:00 am, Ollie starts to sense something strange is happening--he's still getting hit from all sides but he has noticed he's had fewer neighbors and fewer collisions off his right side. He notices he is starting to drift ever so slowly to the right since he is getting the normal punches on his left but fewer off his right. At about 7:04 he notices this drift is starting to get stronger. He has moved an extra inch or so to the right. At 7:06 things get really crazy as this slow crawl to the right rapidly turns into a crowded stampede. Three seconds later things have gotten really violent and Ollie gets a front-row seat to the madness as all of a sudden a blade swings violently toward him. A hard aluminum molecule makes contact and yet another collision is added to Ollie’s list. He surges even harder to the right as the aluminum molecule gets a shove to the left thanks to the swinging follow-through of the prop. They part ways never to encounter each other again.

Through many more random collisions with other oxygen and nitrogen atoms in the ensuing milliseconds, Ollie quickly shares his experience of the blade by giving away some of the energy he received. He still bounces up and down, left and right, and side to side, but overall, he's moving to the right. 

Despite having had a one-in-a-billion chance among his neighbors for a direct collision with what was the propeller of our airplane, only a split-second later Ollie is directed through the never-ending motion of his friends towards the bottom of the wing. He strikes a paint molecule just below the leading edge and gets a kick downward and forward, yet the billions of molecules near Ollie drag him along under the wing like an unwilling country music hater in a Willie Nelson flash mob. Ollie's downward collision from the wing is definitely not unique. In fact, a billion of his closest neighbors also struck the wing, crowding them all closer together to allow a path for the wing to pass through. Being closer together caused even more collisions among themselves. We would recognize this at our scale as a slightly higher pressure. This experience with the wing has forever changed Ollie’s path in life. In fact, he will pass along the presence of the wing in every collision he ever makes after this event. He will band along with trillions of other oxygen and nitrogen atoms that have also encountered the wing and will pass along this information far upstream for other atoms to prepare a path that will allow the wing to pass. This communication is the essence of what allows fluids to create beautiful streamline paths around wings.

To sit back and examine such a scenario seems mind bogglingly complex. Through the chaotic motions and collisions of seemingly innumerable atoms and molecules, a beautiful and consistent flow pattern is created. We have reached the heart of aerodynamics, an incredibly complex study of how airflow can be harnessed to create useful forces like thrust and lift. No human or computer has ever been, or will likely ever be, able to fully predict exactly how these rambunctious particles will behave around a certain shape like a fuselage or a wing. That is what is so incredible about aerodynamics; no full understanding has ever been found and if I can wager a guess I highly doubt one ever will be. 

We must, however, ask ourselves what we have learned from this example.

1. Motion of air is incredibly complex and chaotic on small scales, yet it can be incredibly uniform and orderly on large scales.

2. By means of roundabout communication through elastic collisions, individual atoms share their local understanding of other particles and objects like wings and propellers with other fluid particles at a distance. 

3. Fluid particles always order themselves in a way that is in complete harmony with the laws of nature. 

4. Fluid particles and the greater flow of fluids are never aware of the simplifications humans use to understand or explain aerodynamics. 

If all of these observations are correct, we have to wonder what all of the fuss is about in aerodynamics textbooks. There certainly are some fundamental facts of nature that we use to understand aerodynamics but they aren't the answers to our problems themselves. Historic figures like Bernoulli and Newton gave us some amazing insights, but each of them is just a dim glance at something that is so incredibly beautiful and complex that humans just aren't capable of fully grasping it. So, the next time you hear terms like "Bernoulli's Equation", vorticity, circulation, and Navier-Stokes, know that these are only approximations or parts of something so incredibly complex and amazing that lets us fly and see the world in a way few in all of history have ever been able to witness.

If you haven't been bored to tears yet about fluid motion, take a look at this cool gas simulator by Paul Falstad.

Clear skies and tailwinds,

David

The Gee-Wiz Factor: The Wing and the Wind

In the last article, we discussed how the structure of an airplane behaves and responds to outside forces. These forces included the upward pressure from the pavement on the landing gear and airflow over the fuselage and flying surfaces. We looked more at what the forces did to the airplane internally, instead of how the airplane interacted externally with the forces themselves. Today, let's look at the airplane's interaction with the air through which it travels.

Imagine if you will, what it must be like to be a bird in flight. These flying creatures seem to have an innate understanding of how to maneuver in the air from the moment they first leave the nest. They know what they can and can't get away with when using the air to provide their path. Our winged friends know a whole lot more about aerial maneuvering than even the best of pilots.

Consider though, a poor bird caught out of the roost at night and in the clouds. The poor fellow has no idea where or how high he is or which way he’s pointed. He decides to just keep flying until morning and then hope to see well enough to land safely. In the meantime, all he can do is flap his wings. About the only sense our poor friend can rely on is the rushing of air straight over his beak and back to his tail feathers. Sadly, this is no guarantee that he is flying straight and level and slowly, something in his bird brain senses that he is descending. This is not a concrete sense like his vision could have given him on a clear day--it’s just an inkling. He lowers his tail feathers, and he feels the air shift from hitting him on the beak down to hitting him on the neck. The air flows past him at an angle now instead of from beak to caboose. Our friend has just changed his Angle of Attack (AoA). Unfortunately, this is the only thing he can know for sure.

Now, let's wish our poor feathered friend the best and for the sake of a happy ending we’ll say he made it till morning and broke out of the clouds to find his nest nearby. What we need to take from this story though, is the pure and isolated idea that any heavier-than-air flying machine is governed and limited by its AoA. Try to put away every single property that you might want to see or control. This includes pitch, bank, airspeed, and so on. No matter which direction you are going, whether straight and level, turning, straight up, or slightly down, you will have some AoA. You can be pointed in any direction and have any AoA. In short, only think about the wind and the wing. 

Now we must consider how we control this relationship and the inherent realities and limits behind it. Know that the stick in your hand is purely an aerodynamic control. It is not an up and down control. It definitely is not an airspeed or an altitude control. The stick in your hand controls AoA and that's it. If you need more AoA pull the stick back; if you need less, push it forward. You should have a good idea from your early training that pulling on the stick has its limits. Pull it back so far and you keep getting more AoA. Pull it back too far and the wing now meets the wind at too steep of an angle to get useful lift out of the wing. Remember this can and has happened to many aviators in all sorts of attitudes, not just with the nose being up. Most aviators have never performed anything other than benign straight-ahead stalls at shallow pitch attitudes. It is ingrained in us from early on to not let the nose get too high. The nose gets high and we start to get timid of pulling on the stick in a serious way. Sadly, all of this creates a fallacious link between pitch and AoA. Then, when you start doing aerobatics, or heaven-forbid end up in an inverted unusual attitude emergency all our faulty intuition falls out the window vent. The timidity to pull that came from a nose-up attitude is unconsciously lost. We then haul back on the stick hoping for a miracle. The wing stalls, and what we may have bet our lives on isn't there to save us.

Imagine now you and your airplane replacing our poor bird in our previous example. All you have in the panel is an AoA gauge, nothing else. This, of course, would be a horrible and deadly situation to be in but just imagine for now it's a harmless lab experiment. You are flying along and you pull back hard on the stick for a good thirty seconds. The AoA gauge goes up while you hold it back. Did you pitch straight up? Did you do a loop or a turn? Did you nose up and then pitch forward again? Is up even up anymore? There is no way to know. All you know is that for thirty seconds, the wind met the wing at a steeper angle than it had before you pulled back. Now you push forward on the stick and hold it. The needle on the AoA drops to zero. The air rushes right over the nose of your plane--straight from spinner to tail, leading edge to trailing edge. You push even harder forward, the needle plummets below zero. The air now rushes at the upper camber of the wing and parts the trailing edge heading for beacon light on top of the rudder. Have you done an inside loop? Plummeted straight toward Mother Earth? Pointed straight up? There really is no way to tell.

The moral of this story is that the wing can never meet the wind at more than the critical angle of attack. You could be completely upside down and turning but if the AoA isn't critical yet, you will have control relative to the air. In essence, the air can still be used to bend and shape your flight path. What we need then is a consistent way to know when we are near or at the stall.

The avionics guy's answer to this problem might be an AoA gauge. These are nice to have but are incredibly expensive and rare in GA so they are off the table as a blanket option. Some might say you could rely on an airspeed indicator for this. Sadly, airspeed indicators are pretty lousy at universal stall prediction. Do you know the stalling speed of your airplane inverted at sixty degrees of bank? Good luck calculating that one. We need something else and sadly money can't just purchase it new out-of-the-box.

Let's shift our thinking to the loose nut who is death-gripping the stick. You would be amazed how many pilots get busy and don't even realize they are pulling the stick back dangerously far. Their brain is on autopilot and just trying to keep the attitude "right". This pilot is only thinking about his relationship to the horizon, not the air he is journeying through. Believe me, it happens and more frequently than we might like to admit. In a car, it's incredibly easy to stay on the road, you just keep your eyes forward and turn the wheel as the curves come and go. Try to think about the air as a road, not the land beneath you or the horizon ahead of you. This invisible road can only have turns so tight, and the grades only so steep.

Thankfully, there is an answer to our problem. We need to get our brain into the feedback loop with our arm and hand as they are pulling on the stick. In short, you need something telling you how far back you can pull the stick without stalling out. This is what replaces the needs for instruments or numbers in our head. Start trying to feel for cues in your arm and hand muscles as you are pulling; learn to recognize when you are asking for more from the wing. Know what your arm feels like along your side or on your leg when you've got it back at or near the stall. You may or may not get buffeting cues depending on the airplane. Know what they feel like at 50 knots and 100 knots; in other words, mushiness in the controls is not a guarantee you are near stall. Practicing with many different attitudes and airspeeds will let you build this universal awareness of how much is enough and how much is too much. 

One of my favorite ways to practice is to go up and do high-speed turning stalls. Climb to a healthy altitude, bank steeply, and then pull as hard and as quickly as you dare to enter a turn. It's kind of like the Price is Right for Pilots. The person who pulls the most without going over, wins. Keep in mind this is all about how far back you have the stick. You don't need to measure it with a ruler or find the magical amount of pull to induce a stall. That's the wrong way to approach this. You just need to know how much you've been pulling on the stick and if it's back or not. Then, when you are in the pattern and trying too hard to crank it around the base to final turn, a little voice will travel up your arm to your noggin and say, "Woe is you if you pull back anymore". 

Now for a few side-notes. It amazes me how basic and limited the Private Pilot standards are for stalls. Advanced stalls, the ones that will get you in serious trouble, only have to be demonstrated to the student once. They don't have to practice them and most don't want to! The average new private ticket holder probably only has twenty or thirty stalls to their name. That's hardly an introduction, let alone mastery. Go out and do twenty or thirty stalls in one flight. Your confidence, awareness, and familiarity will vastly improve. Keep track in your logbook and set a lofty goal of say a thousand stalls. Track them like you track your flying time; you will want more. Also, if you don't have a g-meter stay below maneuvering speed. If you do have one put the airframe to the test and venture out beyond the false security of one-g and maneuvering speed. You will be amazed what these flying machines can really do. The same principle applies to AoA gauges; they let you know right away how much you've used and how much is left. You'll be pleasantly surprised at how aggressive you can get and yet stay within limits. Finally, keep in mind that not every airplane can be flown by feel like we are used to in GA. Here again, consciously knowing you are bringing the stick back can be a help in addition to AoA gauges and stick shakers.

Clear skies and tailwinds,

David

The Gee-Wiz Factor: Airplanes Don't Have Eyes

Series Introduction

Over the past four years of flight instruction, I have witnessed many students struggle mightily, at least at some point in their training, with understanding how to control the flying machine they are trying to master. Having also flown with many seasoned pilots, I have found no shortage of backwards, inefficient, puke-inducing, and sometimes downright dangerous, control behaviors. At the core of this problem lies a mixture of some simple physics and applied aerodynamics known as the mechanics of flight.

From my first week in the right seat until now, I have wrestled with how to teach the mechanics of flight to my students. What rules of thumb should I use? What information is fact versus fiction? What level of depth is too much and what is not enough? Why teach aerodynamics if we don't apply the information? The answers and implications to these questions are, in my opinion, monumental. 

What we teach and reinforce in the cockpit, lays the groundwork for what will hopefully be many decades of flying whether for personal enjoyment or professional application. As flight instructors, we have an immense responsibility for the future safety of our students. Some of this responsibility shifts to the newly-minted private pilot to continue self-study and maintain proficiency however, the majority of it still lies in the first lessons we provide as educators.

It is my opinion that we should teach the facts of the mechanics of flight for all they are worth. They should be recognized as complex topics requiring careful study. They should not be ignored or simplified to the point of irrelevance. 

Unfortunately, the facts governing the mechanics of flight can seem unreachable, complex and unintuitive; on top of that they are often buried in the stilted and awkward language of academic writing. Take a glance into books with titles like Fundamentals of Aerodynamics and Aircraft Performance and Control and you will be met with dense paragraphs of mathematical minutia and few, if any, helpful diagrams or pictures. Theory is presented instead of application. Anyone interested in this meeting of the practical with the theoretical must understand both worlds well enough to play a "Where's Waldo?" in the textbooks.

Back in the old days of flying, the flying community knew a whole lot less about many factors governing flying than we know today. This included weather, aerodynamics, weight and balance, radio communications, etc. What I like about the old literature from those days is that when facts were well understood, pilots were expected to read, understand, and apply that knowledge to good effect. My favorite example of this is the Pilots' Powerplant Manual published by the Civil Aeronautics Administration in 1940. This book has more useful information in it about engines than what most A&P mechanic students are coming out of school with these days. If a pilot of that era were to have ingested and comprehended that book, they would have been a one-man army of engine comprehension. That is the kind of guy I would want up front in a DC-3 over some patch of inhospitable terrain at night and in the clouds with an engine issue.

Sadly, even though our collective knowledge of aerodynamics, performance, and control accelerated exponentially through the 20th century in the engineering world, the pilot community has remained in the dark on what is actually happening when we as humans interact with flying machines. There is a little-known, yet beautiful world of understanding that exists between the flying and aerospace engineering communities and is within easy mental reach of anyone smart enough to solo an airplane or pass a college aerodynamics class. 

The purpose of this series of articles is to share the amazing insight a pilot can have about how their airplane flies and will find this information can entirely change the way they control their airplane. Some of the information may or may not have practical use, but that's the cool thing about something free -- you can take or leave it at no charge! That's why I have entitled the series; The Gee-Wiz Factor.

Airplanes Don't Have Eyes

Imagine with me for a moment what it must be like to be born blind. With no ability to sense light, the beauty and detail we are all familiar with would be only a mere concept. Small objects could certainly be felt and maybe some sort of mental "picture" could be built. However, descriptions of larger structures and landscapes would likely have very little meaning. How close would your idea of the horizon, the sky, and land be to reality? I can imagine that if someone born this way were to gain sight, the larger world would be of entirely different proportion than they might have expected. Houses may seem smaller than anticipated, and skyscrapers enormously larger than expected.

The world the blind do live in however, is richly full of touch and sensation. Without a sense of sight to get in the way, these people must have an incredibly refined sense of force and pressure. The slightest air draft, brush of a book's page, or change in seat pressure when going around turns in a car is likely much more noticed and analyzed. 

If you will permit me, let's personify our airplane and imagine what it must be like to experience flight from it's blind perspective. Resting on the ramp, the tarmac presses up into our airplane's wheels as the force of gravity maintains it's ever-present pull upon every molecule of our plane. This pressure on the tires bulges them slightly, transmits this force to the axles, and up through the landing gear which bends slightly to provide the necessary resistance. This bend of the gear slightly twists against the fuselage mounts and stretches the bolts holding the two together. From here, the loads transmit into the aluminum structure and spread like a drop of food coloring in a glass of hot water to the entire airplane. Some parts experience enormous compressions, stretches, twists, and bends. Others feel only a slight reaction. You can be assured though that every part feels something. Our airplane is at rest and only it feels its own aches and pains. The cloud drifting overhead, the sun setting in the distance and even which direction is "up", are all unknown to our airplane.

All of a sudden, a door opens and in a moment extra forces have been added. Pressures run in an instant, down through the seats, across the floor, around in circles within the bulkheads, and down through the gear and tires to the tarmac which now bows ever slightly more into the earth beneath. Our airplane lets out a slight metallic groan; maybe it's only a maintenance technician checking the tach time. Suddenly, electricity flows from the battery, through some circuitry, and into the starter. The engine gallantly resists but is no match for the starter as the prop starts to swing. The conditions are just right in the engine which fires right off. Waves of vibration jump from the engine case, through the rubber mounts, to the engine mount, and finally into the airframe. Every single part of the airplane is now in rhythmic vibratory motion; every rivet and fastener is under constant assault to let go. Our airplane sighs and mutters to itself, "Here we go again."

The throttle cable feels a shove, and transmits the push to the engine which responds with more rapid rotation of the prop. Gooey wax particles on the surface of our airplane feel an increase in collisions and the slipping of tiny air molecules passing over that had just gotten kicked in the pants by the prop. The wax passes off these forces to the paint beneath, which passes it off to the primer, which passes it off to the aluminum below which then shares it with the rest of the structure. By kicking these air molecules backwards, the prop gets an equal and opposite reaction forward which strains against the crankshaft, which passes it off to the case of the engine. The same path taken by the engine's vibration is also taken by this forward tug and over a short time every molecule in the airplane is made aware of the need to move forward. Suddenly, the wings start to feel the collisions of billions of air molecules over and under their outer skins and transmit the message to the landing gear far beneath which no longer have to wage the constant fight against gravity. The rush of air over the entire plane surges faster and faster. More and more reaction is felt through the skin, which transmits to the rest of the structure, which after a while is exactly balancing out the forward tug from the propeller. Our airplane has no idea how fast it is going, which way is up, or how high it is above the ground. All it knows is that the tug-of-war on it's bone structure has stabilized and it only has to worry about the constant ache of vibration.

This is all that mechanical devices like airplanes understand; forces and their accompanying stresses. These are the only commands an airplane's bones and flesh can reply to. A force is enacted, internal stress is created, and in the absence of an opposing force, the airplane surges through acceleration in the direction of that force. Sadly, we as visual humans, have little natural capability to grasp this dark world devoid of everything except mechanical forces. Someday, I would love to put on a magical pair of glasses which would let me see into this mysterious world of actions and reactions. Yet, for the moment, diagrams and drawings in textbooks must suffice. 

In the meantime, start thinking of the airplane's controls as force controllers. The elevator doesn't merely raise and lower the nose like a grocery store grapple game stick moves the grapple from one position to another, it adds and subtracts from the forces at play upon the airplane. If this tug-of-war of forces is dominated by the force you have introduced, changes will happen. More about this battle next time.

2021 Gulf Coast Tour

Hello Friends,

It's been a long time since I posted last. My wife and I recently got back from a 21 day trip to tour the Gulf Coast in the Luscombe. Unfortunately, we didn't have good video capability but we did take a lot of pictures and I think we have shared them in a unique way via Google Earth! Click on the little pins on our route map within Earth and the related picture will appear! See the link below.

I'm hoping to start posting some more and have added a new page entitled "Lessons Learned" and have added some pictures in "Sunrises and Sunsets".

David

Google Earth Link