Transcription
Welcome to the Free Pilot Training Podcast. I'm Josh Campbell. You may have heard that I have the most complete and totally free Private Pilot ground course on YouTube, but a lot of my viewers have been asking me to put together a podcast that they can listen to as well. So, here you go, guys.
And throughout this series, I'll teach you everything you need to know in order to earn your pilot wings. But keep in mind, some things are going to be better learned on video, so be sure to check out my channel when you have a chance. But in this podcast series, I'm going to go over everything we can discuss through an audio format.
In today's lesson, we're going to be covering three different topics: the four forces that act on an aircraft in flight, how an airplane creates lift, and what causes an airplane to stall. So, if that sounds good to you, let's get started by talking about the four forces that act on an aircraft while it's in flight.
This is a super important lesson because if you have a basic understanding of aerodynamics, this is going to give you a really good foundation that you can start building on throughout your training. Plus, these questions might be on the written test. And if you didn't know, there is a written test that you have to take before you can even take the check ride. This is a 60-question multiple-choice test, and you have to score at least a 70% to pass. In fact, this test can be a little bit of a bear, so it's really important to listen to stuff like this and watch videos that'll help you pass with flying colors.
Now, one other thing I want to mention is that in order to take the written exam, you have to be endorsed by an approved instructor. So, a lot of people pay for a ground course or pay an instructor to teach them this stuff that they need to know, and when they're ready, the instructor gives them this endorsement. Another option is to buy my premium ground course, which includes quizzes, practice tests, and reading assignments, and when you're ready, I'll email you that endorsement. You can get all this for $50 on freepilottraining.net, and that's a steal of a deal. Most ground courses out there are closer to $300, and the average cost for an instructor is around $50 an hour.
But anyway, let's get back to this lesson and talk about these four forces that affect our airplane in flight. These forces are lift, weight, thrust, and drag.
The first force you should know about is lift. Lift is the upward force that actually picks the aircraft up away from the ground, and this is caused by basically two things: relative wind and the wing design. We pilots have a super fancy name for this wing design; we call it an airfoil, and we're going to be talking about that in more detail in a minute when we start talking about how an airplane creates lift.
But the next force we need to consider when flying an aircraft is weight. Weight is simply the combined load of all the people on board the aircraft, all the fuel, any baggage or cargo you may have, and the aircraft itself. It includes everything on board the aircraft. And the other thing you should know about weight is that it opposes lift and it acts downward through the center of gravity. We'll discuss the center of gravity more in an upcoming episode, but the center of gravity is basically where the average weight of the aircraft and everything on board is centered. If you could hang an airplane from a string and you were to place the center of gravity directly beneath the string, the aircraft would balance perfectly.
Here's something that may not make a lot of sense yet, but it's important to know: in steady, unaccelerated flight, lift and weight are both equal. We'll come back to that in just a second, but this is something you're going to need to remember.
Next, we have thrust, and this is the force that propels our aircraft forward. Thrust is typically created by some kind of engine and propeller working together, or a rotor for you helicopter nerds out there. The important thing to know about thrust is that in order to move the aircraft forward, you must first overcome drag, which we depict as a rearward force. But really, this force just opposes forward movement, and that drag makes it difficult to move the aircraft forward.
Keep in mind, there are two types of drag that you should be concerned about as a pilot: parasite drag and induced drag. What happens when you stick your hand out of the window of a moving vehicle? The wind pulls your hand back, doesn't it? The faster you drive, the more your hand gets pulled back. This is what we call parasite drag. And just like it does when you stick your hand out of the moving vehicle, parasite drag increases as airspeed or thrust increases. In fact, it actually increases exponentially as you fly faster. That's one of the reasons why we can't fly super fast in a training aircraft; there's way too much parasite drag holding the airplane back at higher airspeeds.
Now, there are actually three different types of parasite drag, and the first one is form drag. In a nutshell, form drag is wind that goes around the shape of the aircraft. How much form drag do you think an aircraft would have if it was shaped like a rectangle? Probably a lot, right? A rectangle is not a smooth, round shape that can allow air to smoothly flow around it, and because of that, something that's really boxy is going to have a lot of drag on it. Boy, I bet those 1980 Chevy pickups have a lot of form drag on them, don't you?
Another type of parasite drag is interference drag. This one might be a little harder to understand, but this type of drag occurs when different strains of air flow create little pockets of turbulence at different places on the aircraft, and this restricts the smooth airflow around the surface of the aircraft.
Then we have skin friction drag, and this type of parasite drag is caused by the surface of the aircraft. If we were able to zoom in with a microscope and look at the skin of the aircraft, what you'd find is that it's not actually as smooth as it looks. It almost looks like sandpaper under a microscope, and wind can still act on these microscopically rough areas and reduce the speed of our aircraft. And as you may have noticed, we typically paint or polish the exterior surfaces of an aircraft to reduce skin friction drag, but it's never completely eliminated.
Induced drag is the other type of drag that affects our aircraft, and that's created as a direct result of lift. I'll be explaining this more in a minute, but our wings direct the relative wind above and below the surface of the wing, and this creates lift, but it also creates induced drag. Induced drag is kind of weird: as airspeed increases, the wind gets more efficient, and because of that, induced drag actually decreases. If we were to look at these two types of drag on a chart, what you find is that parasite drag increases exponentially with airspeed, but induced drag decreases exponentially with airspeed. And because of that, there's a magical little airspeed range on every aircraft where we can achieve the least amount of drag possible when we're out flying around. And a lot of airspeeds you're going to be flying are directly related to this airspeed. Pilots often refer to this as the lift-to-drag ratio, or LD Max.
You might be wondering why it's called the lift-to-drag ratio and not the thrust-to-drag ratio. Well, just like lift and weight in steady, unaccelerated flight, thrust and drag are both equal. But it's important to remember that all these forces are directly related to each other.
You're probably wondering why the heck we're talking about these four forces and how they're related. You just want to get into the plane and start flying. But if you understand this concept, it's really going to help you to understand how to control the aircraft. And anytime we make an adjustment to any one of these four forces, it's going to cause a change in one of the other forces.
Remember what we said earlier: during steady, unaccelerated flight, lift and weight are both equal. But let's say I make a change in one of these forces. For example, let's say I've got my airplane trimmed out for level flight. What do you think will happen if I increase my airspeed by increasing my thrust? Well, by increasing my thrust, more wind flows over the wings, and they create more lift because we're accelerating. And when we have more lift, that lift becomes greater than our weight. And the opposite is true when we slow down. So, during level, unaccelerated flight, lift and weight were equal, but once we accelerate, lift becomes greater than weight. Does that make sense?
Okay, let's say I'm trimmed out for level, unaccelerated flight again. At this point, thrust and drag are equal as well. But what happens if I increase my lift? What do you think will happen to my airspeed if I raise the nose of my aircraft to increase my lift? Well, if I pull back on the yoke and raise the nose of my airplane, my airspeed is going to decrease. Before I increased my lift by raising the nose of my airplane, we were in steady, unaccelerated flight, but then I increased my lift, so I also increased my induced drag, and this caused my drag to be stronger than the thrust, and this slowed down my aircraft.
So, the key takeaway here is that in steady, unaccelerated flight, lift and weight are equal, and thrust and drag are equal as well. Remember this for the test. But if you can understand these basic principles, this is probably the most important step in learning how to control an airplane. And now that you know that, you have an understanding that most pilots don't get in the first ground lesson. In fact, that's a huge problem with a lot of ground courses these days: they'll teach you this stuff so you can pass a test, but they don't explain why you actually need to know this stuff. I don't care what other people say, you do need to know this stuff because when you get it, it's going to help you understand how to control your aircraft.
Okay, next, we're going to talk about how an airplane creates lift because if you want to learn how to fly, you also need to understand how the wings of an airplane work and how they pick up the aircraft and allow it to fly.
Now, you may have noticed this, but maybe not, but airplanes have these things called wings, and wings are an important part of creating lift. Actually, they're the only part. But anyway, our wings use two things to create lift: relative wind and the shape of the wing.
Now, I don't want to insult your intelligence, but as I mentioned a second ago, lift is an upward force that opposes the weight of your aircraft and everything on board. And remember, in steady, unaccelerated flight, lift equals weight. But if we want to climb or create more lift, we have to do something, and we'll get more into that in just a second. But for now, let's take a look at how the airplane creates lift.
Now, there are really two ways that an airplane creates lift, and we're going to discuss both. But for the written exam, they're mainly going to want you to know about something called Bernoulli's principle. So, let's take a closer look at that.
In a nutshell, Bernoulli's principle states that as the velocity of a moving fluid increases, its pressure within that fluid decreases. Bernoulli studied fluids and how they move through tubes and pipes, and what he found was that when a fluid moves through a narrow area in a tube, this forces the fluid to increase its speed. You'll sometimes hear these narrow areas referred to as Venturi, and these Venturi areas decrease the pressure when they increase the velocity of a fluid. In other words, an increase in speed causes a decrease in pressure.
Just like in a Venturi, the curved surface of the upper wing forces the air on top of the wing to travel faster than the air on the bottom of the wing. This creates an area of lower air pressure on the top of the wing and an area of high pressure on the bottom of the wing. And what do you think happens when you have low pressure on the top of the wing and high pressure on the bottom? That's right, that high pressure pushes the wing up, which creates an upward force, and this is what we call lift.
To increase that lift, we can do a few different things. One of the ways is to increase the speed of the air going over the top of the wing. We call this air going over the wing relative wind because this is a combination of multiple different types of wind. First, you've got the wind that's caused by the thrust of the aircraft, and as we increase the thrust that our aircraft is producing, the speed of the air going over the wings increases as well. Then you've got something called prop wash. This is basically the backwards wind that's caused by the propeller pushing the aircraft forward. This is like a fan blowing on you when it's hot outside. But you could also have the actual wind outside, which you have absolutely no control over. Sometimes you have a headwind that helps you create more lift, and sometimes you have a tailwind that takes away some of the relative wind. And this is actually one of the reasons why we take off and land into the wind; this gives us the most amount of relative wind possible so our wings can produce plenty of lift.
The other thing we can do is to change the shape of the wing, and we can do that a couple of different ways. Think back on how our wing is shaped if you're looking at it from the side of the aircraft. Another name for the side angle view of our wing is called an airfoil. And when we change the airfoil design and make it more curved on top, this increases something that we call the camber of the wing. This simply means that the air on top has to travel a further distance than the air on the bottom, so it must travel faster in order to meet the air on the other side. This creates even less pressure on the top of the wing, which creates more lift, but it also has more drag it has to overcome as well. So, increasing the camber increases our lift, but it also creates more induced drag. One of the ways we can increase the camber of our wings is by lowering the flaps on our aircraft. This lengthens that upper surface of the wing, which creates more lift, but it also increases the drag on our airplane.
Another way we can increase the lift that our wing produces is to increase the angle of attack. I'm going to go into a lot more detail on this in a few minutes, but in a nutshell, this just means that we're raising the nose of our aircraft up so that the relative wind has to travel a farther distance around the upper surface of the wing. And by raising our nose, our lift increases because that wind traveling over the top has to travel faster, just like it did when we increased the camber. But once again, increasing our angle of attack also increases our induced drag because we're exposing a bigger portion of the wing to the relative wind.
Another thing that creates lift is Newton's third law, which states that for every action, there is an equal and opposite reaction. We know that Bernoulli's principle creates lift by creating lower air pressure on the upper surface of the wing, but in addition to that, air can strike the lower surface of the wing and push it up. And when relative wind hits the lower surface of the wing, it pushes the wing up and back. And when we raise the wing to a higher angle of attack, there's more surface area on the bottom of the wing that's exposed. This allows relative wind to push the wing up and back even harder. So, as we increase our angle of attack, lift increases, but drag also increases when we do that, right?
Now, you might be thinking, "Who cares? I want to fly airplanes. I don't want to be a NASA engineer." Well, me neither. But check this out: now that we know the things that cause lift, we can start using that knowledge to control the aircraft. Let's say I'm trimmed up for level, unaccelerated flight, but I want to increase my lift. What's something I can do? Well, one option is to increase my airspeed. I don't even have to pull back on the yoke; I can increase my lift by simply increasing my airspeed. So, when I push the throttle up and I don't do anything else with the yoke, my altitude will increase because my wings are creating more lift.
Okay, so what's another way I can increase my lift? Well, another option is to increase the camber of my wings. So, let's say I lowered my flaps. When I do this, my airplane will start to climb because it's creating more lift. But something else happens when we do that as well. Yes, I'm creating more lift, but I'm also inducing more drag. So, my airspeed is going to decrease. So, increasing lift will also reduce my airspeed.
The last thing I can do if I want to create more lift is to increase my angle of attack, and this just means to pitch the nose of my aircraft up. And when I pitch up, you'll notice that this increases the angle of attack, and because of that, my airspeed will start to decrease here as well because I'm inducing more drag. So, once again, to create more lift, we can do three things: one, we can increase our airspeed; two, we can increase our angle of attack; and then three, we can increase the camber of our wing.
And now you know how an airplane creates lift, which is really important for all pilots to know. But before I leave you today, I want to discuss one more topic that's super important for all pilots to know as well, and that is stalls.
We just talked about lift and how we can increase our lift by increasing our angle of attack. But what would happen if we increase our angle of attack too much? Well, first of all, we'd be inducing a lot of drag, so this would make us fly super slow. And at first, this would make our airplane climb like a beast. But there is a point where if we increase our angle of attack too much, the airplane will just fall out of the sky. Oh boy, not a good time.
Now, remember, lift is good, but sometimes we intentionally want to reduce the amount of lift that our airplane is producing, and one of those times is when we're trying to descend for a landing. But something that is not good is losing a bunch of lift all at once. And when our airplane has a rapid loss of lift, this is what we call a stall.
To completely understand stalls and why they cause a rapid loss of lift, we need to take a closer look at the cross-section of a wing, and once again, this is what we call an airfoil. Up at the front of the airfoil, we have the leading edge. Then at the very back, we have the trailing edge. To understand this next part, let's draw an imaginary line from the leading edge of our wing down through the trailing edge, and we actually have a name for this imaginary line; it's called the chord line. And as our aircraft's wing moves forward through the air, that creates airflow over the wing in the opposite direction. As you may remember, this wind is partially caused by the aircraft's forward movement. It's also caused by wind from the propeller, or prop wash, and the actual wind can add or take away from this total wind that goes over our wings. And once again, this total wind is what we call relative wind, and this is what causes our wing to create lift.
Now, when your airplane changes its direction of travel, like when it pitches up or when it pitches down, the relative wind changes as well, because relative wind directly opposes the direction that your airplane is traveling. So, you could say that relative wind acts on the airfoil at different angles, and this is what we call the angle of attack. More specifically, the angle of attack is the angle between the relative wind and the wing's chord line. And this is super important for you to understand.
Remember, when we increase the relative wind or when we increase the angle of attack on our wing, we also increase the lift that our wing produces, as long as there's smooth air flowing around the upper and lower surfaces of the wing. You may not know this, but as air flows above and below the wings, it actually does so in very thin layers. We call this laminar, or airflow, because the definition of the word laminar is thin horizontal layers. Yeah, I guess that may makes sense then. But anyway, at low angles of attack, the wind moving around the wings flows smoothly, and that allows all those little layers to stay separated and flow around the wing in a nice orderly fashion.
But as we increase the angle of attack on our wing, once again, this increases our lift because the air on top of the wing has a further distance to travel, and that creates less air pressure up there. But if we start increasing our angle of attack too much, that air doesn't want to flow around the upper surface of the wing anymore; it wants to flow straight back. And because of that, this air becomes separated from the upper surface of the wing. This is what we call boundary layer separation. And when this happens, the air on the back side of the wing is no longer flowing in a nice orderly fashion; it's kind of doing its own thing, and it's just spinning around back there. It's also breaking up our nice neat little layers and just making a huge mess. So, those nice neat layers aren't nice and neat anymore, and that means we no longer have good laminar flow. And that's not neat at all; that's pretty neat.
Now, even at these high angles of attack where we've got all this turbulence at the back of the wings, they can still produce some lift. Even though we don't have good laminar flow, and as long as the lift that the wing is producing is greater than or equal to the weight of our aircraft, we can just chill these high angles of attack, and the airplane will still fly.
With that in mind, if we continue to increase the angle of attack on our wing, at some point, the wing will no longer be able to support the weight of our aircraft. And when that happens, we'll get a rapid loss of lift. This rapid loss of lift is what we're calling a stall.
Now, something super important for you to keep in mind is that every airplane has a specific angle of attack that it stalls at. And with that in mind, altitude and the gross weight of your aircraft can affect the airspeed that your airplane stalls at, but the angle of attack is always the same.
Now, right before the airplane stalls, it will reach a point where the chord line on the airfoil is at its greatest angle with a relative wind. If this angle increases at all beyond this point, the aircraft will stall. This maximum angle is called the critical angle of attack, and when you exceed the critical angle of attack, this is when your airplane will stall, every single time. In fact, if you don't remember anything else from this lesson, this is the one thing you should remember: when you exceed your wing's critical angle of attack, the airplane will stall. The airspeed at which you stall can change a little bit depending on your altitude, your aircraft weight, and your configuration, but it will always stall at the same angle of attack. And if you're studying for the written exam, this is something really important for you to remember. The FAA wants to make sure that all pilots understand this before they earn their private pilot certificate.
But there's one more thing that you really need to know about stalls. When you exceed the airplane's critical angle of attack and it stalls, the wings are still producing some lift. If they weren't, you would literally fall to your death. Anytime you stall the aircraft, so our wings are still producing some lift. If that's true, how do you think we can fix a stall if we exceed the critical angle of attack? Yeah, all we have to do is reduce the angle of attack. So, to break a stall, the most important thing we can do is to reduce our angle of attack by pitching down or releasing the back stick pressure on the yoke. This will get us below our critical angle of attack so the wings can start producing lift again.
Keep in mind, accidental stalls are some of the scariest situations you can get yourself into. Because of that, you're going to be practicing some different stall recoveries when you start your flight training. And accidental stalls can happen at any point during a flight, but they're most likely to happen when you're flying at a slow airspeed. And because of that, during your training, you're going to be practicing stall recoveries during phases of flight where you typically fly at slow airspeeds.
One of these critical times when you'll be flying at a slow airspeed is during a landing. This means that you'll be practicing something called a power-off stall to simulate that you've stalled the aircraft during an approach. Now, we train you how to do this at a higher altitude, that way, if you do stall during a landing, it's going to be second nature for you to recover the aircraft.
Another time when you can be operating near your critical angle of attack is during a takeoff. During a takeoff, you typically have full power, so we practice takeoff stalls with full power applied. This is what we call a power-on stall. And once again, we do this at a safe altitude so you can learn to recognize when you're getting close to that stall and how to fix the problem before you die. Because believe me, dying is no fun unless you're into that, you weirdo.
All right, those are the first three lessons on aerodynamics that you need to know in order to become a private pilot. But remember, if you need an endorsement for the written exam, be sure to check out my premium ground course at freepilottraining.net, and that's only $50, which is a fraction of the cost of almost every other ground course out there, and I guarantee you, you won't be disappointed with that course.
Thank you so much for listening today, and I hope this training helps you master that written exam and gives you an awesome foundation for your flight training. See ya.