The following consists of a talk Alarn is putting together as an introduction to the principles of flight:
It is all meant to be spoken directly by Alarn to reflect someone talking somewhat off the cuff instead of reading a rigidly prepared script, emotes will be added at a later date once the text is refined.
Almost everyone here has seen one of the alliance’s fine flying machines and wondered what makes them fly. Unfortunately, some of the commonly accepted answers to this question are either misleading or just entirely incorrect. As an example, many descriptions of the physics of lift focus on the shape of the wing’s longitudinal cross-section, being the aerofoil. We’ve all seen that these wings have a bulge on the top and are flat on the bottom so therefore the air must travel farther over the top than under the wing yet we know perfectly well that these wings still defy gravity when upside down; a paradox that is sometimes explained away by having special rules for inverted flight. In reality, the shape of the wing has little to do with how lift is generated and instead determines the wing’s efficiency and characteristics under certain flight conditions.
As a further counter to this description of lift, many aerofoils today are in fact symmetrical, some are ‘supercritical’ where the flat section is on the top and you could even generate lift from a perfectly flat plate.
Many of the incorrect explanations of lift depend on a principle called ‘equal transit time’ which incorrectly asserts that the air going around a wing must take the same amount of time regardless of whether it travelled over the top or bottom; therefore the air going over the top surface of the wing must travel faster than the air travelling over the bottom surface, which has a shorter distance to cover. It is well understood that gas travelling at a higher speed has a lower pressure so this must result in an area of suction where the air is travelling fastest.
While it is correct that a region of low pressure exists on the lift producing side of an aerofoil, to say that this is because of ‘equal transit times’ is completely incorrect, in fact the air flowing around the top side of an aerofoil will reach the trailing edge quicker than if it went around the bottom.
So then, how does a wing actually produce lift?
To begin to understand this we must remember some well-known laws of physics.
“For every action there is an equal and opposite reaction”
And “Every object will remain in its state of rest of uniform motion in a straight line unless it is compelled to change that state by forces acting on it”
As an example, I am lifting this weight –an action, in turn the weight pushes down on me –the reaction; in the absence of some magical intervention, this weight can’t just hover here, it needs to act against something which will push back, to resist gravity. The same is true for a wing producing lift except that it’s pushing down on the air, a whole lot of air! The air that flows over the wing, is pushed down, bending its path into what we call downwash.
The second law we mentioned states that for the air to be bent down (the action) there needs to be a force, pushing down, of course there then must be a reaction to that force which pushes up. This force is equal in magnitude to the change in momentum experienced by the huge amount of air being diverted. Since momentum is defined as mass times velocity, to produce more lift a wing can either push down a greater mass of air, push it down with a greater vertical velocity or both. It’s this downwards vertical velocity that gives the wing lift.
It really is that simple.
Now, what I haven’t mentioned is that the airflow isn’t perfectly parallel with the chord line of the wing; there is some small angle between them called the angle of attack. For the wing to produce lift, it needs a positive angle of attack meaning that the wing is pointing up, to some degree. There are exceptions but that’s generally true.
So, just how much air needs to be moved? We can demonstrate that with a simple back of the envelope calculation. Say we have a small aeroplane in steady level flight that weighs 1,000 kilograms or about 2,200 pounds, it’s travelling at a speed of 220 kilometres per hour or 140 miles per hour and is flying at an angle of attack of five degrees. Now, we’re going to assume that from the perspective of the pilot, the downwash departs the wing at the at the aircraft’s angle of attack of five degrees; while this is not strictly true, it is close enough for this specific example.
So, we know the aircraft’s speed as well as the angle at which the downwash is deflected, from trigonometry we can calculate the downwash’s vertical speed to be around 18 kilometres per hour or 11.5 miles per hour, at the wing. If we assume that the average vertical velocity of all the air in the downwash is half of this, we can calculate that the mass of air deflected is about five metric tonnes per second or 11 thousand pounds every second. That’s nearly four times the mass of the aircraft, every single second; if this much air was picked up by a giant scoop on top of the wing, it would have to be 7.3 meters or 18 feet tall!
But how does it move that much air?
Well, if the air didn’t bend; as the wing was moving through the air it would push air aside and create a void behind it. As we know, nature abhors a vacuum so the surrounding air must be sucked in to fill this space. This bends the air around the top of the wing and pulls in air above it; this attraction is what results in the region of low pressure above the wing as well as the downward acceleration of the downwash, I mentioned earlier.
To reiterate, this low pressure region accelerates air over the top of the wing, this then sucks up air in front of the wing, what we call upwash, and then shoots it out behind the wing and downwards. Of course, when some packet of air moves away, the other air around it must rush in to fill its place. While it is true that the Kutta-Joukowski theorem stipulates that if you know the magnitude of this circulation, you can calculate the lift produced by an aerofoil; the circulation is not the driving force behind lift but is a consequence of it.
So that’s some of the how, now how about the why?
There’s a simple experiment you can do at home, all you need is a round drinking glass and a small stream of water, from a tap. What you do is hold the glass horizontal and move it so that the very edge of its circular shape just barely touches the running water. What you’ll notice is that the water is not pushed away from the glass but is instead attracted toward it and wraps around the outside of the glass before shooting off into your kitchen sink. Now, we know from the physical laws stated earlier that for the water to be bent, there must be a force acting on it and that force is in the direction of where it’s bending; we also know that there must be an equal and opposite force acting on the glass. You might not be able to feel it, but the water is in fact pulling the glass towards itself, rather than pushing it away as one might expect.
Of course, our flying machines don’t soar on cylindrical wings, brushing against jets of water but the physical principles are the same. Sure, that’s all very interesting but it still hasn’t answered why.
Well, the short answer is that it’s because of viscous interactions, pressure and the fact that air is, for the most part, incompressible; at low speeds. Of course, if you fill a piston with air and squish it, it will compress but that’s not what I’m talking about here. If I was to swing my arm about, the air in front of my arm won’t get squished but will instead move out of the way; in that sense we say the flow is incompressible, having a density that is invariant with space. Now, if my arm was travelling at or near the speed of sound, that would be a different story all together and the air flow would be quite compressible; in fact we’d see shockwaves forming and I’d get cooked by aerodynamic heating but that’s neither here nor there.
The long answer is… a bit longer and comes in several chunks. Firstly, due to the incompressible nature of the flow, the volume of the fluid packets stays constant and instead of getting stretched thin and forming voids or squishing up, it simply moves around. Secondly, the ‘streamlines’ communicate with each other. What the heck is a streamline? Well, if we were to take a single particle of fluid and trace out its path, that would be a streamline; while nearby streamlines influence one another, there is no fluid flow between them. Now, these streamlines communicate with one another through pressure and viscosity; pressure is of course the force per unit area that one packet of fluid exerts on its neighbours and viscosity is like friction but between fluid molecules. These streamlines can be thought of as layers, when one is moving, viscosity will then pull on the adjacent layers so they will also move, to a lesser extent, before then acting on their adjacent layers and so on until we’ve moved so far away from the initial layer that its influence is imperceptible. Now, fluid at the very surface of the wing tends to stick and move with the aerofoil, this then slows down the next layer of fluid, which acts on the next and so on; giving what we call a boundary layer. Now imagine, the very first streamline that isn’t stuck to the surface just barely touches the highest point in the aerofoil; if it were then to continue heading straight in the ‘freestream’ direction, the aerofoil surface would sink away from it creating a volume of still air, from the perspective of the aerofoil. Now, viscous interactions between our moving layer and the still air will pull that still air away from the surface and unless we have more air rushing in to replace it, that will result in a lowering of pressure. This lower pressure will then bend our streamline down until it follows the surface; the same process also bends the next layer down onto the first and then the next after that, all the way out until the forces involved become so small they may as well be non-existent.
Therefore, a whole heap of fluid is bent over the top of a wing by the lowering of pressure which propagates out throughout the entirety of the fluid at the speed of sound, which is in essence the speed of fluid communication.
To summarize, the bending of the air causes the reduction in pressure and this reduction in pressure causes the air to be accelerated. Furthermore, this bending and acceleration requires a force which acts in the direction the fluid is bent towards and for the wing to apply a force onto the fluid there must also be an equal and opposite force acting onto the wing, these forces are transferred to the wing via pressure fields.
But our story does not end here.
In all types of wings, be they a conventional shape, symmetrical or merely a flat plate; air is pulled down from above and ejected into the downwash. Regardless of their shape, what they all have in common is an angle of attack with respect to the incident airflow, this angle of attack has the greatest influence on how much lift a wing will produce.
Although this is not true in all cases, such as with cambered aerofoils a simple way of thinking about it would be to say that when the angle of attack is zero, the wing produces no lift, at a positive value it produces lift and at a negative angle, with the wing pointing down, it produces downforce. The amount of lift produced is proportional to the angle of attack when flying under conditions that we call the ‘linear region’, which typically is where all aircraft operate; excursions beyond this flight regime can result in… very bad things.
To reiterate, angle of attack is the most important factor determining how much lift a wing will produce, the aerofoil’s shape only influences the lifting efficiency and what the maximum amount of lift it can produce is before you depart that linear flight regime and experience undesirable flight characteristics, namely a sudden loss of lift coupled with a substantial increase in drag.
Topics to come:
+Power required for lift
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