Understanding Lift - As Non-technical as possible, Bottom-up
Posted: 02 Jul 2016, 07:08
Yes, this is my take on the topic that is the best way to start a bar fight in aviation community.
Kidding aside, it is astonishing how poorly the lift is most often explained. Or actually why it even needs all these explanations, maybe it is that it can mysteriously point upwards that makes it a difficult concept, we being used to things falling down primarily. If I asked someone if he understood how the rudder of a boat works, he likely answers "Of course!" Explaining the same force when it points upwards seemingly requires all kinds of fancy explanations, many of them being just wrong.
There are "Newton-based explanations" and "Bernoulli-based" explanations and so on and so forth. I dislike them all, because they hardly explain anything but are just names for some relationships that play their parts. We are very apt in naming things, and then learning those names so we can supposedly communicate ideas instead of understanding them. Very often these ideas tend to a pitfall in attempting to explain a phenomena over a few sentences where no such explanation exists.
So, to begin our coffee table discussion, have a deep breath and think about it: what is that stuff you actually breath in?
Air is made up of molecules, tiny groups of particles. These particles are so tiny that we don't even want to consider the number of them in that breath you took, it would be enormous! They are actually so tiny that if we looked close enough, the rules that seemingly govern our visible world start to break up with them. However, for a very good approximation, we can say that they are moving around like very tiny balls, moving into random directions and with various velocities, bouncing from each other and from the objects they collide into - just like any balls with some mass and velocity would if allowed for perpetual motion. There are billions of them everywhere, and they are constantly colliding with each other - actually, the mean free path an air molecule travels before colliding into another in room temperature at sea level is in order of 68 nanometers - that's about 2.7 millionths of an inch! Imagine all that going on all around you - and within you!
When these countless tiny particles constantly collide against a wall for instance, we feel that as pressure, the momentum of them. The faster they travel on average, harder they punch the molecules and atoms of the wall, making them jiggle too - we feel that as temperature. One can imagine from that the temperature and pressure of a gas are inherently related. And indeed they are.
The air is constantly exerting pressure on every object immersed in it - and importantly, to itself! Understanding this is the key to understanding lift, transforming it from a mysterious effect into a necessary result of the world around us. It is so important that we want to imagine the mechanism a bit better.
Imagine a solid object going very fast through the atmosphere, say, a cannon shell. If the air was something like sand, it would plow a void in it, which may or may not collapse. But due to the air being made up of these constantly jiggling tiny little particles colliding with each other and everything, something very interesting happens! When the shell passes, it indeed leaves a void behind it, for it went by very fast. Now, some of the air molecules are, by random chance, traveling into that direction. Some ricochet from the other particles into that direction. But in that void, there are no other molecules to collide with! Therefore, on average, the air molecules can travel farther into that direction. They tend to fill up the void, by mere randomness!
Essentially the pressure, as we saw, tells us how much of momentum the colliding particles throw against an object or the air itself. If we had an area of low pressure formed, on average the air molecules which were bounced into that direction were both less often and less forcefully bounced back into other directions. This creates this magnificent property of the air: it tends to flow towards lower pressures! And because of the sheer amount of the particles in it, and the tiny scale of their collisions, we feel the average movement as a flow of matter, behaving as if it was continuous fluid. But to understand the tricks the fluid does, it is necessary to understand what it truly is.
Why the void is not instantly, or extremely quickly at least, filled up? This is because the air, like any matter, has mass. Like Sir Isaac Newton described, the mass and a force interact in accelerating bodies: the mass resisting the change in momentum and the force...well, forcing it. In this ping ball game this is satisfied by the fact that only so-and-so many of the gas molecules can be bounced towards the lesser resistance in a given time, and the resulting flow towards the lower pressure actually reduces the availability of the molecules punching others towards it. So, the amount of the molecules traveling or punching others towards the lower pressure actually reduces when there is flow towards the lower pressure. Effectively the flow takes time to accelerate like any mass. A given drop in pressure results in molecules, on average, accelerating towards it at a predictable rate, the pressure differential pushing the molecules while the mass of the mass of them holding back the acceleration. If the resulting flow was directed towards some air that was stationary on average, the molecules would pile up, kicking back from the increased resistance, and on average, slowing down into that direction.
Sounds familiar, huh? Lower pressure - speeding up, and vice versa. That's the Bernoulli principle! Essentially it is just an extension of Newton's description on how forces accelerate masses applied to the structure of air (or any fluid). In that sense, the "complete explanations" based on "either" the Newton's or Bernoulli's principles are BS, for the principles themselves relate to each other.
Now a Piper Aerostar comes, plowing its way through the immense spectacle created by these molecules. The molecules actually feel its approach in advance. The situation is actually very complicated, but you can imagine how it tends to punch some molecules forward, like a baseball bat. These immediately collide with other molecules, passing the blow ahead at some definite speed dependent on the amount of jiggling and bouncing going on. That would be the speed of sound, and we ought to figure out it must be dependent of the temperature, which measured the average jiggling and bouncing going on, in a way that this blow is transferred ahead in the air slower if there was less jiggling and bouncing going on and vice versa. And indeed, that's exactly how it turns out to be! This also shows that the air has capability of being affected over distances: the information of something happening travels through it, transmitted by that jiggling and bouncing going on. It can really be used to transmit information, in a way of sound.
At around that Aerostar, these properties of air have some interesting and very useful consequences. The airplane has its wings, which are smoothly formed planes that are flown through the air at some angle of attack. The leading edge bounces the molecules away from it, them hitting each other, piling up the air. This puts some extra-pressure to the leading edge, felt as drag because it is un-countered by a similar piling effect on the trailing edge of course. But the air, due to all that jiggling and bouncing going on, wants to get somewhere! It starts to flow around the object, splitting up at the leading edge. Now, the surfaces of the wing...let us say the upper surface to pick an example, form a smooth curve from the stagnation point at the leading edge - the exact center around which the piling effect takes place, and where the molecules have some hard time "deciding" which way to go, eventually getting bounced into one or another. Very quickly the curve of the wing's upper surface starts to curve away from the direction where any molecules were directly bounced by the wing. O-oh.
But the air had this magnificent property! The air molecules very nearby, by their random chances are flying towards the wing and kick others towards it in millions of collisions every millimeter. The air turns to fill up the void which would form otherwise. The flow tends to stick to the surface that way, and there tends to be a smooth, constant deficiency of molecules hitting each other and the wing's upper surface for it being tilted away, and in this case, is nicely curved (it doesn't has to be, but it works much better if it is!). This is felt as a lower pressure on the lift-side of the wing. And because there is that deficiency of molecules the others travel to equalize - thereby the flow, the average movement we feel, speeds up to be there. On the other side of the wing, that is more directed against the flow than away from it in comparison, the molecules hit against it somewhat like before, or may even be piling up a bit. The difference of the molecules hitting the wing is felt as a difference in pressures.
That difference in pressures is called the lift.
By the time the flow hits the trailing edge, there is no more wing to bounce against - but there is the air from the other side! This equalization of the average momenta takes place in the flow having an average velocity downwards at the trailing edge, with both sides of the wings pointing the flow into a common direction, but with the higher pressure wanting to equalize where there is no this common direction of the flow - that would be around the wing tips! A molecule just outside the wing tip only sees the inrush to fill the lower pressure, so it joins that, by being bounced by the other molecules. That's how the wingtip vortices are formed.
The pressure difference along the wing in flight is actually sustained, that is, not allowed to fill up, by the mass of the air resisting the flow to equalize the pressure. If the air had no "weight", it would not be able to sustain that pressure differential, filling it up instantly, and there would be no flight that way (of course, it would have no pressure without weight in atmosphere neither!). The deficiency of the molecules - the low pressure - is therefore sustained by the air's slowness to accelerate as described by Newton for any object, the relationship of these two known as Bernoulli principle. By sheer imagination of the nature, these all, these tiny little "balls" jiggling and bouncing around, the total amount of them getting directed some amount against the lift direction (down if lift was up), them bouncing against the wing and each other when getting pushed there, creating flow acceleration, pressure differentials, even temperature changes as is suggested by those occasional fog clouds over the wings, effective means of transportation and immense amounts of fun, all work out in perfect balance where there would be no one effect without another. It works so perfectly to satisfy what we dare to call the rules of physics - the Newton's laws, the conservation of momentum, the conservation of energy, and most importantly, the numbers of the flight manual and the FAA certification requirements, that it is easy to forget what is behind the names of the phenomena.
So, quite a bit is left out of this, but I'll be back as someone once (or twice or more!) said! It is noteworthy that I could only add stuff, in my mind there is not much to take away. There are no short, easy explanations to much anything in the nature, for the very way she behaves is the tend for a balance, by definition involving all the aspects which are connected in a way or another.
Every so often that strikes me, and stretches my imagination: watching the flags to fly in the wind, and the birds and the airplanes in the air - all that actually happens by mere randomness in a scale so immense that it just has to work that way.
-Esa
Kidding aside, it is astonishing how poorly the lift is most often explained. Or actually why it even needs all these explanations, maybe it is that it can mysteriously point upwards that makes it a difficult concept, we being used to things falling down primarily. If I asked someone if he understood how the rudder of a boat works, he likely answers "Of course!" Explaining the same force when it points upwards seemingly requires all kinds of fancy explanations, many of them being just wrong.
There are "Newton-based explanations" and "Bernoulli-based" explanations and so on and so forth. I dislike them all, because they hardly explain anything but are just names for some relationships that play their parts. We are very apt in naming things, and then learning those names so we can supposedly communicate ideas instead of understanding them. Very often these ideas tend to a pitfall in attempting to explain a phenomena over a few sentences where no such explanation exists.
So, to begin our coffee table discussion, have a deep breath and think about it: what is that stuff you actually breath in?
Air is made up of molecules, tiny groups of particles. These particles are so tiny that we don't even want to consider the number of them in that breath you took, it would be enormous! They are actually so tiny that if we looked close enough, the rules that seemingly govern our visible world start to break up with them. However, for a very good approximation, we can say that they are moving around like very tiny balls, moving into random directions and with various velocities, bouncing from each other and from the objects they collide into - just like any balls with some mass and velocity would if allowed for perpetual motion. There are billions of them everywhere, and they are constantly colliding with each other - actually, the mean free path an air molecule travels before colliding into another in room temperature at sea level is in order of 68 nanometers - that's about 2.7 millionths of an inch! Imagine all that going on all around you - and within you!
When these countless tiny particles constantly collide against a wall for instance, we feel that as pressure, the momentum of them. The faster they travel on average, harder they punch the molecules and atoms of the wall, making them jiggle too - we feel that as temperature. One can imagine from that the temperature and pressure of a gas are inherently related. And indeed they are.
The air is constantly exerting pressure on every object immersed in it - and importantly, to itself! Understanding this is the key to understanding lift, transforming it from a mysterious effect into a necessary result of the world around us. It is so important that we want to imagine the mechanism a bit better.
Imagine a solid object going very fast through the atmosphere, say, a cannon shell. If the air was something like sand, it would plow a void in it, which may or may not collapse. But due to the air being made up of these constantly jiggling tiny little particles colliding with each other and everything, something very interesting happens! When the shell passes, it indeed leaves a void behind it, for it went by very fast. Now, some of the air molecules are, by random chance, traveling into that direction. Some ricochet from the other particles into that direction. But in that void, there are no other molecules to collide with! Therefore, on average, the air molecules can travel farther into that direction. They tend to fill up the void, by mere randomness!
Essentially the pressure, as we saw, tells us how much of momentum the colliding particles throw against an object or the air itself. If we had an area of low pressure formed, on average the air molecules which were bounced into that direction were both less often and less forcefully bounced back into other directions. This creates this magnificent property of the air: it tends to flow towards lower pressures! And because of the sheer amount of the particles in it, and the tiny scale of their collisions, we feel the average movement as a flow of matter, behaving as if it was continuous fluid. But to understand the tricks the fluid does, it is necessary to understand what it truly is.
Why the void is not instantly, or extremely quickly at least, filled up? This is because the air, like any matter, has mass. Like Sir Isaac Newton described, the mass and a force interact in accelerating bodies: the mass resisting the change in momentum and the force...well, forcing it. In this ping ball game this is satisfied by the fact that only so-and-so many of the gas molecules can be bounced towards the lesser resistance in a given time, and the resulting flow towards the lower pressure actually reduces the availability of the molecules punching others towards it. So, the amount of the molecules traveling or punching others towards the lower pressure actually reduces when there is flow towards the lower pressure. Effectively the flow takes time to accelerate like any mass. A given drop in pressure results in molecules, on average, accelerating towards it at a predictable rate, the pressure differential pushing the molecules while the mass of the mass of them holding back the acceleration. If the resulting flow was directed towards some air that was stationary on average, the molecules would pile up, kicking back from the increased resistance, and on average, slowing down into that direction.
Sounds familiar, huh? Lower pressure - speeding up, and vice versa. That's the Bernoulli principle! Essentially it is just an extension of Newton's description on how forces accelerate masses applied to the structure of air (or any fluid). In that sense, the "complete explanations" based on "either" the Newton's or Bernoulli's principles are BS, for the principles themselves relate to each other.
Now a Piper Aerostar comes, plowing its way through the immense spectacle created by these molecules. The molecules actually feel its approach in advance. The situation is actually very complicated, but you can imagine how it tends to punch some molecules forward, like a baseball bat. These immediately collide with other molecules, passing the blow ahead at some definite speed dependent on the amount of jiggling and bouncing going on. That would be the speed of sound, and we ought to figure out it must be dependent of the temperature, which measured the average jiggling and bouncing going on, in a way that this blow is transferred ahead in the air slower if there was less jiggling and bouncing going on and vice versa. And indeed, that's exactly how it turns out to be! This also shows that the air has capability of being affected over distances: the information of something happening travels through it, transmitted by that jiggling and bouncing going on. It can really be used to transmit information, in a way of sound.
At around that Aerostar, these properties of air have some interesting and very useful consequences. The airplane has its wings, which are smoothly formed planes that are flown through the air at some angle of attack. The leading edge bounces the molecules away from it, them hitting each other, piling up the air. This puts some extra-pressure to the leading edge, felt as drag because it is un-countered by a similar piling effect on the trailing edge of course. But the air, due to all that jiggling and bouncing going on, wants to get somewhere! It starts to flow around the object, splitting up at the leading edge. Now, the surfaces of the wing...let us say the upper surface to pick an example, form a smooth curve from the stagnation point at the leading edge - the exact center around which the piling effect takes place, and where the molecules have some hard time "deciding" which way to go, eventually getting bounced into one or another. Very quickly the curve of the wing's upper surface starts to curve away from the direction where any molecules were directly bounced by the wing. O-oh.
But the air had this magnificent property! The air molecules very nearby, by their random chances are flying towards the wing and kick others towards it in millions of collisions every millimeter. The air turns to fill up the void which would form otherwise. The flow tends to stick to the surface that way, and there tends to be a smooth, constant deficiency of molecules hitting each other and the wing's upper surface for it being tilted away, and in this case, is nicely curved (it doesn't has to be, but it works much better if it is!). This is felt as a lower pressure on the lift-side of the wing. And because there is that deficiency of molecules the others travel to equalize - thereby the flow, the average movement we feel, speeds up to be there. On the other side of the wing, that is more directed against the flow than away from it in comparison, the molecules hit against it somewhat like before, or may even be piling up a bit. The difference of the molecules hitting the wing is felt as a difference in pressures.
That difference in pressures is called the lift.
By the time the flow hits the trailing edge, there is no more wing to bounce against - but there is the air from the other side! This equalization of the average momenta takes place in the flow having an average velocity downwards at the trailing edge, with both sides of the wings pointing the flow into a common direction, but with the higher pressure wanting to equalize where there is no this common direction of the flow - that would be around the wing tips! A molecule just outside the wing tip only sees the inrush to fill the lower pressure, so it joins that, by being bounced by the other molecules. That's how the wingtip vortices are formed.
The pressure difference along the wing in flight is actually sustained, that is, not allowed to fill up, by the mass of the air resisting the flow to equalize the pressure. If the air had no "weight", it would not be able to sustain that pressure differential, filling it up instantly, and there would be no flight that way (of course, it would have no pressure without weight in atmosphere neither!). The deficiency of the molecules - the low pressure - is therefore sustained by the air's slowness to accelerate as described by Newton for any object, the relationship of these two known as Bernoulli principle. By sheer imagination of the nature, these all, these tiny little "balls" jiggling and bouncing around, the total amount of them getting directed some amount against the lift direction (down if lift was up), them bouncing against the wing and each other when getting pushed there, creating flow acceleration, pressure differentials, even temperature changes as is suggested by those occasional fog clouds over the wings, effective means of transportation and immense amounts of fun, all work out in perfect balance where there would be no one effect without another. It works so perfectly to satisfy what we dare to call the rules of physics - the Newton's laws, the conservation of momentum, the conservation of energy, and most importantly, the numbers of the flight manual and the FAA certification requirements, that it is easy to forget what is behind the names of the phenomena.
So, quite a bit is left out of this, but I'll be back as someone once (or twice or more!) said! It is noteworthy that I could only add stuff, in my mind there is not much to take away. There are no short, easy explanations to much anything in the nature, for the very way she behaves is the tend for a balance, by definition involving all the aspects which are connected in a way or another.
Every so often that strikes me, and stretches my imagination: watching the flags to fly in the wind, and the birds and the airplanes in the air - all that actually happens by mere randomness in a scale so immense that it just has to work that way.
-Esa