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  • 3WE
    replied
    Originally posted by Gabriel View Post
    Loss of vertical damping. AF had lift approx = weight most of the time.
    It was unable to produce more lift to get upward acceleration- which could happen on an unstalled wing.

    The crisp descent initiated when lift the was lost when it stalled

    (And yes, the accelerations described are transient- some lift is restored from higher downward velocity dragging the wings upward- but, the stalled wing can’t get enough lift to arrest the descent NOR ADDITIONALLY gain back altitude.)

    Leave a comment:


  • Gabriel
    replied
    Originally posted by 3WE View Post
    Gonna argue this one...

    I think “loss of lift” is very much what did in Air France. Not_loss of control...

    It’s also a major issue with the forgiving Cessnas. Tommys, not so much.
    Loss of vertical damping. AF had lift approx = weight most of the time.

    Leave a comment:


  • Gabriel
    replied
    This is the best material FOR PILOTS that I have ever found.

    It was written by a person pretty much related with science both from the academic (he has a PhD) and professional (worked in rhe AT&T Bell Laboratories) point of views, in both cases involving Physics (and other stuff) but not aerodynamics.

    He is also a Commercial Pilot, Flight Instructor, and Ground Instructor. He also is (or was?) an FAA Aviation Safety Counselor.

    Undoubtedly, he could use his experience and knowledge in aviation coupled with his scientific approach to prepare this material in a way that is correct, avoiding all of the common misconceptions, errors and voids, and yet you don;t need to solve the Navier Stokes equations to understand it. Right concepts, explained mostly conceptually, with not a lot of Maths (and where Maths is uses it is mostly for practical tricks like how much you can climb if you bleed X knots of speed rather than to solve the Physics, although you have a bit of that too).

    From this work I, don't know if learned, but realized, for example that the loss of lift is not your biggest problem in a stall, and that to recover of a spiral dive you should not do it as if it was a normal dive.

    THIS is the kind of stuff that I would like every pilot to know and understand (which is not the same).

    The material is quite old, the site is a simple HTML page, it is not filled with eye-catching slides and videos, so it might be a bit boring for current standards. But it is great material. As I said, the best I found for pilots.

    And it is free.

    How is the work named? "See how it flies".

    Add the picture in the "cover"?

    Click image for larger version

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    https://www.av8n.com/how/

    Leave a comment:


  • 3WE
    replied
    Originally posted by Gabriel View Post
    Myths and misconceptions:
    - The dangers of stall (hint, loss of lift is not your main problem).
    Gonna argue this one...

    I think “loss of lift” is very much what did in Air France. Not_loss of control...

    It’s also a major issue with the forgiving Cessnas. Tommys, not so much.

    Leave a comment:


  • 3WE
    replied
    Evan:

    Please read stick and rudder.
    Please ride a bicycle.

    They are insightful.

    I too cringe that you continue to say “we don’t know how lift is generated”.

    Leave a comment:


  • Gabriel
    replied
    Originally posted by Evan View Post
    However... if we conduct a one minute Google search...
    Oh, a google search of "some other site" and "this one also". Proof that the scientific community doesn't agree and have competing theories on how lift is generated.

    Professor Holger Babinsky, Cambridge University Department of Engineering is right. These pressure differences is what makes the wing push the air down and what makes the air push the wing up. Pressure is the way that fluids make and receive force (also viscous friction). "The wing pushed the air down" and "The air pushed the wing up" HAS TO MEAN that the pressure above the wing is lower than below the wing. In which other way would this action-reaction pair take place?

    A second-year MIT student is also right. The viscous nature of the flow in the boundary layer is what causes the separation in the trialing edge, the separation of the flow in the trailing edge is what causes the circulation, and this circulation is what causes the air to go faster above than below, and this speed difference is what causes the pressure difference, and this pressure difference is what pushes the air down and the wing up, and that is lift.

    I explained all this in the post #33 I believe.

    "Some other site" doesn't understand Bernoulli or lift at all. The speed will be higher and the pressure will be smaller near the upper surface than farther up. What he says is more fucked up than the air meeting at the trailing edge which, although not true, at least would explain some lift if it was true (not as we know it). This "other site's" explanation, if you think of it, has the being pushed up by the wing, not down.

    And yes, "this one also" is correct that the Meeting-at-the trailing-edge theory is a persistent myth (outside academy). Why? I don't know. It is known to be wrong since the times that the fluidmechanicist got the Kutta condition right and discovered that they were wrong about generation of lift being impossible. The very first physical/mathematical models that were able to predict lift (in the very early 20th century, around the same time Einstein was publishing the Special Theory or Relativity and the Wright Brothers were achieving the 1st powered flight) have the air not-meeting-together at the exit. Do you know what other very persistent myths are out there? The turning-to-downwind dangers and the pendulum effect of high-wing planes. Einstein believed the equal-transit? I'd love to see that. It would be more interesting than surprising. He basically invented quantum mechanics with his electrodynamics theory that won him the Nobel prize (published before any relativity stuff) and yet later got all Quntum Mechanics wrong. He even got wrong the cosmological constant in his General Relativity equation, twice: First by postulating that such a constant needed to exist to keep the universe basically static (not contracting nor expanding) and later, when Hubble proved that the universe was expanding, by saying that the cosmological constant was the biggest blunder of his career and deleting it from the equation (and now we are using it back to account for dark energy and how the universe is not only expanding but doing so at an ever faster rate too).

    You know why? Because:
    a) Einstein was not an expert in Quantum Mechanics
    b) Einstein was not an expert in Mechanics of Fluids and
    c) Even experts are wrong some times (that's how he got the cosmological constant wrong in a field where he was the expert).

    And c) applies even for the greatest known genius that the Universe had come up with so far.

    Again, Gabriel, I'm not challenging your expertise. I'm pointing out how confusing it is out there for someone who is not aerodynamicist, fluidmechanicist or aerospace engineer to sort this all out, because a clear explanation with widespread consensus doesn't seem to exist outside of the engineering world, and more specifically is not well-known to those people who fly commercial airplanes for a living.
    That I can agree with, it happens with many things. People is in general ignorant and there are myths and misunderstandings around everything.

    But this is what I am complaining about:

    [How lift is generated] is also a debatable science based on prediction and observation, which fosters argument within the science community itself and leads to somewhat divergent theories. [...] Nobody absolutely knows How Airplanes Fly
    There may be [ok, there are] myths and misunderstandings in the general public and among some pilots and engineers. But the science behind the generation of lift is not debatable, it is as based on prediction and observation as any science (look up the scientific method), it doesn't foster any argument within the scientific community, there are no divergent scientific theories, and some people knows so well how airplane flies that they can model a FULL airplane and fly it in the simulator before the first rivet is built, only to find out that, when the plane is finally built and test flown, it matches the model surprisingly well. What else do you need to give credit that it works to the only one generation-of-lift theory that exists in the scientific community?

    Leave a comment:


  • BoeingBobby
    replied
    I have to say, no you know what F it.

    Leave a comment:


  • Evan
    replied
    Originally posted by Gabriel
    Wing pushes air down, so air pushes wing up, is 100% accurate and accounts for 100% of the lift.
    However... if we conduct a one minute Google search...

    Originally posted by Professor Holger Babinsky, Cambridge University Department of Engineering
    “What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes – lower pressure on the upper surface and higher pressure on the lower surface.”
    And then there is the question of WHAT CAUSES that pressure differential...

    Originally posted by A second-year MIT student
    The effects of viscosity lead to the formation of the starting vortex, which, in turn is responsible for producing the proper conditions for lift. The starting vortex rotates in a counter-clockwise direction. To satisfy the conservation of angular momentum, there must be an equivalent motion to oppose the vortex movement. This takes the form of circulation around the wing. The velocity vectors from this counter circulation add to the free flow velocity vectors, thus resulting in a higher velocity above the wing and a lower velocity below the wing.
    Or...

    Originally posted by Some other site
    If the wing has an angle of attack or if it has the classic airfoil shape, the air stream going over the wing will traverse well above the wing's upper surface, ie. making an arc. Here's where Bernoulli steps in: The air which is closer to the wing's upper surface is travelling slower than the stream of air making the arc over the wing. Because of the Bernoulli effect this causes a low-pressure pocket on the upper surface of the wing.
    And this one also claims that the incorrect Bernoulli / Meeting-at-the trailing-edge theory is:

    ...one of the most persistent and widespread aerodynamic physics myths in modern history. This myth is so prevalent and persistent that even Albert Einstein, one of the most brilliant physicists of all time, believed it and got it all wrong.
    Again, Gabriel, I'm not challenging your expertise. I'm pointing out how confusing it is out there for someone who is not aerodynamicist, fluidmechanicist or aerospace engineer to sort this all out, because a clear explanation with widespread consensus doesn't seem to exist outside of the engineering world, and more specifically is not well-known to those people who fly commercial airplanes for a living.

    Leave a comment:


  • Gabriel
    replied
    Evan from all the disappointing things you are saying, this is the most disappointing one:

    [the manner in which a wing generates lift] it's also a debatable science based on prediction and observation
    I don't know exactly what you mean, but in what way and opposed to what other science that doesn't suffer this shortcoming?

    which fosters argument within the science community itself and leads to somewhat divergent theories.
    No it doesn't. Unless you think that the shape of the Earth, how may zeroes does the age of the Earth have, and evolution foster arguments within the science community itself and leads to somewhat divergent theories.

    The most popular explanation by far of How Airplanes Fly is completely wrong!
    Most popular among whom? Since wen truth is based on popularity and ignorance? Show me ONE scientific textbook used by aerodynamicist, fluidmechanicist or aerospace engineers that says that the travel time is the same above and below. There may be some, but I have read quite a few and I have never seen such a thing.

    Go to youtube and you will see that most Americans (and I would say most of the world population, but these videos were about Americans) can't answer simple things like how seasons happen, what creates the phases of the moon, how the fall of a heavy ball compares with a light ball, or, I don't know, what is the Capital city of the Untied States, name a country whose name stars with U or in what state was KFC founded. Maybe science is not sure whether United States stars with U or Kentucky Fried Chicken was founded in Kentucky?

    Because no one** can explain it in a clear and concise manner.
    It would be much easier face-to-face, with a piece of paper and some markers on the table, being able to talk and exchange words more fluidly, and being able to make a gesture of a hand at an angle of attack, than with plain text as the only resource (and I did take the time to make a drawing in Paint and attach it, showing in I hope a VERY intuitive way why, after a bit of reflection on it, the same-transit-time postulate should make no sense whatsoever even to a 5-years-old, as it didn't make sense to me when I first saw it in a BOOK FOR PILOTS well before having my 1st class in aero or fluid dynamics).

    By the way, NOBODY can TOTALLY explain ANYTHING in a clear in concise manner. Or better, nobody can explain totally anything. The chain of "why and how" about ANYTHING will eventful reach the limit of our knowledge, and that will never change no matter how many new and better theories we discover about anything. And even within the realm of our knowledge, the deeper tyou go the most complex things get in ANUY FIELD of knowledge. Yes, you can explain quantum mechanics in a layman-friendly way. But as soon as you want to go deeper into the details and in accuracy, it is not so simple anymore. There is a reason why it takes YEARS of HARD STUDY to become an aeronautical engineer or a Physicist. With that said: Wing pushes air down, so air pushes wing up, is 100% accurate and accounts for 100% of the lift. And I hope that is easy enough to grasp.

    Are you putting different standards for the generation of lift than for any other field of knowledge?

    Leave a comment:


  • Evan
    replied
    Originally posted by brianw999 View Post
    The most astonishing aspect of this thread is that two or three educated people who have had what seems to be a good education are discussing the facts of complex aerodynamics !
    Which brings me to the reason I started this little diversion. We see a pilot (who may or may not be BoeingBobby) reading a book called How Airplanes Fly. To which I replied:

    Originally posted by Evan
    Truly. How many pilots could accurately describe why a wing generates lift? I wonder if you could?
    Now, notice that I said "describe", not "know". How easy is it to explain?

    As it turns out, it is very difficult (if not impossible) to explain the manner in which a wing generates lift without either brushing over broad concepts or getting into bewildering technical discussions involving things like viscosity and velocity vectors and friction and how a boundary layer forms and the complex reasons that cause an airflow to change pressure when being diverted by an airfoil shape. In order to fully understand it all, you need to go back and study fluid dynamics and Bernoulli and Euler and understand fairly complex equations. I freely admit that post #33 brought on intense dizzyness and I might have passed out for a while.

    And, in addition to it being so difficult to explain in simple language using concepts that laypeople can grasp, it's also a debatable science based on prediction and observation, which fosters argument within the science community itself and leads to somewhat divergent theories. The most popular explanation by far of How Airplanes Fly is completely wrong! Nobody absolutely knows How Airplanes Fly. Because no one** can explain it in a clear and concise manner.

    Which brings me full circle to the reality. Most pilots probably could not correctly and definitively explain How Airplanes Fly.

    (Although I guess I would consider it satisfactory if they just answer, "By maintaining healthy airspeed and attitudes without breaking off the rudder.")

    ** I'm still digging in this respect and hold out some hope for the future.

    Leave a comment:


  • Gabriel
    replied
    Originally posted by brianw999 View Post
    The most astonishing aspect of this thread is that two or three educated people who have had what seems to be a good education are arguing about the facts of simple aerodynamics !
    Aerodynamics and how plane flies, for pilots and the general educated public, is full of misconceptions, myths, and things that most educated people don't know:
    Myths and misconceptions:
    - That the air separated at the trailing edge must reach the trailing edge together.
    - That the fulcrum effect is what makes a high-win more stable than a low-wing (even Mentour Pilot fell in that myth).
    - The dangers of turning downwind.
    Things not well known or not well understood
    - The wing-induced angle of attack in the tail.
    - The dangers of stall (hint, loss of lift is not your main problem).
    - The pitch down moment induced by pitching up.
    - Phugoid (and how to really control your plane with no elevator or trim).
    - How to recover from a spiral dive (hint, it is almost the opposite of how to recover from a garden-variety dive).

    And the list goes on....

    And THEN you have the not-so-well educated people, of which even the aeronautical engineers pilots communities have their its share, that say totally absurd thins. These are things told to me by ACTUAL pilots and aeronautical engineers:
    - The Tomahawk doesn't behave so well flying with a tailwind because of its T-tail.
    - We are taking off with a strong headwind so we need to turn with care because the bottom of the wing will catch all that wind when we face the underside to it.
    - We are fast and high for the landing but I will not add flaps because this plane tends to float too much with flaps down (and he didn't go around either and we ended up touching down in the last 1/3 of the runway and stopping at the very end of the paved surface.
    - A 747 glide like a piano. And a Cessna? Like a pianola.

    And, again, the list goes on...

    So, Brian, to be honest, I find this discussion with Evan and 3WE being of much higher level than what I had with some supposedly educated pilots and aeronautical engineers.

    Leave a comment:


  • brianw999
    replied
    The most astonishing aspect of this thread is that two or three educated people who have had what seems to be a good education are arguing about the facts of simple aerodynamics !

    Leave a comment:


  • BoeingBobby
    replied
    It's not like this is something new here folks. https://en.m.wikipedia.org/wiki/Bernoulli's_principle

    Leave a comment:


  • Gabriel
    replied
    Originally posted by Evan View Post
    So, are you saying that, at zero AoA, the airstreams depart the trailing edge at the same time?
    Did I say zero? No, I said that if the upper flow wins at one AoA and the lower flow wins at another AoA, there HAS to be some AoA somewhere between these 2 where the 2 flows arrive together. I am not making any claim of what AoA would that be, it will be different from airfoil to airfoil and, while most likely it will be a small AoA, it doesn't need to be and in general it will not be zero (unless it is a symmetrical airfoil). This particular AoA can be found, for any given airfoil, experimentally and theoretically. This is another prediction that the theory is capable to make with very good accuracy.

    So, are you saying an AoA below zero (negative AoA) is capable of generating positive lift?
    No I am not saying that. I am saying that an airfoil that is not symmetrical, at the previously found AoA (that doesn't need to be zero), will typically still generate a bit of positive lift. That means that there will be an AoA even lower than that where the lift will be zero, and any AoA in between these 2 will have positive lift with the lower flow winning the race to the trailing edge.

    That said, I am telling you NOW (well, I've already told you a few posts ago, in the long explanation of airfoils) that, if you define the AoA in the conventional way (the angle between the free stream and the chord line of the airfoil), then yes, most asymmetrical airfoils have positive lift at some smallish negative AoA (or, the AoA of zero lift is negative), because they are cambered, and the camber displaces the lift vs AoA curve upwards.

    Because it served to explain how the asymmetrical shape of the wing chord FORCES the air to accelerate IN ORDER to meet that requirement.
    Yes, but it is a false explanation.

    Because the question that remains unanswered (or unsatisfactorily answered) is WHY does the airflow across the top of an assymetrical wing chord accelerate?

    Perhaps this is the answer I'm questing for?

    What is causing this vacuum? The air displacement from the leading edge? The friction along the boundary layer that pulls it back into place?
    We can have several didactic explanations, none of them completely accurate. 3WE gave you one. I gave you another one (for the one AoA where both flow reach the trailing edge together, the air flowing above will still have a longer path and hence will need more speed to reach together).

    But the accurate explanation may be not satisfactory for you. It is the only thing the air can do to change its constant speed profile of the free stream (with all the molecules moving in straight parallel lines at the same velocity) to a profile that goes around the airfoil and separates at the trailing edge. If the air did anything else it would be violating F=m*a for the individual elements of air, or it would not be going around the airfoil.

    But let me make an analogy (also imperfect). Imagine that you have train tracks follow a straight line but at some point they have to deviate to ago around a mountain and then come back to rejoin the original straight line. Let's say that there is no friction and the track is horizontal so the velocity will remain constant. However, the speed vector will need to change its direction because the train NEEDS to remain on the track, which changes direction. Now you go and measure the force between the train and the track at each point during the path around the mountain and ask... Why is the force precisely what it is at each point? There is no "outside" reason for the force being what it is. It is what it NEEDS to be to remain on the tracks and comply with F=m*a at the same time (and it can be predicted theoretically using F=m*a and compared with the experimental result, and you will find a close match). If the force wasn't exactly what it is at each point along the tracks, the train would either not follow the tracks or violate F=m*a.

    The same happens with the air around an airfoil, with 2 caveats.
    - The force applied on the air for it to deviate around the airfoil is a pressure. But pressure, by it own nature, acts in every direction. You cannot thus, in general, just deviate a flow but keep its velocity constant. The same gradient of pressures that makes it turn around the obstacle will also make it increase or decrease the speed.
    - The train doesn't "know" that there is a curve ahead in the tracks. The air (subsonic) does know that there is an obstacle ahead because the pressure disturbances propagate at the speed of sound. That's why the stream lines react to the airfoil ahead well below reaching the airfoil itself. You have variations of pressures before the air reaches the airfoil. The tracks are actually the stream lines, not the airfoil itself (although for the air actually touching the airfoil, the stream line needs to match the contour of the airfoil when going around it).

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  • Evan
    replied
    Ok, there are what I feel are a few assumption here:

    Originally posted by Gabriel
    So let's say that we play with the AoA until we find the one that makes that happen. What do you expect the lift to be at that AoA"? Zero?

    Well, then you are wrong, unless the airfoil is a symmetrical one. Otherwise, the airfoil in that condition will still produce a bit of positive lift. Why? Ok, take what I say with a grain of salt, but just for the sake of visualization, let's say that the air flowing above the airfoil will still have to travel a longer distance than the air flowing under the airfoil due to the longer geometry of the upper surface, and since now both particles of air are taking the same time to make their trips (not because that NEEDS to happen but only because we adjusted the AoA until we found the one that makes it happen), the air flowing above will need to go faster which means lower pressure as per Bernoulli.
    So, are you saying that, at zero AoA, the airstreams depart the trailing edge at the same time?

    Originally posted by Gabriel
    Now I have the airfoil making a bit positive lift and with the air flowing above and below reaching together the trailing edge. For sure there will be an AoA smaller but close enough to the current one where the lift will be lower but still a little bit positive.
    So, are you saying an AoA below zero (negative AoA) is capable of generating positive lift?

    Originally posted by Gabriel
    The real question is, why would anybody expect that for lift to exist the air flowing above the airfoil should reach the trailing edge at any particular time compared to the air flowing below the airflow? Because whatever that reason is, it is wrong.
    Because it served to explain how the asymmetrical shape of the wing chord FORCES the air to accelerate IN ORDER to meet that requirement. And the acceleration explains the pressure differential (And, yes, we know this is not true). But, as you pointed out, this science is based on conclusions drawn from observation and deduced logic, and certain problems remain if you go far enough down the path.

    Because the question that remains unanswered (or unsatisfactorily answered) is WHY does the airflow across the top of an assymetrical wing chord accelerate?

    Perhaps this is the answer I'm questing for?

    Originally posted by 3WE
    The upper stream is accelerated because a 'vacuum' PULLS it down and back and into the low pressure area that even goes behind the wing.
    What is causing this vacuum? The air displacement from the leading edge? The friction along the boundary layer that pulls it back into place?

    Leave a comment:

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