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Mightyflyer86
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Wed Feb 12, 2020 8:35 pm

Ertro wrote:
Another very simple question which almost nobody can answer thoroughly is why a small single or twin-engine propeller airplane which experiences an engine failure just after taking off into headwind is prone to stalling immediately after it has first stabilized its airspeed going forward and then tries to turn 180degrees back to the airfield into a direction where it has tailwind.

I have seen this question raise violent discussions on many discussion forums where each participient can be expected to hold some kind of pilots certificate but pretty much nobody can explain this problem completely and more worriyngly almost half pilot license holders seem believe all kinds of absolute total BS related to this matter so that as end result nobody agrees with nobody else what is really going on.

Whether there is a problem being prone to stalling is not the question. Pretty much everybody agrees that this is something that can easily happen but exactly WHY it happens is the question.


Have you ever been on a commercial flight and right after the pilot starts a right or left turn, you begin to feel heavier as if you were on a roller coaster for a few seconds? That force is what we engineers call “Gs” as in gravity, it happens because when an airplane is flying level it is using all the lift produced by the wings, fuselage etc to maintain flight. Now when you start to turn, part of that lift is being used to turn so the airplane starts losing altitude and the pilot has to compensate by raising the nose/increasing the angle if attack which makes you feel like you are in a roller coaster.

Short answer: think of it (lift) as available energy. It can only be “increased” by raising the nose of the airplane and flying faster via increased engine thrust. Turning requires the airplane to use the available energy to turn and to maintain flight thus without engine thrust it won’t be able to fly at or above the minimum speed and will stall once it runs out of the minimum energy required to stay airborne (stall speed).
 
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VirginFlyer
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Wed Feb 12, 2020 8:36 pm

B737Captain1980 wrote:
I don't know who is more dumb, the person that wrote the article, or the genius that is listening to him.

My vote would be for the people who deride the article without any explanation; makes them sort of sound like the people a century and a bit ago who derided heavier-than-air flight as “that’s just crazy, it will never happen.” But perhaps you can enlighten us on what you perceive as the flaws in the article?

Personally, I think it is interesting this is getting aired to a more general audience; it was something I had learned about in a university aerodynamics course. According to that course, the only model that made mathematical sense and explained everything that was happening involved using imaginary concepts such as the “rationality” of air. In a way I see this as a lot like electrical systems, where our explanations of their workings involve use of imaginary numbers.

I think the important take-away from all of this, applicable to pretty much any area of life, is that it is possible to understand how to manipulate and utilise something (be it aerodynamics or electricity - things that we definitely know how to manipulate and utilise) without knowing in precise detail how they actually work at a fundamental level. When we think like this I believe it helps us both realise the limitations of our knowledge, and also not reject what someone does know on the basis of them not knowing everything: provided what they tell me is borne out by experiment and demonstration, I am not going to trust the aerodynamicist or the electrical engineer any less just because they can’t tell me at the fundamental level what is actually going on with the device.

V/F
It is not for him to pride himself who loveth his own country, but rather for him who loveth the whole world. The earth is but one country, and mankind its citizens. —Bahá'u'lláh
 
WeatherPilot
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Wed Feb 12, 2020 8:41 pm

So, is this why I was taught to tie my airplane down? One day it might just float away without explanation? Who knew...
 
tomcat
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Wed Feb 12, 2020 8:55 pm

The article ends with this sentence:

“One apparent problem is that there is no explanation that will be universally accepted,” he says. So where does that leave us? In effect, right where we started: with John D. Anderson, who stated, “There is no simple one-liner answer to this.”


I don't see any problem with the fact that there is no one-liner explanation to describe lift. It's a complex phenomenon which requires many words to be precisely explained.

Here is another article which in a way is an answer to this SA article. Here is its wise conclusion:
"The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems."


https://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html?fbclid=IwAR03TjvXQqhnDYO5GJHnThqPBOF20PndzPUV6eNzh_vE8nb9w__MjXOHz-8
 
TYWoolman
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Wed Feb 12, 2020 9:18 pm

So since we truly don't know why planes fly, has aviation all these years been on a failed trajectory of efficiency and progress that will forever limit our interstellar travel ambitions (?) Are we to be stuck on this blue marble forever? Say it ain't so!
 
MRYapproach
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Wed Feb 12, 2020 9:44 pm

I loved this story. I always struggle to justify the "air moving over the top of the airfoil moves farther, so faster, so lower pressure, so upward lifting force" line of thinking when explaining how flight works (at least to me...and I have a degree in mechanical engineering) to others. I forwarded this sotry to a co-worker who is an aerodynamics engineer, and he said this argument took up about a year of his education. As is often the case, truth is stranger than fiction!
 
mxaxai
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Wed Feb 12, 2020 9:54 pm

hivue wrote:
mxaxai wrote:
Isn't this interrelation exactly what the Navier-Stokes Equations descibe? If you set the correct boundary conditions (undisturbed airflow, wing geometry, no-slip condition) and solve the NSE you'll get the pressure distribution along the wing, which can be converted to lift. The NSE are based on some fundamental physics (conservation of mass, energy, momentum) so that's your why.
Any why beyond that is bound to become philosophical.


The NSE takes care of the math. I think what the article is about is what is the physical explanation for the math. When Plank discovered quantization of energy in 1900 his equation worked, but everyone thought it was just a mathematical convenience. It was some years before anyone came up with a physical explanation.

Youtube has a number of videos explaining various experts' opinions on how airfoils actually work. There are some folks who think it has to due with forces, specifically the centrifugal force acting on molecules in the bending air stream causing them to be jammed up under the wing and dispersed above the wing. Others place more emphasis on the Coandă effect and all its possible implications. Others say the Coandă effect has nothing to do with anything. You can take your pick.

The NSE are a physical explanation. They just don't have an analytic solution yet. Coanda effect, etc. are all macroscopic effects that can be easily visualised, but they are fully described by the NSE.

The point of those videos - and IMHO of this article - is to provide an explanation that people unfamiliar with aerodynamics can easily grasp. A pilot doesn't need to solve the NSE to keep his plane in the air (gosh that would be annoying, I'd much rather enjoy the scenery ...), they only need to know that air goes over the wings and creates lift under certain conditions - conditions that they must meet during a normal flight.
It's like asking "why does an egg get hard?" - because of the excitation of atoms which changes the properties of the molecules that constitute the egg? Or because I turned on the stove?

On a more fundamental level, this answer is correct:
Exeiowa wrote:
As a chemist I understand everything in terms of molecules, so to me when I think of this problem there are more molecules hitting the bottom of the wing than the top and if you add the momentum (so speed of the molecules is a factor) that they impart net more energy than on the top and there by push you up, now they would also have a backwards component hence "drag".

The NSE are derived for continuum mechanics, so at very small scales (or very low densities, or very high temperatures) they lose their validity - you're looking at single molecules, not a continuum. If you want to go there, you can try to use the Boltzmann equation that replaces the continuum with a probability distribution for each particle. The NSE can be derived from it. At it's core you still have the basic laws of physics:
Conservation of mass, momentum, energy and charge.
 
Tailwinds
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Wed Feb 12, 2020 10:07 pm

The Bernoulli effect doesn't work because of the shape of the wing, which is why the whole inverted argument misses the point. Bernoulli says that pressure drops when a fluid moves faster. The whole curved wing = faster air is a bit of a myth. It doesn't explain where the energy comes from that accelerates that air. That energy comes from the vacuum created in the shadow of the wing. It pulls air toward the wing, accelerating it, which adds to the already low pressure there. The camber affects the efficiency of this process, but it isn't the process. Fly a plane inverted and the same process will work, it'll just be less efficient because the camber on the bottom of the wing isn't optimized for the purpose.

At the end of the day though, all this is just one of the ways a plane imparts a net downward momentum to the air around it. Newton is *why* a plane stays aloft, that being the reaction for the action of flinging air downward, Bernoulli is one of the part of the *how*.
 
meecrob
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Wed Feb 12, 2020 10:39 pm

Tailwinds wrote:
The whole curved wing = faster air is a bit of a myth. It doesn't explain where the energy comes from that accelerates that air. That energy comes from the vacuum created in the shadow of the wing.


No, the energy comes from ramming an airfoil through a fluid such as air. The by-product of this is induced drag. Windmills work like this in reverse - accelerate air around an airfoil and the airfoil moves.
 
TWAElite1
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Wed Feb 12, 2020 11:35 pm

We still don't fully understand gravity either. Maybe when we figure these two things out, they'll both quit working? :)
 
144modeller
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 12:25 am

The four relevant forces are: Load, Lift, Thrust, and Drag. This does not conflict with anything already said. Thrust forces the aircraft forwards, providing the airflow around the aerofoil to produce the lift. The lift has to be sufficient to overcome the load, and the thrust has to overcome the drag which is the disadvantage of moving through air. It is obvious, but needs to be mentioned for the context, that the plane's speed relevant to the ground is not important: it is the speed relative to the air that matters.
So, when your light aircraft suffers engine failure immediately after takeoff, its forward speed may not be sufficient to overcome the drag, and the air will therefore slow it down, reducing the lift. If it manages to turn 180 degrees, the forward movement will be in the same direction as the wind, so the all-important airspeed will be reduced further.
Lift is at 90 degrees to the plane of the wing, i.e. directly opposite to gravity when the aircraft is flying level. When banking in a turn, the angle of lift becomes more shallow, relative to gravity, reducing the efficiency of the lift. Take this idea to extremes, and when the craft is completely on its side, there is no lift at all in opposition to gravity.
Can an aircraft take off from a conveyor? Yes, if the conveyor can reach the normal take-off speed of the craft for long enough. My brother's sailplane was given forward thrust by the Land Rover that towed it along the ground. It's the same principle. It has been known for a stationary light aircraft to lift off the ground if the windspeed is head-on and great enough, but without a pilot to steady it, this usually results in a write-off.
Sailplanes and hang gliders make use of upward-moving air to prolong their flights. This can be found when warm air rises (thermals), or wind rises to overcome hills, as well as in storms (but not recommended).

I am not trained in aeronautical science, but I think my everyday language might appeal to those who don't understand the tech talk.
 
VS11
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 2:43 am

VirginFlyer wrote:
B737Captain1980 wrote:
I don't know who is more dumb, the person that wrote the article, or the genius that is listening to him.

My vote would be for the people who deride the article without any explanation; makes them sort of sound like the people a century and a bit ago who derided heavier-than-air flight as “that’s just crazy, it will never happen.” But perhaps you can enlighten us on what you perceive as the flaws in the article?

Personally, I think it is interesting this is getting aired to a more general audience; it was something I had learned about in a university aerodynamics course. According to that course, the only model that made mathematical sense and explained everything that was happening involved using imaginary concepts such as the “rationality” of air. In a way I see this as a lot like electrical systems, where our explanations of their workings involve use of imaginary numbers.

I think the important take-away from all of this, applicable to pretty much any area of life, is that it is possible to understand how to manipulate and utilise something (be it aerodynamics or electricity - things that we definitely know how to manipulate and utilise) without knowing in precise detail how they actually work at a fundamental level. When we think like this I believe it helps us both realise the limitations of our knowledge, and also not reject what someone does know on the basis of them not knowing everything: provided what they tell me is borne out by experiment and demonstration, I am not going to trust the aerodynamicist or the electrical engineer any less just because they can’t tell me at the fundamental level what is actually going on with the device.

V/F


The article is annoying because it is conflating and confusing, either on purpose or due to the author’s own confusion, two separate issues. One issue is what causes planes to fly, and the other is what causes the difference in pressure above and below wing when the plane is moving. We do understand very well why and how planes fly even if there is a debate about the pressure differential. The article was just looking for some cheap sensationalism while being unnecessarily long and repetitive even though the core question is in itself very interesting.

It is like saying we don’t fully understand why toasters work because we don’t know what causes particles to exist ( which is probably the ultimate explanation on everything).
 
11C
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 4:32 am

scbriml wrote:
Well, it’s clearly understood that helicopters don’t fly - they’re so ugly the Earth repels them!


Eye of the beholder. I’ve been lucky to fly with a few guys who made helicopter flying quite beautiful, and amazingly precise. And I imagine the guys who were lifted out of battle after being wounded in Vietnam (or any war zone) found them to be quite beautiful, as well. But, I agree, from a physics standpoint, it’s not as elegant.
 
ikramerica
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 5:40 am

“Stop Abusing Bernoulli” is a book I tried to read years ago that explains the lift lie is used to explain things to non-scientists.
Of all the things to worry about... the Wookie has no pants.
 
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N14AZ
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 6:43 am

Amiga500 wrote:
Bernoulli is a a principle of connecting strictly subsonic airflow velocity with pressure. It does not explain why the velocities are what they are.

I do not understand your comment about the velocities. In still air the velocity on the downside is more or less the velocity of the aircraft and on the top of the wing it’s the speed of the aircraft plus the extra velocity necessary to compensate for the longer way. But maybe I misunderstood your comment about the velocities.
 
 
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SomebodyInTLS
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 7:58 am

Tugger wrote:
The wing pushes the air down, resulting in a downward turn of the airflow. The air above the wing is sped up in accordance with Bernoulli’s principle. In addition, there is an area of high pressure below the wing and a region of low pressure above. [...] But it is the interrelation among these four elements that is the most novel and distinctive aspect of McLean’s account. [...]


Pretty cool to learn this, that flight is still a mystery (in science).


Now I don't feel so stupid for struggling with aero courses as my worst subjects in uni. In fact, it confirms my feeling that it never quite made sense!

I seem to remember one professor stating that it's all about the difference in pressure while in another course the textbook flat out stated that people are wrong to interpret it being about pressure; it's actually all about the circulation. I could never quite wrap my head around the contradictions - not helped by having to re-sit exams biased one way while also sitting exams biased towards the other... in between lectures on the supersonic and hypersonic stuff which is literally an entirely different set of theories!
Last edited by SomebodyInTLS on Thu Feb 13, 2020 8:01 am, edited 1 time in total.
"As with most things related to aircraft design, it's all about the trade-offs and much more nuanced than A.net likes to make out."
 
WIederling
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 7:59 am

frmrCapCadet wrote:
I read the article and found it puzzling. I wonder if the question is a candidate for AI neural net approach, just collect wind tunnel data and let the computer find the pattern. After which, I worry, the computer knows but I still am puzzled.


AI neural net approach:
Misconception pushed by many.
AI finds patterns but not rules ( i.e. the why of the patterns.)

Flying:
You start with actio == reactio
to have the plane counter gravity without being supported by contact to solids
you have to create impulse up by moving mass down ( keeps the plane up.)

Next step is you can go various path to explain how moving that mass down is achieved.
You can refine the model to infinity. ( Just like in any other physics topic than can be onionized to no end.
they haven't found a final end for subatomic particles yet :-)
There is a limit to how much refining makes sense.

Now Why " actio == reactio" works is again susceptible to physics onionizing to no end. :-)))))))
Murphy is an optimist
 
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VirginFlyer
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 8:50 am

SomebodyInTLS wrote:
Tugger wrote:
The wing pushes the air down, resulting in a downward turn of the airflow. The air above the wing is sped up in accordance with Bernoulli’s principle. In addition, there is an area of high pressure below the wing and a region of low pressure above. [...] But it is the interrelation among these four elements that is the most novel and distinctive aspect of McLean’s account. [...]


Pretty cool to learn this, that flight is still a mystery (in science).


Now I don't feel so stupid for struggling with aero courses as my worst subjects in uni. In fact, it confirms my feeling that it never quite made sense!

I seem to remember one professor stating that it's all about the difference in pressure while in another course the textbook flat out stated that people are wrong to interpret it being about pressure; it's actually all about the circulation. I could never quite wrap my head around the contradictions - not helped by having to re-sit exams biased one way while also sitting exams biased towards the other... in between lectures on the supersonic and hypersonic stuff which is literally an entirely different set of theories!

I suspect it is all probably much more nuanced than A.net likes to make out. If you thought Airbus v Boeing was bad, wait until Newton v Bernoulli sets in!

V/F
It is not for him to pride himself who loveth his own country, but rather for him who loveth the whole world. The earth is but one country, and mankind its citizens. —Bahá'u'lláh
 
rrlopes
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 10:22 am

dopplerd wrote:
Tugger wrote:
cedarjet wrote:
Bernoulli (the curved upper surface and flat lower surface creates a vacuum above the wing) doesn’t work because it would cause an inverted plane to fall earthwards. Well it helps us get airborne in one way — by being able to recite it in an exam they give you a pilots licence.

Have they ever tested flying a plane with a wing installed inverted? Tried to take off with it like that?
Just curious.

Tugg


This is cray cray. AoA plays as big if not bigger role than wing camber.

Aerobatics aircraft have a pretty symmetrical wing top to bottom in cross section. The lift when flying inverted or wheels down is managed through angle of attack. Bernoulli still works because of changes in AOA and relative wind to the wing change the air speed on the opposite sides of the wing. Essentially the aoa changes the break point on the leading edge at what point air goes over or under the wing.

An airliner will have a nonsymmetrical wing since they are not intended to fly upside down. This allows designers to optimize wing shape and angle for better lift and efficiency than a symmetrical airfoil.

An upside down winged airliner might be able to fly but with very poor performance and high fuel consumption due to the aoa necessary to generate lift.


Absolutely. I never forget a quote from one of my fluid dynamics textbook that said that "... even a door with an angle of attack generates lift".

Let us also not forget that even the aircraft fuselage when flying with a larger than zero angle of attack (with respect to the fuselage axis) will also generate lift.
 
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T18
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 12:25 pm

Tugger wrote:
cedarjet wrote:
Bernoulli (the curved upper surface and flat lower surface creates a vacuum above the wing) doesn’t work because it would cause an inverted plane to fall earthwards. Well it helps us get airborne in one way — by being able to recite it in an exam they give you a pilots licence.

Have they ever tested flying a plane with a wing installed inverted? Tried to take off with it like that?
Just curious.

Tugg


Well kinda as an inverted airfoil is what the wings on a indycar or F1 car are. Oddly those stay well panted unless you swap the direction of the relative wind 180 degrees then they fly, but I would argue that is related to the AoA being inverted with that change.
“Racing's important to men who do it well. When you're racing, it's life. Anything that happens before or after is just waiting.” ― Steve McQueen (Le Mans) 1971
 
mxaxai
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 12:40 pm

N14AZ wrote:
Amiga500 wrote:
Bernoulli is a a principle of connecting strictly subsonic airflow velocity with pressure. It does not explain why the velocities are what they are.

I do not understand your comment about the velocities. In still air the velocity on the downside is more or less the velocity of the aircraft and on the top of the wing it’s the speed of the aircraft plus the extra velocity necessary to compensate for the longer way. But maybe I misunderstood your comment about the velocities.

Except that there is no law that requires the air on the top to meet the air on the bottom. The different geometry leeds to different velocities, yes, but not to 'compensate' anything.
 
AleksW
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 1:03 pm

I think we (I mean aerodynamic engineers) do fully understand how planes fly and how wings generate its lift. It's just that there are few ways to explain it, and it might create some confusion.
 
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flyingturtle
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 1:41 pm

In the end, air must be accelerated downwards so the aircraft can be accelerated upwards, negating the force of gravity.
Reading accident reports is what calms me down
 
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NeBaNi
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 4:17 pm

mxaxai wrote:
N14AZ wrote:
Amiga500 wrote:
Bernoulli is a a principle of connecting strictly subsonic airflow velocity with pressure. It does not explain why the velocities are what they are.

I do not understand your comment about the velocities. In still air the velocity on the downside is more or less the velocity of the aircraft and on the top of the wing it’s the speed of the aircraft plus the extra velocity necessary to compensate for the longer way. But maybe I misunderstood your comment about the velocities.

Except that there is no law that requires the air on the top to meet the air on the bottom. The different geometry leeds to different velocities, yes, but not to 'compensate' anything.

To answer Amiga500's point, I thought the explanation of why the velocities are what they are is conservation of energy? There is lower pressure on the upper surface of the wing, so the speed on the upper surface has to increase from the freestream speed to account for this lower pressure, and hence conserve energy.

To answer mxaxai's point, I don't get it. The air on the top has to meet the air on the bottom at the trailing edge because at the trailing edge, you don't have physical separation, and the pressures have to be equal. The Kutta condition (see https://en.wikipedia.org/wiki/Kutta_condition) explains this in terms of circulation. Also, Bernoulli's equation (1/2 rho V^2 + p = const) applies to steady flows along streamlines. So it is incorrect to apply it the same way along two streamlines.
 
mxaxai
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 4:53 pm

NeBaNi wrote:
To answer mxaxai's point, I don't get it. The air on the top has to meet the air on the bottom at the trailing edge because at the trailing edge, you don't have physical separation, and the pressures have to be equal. The Kutta condition (see https://en.wikipedia.org/wiki/Kutta_condition) explains this in terms of circulation. Also, Bernoulli's equation (1/2 rho V^2 + p = const) applies to steady flows along streamlines. So it is incorrect to apply it the same way along two streamlines.

Of course the air 'meets' again at the trailing edge - there is no vacuum. And yes, at the trailing edge the velocities of particles that went over or under the airfoil are identical.

But imagine two air particles next to each other. One of them happens to go over the wing, and the other goes under it. There is no guarantee that these two particles will end up next to each other again. In fact, it is quite likely that each particle will have found a new neighbor because the time required to go over the wing is often different from under the wing.
This simulation illustrates it quite well: https://commons.wikimedia.org/wiki/File ... refftz.gif There are similar wind tunnel tests but I couldn't find any good videos.
 
Amiga500
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 5:02 pm

NeBaNi wrote:
mxaxai wrote:
N14AZ wrote:
I do not understand your comment about the velocities. In still air the velocity on the downside is more or less the velocity of the aircraft and on the top of the wing it’s the speed of the aircraft plus the extra velocity necessary to compensate for the longer way. But maybe I misunderstood your comment about the velocities.

Except that there is no law that requires the air on the top to meet the air on the bottom. The different geometry leeds to different velocities, yes, but not to 'compensate' anything.

To answer Amiga500's point, I thought the explanation of why the velocities are what they are is conservation of energy? There is lower pressure on the upper surface of the wing, so the speed on the upper surface has to increase from the freestream speed to account for this lower pressure, and hence conserve energy.

To answer mxaxai's point, I don't get it. The air on the top has to meet the air on the bottom at the trailing edge because at the trailing edge, you don't have physical separation, and the pressures have to be equal. The Kutta condition (see https://en.wikipedia.org/wiki/Kutta_condition) explains this in terms of circulation. Also, Bernoulli's equation (1/2 rho V^2 + p = const) applies to steady flows along streamlines. So it is incorrect to apply it the same way along two streamlines.


You may be talking across each other there.

Air from the top does meet air from the bottom (Kutta condition) - however two molecules that are side by side prior to the leading edge stagnation point (one going above, one going below aerofoil) do not (unless by astronomical fluke) meet up at the trailing edge.


edit: Ah, was away from computer for 15 mins in middle of composing this. Since been ninja'd.
 
BooDog
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 5:25 pm

This article explains the different airfoil shapes without all the fluff. https://amaflightschool.org/getstarted/ ... c-trainers
I encourage reading about the design of wind turbine blades. Looking at lift without having to think about forward momentum made the concept of lift easier to understand for me.
B1B - best looking aircraft ever.
 
mpdpilot
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 5:37 pm

Ertro wrote:
Another very simple question which almost nobody can answer thoroughly is why a small single or twin-engine propeller airplane which experiences an engine failure just after taking off into headwind is prone to stalling immediately after it has first stabilized its airspeed going forward and then tries to turn 180degrees back to the airfield into a direction where it has tailwind.

I have seen this question raise violent discussions on many discussion forums where each participient can be expected to hold some kind of pilots certificate but pretty much nobody can explain this problem completely and more worriyngly almost half pilot license holders seem believe all kinds of absolute total BS related to this matter so that as end result nobody agrees with nobody else what is really going on.

Whether there is a problem being prone to stalling is not the question. Pretty much everybody agrees that this is something that can easily happen but exactly WHY it happens is the question.


So this question and the one posted in the article illustrate a very important point. Just because there is a discussion about the correct answer doesn't mean we don't know the answer. Just because smart people disagree, it doesn't make them both right. Even if we assume for a moment that "We don't actually fully understand how planes fly" that doesn't mean there isn't a reason. There is a reason, just because there is debate doesn't make it a mystery.

There are people who can raise violent discussions about whether we landed a man on the moon in 1969, that doesn't mean "we don't actually know if we landed on the moon".
One mile of highway gets you one mile, one mile of runway gets you anywhere.
 
mxaxai
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 6:56 pm

mpdpilot wrote:
There are people who can raise violent discussions about whether we landed a man on the moon in 1969, that doesn't mean "we don't actually know if we landed on the moon".

And just because we can never be sure why we landed a man on the moon, we certainly know how to land a man on the moon.
 
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NeBaNi
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 10:14 pm

Amiga500 wrote:
NeBaNi wrote:
mxaxai wrote:
Except that there is no law that requires the air on the top to meet the air on the bottom. The different geometry leeds to different velocities, yes, but not to 'compensate' anything.

To answer Amiga500's point, I thought the explanation of why the velocities are what they are is conservation of energy? There is lower pressure on the upper surface of the wing, so the speed on the upper surface has to increase from the freestream speed to account for this lower pressure, and hence conserve energy.

To answer mxaxai's point, I don't get it. The air on the top has to meet the air on the bottom at the trailing edge because at the trailing edge, you don't have physical separation, and the pressures have to be equal. The Kutta condition (see https://en.wikipedia.org/wiki/Kutta_condition) explains this in terms of circulation. Also, Bernoulli's equation (1/2 rho V^2 + p = const) applies to steady flows along streamlines. So it is incorrect to apply it the same way along two streamlines.


You may be talking across each other there.

Air from the top does meet air from the bottom (Kutta condition) - however two molecules that are side by side prior to the leading edge stagnation point (one going above, one going below aerofoil) do not (unless by astronomical fluke) meet up at the trailing edge.


edit: Ah, was away from computer for 15 mins in middle of composing this. Since been ninja'd.

I agree with all of that, and that just means the "side by side molecules have to meet" explanation is incorrect. But that doesn't mean we don't know how lift works.
 
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TWA772LR
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Thu Feb 13, 2020 10:30 pm

cledaybuck wrote:
NonTechAvLover wrote:
I have recently come to think that planes actually do not “fly” per se, but simply avoid falling (by the use of speed plus angling the plane that is the underside of the plane) until descent and landing, at which point they fall in a controlled manner. The only things that should be deemed to be flying are things lighter than air. But this is semantics. I was once told that the Russian word for aeroplane (“samalyot”) literally means “that flies by itself.” When you tell Russian speakers that birds also fly by themselves, they generally respond “but that’s different.”

How do you explain takeoff and ascent as "avoid falling"?

Think about a human jumping in the air. We have enough raw power to actually leave the ground. But what goes up must come down. The airplane uses its raw power on takeoff and climb to get in the air (lift plays a large factor but jet engines get get in to the air on their own under enough power and the correct angle at the end of the day). Thanks to thrust creation, the airplane can sustain its "jump" for a lot longer and the "controlled fall" stated by cledaybuck is the planes return to Earth, just as a human on the 2nd half of the jump.

But at the end of the day something is always mystery and that's why we have science/religion/real life experiences and wonders. Some things are left up to faith.
When wasn't America great?


The thoughts and opinions shared under this username are mine and are not influenced by my employer.
 
cledaybuck
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Fri Feb 14, 2020 12:41 am

TWA772LR wrote:
cledaybuck wrote:
NonTechAvLover wrote:
I have recently come to think that planes actually do not “fly” per se, but simply avoid falling (by the use of speed plus angling the plane that is the underside of the plane) until descent and landing, at which point they fall in a controlled manner. The only things that should be deemed to be flying are things lighter than air. But this is semantics. I was once told that the Russian word for aeroplane (“samalyot”) literally means “that flies by itself.” When you tell Russian speakers that birds also fly by themselves, they generally respond “but that’s different.”

How do you explain takeoff and ascent as "avoid falling"?

Think about a human jumping in the air. We have enough raw power to actually leave the ground. But what goes up must come down. The airplane uses its raw power on takeoff and climb to get in the air (lift plays a large factor but jet engines get get in to the air on their own under enough power and the correct angle at the end of the day). Thanks to thrust creation, the airplane can sustain its "jump" for a lot longer and the "controlled fall" stated by cledaybuck is the planes return to Earth, just as a human on the 2nd half of the jump.

But at the end of the day something is always mystery and that's why we have science/religion/real life experiences and wonders. Some things are left up to faith.

No. It’s not a rocket. A plane without wings isn’t taking off. The engines are there to provide the speed to the air going across the wings to provide lift to get the plane off the ground.
As we celebrate mediocrity, all the boys upstairs want to see, how much you'll pay for what you used to get for free.
 
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SheikhDjibouti
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Fri Feb 14, 2020 1:12 am

TWA772LR wrote:
The airplane uses its raw power on takeoff and climb to get in the air (lift plays a large factor but jet engines get get in to the air on their own under enough power and the correct angle at the end of the day).

Er...no. Not even close.

The benchmark for raw power at take-off is surely the AV-8.
This is powered by a RR Pegasus of 23,500 lbf thrust.
The AV-8B has an MTOW of 31,000 lb, except if you want a vertical take-off, this is limited to 20,755lb
It's not a coincidence that the MTOW has to be lower than the thrust available.

Now let's take a Boeing 737-800
Thrust is (at best) 27,000 lb x 2 = 54,000 lbf, and MTOW is 174,200 lb
If (and it's a big if) the engines delivered their thrust vertically (as with the AV-8B), all 100% of that 54,000 lbf would be capable of lifting around 50,000 lb.
That still leaves 124,200 lb of 737 that ain't going no-where.
However, unlike the AV-8B, there is no vectored thrust available, and the best you can hope for is a 7-8deg climb angle, which results in a vertical thrust component of just 4,320 lbf.
That's barely 2% of the 737s MTOW.
The other 98% of lift comes from the wings.
Nothing to see here; move along please.
 
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TWA772LR
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Fri Feb 14, 2020 2:04 am

cledaybuck wrote:
TWA772LR wrote:
cledaybuck wrote:
How do you explain takeoff and ascent as "avoid falling"?

Think about a human jumping in the air. We have enough raw power to actually leave the ground. But what goes up must come down. The airplane uses its raw power on takeoff and climb to get in the air (lift plays a large factor but jet engines get get in to the air on their own under enough power and the correct angle at the end of the day). Thanks to thrust creation, the airplane can sustain its "jump" for a lot longer and the "controlled fall" stated by cledaybuck is the planes return to Earth, just as a human on the 2nd half of the jump.

But at the end of the day something is always mystery and that's why we have science/religion/real life experiences and wonders. Some things are left up to faith.

No. It’s not a rocket. A plane without wings isn’t taking off. The engines are there to provide the speed to the air going across the wings to provide lift to get the plane off the ground.

Im not saying it's a rocket. Jet engines have gotten flying machines in to the air without wings (F35, Harrier, Apollo lunar training craft). The engines maintain the speed of the plane thus helping the wing generate lift. A plane isnt just going to float off the runway (eventually it should according to our knowledge of physics if it has consistent positive acceleration, but not immediately at Vr) on a takeoff roll with out thrust being applied to make the plane go in an upward direction.
When wasn't America great?


The thoughts and opinions shared under this username are mine and are not influenced by my employer.
 
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DocLightning
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Fri Feb 14, 2020 6:08 am

If you actually read McLean's book, he does an excellent job of explaining it.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
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SomebodyInTLS
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Fri Feb 14, 2020 2:27 pm

VirginFlyer wrote:
I suspect it is all probably much more nuanced than A.net likes to make out.


Ha ha! I see what you did there... :)
"As with most things related to aircraft design, it's all about the trade-offs and much more nuanced than A.net likes to make out."
 
WIederling
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Re: We don't actually fully understand how planes can fly!.- Who knew?!

Fri Feb 14, 2020 3:12 pm

mxaxai wrote:
And just because we can never be sure why we landed a man on the moon, we certainly know how to land a man on the moon.

that probably is contestable?
Jerry Pournelle: "I always knew I would live to see the first man on the moon. I never thought I would outlive the last one. "
:-)))
Murphy is an optimist

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