Moderators: jsumali2, richierich, ua900, PanAm_DC10, hOMSaR

 
User avatar
Tugger
Topic Author
Posts: 10608
Joined: Tue Apr 18, 2006 8:38 am

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

Wed Feb 12, 2020 3:38 pm

Well, something new to me! We don't actually fully understand how and why planes can fly, how/why wings work!

Who else here knew this? I had thought it was a very basic and clearly understood process in physics, but apparently not. I have included a few snapshots from the article but I suggest reading the full version in the Scientific American link below. It's a good read, not too long (and I recommend SA to anyone who likes to geek out about science and physics as I suspect most lovers of flight do).
On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs.
There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations.
Aerodynamicists have recently tried to close the gaps in understanding. Still, no consensus exists.

https://www.scientificamerican.com/arti ... n-the-air/

, however, leading aerodynamicist Doug McLean has attempted to go beyond sheer mathematical formalism and come to grips with the physical cause-and-effect relations that account for lift in all of its real-life manifestations. McLean, who spent most of his professional career as an engineer at Boeing Commercial Airplanes, where he specialized in CFD code development, published his new ideas in the 2012 text Understanding Aerodynamics: Arguing from the Real Physics.

Considering that the book runs to more than 500 pages of fairly dense technical analysis, it is surprising to see that it includes a section (7.3.3) entitled “A Basic Explanation of Lift on an Airfoil, Accessible to a Nontechnical Audience.” Producing these 16 pages was not easy for McLean, a master of the subject; indeed, it was “probably the hardest part of the book to write,” the author says. “It saw more revisions than I can count. I was never entirely happy with it.”


And McLean's explanation in a nutshell:
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. This means that there are four necessary components in McLean’s explanation of lift: a downward turning of the airflow, an increase in the airflow’s speed, an area of low pressure and an area of high pressure.

But it is the interrelation among these four elements that is the most novel and distinctive aspect of McLean’s account. “They support each other in a reciprocal cause-and-effect relationship, and none would exist without the others,” he writes. “The pressure differences exert the lift force on the airfoil, while the downward turning of the flow and the changes in flow speed sustain the pressure differences.” It is this interrelation that constitutes a fifth element of McLean’s explanation: the reciprocity among the other four.


But there is disagreement on this as noted in the article. Pretty cool to learn this, that flight is still a mystery (in science).

Tugg
I don’t know that I am unafraid to be myself, but it is hard to be somebody else. - W. Shatner
There are many kinds of sentences that we think state facts about the world but that are really just expressions of our attitudes. - F. Ramsey
 
NonTechAvLover
Posts: 81
Joined: Tue Dec 17, 2019 5:09 am

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

Wed Feb 12, 2020 3:47 pm

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.”
 
NonTechAvLover
Posts: 81
Joined: Tue Dec 17, 2019 5:09 am

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

Wed Feb 12, 2020 3:56 pm

Tugger wrote:


....McLean, who spent most of his professional career as an engineer at Boeing Commercial Airplanes, where he specialized in CFD code development, published his new ideas in the 2012 text Understanding Aerodynamics: Arguing from the Real Physics.

Considering that the book runs to more than 500 pages of fairly dense technical analysis, it is surprising to see that it includes a section (7.3.3) entitled “A Basic Explanation of Lift on an Airfoil, Accessible to a Nontechnical Audience.” Producing these 16 pages was not easy for McLean, a master of the subject; indeed, it was “probably the hardest part of the book to write,” the author says. “It saw more revisions than I can count. I was never entirely happy with it.”




And I worry about this part: how long before a user writes: “Even the most senior Boeing engineers do not understand how planes fly...? This is dangerous material for A.net, I am afraid.
 
User avatar
nighthawk
Posts: 4889
Joined: Sun Sep 16, 2001 2:33 am

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

Wed Feb 12, 2020 4:02 pm

We don't even understand if a plane can take off from a conveyor belt... so why would we understand the forces that keep it in the air? :stirthepot:
 
cledaybuck
Posts: 1708
Joined: Thu Aug 18, 2016 6:07 pm

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

Wed Feb 12, 2020 4:04 pm

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"?
As we celebrate mediocrity, all the boys upstairs want to see, how much you'll pay for what you used to get for free.
 
Ertro
Posts: 131
Joined: Thu Apr 04, 2019 9:28 pm

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

Wed Feb 12, 2020 4:13 pm

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.
 
User avatar
scbriml
Posts: 19109
Joined: Wed Jul 02, 2003 10:37 pm

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

Wed Feb 12, 2020 4:15 pm

Well, it’s clearly understood that helicopters don’t fly - they’re so ugly the Earth repels them!
Time flies like an arrow. Fruit flies like a banana!
There are 10 types of people in the World - those that understand binary and those that don't.
 
Karlsands
Posts: 229
Joined: Sat Sep 16, 2017 2:53 pm

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

Wed Feb 12, 2020 4:25 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.


This happens because without an increase in power or back pressure an aircraft WILL lose lift in a turn. When you are gliding below 1000 feet in a traffic pattern and attempt to return to the field you’re sacrificing more altitude the higher the degree of the turn is. Obviously if one tried to trim nose up and or keep back pressure while turning without any sufficient power the aircraft will stall and enter a low altitude spin. There is no mystery to this
 
Exeiowa
Posts: 341
Joined: Fri Jul 06, 2018 4:49 pm

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

Wed Feb 12, 2020 4:26 pm

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" But if you think in this terms low and high pressure become irrelevant and the downward flow part is the equal and opposite portion of the conservation of energy laws. Calculation in these terms are probably unhelpful but in understanding it makes perfect sense to me.
 
meecrob
Posts: 163
Joined: Sun Jun 19, 2016 6:15 pm

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

Wed Feb 12, 2020 4:29 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.


Your problem is that you are listening to pilots.
 
NonTechAvLover
Posts: 81
Joined: Tue Dec 17, 2019 5:09 am

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

Wed Feb 12, 2020 4:30 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"?


I was referring to cruising. Takeoff and ascent are more like what a rocket does, not really flying either. Joking aside, there is something in flying that implies floating (being lighter) and that sense is violated when a metal object takes to the skies. Anyone who has put his hand out of a speeding car and angled his hand (almost always boys) understands how planes fly, I think, at least feels it.
 
Ertro
Posts: 131
Joined: Thu Apr 04, 2019 9:28 pm

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

Wed Feb 12, 2020 4:33 pm

Karlsands wrote:
This happens because.... There is no mystery to this


This is exactly what everybody thinks. The problem is that nobody writes the explanation in any way that are similar to nobody else and violent disagreement follows.
 
frmrCapCadet
Posts: 4223
Joined: Thu May 29, 2008 8:24 pm

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

Wed Feb 12, 2020 4:35 pm

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.
Buffet: the airline business...has eaten up capital...like..no other (business)
 
User avatar
Moose135
Posts: 3176
Joined: Mon Oct 04, 2004 11:27 pm

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

Wed Feb 12, 2020 4:48 pm

How do planes fly? It's FM...

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

They beat the air into submission!
:rotfl:
KC-135 - Passing gas and taking names!
 
Amiga500
Posts: 2645
Joined: Tue Mar 03, 2015 8:22 am

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

Wed Feb 12, 2020 4:57 pm

... and there was me thinking the starting vortex (which obviously then expands to include the other 3 in the system, bound vortex and two trailing vortices) with Kutta condition was well understood...

https://www.youtube.com/watch?v=bvV7-9wAXc0

Perhaps Mr McLean is searching for an explanation of why these form?

Bernoulli is a a principle of connecting strictly subsonic airflow velocity with pressure. It does not explain why the velocities are what they are.
 
User avatar
TVNWZ
Posts: 2268
Joined: Fri Feb 17, 2006 9:28 am

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

Wed Feb 12, 2020 5:08 pm

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


Because sometimes they don't?
 
mxaxai
Posts: 1885
Joined: Sat Jun 18, 2016 7:29 am

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

Wed Feb 12, 2020 5:22 pm

Tugger wrote:
“The pressure differences exert the lift force on the airfoil, while the downward turning of the flow and the changes in flow speed sustain the pressure differences.” It is this interrelation that constitutes a fifth element of McLean’s explanation: the reciprocity among the other four.

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.
 
bhill
Posts: 1829
Joined: Thu Sep 13, 2001 8:28 am

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

Wed Feb 12, 2020 5:31 pm

Odd...I posted this topic in Tech Ops yesterday...where did it go???
Carpe Pices
 
Karlsands
Posts: 229
Joined: Sat Sep 16, 2017 2:53 pm

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

Wed Feb 12, 2020 5:49 pm

Ertro wrote:
Karlsands wrote:
This happens because.... There is no mystery to this


This is exactly what everybody thinks. The problem is that nobody writes the explanation in any way that are similar to nobody else and violent disagreement follows.

Because it’s true
 
cedarjet
Posts: 8804
Joined: Mon May 24, 1999 1:12 am

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

Wed Feb 12, 2020 5:56 pm

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.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
 
User avatar
Seabear
Posts: 322
Joined: Fri Jun 17, 2016 3:05 pm

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

Wed Feb 12, 2020 5:58 pm

I can't wait to see how the Media spins this "shocking revelation"...."Is Your Next Flight At Risk of Falling From The Sky? Film at 11..."
 
Ertro
Posts: 131
Joined: Thu Apr 04, 2019 9:28 pm

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

Wed Feb 12, 2020 6:01 pm

Karlsands wrote:
Ertro wrote:
Karlsands wrote:
This happens because.... There is no mystery to this


This is exactly what everybody thinks. The problem is that nobody writes the explanation in any way that are similar to nobody else and violent disagreement follows.

Because it’s true


Yeah. It is true, but the problem is that your explanation does not explain why this particular situation and turn from headwind into tailwind is especially dangerous. Your explanation was more about how turning in general is problematic. Millions of turns happen every day at low altitudes without stalling when this particular turn from headwind into tailwind has really ugly percentages for failure that would need some specific explanation.
Last edited by Ertro on Wed Feb 12, 2020 6:05 pm, edited 1 time in total.
 
User avatar
Tugger
Topic Author
Posts: 10608
Joined: Tue Apr 18, 2006 8:38 am

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

Wed Feb 12, 2020 6:02 pm

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
I don’t know that I am unafraid to be myself, but it is hard to be somebody else. - W. Shatner
There are many kinds of sentences that we think state facts about the world but that are really just expressions of our attitudes. - F. Ramsey
 
peterinlisbon
Posts: 1804
Joined: Wed Feb 01, 2006 3:37 am

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

Wed Feb 12, 2020 6:02 pm

The reason that nobody really knows how planes fly is that it's very hard to see the strings that are holding them up.
 
User avatar
casinterest
Posts: 11530
Joined: Sat Feb 12, 2005 5:30 am

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

Wed Feb 12, 2020 6:09 pm

I feel like the article and the explanations are all tossing out one of the constants required to get lift. Thrust. That low pressure would not exists without speeding up the air flow surrounding that wing. The low pressure is the result of Thrust/Energy exerted to get lift. That low pressure is not powerful enough until the flow over and under that wing is strong enough to produce a lower pressure/vacuum to overcome drag and gravity .
Where ever you go, there you are.
 
User avatar
casinterest
Posts: 11530
Joined: Sat Feb 12, 2005 5:30 am

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

Wed Feb 12, 2020 6:13 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



Here is the real question, what is the stall speed of an inverted plane, and can you keep that plane level without moving the flaps or stabilizers.

We know cylindrical rockets can launch, roll, and still go to space, but they fall better than they glide.
Where ever you go, there you are.
 
User avatar
Tugger
Topic Author
Posts: 10608
Joined: Tue Apr 18, 2006 8:38 am

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

Wed Feb 12, 2020 6:14 pm

casinterest wrote:
I feel like the article and the explanations are all tossing out one of the constants required to get lift. Thrust. That low pressure would not exists without speeding up the air flow surrounding that wing. The low pressure is the result of Thrust/Energy exerted to get lift. That low pressure is not powerful enough until the flow over and under that wing is strong enough to produce a lower pressure/vacuum to overcome drag and gravity .

This is mentioned in the article:
How is it possible for each element of the interaction to sustain and reinforce all of the others? And what causes this mutual, reciprocal, dynamic interaction? McLean’s answer: Newton’s second law of motion.

Newton’s second law states that the acceleration of a body, or a parcel of fluid, is proportional to the force exerted on it. “Newton’s second law tells us that when a pressure difference imposes a net force on a fluid parcel, it must cause a change in the speed or direction (or both) of the parcel’s motion,” McLean explains. But reciprocally, the pressure difference depends on and exists because of the parcel’s acceleration.


Tugg
I don’t know that I am unafraid to be myself, but it is hard to be somebody else. - W. Shatner
There are many kinds of sentences that we think state facts about the world but that are really just expressions of our attitudes. - F. Ramsey
 
B737Captain1980
Posts: 41
Joined: Tue Jun 21, 2016 11:14 pm

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

Wed Feb 12, 2020 6:18 pm

I don't know who is more dumb, the person that wrote the article, or the genius that is listening to him.
 
User avatar
casinterest
Posts: 11530
Joined: Sat Feb 12, 2005 5:30 am

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

Wed Feb 12, 2020 6:18 pm

Tugger wrote:
casinterest wrote:
I feel like the article and the explanations are all tossing out one of the constants required to get lift. Thrust. That low pressure would not exists without speeding up the air flow surrounding that wing. The low pressure is the result of Thrust/Energy exerted to get lift. That low pressure is not powerful enough until the flow over and under that wing is strong enough to produce a lower pressure/vacuum to overcome drag and gravity .

This is mentioned in the article:
How is it possible for each element of the interaction to sustain and reinforce all of the others? And what causes this mutual, reciprocal, dynamic interaction? McLean’s answer: Newton’s second law of motion.

Newton’s second law states that the acceleration of a body, or a parcel of fluid, is proportional to the force exerted on it. “Newton’s second law tells us that when a pressure difference imposes a net force on a fluid parcel, it must cause a change in the speed or direction (or both) of the parcel’s motion,” McLean explains. But reciprocally, the pressure difference depends on and exists because of the parcel’s acceleration.


Tugg

I Feel like this is a complex article trying to fool us into forgetting what we already know. You can't fly without power.
Where ever you go, there you are.
 
hivue
Posts: 2076
Joined: Tue Feb 26, 2013 2:26 am

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

Wed Feb 12, 2020 6:19 pm

casinterest wrote:
I feel like the article and the explanations are all tossing out one of the constants required to get lift. Thrust.


So how do sailplanes fly?
"You're sitting. In a chair. In the SKY!!" ~ Louis C.K.
 
carlokiii
Posts: 183
Joined: Sun Oct 04, 2015 11:03 am

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

Wed Feb 12, 2020 6:21 pm

mxaxai wrote:
Tugger wrote:
“The pressure differences exert the lift force on the airfoil, while the downward turning of the flow and the changes in flow speed sustain the pressure differences.” It is this interrelation that constitutes a fifth element of McLean’s explanation: the reciprocity among the other four.

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.

Exactly. I honestly thought I had the scientific explanation of flight understood thanks to a few courses in fluid dynamics, but this article felt like it was looking for answers to questions that weren’t being asked... and I was wondering how much further molecular was the explanation needed to be? Until I realized it may actually be deemed philosophical instead of scientific.

Nonetheless, very interesting article. Brought me back to concepts I had last encountered in university (thankfully!).
 
User avatar
casinterest
Posts: 11530
Joined: Sat Feb 12, 2005 5:30 am

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

Wed Feb 12, 2020 6:22 pm

hivue wrote:
casinterest wrote:
I feel like the article and the explanations are all tossing out one of the constants required to get lift. Thrust.


So how do sailplanes fly?



They glide, and they don't get to the beginning of their "flight" without a massive input of power/Thrust to get them to their starting point.
Where ever you go, there you are.
 
m007j
Posts: 142
Joined: Mon Aug 15, 2016 3:05 pm

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

Wed Feb 12, 2020 6:27 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


Very few airfoils have flat lower surfaces, they're all about curved. You're forgetting about angle of attack-Bernoulli certainly still works if you attack the air correctly with an inverted airfoil. Aerobatic planes generally have symmetrical airfoils to allow inverted flight- you'll notice that even while inverted, these planes have a slight nose up attitude to keep their AoA in the zone to produce lift and remain level.

You can certainly take off with an airfoil' "bottom" edge on the "top", but you'd have to change the AoA or produce enough thrust to overcome gravity (like a rocket) to get airborne.
 
hivue
Posts: 2076
Joined: Tue Feb 26, 2013 2:26 am

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

Wed Feb 12, 2020 6:30 pm

mxaxai wrote:
Tugger wrote:
“The pressure differences exert the lift force on the airfoil, while the downward turning of the flow and the changes in flow speed sustain the pressure differences.” It is this interrelation that constitutes a fifth element of McLean’s explanation: the reciprocity among the other four.

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.
"You're sitting. In a chair. In the SKY!!" ~ Louis C.K.
 
dopplerd
Posts: 103
Joined: Fri Jul 01, 2016 7:30 pm

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

Wed Feb 12, 2020 6:31 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


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.
 
bigb
Posts: 1125
Joined: Fri Nov 07, 2003 4:30 pm

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

Wed Feb 12, 2020 6:31 pm

Ertro wrote:
Karlsands wrote:
Ertro wrote:

This is exactly what everybody thinks. The problem is that nobody writes the explanation in any way that are similar to nobody else and violent disagreement follows.

Because it’s true


Yeah. It is true, but the problem is that your explanation does not explain why this particular situation and turn from headwind into tailwind is especially dangerous. Your explanation was more about how turning in general is problematic. Millions of turns happen every day at low altitudes without stalling when this particular turn from headwind into tailwind has really ugly percentages for failure that would need some specific explanation.


Lol its true and it doesn't matter if its a tailwind or headwind. It can be calm winds. Any time an airplane enters into a turn, the horizontal component of lift always decreases in a tradeoff with the plane's vertical component of lift which requires an increase of total lift to main altitude (i.e. more thrust required and additional backpressure). In a turn, depending on how steep, an aircraft stall speed increases as well.

Takeoff is one of the most critical phases of flight especially in an SE aircraft because the airplane is in a low energy state with low airspeed. Most single-engine pistons climb speed range in 65-80 knot range depending on configuration with the average wings level full break stall speed in the 35-45 knot range depending on the configuration. A pilot who attempts to make a 180-degree turn under 1000 AGL in a single-engine piston plane is playing with fire because one they are have lost the thrust they need to compensate of the lost of horizontal component of lift and they end up stalling and spinning because in a turn the stall speed increases. 1000 feet is not enough altitude to recover from a spin. Hence is why 180-turns are most of the times not successful unless you understand that if you attempt it, you are pushing the nose down to decrease the AOA and inncrease speed to trade-off altitude to make that turn.

This is basic private pilot 101 aerodynamics.
 
User avatar
seahawk
Posts: 9630
Joined: Fri May 27, 2005 1:29 am

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

Wed Feb 12, 2020 6:34 pm

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.


The position of the airfoil in the airstream does not depend on the orientation of the plane.
 
iberiadc852
Posts: 301
Joined: Thu May 26, 2005 8:23 am

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

Wed Feb 12, 2020 6:35 pm

casinterest wrote:
hivue wrote:
casinterest wrote:
I feel like the article and the explanations are all tossing out one of the constants required to get lift. Thrust.


So how do sailplanes fly?



They glide, and they don't get to the beginning of their "flight" without a massive input of power/Thrust to get them to their starting point.


So you have an aircraft on a platform and expose it to a 300 km/h air stream and it doesn't lift?
variety is the spice of life; that's what made the "old times" so good
 
UpNAWAy
Posts: 681
Joined: Thu Aug 11, 2016 12:42 pm

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

Wed Feb 12, 2020 6:38 pm

Pigs can fly.................................................................If they have enough thrust!!!!!!!!!!
 
rbavfan
Posts: 3619
Joined: Fri Apr 17, 2015 5:53 am

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

Wed Feb 12, 2020 6:38 pm

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



lol Interesting concept. So how do certain talk show host stay on the ground?
 
User avatar
Tugger
Topic Author
Posts: 10608
Joined: Tue Apr 18, 2006 8:38 am

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

Wed Feb 12, 2020 6:41 pm

iberiadc852 wrote:
casinterest wrote:
hivue wrote:

So how do sailplanes fly?



They glide, and they don't get to the beginning of their "flight" without a massive input of power/Thrust to get them to their starting point.


So you have an aircraft on a platform and expose it to a 300 km/h air stream and it doesn't lift?

Interestingly, if you look at hang gliding, the energy input is from "running" into the wind, then it needs to hit an updraft of sufficient energy to allow it to keep "falling through the air".

... kinda, sorta... :spin:

Tugg
I don’t know that I am unafraid to be myself, but it is hard to be somebody else. - W. Shatner
There are many kinds of sentences that we think state facts about the world but that are really just expressions of our attitudes. - F. Ramsey
 
rbavfan
Posts: 3619
Joined: Fri Apr 17, 2015 5:53 am

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

Wed Feb 12, 2020 6:51 pm

Karlsands wrote:
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.


This happens because without an increase in power or back pressure an aircraft WILL lose lift in a turn. When you are gliding below 1000 feet in a traffic pattern and attempt to return to the field you’re sacrificing more altitude the higher the degree of the turn is. Obviously if one tried to trim nose up and or keep back pressure while turning without any sufficient power the aircraft will stall and enter a low altitude spin. There is no mystery to this



Part of the reason is when the heavy engine is producing airflow, that airflow helps counter the weight of the engine. When the engine does not produce flow its fwd dead weight fwd which can create stall at lower weights..
 
flilot
Posts: 22
Joined: Sun Aug 12, 2007 1:45 am

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

Wed Feb 12, 2020 6:55 pm

This is why I scoff at "scientists" wheeled out by news corps etc on TV or in print media to "prove" things like Global Warming.
"Science" is mostly a bunch of theories with no actual understandings or proof. Some people with fancy letters after their name often proffer theories as facts, and some other people lap it up as so. Please don't.

Question everything.
 
rbavfan
Posts: 3619
Joined: Fri Apr 17, 2015 5:53 am

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

Wed Feb 12, 2020 6:58 pm

Ertro wrote:
Karlsands wrote:
This happens because.... There is no mystery to this


This is exactly what everybody thinks. The problem is that nobody writes the explanation in any way that are similar to nobody else and violent disagreement follows.


A pilot write the way pilots think, an engineer & programmers write based on their knowledge. Few of the three groups can write a manual that you can read. Same reason people have issues learning programming or use of a computer app. They do not think like the programmers do. Apple used to have a group that knew how to write manuals for lay people. Now days they have fallen back on this problem as well. But at least they know how to do it, so customers can understand..
 
User avatar
Tugger
Topic Author
Posts: 10608
Joined: Tue Apr 18, 2006 8:38 am

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

Wed Feb 12, 2020 7:00 pm

flilot wrote:
This is why I scoff at "scientists" wheeled out by news corps etc on TV or in print media to "prove" things like Global Warming.
"Science" is mostly a bunch of theories with no actual understandings or proof. Some people with fancy letters after their name often proffer theories as facts, and some other people lap it up as so. Please don't.

Question everything.

Uhh... I think you fundamentally misunderstand science and what it is. And are thereby confused. The entire basis of science IS "question everything".

My question to you is: Why don't you question those who say "It is not happening"?

Question everything.

Tugg
I don’t know that I am unafraid to be myself, but it is hard to be somebody else. - W. Shatner
There are many kinds of sentences that we think state facts about the world but that are really just expressions of our attitudes. - F. Ramsey
 
User avatar
casinterest
Posts: 11530
Joined: Sat Feb 12, 2005 5:30 am

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

Wed Feb 12, 2020 7:03 pm

Tugger wrote:
iberiadc852 wrote:
casinterest wrote:


They glide, and they don't get to the beginning of their "flight" without a massive input of power/Thrust to get them to their starting point.


So you have an aircraft on a platform and expose it to a 300 km/h air stream and it doesn't lift?

Interestingly, if you look at hang gliding, the energy input is from "running" into the wind, then it needs to hit an updraft of sufficient energy to allow it to keep "falling through the air".

... kinda, sorta... :spin:

Tugg



A plane can take off the energy to get the air over the wings to 300 MPH exists. However you are still getting power/Thrust from somewhere, and in this case it would be reverse thrust as instead of propelling the plane, you have air propelled .to the plane.
Where ever you go, there you are.
 
UpNAWAy
Posts: 681
Joined: Thu Aug 11, 2016 12:42 pm

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

Wed Feb 12, 2020 7:11 pm

This is kind of like asking is black and white colors, or do objects have color? It is a known thing just not easily explained.
 
hivue
Posts: 2076
Joined: Tue Feb 26, 2013 2:26 am

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

Wed Feb 12, 2020 7:46 pm

m007j wrote:
Very few airfoils have flat lower surfaces, they're all about curved. You're forgetting about angle of attack-Bernoulli certainly still works if you attack the air correctly with an inverted airfoil. Aerobatic planes generally have symmetrical airfoils to allow inverted flight- you'll notice that even while inverted, these planes have a slight nose up attitude to keep their AoA in the zone to produce lift and remain level.

You can certainly take off with an airfoil' "bottom" edge on the "top", but you'd have to change the AoA or produce enough thrust to overcome gravity (like a rocket) to get airborne.


Airfoils come in all shapes: curved top surface, curved bottom surface, curved both surfaces, blunt lead edge, sharp leading edge, etc. One thing they all have in common, though, is a sharp trailing edge. Apparently this causes air from the bottom surface wanting to move around the trailing edge to the top surface to be foiled (parden the pun) in this effort due to the effectively infinitely small turning radius, asisting in creating the low pressure area on the top surface.
"You're sitting. In a chair. In the SKY!!" ~ Louis C.K.
 
bigb
Posts: 1125
Joined: Fri Nov 07, 2003 4:30 pm

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

Wed Feb 12, 2020 8:29 pm

rbavfan wrote:
Karlsands wrote:
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.


This happens because without an increase in power or back pressure an aircraft WILL lose lift in a turn. When you are gliding below 1000 feet in a traffic pattern and attempt to return to the field you’re sacrificing more altitude the higher the degree of the turn is. Obviously if one tried to trim nose up and or keep back pressure while turning without any sufficient power the aircraft will stall and enter a low altitude spin. There is no mystery to this



Part of the reason is when the heavy engine is producing airflow, that airflow helps counter the weight of the engine. When the engine does not produce flow its fwd dead weight fwd which can create stall at lower weights..


Wait what? No! This has nothing to do with the weight of the engine. Lol. See my response above, this is aerodynamics 101, private pilot level.
 
Mightyflyer86
Posts: 26
Joined: Sat Dec 28, 2019 3:50 pm

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).

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos