Why Do Aircraft fy? Flat Plate Lift Vs Bernoulli?
With or without a camber on the upper surface of a wing, the pushing force on the underside of a wing is not increased as a result of the lowering of static pressure on the upper surface, but the static force acting downwards on the upper surface of the wing is reduced as a result of reducing static pressure,
...thereby allowing the wing to be drawn (sucked/pulled/whatever) into the reduced pressure zone.
OK I think I know how to make my point.
You have a long straw 1 metre long and made of glass. You put the end of the straw in a bowl full of mercury and start sucking on the straw. When you do this you notice the mercury travel up the straw towards you mouth. It seems the harder you suck, the further you are pulling the mercury towards you mouth.
You think to yourself if I suck really really hard I will be able to pull the mercury right up to the top of the straw. But as hard as you try, you can't suck hard enough for it to reach the top and your mouth starts to hurt for the extra effort you made.
If you are determined to get the mercury to the top of the straw, you might decide to let a mechanical pump to take over the sucking. Surely if you had a pump strong enough, you would be able to suck/pull/draw the mercury to the top of the metre-long straw!
So that is what you do. You connect a very powerful pump which is able to create a vacuum so strong, it causes 99.99% of air in the straw to be removed. It's almost a perfect vacuum inside the straw now.
So guess what? The mercury still didn't reach the top of the straw! In fact it only travelled 76 cm above the level of mercury in the bowl. Now does 76 cm mean anything to you? No? How about if you convert it to inches?
Now I ask you? Is the mercury in the straw being sucked/pulled/drawn up the straw, or is it being pushed from below?
What I am asserting is that the sucking of air up and out of the top of the straw, is simply the removal of the opposing downward force on the mercury from that air pressure on the mercury surface on the inside of the straw. The force of air pressure pushing the mercury up the tube from below does so until the weight of mercury in the tube above the level of mercury in the bowl plus the remaining downward pressure in the partial vacuum is equal to the pressure pushing down on the surface of the mercury in the bowl and up the tube.
Same thing with the top surface of the wing. You are reducing the downward force of air pressure on the top surface of the wing. But there is still a downward pressure! It's just that it is not enough to balance the upward force on the underside of the wing.
Last edited by Blip; 23rd Dec 2008 at 00:05.
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An aircraft cannot fly unless the paperwork equals or exceeds the weight of the craft. Also noise generators are required for propulsion. More noise, the faster/better it flies.
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Haven't you guys ever heard of the passenger theory?
Most aeronautical engineers and the general public associate the lift generated by a wing with the differential pressure between the upper and lower surfaces of the wing. Nothing could be further from the truth. In reality, the lift required to make a commercial aircraft airborne is furnished by the passengers. Further, the lift is inversely proportional to both the wing size and the distance to be traveled. Farther further, the distance to be traveled has a non-linear relationship to lift, as will become clear in the following explanation.
1. How passengers provide lift for commercial aircraft:
The lift required for an aircraft to take off is furnished by the passengers pulling up on their seat armrests.
2. How take-off lift is initiated by the pilot:
After the aircraft reaches the end of the runway preparatory to take-off, the captain will advance the throttles on the engines. This action has two purposes: a) to provide horizontal thrust to propel the aircraft down the runway, and b) to increase the Passenger Aggregate Fear Level (PAFL) by raising the noise level in the cabin. The consequent rise in PAFL causes the passengers to strenuously lift up on their seat armrests, thus imparting lift to the aircraft. As we can readily see, the engines have two purposes, to move the aircraft horizontally and to scare the bejabbers out of the passengers.
3. How the duration and degree of lift is modulated by the pilot:
Once cruising altitude is reached, the pilot will throttle the engines back to lower the noise level. The reduction in noise level results in a reduction in PAFL, with a consequent decrease in lift. It is necessary for the pilot to make only minor changes in noise level to maintain straight and level flight. In some instances where the PAFL does not decrease sufficiently to prevent further climbing, the captain may order that free drinks be passed around, thus further relaxing the passengers and lowering the PAFL.
One may observe that on most aircraft the first-class passengers are automatically anaesthetized by the use of free booze. Clearly first-class passengers are a source of surplus lift and must be dealt with accordingly.
While the airline industry will never admit it, passenger seating assignment is governed by national characteristics. For instance, Italian males are hardly ever upgraded to first class since they are easily excitable, respond very quickly to outside stimuli and provide almost immediate changes in lift. Clearly one would not want to get the Italians drunk. One difficulty associated with using Italians in this manner is their clannish nature; getting them evenly distributed (left and right, front and back) within the cabin can sometimes be difficult. Stewardesses will often resort to eyelash batting and hip wiggling to move the Italians about the aircraft.
While at first blush it may seem that the French would also be a good source of lift, their uncooperative nature makes lift modulation difficult. One should never fly on an aircraft containing more than 45 percent (by volume) Frenchmen.
The reader will note that Lufthansa, SAS and KLM fly only very large aircraft. Raising the PAFL for the stolid Germans, Swedes and Dutch is notoriously difficult, requiring as many people as possible in each aircraft. The British never fly.
The high takeoff-accident rate for Aeroflot can be attributed to the fact that Russians are generally drunk before they get on the aircraft and are not a reliable source of PAFL-induced lift.
Descent and landing are accomplished using a combination of fatigue and passenger discomfort. It is a happy coincidence that travel over greater distances takes a correspondingly longer time. Even the most casual observer will note that after the aircraft reaches cruising altitude the plane will begin a slow and gradual descent for the balance of the trip. This descent is due to passenger fatigue and discomfort. A detailed explanation of the fatigue factor is unnecessary; suffice to say that with time one's arms get tired and the upward pull on the armrests is reduced. By reducing leg and hip room, passenger discomfort is increased with time, and this distraction is also sufficient to reduce the Passenger Induced Lift, or PIL. The common airline practice of showing only the most boring of in-flight movies is also a lift-modulation technique.
Nota bene: The decrease in the amount and quality of airline food has not been found to be an effective method of PAFL modulation; biogas production offsets any decrease in lift. (See Hindenburg disaster, reference no. 75.)
Several recent instances of sudden aircraft descent have been attributed to air pockets. The air pocket explanation is clearly a feeble attempt on the part of the aircraft crews to avoid blame. In reality the crew neglected to closely monitor passenger fatigue, discomfort or degree of inebriation. Luckily sudden decreases in altitude are self-correcting due to the consequent rise in panic levels and increase in PAFL-induced lift.
Most passengers and the general public believe that the oft experienced practice of circling the airport many times prior to landing is caused by the weather. This is not wholly the case. During bad weather the PAFL increases as the aircraft reaches its destination. This undesirable increase in PAFL and consequent increase in lift must be dissipated by prolonging the flight and further tiring the passengers.
4. Historical basis for this theory and the role of PAFL in aircraft design:
As your may recall from early aeronautical history, the Wright Brothers' aircraft had four wings with a very large surface area. The large surface area of the wings inspired great confidence in Wilbur and Orville, decreasing their PAFL and, as a consequence, decreasing the altitude and flight duration capabilities of the Wright Flyer. As aircraft design advanced, it was found that smaller wing surfaces inspired greater PAFL, with a resultant increase in aircraft performance. Indeed it was not until the advent of the multipassenger aircraft (with a higher PAFL factor) that increases in range and altitude were possible. The only reason wings (albeit very small ones) are still included on aircraft is that they look nice.
It is a little-known historical fact that the general unpopularity and eventual demise of the supersonic passenger aircraft were brought about by the fact that as soon as the aircraft reached supersonic speeds, the passengers could no longer hear the engines. No noise, no PAFL - and no PIL. The aircraft would drop like a rock, causing the PAFL to spike drastically, and the aircraft would then climb precipitously to a supersonic altitude, with a consequent loss of engine noise. The process would then repeat. The resultant sinusoidal altitude and speed changes have rendered supersonic travel impractical.
While further research by really annoying and pedantic people may bring my theory into disrepute, one must keep firmly in mind that even with all of the efforts to reduce personal space aboard commercial airliners, they have yet to remove the armrests. Think about it.
Hope that helps!
Most aeronautical engineers and the general public associate the lift generated by a wing with the differential pressure between the upper and lower surfaces of the wing. Nothing could be further from the truth. In reality, the lift required to make a commercial aircraft airborne is furnished by the passengers. Further, the lift is inversely proportional to both the wing size and the distance to be traveled. Farther further, the distance to be traveled has a non-linear relationship to lift, as will become clear in the following explanation.
1. How passengers provide lift for commercial aircraft:
The lift required for an aircraft to take off is furnished by the passengers pulling up on their seat armrests.
2. How take-off lift is initiated by the pilot:
After the aircraft reaches the end of the runway preparatory to take-off, the captain will advance the throttles on the engines. This action has two purposes: a) to provide horizontal thrust to propel the aircraft down the runway, and b) to increase the Passenger Aggregate Fear Level (PAFL) by raising the noise level in the cabin. The consequent rise in PAFL causes the passengers to strenuously lift up on their seat armrests, thus imparting lift to the aircraft. As we can readily see, the engines have two purposes, to move the aircraft horizontally and to scare the bejabbers out of the passengers.
3. How the duration and degree of lift is modulated by the pilot:
Once cruising altitude is reached, the pilot will throttle the engines back to lower the noise level. The reduction in noise level results in a reduction in PAFL, with a consequent decrease in lift. It is necessary for the pilot to make only minor changes in noise level to maintain straight and level flight. In some instances where the PAFL does not decrease sufficiently to prevent further climbing, the captain may order that free drinks be passed around, thus further relaxing the passengers and lowering the PAFL.
One may observe that on most aircraft the first-class passengers are automatically anaesthetized by the use of free booze. Clearly first-class passengers are a source of surplus lift and must be dealt with accordingly.
While the airline industry will never admit it, passenger seating assignment is governed by national characteristics. For instance, Italian males are hardly ever upgraded to first class since they are easily excitable, respond very quickly to outside stimuli and provide almost immediate changes in lift. Clearly one would not want to get the Italians drunk. One difficulty associated with using Italians in this manner is their clannish nature; getting them evenly distributed (left and right, front and back) within the cabin can sometimes be difficult. Stewardesses will often resort to eyelash batting and hip wiggling to move the Italians about the aircraft.
While at first blush it may seem that the French would also be a good source of lift, their uncooperative nature makes lift modulation difficult. One should never fly on an aircraft containing more than 45 percent (by volume) Frenchmen.
The reader will note that Lufthansa, SAS and KLM fly only very large aircraft. Raising the PAFL for the stolid Germans, Swedes and Dutch is notoriously difficult, requiring as many people as possible in each aircraft. The British never fly.
The high takeoff-accident rate for Aeroflot can be attributed to the fact that Russians are generally drunk before they get on the aircraft and are not a reliable source of PAFL-induced lift.
Descent and landing are accomplished using a combination of fatigue and passenger discomfort. It is a happy coincidence that travel over greater distances takes a correspondingly longer time. Even the most casual observer will note that after the aircraft reaches cruising altitude the plane will begin a slow and gradual descent for the balance of the trip. This descent is due to passenger fatigue and discomfort. A detailed explanation of the fatigue factor is unnecessary; suffice to say that with time one's arms get tired and the upward pull on the armrests is reduced. By reducing leg and hip room, passenger discomfort is increased with time, and this distraction is also sufficient to reduce the Passenger Induced Lift, or PIL. The common airline practice of showing only the most boring of in-flight movies is also a lift-modulation technique.
Nota bene: The decrease in the amount and quality of airline food has not been found to be an effective method of PAFL modulation; biogas production offsets any decrease in lift. (See Hindenburg disaster, reference no. 75.)
Several recent instances of sudden aircraft descent have been attributed to air pockets. The air pocket explanation is clearly a feeble attempt on the part of the aircraft crews to avoid blame. In reality the crew neglected to closely monitor passenger fatigue, discomfort or degree of inebriation. Luckily sudden decreases in altitude are self-correcting due to the consequent rise in panic levels and increase in PAFL-induced lift.
Most passengers and the general public believe that the oft experienced practice of circling the airport many times prior to landing is caused by the weather. This is not wholly the case. During bad weather the PAFL increases as the aircraft reaches its destination. This undesirable increase in PAFL and consequent increase in lift must be dissipated by prolonging the flight and further tiring the passengers.
4. Historical basis for this theory and the role of PAFL in aircraft design:
As your may recall from early aeronautical history, the Wright Brothers' aircraft had four wings with a very large surface area. The large surface area of the wings inspired great confidence in Wilbur and Orville, decreasing their PAFL and, as a consequence, decreasing the altitude and flight duration capabilities of the Wright Flyer. As aircraft design advanced, it was found that smaller wing surfaces inspired greater PAFL, with a resultant increase in aircraft performance. Indeed it was not until the advent of the multipassenger aircraft (with a higher PAFL factor) that increases in range and altitude were possible. The only reason wings (albeit very small ones) are still included on aircraft is that they look nice.
It is a little-known historical fact that the general unpopularity and eventual demise of the supersonic passenger aircraft were brought about by the fact that as soon as the aircraft reached supersonic speeds, the passengers could no longer hear the engines. No noise, no PAFL - and no PIL. The aircraft would drop like a rock, causing the PAFL to spike drastically, and the aircraft would then climb precipitously to a supersonic altitude, with a consequent loss of engine noise. The process would then repeat. The resultant sinusoidal altitude and speed changes have rendered supersonic travel impractical.
While further research by really annoying and pedantic people may bring my theory into disrepute, one must keep firmly in mind that even with all of the efforts to reduce personal space aboard commercial airliners, they have yet to remove the armrests. Think about it.
Hope that helps!
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I have thought five times before joining in on this thread as the whole Bernoulli/Newton thing has been examined by better minds than mine for many years.
However as a young lad I was fortunate to be educated in a practical world of wind tunnels and flight test where one’s day job was primarily to measure and observe things.
Because of this I would like to extend my condolences to VinRouge regarding his cry from the heart and to explain why the Bernoulli ‘notion’ works as well with a flat plate as it does with a traditional curved aerofoil. (Something actually not surprising given so many aircraft use flat plate tailplanes and fly very nicely).
Understanding why this is so becomes easy if you examine the practical evidence concerning two terms not often bandied about on PPRuNe ‘stagnation streamline’ (SS) and ‘stagnation point’ (SP).
I trust the above diag shows how the SS is just an imaginary streamline that defines the plane of separation above which all air goes over the wing and below which it all goes underneath and the SP is the point where the SS meets the aerofoil.
Now consider a flat plate at 90 AOA as in the diag below where the SS and SP are as shown:
If you now reduce the AOA the SP moves forward but remains behind the leading edge until the AOA is exactly zero meaning that at say 10 AOA the air above the SS has further to travel that that below. QED for the folk who follow Mr B
Sorry! I did not intend the diags to be as large and offensive as that!
However as a young lad I was fortunate to be educated in a practical world of wind tunnels and flight test where one’s day job was primarily to measure and observe things.
Because of this I would like to extend my condolences to VinRouge regarding his cry from the heart and to explain why the Bernoulli ‘notion’ works as well with a flat plate as it does with a traditional curved aerofoil. (Something actually not surprising given so many aircraft use flat plate tailplanes and fly very nicely).
Understanding why this is so becomes easy if you examine the practical evidence concerning two terms not often bandied about on PPRuNe ‘stagnation streamline’ (SS) and ‘stagnation point’ (SP).
I trust the above diag shows how the SS is just an imaginary streamline that defines the plane of separation above which all air goes over the wing and below which it all goes underneath and the SP is the point where the SS meets the aerofoil.
Now consider a flat plate at 90 AOA as in the diag below where the SS and SP are as shown:
If you now reduce the AOA the SP moves forward but remains behind the leading edge until the AOA is exactly zero meaning that at say 10 AOA the air above the SS has further to travel that that below. QED for the folk who follow Mr B
Sorry! I did not intend the diags to be as large and offensive as that!
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go down to the biology lab at a university, and have a look at a birds wing cross section.. it is curved.. and im sure if you look at a discovery channel doco about insects, you will see slo motion video of a fly, flying, its wings are flat, yet at such a high AOA, that they create a vortex above their wings to create lift.
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How about this. A 747 is at the end of a runway about to take off. Now ask the question "what makes it fly?" For me, the answer has to be "brute force", the engines, and that has to be Newton. If we have enough Newtons, we can send it into the air like a rocket. Bernoulli is really there to help reduce the amount of force needed to get it airborn and to help control it.
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Lift is created by reducing the cross-sectional area of a stream flow over an aerofoil. This convergent streamtube accelerates a mass of air, and due to bernouilli theorem, causes its pressure to decrease and therefore sets up a pressure differential between top and bottom surface. It is this pressure differential that causes the upwards force (by newtons).
The bottom surface similarly reduces the pressure of the air, which is why a symmetrical aerofoil at 0 AoA has a zero lift coefficient.
The bottom surface similarly reduces the pressure of the air, which is why a symmetrical aerofoil at 0 AoA has a zero lift coefficient.
Last edited by cc2180; 23rd Dec 2008 at 16:42.
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We've all been said that airplanes fly because of the shape of the airfoil- forget the engines, airplanes can glide-, the upper camber being longer
than the lower one, thus, creating a low pressure area above the
wing and a high pressure area below, which makes the airplane fly.
I was told once by an aeronautical engineering professor that the shape
of the wing created a suction force on the upper surface. But, I've been
asking myself for a long time that if this is so, why airplanes can fly
inverted or at a 90 degree bank angle.
than the lower one, thus, creating a low pressure area above the
wing and a high pressure area below, which makes the airplane fly.
I was told once by an aeronautical engineering professor that the shape
of the wing created a suction force on the upper surface. But, I've been
asking myself for a long time that if this is so, why airplanes can fly
inverted or at a 90 degree bank angle.
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KISS. Newton's third law. To every action is an equal and opposite reaction.
That is the answer in a nutshell.
Aircraft fly because the upward force opposite to their weight is equal.
The way aircraft produce such opposing force is by deflecting air downwards. Now whether it is by aerofoils , bernoulli, coanda, flat plates or pure chance, it makes no odds. It is simply the downward deflection of the air that causes an opposite force upward on the aircraft.
That is the answer in a nutshell.
Aircraft fly because the upward force opposite to their weight is equal.
The way aircraft produce such opposing force is by deflecting air downwards. Now whether it is by aerofoils , bernoulli, coanda, flat plates or pure chance, it makes no odds. It is simply the downward deflection of the air that causes an opposite force upward on the aircraft.
Last edited by indiscipline_girl; 24th Dec 2008 at 10:30. Reason: Can't spell bernoulli but then who can?
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Beechbum
But, I've been
asking myself for a long time that if this is so, why airplanes can fly
inverted or at a 90 degree bank angle.
asking myself for a long time that if this is so, why airplanes can fly
inverted or at a 90 degree bank angle.
Understand, the cambered planes you fly... the bottom surface also produces "lift" towards the ground, its just that it's overcome by a greater uplift of the upper surface.
E.g Acrobatic/Military planes use a more symmetrical wing to help with this as one advantage.
Planes can fly at extreme attitudes only when they have powerful engines where pure thrust is used to overcome the lack of vertical lift from the wing (because its at 90 degress).
A red bull racer can fly straight and level at 90 degrees bank. The symmetrical wing of such an acrobatic plane means it hardly changes heading when banked (achieved with elevator input), whilst the huge thrust of the engine compensates for the weight.
VinRouge
Sorry that I can not quote the exact link but I did come accross a NASA link once on this very topic which suggested that about 20% of total lift came from Bernoulli and the rest from reaction ( ie Newton) and the flat plate effect. I think the Bernoulli bit has a lot to do delaying the flow separation which is very bad on a flat plate.
Sorry that I can not quote the exact link but I did come accross a NASA link once on this very topic which suggested that about 20% of total lift came from Bernoulli and the rest from reaction ( ie Newton) and the flat plate effect. I think the Bernoulli bit has a lot to do delaying the flow separation which is very bad on a flat plate.
Indiscipline_girl
Exactly
wing/rotor/prop/turbine etc pushes air down / back/ forward
air pushes wing up/forward /back
any further requires mathematical obscenity
also, most students who start basic aerodynamics class wearing Gucci sunglasses and begin their introduction by ignorantly 'spouting utter tosh'
about Coanda effect blade element theory etc. receive a F
Lester
Exactly
wing/rotor/prop/turbine etc pushes air down / back/ forward
air pushes wing up/forward /back
any further requires mathematical obscenity
also, most students who start basic aerodynamics class wearing Gucci sunglasses and begin their introduction by ignorantly 'spouting utter tosh'
about Coanda effect blade element theory etc. receive a FLester
viscosity vs. density
I keep hearing that viscosity is essential. Yet, point a firehose at an inclined flat plate and it will move. It moves because of fluid's inertia, action, reaction, etc. No viscosity needed.
It sounds like viscosity is something needed for Bernoulli effect. An added bonus.
Perhaps cambered airfoil and related Bernoulli effect figure so prominently in the aerospace curricula because when the science was being created, the starting point was what we saw on birds. Also, early on, power-to-weight of the available powerplants was so low, that, indeed, the airplanes would not fly without the benefit of camber.
Off course, we should keep the camber; it improves performance and saves money, but it is still a rather complex starting point for students, and the resulting confusion abounds.
So, I would like to pose a corollary question:
On Cessna ... ok, ok.. this is a "professional" forum... on Boeing 737-xxx, how much lift % would we loose if we:
1. "turned off" air viscosity?
2. "turned off" air density?
I'm guessing that the above distribution will be a function of speed & weight but any data will help me (and others?) getting sense of order of things.
It sounds like viscosity is something needed for Bernoulli effect. An added bonus.
Perhaps cambered airfoil and related Bernoulli effect figure so prominently in the aerospace curricula because when the science was being created, the starting point was what we saw on birds. Also, early on, power-to-weight of the available powerplants was so low, that, indeed, the airplanes would not fly without the benefit of camber.
Off course, we should keep the camber; it improves performance and saves money, but it is still a rather complex starting point for students, and the resulting confusion abounds.
So, I would like to pose a corollary question:
On Cessna ... ok, ok.. this is a "professional" forum... on Boeing 737-xxx, how much lift % would we loose if we:
1. "turned off" air viscosity?
2. "turned off" air density?
I'm guessing that the above distribution will be a function of speed & weight but any data will help me (and others?) getting sense of order of things.
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how much lift % would we loose if we:
2. "turned off" air density?
1. "turned off" air viscosity?
Making viscosity equal to zero makes Reynolds number infinite.
What will happen to the coefficient of lift of a fixed airfoil shape when the Reynolds number is increased without limit? (This can be done by the unphysical method of "turning off" air viscosity, or by merely impractical method of scaling up the airfoil to absurd sizes)
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According to a book wot I got called 'Flight of Fancy' (tries to explain theory of flight in laymans terms)
On page 47 it says "Flying thus depends on gaseous friction' The point being made is- no friction= no flight also
no friction= no thrust, therefore no flight.
Ask Sophie!
On page 47 it says "Flying thus depends on gaseous friction' The point being made is- no friction= no flight also
no friction= no thrust, therefore no flight.
Ask Sophie!
Originally Posted by chornedsnorkack
how much lift % would we loose if we:
2. "turned off" air density?
That one is trivial. 100 %. No lift in vacuum.
2. "turned off" air density?
That one is trivial. 100 %. No lift in vacuum.
Perhaps I could rephraze the questions to highlight what I'm after:
1. "reduce" air viscosity by a factor of... (pregnant pause.. + Dr. Evil cunning facial expression..) 1000.
2. "reduce" air density by a factor of 1000.
Originally Posted by BarbiesBoyfriend
no friction= no flight
Am I creating something unrealistic with my simplifications?
Last edited by balsa model; 23rd Dec 2008 at 21:18. Reason: clarify what I am trying to ask, maybe
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Personally I think Dash&Thump in post #9 already has hit it on the head.
No viscosity, no trailing edge separation.
So, for a flat plate at an angle of attack, lower pressure on top, same higher pressure on the bottom.
No lift, just drag.
He's probably right, otherwise why would we fuss so much about Reynolds numbers?
CJ
The viscosity is the property that allows the air to seperate at the trailing edge and not come back up and join the other stream in the opposite place to the separation point at the front. I think....
So, for a flat plate at an angle of attack, lower pressure on top, same higher pressure on the bottom.
No lift, just drag.
He's probably right, otherwise why would we fuss so much about Reynolds numbers?
CJ