Simple AoA question
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Nice one Keith.
That explanation is simple enough and not entirely inaccurate.
However, the entirely inaccurate lift theorem needs to stop... As pointed out by NASA courtesy of oggers:
You've spent a bit of time constructing that post and I'm honoured to have precipitated it. Kind regards, Derfred.
That explanation is simple enough and not entirely inaccurate.
However, the entirely inaccurate lift theorem needs to stop... As pointed out by NASA courtesy of oggers:
The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory
Definitely Lift Demons - as described originally by Mary Shafer at NASA.
To abstract from the full paper:
HTH,
PDR
To abstract from the full paper:
According to Shafer "Lift is caused by Lift Demons. These little, invisible demons hold on to the leading and trailing edges of the aircraft and lift it into the air by flapping their wings (so, in a reductionist sense, lift is actually caused by feathers). Some of the demons are a little confused and they hold on backwards, causing drag. The reason that planes stall at high alpha is that the leading edge demons get scared and let go when they can't see the ground anymore. Lift demons have good taste and don't like to look at ugly aircraft, so they hold on backwards on ugly planes. That's why gliders have so much lift and so little drag and why F-4s have lots of drag." This, however, did not address what gives lift to the Lift Demons' wings. Again, according to Shafer "Feathers. The multiple filaments on feathers trap the air molecules and they struggle to escape, which causes the action-reaction that we call lift. Bat wings don't have feathers but they're hairy and that works just about as well (air molecules are a little claustrophobic)."
Because Lift demons hold on backwards on ugly planes to avoid looking at the planes, drag can be calculated using CDU (Coefficient of Drag due to Ugliness) used in correlating aircraft ugliness against Lift Demons' unwillingness to hold on forwards and provide lift.
It was originally believed that Lift Demons got their lift from smaller Lift Demons whose lift was, in turn, produced by even smaller Lift Demons leading to the "Infinite Demons Theory" as proposed by the great Greek philosopher-scientist Miasma. However, with the revival of scientific knowledge that eventually ended the Dark Ages, it was realised that this situation unresolvable according to Zeno's paradox. The "Infinite Demons Theory" works in many problems of engineering significance, however, real understanding requires that the ether be introduced into the analysis at some point. The "Ether Concept" explains why planes fly more efficiently at higher altitudes. This is an absolute necessity when studying orbital and interplanetary travel where, it is believed, many of the Lift Demons are unable to breathe.
Because Lift demons hold on backwards on ugly planes to avoid looking at the planes, drag can be calculated using CDU (Coefficient of Drag due to Ugliness) used in correlating aircraft ugliness against Lift Demons' unwillingness to hold on forwards and provide lift.
It was originally believed that Lift Demons got their lift from smaller Lift Demons whose lift was, in turn, produced by even smaller Lift Demons leading to the "Infinite Demons Theory" as proposed by the great Greek philosopher-scientist Miasma. However, with the revival of scientific knowledge that eventually ended the Dark Ages, it was realised that this situation unresolvable according to Zeno's paradox. The "Infinite Demons Theory" works in many problems of engineering significance, however, real understanding requires that the ether be introduced into the analysis at some point. The "Ether Concept" explains why planes fly more efficiently at higher altitudes. This is an absolute necessity when studying orbital and interplanetary travel where, it is believed, many of the Lift Demons are unable to breathe.
PDR
The type of explanation which is most appropriate depends upon the audience and the purpose of the discussion. For PPL and the initial stages of ATPL, Principles Of Flight training, the explanation which I produced in my initial post (post 7) to this thread is perfectly adequate. Note that I did not actually include the requirement for equal transit time for the two air streams. This omission was quite deliberate, because I am aware of the fact that this line often results in heated arguments, particularly when the audience includes Aerospace Engineering Graduates.
In deciding what is or is not an acceptable explanation when teaching a subject I think that we need to consider the following basic requirements:
1. The explanation must be sufficiently simple to enable the students to understand it. If they cannot understand it then it worthless and possibly harmful.
2. The explanation should aid the understanding of related subjects which will be covered in subsequent lessons. As an example the “lift Fairies” explanation may amuse the students, but it would not help them to understand more complex aspects of lift theory such as stalling or induced drag.
3. The simplification process must not result in dangerous misunderstandings.
Derfred
I do understand why you are so vehemently opposed to the “long path/short path - equal transit time” argument. For most students it is easy to understand, is quite convincing, and fits in well when moving on to the effects of increasing angle of attack, stagnation point movement and stalling.
NASA did not actually argue that this explanation should be eliminated from use. They simple said that it is one of the most commonly used. It is worth considering why it is so popular. Could it be that it satisfies all of the above requirements?
A far worse, but very common incorrect explanation is:
“Propellers produce thrust by accelerating air rearwards.”
Not only is this untrue, but it suggests that the rearward acceleration of the air is a good thing, when in reality it is the means by which vast amounts of energy are wasted.
A far better explanation would be something along the lines of:
“Propellers produce thrust by exerting a rearward force on the air. This causes the air to exert a forward force (the thrust) on the propeller as predicted by Newton’s Third Law.
Unfortunately the air is not rigid enough to resist this rearward propeller force, so it is accelerated rearwards, thereby giving kinetic energy to the air. This transfer of energy from the aircraft to the air represents a waste of fuel. We would get far more miles for each gallon of fuel if we were able to exert the rearward force on something which was rigid enough to remain stationary.”
A campaign to eliminate the lie that thrust is produced by accelerating air rearwards is far more justifiable than your campaign to eliminate the “long path /short path” explanation of lift.
In deciding what is or is not an acceptable explanation when teaching a subject I think that we need to consider the following basic requirements:
1. The explanation must be sufficiently simple to enable the students to understand it. If they cannot understand it then it worthless and possibly harmful.
2. The explanation should aid the understanding of related subjects which will be covered in subsequent lessons. As an example the “lift Fairies” explanation may amuse the students, but it would not help them to understand more complex aspects of lift theory such as stalling or induced drag.
3. The simplification process must not result in dangerous misunderstandings.
Derfred
I do understand why you are so vehemently opposed to the “long path/short path - equal transit time” argument. For most students it is easy to understand, is quite convincing, and fits in well when moving on to the effects of increasing angle of attack, stagnation point movement and stalling.
NASA did not actually argue that this explanation should be eliminated from use. They simple said that it is one of the most commonly used. It is worth considering why it is so popular. Could it be that it satisfies all of the above requirements?
A far worse, but very common incorrect explanation is:
“Propellers produce thrust by accelerating air rearwards.”
Not only is this untrue, but it suggests that the rearward acceleration of the air is a good thing, when in reality it is the means by which vast amounts of energy are wasted.
A far better explanation would be something along the lines of:
“Propellers produce thrust by exerting a rearward force on the air. This causes the air to exert a forward force (the thrust) on the propeller as predicted by Newton’s Third Law.
Unfortunately the air is not rigid enough to resist this rearward propeller force, so it is accelerated rearwards, thereby giving kinetic energy to the air. This transfer of energy from the aircraft to the air represents a waste of fuel. We would get far more miles for each gallon of fuel if we were able to exert the rearward force on something which was rigid enough to remain stationary.”
A campaign to eliminate the lie that thrust is produced by accelerating air rearwards is far more justifiable than your campaign to eliminate the “long path /short path” explanation of lift.
Megan you are confusing cause and effect.
The propeller exerts a rearward force on the air. This force has three effects:
Effect 1. The air exerts an equal and opposite forward force (thrust) on the propeller.
Effect 2. The air is accelerated rearwards.
Effect 3. The rearward acceleration gives the air kinetic energy, which must be provided
by the engine.
The situation when generating lift is similar, just the directions have changed from rearward/forward to downward/upward.
Let’s examine this subject by looking at a few scenarios.
Scenario 1.
We take a helicopter and park it on a concrete pad. The helicopter exerts a downward force (its weight) onto the concrete. The concrete reacts to this by exerting an equal and opposite upward force as predicted by Newton’s Third Law. Because no energy is being transferred from the helicopter to the concrete, the engines do not need to be running and the helicopter can sit there for as long as we want without ever using any fuel.
But a parked helicopter is nothing more than a very expensive shed, so let’s start it up and take it into the hover. The main rotor now exerts a downward force (equal to the helicopter’s weight) onto the air. The air reacts be exerting an equal and opposite upward force (the lift) as predicted by Newton’s Third law. But because the air is unable to resist the downward force, it is accelerated downwards. This acceleration gives the air kinetic energy which must be provided by the engines. The helicopter can only remain in the hover until its fuel supply runs out, because it must provide a constant supply of energy to the air.
The required lift can be generated with or without accelerating anything downwards, but why are the two results so different? Because accelerating the air downwards involves energy being transferred from the helicopter to the air.
Scenario 2.
We want to take a large advertising board and suspend it high above a city. We attach the board to a helium balloon, which is more than able to support the weight of the board, the balloon and its tethering rope. We permit the balloon to float up to the required height then tie the tethering rope to a post. The combined balloon and board exert a downward force on the surrounding air. The air reacts by exerting an equal and opposite upward force on the balloon and board. But the air is not accelerated downwards, so no energy is being lost to the air. The balloon and board can remain in place for as long as we want, without ever using any fuel.
We take the same board and attach it to a helicopter, start the engines and take it into the hover at the required height. The main rotor now exerts a downward force (equal to the helicopter’s weight) onto the air. The air reacts be exerting and equal and opposite upward force as predicted by Newton’s Third law. But because the air is unable to resist the downward force, it is accelerated downwards. This acceleration gives the air kinetic energy which must be provided by the engines. The helicopter can only remain in the hover until its fuel supply runs out because it must provide a constant supply of energy to the air.
Once again the required lift can be generated with or without accelerating air downwards, but the method which does not accelerate anything downwards, is the most efficient.
Scenario 3.
You drive your car along the road at 70 mph. The driving wheels exert a rearward force on the road. The road reacts by exerting an equal an opposite forward force on the wheels. This force pushes the car forward. Because the road surface is rigid enough to resist the rearward force, it is not accelerated rearward. This means that no energy is transferred from the car to the road. This in turn means that all of the energy available at the driving wheels is used to move the car forward.
You modify your car so that the engine does not drive any of the wheels, but instead drives a propeller. The propeller exerts a rearward force on the air. The air reacts by exerting a forward force on the propeller. This forward force pushes the car forward. But the rearward force on the air causes it to accelerate rearwards. This gives kinetic energy to the air. This energy must be provided by the engine, so there is less energy available to do the work of driving the car forward.
The required thrust can be generated with or without accelerating anything rearwards, but the method which does not involve acceleration is the most energy-efficient.
These three scenarios illustrate three facts:
Fact 1. It is not necessary to accelerate anything backwards to create forward thrust.
Fact 2. It is not necessary to accelerate anything downwards to create lift.
Fact 3. Thrust and lift systems which do not accelerate the air are more energy-efficient than those which do.
In designing aircraft to provide the operational flexibility, mobility, speed, manoeuvrability and agility which we need, the propulsion and lift must be provided by interacting with the surrounding air. And because the air is a fluid, these interactions cause it to be accelerated. There are precise mathematical relationships between the accelerations and the thrust or lift generated and these relationships have enabled us to devise powerful equations and analytical tools. But the fact remains that these accelerations are the waste-products of our propulsion and lift systems. They are not the means by which we produce thrust and lift.
To argue otherwise is the equivalent of saying that human being produce waste products in order to obtain the energy needed to life. The truth is that human beings eat in order to obtain the energy needed to live. Human waste products are just an unfortunate consequence of this process.
The propeller exerts a rearward force on the air. This force has three effects:
Effect 1. The air exerts an equal and opposite forward force (thrust) on the propeller.
Effect 2. The air is accelerated rearwards.
Effect 3. The rearward acceleration gives the air kinetic energy, which must be provided
by the engine.
The situation when generating lift is similar, just the directions have changed from rearward/forward to downward/upward.
Let’s examine this subject by looking at a few scenarios.
Scenario 1.
We take a helicopter and park it on a concrete pad. The helicopter exerts a downward force (its weight) onto the concrete. The concrete reacts to this by exerting an equal and opposite upward force as predicted by Newton’s Third Law. Because no energy is being transferred from the helicopter to the concrete, the engines do not need to be running and the helicopter can sit there for as long as we want without ever using any fuel.
But a parked helicopter is nothing more than a very expensive shed, so let’s start it up and take it into the hover. The main rotor now exerts a downward force (equal to the helicopter’s weight) onto the air. The air reacts be exerting an equal and opposite upward force (the lift) as predicted by Newton’s Third law. But because the air is unable to resist the downward force, it is accelerated downwards. This acceleration gives the air kinetic energy which must be provided by the engines. The helicopter can only remain in the hover until its fuel supply runs out, because it must provide a constant supply of energy to the air.
The required lift can be generated with or without accelerating anything downwards, but why are the two results so different? Because accelerating the air downwards involves energy being transferred from the helicopter to the air.
Scenario 2.
We want to take a large advertising board and suspend it high above a city. We attach the board to a helium balloon, which is more than able to support the weight of the board, the balloon and its tethering rope. We permit the balloon to float up to the required height then tie the tethering rope to a post. The combined balloon and board exert a downward force on the surrounding air. The air reacts by exerting an equal and opposite upward force on the balloon and board. But the air is not accelerated downwards, so no energy is being lost to the air. The balloon and board can remain in place for as long as we want, without ever using any fuel.
We take the same board and attach it to a helicopter, start the engines and take it into the hover at the required height. The main rotor now exerts a downward force (equal to the helicopter’s weight) onto the air. The air reacts be exerting and equal and opposite upward force as predicted by Newton’s Third law. But because the air is unable to resist the downward force, it is accelerated downwards. This acceleration gives the air kinetic energy which must be provided by the engines. The helicopter can only remain in the hover until its fuel supply runs out because it must provide a constant supply of energy to the air.
Once again the required lift can be generated with or without accelerating air downwards, but the method which does not accelerate anything downwards, is the most efficient.
Scenario 3.
You drive your car along the road at 70 mph. The driving wheels exert a rearward force on the road. The road reacts by exerting an equal an opposite forward force on the wheels. This force pushes the car forward. Because the road surface is rigid enough to resist the rearward force, it is not accelerated rearward. This means that no energy is transferred from the car to the road. This in turn means that all of the energy available at the driving wheels is used to move the car forward.
You modify your car so that the engine does not drive any of the wheels, but instead drives a propeller. The propeller exerts a rearward force on the air. The air reacts by exerting a forward force on the propeller. This forward force pushes the car forward. But the rearward force on the air causes it to accelerate rearwards. This gives kinetic energy to the air. This energy must be provided by the engine, so there is less energy available to do the work of driving the car forward.
The required thrust can be generated with or without accelerating anything rearwards, but the method which does not involve acceleration is the most energy-efficient.
These three scenarios illustrate three facts:
Fact 1. It is not necessary to accelerate anything backwards to create forward thrust.
Fact 2. It is not necessary to accelerate anything downwards to create lift.
Fact 3. Thrust and lift systems which do not accelerate the air are more energy-efficient than those which do.
In designing aircraft to provide the operational flexibility, mobility, speed, manoeuvrability and agility which we need, the propulsion and lift must be provided by interacting with the surrounding air. And because the air is a fluid, these interactions cause it to be accelerated. There are precise mathematical relationships between the accelerations and the thrust or lift generated and these relationships have enabled us to devise powerful equations and analytical tools. But the fact remains that these accelerations are the waste-products of our propulsion and lift systems. They are not the means by which we produce thrust and lift.
To argue otherwise is the equivalent of saying that human being produce waste products in order to obtain the energy needed to life. The truth is that human beings eat in order to obtain the energy needed to live. Human waste products are just an unfortunate consequence of this process.
So how does a flying aircraft produce lift, or a prop thrust, if its not by accelerating the air mass in the required direction.
You saying NASA don't know what they're talking about?
PS: Scenario 2, you are confusing aerodynamic lift with buoyancy.
If we assume still air conditions and ignore any meteorological factors, then the static pressure at any point in the atmosphere is determined by the mass of the column of air above that point.
Let’s consider two columns, the one immediately above the bottom of the balloon and one immediately adjacent to it.
For any position immediately adjacent to the bottom of the balloon, the column above will be entirely made up of air, which will exert a certain static pressure.
For the column directly above the bottom of the balloon, most of the column will be occupied by air but some will be occupied by the balloon. The balloon is filled with helium which is less dense than air, so its overall density will be less than that of air. This means that the static pressure which this column is able to exert at the bottom of the balloon will be less than that immediately adjacent to it.
Air moves from areas of high pressure to areas of lower pressure, so the air surrounding the bottom of the balloon will tend to move under the balloon, thereby increasing the local static pressure. We now have a situation where the static pressure pushing upwards at the bottom of the balloon is greater than that which can be exerted by the column of air plus helium balloon above. Unless the balloon is tethered it will move upwards. So we have an upward acting lift force which is being generated without the need for any acceleration of the air.
Scenareo 3, the motor vehicle does accelerate the earth in the opposite direction to that of its travel. Due to conservation of momentum, every force on the Earth’s surface has some effect on its rotation. The gradual slowing of the earths rotation is caused by wind friction, not tidal forces as previously thought.
