An aerodynamics question (for experts only)
ENTREPPRUNEUR
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The intent is to simplify as far as possible while still being able to generate useful predictions of the actual system behavior.
Spot on!
There are not one, two , three, four or whatever forces. There is whatever you choose to best explain what is going on.
(If anyone else doesn't understand this just try putting your hand out of each door and window in turn during the cruise. Then write back and tell us there are no forces...)
Spot on!
There are not one, two , three, four or whatever forces. There is whatever you choose to best explain what is going on.
(If anyone else doesn't understand this just try putting your hand out of each door and window in turn during the cruise. Then write back and tell us there are no forces...)
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For the plane to be in S&L flight TWO things must be true...
1). The sum of all the TRANSLATIONAL (straight line) forces must be zero
AND
2). The sum of all the TORQUES/ROTATIONAL forces must be zero.
If you define LIFT(cog), THRUST(cog), WEIGHT(cog) and DRAG(cog) as the orthogonal (right angles) translational components of all the forces that act through the center of gravity then it's true that they balance to zero.
But then you have to look at the other rotational forces. If the true center of drag is a bit above the CoG then that causes a rotational pitch-up. A high thrust line creates a pitch down. The wings pitching moment causes a pitch down etc etc. All these rotational forces must also balance to zero or the plane will be rotating about it's CoG - as it flys along S&L.
Consider what happens when you put the gear down. The translational component of drag increases so you must apply more power to maintain speed. However the center of drag also moves downwards. This causes a rotational force (pitch down) which may require a trim change. The trim change adds up elevator restoring the balance between rotational forces.
The TWO types of force must balance or the plane will either accelerate in some direction or rotate.
1). The sum of all the TRANSLATIONAL (straight line) forces must be zero
AND
2). The sum of all the TORQUES/ROTATIONAL forces must be zero.
If you define LIFT(cog), THRUST(cog), WEIGHT(cog) and DRAG(cog) as the orthogonal (right angles) translational components of all the forces that act through the center of gravity then it's true that they balance to zero.
But then you have to look at the other rotational forces. If the true center of drag is a bit above the CoG then that causes a rotational pitch-up. A high thrust line creates a pitch down. The wings pitching moment causes a pitch down etc etc. All these rotational forces must also balance to zero or the plane will be rotating about it's CoG - as it flys along S&L.
Consider what happens when you put the gear down. The translational component of drag increases so you must apply more power to maintain speed. However the center of drag also moves downwards. This causes a rotational force (pitch down) which may require a trim change. The trim change adds up elevator restoring the balance between rotational forces.
The TWO types of force must balance or the plane will either accelerate in some direction or rotate.
Last edited by cwatters; 14th Jul 2002 at 17:41.
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GoneWest why don't you take thrust out of the equation and consider a glider. It has a downward force due to its Mass and Gravity.
Newtons third law of motion states that every force has an equal and opposite force known as the reaction.
So now we have a glider with weight acting vertically down and the reaction acting vertically upwards. 2 Forces.
For convenience, we resolve the reaction into 2 "man made" forces called lift and drag to explain how the glider flies.
If we add a third force Thrust, there will be an equal and opposite reaction to that force however, the two reactions do not coincide with the established definitions of Lift and Drag. If we combine our reactions into a total reaction, there are now 3 forces acting on our aeroplane, one of which can be resolved into the two forces we require for the purpose of explanation thus giving us the conventional 4 forces.
The reference to 3 forces is to make you think about the forces acting and realise that Lift and Drag are not equal and opposite reactions.
Newtons third law of motion states that every force has an equal and opposite force known as the reaction.
So now we have a glider with weight acting vertically down and the reaction acting vertically upwards. 2 Forces.
For convenience, we resolve the reaction into 2 "man made" forces called lift and drag to explain how the glider flies.
If we add a third force Thrust, there will be an equal and opposite reaction to that force however, the two reactions do not coincide with the established definitions of Lift and Drag. If we combine our reactions into a total reaction, there are now 3 forces acting on our aeroplane, one of which can be resolved into the two forces we require for the purpose of explanation thus giving us the conventional 4 forces.
The reference to 3 forces is to make you think about the forces acting and realise that Lift and Drag are not equal and opposite reactions.
GoneWest why don't you take thrust out of the equation and consider a glider. It has a downward force due to its Mass and Gravity.
Newtons third law of motion states that every force has an equal and opposite force known as the reaction.
So now we have a glider with weight acting vertically down and the reaction acting vertically upwards. 2 Forces.
Newtons third law of motion states that every force has an equal and opposite force known as the reaction.
So now we have a glider with weight acting vertically down and the reaction acting vertically upwards. 2 Forces.
In the case you cite, the 'reaction' is the gravitational force that the glider applies to the earth, equal to the weight of the glider and applied vertically upward -- it has nothing whatsoever to do with the aerodynamic forces.
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I think the tree-force-model has come from the aircraft and wings designing department.
Since both drag and lift are equally subject to airspeed and angle of attack for a specific wing or entire aircraft, it does make sense to calculate with one resultant vector. Otherwise, you would have to calculate lift and drag seperately and the put them together again. For all the graphs it's much easier with one resulting force.
But in the end, both models still describe the same effects, don't they? (Yes, it does fly - and yes, you need some energy to keep it doing so)
Since both drag and lift are equally subject to airspeed and angle of attack for a specific wing or entire aircraft, it does make sense to calculate with one resultant vector. Otherwise, you would have to calculate lift and drag seperately and the put them together again. For all the graphs it's much easier with one resulting force.
But in the end, both models still describe the same effects, don't they? (Yes, it does fly - and yes, you need some energy to keep it doing so)
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We're going round and round saying the same things - and this is from folks who understand the situation. Maybe the first real test might be to see how well a trainee grasps the concept. Perhaps its time we heard from them?
I suspect that we might be getting away from the "KIS" principle that has served us all so well, ever since the Wright Bros flew at Kittyhawk.
I have always referred to a "Total Reaction" force in a climb, for example, but this is a completely different situation to S+L flight. It's quite easy to introduce that concept at that stage because the trainee already has a grasp of the fundamentals that apply in S+L. I'm just not sure it's such a good idea to introduce the TR force in S+L, but I'm happy to accept that others can make it work without bamboozling a trainee.
I suspect that we might be getting away from the "KIS" principle that has served us all so well, ever since the Wright Bros flew at Kittyhawk.
I have always referred to a "Total Reaction" force in a climb, for example, but this is a completely different situation to S+L flight. It's quite easy to introduce that concept at that stage because the trainee already has a grasp of the fundamentals that apply in S+L. I'm just not sure it's such a good idea to introduce the TR force in S+L, but I'm happy to accept that others can make it work without bamboozling a trainee.
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Does a 3 force aircraft fly faster or slower than a 4 force aircraft???????????
Look in you student's, or examiner's face. If you see understanding - you explained it the correct way. If not - try something different.
Look in you student's, or examiner's face. If you see understanding - you explained it the correct way. If not - try something different.
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On the subject of drag, I believe it manifests itself in three somewhat unrelated forms, one only of which is related to angle of attack/lift.
It cannot be analytically useful to consider only three forces, thrust, weight and was it TR?, but it is useful to consider thrust overcoming drag, producing speed through the air, thus acting on the airfoil and producing lift to overcome the weight, while unfortunately generating various drag forces which in turn reduce the resultant force available to produce speed through the air.
Equally unfortunatey none of these effects is linear so the equations relating to the above simplified consideration of flight are both multivariable and non linear.
It cannot be analytically useful to consider only three forces, thrust, weight and was it TR?, but it is useful to consider thrust overcoming drag, producing speed through the air, thus acting on the airfoil and producing lift to overcome the weight, while unfortunately generating various drag forces which in turn reduce the resultant force available to produce speed through the air.
Equally unfortunatey none of these effects is linear so the equations relating to the above simplified consideration of flight are both multivariable and non linear.
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I'm with bookworm on this one.
Add them, subtract them, do what ever you want. Just choose the method that explains most easily the behaviour you are trying to explain.
When teaching PofF I always used to avoid starting a discussion of the forces by looking at straight and level. Its too much of a special case. People get the wrong idea really easily about what orientation the forces are defined in, then you are really in trouble.
CPB
Add them, subtract them, do what ever you want. Just choose the method that explains most easily the behaviour you are trying to explain.
When teaching PofF I always used to avoid starting a discussion of the forces by looking at straight and level. Its too much of a special case. People get the wrong idea really easily about what orientation the forces are defined in, then you are really in trouble.
CPB
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By definition a vector is a force with a direction (magnitude, sens and orientation).
Drag as always been a force (see defintion) in every aerodynamic books. Don't trust flight instructors but rather engineers for those issues.
cheers
Drag as always been a force (see defintion) in every aerodynamic books. Don't trust flight instructors but rather engineers for those issues.
cheers
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Bookworm,
you say: "that's an abuse of Newton's third law"
-------------------------------------------------
NASA don't seem to be too upset by such abuse:
Quote from:
http://www.lerc.nasa.gov/WWW/K-12/airplane/newton.html
"The third law states that for every action (force) in nature there is an equal and opposite reaction. In other words, if object A exerts a force on object B, then object B also exerts an equal force on object A. Notice that the forces are exerted on different objects. The third law can be used to explain the generation of lift by a wing"
you say: "that's an abuse of Newton's third law"
-------------------------------------------------
NASA don't seem to be too upset by such abuse:
Quote from:
http://www.lerc.nasa.gov/WWW/K-12/airplane/newton.html
"The third law states that for every action (force) in nature there is an equal and opposite reaction. In other words, if object A exerts a force on object B, then object B also exerts an equal force on object A. Notice that the forces are exerted on different objects. The third law can be used to explain the generation of lift by a wing"
I start most of my technical groundschool by saying:..' now this is not 100% technically correct but it is the easiest way to understand..'
Usually I get the point across.
I also keep a book from the University of Delft(Holland) from the division of aeronautics as a backup for the occasional smart-ass student....'but what if..'
In that case I get the book out,show them all the formulas and tell them they are only required to know the simple version.(Right?)
One vector split in two doesn't really change the right of existence of the original one now does it?
Our whole system of mathematics is man-made and a model that exists by the grace of not been disproved....yet.
Thrust and weight can also be conbined in a single vector resulting in a single angled force.Counteracted by the lift/drag combo,back to 2 vectors.
Question: Why do we need more power in slow flight,in fact almost as much as in cruise? (Single engine GA)
Most frequent answer: high angle of attack resulting in high lift therefore higher induced drag....
How about this one: as lift works perpendicular to the wing right?
In a turn we can split the lift into a horizontal comp. and a vertical comp.
The horizontal comp. makes the airplane turn and the vert. keeps the plane in the air.
Now for the shocker....the same applies in slow flight.
The horizontal component works in the same direction as drag in the 4-forces model,therefore increasing it.Hey presto more thrust needed.
Never even thought about this till the FAA examiner for my CFI mentioned this.I will forever be grateful.......
Taught most of my students the same thing.Makes great sense after you've experienced slow flight.
Now for the big question...
Does the 1st answer disprove or overrule the 2nd one?
No ....they co-exist.
Some people tend to think their way is the only way
it's not..
I never claim I'm a 100% right,I'm just less wrong than the the people I teach and I hope the'll end up being equally right.
Not more,that's embarrassing.
To finish: Confucius say: ..many thoughts lead to a cluttered brain....
Usually I get the point across.
I also keep a book from the University of Delft(Holland) from the division of aeronautics as a backup for the occasional smart-ass student....'but what if..'
In that case I get the book out,show them all the formulas and tell them they are only required to know the simple version.(Right?)
One vector split in two doesn't really change the right of existence of the original one now does it?
Our whole system of mathematics is man-made and a model that exists by the grace of not been disproved....yet.
Thrust and weight can also be conbined in a single vector resulting in a single angled force.Counteracted by the lift/drag combo,back to 2 vectors.
Question: Why do we need more power in slow flight,in fact almost as much as in cruise? (Single engine GA)
Most frequent answer: high angle of attack resulting in high lift therefore higher induced drag....
How about this one: as lift works perpendicular to the wing right?
In a turn we can split the lift into a horizontal comp. and a vertical comp.
The horizontal comp. makes the airplane turn and the vert. keeps the plane in the air.
Now for the shocker....the same applies in slow flight.
The horizontal component works in the same direction as drag in the 4-forces model,therefore increasing it.Hey presto more thrust needed.
Never even thought about this till the FAA examiner for my CFI mentioned this.I will forever be grateful.......
Taught most of my students the same thing.Makes great sense after you've experienced slow flight.
Now for the big question...
Does the 1st answer disprove or overrule the 2nd one?
No ....they co-exist.
Some people tend to think their way is the only way
it's not..
I never claim I'm a 100% right,I'm just less wrong than the the people I teach and I hope the'll end up being equally right.
Not more,that's embarrassing.
To finish: Confucius say: ..many thoughts lead to a cluttered brain....
StrateandLevel
The quote from NASA is correct. But if you read the pages linked to from the one you cite, you'll find that at no point do they use Newton's Third Law to assert that lift, drag, or total aerodynamic force is a 'reaction' to gravity, which is what you implied.
B2N2
They don't coexist at all. The assertion that the direction of lift (or total aerodynamic force) is perpendicular to the chord line is simply wrong, and you can see that it's wrong by either:
a) inspecting any drag polar and noting that D/L is not equal to the angle of attack
b) thinking about how that rule might work when you know that changing the wing span changes the drag coefficient for th same AOA.
The problem is not that it's a little inaccurate or not 100% correct. It's not even close.
If you want to claim that your rule of thumb is a good aide-memoire for those who don't understand mechanics then fine, go ahead and teach it. But it doesn't make the physics correct, or more importantly it doesn't make the physics as correct as a proper explanation of induced drag!
The quote from NASA is correct. But if you read the pages linked to from the one you cite, you'll find that at no point do they use Newton's Third Law to assert that lift, drag, or total aerodynamic force is a 'reaction' to gravity, which is what you implied.
B2N2
Now for the big question...
Does the 1st answer disprove or overrule the 2nd one?
No ....they co-exist.
Does the 1st answer disprove or overrule the 2nd one?
No ....they co-exist.
a) inspecting any drag polar and noting that D/L is not equal to the angle of attack
b) thinking about how that rule might work when you know that changing the wing span changes the drag coefficient for th same AOA.
The problem is not that it's a little inaccurate or not 100% correct. It's not even close.
If you want to claim that your rule of thumb is a good aide-memoire for those who don't understand mechanics then fine, go ahead and teach it. But it doesn't make the physics correct, or more importantly it doesn't make the physics as correct as a proper explanation of induced drag!
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> How about this one: as lift works perpendicular to the wing right?
Well err no. There is no such thing as a single lift vector..
Sum ALL the force vectors acting on the wing and you get ONE vector pointing upwards and backwards. You can split this into two orthogonal (right angles) vectors pointing in any direction you like...
But for convenience it makes sense to choose one pointing upwards at right angles to the airflow because weight normally acts downwards, also at right angles to the airflow (unless you happen to be wave or ridge soaring etc).
You could define your lift vector to act perpendicular to the wing but you don't have to.
Well err no. There is no such thing as a single lift vector..
Sum ALL the force vectors acting on the wing and you get ONE vector pointing upwards and backwards. You can split this into two orthogonal (right angles) vectors pointing in any direction you like...
But for convenience it makes sense to choose one pointing upwards at right angles to the airflow because weight normally acts downwards, also at right angles to the airflow (unless you happen to be wave or ridge soaring etc).
You could define your lift vector to act perpendicular to the wing but you don't have to.
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Again, I'm with Bookworm.
It really is depressing how much people misunderstand Newton.
Even the RAF have it wrong (I used to teach air cadets, and their manual has the same mix up!).
The equal and opposite the the gravitional force of the earth on the aircraft is:
The gravitational force of the aircraft acting on the earth.
Think about the moon. Not only is it pulled towards the earth by gravity, but the Earth is pulled towards the moon.
Good thing its in Orbit really (Itchy and Scratchy).
If you jump out of an aircraft, not only do you fall towards the Earth, but the Earth falls towards you. The gravitational force is equal and opposite.
Thing is, Earth has more mass, you have relatively little. Newton also tells us F=MA, so you do most of the accelerating.
Eat lots of crewfood till you weigh the same as the Earth. Then, if you jump out, you'll meet the Earth half way.
Acheo.
Vector is an amount and a direction, not a Force and a direction.
A force is a vector, but there are many other vector quantities.
Velocity. Acceleration. Momentum.
All the rotational equivalents, to name a few.
CPB
It really is depressing how much people misunderstand Newton.
Even the RAF have it wrong (I used to teach air cadets, and their manual has the same mix up!).
The equal and opposite the the gravitional force of the earth on the aircraft is:
The gravitational force of the aircraft acting on the earth.
Think about the moon. Not only is it pulled towards the earth by gravity, but the Earth is pulled towards the moon.
Good thing its in Orbit really (Itchy and Scratchy).
If you jump out of an aircraft, not only do you fall towards the Earth, but the Earth falls towards you. The gravitational force is equal and opposite.
Thing is, Earth has more mass, you have relatively little. Newton also tells us F=MA, so you do most of the accelerating.
Eat lots of crewfood till you weigh the same as the Earth. Then, if you jump out, you'll meet the Earth half way.
Acheo.
Vector is an amount and a direction, not a Force and a direction.
A force is a vector, but there are many other vector quantities.
Velocity. Acceleration. Momentum.
All the rotational equivalents, to name a few.
CPB
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The NASA quote...
...can be used to explain lift by a wing. The wing accelerates air downwards and forwards (by applying a downwards and forwards force to it), causing the wing to experience an upward (lift) and backward (lift drag) force. The more you accelerate it (by increasing the angle of attack), the greater the forces are.
What is actually going on when an aeroplane flies is enormously complicated. In order to understand it, we build simplified models. There are no 'right' or 'wrong' models; however some models explain certain aspects of flight better than others. Therefore, it is sometimes easier to think of an aeroplane as having thrust, drag, lift and weight, and sometimes it better to think of it as other forces (for example splitting the lift force into a horizontal and vertical component when the aeroplane is turning).
I think you should be comfortable with both the 3 and the 4 force model (and perhaps models with a few more forces as well to handle turning and spins and so on), but don't think that one is correct and the other is somehow wrong.
A good model should:-
Be simple as possible
Be able to explain observer results
Be able to predict future results
The third law states that for every action (force) in nature there is an equal and opposite reaction. In other words, if object A exerts a force on object B, then object B also exerts an equal force on object A. Notice that the forces are exerted on different objects. The third law can be used to explain the generation of lift by a wing
What is actually going on when an aeroplane flies is enormously complicated. In order to understand it, we build simplified models. There are no 'right' or 'wrong' models; however some models explain certain aspects of flight better than others. Therefore, it is sometimes easier to think of an aeroplane as having thrust, drag, lift and weight, and sometimes it better to think of it as other forces (for example splitting the lift force into a horizontal and vertical component when the aeroplane is turning).
I think you should be comfortable with both the 3 and the 4 force model (and perhaps models with a few more forces as well to handle turning and spins and so on), but don't think that one is correct and the other is somehow wrong.
A good model should:-
Be simple as possible
Be able to explain observer results
Be able to predict future results
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You're making it too complicated
All forces acting on a body at any instant can be resolved into a single force vector.
If the aircraft is at constant altitude then the weight and lift forces cancel out.
If the aircraft is flying 'straight and level' then the lateral forces must also cancel.
And if it is travelling at constant speed -- and thus not accelerating -- then the longitudinal forces cancel.
Therefore there is no overall force acting on the aircraft. Perfectly in line with Newton's first and second laws.
QED
If the aircraft is at constant altitude then the weight and lift forces cancel out.
If the aircraft is flying 'straight and level' then the lateral forces must also cancel.
And if it is travelling at constant speed -- and thus not accelerating -- then the longitudinal forces cancel.
Therefore there is no overall force acting on the aircraft. Perfectly in line with Newton's first and second laws.
QED
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Konkordski
Perfectly true and perfectly useless. I won't quote the usual joke.
Your statement applies equally to a stone resting on the ground.
The objective is to understand the forces acting on an airplane, taking a simplified approach at first and eventually a more refined and complex approach in order, in the case of a pilot, to understand the interreaction of the controls available to the pilot in order to equip the pilot with the ability to fly proficiently and safely.
The four force explanation of straight and level flight carries out the first steps of this path to full understanding .
A resting stone analysis fails to do this.
Perfectly true and perfectly useless. I won't quote the usual joke.
Your statement applies equally to a stone resting on the ground.
The objective is to understand the forces acting on an airplane, taking a simplified approach at first and eventually a more refined and complex approach in order, in the case of a pilot, to understand the interreaction of the controls available to the pilot in order to equip the pilot with the ability to fly proficiently and safely.
The four force explanation of straight and level flight carries out the first steps of this path to full understanding .
A resting stone analysis fails to do this.
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It seems to me we're all taking the same language. The original post was about...
><snip> actually there are only three forces. Weight, thrust and
>total reaction (TR). This TR can be broken down, by the
>scientists, to show a horizontal vector (not a force) which we
>call drag - and a vertical vector which we call lift.
Actually their are an infinite number of forces acting on a plane that sum to zero in S&L constant velocity fight - but we prefer to resolve them (break them down into) components that act in specific directions.
You could say...
The sum of all forces = 0
or
W + T + D + L = 0
or
W + T + L + TR = 0
or
W + everything else = 0
It's all means the same thing.
The important bit is that the sum = zero. If it doesn't then there
is a residual force acting on the plane that will cause it to depart
from S&L constant velocity flight.
><snip> actually there are only three forces. Weight, thrust and
>total reaction (TR). This TR can be broken down, by the
>scientists, to show a horizontal vector (not a force) which we
>call drag - and a vertical vector which we call lift.
Actually their are an infinite number of forces acting on a plane that sum to zero in S&L constant velocity fight - but we prefer to resolve them (break them down into) components that act in specific directions.
You could say...
The sum of all forces = 0
or
W + T + D + L = 0
or
W + T + L + TR = 0
or
W + everything else = 0
It's all means the same thing.
The important bit is that the sum = zero. If it doesn't then there
is a residual force acting on the plane that will cause it to depart
from S&L constant velocity flight.