Why do airplanes need wings?

[Danny Uncanny7]

Bear with me for a second before throwing out the standard grade school answer. If you consider any heavier than air flying object/vehicle. In the most general sense, it stays aloft by pushing air down. Specifically, it needs to constantly accelerate a mass flow of air equal to its weight

Some people explain wings as working by developing suction on the upper surface. But really that's just describing the specific mechanism it uses to vector the oncoming air downwards. So if planes are ultimately using their energy to overcome drag and push air downwards (overcoming the additional pressure drag from the wings), why is it that they can't simply angle the engines and get the same result.

[Hawknc]

Because the moment you turn the engines off, you fall out of the sky. Very, very quickly.

An aircraft without wings is basically a missile, and obviously missiles fly just fine, but if you want to move cargo and people around efficiently and safely it's not ideal. Wings basically provide a mechanism by which you can sacrifice a small amount of forward momentum and use it to counteract gravity, which is more efficient than having a component of thrust continuously dedicated to the same task.

[EdgarJPublius]

why is it that they can't simply angle the engines and get the same result.

Because then it would become a helicopter.

[Danny Uncanny7]

Yeah obviously I understand these things. But I am trying to address the physical understanding in my head. After a bit of thought, I think what my question boils down to is how does the lift/drag coefficient not violate conservation of momentum? If lift and drag are forces, then they are equal to a momentum change in a mass flow. So from the perspective of the mass flow of air being affected by the wing, it seems like the lift momentum change is not equal to the drag momentum change. Basically vertical momentum is appearing from nowhere... or is it?

[Meteoric]

why is it that they can't simply angle the engines and get the same result.

Well, they can, it just isn't called a plane anymore when they do so. The question you seem to be meaning to ask is "why don't they always do that", which I would guess has to do with fuel efficiency or something.

The vertical momentum isn't appearing from nowhere - air pressure is exerting a net upward force, so as the air pushes the plane up, the plane pushes air down.

[Tass]

An airplane moves very fast trough the air and the wings are big, so the wings push on a lot of air in a given time interval. This means that the air does not need to be given that much downwards velocity in order to give the force needed to keep the plane up. Momentum scales directly with velocity, energy scales with velocity squared. Therefore it does not cost that much energy to stay up.

The engines push very strongly on a relatively small amount of air, the wings is a simple way of turning a bit of this into the big (but performing no work) force needed to stay up.

Planes typically has a lift-to-drag ratio of something like ten. This means that the air resistance braking forward movement is only one tenth of the lift force perpendicular to the movement. It is also know as the glide ratio. A plane could in theory use the potential energy of one km height to keep up the speed for 10km of forward flight. Really good sailplanes manage something like 60 or 70.

VERY few airplanes have ever had engines powerful enough to actually lift the entire weight of the plane. It would be a total waste when wings are so relatively simple and cheap to construct.

http://en.wikipedia.org/wiki/Lift-to-drag_ratio

Yeah obviously I understand these things. But I am trying to address the physical understanding in my head. After a bit of thought, I think what my question boils down to is how does the lift/drag coefficient not violate conservation of momentum? If lift and drag are forces, then they are equal to a momentum change in a mass flow. So from the perspective of the mass flow of air being affected by the wing, it seems like the lift momentum change is not equal to the drag momentum change. Basically vertical momentum is appearing from nowhere... or is it?

Momentum is a vector quantity. Drag momentum and lift momentum are perpendicular to each other and therefore has nothing to do with each other. What you want to conserve is energy, and that is taken care of by the differences in velocities.

[jaap]

why is it that they can't simply angle the engines and get the same result.

Well, they can, it just isn't called a plane anymore when they do so.

Except for the Harrier. In the case of a jet like this, the fuel inefficiency of such a take off or landing is outweighed by the military advantages. For passenger planes the cost (in fuel and in maintanance of the more complex system) is not worth it.

[Zamfir]

You know what they say about helicopters, that the blades are just wings that they turn around by the engine?

You can think of that in reverse as well: the wings of a plane are like propeller blades. Instead of turning them, the plane moves constantly forward to move air over the wings. That's good because:

A. you can build huge 'blades' this way, which is more efficient (compare helicopter blades to Harrier engine blades)
B. quite often, you want to go forward at high speed anyway

Aircraft with stationary targets (like reconaissance drones) fly around in circles. That way the whole plane becomes effectively a big single helicopter blade.

[Carnildo]

Bear with me for a second before throwing out the standard grade school answer. If you consider any heavier than air flying object/vehicle. In the most general sense, it stays aloft by pushing air down. Specifically, it needs to constantly accelerate a mass flow of air equal to its weight

Some people explain wings as working by developing suction on the upper surface. But really that's just describing the specific mechanism it uses to vector the oncoming air downwards. So if planes are ultimately using their energy to overcome drag and push air downwards (overcoming the additional pressure drag from the wings), why is it that they can't simply angle the engines and get the same result.

See lifting body and rocket.

[scootwhoman]

Bear with me for a second before throwing out the standard grade school answer. If you consider any heavier than air flying object/vehicle. In the most general sense, it stays aloft by pushing air down. Specifically, it needs to constantly accelerate a That Mass flow of air equal to its weight

Some people explain wings as working by developing suction on the upper surface. But really that's just describing the specific mechanism it uses to vector the oncoming air downwards. So if rockets are ultimately using their energy to overcome drag and push air downwards (overcoming the additional pressure drag from the wings), why is it that they won't simply angle the engines and get the same result.

Um, I think that you may have confused aerodynamic lift with reaction. Rockets work on the principle of ejecting mass to move in the opposite direction. Pushing air has nothing to do with it.

[Seraph]

Isn't the answer to this question the fact that kinetic energy is proportional to velocity squared? My understaning is that in order to stay airborne over a given period of time you need to transfer a certain amount of momentum to the air (i.e. to maintain flight at any particular instant you need to be applying a force that counteracts the force of gravity). So if you halve the amount of air you're acting on then you need to give it twice as much velocity, and therefore need double the amount of energy?

[mfb]

That is important, too, right. Wings increase the amount of air which is pushed down, and increase the lift to drag ratio.

[Puppyclaws]

Planes fly on the power of belief. Most plane passengers believe that only things that have wings are capable of flight. Therefore, planes are designed with unnecessary appendages we dub "wings" in order to help people believe they are capable of flight, thus actually helping to keep them aloft.

...or the things that people are saying above.

[gorcee]

Also, jet engines require an inlet velocity to operate for extended periods at high thrust. So, there's that, too.

[johnny_7713]

Bear with me for a second before throwing out the standard grade school answer. If you consider any heavier than air flying object/vehicle. In the most general sense, it stays aloft by pushing air down. Specifically, it needs to constantly accelerate a That Mass flow of air equal to its weight

Some people explain wings as working by developing suction on the upper surface. But really that's just describing the specific mechanism it uses to vector the oncoming air downwards. So if rockets are ultimately using their energy to overcome drag and push air downwards (overcoming the additional pressure drag from the wings), why is it that they won't simply angle the engines and get the same result.

Um, I think that you may have confused aerodynamic lift with reaction. Rockets work on the principle of ejecting mass to move in the opposite direction. Pushing air has nothing to do with it.

Aerodynamic lift is just a fancy way of saying that there are more air molecules pushing on one side of an object than on the other (per time unit), since that is what a pressure difference means. Newton's third law explains that if the air molecules are pushing the wing, the wing must necessarily be pushing back on the air molecules (which indeed it does).

[gorcee]

Bear with me for a second before throwing out the standard grade school answer. If you consider any heavier than air flying object/vehicle. In the most general sense, it stays aloft by pushing air down. Specifically, it needs to constantly accelerate a That Mass flow of air equal to its weight

Some people explain wings as working by developing suction on the upper surface. But really that's just describing the specific mechanism it uses to vector the oncoming air downwards. So if rockets are ultimately using their energy to overcome drag and push air downwards (overcoming the additional pressure drag from the wings), why is it that they won't simply angle the engines and get the same result.

Um, I think that you may have confused aerodynamic lift with reaction. Rockets work on the principle of ejecting mass to move in the opposite direction. Pushing air has nothing to do with it.

Aerodynamic lift is just a fancy way of saying that there are more air molecules pushing on one side of an object than on the other (per time unit), since that is what a pressure difference means. Newton's third law explains that if the air molecules are pushing the wing, the wing must necessarily be pushing back on the air molecules (which indeed it does).

Yes, this is strictly true from a molecular physical standpoint.

It is, however, misleading to describe the phenomenon in this fashion on a macro scale. Wings do not simply fly by pushing the flow field downward. While an airfoil produces downwash, the lift is not typically considered simple a product of momentum exchange (because we would have to account momentum terms in 3-D effects such as trailing sheet and tip vortices all the way from rest!).

Instead, we talk about pressure differences, and then we use momentum conservation in other analyses.

In other words, lift is not achieved by pushing large quantities of air in a mostly downward direction. Lift is achieved by creating a pressure differential between the upper and lower surfaces of the wings. Of course, at very small scales, pressure is an inter-molecular phenomenon, but this is not what we talk about.

[SU3SU2U1]

In other words, lift is not achieved by pushing large quantities of air in a mostly downward direction. Lift is achieved by creating a pressure differential between the upper and lower surfaces of the wings

These are just different words, a different type of analysis, for the same thing. You can use whatever is convenient, but you can understand lift either way. Personally, I find it qualitatively easiest to understand lift in terms of air moving down. Quantitatively pressure works a bit better, but quantitative stories of lift are difficult (the flow is very turbulent).

[Daniel Cohen]

A lot of airplanes have bodies designed to provide enough lift to stay airborne at their regular design speed.

For examples:
- On the F-104's engine intake, the upper surface is shaped to provide lift
- On the Valkyrie, the air pressure from the sides of the body pushes up under the wing.
- The F-22's entire body is shaped for lift.

These airplanes don't need their wings at design speeds, and will run at zero angle-of-attack.

However they usually have to land eventually. At that slower airspeed, then they do need their wings to keep from falling down.

[Danny Uncanny7]

Sorry to drag this up from back, but I did some more thinking about the matter, and still am not really satisfied with the result.

So planes fly because their wings are airfoil shaped to provide more lift than drag at certain angles right? So it might only take 1 unit of thrust to generate 10 units of lift, enough to keep the whole thing in the air.

Yeah okay, but where is that thrust coming from? At least in propeller driving planes, it's coming from other little airfoils spinning around in circles. So these airfoils are also getting the same lift/drag ratio, except their lift becomes the planes thrust. So the only force going in is the engine torque to overcome the drag of the propeller, which in turn overcomes the drag of the aircraft. It's like multiplying the lift/drag ratio of the propeller and the wings And apparently this is more efficient than a helicopter which just directly drives the airfoil skipping the extra intermediary airfoils on the prop. A plane flying in a circle uses less fuel per kg than a helicopter hovering.

So if adding more airfoils at right angles to eachother keeps leveraging up the lift/drag ratio, would it then be even more efficient to have little propeller blades on the ends of the propeller driving that around to drive the propeller to provide thrust?

[gorcee]

Sorry to drag this up from back, but I did some more thinking about the matter, and still am not really satisfied with the result.

So planes fly because their wings are airfoil shaped to provide more lift than drag at certain angles right? So it might only take 1 unit of thrust to generate 10 units of lift, enough to keep the whole thing in the air.

Yeah okay, but where is that thrust coming from? At least in propeller driving planes, it's coming from other little airfoils spinning around in circles. So these airfoils are also getting the same lift/drag ratio, except their lift becomes the planes thrust. So the only force going in is the engine torque to overcome the drag of the propeller, which in turn overcomes the drag of the aircraft. It's like multiplying the lift/drag ratio of the propeller and the wings And apparently this is more efficient than a helicopter which just directly drives the airfoil skipping the extra intermediary airfoils on the prop. A plane flying in a circle uses less fuel per kg than a helicopter hovering.

So if adding more airfoils at right angles to eachother keeps leveraging up the lift/drag ratio, would it then be even more efficient to have little propeller blades on the ends of the propeller driving that around to drive the propeller to provide thrust?

There is no reason for the propeller blades to have the same L/D ratio. In fact, they do not. In addition, airplane wings more or less have the same freestream velocity over every point of the wing (because the airplane flies straight). But prop blades go in a circle, so the tip moves faster than the middle, for instance.

Lift/Drag is not a fixed quantity, it's a derived quantity that can -- and does -- change at different flight conditions.

But, given some L/D at some fixed flight condition, all you're saying is that the generation of L pounds of lift comes at the expense of D pounds of drag. To overcome that drag, you need an engine that provides at least D pounds of thrust.

For a prop driven plane, you then take that thrust calculation, and you grab a propeller, and you figure out how many horsepower you need. Remember that the engine doesn't generate any lift, really. The "lift" direct of the propeller blades is the "thrust" direction of the airplane. And the "drag" direction of the prop blades is always tangential to the orientation of the blade -- this means that the "drag" on the propeller is not counter to the lift of the aircraft, but it's a rotational force on the engine -- it's just engine loading.

Therefore, the equivalent to L/D for the engine is something like Thrust * radius/Loading. And you can tweak that by throwing in a bigger/smaller engine, without at all affecting the L/D characteristics of the airplane.

Remember -- an airplane with a broken engine is going to have the same (pretty much, let's not get into windmill drag) L/D as the powered aircraft.

And to answer your last question -- no, it would not. One, because it would be nearly impossible to machine such a thing to prevent it from breaking. Two, such mechanisms carry a weight penalty. Three, the tip of the prop blade is only in an orientation that would add to the lift of the vehicle ~ 50% of the time. The other 50% of the time it's in the gravity direction. At best, it would cancel out.

That said, there *are* technologies that can be installed on the prop blade to increase engine efficiency that work by modifying the flow around the blade. These, however, do not work by simply adding lift to the vehicle. Rather, it gets into a more complicated area of fluid mechanics that involves control of tip vortices and flow separation.

[Danny Uncanny7]

I think you kind of missed my point in the last post. The original question was why do planes need wings instead of just pointing their engines downwards, because in the end you're just staying up there by accelerating air downwards. It came up that for a prop driven plane at least, this is basically what a helicopter is. So the question became, why are planes more efficient than helicopters. Flying a plane in a circle used a lot less power/kg than hovering a helicopter.

After a bit more thought, what it really comes down to is the mass of air being moved by the lifting surfaces. The larger the mass flow rate you are accelerating downwards, the less power you need to stay in the air. All the wings on a plane are doing really is taking the small high speed air flow being accelerated from the engine, and converting it into a larger lower speed air flow by the wings. But really, planes get a big mass flow efficiency boost because the faster they move, the larger the mass of air they "encounter" every second.

[gorcee]

The larger the mass flow rate you are accelerating downwards, the less power you need to stay in the air.

That's really actually the opposite of what is true. And helicopters in forward flight behave a whole lot more like conventional airplanes than a vehicle that just points the engines upward.

Of course, air vehicles follow Newtonian mechanics, but if you think of flight in pure terms of "equal and opposite reaction", you need to take into account many, many, many factors beyond what are immediately obvious.

It is strictly physically true that wings "push air downward". But this does not jive with the common intuitive sense of, say, momentum conservation (ie, a rocketship). If you want to think of flight in these terms, you mustn't think of it in terms of shoving X pounds of air downward. This is only really true for hovering flight.

[SU3SU2U1]

If a plane is flying and not accelerating (either up or forward), there is a force pushing up on the airplane equal to the weight of the plane.

That means there is a force downward on the air equal to the weight of the plane. Now, you can accelerate a lot of mass by a little bit to achieve this force, or you can accelerate a little bit of air a whole lot.

The problem is, generally as you push air down, you are also pushing air forward. This creates the drag force on the plane (which the engines must overcome).

The goal of a good wing/aerofoil design is to move as much air as possible downward, and as little air as possible forward. The power you need from the engines (at cruising speed/altitude) is the power needed to overcome the drag.

Now, the more air you can push DOWNARD FOR A GIVEN DRAG, the less power you'll need from your engines because you can achieve the lift needed to stay aloft at a lower drag. However, to maintain altitude, a fixed weight will always need the same momentum flow downward (lift=weight).

It is strictly physically true that wings "push air downward". But this does not jive with the common intuitive sense of, say, momentum conservation (ie, a rocketship).

Its not a rocket, but that doesn't mean it doesn't jive with common sense. You can make an inefficient airplane with simple angle of attack wings, I made one out of an old coke box and a cheap radioshack motor, which does get off the ground (it also is prone to stalls). You can also stick your hand out a car window at different angles and feel the lift/drag as you change the angle of your hand. Personally, I find this very intuitive.

Unfortunately, this approach is god-awful for calculation, and its easier to work in a fluid-flow regime. But calculations for flying craft are awful in general, there are important issues hidden in the turbulent fluid-flow

[gorcee]

If a plane is flying and not accelerating (either up or forward), there is a force pushing up on the airplane equal to the weight of the plane.

That means there is a force downward on the air equal to the weight of the plane. Now, you can accelerate a lot of mass by a little bit to achieve this force, or you can accelerate a little bit of air a whole lot.

The problem is, generally as you push air down, you are also pushing air forward. This creates the drag force on the plane (which the engines must overcome).

The goal of a good wing/aerofoil design is to move as much air as possible downward, and as little air as possible forward. The power you need from the engines (at cruising speed/altitude) is the power needed to overcome the drag.

Now, the more air you can push DOWNARD FOR A GIVEN DRAG, the less power you'll need from your engines because you can achieve the lift needed to stay aloft at a lower drag. However, to maintain altitude, a fixed weight will always need the same momentum flow downward (lift=weight).

It is strictly physically true that wings "push air downward". But this does not jive with the common intuitive sense of, say, momentum conservation (ie, a rocketship).

Its not a rocket, but that doesn't mean it doesn't jive with common sense. You can make an inefficient airplane with simple angle of attack wings, I made one out of an old coke box and a cheap radioshack motor, which does get off the ground (it also is prone to stalls). You can also stick your hand out a car window at different angles and feel the lift/drag as you change the angle of your hand. Personally, I find this very intuitive.

Unfortunately, this approach is god-awful for calculation, and its easier to work in a fluid-flow regime. But calculations for flying craft are awful in general, there are important issues hidden in the turbulent fluid-flow

Again, all of these things are strictly true in a physics sense, but they are not true in the same intuitive sense. Yes, the airplane is suspended on air molecules exerting electromagnetic repulsion against the skin of the wing!

But if you fix your coordinate frame on the airplane and visualize the flowfield around the wing, what do we see? We don't see any air moving forward! We see some air moving down, but not by that much. And if we compute the streamlines far enough aft of the trailing edge, using the pressure field, we see air moving up!

So, how can this be? There's no air moving forward, and there's all sorts of air moving up and down!

If we look at a jet engine or a rocket in a coordinate frame fixed on the engine/rocket, however, things are more clear. Rocket go one way, hot gas go the other!

How do we reconcile this? The solution, as I've been saying, is that we cannot simply look at things the same way. Yes, airfoils induce downwash. Yes, there is air that moves down. But if you attempt to extrapolate the "wing make air go down" intuition just a little farther in a vehicle-centric frame, then you might say, "well, let's just angle the wings a little more, and then we'll get more engine thrust, and we should totally improve our L/D ratio, even at the expense of added drag." And then the wing stalls and you fall out of the sky and die.

"Air goes downward, airplane goes upward" only makes sense in hovering flight, or when taking account for the total momentum balance from the very instant of forward flight, which we never do, because that is not only really complicated, but it defies intuition that I should still be considering fluid flow in Los Angeles while I'm over Kansas.

Momentum balance is a very simple thing to understand intuitively; however, it is totally false when thinking about an airplane in flight in the common fashion. It only becomes correct once you factor in many other things that are quite unintuitive.

[Argency]

But if you fix your coordinate frame on the airplane and visualize the flowfield around the wing, what do we see? We don't see any air moving forward! We see some air moving down, but not by that much. And if we compute the streamlines far enough aft of the trailing edge, using the pressure field, we see air moving up!

So, how can this be? There's no air moving forward, and there's all sorts of air moving up and down!

Hang on, something's fishy here. Of course from the frame of the plane no air gets accelerated so much it ends up going forward, but that doesn't say anything about the air's momentum. Some of the air coming towards the plane gets slowed down a great deal, which amounts to imparting negative backward momentum to it, which amounts to imparting forward momentum to it. It's just that you don't impart enough forward momentum to overcome its substantial backward momentum.

Similarly, from the air's point of view the plane doesn't end up going backwards - you might as well have turned it around and said that his description didn't make intuitive sense because the air doesn't see the plane going backwards. Of course, that's irrelevant too because the plane gets slowed down quite a bit, which is the same, from it's point of view, of having backward momentum imparted to it. Momentum is conserved in all frames so as long as you pick a frame and stick to it, you can't go wrong. I think this description seems un-intuitive to you because you're getting your frames muddled. Which is a completely legitimate error, which even James Bond can make: http://xkcd.com/123/

WAIT ONE SECOND. Planes. Reference frames. Thrust/drag. This is becoming suspiciously like a certain argument involving a treadmill. So, even if its probably a false alarm, my anti-troll protocols take hereby take effect: you've heard my opinion and now I won't post again. :p

[SU3SU2U1]

But if you fix your coordinate frame on the airplane and visualize the flowfield around the wing, what do we see? We don't see any air moving forward!

But, as the previous poster mentioned, we see air slowing way down. Thats the drag in this frame.

We see some air moving down, but not by that much.

But what is important is we see a LOT of air moving down by not-that-much.

And if we compute the streamlines far enough aft of the trailing edge, using the pressure field, we see air moving up!

Hopefully we can agree that only whats happening right at the plane matters- the forces in question are local.

But if you attempt to extrapolate the "wing make air go down" intuition just a little farther in a vehicle-centric frame, then you might say, "well, let's just angle the wings a little more, and then we'll get more engine thrust, and we should totally improve our L/D ratio, even at the expense of added drag." And then the wing stalls and you fall out of the sky and die.

Why do wings stall? What is fundamentally happening? What happens is that in low-angle flow, the air flow nicely follows the contour of the aerofoil which means it flows down along the edge and down at the trailing edge. As you near the critical angle, the flow begins to separate from the top of the aerofoil, which means its not flowing downward, hence less lift.

If you try to extrapolate from the lower-angle/laminar flow situation to higher angle-of-attack, you'll get it wrong regardless of whether you are thinking in terms of momentum balance or fluid dynamics.

Also, importantly, nowhere has anyone suggested you can calculate the air flow from simple momentum considerations-only that given an airflow air-goes-down=lift. But keep in mind,calculating the airflow is pretty much impossible- its turbulent around the wings.

"Air goes downward, airplane goes upward" only makes sense... when taking account for the total momentum balance from the very instant of forward flight, which we never do... it defies intuition that I should still be considering fluid flow in Los Angeles while I'm over Kansas.

It defies intuition because its not true. Only flow RIGHT AT THE PLANE matters. The forces in question are basically contact forces. What the air does after it leaves the vicinity of the plane is completely irrelevant, just like the rocket- it doesn't matter where the exhaust goes.

Momentum balance is a very simple thing to understand intuitively; however, it is totally false when thinking about an airplane in flight in the common fashion.

No, no it isn't. The core mechanics of flight of all kinds is air-goes-down/plane-goes-up. Now, lots of different kinds of flight approach the problem of pushing lots of air down differently- an airplane is a very different approach to pushing air down than a helicopter (or a bird, or a bee). But mechanics are still air-down/plane-up.

[Zamfir]

So if adding more airfoils at right angles to eachother keeps leveraging up the lift/drag ratio, would it then be even more efficient to have little propeller blades on the ends of the propeller driving that around to drive the propeller to provide thrust?

There is a simple answer to this: because you don't have to.

A direct mechanical link (like from an engine to the blades) is more efficient in providing the necessary force than propellers. That's why car engines drive the wheels, not a propellor.

A propellor has aerodynamic losses of its own: friction drag, and unavoidably the kinetic energy that has been put in the backward-accelerated air. Propeller-provided thrust is therefore less efficient ( in the order of 30% or so) than the same thrust provided by for example wheels on the ground, or a cable pulling the aircraft forward. It's just that wheels or cables are inpractical in aircraft, so we accept this loss.

Putting little propellers on the tips of regular propellors is therefore a bit silly: you would just be adding another stage of losses.

People did experiment with something like it, though. But in helicopters, where this trick takes away the need for a tail propeller and makes the connection between rotor and fuselage simpler

It is driven by ramjets at the end of the rotors. Apparently, it produces a truly nasty amount of noise.

[gorcee]

No, no it isn't. The core mechanics of flight of all kinds is air-goes-down/plane-goes-up. Now, lots of different kinds of flight approach the problem of pushing lots of air down differently- an airplane is a very different approach to pushing air down than a helicopter (or a bird, or a bee). But mechanics are still air-down/plane-up.

You're apparently completely missing the part where I don't debate this at all. I'm just arguing that it's a totally different intuitive mechanism than the common intuition for fixing your attention on a little envelope vehicle, and expecting lift purely by pushing air down, as a rocket. Because this intuition does not explain vortical behavior.

What I'm saying is that the intuitive sense of "air down, plane up" is perceiving things like a firehose -- fluid goes out, and you get pushed the opposite way. This intuition is necessarily one of momentum balance: even if you don't really know what momentum is or how to calculate it, it does make sense from everyday life. Maybe you disagree on the nature of what I call this common intuition. But if you agree with this intuition, then you must admit that it is one that only makes sense in a momentum-conservation sense. And to account for the fluid momentum, which I agree, is impossible, even if we neglect turbulence, then you have to essentially go back to t=0.

[SU3SU2U1]

And to account for the fluid momentum, which I agree, is impossible, even if we neglect turbulence, then you have to essentially go back to t=0.

No, you really don't go back to t=0. Thats like saying to understand a rocket, you have to keep track of all of its exhaust. You only have look at the exhaust as it leaves the rocket.

Consider a system that is just air and an airplane, and you are working in the frame of the plane. In the absence of the plane, all the air is flowing horizontally. I contend that if you draw a cube around the airplane that just barely fits the plane, and calculate the flow of air into and out of the cube, you can calculate the drag and the lift on the plane. Do you disagree with this?

[Zamfir]

Su, that's surely wrong? If you draw a small box, you need both pressures and flows on the boundaries of the box. As you take the boundary closer and closer, the pressure terms become dominant, until a box fitted to the shape of the aircraft has only pressure terms and no fluid momentum exchange on the boundary.

That's different from a rocket, where the exhaust overpressure is often a small contribution to net the thrust. If you draw a small box around a rocket and only look ar momentum exchange, you often get a reasonable approximation of what's going on.

[SU3SU2U1]

I put the whole plane in a cube, because I wanted to avoid getting close to the boundary of the plane as it complicates the matter. What I want to get at is that the pressure acting on the aircraft (the usual way of thinking about it) is intimately related to the change in momentum of the air (the air-goes-down/plane-goes-up way of thinking about it).

For an analogous situation, imagine you are spraying a firehouse at a planck of wood (in the absence of gravity ). If you draw a box around the planck of wood, and calculate the water flow in and out, you get the force on the planck of wood.

You could also just calculate pressure from the water right at the surface of the wood and get the same answer.

The plane is analogous, though not completely. Gravity acts on the air, which creates a natural pressure gradient with the height of the box. But apart from that, any changes in the momentum flow of the fluid have to be imparted to the plane.

[Zamfir]

Of course, but momentum exchange at the boundary is not enough to calculate the net force on the plane (or on the box). You also have to include the pressures at the boundaries.

In general, an arbitrary volume around a lifting body will not be bounded be an isobaric surface. So the net force on the box consists of both the integated momentum exchange over the boundary, and the integrated pressure. The integrAted pressure around an arbitrary volume will not just yield the buoyancy, it contributes significantly to the net lift and drag.

[mfb]

"I need a velocity != 0 to reach my target."
"Velocity does not help you, you need kinetic energy!"
"I don't care about kinetic energy, I need a force which accelerates me if my current velocity is 0."
"Forget all that stuff, what you need is momentum."

Different points of view, but the same thing (assuming that the subject of the first statement is not massless ). And you can explain your favorite model over and over again, it will not make it "more right" or the others "less right".

[SU3SU2U1]

Of course, but momentum exchange at the boundary is not enough to calculate the net force on the plane (or on the box). You also have to include the pressures at the boundaries.

Yes, absolutely. I've done this sort of calculation a few times in simulations, but I am by no means an aerospace expert. I was/am under the impression that for planes in flight in calm air, the flow effects dominate the pressure terms for volumes of order the size of the plane (at least for magic planes that don't use engines for thrust ), but I am generalizing from only a handful of situations I've worked on.

I am also under the impression that the same is true for volumes centered around the wings in the laminar flow regime.

[Zamfir]

Hmm, which effect dominates at which distance might be tricky, actually. Obviously, it's pressure that dominates very close to the body. At large distances in a simulation, the result will be shaped by the choice of artificial boundary conditions. In reality, turbulence makes the difference unanswerable at larger distances.

It could be that at some distance scale in between, pressure differences typically already reduce to turbulent-fluctuation levels, while flow differences are still macroscopically significant. For the moment, I don't see an obvious reason why this should necessarily be the case, though. And 3-d cases will behave different from 2-d cases, because of three-dimensional vortices.

It's been a while for me too that I 've made such simulations, and I can't say I remember flow fields in enough detail for this. I am pretty sure that at least one compressible, inviscid simulation applied a fixed flow as far-field boundary condition and calculated drag by integrating pressure at the far-field boundary. But I never integrated pressure and momentum flow separately on volumes close around a shape.

[Soralin]

Well the question really, from the context, is really "Why do you need wings, rather than just large engines?" And in that case, that's where the whole rocket stuff comes from. If you want some amount of delta-v, there's two main ways you can go about it. You can throw a small amount of mass at high speed, which doesn't take much mass, but it takes a lot of energy. Or you can throw a large amount of mass at low speed, which doesn't take much energy, but takes a whole bunch of mass.

Now, given that an airplane has an abundant supply of reaction mass available to it(air), the mass requirement isn't a problem, so it's just a matter of energy. And in that case, the more mass you can apply a force to, the less energy you need for the same amount of acceleration. So, if you use wings to direct downward a huge amount of air, with a small force, it takes much less energy than if you're hovering on a jet exhaust (a small amount of air moved at high speed), to get the same result.

That runs deeper than simply mechanical factors or engineering, it's a matter of physics. It will hold true no matter what methods you use to move the air. (I think this is all correct at least)

[gorcee]

Hmm, which effect dominates at which distance might be tricky, actually. Obviously, it's pressure that dominates very close to the body. At large distances in a simulation, the result will be shaped by the choice of artificial boundary conditions. In reality, turbulence makes the difference unanswerable at larger distances.

It could be that at some distance scale in between, pressure differences typically already reduce to turbulent-fluctuation levels, while flow differences are still macroscopically significant. For the moment, I don't see an obvious reason why this should necessarily be the case, though. And 3-d cases will behave different from 2-d cases, because of three-dimensional vortices.

It's been a while for me too that I 've made such simulations, and I can't say I remember flow fields in enough detail for this. I am pretty sure that at least one compressible, inviscid simulation applied a fixed flow as far-field boundary condition and calculated drag by integrating pressure at the far-field boundary. But I never integrated pressure and momentum flow separately on volumes close around a shape.

That's basically how it works, more or less. No one integrates momentum flow, because it's basically computationally impossible, even for low Reynolds number flow. If you look at old aero books, they talk about Eulerian vs. Lagrangian frames -- one of which follows a fluid particle, and one of which fixes on an object that fluid flows around (I forget which frame is which). You can do momentum arguments if you follow the particle, but those become way more difficult in the other frame (although the two frames are shown to be equivalent in inviscid flow because of the symplectic nature of the Euler equations).

Consider a system that is just air and an airplane, and you are working in the frame of the plane. In the absence of the plane, all the air is flowing horizontally. I contend that if you draw a cube around the airplane that just barely fits the plane, and calculate the flow of air into and out of the cube, you can calculate the drag and the lift on the plane. Do you disagree with this?

Not at all. But you can't compute it with a simple momentum-based "air goes down" argument, because you cease the ability to track the momentum of a fluid element once it leaves your cube.

[gorcee]

Su, that's surely wrong? If you draw a small box, you need both pressures and flows on the boundaries of the box. As you take the boundary closer and closer, the pressure terms become dominant, until a box fitted to the shape of the aircraft has only pressure terms and no fluid momentum exchange on the boundary.

That's different from a rocket, where the exhaust overpressure is often a small contribution to net the thrust. If you draw a small box around a rocket and only look ar momentum exchange, you often get a reasonable approximation of what's going on.

Also, in a rocket, the fuel is stored onboard. So you can compute it using momentum considerations because the momentum is internal to the body. This is not the case in an airplane, where the fluid momentum is external to the body.

[Danny Uncanny7]

The larger the mass flow rate you are accelerating downwards, the less power you need to stay in the air.

That's really actually the opposite of what is true.

How so? I think we have all agreed that heavier than air flight ultimately requires the acceleration of air downwards for the plane to stay up. Anyways, the weight is fixed so the net force is fixed. This means on an ongoing basis, accelerating a small mass a lot or a larger mass a little. The minimum energy required increases with the velocity change. For an enormous mass flow to push against it would require miniscule power to stay aloft even before considering the added fluid drag of that velocity change.

Supersonic jets only need tiny stubby wings because they already get an enormous air flow from their velocity. Almost none of their thrust is going to lift induced drag compared to slower aircraft. In other words, the difference in drag between a supersonic jet flying with no lift (some sort of parabolic trajectory), and one with the flaps set for enough lift to keep it in the air indefinitely, will be much less than the equivalent difference in drag for a slower plane of the same mass. Of course the total drag is much higher for the faster plane, but not the lift induced drag.

[Zamfir]

Su, that's surely wrong? If you draw a small box, you need both pressures and flows on the boundaries of the box. As you take the boundary closer and closer, the pressure terms become dominant, until a box fitted to the shape of the aircraft has only pressure terms and no fluid momentum exchange on the boundary.

That's different from a rocket, where the exhaust overpressure is often a small contribution to net the thrust. If you draw a small box around a rocket and only look ar momentum exchange, you often get a reasonable approximation of what's going on.

Also, in a rocket, the fuel is stored onboard. So you can compute it using momentum considerations because the momentum is internal to the body. This is not the case in an airplane, where the fluid momentum is external to the body.

Just to be sure I get your main point: your main objection to momentum explanations is that while you can calculate lift if you know momentum exchange and pressures on a boundary, it doesn't actually give you anything to figure out those values, right? Unlike a rocket, where you can say "it's tossing out X kg/s at m/s, therefore it produces roughly Z force". And no matter whether you draw a boundary close or far from the rocket, you will indeed find roughly that same momentum flow if you integrate over the boundary.

If that's roughly your point, I can only agree. Downward-deflected air is part of the picture that you need in your head to understand lift, but it's not enough.