Birds in a truck - I'm not getting it

[Dale]

But just to help me see something about your most recent comment, what would be the total system OW in the event that I replace my bird with a small remote controlled powered aeroplane. Let me place that aircraft into a tight circle with wings vertical and held at a constant speed?

Is the circle in the horizontal plane or in a vertical plane?

As far as I can see, the COM is on average static in the vertical frame which is the one we care about, and it is not accelerating.

Yes, the COM is static on average in the vertical direction. If the circle is in the horizontal plane then the COM is always static (not just on average) in the vertical direction.

My understanding of aerodynamic forces suggests that lift is expressed horizontally in this example, so which force now applies to the floor of the box? Remember that earlier, we agreed that forces applied to the walls of the box do not contribute to OW.

Putting an airplane into a vertical orientation basically turns it into a helicopter with the propeller as the rotor. If the propeller cannot generate enough thrust to climb vertically then it will crash, as has been mentioned by others. The force applied to the floor is the wash from the propeller.

 

[russ_watters]

Dale, the way I read it (the second time) was that it is turning in tight, horizontal circles, with the wings vertical...

 

[Dale]

That is kind of what I thought too, but I wasn't sure. If so, it is basically either a helicopter or a crash.

 

[Graeme M]

I disagree. I am no aeronaut, but an aircraft in a tight turn maintains altitude through the work of the engine providing thrust via the agency of the propellor and the use of rudder to maintain nose-up. The process of flying is effectively an exercise in fluid dynamics I suspect. The fluid, in this case the air, provides the aerodynamic forces to offer lift. That lift is expressed in relation to the direction of fluid flow and the plane of the airfoil.

In a vertical turn (ie wings vertical in relation to the ground or floor), the thrust is directed behind the aircraft, ie horizontally. Lift is provided in this case horizontally as well. The only 'vertical' force would be that of the rudder but again that's largely an aerodynamic force (though I might be quite wrong in how I think an aeroplane flies).

It's the engine doing work that provides the thrust to allow the aircraft to create a fluid flow and that flow proffers lift in whatever plane the aircraft is in. Naturally, a tight turn without more power or rudder will result in the steady deterioration of the circle, but providing we have enough lift and power, the circle should not deteriorate.

In effect, the force of lift is cancelling the attraction of gravity, and it's by doing work that we can control the amount of lift.

It's a little like firing a rocket across the face of the Earth - with enough thrust we can counter gravity and create a largely straight path. With too little power, gravity will take over and decay the path toward the centre of gravity of the earth,

That's my take, anyway.

 

[Graeme M]

And yes, turning horizontally with wings vertical.

 

[russ_watters]

I disagree. I am no aeronaut, but an aircraft in a tight turn maintains altitude through the work of the engine providing thrust via the agency of the propellor and the use of rudder to maintain nose-up.

Oh, c'mon! [Dale wins]

If the propeller is providing the force that keeps the plane aloft, then it is just a complicated helicopter and the force to keep the plane aloft is still directed to the bottom of the truck.

Unimportant, but what you describe isn't how it typically works. Planes usually like to fly coordinated turns where the lift force vector is perpendicular to the wings. A plane in the configuration you are describing is side-slipping.

http://en.wikipedia.org/wiki/Coordinated_flight

The only 'vertical' force would be that of the rudder but again that's largely an aerodynamic force (though I might be quite wrong in how I think an aeroplane flies).

Er, you just said [correctly] in the quote above that the propeller provides the vertical force.

The rudder actually provides a downward vertical force.

It's a little like firing a rocket across the face of the Earth - with enough thrust we can counter gravity and create a largely straight path. With too little power, gravity will take over and decay the path toward the centre of gravity of the earth...

Only if the rocket is propelled at 25,000 mph. At any slower speed, it must pitch up and use the engine to provide lift.

 

[Dale]

providing we have enough lift and power, the circle should not deteriorate.

Regardless of how you orient the plane, in order to maintain altitude you must push the air down with a force equal to the weight of the plane, and this force in turn pushes on the box. The COM is stationary (vertically) so the OW is equal to the GW.

 

[Graeme M]

I should apologise if my tone is sometimes not appropriate, I make these posts in snatches between my normal stuff. So sometimes I miss things that have been said or I say things in a less than considered way.

Now, I did not at any stage say that an aircraft's propellor provides a vertical thrust when it is turning. What I am suggesting is that an aircraft flying in air uses aerodynamic forces to obtain lift and those forces are not necessarily directly relevant to gravity.

The propellor provides the thrust to make the plane move through the air - all that is doing is creating a situation analagous to the airfoil remaining stationary in a moving flow. It's only the flow of air over the airfoil that creates lift. The propellor in this case is giving us movement of the aircraft through the air and allowing the airfoil to generate lift in an otherwise static fluid (using our box model).

You'll need to show me somewhere how an aircraft doing a tight turn is transferring any force at all to the ground.

You said "Regardless of how you orient the plane, in order to maintain altitude you must push the air down with a force equal to the weight of the plane, and this force in turn pushes on the box. " I'm saying that the aerodynamic force of lift is relative to the plane of the aircraft and that the forces are expressed horizontally in the main. Yes gravity is still pulling down, but the forces of thrust and lift are sufficient to overcome gravity.

The rocket example demonstrates this. Yes you may need to go at 25000 mph because the rocket is using very little aerodynamic lift, but the thrust of the engine is what allows it to go faster enough for that force to overcome gravity. A rocket zooming across the face of the moon would exert no force on the moon's surface even though there is a gravitational attraction and hence the rocket has a gravitational weight.

 

[Graeme M]

PS when I get a moment I'll read your wikipedia reference to coordinated turns! :)

 

[russ_watters]

Now, I did not at any stage say that an aircraft's propellor provides a vertical thrust when it is turning.

You said (basically correctly):

[previous post]
...an aircraft in a tight turn maintains altitude through the work of the engine providing thrust via the agency of the propellor and the use of rudder to maintain nose-up.

So to put it another way, if the airplane is angled upward, the propeller directs a force downward, which is what holds-up the plane (if it can).

What I am suggesting is that an aircraft flying in air uses aerodynamic forces to obtain lift and those forces are not necessarily directly relevant to gravity.

Aerodynamic forces on what? You said the wings are vertical, so they can't provide an aerodynamic force to keep the plane aloft. You said clearly that the force that keeps the plane aloft comes form the engine. Yeah, the propeller uses aerodynamic forces.

The propellor provides the thrust to make the plane move through the air...

In which direction? Before, it had both horizontal and vertical components. But it sounds like you are hedging/backtracking on that.

It's only the flow of air over the airfoil that creates lift.

As you pointed out, if the wings are vertical, the lift is horizontal, so it can't be keeping the plane aloft.

You'll need to show me somewhere how an aircraft doing a tight turn is transferring any force at all to the ground.

It sounds to me like you are changing the scenario. You very clearly said before that it is angled upwards, so the engine is directing a force downward. If you want the plane (fuselage) horizontal, and the wings vertical, then there is nothing keeping it aloft: it death-spirals into the ground.

I'm saying that the aerodynamic force of lift is relative to the plane of the aircraft and that the forces are expressed horizontally in the main. Yes gravity is still pulling down, but the forces of thrust and lift are sufficient to overcome gravity.

You really white-washed that. If the plane is not dropping, there must be a force directed downward. Before, you said it came from the propeller. If now you are saying that the propeller isn't providing that force, then clearly you have a self-contradiction.

The rocket example demonstrates this. Yes you may need to go at 25000 mph because the rocket is using very little aerodynamic lift, but the thrust of the engine is what allows it to go faster enough for that force to overcome gravity.

No. That situation is a very specific one: that's escape velocity (actually, it could be a little lower and be in orbit). The rocket is still falling, but it is traveling along the Earth's surface and covering enough distance that the Earth drops away from the rocket faster than the rocket drops toward the earth.

 

[Simon Bridge]

1. A sealed box with the air inside at normal atmospheric pressure, placed on a scale, weighs X.
2. Add a bird of weight Y so that the total weight now equals X+Y.
3. If the bird takes to the air inside the box, flies around for awhile, and then resettles, the total weight of the box and bird 'system' is always X+Y.
4. The reason is that the birds flapping produces a downwards force that registers on the floor of the box as a force equal to the weight of the bird.

Is that a fair summary? If so, I have a couple of questions. If not... well... :)

I would have said "yes and no" ... this is fair for the situation in static equilibrium - which is an implicit assumption.
The questions immediately following do not involve a static equilibrium.

My vet has a large scale for weighing large dogs, small horses etc.

I stand on a platform and it reads my weight - quite accurately I think since all other scales have me a half-kg heavier.
If I jump up and down on that platform, the scale wobbles about - and the vet tells me off.
If I stand still, the scale is still.

Drive an RC car around the scale-platform and it stays the same - so the distribution of weight across the scale does not matter.

However, there are lots of different ways to move around a closed box - they are not all equivalent.

 

[Graeme M]

Russ, great answer and I really want to take issue with you over most of it. Just no time right now. But we are getting closer to the crux of the matter from my point of view.

Bear in mind I am just going with my thoughts here, but this discussion is great in that it's making me THINK and I love that. So thanks for the patience so far.

But I'm going to rip into your last comment just as soon as I can. :)

Simon, agreed, but that too is getting close to the crux of the matter. What is weight, how is it expressed, and what part does an atmosphere play in the equation.

 

[berkeman]

Be careful how you "rip". Other Mentors are watching this thread now.

EDIT -- let me clarify. Other less patient Mentors than Russ. Please be careful to base your questions on actual science and aerodynamics, and please explain your questions fully, so others do not have to guess at what you are asking or saying. Thank you.

 

[rcgldr]

You'll need to show me somewhere how an aircraft doing a tight turn is transferring any force at all to the ground. ... I'm saying that the aerodynamic force of lift is relative to the plane of the aircraft and that the forces are expressed horizontally in the main. Yes gravity is still pulling down, but the forces of thrust and lift are sufficient to overcome gravity.

If the aircraft's wings are vertical, then the plane is generating lift in "knife edge" fllight by using the fuselage of the aircraft as an airfoil, diverting the relative air flow downwards. The prop axis is angled (yawed) upwards, so some component of thrust is also downwards. In order for the aircraft's COM to not be accelerating downwards, the aircraft has to generate enough vertical component of aerodynamic force to keep the aircraft from accelerating downwards.

 

[Graeme M]

Berkeman, not sure quite what you mean regarding the mentoring thing - I AM 'basing' my questions on science and aerodynamics, as best I understand them. But I freely admit my understanding is weak and my grasp of terminology similarly uncertain. What I am doing is as much aimed at querying my own ideas as anyone elses. So while I might engage in a somewhat intensive manner, I am not trying to 'prove' I'm right. But equally I want to test the claims others make. By the way, is that you on the bike in your avatar?

rcgldr, I will need to think about what you've posted. First up, a curved airfoil is needed to divert the airstream in a way that generates lift. I thought that a perfectly symetrical airfoil will not generate lift. If the aircraft is canted with wings vertical, I believe the fuselage cross section is perfectly symetrical and therefore unlikely to contribute much 'lift' in the vertical plane. Secondly, how is the prop axis angled up? Are you saying that in such a manoeuveur, the aircrafts actual longitudinal axis is slightly 'up' to the horizontal as it balances forces with control surfaces? If so, I guess that may explain the notion that the fuselage is contributing to lift as then we'd have a deflected airstream. But I suspect that the vertical lift component in this case is substantially less than the vertical component of level flight.

 

[A.T.]

First up, a curved airfoil is needed to divert the airstream in a way that generates lift...

You are getting lost in details here. To maintain constant altitude, any flying machine/bird must somehow transfer vertical downwards momentum to the air. It doesn't matter how this it is achieved in detail.

 

[rcgldr]

symetrical airfoil

Aerobatic aircraft use symetrical airfoils and fly just fine. They can also fly in knife edge flight. Radio control models can do knife edge loops. youtube_knife_edge_loop If you want to discuss aircraft, lift and maneuvers, please create a new thread.

Getting back on topic, a sealed truck, the air, and anything inside the sealed comparment are part of a closed system, and if the COM if not accelerating vertically, the weight equals the sum of the weight of the components in the closed system.

As posted previously, air exerts it's weight and the weight of any object supported by the air via a pressure differential, where pressure decreases with height. The pressure at the top is less than the pressure at the bottom, and the net downforce due to this pressure differential equals the weight of the air and any object that the air is supporting (as long as the COM of the system is not accelerating vertically).

This is also true for a object "floating" in the air due to buoyancy. The object displaces the air, which results in an increase in the density and pressure differential within sealed container, so again, the total weight of the closed system equals the sum of the weight of the components.

 

[sophiecentaur]

Ye gods, this thread certainly has taken off.
Before introducing the action of an aerofoil and aerodynamics, there is one thing you just cannot ignore. That is the fact that you cannot (so far, at least) make an anti-grav machine or a reactionless drive. If the bird / plane is in the air, it must be subject to an upwards force from somewhere. The only thing that can be supplying tht force must be that the momentum change of the air molecules underneath it is greater than the momentum change of the air molecules above it. Molecules must be pushed downwards, to give them this required momentum, one way or another. However this is achieved, this must result in a net flow of air downwards from the wing / rotor etc. If you want to argue against this then you have a non-starter. Aerodynamics is a system for describing what happens to the air around a wing in terms of pressure and velocity and it is just an approximate model of what is going on in the immediate vicinity (it's statistical, in the end). It 'ignores' Newton's laws of motion because the error involved, when spread over a large volume of air, is so small. The momentum of the downward moving air molecules has to be transferred ( second / third / fourth etc. hand) and randomised, to molecules way below and over a wide area in the form of pressure. The mean pressure on the ground or the floor of the box must be increased and the total extra load on the floor must be equal to the weight of the plane plus ambient air pressure.
Nothing magical has happened, just because the movements of air molecules are dispersed throughout the box. Just because we are dealing with moving particles, there is no essential difference between the case where the birds in the box are all suspended on tiny strings or vertical rods and when they are flying. It's just that the flying situation is much more complicated.
Before launching into an ever widening argument about what is 'really' happening, it is a good idea to hang onto a few things that 'must' be happening - fundamental principles cannot be ignored by using smoke and mirrors arguments. Newton rules in this case and any explanation cannot ignore him.

 

[rcgldr]

If the bird / plane is in the air, it must be subject to an upwards force from somewhere. The only thing that can be supplying that force must be that the momentum change of the air molecules underneath it is greater than the momentum change of the air molecules above it.

There just needs to be a downwards change in momentum of the air. It doesn't matter how much of this change occurs from above (due to reduced pressure) or below (due to increased pressure) a wing.

 

[sophiecentaur]

There just needs to be a downwards change in momentum of the air. It doesn't matter how much of this change occurs from above (due to reduced pressure) or below (due to increased pressure) a wing.

Kinetic theory says that pressure in a gas is due to momentum change at the surface. I was including all momentum changes in my statement. Of course, 'net' momentum change is downwards.

 

[Graeme M]

Hmmm.. I had to go and read the wikipedia article on aerodynamics and I think there are discrepancies between that and comments here. But I see I have been asked not to investigate aerodynamics further, which is a pity.

I don't understand the matter of change in downward momentum of air. How is this relevant? Or can you point me to a reference in this respect?

My point was that aerodynamic lift is mostly in the direction perpendicular to the flow of air over the airfoil, according to wikipedia. There may be small amounts of lift from other surfaces but the main component of lift is not always expressed vertically. So it eludes me how this lift force gets expressed in its entirety on the floor of the box in the event that our aeroplane is not in straight and level flight.

That said, I have finally formed my mental model of what's going on. It is at odds with what has been said here. I shall try to describe succinctly Graeme M's Theory of Birds in a Truck. It may take me a while so you may have to wait a day or two.

I shall look forward to the deconstruction of my theory! :)

 

[russ_watters]

I don't understand the matter of change in downward momentum of air. How is this relevant? Or can you point me to a reference in this respect?

Momentum change is one definition of force, in Newton's 2nd law:
http://en.wikipedia.org/wiki/Momentum#Relation_to_force

My point was that aerodynamic lift is mostly in the direction perpendicular to the flow of air over the airfoil, according to wikipedia. There may be small amounts of lift from other surfaces but the main component of lift is not always expressed vertically. So it eludes me how this lift force gets expressed in its entirety on the floor of the box in the event that our aeroplane is not in straight and level flight.

I agree that you are getting lost in details here. As said, regardless of what other things are going on or how exactly it happens, if a plane is being held aloft, there must be a force in the vertical direction equal to its weight. Whether it is generated by a fuselage lifting-body effect or by the engine (or both) is irrelevant to the issue of the thread. Either way, it has to exist. And if the wings are vertical, they don't participate in generating that force.

That said, I have finally formed my mental model of what's going on.

Agreed, and it is pretty clear to me that this "mental model" of yours is wrong. You are trying to weasel your way out of the existence of a vertical force holding up the plane/bird.

It is at odds with what has been said here. I shall try to describe succinctly Graeme M's Theory of Birds in a Truck. It may take me a while so you may have to wait a day or two.

I shall look forward to the deconstruction of my theory! :)

Please note: PF doesn't do "my theory"s. We discuss real science here. The reason is that learning by deconstruction of wrong ideas is not the most effective way to learn - it often stands in the way of learning.

 

[sophiecentaur]

That said, I have finally formed my mental model of what's going on. It is at odds with what has been said here. I shall try to describe succinctly Graeme M's Theory of Birds in a Truck. It may take me a while so you may have to wait a day or two.

In that case, it will have flaws. You are really pushing your luck to disagree with Newton's Laws of motion, in a context like this - and that's what all this boils down to. You might spend your time more usefully reading elsewhere (if PF seems not to be helping) and see how the very basics apply.
Aerodynamics is not 'verboten'; it's just been pointed out, correctly, that it is not relevant here to the basic situation. It's just a mechanism linking the force from the floor and the force supporting the birds. The presence of the forces is not in doubt. Would you be any happier with a Helium Balloon supporting the birds?
There are hours and hours-worth of wasted posts by people who would prefer have their personal theories deconstructed rather than learn mainstream Physics. It's only in the indulgent environment of the Internet Forum that this method can be attempted. The only alternative used to be the highly paid personal tutor, on call 24/7.
A situation like this is not on the frontiers of Science, where the personal opinions of highly able Scientists are at odds with each other. It's clear cut and well within the envelope of classical Science. This looks like a case of "Everyone's out of step but our Alfred".

 

[Graeme M]

A good suggestion there. I did note earlier this medium wasn't going to get me where I wanted to go, and the idea that I could hire a physics tutor is an excellent one. It hadn't occurred to me. I shall look into doing that.

As for getting lost in details, no I don't think so. The detail is critically important. At least to me. And it was MY thread, you didn't have to answer.

Let me leave you with one last question. If I replace my birds with a motorcycle in a larger box. I create a near vertical ramp and launch my motorcycle upwards. It will go so high, then gravity will drag it back down. Is the OW different at any time in that process other than at the moment of launch and the moment of impact?

 

[A.T.]

If I replace my birds with a motorcycle in a larger box. I create a near vertical ramp and launch my motorcycle upwards. It will go so high, then gravity will drag it back down. Is the OW different at any time in that process other than at the moment of launch and the moment of impact?

This was answered by DaleSpam in post #9. Look at the COM.

 

[russ_watters]

...I could hire a physics tutor...

Note, that suggestion wasn't exactly a recommendation. The tutor is also likely to tell you that you are approaching learning wrong and you may have some difficulty getting one who will indulge you. Ultimately I'm sure you can find one though, since if you're paying, someone will do what you want, even if it isn't productive. Similarly, in another thread, an electrician is asking about how to build an EM radiation shield for a client's house. Completely pointless, but if someone wants to pay him to do pointless work, he'll do it.

As for getting lost in detailes, no I don't think so.

Your approach works like this:
1. Think of a clever way/scenario to hold an object aloft in a truck.
2. Examine it to see if it makes the truck appear lighter on a scale.
3. Find out that it doesn't.
3a. Modify it slightly, without telling anyone, to try to hide the flaw (repeat 3 and 3a as necessary).
4. Repeat. Forever.

The obvious problems with this approach are:

1. You never reach the point of accepting (learning) the scientific principle you are examining.
2. The scientific principle you are examining, if accepted first, would be (is) a valuable tool in examining the scenarios.
3. Each scenario is more complicated than the last, and the goal really becomes to confuse yourself into not seeing the force, which then convinces you it isn't there. That's what's happening with your airplane. That is how all perpetual motion machines "work".
4. It is insulting to the people you are [not] learning from. And not just the insult of not valuing someone's time (which can be alleviated by paying them for it): it is insulting because you aren't trusting their expertise or the expertise of every other scientist/engineer who has ever lived. You're basically telling us we're all wrong and trying to enlist our help to prove we're wrong.

Most people who are on the road to becoming crackpots don't recognize these issues, especially the last one (which is why people don't understand when they get negative reactions to such lines of questioning). You're on the road to becoming a crackpot. I don't think you are there yet (otherwise this thread would have been closed already). Please turn around/get off that road.

[Edit] Oh, and the motorcycle? The scale registers a higher force when the motorcycle hits the ramp and lands and a lower force while it is in the air.

 

[sophiecentaur]

Let me leave you with one last question. If I replace my birds with a motorcycle in a larger box. I create a near vertical ramp and launch my motorcycle upwards. It will go so high, then gravity will drag it back down. Is the OW different at any time in that process other than at the moment of launch and the moment of impact?

Using the term OW brings up questions like this - as I said it would - and it is unnecessary because the only force that counts and is consistent is Weight. If you have a motorcycle and a van then their two weights are separate and the values can be used to predict (/ understand) what happens under any circumstance. The motions of van and bird can be discussed independently and their detailed interactions could involve aerodynamics.

BTW, did you consider the cost of a live-in tutor? Sounds like a system used by an Ancient Greek nobleman or a Medieval king. If you're offering, and could find room for my wife as well, I could apply for the job. I could picture you in a large mansion with access to the sea, moorings for my boat and plenty of trout fishing available. (I would be much less grumpy if you were paying me I would be very very polite, in fact.)

 

[cjl]

My point was that aerodynamic lift is mostly in the direction perpendicular to the flow of air over the airfoil, according to wikipedia. There may be small amounts of lift from other surfaces but the main component of lift is not always expressed vertically. So it eludes me how this lift force gets expressed in its entirety on the floor of the box in the event that our aeroplane is not in straight and level flight.

If you insist on looking at knife-edge flight, in a tight horizontal turn, you are correct that the main component of lift is probably not expressed vertically. However, the total aerodynamic force will have a magnitude many times larger than the weight of the aircraft in this situation. The component of the aerodynamic force holding the aircraft up (assuming a horizontal turn, with no change in altitude) will equal the weight of the aircraft, and the rest of the aerodynamic force is going into maintaining the tight turn. Thus, the lift force does not get expressed in its entirety on the floor of the box. Most of it gets expressed on the sides of the box, due to the aircraft's turning. However, exactly enough gets expressed on the bottom of the box to counter the aircraft's weight.

As stated by others above, you are getting bogged down in the details, when no matter what situation you come up with, it still must obey the same basic, fundamental physical relations.

 

[A.T.]

If you insist on looking at knife-edge flight, in a tight horizontal turn, you are correct that the main component of lift is probably not expressed vertically. However, the total aerodynamic force will have a magnitude many times larger than the weight of the aircraft in this situation. The component of the aerodynamic force holding the aircraft up (assuming a horizontal turn, with no change in altitude) will equal the weight of the aircraft, and the rest of the aerodynamic force is going into maintaining the tight turn. Thus, the lift force does not get expressed in its entirety on the floor of the box. Most of it gets expressed on the sides of the box, due to the aircraft's turning. However, exactly enough gets expressed on the bottom of the box to counter the aircraft's weight.

As stated by others above, you are getting bogged down in the details, when no matter what situation you come up with, it still must obey the same basic, fundamental physical relations.

A picture might help:

Source : http://avstop.com/ac/1-24.html

The vertical component of lift still equals the weight. So the air still gets the same downward momentum, as with a hovering helicopter.

 

[Graeme M]

I think you are all misunderstanding a critical aspect to this. I'm not trying to prove YOU are wrong, I'm trying to see why I am wrong. And there are lots of details involved, because every time one of you offers some angle on the explanation, I immediately see a hole in it. Now obviously that hole can be plugged, I just need to see how. What I really need is to be sitting around where I can talk and draw and the discussion happens more freely and relevantly.

Look it doesn't matter if I don't get it. And no I am not trying to prove a pet theory. I just want to niggle away at the problem till I get it. Geez, just this morning I spent an hour before I got up thinking it through.

I repeat, it doesn't MATTER if I don't get it. And what IT is, is what it is without my attention.

I just want to have a go at figuring it out, that's all. And I have learned a whole lot in the process. But as I've observed before, we probably have a couple of problems. One is that the medium is not suitable to my needs and two I'm not smart enough to follow some of the arguments.

 

[rcgldr]

But I see I have been asked not to investigate aerodynamics further, which is a pity.

You were just asked to start a new thread about the aerodyanamics, instead of mixing up subjects in this thread.

I don't understand the matter of change in downward momentum of air.

A basic physics law is: impulse = force x time = change in momentum, in this case, the downwards (relative to aircraft) force applied to the air (related to lift) x time a wing interacts with a particular volume of air equals the change in momentum of that air. The vertical (with respect to gravity) component of that force will correlate to the vertical (with respect to gravity) change in momentum of the affected air.

 

[A.T.]

I'm trying to see why I am wrong. And there are lots of details involved,

There is no way around momentum conservation, no matter how many details you bring into it.

because every time one of you offers some angle on the explanation, I immediately see a hole in it.

It's the other way around: Every time you bring up a new scenario to prove your point, people immediately point out the error in it. Then you go to the next one: birds, helicopter, falling weight, turning aircraft, jumping motor cycle... it doesn't matter what you come up with. The CoM-explantion from post #9 applies to all of them.

 

[Nugatory]

I just want to have a go at figuring it out, that's all. And I have learned a whole lot in the process. But as I've observed before, we probably have a couple of problems. One is that the medium is not suitable to my needs and two I'm not smart enough to follow some of the arguments.

That's fair, and of course we all went through this at some point ourselves.

Although learning styles are notoriously non-portable, so what works for me may not work for you, I find that the best way of figuring out this sort of problem is to start with the simplest and most idealized version of the problem, get it clear in your head, and the start adding the complications.

When I was learning how to attack this problem, my then-teacher gave me a series of questions (and interestingly, used a hovercraft as CWatters suggested - that's why I'm so partial to that example). It was something like:
1) There's a hovercraft, not hovering, motors off, just sitting on the floor of the truck.
2) Next, the hovercraft fires up its motor, lifts itself 1/16" off the floor and stabilizes. What does the scale read after it's stabilized again? Why?
3) What do you expect the needle of the scale to do if the hovercraft rises again? Why?
4) Suppose there's a whole bunch of them, moving more or less at random, no coordinated activity?
5) Suppose there's just one, but its mass is fairly large and it's moving violently, as in Voko's cannon example from #3?

In doing this, you'll likely find that the textbook answer that prompted you to start in on the investigation was based on some unstated simplifying assumptions. That's certainly the case here; way back in post #1 we have "4. The reason is that the birds flapping produces a downwards force that registers on the floor of the box as a force equal to the weight of the bird" and if there's one thing that's clear from the subsequent discussion, that's only true if we make a bunch of simplifying assumptions (steady state, hovering bird, ...), basically case two above.

 

[Graeme M]

AT I take your point, but it isn't really me bringing up scenarios to prove my point. It's just then what passes for a mind in my case tries to assess what's been offered and it spots something that doesn't fit (to me).

But I think I've got it sussed. It's the air, stupid.

 

[berkeman]

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