Birds in a truck - I'm not getting it

[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]

Thread closed for Moderation...

 

[Dale]

The mentors have discussed and the thread will remain closed. The correct physics has been explained and now the thread is just going in a circle and heading towards personal speculation.

The detail is critically important. At least to me.

The question is which details are important? Do you think that the color of the box makes a difference? Or perhaps whether the box is arranged with the length north/south vs. east/west? Perhaps you should worry about different types of birds, maybe pigeons vs. doves, or maybe different colors of the same species?

For any physical scenario there are important details and there are unimportant details. For this scenario there is only one detail which is important, the vertical acceleration of the center of mass of the system (truck plus contents). All other details are unimportant. Given the GW of the system and given the vertical acceleration of its COM you can immediately find the OW, regardless of any other details.

 

[berkeman]

The thread will remain closed.

@Graeme M -- Please do some more reading about aeronautics to help your understanding of the things you were told in this thread. After you have done some reading, if you have specific questions about what you have read, please feel free to post in the Engineering/Aeronautics forum here on the PF. Posts like "I believe this, how am I wrong" are not generally permitted here on the PF. Specific questions about things you have read at credible websites certainly are allowed.