Birds in a truck - I'm not getting it

[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.