I'm not trying to annoy you or be patronising, I just want to be clear.

So what is supporting the reacting air which is directly under the wing?

Any other engineers/physicists on here fancy one of my biccies? A nice drop of red?

I'm still at fechin work.

The air under the truck would be enough to support the air in the truck, but the truck bed is there too… If the truck is sealed then there is a contribution from the air around the truck thereby cancelling out the weight of the air.

I'm not at work, just got back in from an interval session in the gym 🙂

molgrips – Member
People only respond to questions with questions when they are unsure of their position..

Or perhaps they did a liberal arts degree?

Or perhaps they did a liberal arts degree?

What's a liberal arts degree?

The air under the truck would be enough to support the air in the truck, but the truck bed is there too… If the truck is sealed then there is a contribution from the air around the truck thereby cancelling out the weight of the air.

And this air what is supporting it?

Newtons Third Law:

The mutual forces of action and reaction between two bodies are equal, opposite and collinear. This means that whenever a first body exerts a force F on a second body, the second body exerts a force ?F on the first body. F and ?F are equal in magnitude and opposite in direction. This law is sometimes referred to as the action-reaction law, with F called the "action" and ?F the "reaction".

In the example being discussed there are two important factors

1. Are the birds in cages and are they mesh or solid?
2. Is the floor of the truck mesh or solid?

If the downward air from the birds flapping is stopped by the solid floor (and re-directed horizontally), the floor should exert a similar force upwards and it feels the same force downwards. This is the reason why, if there is a floor, the birds hovering do not lighten the truck.
But if the downward air can continue until the road, through mesh cages and a mesh platform truck, then the downward force is exerted on the road and not on the truck.

Either you suppose that the air is completely stopped or that the air goes through the mesh freely.

What if the cargo area of the truck was a vacuum and the truck had just driven off a cliff and was falling vertically through space?

Any unrestrained object within a falling container will surely become 'weightless'?
show working

Right.. so if the pigeons are standing on the floor initially, then the weight reading on the scales is truck + birds. Once the birds take off and are gliding, the reading on the scales is just truck.

LOL – which polytechnic was that degree from ?

some good craic though – so it will make a diff whether the birds are gliding or flapping ? they don't fly like airplanes fly you know …

don't forget I'm assuming the same mass of air stays in the lorry at all times, otherwise we are in BS territory …

btw, the footy was OK, and the beer was cold – looking forward to tonight's !

And this air what is supporting it?

The earth.

LHS – what you say is true and was covered much earlier in the thread. However birds do not always fly by flapping and pushing air downwards to gain a reaction. If a bird is gliding it's being supported by the aerofoil effect i.e. Bernoulli's principle (partly).

Greyman – in this model they do. This is about if, in theory, the birds become partially buoyant, does it have an effect on the weight reading of the scales. Not about how birds actually fly.

My degree is from Cardiff University btw, and we did not cover trucks full of birds, nor did we spent much time talking about Newtonian Physics.

What's the principle discussed recently on STW where people lower down the intelligence scale don't realise how stupid they are? 😉

LOL – touchee monsieur

Dunning Kruger I think – stating the bleedin' obvious really, although of course inherently ironic don't cha think ?

🙂

Have you lot not seen a pigeon carrying truck? I know the question is hypothetical, (maybe assuming that the cages are airtight) in which case the weight is constant, but they are pigeons! so it doesn't really work very well as a model, if you bring common sense into it the pressure created by the flapping wings (read as weight on the scales once birds are a'flappin) which would have to be equal or greater the the weight of the bird only effects an overall weight if all pressure is directed downwards, which of course it isn't, as the pressure outside the lorry (through the cage type thing) is less than the pressure created by the downward pressure of the wings innit. The argument here is hypothetical v's reality. Buy of of these, fill it with pigeons and put it on a weigh bridge and see if i'm wrong. You could do it on a smaller scale with a budgie in a cage and some kitchen scales – anyone game? 😛

Shoefiti – posting a real pigeon truck.. now you're just being silly!

Dunning Kruger – that's the chap. And yes, ironic 🙂

If the front wall/bulkhead of the cage were also mesh and the truck were travelling at speed x (where x is is fast enough to sustain gliding but not so fast that the birds couldn't glide), then the birds could glide, thus not creating any downdraft on the floor of the cargo area, not placing any weight directly on the cargo area and not beating their wings.

Then what?

kona, I am suggesting that if they are gliding then there's hardly any downdraught anwyay. That's what I am talking about – the scales only read the truck not the birds.

Then what?

liking it – you do realise in that scenario, if the pigeons spread their wings and held on to the perches – the truck would fly !

no conveyor belt required.

outstanding. 😉

It'd only fly for a bit, since the wheels would leave the ground and slow it down a bit. We're straying into plane/conveyor belt territory here!

I was busy writing my hollywood screenplay in starbucks when i happened upon this thread.
Most entertaining.

if the pigeons spread their wings and held on to the perches – the truck would fly !

Omigod – if that could save even 1% of fuel consumption, the value of the diesel saved across an entire fleet of trucks would hugely outweigh the cost of birdseed required to feed the pigeons! The commercial applications are massive – if you could provide a one-stop solution for pigeon management to equip every vehicle with a flock and market it as a costsaver to fleet managers, it would be hugely profitable. The growth would be immense. Best of all, you could franchise the idea to have dual streams of income.

Tell you what, greyman – how does 60/40 sound to you? Obviously we'd need to attract mezzanine investment and eventually we'd have to float on AIM, but in the meantime, you and I could make MILLIONS!

Truck gets lighter = Myth! Busted… 😛

Without trawling back through all the posts, has nobody pointed out that this theory was put to test on the discovery channel show "mythbusters". I remeber watching it and the test was pretty convincing. It was back in March 2007. They put each wheel of a truck on load cells and managed to get 11lbs of pigions to fly in the back all at once, with no change in weight, although the readings where a little noisey. So to go a step further, they flew a RC helicopter in the back of the truck….no weight change.

http://kwc.org/mythbusters/2007/04/episode_77_birds_in_a_truck_bi.html

Video here! http://www.youtube.com/watch?v=p7qjSP78T8M&feature=related

LOL at Konabunny

Dragons Den here we come ?

African Swallow or European ones do you think ?

Sorry I've hardly been paying attention to this thread but I'm getting a general gist that someone is arguing that a bird flying around in the air is pushing the air down so that the air puts the same force on to the ground as if the bird was sitting on the ground?

Have you ever had an aeroplane or a helicopter fly overhead? Did your house collapse?

If you're talking about a situation where there is a truck, with a bird in it, then the reaction force the ground has on the truck is (bird + truck) x g. Then when the bird takes off the reaction force will increase slightly (by the amount needed to accelerate the bird – eg: the force from its legs). And then the reaction force will just be the truck x g.

Ok now that we have cleared that up.. 😀

(Basically air can not carry a force (if you will) all the way to the ground – the force will just move the air about a bit immediately by the bird, then turn into heat..)

But if the air obeys as an ideal gas, assuming the truck is airtight and the volume of gas cannot change, and if the truck is a perfect insulator, the pressure will rise with the temperature, until eventually the bird melts/is crushed/the truck explodes.

Grimy – you miss the point.

If the pigeons are flapping about in the truck, then sure the downdraught is equal to their weight so the weight doesn't change.

This is about the hypothetical situation where the truck is big enough for the pigeons to glide. My assertion is that a gliding pigeon is slightly buoyant, so it'd be like putting a helium balloon in the truck…

PS sorry to break this to you but Mythbusters, whilst a great show, is often profoundly unscientific. And Adam and Jamie know it too, but it's glossed over 🙂

BS of the week.

oh the ironing.

I never meant it that way steve.

Is this going to become BS of next week too then?

I haven't read most of this but I get the jist.

An example that whilst not the same, illustrates the point.

You have a sail boat with a large sail. You have a large fan on the boat(petrol powered for arguments sake) you point the fan at the sail at full blast, but the boat will not move. Put the fan on another boat behind and point it at the sail, the boat will move.
Put the fan on the boat. Pointed away from the sail. The boat will move.
Do you understand why this is the case? Please don't make me explain! 🙁

On the other hand, what if they were swallows?

Funky Monkey – see Dunning Kruger. You're not understanding the problem I'm trying to address 🙂

A victim of my own success 🙁

Molgrips, I know the show dosnt exactly conduct experiments to the scientific n'th degree, but the experiments are usually carried out with sufficent accuracy to dispell certain myths, and even then, people will change the argument and still stand by some theoretical minute point thats almost imposible to dispell just to suit their own argument. I think Adam and Jamie do a fairly good job of testing out stuff in a "real world" environment and with what is reasonably practical circumstances.

I'd have thought that the blades of a helicopter cut through the air and create lift in pretty much the same way as a gliding bird anyway, dispelling that theory too. But i'm really not interested in arguing that one out….youll only change the argument again to suit your needs! 😉 😛

A fair few of the experiments have MASSIVE holes in them from the scientific point of view. Most don't tho, admittedly.

Helicopter blades are static and result in a downward current of air below the helicopter – unlike the bird.

I haven't changed the argument to suit my needs, wtf!? We discussed the original case and then I thought about a special case which might produce a counter-intuitive result, for amusement purposes.

Jeez, you just can't play intellectual games with some folk…

Sorry molgrips, I'm just having a little interllectual fun too!

Although I said I wasnt going to argue over the similaritys of a helicopter blade and a birds glide, I will debate it with you! 😛

The way I understand helicopter flight, is not as you describe. It does not fly as a result of forcing air down like a harrier jump jet, but as a result of foiled rotating wings cutting through the air, creating less density above the wing than bellow, and thus achiving Lift, not thrust. Much in the same way a plane or bird glides. So if the experiment showed no weight change for a helicopter flying in a truck, then, the prinipal of flight being the same for a bird, the same results would apply?

ne'er mind al.

look at it as a win instead.

Ok, well if we're gonna get into this.. 🙂

Aeroplane wings and presumably helicopter wings work with a combination of the Bernoulli effect (the low pressure above the wing bit) and just forcing air downwards by being angled up (the angle of attack of a plane wing is critical).

I suspect that it's a little different for a helicopter though because it hovers in place. The low pressure above the wing ends up sucking air downwards from above, and coupled with the angled blade ends up forcing it downwards. Because it's hovering, this sets up a column of air with momentum downwards which is what keeps it up.

I suspect that an aeroplane (and bird maybe) has more of a component of lift generated by the buoyancy of the low pressure area above the wing. Since the wing is moving forward through air all the time, the pressure differential will be only present for a short moment, and would tend to equalise after the plane's gone, there by resulting in less of a downward moving pressure wave…

But I really don't know for sure – any aeronautical engineers on here?

You could do the same experiment with a small radio controlled plane in the truck, and see if the truck got lighter…

Or a hot air balloon…

I cant argue with that theory molegrips, It certainly sounds convincing. I wouldnt like to put money on it either way?

Me neither 🙂

Anyone had a plane fly really low over them, and felt a downdraught?