Sheesh, I wish I'd never come across this thread...
 

...it's kept me awake for three nights running.

That bird in the cabin again. So it's argued above that in an enclosed/pressurised cabin the bird, when it's flying, still contributes to the weight of the aircraft.

I'm still not sure that's true, but what about the other bit of my question - does it contribute to the weight if the cabin is not enclosed/pressurised (not sure which one of those is the significant one.) Clearly not, I'd suggest, as a moment's reflection on a bird sitting on your lap in a Tiger Moth will make obvious.

So what exactly is the difference when the cabin is enclosed/pressurised? I guess supporters of the bird-weight theory will say it's still the downdraft, but I have to say I found the response to my question as to what happens if the downdraft is artifically blown away by a fan unconvincing.

I also suspect that for the same reasons, some of you will say the Tiger Moth situation described above is not so simple. OK, but then the question arises as to how far does the bird have to go before it no longer contributes to the aircraft weight.

(Presumably this could in fact be tested on the ground - I wonder if it's ever been done?)

Google is your friend

MythBusters (season 5) - Wikipedia, the free encyclopedia

The significant part is the cabin being enclosed. Pressurization is not important.

Consider two aircraft in formation flight, or one flying and the other on ground. If the downdraft generated by one aircraft hits the other craft, the weight of the plane flying/located in the downdraft increases, because some of the weight of the other plane rests on it. But so long as both craft are in external airflow and not in enclosed cabin, both are generating some lift and their propwash or jet blast is escaping. Now, when an aircraft flies into the cabin or cargo hold of another plane and the cargo door is closed, its downwash no longer escapes the cabin, so exactly its whole weight is transferred to the other craft, whether the inside aircraft is hovering or resting on cabin floor.

If the cylinder is spinning in still air, there is no lift. Lift is only developed if there is relative airflow over the cylinder - and in which case there will be downwash at the "trailing edge" of the cylinder - check your Navier-Stokes notes...


Yes, but only in the two dimensional cross section of the wing that you are examining. There will also be an axial flow towards the wingtip (in the case of a non-infinite aspect ratio!) and at the wing-tip there will be vortices with very large downwash.

Yes, this is always the case (ignoring the vertical component of engine thrust of course - which is far from negligible especially during take-off).

I'm aware that there are some books around, like Stick and Rudder, that say lift is entirely down to downwash and nothing to do with pressure distribution. I reckon this is wrong - but have I missed something?

The only thing that you are missing is the vertical component of engine thrust, but otherwise you are correct in straight and level flight, that the aerodynamic component of lift is equal to aerodynamic downwash (conservation of momentum) and that the weight of the aircraft is equal to the net vertical force created by the pressure distribution around the entire airframe

CirrusF and everyone else who has replied to my initial question - thanks very much.

I understand the thrust bit, but didn't want to complicate things initially.

Here's an analogy that helps me.

Imagine a rubber ball thrown against a wall. The ball exerts a force on the wall, for a time, during its collision. Why?

A) The ball deforms when it is in contact with the wall, causing pressure to be applied to the wall by the rubber where it is in contact.

B) The momentum of the ball changes between the time it is moving towards the wall and the time it is moving away. Therefore there must be an impulse applied to the ball and consequently a force applied both by the ball to the wall and the wall to the ball.

Which explanation is "correct"? Well they both are correct. They're just models at different scales. If you were able to make appropriate measurements of deformation of the ball, you'd find the total impulse applied to be consistent with the change in momentum.

The lift models are a little like that. You can look at the pressure acting on the surface of the wing at every point, a bit like A. Or you can look at the momentum change of the air that has been turned by the wing, a bit like B. The lift predicted by either model will be the same. The only difference is in the practicality of the calculation. In the case of the rubber ball, it's obviously simpler to measure the velocity before and after and calculate the impulse that way. With wings, it tends to be easier to look at the pressure distribution on the wing than to calculate the momentum change of every relevant element of airflow. Hence aerodynamicists tend to think of lift as being "caused" by the pressure distribution, which is in turn "caused" by velocity differences in the airflow around the wing.