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I don't believe that lift can occur without downwash. The first two cases you quote, "spinning cylinder and reflex trailing edge", both turn the wake downwards.
A cylinder spinning backwards and travelling forwards has a delayed boundary layer separation point on the upper surface, where the boundary layer is moving in the same direction as the free stream flow, and a premature separation point on the lower surface where it is moving against the free stream flow. This deflects the whole cylinder wake downwards, i.e. it creates downwash, and hence lift.
This is the same effect as a spin bowler uses to curve the flight of a cricket ball - it works for spheres, too!
An aerofoil wing section with a reflex trailing edge will still generate downwash and lift, simply less of each because it is not an optimum shape for high lift coefficient.
I'm afraid I don't understand what you mean by "lift without power for infinite spans", so cannot comment.
As one who has had the experience of standing beneath a Sea Stallion Helicopter as it turned through 90 degrees at 50ft altitude just before landing,
I can vouchsafe that it felt to me as if the air mass equivalent to the whole weight of the machine plus a percentage more to allow for the turn was washed down by the rotors as cyclic pitch plus power was applied by the pilot. Not very scientific, I agree, but a practical demonstration I shall never forget.
You can't have lift without downwash. If you are able to produce lift without downwash you have your fortune made, helicopter manufacturers for one will beat a path to your door. A couple of examples I hope may illustrate.
Example 1 Consider the fin/rudder of an aircraft flying without any induced sideslip from other causes. The rudder will be faired with the fin (0° deflection) and the angle of attack is zero. In this condintion we have a symetrical airfoil and on either side of the fin/rudder will be a pressure field induced by the airlow being accelerated over the curved surface. The pressure fields on either side of the fin/rudder are equal and are thus unable to effect any turning of the airflow to create a downwash. The instant rudder is applied a positive angle of attack is created which then creates an imbalance in the pressure field on either side of the fin/rudder. The imbalance in the pressure field causes downwash and so you then have lift. You may have seen the computer modelling of the pressure fields about an aircraft where the pressure level is depicted by different colours. An aircraft experiences pressures of varying levels on all external surfaces, but unless there is an ability of the pressure field to induce a downwash there is no lift.
Example 2 A propellor is nothing but a wing that rotates, rather than travelling through the air in a straight line as does an aircrafts wing (for all intense purposes). As a pilot you intuitively know that unless the propellor is blowing air back you have no thrust. Thrust and lift are exactly the same and is the result of an airfoils movement through the air at an angle of attack sufficient to induce downwash, thrust being the lift produced by a propellor, and lift the thrust produced by a wing. In the propellors case the downwash is seen as a cylinder of air being thrown to the rear, and in the wings case, as a mass of air being thrown towards the ground.
As kids we used to live on a small hill directly under the approach path. The heaviest aircraft we used to see was the DC-3 and they would pass over the top at a couple of hundred feet. On a calm day a party trick with school mates was to place a sheet of newspaper on the ground and shortly after the -3 passing over the paper would move as if by an invisible hand.
"There is a misconception held by some that lift does not require power. This comes from aeronautics in the study of the idealized theory of wing sections (airfoils). When dealing with an airfoil, the picture is actually that of a wing with infinite span. Since we have seen that the power necessary for lift is proportional to one over the length of the wing, a wing of infinite span does not require power for lift."
This is the bit I'm struggling with. The rest is quite comfortable.
I've looked again at the spinning cylinder and its clearer now.
Since we have seen that the power necessary for lift is proportional to one over the length of the wing, a wing of infinite span does not require power for lift
An infinite span - It's an absurdity on the face of it, right?
Simply postulating a mathematical asymtotic extrapolation doesn't mean we have to build the damn thing.
Matter of fact, if we build a wing with slightly over a 40 million meter span, the left tip will meet the right tip, there will be no tip vortices, and it can float at zero IAS.
Its just hard to understand why that would be. Surely a wing needs some force on it to keep it floating, and without the mechanism that produces that force, surely it would not have a force acting on it.
I realise it fits the maths well, but it doesn't fit reality surely?
Milt says: "Interesting to explain wing lift scientifically/mathematically/physically or whatever but the bird flying around in the pressurised cabin of an aircraft nevertheless transfers its weight to the aircraft by air pressure waves and the aircraft then transfers its weight, which includes that of the bird, to the earth's surface by more pressure waves."
OK, so when the bird is flying its weight is transferred by "air pressure waves" - maybe - and when the bird sits in seat 18B it is effectively part of the aircraft. So far, maybe, so good.
So how much does the combination weigh while the bird is descending inside the aircraft with its wings folded? There are then no "air pressure waves", and it is not "part of the aircraft".
Or try this. Suppose the bird is in its seat on the aircraft sitting stationary on the ground. We weigh the combination and get a figure. Then the bird takes to the air inside the aircraft. How much do the scales now register? Does it matter if the cabin has a roof or not? And if we (highly hypothetically) performed the same experiment with the aircraft in flight, now does it matter if there is a roof or not?
Surely a wing needs some force on it to keep it floating, and without the mechanism that produces that force, surely it would not have a force acting on it
Think of the rings of Saturn. Now think of all those millions of ring particles connected to each other in a solid mass, with the centre of mass of this ring object coinciding with the planet's centre.
Now freeze the ring so it's stationary WRT the planet, not orbiting around it. Gravity will keep the two centres of mass together!
(What does this have to do with aerodynamics? gee--- duh--- )
The maths behind zero power for an infinitely long wing don't take account of the fact that the Saturn analogy is a closed loop. Imagining a flat plane with an object above it, infinitely long in all directions, then surely if there is a gravitational force, the object and the plane attract each other. Unless they orbit each other, (like Saturn's rings orbit Saturn), then they will move toward each other. Surely the particles in Saturn's rings are in perpetual freefall, just like satellites orbiting earth? Without that motion around each other, the infinite wing would still need something to keep it in the air?
Obviously with a wing around the earth as a single body, the pull on one side would be counteracted by the pull on the other and it would float, or in other words the earth would be pulled toward each part of our ring equally. The difference being that this is a single body and that Saturn's rings are many bodies.
I realise it works mathematically, I'm just saying I find it hard to conceptualise how it would work in reality.
Yup, that sounds right. But I don't think it's analogous to the bird in the cabin. Surely the bird hanging (getting slightly surreal here) from the cabin roof is not the same as it being in lift-generating flight inside the cabin?
But if it's not, then when it's in flight does it add to the weight of the 'aircraft' as suggested earlier. And if it does, then does that weight go away from the 'aircraft' while the bird is in non-lift-generating descending-flight?
Hmm, well maybe it's because of the three glasses of riesling I just had, or maybe it's because I just find it hard to believe anyone called chornedsnorkack but I still have problems with this.
However, maybe we stumbled across something interesting. Think of the aircraft on the ground and with the bird (or hovercraft or helicopter) being suspended inside the aircraft from a cable which is hanging from a totally separate crane outside the aircraft. If the crane lowers the bird onto its seat again then the aircraft will weigh more. But if it picks it up then clearly it will weigh just the weight of the aircraft.
But, you say, and other people in the thread say, that if the bird is flying (at least in an enclosed cabin) then the downwash from the bird will make the aircraft weigh more. So that seems to go back to the original nice and simple question that started the thread - aerodynamic lift is a different force from the tension in the cable attached to the crane. So it must be the downwash that results in lift.
But suppose the downwash is insufficiently strong to reach the floor of the cabin (or it's blown horizontally by a fan for example) then is there no lift anymore?
Don't feel obliged to answer this - perhaps you have a busier life than mine over there in Estonia.
Correct me if I'm worng, but.... The reaction of the ground isn't anything to do with the creation of lift at the source, its just that the momentum has to go somewhere eventually.
If your bird on your outside crane started flapping, the tension in the wire would decrease, leading to a decrease in the total weight of the crane reacted through its wheels.
This lift would then have to be reacted somewhere and it would be the floor of the aeroplane the bird is in (making it heavier). If a fan blew the downwash away, the downward momentum would still be there and would have to be reacted somewhere, perhaps further down the cabin, or if the aeroplane had an opening tail, further down the airport.
its just that the momentum has to go somewhere eventually.
Watch out. Momentum conservation is not like a set of accounts that has to balance at the end of the tax year. It is balanced instantaneously, as a consequence of Newtons second and third laws.
Returning to the example of an item dropped from the ceiling of a plane - yes, the momentum transferred to it by force of gravity balances instantaneously, by third law. It balances by accelerating the item, in free fall, by second law.
No.
The momentum is not "balanced by the item accelerating".
Gravitational attract is occuring between two masses, one very big (the Earth) and one very small (the item you dropped). So, not only is the item accelerated towards the earth, but an equal and opposite gravitational attraction (Newton 3) is pulling the Earth towards the item. As a result, both accelerate towards on another.
However, since the Mass of the Earth is very very Huge, the resulting acceleration (newton 2) is very small. The item though has a much smaller mass and therefore accelerates a lot more.
Newton 3 tells us the gravitational attraction is equal and opposite.
Newton 2 tells us each item experiences a rate of change of momentum equal to the force.
The upwards momentum of the earth is equal in size to the downwards momentum of the item at all times. Added together as vectors they cancel one another out, and the total vertical momentum of the item and the Earth is zero, at all times.
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?)
Looking at lift on a spinning cylinder, the upwash and downwash are identical, so surely there cannot be any net downwash?
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...
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Also, one can turn the trailing edge of a wing up and still produce lift, albeit not very efficiently, but without downwash.
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.
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So have I missed something? Is lift = downwash = integrated pressure around the whole body?
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
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.
About that bird in the aircraft...
but I still have problems with this.
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