Lift
 

Just as I suspected, with one or two illustrious exceptions -

I've been putting my life in the hands of pilots for years, and you still haven't agreed how the wings work ! :)


I've always gone for a mix of the ' airflow deflected down ' idea- I've been under low flying fast jets, and that's how it seemed to me ! Mind you they were at relatively high alpha, flaps & power on, for chase work.

Also the ' baby sucking on a straw ' pressure theories, but have only studied aeronatics slightly compared to most posting here.

I once attended a lecture by Bill Bedford, when he was asked by an idiot ( who naturally qualified later as a BAe 'manager' for about 5 minutes ) " what new developments are in the pipeline, and will we see the P-1154 ?"

This was 1978 ! Bill gave him a withering look, and replied " well we haven't got anti-gravity paint worked out yet ! "

So what I wrote above would be correct - forces are transferred to the wing by pressure forces, caused by turning the air. These pressure forces are as a result of conserving momentum, energy and mass. The body feeling the reaction of the force imparted to the flow.

I'm quite happy with that, with two exceptions.

How does one explain lift without downwash (spinning cylinder and reflex trailing edge) and lift without power for infinite spans?

I don't think "lift is produced by throwing air at the ground" is quite as accurate as that. Surely "lift is produced by turning air and feeling the reaction" would be better - albeit with the ground (or other solid objects) eventually taking the momentum away from the air.

As I said I am no expert ( did play with a wind tunnel a few times ) but surely what you're describing is ground effect, not true wing lift - velocity, chord & all that ?

Jetstream Rider (Post #40):

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.

Helicopter Downwash
 

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.

"lift without power for infinite spans" comes from the link above

here
How Airplanes Fly: A Physical Description of Lift c

"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.

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. :rolleyes:

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?

About that bird in the aircraft...
 

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?

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.

Consider it this way: look at a freighter with a crane installed in its ceiling (I think several freighters have them) and a heavy item suspended from that crane.

So long as the item remains hooked at the crane, the weight that works on the landing gear is that of the plane with all its loads - including the item.

Now, a clumsy loadmaster drops the item.

So long as the item is in free fall inside the plane, the weight of the plane decreases and is the weight of the plane and load without the item in free fall.

When the item hits the floor, the weight of the plane increases, and becomes bigger than the weight of the plane and load including the item. This lasts until the item comes to a stop (or breaks through the bottom of the plane). After the item has come to a stop, the weight of the plane is again exactly the weight of the plane and load including the item.

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?

Imagine having a small helicopter, hovercraft and other vehicles in the cargo hold of a big freighter plane - whether the plane is resting on tarmac, in steady taxying or in steady cruise.

When a vehicle is inactive and just sits in the hold, its weight is transferred to the cargo hold through its landing gear.

The weight is actually already carried by air. A vehicle is supported by air trapped in its suspension. And the same vehicle is also supported by air supported inside its tires. (No power is needed to keep it there, because it cannot escape).

Now, put power on the hovercraft. It rises up on air cushion. A hovercraft in steady hover has precisely its entire weight supported by the same cargo floor, because of the extra pressure of air cushion to the floor. The hovercraft does spend some power to hover because the air in air cushion can escape, unlike that inside tires or suspension.

Now, power up your small chopper. It creates downwash. Its weight is no longer supported by its landing gear - but the downwash transfers exact same weight on floor, and perhaps ceiling. So long as it is a steady hover.

If your helicopter is in unsteady flight them the weight of the plane will change. When the copter is in free fall, accelerating downwards, its weight goes away. When the helicopter is accelerating upwards, the weight of airplane will be more than that including the helicopter. Although you should remember to count the acceleration of air inside - the plane does not acquire the weight as soon as the helicopter increases lift, but only when the extra downwash so created hits floor (or ceiling).

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.

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.

pb

Both are right.

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.

But the item cannot get out of the plane. When it hits the floor and comes to stop, its momentum returns to zero. That´s when the entire weight and momentum goes back to the airframe.

So, when you count the momenta of moving parts, the momentum must be conserved instantly. But since internal momentum cannot be nonzero in long term, eventually the momentum must be conserved without any allowance for moving parts.

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.