Theory on lift

[Lyman]

Hi HN

Point of View without question. But my comment is framed from the Bernoulli position, that locale is all important, to the exclusion of logic, and entertaining assumptions that require the suspension of physics.....

Inviscid, ideal liquid, "virtual scoop", Accelerated airmass, etc....

I thought the global view was reserved for the Physics shair at Oxford, not Bernoulli?

Am I winning you over?

[roulishollandais]

1. ...take care to that mistake (sleep better !)
Drag and Lift are not connected to gravity direction, but to airspeed direction.

Weight and Power come in balance (or unbalance) with Drag and Lift.

2. For these who find the discussion too difficult, stay with Gentry

3. For our PPRuNe's Flight Gods, perhaps they don't understand fractals, equations, aso. but with their brain and body they understand Lift, Drag, Weight, Power, and evolutions in the air better than all of us.

4. Lift and Drag and viscosity are some of the most difficult things to understand. Our best models are still far away from reality... don't desesperate !

[FlightPathOBN]

Airfoil database.... UIUC Airfoil Data Site

[Lyman]

Bonjour roulis


A theoretical for you. As to streamlines, it is accepted that no matter may pass through them, so as to wing area ( Span x mean Chord). Do these phenomena imcrease the effective area of the wing? We see that the two streamlines, when reconnected, prevent for a short distance any alteration in flow, yes? It is Kutta, at the other end?

Also, this.....

A wind tunnel is created that separates two areas, one, the lower, is air, the upper (above the wing) is completely evacuated, though the pressure is maintained at constant level throughout the tunnel, (bear with me).

The wing is in constant contact with the lower (moving) airmass, there is no mass above the leading edge. Is there "lift"?

[Owain Glyndwr]

@FlightpathOBN

Nice pictures, but I asked for an explanation of how the vortices developed using Newtonian arguments.

The aircraft is planing through the air, the weight of the aircraft, with angle of attack of the wings, compresses the airmass to provide level flight. (notice as the aircraft fuel load lightens, the angle of attack to fly level is reduced

So airspeed doesn't come into it?

The A&E paper says:

The key points are that the lift of the wing is proportional to mdot & vv and the induced power is proportional to Lvv. Also, mdot is proportional to the wing's area and speed, and the air density, while vv is proportional to the wing's speed and angle of attack

.


That is mdot (mass flow rate) not air mass, and there is no mention of any air mass being compressed. When we get to details that A&E paper again says:

Thus, a Cessna 172 at cruise is diverting about five times its own weight in air per second to produce lift. A 250-ton jumbo jet in cruise is diverting about its own weight per second.

How do you explain that, and why isn't the Cessna a danger to the jumbo not the other way round?

The wing section is constantly changing, with the leading edge sweep angle, thickness, and width.

So what about an unswept wing, constant chord and t/c? Doesn't that have tip vortices?

The combination of angle upward of the bottom of the wing, and the component airflow direction over the top, causes the rollup,

Why should the angle upward of the bottom of the wing have any effect on the rotational velocity? What causes the "component airflow direction over the top"? Doesn't a change of direction (from freestream) imply some sort of force (Newton's first law)? So where does this force come from in Newtonian logic?

[FlightPathOBN]

Owain,

Will have to take some time to look through much more of the thread. So many issues have been brought up, and by so many people, it is difficult to see which question(s) were answered, and who asked...

and nothing has tip vortices...

[Owain Glyndwr]

Flightpath

and nothing has tip vortices...

Sorry, I thought that was common usage - trailing vortices originating at the tips of a wing with no flap or control surface deflection be OK?

BTW, I've just noticed that on your bottom picture showing how the wing vorticity behind the aircraft rolls up into two concentrated vortices, there are symbols +/- capital Gamma. Capital Gamma (sorry I can't find a way of inserting the Greek letter) is the standard symbol for circulation, so your picture is actually confirming that wake turbulence is proportional to circulation (and wing lift)

[awblain]

Newton's vortices?

How does Newton account for vortices?

Air that's flowed over the wings is going down. Air that's closer to the centerline is going down more, air that's further from the centreline is going down less. Hence shear, which is skating perilously close to rotation.

[Owain Glyndwr]

Air that's flowed over the wings is going down. Air that's closer to the centerline is going down more, air that's further from the centreline is going down less.

Just take a look at Flightpath's pictures - especially the one immediately under "The aircraft wake is compressed, and with the combination of the 2 counter-rotating vortices"

That little symbol 'w' is downwash and the picture shows that the air closer to the centreline is going down a lot less than that near the tip. So I'm afraid your argument is not supported by the facts.

[TheChitterneFlyer]

I'll bet 'extricate' wishes he'd never started this!

I'd like to bet that everyone else wishes that he hadn't started this!

Ask a simple question to PPruners and you get "rocket science answers"... rocket science it aint; but there are many PPruners who wish to go on a "points scoring" trip! However, it would be great if the "point scorers" could write in "correct" grammatical English that everyone can understand.

[awblain]

Look at the trough in the cloud behind the C17

That's the best picture to see the effect on airflow from the passage of the aircraft: the cloud is lowest along the keel, not in two tracks behind the wingtips.

See the 757 wake smoke too - ten lengths back, you have a dipole-field-like flow.

Alongside the posted picture you reference: there is almost no compression of the air. Dynamic pressure, certainly, but no bulk compression.

[HazelNuts39]

Originally Posted by Owain Glyndwr

Just take a look at Flightpath's pictures - especially the one immediately under "The aircraft wake is compressed, and with the combination of the 2 counter-rotating vortices"

I think that picture shows the distribution of downwash velocity at some distance behind the wing (10x wingspan), where the trailing vortex sheet has 'rolled up' into two discrete vortices. Would the distribution closer to the wing be more even (i.e. closer to a constant downwash velocity across the span)?

[Lyman]

Hi awblain...

I may be pedantic, but I think it is important. The aircraft hasn't an "effect" on airflow, the air is still prior to the entry of the airplane. It is the energy of the a/c that develops airflow. The visible effects are the results of energy being imparted to the airmass. Some may think it unimportant whether the air is magically flowing, or is instead disturbed by, the mass driving through it. Strictly speaking, it is the wing that is "flowing" (moving) not the air. The streamlines move downward (wash) long after the wing is gone. It is this transition in angle of streamline that is sensitive to rate of development of pressure, or, velocity of the wing relative to the airmass. How quickly it turns (quantity of lift) is related to velocity and angle of attack.

Dynamic pressure is just compression expressed as a result of motion. To deny there is compression of the airmass as the wing quickly enters is not correct, imho. Mass moves as a result of collision, Third Law? The Air Mass resists motion, First Law, viscosity. Streamline curves, reluctantly, First Law? It retains its shape, Second Law?

There is no wing tip in the standard model drawing, don't know what that's about. Not sure what vortices have to do with lift. Like waterspouts, they are interesting.

Regards

[HazelNuts39]

Originally Posted by Lyman

Strictly speaking, it is the wing that is "flowing" (moving) not the air.

Physically it doesn't really matter whether you place yourself in the airplane and describe the airflow moving by, or you place yourself in air at rest and look at the airplane flying past you. If you want to discuss from the perspective of the "air's rest frame":

Dynamic pressure is just compression expressed as a result of motion. To deny there is compression of the airmass as the wing quickly enters is not correct, imho. Mass moves as a result of collision, Third Law? The Air Mass resists motion, First Law, viscosity.

Place yourself in the air molecule that meets the approaching wing at the 'stagnation point' close to the leading edge. When the wing has reached you, you will be moving at the same speed as the wing, and the pressure around you will have increased by the 'dynamic pressure'. Your acceleration from rest to the speed of the moving wing is not caused by 'collision' with the wing, but by the increasing pressure as the wing is approaching, because that pressure propagates forward in front of the wing (assuming the airplane is moving at subsonic speed). Sitting at the stagnation point of the moving wing, your air molecule has the choice of moving along either the lower or the upper surface. Whichever way your molecule goes, it will initially be 'pushed' along by the pressure reducing downstream from the stagnation point. Going along the lower surface, the pressure will reduce until at the wing trailing edge it has reached approximately the pressure of the surrounding air at rest, and the molecule will be moving rearward along the wing surface at about the same speed as the wing is moving forward, i.e. it will be approximately at rest relative to the surrounding air. If, OTOH, the molecule chooses to go over the top of a lift-producing wing, it's a different story. The pressure will reduce much more rapidly until it is well below the ambient pressure, by several times the dynamic pressure, and the molecule speed rearwards is much greater than that of the wing moving forward. However, past the point of minimum pressure the molecule decelerates because it is now moving against increasing pressure, until it finally reaches the trailing edge at about the same speed and pressure as the molecules that passed under the wing.

So, in summary, part of the lift is indeed due to the pressure on the lower surface being higher than the ambient pressure, but that contribution is small compared to the pressures acting on the upper surface being lower than the ambient pressure, commonly called 'suction'.

Bernoulli relates local pressure to local airspeed. Other than that, it does not explain why pressures and speeds change as they do. To explain that, you need viscosity and vorticity.

[Lyman]

Hi HN

I really like that explanation. However, I did not say that all the air collides with the wing, that would not be possible, and here is where I like Bernoulli, for as you say, the theory describes focal regions of pressure/gradient

I think you are also differentiating pressure zones as pairs, above, and beneath the wing. For your purpose, I will agree to the use of the word suction, though it is a bit creepy; I was taught to ignore the concept as spurious.

In any case, any differential that works on one streamline works on the other, on a monoplane, they are mechanically connected, No?

I also appreciate the description of velocities as returning to pre entry values and that is why I prefer to see the wing as the donor, the air as recipient.

As to suction. Above the wing, the low is attractive of the open and ambient zone above it, and the closed boundary beneath, as streamline or wing skin? The wing is pressurized beneath, and moves toward the low due to the ambient high beneath the wing? One cannot distinguish which surface is attracted? The Streamline? Or the skin? (which is of course attached to the aircraft?)

Thanks for your kindness, and your patience.....

Cheers

[Owain Glyndwr]

@HN39

Would the distribution closer to the wing be more even (i.e. closer to a constant downwash velocity across the span)?

Hi HN,
You are right of course; the picture didn't say how far back and I didn't think it through. Write in haste, repent at leisure.
Close up to the wing the downwash will reflect the span loading which may or may not be a constant downwash -but then a constant downwash would not give any shears.

@awblain

As you see, my posted comments were unsound - my apologies.
However I still believe that you cannot generate rotational flow by the action of linear shears

PS - I wonder how many points I lose from that brick

[John Farley]

drag, which is a retarding force, the effect of which is proportional to the speed of the airplane

When I was at school it was proportional to speed squared.

Wonder when it all changed.

[Lyman]

Hi Sir.

I think both statements are correct, though they may appear to conflict...

With the greatest respect, I have wanted to ask the following question.

Several times I have been present at a demonstration of the Harrier. I have a friend whose first experience with one is an interesting tale, but for now....

In the hover, one can easily see the application of the Newton Third Law.

Disregarding the dozens of airfoils providing "lift" from Pegasus, the reaction and work is readily apparent.

As you ease into aerodynamic flight, when exactly, does Newton fade away, and Bernoulli replace him? Can Newton yet apply to and through the "transition"?

Most grateful Sir



HazelNuts39

"Physically it doesn't really matter whether you place yourself in the airplane and describe the airflow moving by, or you place yourself in air at rest and look at the airplane flying past you."

Yes, of course. However. This came up as a response to my initial post that posited the wing as the source of energy in the standard diagram seen in the paper under discussion....

The authors purpose in writing the paper was to generate interest in evolving the basic introduction to the Physics of Flight. It is my view that an introduction should be basic, elemental, and exquisitely explicit.

I share that with the authors, imo. So why introduce 'latitude' ("it makes no difference") when restraint is required? To what end? Without a purpose, offering alternate suppositions to frame of reference is gratuitous, and opens the gate to erroneous knowledge.

My drawing has the thrust arrow "within" the wing, denoting motion of the airfoil, not "in front of" the leading edge. The wing is not passive, yet that is the almost universal conclusion students make when first introduced to the "standard" diagram.

Now Camber. The authors show in an elegant way why camber is not necessary in forming a teaching model. They intimate the importance of shape, but dismiss shape as a fundamental necessity, I agree.

The authors show, again simply, why Bernoulli assumes a closed system. As a practical matter, that is deceptive, again for the beginning student.

So off we always go, as the scientists impatiently require a long list of illogical assumptions of their audience, and then recoil in anger as some refuse.

Newton does not 'fail' in any way, throughout the flight envelope, imo....


to awblain..... Thank you indeed for introducing the word 'keel' to the discussion....

[John Farley]

Lyman

As you suggest jet engines are very much Sir Isaac in action.

With the Harrier in the hover the thrust and hence Sir Isaac are supporting 100% of the weight.

As the nozzles are rotated aft the vertical component of thrust reduces as the sine of the angle.



As IAS increases the wing provides an ever increasing lift (as with any other wing) helped by the fact that it has an angle of attack of some 8 deg with the aircraft in the hover attitude. The hover pitch attitude is determined by the undercarriage and the need to land mainwheels first to avoid overstressing the nose leg.

The above diag shows how trig works in the pilot's favour when stepping off from the hover. The first 15deg of nozzle rotation produces a .26g accel while only costing some .03g downwards. Something for (nearly) nothing!

The pilot rotates the nozzles at a rate that gives him his chosen accel path - too fast and a sink will occur - too slow and height will be gained. With a level transition the nozzles will be aft after about 13sec.

All extremely easy to do.

[awblain]

Lyman, sir.

`Newton' and `Bernouilli' are both right in their way. `Newton' is couched in terms of conservation of momentum, `Bernouilli' in terms of conservation of energy. Both need to hold.

On a hovering Harrier, where does the lifting thrust act? Outwards and upwards equally at the elbow in the nozzles, where Mr Newton's second legal adviser would recommend his engineers to toughen the structure. The fuselage structure is on the rack, with seven tons pulling up, and seven tons pulling the machine apart abeam. As it starts to move forward, the flow over the wings takes some of the upward load, and the nozzles swivel back, rotating those 45 degree forces at the elbows forward, pulling the Harrier along instead of holding it up, while still trying to rip it apart acrossways.

Owain,

I fully accept your point that linear shear is not rotation, but once you account for the descending slab of air behind the aircraft, with the wingtip vortices to connect it to the non-descending air far from the flightpath, it's not a big stretch.

Close to the wing, I agree that the air that moving downwards the fastest is descending round the flap edge in that 757 picture, but that high-speed flow is part of a tight vortex - it spreads and slows behind, making the large scale dipole flow from the C17 wake picture, also effectively seen in the 747 contrail shot.

If the wing was all the way across a smooth windtunnel section, with no tips, the air behind the wing would have a uniform descending velocity component - there would be lift - and yet no rotation. I suspect almost the same is true for a sailplane wing - the downstream airflow is almost all descending slab, and almost no wingtip vortex.