Theory on lift
 

To all aviators,

Need some help here.

I'm trying to understand this concept on lift but i still can't understand the concept when they use flow of air against a moving cylinder. It's regarding the Magnus effect.

I think the whole point of using the cylinder theory is to prove that air flow to the top of the wing has a higher velocity. But how do i visualize it with a moving cylinder and a moving liquid?

Pardon me, this isn't a very technical ques.

Page 3
http://www.faa.gov/library/manuals/a...apter%2003.pdf

http://i50.tinypic.com/2ef6qhg.jpg

The air obviously moves (is accelerated) more rapidly vice the surface in the rotating condition, so the comfortable assumption is to do with friction, and boundary layer? Why am I thinking something to do with Coanda?

Lift? A wing "lifts"? Doesn't it Push? By "fluid" do you mean liquid?

Is "Inviscid" in there anywhere?

The top of the cylinder, moving with the airflow, speeds up the air at any point compared with a non-rotating cylinder, while the bottom of the cylinder, moving against the airflow, slows the air down. Faster air corresponds to lower pressure, slower air to higher pressure, from Bernoulli along the streamline, and so it means there's a net lift force on the cylinder towards the top.

There's also drag pushing/pulling the cylinder in the direction of the flow.

Lyman:
No, the air lifts the wing via a pressure differential. Vector sums.
No. A prop pulls, a jet pushes. A wing is the surface area which is pushed by the fluid.
No. Air is a fluid. Smoke is a fluid. Water is a fluid. That said, the diving planes on a submarine use similar flow characteristics to control depth and trim.
Is that Spiderman's next arch enemy in the soon to be released Hollywood blockbuster, "Spiderman 5?" :E I'll be the guy near the fire exit in a gas mask ... uh, maybe too soon for that joke. :sad:

It's not you.

It is a very confusing page. There are similarities between rotating cylinders, but it's not helpful to use them for an introduction to aerodynamics.

Probably easiest to google theory of lift on wiki.

And if you become interested in Flettner rotors, do the same for fanwing.

Do not, under any circumstances do the same for helicopters. For they are evil.

Got that JT.

I think Boguing is entirely right.

That picture you posted is wrong - after the air has passed the rotating cylinder its net direction is at a downward angle to the horizontal. There's an upward force on the rotating cylinder, so there's a downward force on the air. The bunching up of the streamlines is weird too: when the flow speed increases, and density remains the same, the transverse separation of the streamlines is less.

The FAA graphic is a tad useless as it doesn't show the extent of streamline displacement as the cylinder speed cranks up. Indeed, it is more confusing than illuminating in my isolated view.

If one compares the situation with the main field flowing (cylinder off/on) the rotating cylinder effectively constitutes a vortex within the main flow field (don't fuss too much about how you might get a vortex to sit in the middle of a steady flow without being blown away .. mathematics stuff).

The resulting interaction of the two flows is to force the main flow (in the region of the rotating cylinder) to be deflected so that the incoming stream approaches the cylinder from in front and below and departs to the rear and below. This gives a significant change in flow direction to the local main flow and that causes a significant change in momentum .. which provides a vertical force (up/down according to the direction of cylinder rotation) which we call lift (for some strange reason).

The faster the cylinder rotates, the more pronounced is the lift effect.

If you now look at the flow past a wing (say with LE and TE devices), it is pretty similar. This idea then leads to one of the ways of describing lift mathematically using vortex flows.

You might like to have a play with the Jave applet in this NASA page and you'll see the effect far better than I can explain in words.

Easiest way to play is to move the slider bar to the right and watch the spin rate increase, the flow interaction increase, and the lift force increase. The graph to the right of the flow graphic has a little red dot which slides up and down the line to give you an idea of the force developed.

Neat presentation I thought.


I think the whole point of using the cylinder theory is to prove that air flow to the top of the wing has a higher velocity

One of the problems in teaching is trying to match the description to the needs of the person who is trying to come to grips with the problem. Hence we tend to dumb things down so we can emphasise some of the important things happening.

The speed over the top idea is part of the usual way we talk about Bernoulli (and Euler - although he doesn't get any credit in pilot training) flow to emphasise some aspects.

For the cylinder trick, the important thing to emphasise is not the local flow speeds (and your thought is fine - the top flow close to the cylinder will go faster than that on the bottom) but the fact that the spinning cylinder causes the flow to be distorted so that it approaches from front/below and departs to the rear/below.

To me, this is a much more useful way of explaning lift as it gets down to simple action and reaction stuff which we see in lots of places in everyday life.

And, as to the "cylinder theory", it's not a case of its just being theory. If you put a cylinder in a smoke tunnel and spin it up .. the streamline smoke tell tails do JUST what the NASA applet shows.

Figuring out clever ways to change flow direction is all about how you develop desirable forces (lift) in fluids while minimising undesirable forces (drag) .. and this is what aerodynamicists get paid for.

What about a pusher prop?

Hiya Lonewolf. You must be a helicopter pilot. Twerp the Swiss plumber hisself, Bernoulli, what used the term, Inviscid. He was a contemporary of Bram Stoker, so may heap he coined the term in honor of some bloody beast. He used the term to describe....air. Not solid, no viscosity. He opined that at the velocity of 266.377 knots, air became compressed to serve as a foil on a par with liquids, and solids....I think.

Yes, a wing is acted upon with a push, there is no such thing as a pull in nature, nor is there "vacuum". Differential pressures, SI.

Thrust is what we want, there is no pull, push is what keeps us airborne. Jet, prop, farts, it's all thrust.

Helicopters fly by converting vibration into fear, which levitates the machine and it's contents. Or summat.

Quote:
What about a pusher prop?
Good question:p
I guess the prop just produces thrust:O. If the prop has the cranky thing stuck in its backside, then it pulls. If the cranky thing is stuck into its front-side, then it pushes.

The magnus effect, the one that bends the trayectory of a football ball (the round ones, not the other ones, those are rugby) when a football player kicks it in the side, ia used to explain how lift can exist, but you can live without that didactic trick and know what lift is.

Bernouilli is a traditional explanation of lift, and totally unsatisfactory. Newton, however, explains it much better. If the wing pushes the air downwards, the air will push the wing upwards. That's it.

Exactly. All this time, Nature has been abhorring something that doesn't actually exist. :O

Actually, you can consider the prop blade to be a wing. In which case "A wing/propeller is the surface area which is pushed by the fluid."

Using a rotating cylinder to explain lift is a difficult approach in the first place, and the images you have attached are doing a poor job in detailing what is really going on there.

Here's a technology demonstrator displaying the concept as an actual flying model:


Here's a more in-depth article:


... I'd hate to see the glide ratio if the engine stops :uhoh:

Bernoulli's principle is generally explained by the argument that the faster speed of the air along the top of the wing leads to reduced air pressure above and hence produces a lift.
Given this, simplistically, one would rationally surmise that the fuselage negates this basic couple.

If we look at Bernoulli's Laws, or what is the common explanation of wing aerodynamics. Airfoils are curved on top and flat below, and therefore the air follows a longer path above than below. Since the upper surface of the wing is longer, it causes the upper air to flow faster than the lower, which (by Bernoulli's principle) creates lower pressure above.
According to this pure principle, a wing can create lift at zero attack angle, and do not deflect air, the air behind the wing is flowing the same as the air ahead.
Are there any examples of the wing section at zero attack angle providing lift?


We have all seen the diagrams, with the airflow coming together nicely at the back of the wing, in a straight line with the air from below.

To add further issue, given what we have seen in wake vortex creation, does that seem plausible?


Then again,
If we look at Newtons Laws, using principles of Newtonian Angle, wings are forced upwards because they are tilted and they deflect air. The air behind the wing is flowing downwards, while the air far ahead of the wing is not.
Both the upper and lower surfaces of the wing act to deflect the air.
The upper surface deflects air downwards because the airflow "sticks" to the wing surface and follows the tilted wing, called the "Coanda Effect" (marine thrusters and ducted fan UAV's).
(note: while flaps radically effect lift, they add no surface length over the wing, this appears counter to Bernoulli)
For this to be applicable, air's inertia is critical, so after the wing has passed by, air must remain flowing downwards...sound familiar, ie wake vortex creation?

Newtonian physics also explains the lift generated by the center wing section, while Bernoulli does not..


Given that....

Newton and Bernoulli do not contradict each other. Newton's Laws based on air deflection explain 100% of the lifting force. Bernoulli's Laws based on air velocity also explain 100% of the lifting force.

Bernoulli doesn't say anything about where the streamlines go, since it has no description of momentum conservation, although it does a good job of describing what happens along them, since it's describing energy conservation.

Newton deals with momentum.

You need both conservation laws to describe the full picture. It's also hard to picture lift without dealing with drag at the same time, as the air is slowed/made turbulent and deflected downward behind the lifted object.

You are all mad; I am the only sane one.

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

OK, Marker, your lesser case is different from the general case, and was IIRC what Wilbur and Orville used in their early craft. Like ship propellers, eh? :ok:

Lyman:
Since we helicopter pilots typically combine vibration and thrust, the ladies find us very popular. :E

As to "helicopters are evil" I'll suggest to JT that there is at least a grain of truth in that, given the infamous "helicopter pilots are different" piece by Harry R, and the generally accepted 10X cost per pound gross weight to build a helo versus fixed wing. Such is the price we pay for hovering, or being able to fly backwards, which is a good thing, albeit expensive.

Further for aerodynamicists, the complex interactions of lift production and its byproducts on the multiple rotating wings of a helicopter is an ever fascinating subject, but likely beyond the scope of this thread.

All that considered, it's still a wing with a fluid flowing over it.

As is McD NOTAR...

There are elements of Coanda, Blown Flap, and Bernoulli abounding....

As to Bernoulli, after staring at the ugly leading edge of the P-80, I tested an exaggerated camber wing. The upshot is, without AoA Bernoulli sucks eggs, and Newton rules. The difficult proposition is to mathematically calculate "o" AoA.

One cannot introduce enough airstream onto the leading edge to counter the equal split of drag at O AoA. The lifting force at 0 AoA is parallel the chordwise neutral line, and in opposition to the airstream.

now you understand why winglets look like they do....

The funny thing is that both theories (Newton and Bernoulli) are indirectly linked to each other.
Newton assumes that air behind the wing is accelerated downward. If you follow the air movement further downstream at one point the diretion of flow will be longitudinal in line with free stream (vortices aside).
The vertical distance between the TE and this point will a) relate to the vertical component of the mass volume flow (Newton) and b) relate to the horizontal volume flow and thus the acceleration of the air above the wing (Bernoulli).

Edit: It is worth looking at different wing polars to cross check. E.g. NACA4506 vs. 2Rz12.
The former being a very thin profile with cl > 0,3 at Alpha = 0°
The latter being a rather thick profile with cl < 0,05 at Alpha = 0°.

The accleration of the air above the wing does not only depend on the thickness of the profile alone but rather on the entire 'expansion' of the flow also behind the wing itself. And that is where Bernoulli will meet Newton.

Lift is very complicated. Nobody currently knows why a wing can create lift. It's similar to the fact that nobody currently knows why there is inertia. We have detailed theories and laws where we can accurately predict how inertia will affect an object and we know inertia is proportional to mass but we don't know why. To get the best understanding of lift I've found that you need to understand as many theories regarding lift as you can. All the theories tell the story of lift that's not complete and all from different angles. When you analyze lift from all these different angles you start to build a pretty decent understanding of lift. I'd recommend getting the basic understanding of all the theories (or as many as you can understand) and then keep going over them, and every time that you do go over them you should be able to go further in depth.

Regarding the rotating cylinder: it's showing that a rotating cylinder has a similar effect as an airfoil does. They both produce lift, they both increase the speed of flow over the top and decrease the flow over the bottom. There is upwash and downwash. This will tie into Kelvin's circulation theorem. Eventually this Magnus effect should seem somewhat intuitive. Magnus effect - Wikipedia, the free encyclopedia

John Tulla posted a nice explanation here too.

That's hard to define. I think it's easy to see that a jet pushes as it accelerates a flow of air behind it at a higher velocity. But a propeller actually pulls 50% and pushes 50%. Consider a prop that accelerates the local flow velocity by 100 m/s. There will be a 50 m/s increase by the time the air reaches the propeller and the last 50 m/s increase will happen somewhere down stream of the propeller. So with either a Cessna 182 or a Piaggio Avanti II, they both pull AND push.

http://www.gidb.itu.edu.tr/staff/emi...R_THEORIES.pdf

Equation 13 and the paragraph below that show this. I like that document as the way lift is created for an airplane is the same way thrust is created for a ship. A different way of looking at it might make it clearer. The document is orientated more towards the physics inclined person.

Hi Italian....

"That's hard to define. I think it's easy to see that a jet pushes as it accelerates a flow of air behind it at a higher velocity. But a propeller actually pulls 50% and pushes 50%. Consider a prop that accelerates the local flow velocity by 100 m/s. There will be a 50 m/s increase by the time the air reaches the propeller and the last 50 m/s increase will happen somewhere down stream of the propeller. So with either a Cessna 182 or a Piaggio Avanti II, they both pull AND push. "

A turbojet does the same push/pull...

In extreme cases, the intake can produce well over half of the thrust of the exhaust.

In such a case, the exhaust can be redirected to flow against the a/c forwards, and the aircraft will still fly. A jet engine is full of propellors

Put propellers in a tube, and spin them, will there be thrust? Yes.

Power these propellers with an engine that drives them mechanically, still, thrust.

Inject fuel behind the propellers, and ignite it. Thrust.

Lyman...

A jet engine's blades aren't there to propel the airplane forward (assuming we're talking strictly about a turbojet). They're called compressor blades because they compress the air then fuel is added and the air is ignited. The expansion of the fuel/air mixture is directed out the rear of the engine which produces the thrust that propels the airplane forward. A turbojet is really not related to a propeller.

Engine Pressure Variation - EPR

Do you have a reference for that on a turbojet engine? - specifically where it says that it produces "well over half the thrust".

I'm assuming you're talking about ram effect. There can be a noticeable increase in thrust due to the compression of the air prior to entering the engine (ram effect) if the airplane is traveling at high speeds. But I'm unsure what you're trying to point out.

I am proposing negative ram. The tailpipe is dramatic, but the gas comes from somewhere, in the case of the over square intake, cool air comprises more mass than the exhaust. If you could paint the exhaust red, and the intake blue, the huge blue cone in front of the engine is lower in pressure than the small red cone in the back. The aircraft is falling forward into its own 'blue hole', and the front of the intake disc is massively more involved in propulsion than the doughnuts on a rope in back.

Pratt/Whitney. J58.

Regarding the J58:

A ramjet is not the same as "ram effect" on a turbojet engine. You will not get even near half the thrust from ram effect on a turbojet.

specifically where it says that it produces "well over half the thrust".

If one looks throughout an engine, we get a bunch of pressure variation. Integrating over the various surfaces lets us express that in terms of normal forces. The observation is that, in a well designed installation, a large proportion of the nett forward thrust can be achieved from surface pressure contributions.

As to proportions, this will vary between installations.

The Concorde thread had some discussion on the subject so far as it relates to the Olympus installation and may be worth a read.

This page suggests that the Concorde had a prodigious proportion of non-engine thrust contribution in supersonic flight .. something in excess of 90%.

... standard engine installation design consideration, regardless of which OEM and model.

Hi Italian

"The turbojet components of the engine thus provide far less thrust, and the Blackbird flies with 80% of its thrust generated by the air that bypassed the majority of the turbomachinery undergoing combustion in the afterburner portion and generating thrust as it expands out through the nozzle and from the compression of the air acting on the rear surfaces of the spikes."

To be precise, the J58 is at no time a "ramjet". The 80% thrust referenced above is not combusted air, but intake air, isolated from the turbo machinery and the after burner. It has more in common with "high bypass" turbofans in this mode than with a pure ramjet, a device that has no internal compressor section, only ignition. It is in this regard that the division of pressure at the intake cone resembles the propellor, though the zones are defined more by geography within the ducting than the metal stators, due the intense differentials.

IMO. What say you?

As to whether a given engine "pushes" or "pulls" - simply put a stressmeter on the connection between engine and airframe and determine whether the stress is in tension ("pulls") or compression ("pushes.")

You can fly with a barn door for a wing - with enough power. Water skis provide lift with little or no Bernoulli input. Just downwash and AoA.

However, Bernoulli effect makes the process much more efficient, and makes the difference between being able to fly, and being able to fly functionally (with a useful amount of payload capacity and range).

With barn doors for wings, Orville and Wilbur would have needed closer to 1,200 hp to get off the ground, rather than 12.

John...

I should have been more clear. I was talking about ram effect on a basic turbojet (subsonic) engine which would provide a slight rise in compression at the entrance to the engine which would produce more thrust as it went through the burner stage.

The Concorde engine is quite a bit different than a basic turbojet. I don't fully understand the engine but it's quite interesting! I'll have to read through some of those Concorde threads... there are some very knowledgable people on here.

Lyman...

Reference this: Ramjet - Wikipedia, the free encyclopedia

You will see this quote: "The SR-71's Pratt & Whitney J58 engines act as turbojet-assisted ramjets at high-speeds (Mach 3.2)."

Another reference: Factsheets : J58 Turbojet Engine

It's a turbo-ramjet. A ramjet won't function when there is no airflow. The turbojet is what propels the airplane up to speeds that will have the ramjet part of the engine function. The J58 is essentially a turbojet inside a ramjet.

Not what I've read. That hot, compressed intake air is fed to the afterburner section.

I was talking about ram effect

Understood .. but that is a secondary consideration.

The end result of the pressure distribution acting on ancilliary structure can produce a significant force on that structure. Include some clever design .. and one gets a lunch heavily subsidised by Mother Nature.

CliveL, djpil (and, no doubt, others whose specific backgrounds I am not familiar with) being experienced aerodynamicists .. can speak on such things for ever and a day, I'm sure.

For an authentic explanation of how lift is developed check this one out:
The Origins of Lift

It is written around sails, but the explanation is equally valid for wings. Arvel Gentry is a practical professional aerodynamicist so can be believed. After 40 years of accepting the "Bernouilli" explanation he certainly opened my eyes!

PS: the Olympus 593 (Concorde engine) is a pure conventional turbojet and the "thrust" coming from the intake at Mach 2.0 was about 75% of the total.

Owain...

Excellent article! There is a lot more to lift than Bernoulli.

Back to thrust produced by the intake...

Is there a good article that explains how this all works?

My understanding is that the air is compressed by the intake as the intake slows the air to subsonic speeds. The air is heated as a result of the compression. The resulting air is directed around the 'core' of the engine and put into the afterburner section where fuel is added to the hot air and ignited, producing significant thrust.

I had read that the air gets heated to a very high temperature and was led to believe that if fuel was introduced to the air (in the afterburner section), it would auto-ignite. Is that correct?

Since it's the intake that compresses the air that gets burned and produces 63% of the thrust on the Concorde, they say "the intake produces 63% of the thrust". Is that correct?

Newton is the man.

All you have to find is what does the air do when a wing passes through it. It accelerates the downwards and forward, so the air gives the wing an equal force upwards and backwards.

The more air there is, the more lift (either by increased density, wing surface or angle of attack)
the faster the air is, the more air is accelerated,
the faster the air is, the more accelerated it is

hence, lift is proportional to density, wing surface and angle of attack, and to squared speed. then you use the Lift Coefficient as the constant of proportionality, only it depends on the angle of attack, and that is the formula

Is there a good article that explains how this all works?

Really only necessary if you need to be able to run some calculations for design work.

To get a general overview idea, think ...

(a) structure exists .. and we are looking more at the region around and outside the engine itself but still subject to airflow due to the engine.
(b) air is flowing over the structure due to the engine's good work and the aircraft's flight speed
(c) air has pressure which varies with local speed
(d) hence the air over different bits of structure will have slightly different pressure
(e) for each little bit of structure, the force associated with the air pressure acts normal (perpendicular) to the little bit of structure
(f) resolve the little bits of force into whatever directions are of interest (ie typically forward and aftwards are what we are after)
(g) integrate (add up) all the little bits of contributing force
(h) end result gives a nett thrust or drag
(i) the thrust or drag varies through the engine so, in some regions, we are getting useful thrust (nacelle and nozzle) while, in others, it's all drag.

One needs to keep in mind that this is the province of the aeroplane designers rather than the engine folk. The engine has to hang inside some structure and have suitable airflow conditions presented at the front and somewhere for the hot bits to get out at the back.

A similar argument can be structured for what happens inside the engine proper.

@Microburst

I'm not going to disagree with your overall position that Newton's laws can be used to calculate lift, but that is not quite the same as saying that they explain how lift is generated. If I may take an extract from Gentry's article:

WRT your specific remarks:

@Italia

This is a turbojet we are talking about - there is only the "core", no fan. Fuel is not put into the "afterburner" - at least not in cruise. It goes into combustion chambers aft of the compressor and before the turbine. The 'engine' contribution to thrust is carried on the turbine blades which, contrary to one of your earlier posts, behave very much like a multi-stage propeller.

I dunno, but since the afterburner sits behind the turbine (which has a very high exit temperature anyway) the question has no real meaning

Picking up on JT's comments, the intake on a supersonic aircraft is essentially a convergent/divergent passage. In the first bit (convergent) the air is slowed down from freestream speeds to Mach 1.0. In the divergent bit it is slowed from Mach 1.0 to about 0.6M or whatever the engine can accept. Over the front bit the compression generates pressure forces on forward facing surfaces and gives drag. In the subsonic divergence the pressure increases steadily going towards the engine. This part of the intake has aft facing surfaces and the increased pressures generate thrust. Whether you get a net drag or thrust depends on the Mach number and the intake design - hence Brian Abraham's comments.

The Concorde thread is a good source of information on this.

Owain...

I wasn't talking about a turbojet, I was asking about the intake system that creates 63% of the thrust at cruise speed.

Regarding the multi-stage propeller bit: I understand that the blades act like a multi-stage propeller but I was saying their purpose is to compress the air and not to directly provide thrust like a propeller.

This image shows the Concorde's engines while in supersonic flight.

http://www.concordesst.com/graphics/engineairflow2.jpg

What is the "inlet air"? Where does that "inlet air" go? In the picture it appears that some air goes along the top, following the first arrow and bypassing the engine intake, through a tiny passage and then gets dumped at the rear of the engine right before the divergent exit. There is also another passage on the bottom of the engine where air bypasses the engine intake and meets at the rear of the engine right before the divergent exit. Does the air that exits out of those two passages contribute to the "inlet thrust"?

Edit: 8% of the thrust is from the engine in cruise - does that mean that the fuel burned is only related to that 8% that is from the engine? NONE of the thrust from the inlet is made by combustion?

The turbine removes energy from the exhaust gases in order to power the compressor. In no way does the turbine provide thrust, so the propeller analogy is wrong!