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-   -   Theory on lift (https://www.pprune.org/tech-log/491335-theory-lift.html)

extricate 24th Jul 2012 17:11

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

Lyman 24th Jul 2012 20:44

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?

awblain 24th Jul 2012 21:30

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.

Lonewolf_50 24th Jul 2012 21:51

Lyman:

Lift? A wing "lifts"?
No, the air lifts the wing via a pressure differential. Vector sums.

Doesn't it Push?
No. A prop pulls, a jet pushes. A wing is the surface area which is pushed by the fluid.

By "fluid" do you mean liquid?
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 "Inviscid" in there anywhere?
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:

boguing 24th Jul 2012 21:56

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.

awblain 24th Jul 2012 22:10

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.

john_tullamarine 24th Jul 2012 23:34

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.

MarkerInbound 25th Jul 2012 00:24


No. A prop pulls, a jet pushes.
What about a pusher prop?

Lyman 25th Jul 2012 01:01

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.

RCav8or 25th Jul 2012 04:18

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.

Microburst2002 25th Jul 2012 07:20

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.

DaveReidUK 25th Jul 2012 08:00


there is no such thing as a pull in nature, nor is there "vacuum"
Exactly. All this time, Nature has been abhorring something that doesn't actually exist. :O

Radu Poenaru 25th Jul 2012 08:22


Originally Posted by Lonewolf_50 (Post 7313001)
No. A prop pulls, a jet pushes. A wing is the surface area which is pushed by the fluid.

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.

Checkboard 25th Jul 2012 09:00

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:

FlightPathOBN 25th Jul 2012 16:11

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.

awblain 25th Jul 2012 21:41

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.

fantom 25th Jul 2012 21:46

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

Type1106 25th Jul 2012 21:53

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

Lonewolf_50 25th Jul 2012 21:57

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:

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

Lyman 25th Jul 2012 22:09

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.


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