417mack

What part of the wing generates the most lift, wingtip middle or near the fuse. I had a discussion with a fellow rcer at the airstrip today and nether of us had any idea.

BMatthews

The spanwise lift distribution curves I've seen always have the most lift around the center to center plus mid span area of the wing. The variation is due to different taper ratios and shapes. The fuselage doesn't seem to affect that. It then rolls off in a curve to zero right at the tips in all cases.

Lnewqban

............And this thread discussed the reasons for that distribution, as well as the dependence of the wing planform shape:

http://www.rcuniverse.com/forum/fb.asp?m=9530648

417mack

thankyou for the replys and the link this should settle the discussion.

HighPlains

My Feedback: (1)

Front half of the wing makes most of the lift.

Shoe

As BMatthews points out, the lift distribution always goes to zero at the wing tips. Suppose you had a rectangular planform wing that had constant chord and wing section, and no twist (the simplest wing you could build). If you were to look at the lift distribution, the center sections would "generate" more lift than the outboard sections. Suppose you were to cut the tips off of the wing so that the span was reduced by half. The lift at the new wing tips would now be zero. I think this is interesting because it is easy to look at a plot of a lift distribution and see that the center sections carry more lift. However, without the outboard sections there, the inboard sections wouldn't generate nearly as much lift. So I would make the argument that, although the outboard sections "carry" less lift, their presence is responsible for some of the lift carried by the center sections. Lift and drag can be slippery topics to get your brain around...

opjose

An article in one of the U.K. RC publications basically threw out lift via pressure differential, citing some rather compelling examples...

Instead they ascribed lift to Magnus Effect and associated characteristics... an interesting read...

[link=http://www.grc.nasa.gov/WWW/K-12/airplane/cyl.html]Lift of a cylinder[/link]

As a rough synopsis, the article postulated that lift is the result of a rotational force on the air, giving rise to the Magus effect, and not related or dependant upon classic wing zone pressure differentials at all.

nmking09

My Feedback: (61)

rotation leads to pressure differential leading to lift.

opjose

I wish I could reprint or reproduce the article here...

According to the article: ( and I'm paraphrasing as I write this ... )

The pressure differential is insufficient to achieve the lifting forces. When measured the change in pressure is miniscule compared to the amount of lift that results.
The article goes on to say that it is the rotational vector itself that causes the lift, not any pressure differential.

Several practical test examples are given, which are pretty convincing...

BMatthews

Back in the day NACA tests were done with manometers on wings and it was shown that when the pressure drop on the upper side and pressure rise on the lower side were added up overall that lo and behold there was enough lift to hold the plane up. And thus we all learned that this pressure difference is what holds the plane in the air. Some rather bright science whizes proved this without any room for error.

Later some bright wig showed that the air approaching the wing is split so as to both accellerate up and over the wing as well as downwards on the lower side. When looked at the final downward accelleration of the mass of air affected by the wing was able to show that lift is a Newtonian effect with the energy imparted to pushing the air mass down being equal to the energy needed to maintain flight. And thus we learned that lift is generated by a newtonian reaction due to accellerating a mass of air downward to generate an equal and opposite reaction that holds the plane up.

Then somewhere along the way we found a guy that liked going around in circles that showed that the airflow ahead of, around and behind the wing could be described using a circular airflow. It's a stretch that makes no sense to me but for those able to understand the math I gather it makes sense.

So now we have what appears to be a fourth way to explain the wing's lift. And much like the other three this one appears to be saying that the others are wrong.

Each explanation came from highly respected research. And each suggests that their method is the only valid one. But it's all the same lift that keeps adding up to "1" despite how you add them up. And there IS only ONE lift that seems to have multiple ways of showing itself. Seems like Mother Nature enjoys a practical joke.

I know about the Magnus effect on a rotating cylinder but I'm not sure how you can apply that to a fixed wing. I guess I'd have to read the article or they are calling it that due to the manner in which the air flows around the wing or some such thing. I also question how they can claim that the pressure differential doesn't add up to the lift needed when early NACA experiments proved that it does.

Lnewqban

Very true![sm=thumbs_up.gif][sm=thumbs_up.gif][sm=thumbs_up.gif]

The many explanations to lift remind me of the "Blind men and the elephant" story......

http://en.wikipedia.org/wiki/Blind_men_and_an_elephant

Shoe

BMatthews,

I don't think the essential aspects of the different explanations are mutually exclusive. The suggestion that the sum of the surface pressures (to be more precise, the surface "tractions"... including shear/tangential as well normal forces) isn't equal to the net aerodynamic force on a body is difficult to support. The statement of equality between the surface tractions and the net aerodynamic force is really more of an "identity" than explanation. The interesting question is how/why the motion of the air around a body results in the forces on its surface. Explaining lift in terms of "circulation" doesn't exclude the possibility that lift results from momentum transfer, and vice versa. And certainly neither of those explanations contradicts equality between the sum of the surface forces and the net aerodynamic force.

The underlying physics of fluid flow are pretty well understood. The complication in solving airplane-scale problems has more to do with shortcomings in book-keeping than in understanding the forces at work. I think a reasonable analogy would be that it is possible to have a good understanding of how humans interact with each other financially, but not be able to reliably predict the behavior of a large scale economy. There are so many moving pieces and there is so much interaction that keeping track of what's going on is the real challenge

Rick.

hee hee yeah, its ALL true! The only "theory" i chuck out is the old school text book explanation of lift via Bernie's Nelly principle, you know, the air has to travel faster over the top to meet the air from the bottom at the trailing edge and the lower pressure in the faster moving air provides lift - bunkum as far as i can see! I think it been proved that it doesn't go faster over the top anyway, also it failes to explain flat plate wings and symmetrical airfoils. I did used to think it was to do with the air having to lose pressure/ get thinner as it expanded to fill the leeward side of max thickness - but then a theorist told me that thecentre of low pressure occurred above the max thickness zone, so that scuppered my simple view!
Having said that I understand that the CP moves back as airspeed increases. So i don't know what to believe now........... as long as it flies, eh!!!!!

Paper Cub

SEE ALSO http://www.av8n.com/how/

Doc.316

Wow...you guys could get into a "heated discussion" on this one.....
I am going to stay out of it...but:
1. One theory is pressure...classical.
2. Then there is the source/sink deal (what I learned when going through engr school)....ouch that class was painful...ya know with the rotation and such...
3. Then there is the "what go's up must come down" theory...lol....the amount of air displaced downward equals the lift upward....
4. Then there is the combination....

The bottom line is that the wing does lift...and it follows some equations that model that lift....you throw in some fudge factors here and there derived from test data...and you can calculate the performance of the "thing"...and that is what everyone wants to know...

Steve

Bozarth

My Feedback: (15)

I thought it was how fast the wing is going through the air, not how fast the air is going over the wing?

Grasshopper

da Rock

Of course it is, but we so often talk about it either way because wind tunnels work that way and provide pictures of airflow.

Bozarth

My Feedback: (15)

I love asking rhetoricals, and da Rock loves to answer them.

But seriously: I think one might better understand Bernoulli if they keep in mind that the wing is moving through the air, especially when trying to understand the top and bottom speed differential concept.

Kurt

Lnewqban

Agree.

Stretching the concept a little more:

We should understand that lift is not some magic thing that keeps airplanes flying and gliding, the force of lift is proportional to the work that the wing performs on the mass of air.

In other words, in order to be lifted by the air with a force enough to sustain level flight, the combination engine-wing must give an equivalent amount of energy away to that air.

The airplane acomplishes that by forcing its way through the air overcoming the draft forces (due to friction and lift).

eddieC

My Feedback: (2)

It's a combination of the forces - Bernoulli principle, Newton (angle of attack), etc.

As far as Bernoulli goes, it's been proven (at UND and others) that there is no motive force that 'makes the air molecules meet' at the trailing edge. They don't meet back up, so pressure differential alone can't acount for it all. Likewise, angle of attack can't do it all either. No one theory explains it all, but a combination does.

Lots of things contribute or detract from lift. Speed, angle of attack, airfoil shape, aspect ratio, laminar flow, slats, flaps, spoilers, flow devices like vortex generators and fences, etc. Change just one, and you've made a difference. Change two or more, and it gets complicated fast.

I've been teaching full-scale students for a number of years, and I like to keep things simple. They're trying to learn enough not to crash, not pass an engineering exam.

Himat

With aerodynamics as a physics subject I only spend my leisure time on I would put it this way:

Air have some basic properties, when it comes to model aeroplane lift, mass, densinity and viscosity is the important ones.
With the wing creating lift some observations can be done and math formulas formulated to explain these observations.

Lift is proportional with angle of attack until the wing stall. The AoA-Lift slope is straight a certain AoA range. Makes lift=AoA

If pressure is measured under and above the wing a pressure difference is measured. Multiplying wing area with average pressure difference the total lift is found.

Makes lift=area*pressure difference

Using the Bernoulli theorem, the speed differences between air passing under and over the wing can be found (calculated). Or if the speed is known, the pressure and hence lift found. Fits nice with the above expression, lift=area*pressure difference

Observing a wing in a wind tunnel with smoke markers, the air passing over the wing is observed to "speed up". In real life the air is stationary and the wing passes by. Thinking about it the air "rotates" around the wing, and there you have the idea of the rotation math. The math is then an abstraction used to explain lift.

As I see it, none of the above is mutually exclusive. It's different ways to explain the observations.

Red B.

Newton and Bernoulli do not contradict each other. Newton's laws and on Bernoulli's theorem are completely compatible. For the most part they're just two different ways of simplifying a single complicated subject.

Bernoullis theorem does not try to explain lift, instead it simply states that in the absence of friction and compressibility, the total energy of the fluid is a conserved quantity, nothing more, nothing less.

AndyW

My Feedback: (1)

The lift provided by a "plane" isn't complicated in the least. Picture a plane, (Wiki definition, "abstract surface which has infinite width and length, zero thickness, and zero curvature") and of course, for this discussion, our "plane' does have a defined length and width, and for now, it is also of zero thickness.

So now, picture our plane traveling through deep space. Space is actually not a perfect vacuum, there ARE random molecules floating around. Not many, but they're there. So our plane travels through space with the surface perpendicular to the direction of travel. Eventually, the plane encounters a molecule of air.

So what happens when that molecule hits the plane? Depends on where it hits. If the molecule hits the dead center of the area of the plane, Newton's law of action/reaction dictates that the molecule will resist being displaced and a small force will be imparted on the plane, slowing it down. The molecule itself will be deflected straight backwards, in effect, will bounce off the plane and now travel backwards, reversing its direction. This can be defined as drag.

Let us now tilt the plane 90 degrees so that it travels edge on. With zero thickness, it'll encounter no air molecules, ever, and nothing will happen to the plane. Now lets tilt the plane exactly 45 degrees and now what happens to our theoretical molecule that hits the plane dead center? It's now deflected NOT backwards against its original trajectory, but will now be deflected 90 degrees. Newton says that the molecule will resist this change of direction and impart a force on to the plane, a force that is exactly 45 degrees to its direction of travel. Let's throw in a handful of molecules and for the sake of simplicity, assume they all hit the plane at the same time.

This is lift. Not in space of course, but on earth, where the force imparted is intended to counter the force of gravity.

One earth, with a virtual sea of molecules, our plane traveling at 45 degrees, will exhibit a great deal of lift along with a great deal of drag. Too much lift along with too much drag. So we reduce the tilt so that we find a sweet spot where we maximize lift and minimize drag. That sweet spot is the angle of incidence. What maintains that angle is a thing called a fuselage and another, smaller plane (stabilizer)that functions as a lever to keep the main plane oriented correctly.

This is a simple plane traveling through the air, deflecting that air downwards, with the impact of billions of molecules imparting a force ala Newton to counteract the force of gravity.

In the real world, no plane can have zero thickness so we have that and to minimize drag, we shape the thickness part of our plane into something we call an airfoil.

Simply put, a plane provides lift by having molecules of air bouncing off the bottom to counteract the force of gravity. We're simply "scooping" the air, bouncing it off the plane in the correct direction.

All these air molecules that are being deflected will bounce around, bumping into one another and we will discover EFFECTS. Some of these effects are incorrectly defined as forces such as downwash. Some of these are correctly defined, such as the Coanda effect or laminar flow. They're all just effects. Newton rules the day here, not Bernoulli although he has a minor roll to play depending on the shape of the airfoil used.

If the airfoil we used was rectangular, you can see that a percentage of molecules would be deflected straight back by the flat leading edge and induce significant drag. We want to minimize the drag so we round off the leading edge and narrow the trailing edge to a fine point because birds do it that way and they learned that the hard way through evolution so we copy what works. We also modify what works depending on the characteristics we want our plane to have. Red Bull racers will not use the same airfoil as a DC3 for obvious reasons.

In its purest form, (best) lift is provided by a plane, tilted just right, traveling at just the right speed through a sea of molecules. Everything else is just details.

I had to chuckle. At the Ottawa air museum, they had a display with a Clark Y type airfoil plane, inside a clear, plastic box with air passing through. They used Bernoulli to explain lift, yet they had you tilt the plane to illustrate the lifting force developed.

When I instruct, youngsters in particular may ask how a plane flies. I just tell them that the wing scoops the air and it's amazing how well that satisfies their question. Some will want more details and mention Bernoulli from their high school physics books. That's when they find out that just because it's in a book or written by someone, that doesn't guarantee that it's right or the truth. Just someone's opinion and that all this is,, JMO.

Lnewqban

So, Andy, according to your concept of lift, what makes a stalled wing lift much less that the same wing non-stalled flying at a lower AOA?

The first one still scoops more air than the second one; how it comes that it flies less?

AndyW

My Feedback: (1)

It may scoop more air but it also has much, much more drag. The sweet spot, will be the best lift to drag ratio. I didn't put it that way exactly, but that's the idea.

Take your infinitely thin plane and fly it edge on to the air with no angle of attack. No lift, no drag. Increase the angle of attack one degree. Some lift, some drag. Increase that and you'll get increasing lift and increasing drag. Increase that to the point where your lift starts to diminish but your drag continues to increase. Back off a hair and that's your sweet spot.

In the real, physical world, there's no such thing as an infinitely thin wing, we need structure. That can be a simple, rectangular airfoil. This can do the job, we see that in indoor flyers. We even see that in the Delta Dart. But that kind of airfoil produces a lot of drag with its blunt leading edge. So we round off that edge and taper the airfoil to a sharp point at the trailing edge. Birds showed us the way.

Airfoils of various crossections give us varying characteristics according to the job we need done. Hauling the freight requires a different airfoil and force arrangement from a Red Bull racer or Yak 54. But all aircraft need to have their plane (wing) scoop the air to fly, simple as that. It's all Newtonian though Bernoulli plays a minor roll in some cases. Inertia causes air to resist being displaced. That measure of resistance of billions of molecules wanting to stay put is what provides lift. All other elements are just the effects of those billions of molecules colliding with the surface of the wing and colliding with each other. Laminar flow, Coanda effect, downwash are all just effects. Roughly speaking.

Again, when I tell the kids, "scoops the air" you can see the light go on in their eyes and they're satisfied.