How does split flap increase lift?

[AvEnthusiast]

Please don't just answer by saying "it changes angle of attack". if you say that then you may need to explain me the AOA. Ok we say that AOA is the angle between the relative wind and chord line of the wing and that's the concept we have learned mechanically.For me the AOA an imaginary angle doesn't seem to be affected by changes under the wing at trailing edge. because once the streamline hits the airfoil then the air flows on upper and lower area just from stagnation point. And this split flap neither increase the wing area nor changes the curveture to delay the separation of airflow. Then how this split flap can increase lift apart from increasing drang while brings no changes to surface area or curveture of the wing?

[Pilot DAR]

You have hit upon an observation asked by many, and answered by few!

Three aircraft I have flown were equipped with split flaps (Cessna 310/340, DC-3). Each of these aircraft specified the use of the first flap setting for takeoff, when I presume additional low speed lift, but not drag, is desired. My (non-engineer) observation is that there was a small increase in lift with these flap settings, which was probably accompanied by undesired drag. This lift was probably mostly the result of making the best of the ground effect cushion, and moving the center of pressure aft a little so extreme nose high attitudes were not required to rotate the aircraft.

Split flaps are mechanically the easiest of all flap systems, and are very good for creating drag, which the older, simpler aircraft needed. Like many things in aircraft developement, a better way (plain or fowler flaps) was found. Since then, you really have not seen new aircraft designs come out with split flaps in the recent decades.

Pilot DAR

[Wizofoz]

The chord line is the AVERAGE of the camber of the wing. By deploying a split flap, you are changing the angle between the Fuselage and the chordline by changing the camber of the wing.

For a given attitude, this WILL increase A of A.

Split-flap DOES change the curvature (camber) of the wing- Look at a picture of an aerofoil with and without split flap deployed- when they are out, the wing has (on average0 greater curve.

[ReverseFlight]

Wizofoz, maybe I had too much to drink at dinner tonight ... from my limited knowledge of aerodynamics, I thought:

the "chord" is the straight line between the leading and trailing edges of an aerofoil, while the "camber line" is the average of the cambers of the top and bottom surfaces of the aerofoil

the angle between the longitudinal axis of the fuselage and the chord is the "angle of incidence" and is distinguished from the "angle of attack" which is the angle between the chord and the relative airflow meeting the aerofoil

:confused:

[mavisbacon]

Lift = 1/2. Cl. Rho.Vsq.S it's the Cl bit that changes (I fink)

[Lodown]

Despite dated textbooks mentioning that lift is developed predominately from a low pressure region on top of the wing; it's not. The reality is that an aircraft's lift is developed as an opposite reaction to the wing moving air down in accordance with Newton's 3rd Law. The low pressure is part of the process of getting air to move down. (Air is forced away from a region and creates a vacuum in its place.) Look at it this way and it might be a little easier to understand. Deploying split flaps at a given speed results in more air being pushed down, therefore more lift (and drag), resulting in the pilot either lowering the nose or reducing speed, or both, to maintain the flight path.

I'm sure someone will disagree and someone else will elaborate.

[Wizofoz]

Reverse,

A quick Google shows that you are correct wrt the definition of chordline. It then makes split-flaps a little complex in that, when deployed, the wing now effectivley has two trailing edges, and therefore two chord-lines!

As too your second point, that is basically what I said. An increase in incidence (which is basically what deploying a flap does) leads to an increase in A of A For a given attitude.

mavis- You fink correctley- we're just working out WHY cl changes.

Lowdown- to dicuss whether the pressure differential between upper and lower surface or the reaction to down-wash is the major contributor to lift is a bit like chicken V egg- one automatically leads to the other. I don't dis-agree, but hope I have elaborated!!

[Lodown]

Wiz, you may have commented while I was in the process of editing one of my numerous rewrites so that my comment made sense. I just remember how it was taught to me and I could never understand how air simply speeding up over the top surface of the wing created enough low pressure to lift an aircraft. Many of the diagrams at the time showed air in parallel paths moving aside to make way for a wing and then getting restored to order once the wing had passed. It wasn't until much, much later when I was "educated" that the air is moved downwards. It makes it a lot easier to understand many things about how the wing works.

[toolowtoofast]

Lift = 1/2. Cl. Rho.Vsq.S it's the Cl bit that changes (I fink)

try L=Cl (1/2rhoVsq)S

lodown is just abusing bernoulli :) i have video proof of bernoulli in action over a wing. and i don't think it should be explained as the 'low pressure lifting the wing', (at a very basic level) more that the higher px is trying to move to the low px area, but the wing is in the way - that conveniently explains spanwise flow nicely too

here's what i reckon:

the average chord will change with split flap, but angle of attack should remain the same, just at a lower IAS, for the same nose attitude as the higher IAS. no different than extending 'simple' flap really.

[djpil]

As Lodown stated, the split flap turns the air so just like everything else, it develops lift by pushing the air down (http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html)- Newton's law.

[Dave Incognito]

toolowtoofast,

while the 1/2pVsq is often grouped together to represent dynamic pressure, wouldn't you agree that the maths works out exactly the same?

[flog]

On the Yak the spilt flap turns me from dropping like a stone on final to dropping like a pointy, streamlined, even faster, stone - all be it a little slower over the ground.

I'm not 100% sure they had lift in mind when they were designed for the Yak...

[Hempy]

more that the higher px is trying to move to the low px area, but the wing is in the way

:ok::ok:

^^ best way to get your head around it imo.

[Reluctant737]

The simple answer is -

It barely affects the pressure distribution on the upper surface (if at all), but creates a higher pressure acting under the wing, and the CP moves aft.

Cheers, Ad

[Peter Fanelli]

Three aircraft I have flown were equipped with split flaps (Cessna 310/340, DC-3). Each of these aircraft specified the use of the first flap setting for takeoff,


I'd like to disagree with that for the 310, I don't think there's anything in the book about using flaps for take off .

[Cloud Basher]

We could go into circulation theory as well if you like but we won't complicate things. Simply put Lowdown is spot on. It is Newton's third law in action. For every action there is an equal and opposite reaction.

Bernoulli's principal is simply a way of quantifying the amount of air displaced by the wing according to Newtons Third law. I.e. for a 2000kg aircraft to fly straight and level 2000kg's of air must be displaced at any one time for the aircraft to remain flying level. Therefore they are both correct.

However according to CASA all you need to know is Bernoulli’s theorem so don't go getting confused by all the rest of the (correct) theories. I believe CASA exams still state that two particles of air that split at the front of the wing meet again at the trailing edge? (May not be anymore, but it was when I did my theory). What a load of hogwash! But again don't worry about it, as the theory works regardless of the fact it is wrong.

As stated in other posts a Split flap increases the amount of air displaced and thus increases lift (does stuff all to the airflow over the top of the wing). It does also increase drag so you need to overcome this to maintain speed (more power) or you can reduce speed and get the 2000kgs of lift and thus stay straight and level. Think about it this way, all an increase in AoA is doing is presenting more of the bottom of a wing to the airflow, therefore it displaces more air down thus creating more lift. You can also increase lift by increasing speed iaw L=0.5(rho)V^2 S Cl . Increasing speed again works with Newton as during whatever period of time, the amount of are displaced increases with speed. Again Bernoulli's equation simply quantifies this.

There are other factors that we can go into but you don't need to worry about them unless you are enjoying the fun of studying for an Aeronautical Engineering degree.

So there you have it, a rather verbose explanation, but just remember that whatever theory you adopt they are in fact all correct (except for the two particle theory - an abuse of Bernoulli's work and Bernoulli never ever said that). However they do all simply come back to Newtons Third Law

QED

Any questions please fire away. I love this Sh*t!!! (yes I am a sadistic Aero Eng...)

Cheers
CB

Edited to add, The top shape of the wing is largely irrelevant. Essentially all the top of an aerofoil is good for is reducing the drag. A flat plate will produce lift (but a massive amount of drag), thus to ensure that our C172 flies with 150HP we have an aerofil shape that controls the airflow across the top of the wing reducing drag so we don't have to fit a merlin to a 172 (although that would make it sound better!). That should put the cat amongst the purist's

[Andy_RR]

Bernoulli's principal is simply a way of quantifying the amount of air displaced by the wing according to Newtons Third law. I.e. for a 2000kg aircraft to fly straight and level 2000kg's of air must be displaced at any one time for the aircraft to remain flying level. Therefore they are both correct...

...yes I am a sadistic Aero Eng...)


anyone who would accepts this drivel doesn't deserve to pass any exams and qualify as an engineer.

[Pilot DAR]

I'd like to disagree with that for the 310, I don't think there's anything in the book about using flaps for take off

Though I have not flown the 310 in 25 years, and no longer have the flight manual, I do recall flaps 15 for a short or soft field takeoff was a normal procedure in the flight manual. A review of information I can find through Google seems to confirm that. I agree that flaps zero is the procedure for normal takeoffs, but I did many at flaps 15 out of shorter paved and grass runways, and would not have done them if the flight manual had not so instructed.

It did seem to shorten the ground roll. I always did wonder why, and just credited pushing a bubble of air ahead of the flaps, and thus getting good ground effect.

Pilot DAR

[OZBUSDRIVER]

Split flaps, interesting! More drag than any other attachment. Was described once to me that the wing has good enough aerodynamic qualities for the entire envelope so the need for only split flaps. 402 and 402B have split flaps as well. 404 went to fowlers. This goes the same for me as trying to explain how those barn door Kruger Flaps actually increase lift???? swinging forward from under the leading edge??? dropping vertically down??? Wouldn't that kill any smooth flow? Does the turbulent air behind the flap, both Split and Kruger, form a boundary that acts like an airfoil shape?

[Joker 10]

pilot DAR 100% right flaps 15 only for short field T/O normal no flap

[Flyer517]

I'm not going to disagree with anyone here because frankly I don't know the correct answer, but......

I have a video somewhere what shows a wing in a wind tunnel with smoke being puffed over it. The marker in the smoke that passes over the wing reaches the trailing edge before the smoke that passes under the wing. At least that is how I remember it. That would seem to indicate the old theory about air passing over the top of the wing needing to travel faster and hence creating a low pressure area, etc, etc.

I also seem ro recall seeing a video on Youtube that showed a flat plate in a stream of water from a tap being sucked in to the stream. This again seems to indicate that the high / low pressure theory have some legitimacy.

Edit - here it is YouTube - Lift from water over a spoon (http://www.youtube.com/watch?v=KNGyNeGM4KE)

I may be off the reservation here but some of the comments so far seem to indicate that this pressure differential theory is bogus.

Go ahead - shoot me down!!!! :)

[currawong]

I'm with Andy RR.

The aerofoils most of us here use do not work as a result of the "inclined plane" theory.

The aerofoils we use work in the same manner as a venturi. Albeit one sided.

To say that the upper surface of a wing is shaped as it is to reduce drag is incorrect.

Try flying with this camber disrupted - you will see how critical it is.

[Pilot DAR]

This goes the same for me as trying to explain how those barn door Kruger Flaps actually increase lift???? swinging forward from under the leading edge??? dropping vertically down??? Wouldn't that kill any smooth flow?

Yes, you'd think. But years ago, I stumbled upon a simple variation on this theme in a Cessna 150. If you're (I think the UK term was "bimbling") along at 80 MPH or so, and you force open one of the cabin doors a quarter the way or so - let's say the left one, so you're not scaring your passenger - the plane will gently turn right. Why? I wondered, with more drag on the left side, it should turn left...

It seems that forcing the door open, forces more air to go over the wing in that local area, and more air going over, means more lift, so the left wing lifts more, so the plane turns right. Like a Kruger flap, I'm sure this has it's practical limitations, and probably I reached them doing what I was doing, but it proved a lesson to me that increased lift does not always come from a streamined something altering the airflow.

Pilot DAR

[AerocatS2A]

To say that the upper surface of a wing is shaped as it is to reduce drag is incorrect.

Try flying with this camber disrupted - you will see how critical it is.
All you've proved is that drag is critical.

To those who think the Newtonian explanation of lift is wrong, I challenge you to design a working wing that does NOT accelerate a mass of air down in accordance with Newtonian predictions.

To those who think that the Bernoulli explanation is wrong, I challenge you to create a wing that does NOT accelerate a mass of air over the top of the wing and that the subsequent change in air pressure over the top of the wing does NOT fit with Bernoulli's equations.

They're both simplified explanations that focus on different parts of the same process. And they're both equally right (except the part about the air particles having to meet up at the trailing edge, that's just wrong.)

This is from the NASA website (http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html) (my emphasis.)

When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object. Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.

So what you guys are doing is arguing over nothing really. One of you is saying the apple is green and the other is saying, no it's not, it's round!

[Cloud Basher]

Well please explain to me then Andy_RR how the drivel should be expressed?

CB

Edited to add, I do love it when the Bernoulli lovers say Newton cannot explain lift. EVERYTHING in motion is explained by Newtons three laws. Everything can be derived from what these three laws describe. To say otherwise is denying the laws which govern motion and energy in our universe.

Perhaps I was simplifying a little too much to say lift only comes from the underside of the wing and the camber of the top surface has nothing to do with it. Indeed it does, but again the camber or shape of the top surface acts to accelerate the airflow DOWNWARDS!!!!!!! These issues always arise when you try and dumb down an explanation so that those who are not engineer amongst us and who ask a seemingly simple question can at least partially explain it. Have a look in the wind tunnel videos and you will see this. The opposite reaction is for a force (known as Lift) to be imparted on the wing and thus an aircraft flies. I would dearly love to know how Newton is incorrect! If he is then there are a few things which will be difficult to explain, but like any good scientist and engineer I do have an open mind. And Andy_RR I guess NASA is incorrect as well. As far as I can tell, what I said (in very very simplified terms), is the exact same way that NASA explains how lift is produced?

Anyway I do love a good debate. Chicken or egg next?

[jimshutt]

Don`t you think that we are all wrong, the air does not flow over the wing, it remains stationary relatively speaking and the airfoil just parts it.
The airflow is deflected downwards creating a place for newtons 3rd rule by the down wash and at the same time producing greater profile drag.

[Cloud Basher]

Flyer 517,
Your yourtube video link actually proves what I am trying to say. The water is accelerated over the "aerofoil" (spoon) and is deflected downwards (sideways) you can actually see this towards the end of the video. Thus the equal reaction is for the spoon to move in the opposite direction. As I said earlier, bernoulli's equation puts a numerical value on what you are seeing. There is other forces at play but we will ignore them for the sake of simplicity.

Suffice to say Nature abhors a vacuum and so will try to equalise the lower pressure by pushing (or sucking) the wing up. This comes from the water accelerating downwards and thus Newton states the spoon must move into the water flow in order to react to the water going downwards. So the high/low pressure is spot on. You can even mesure the poressure differential very very accurately. Bernoulli's equation is derived from this. BUT the high.low pressure comes from the fact that the water is being deflected downwards, so the opposite reaction is necessary to equalise the pressure. It is all a big circle and as the NASA quote (and as I believe I stated earlier) they are both correct!

Cheers
CB

[HardCorePawn]

I remember reading another 'discussion' about Bernoulli vs. Newton... and someone pulled out this gem:

One is a Theory... the other is a Law :E

Having started my flying in the world where it was all about High and Low pressure differentials... and then came back about 7 years later where it was all about displacing air (which I might add is much like how boats, made of heavy objects like steel, float by displacing water)... it was a bit difficult to wrap my head around it... :ugh:

I guess it is very much like when someone decided the earth was round... took people a while to let go of what they knew to be the "The Truth''(tm)...


Can I have a fiver on Newton? :}

[AerocatS2A]

Gravity is a theory as well ;).

Don`t you think that we are all wrong, the air does not flow over the wing, it remains stationary relatively speaking and the airfoil just parts it.
The airflow is deflected downwards creating a place for newtons 3rd rule by the down wash and at the same time producing greater profile drag.
There is no difference between a stationary wing with air flowing around it and a wing moving in stationary air. That's why wind tunnels work. Air flow around a wing causes a velocity change (Newton) and a pressure change (Bernoulli.) They are just measuring different things.

To Cloudbasher. You're statement that the upper surface of the wing doesn't matter except for reducing drag is wrong. The shape of the upper surface of the wing has MORE to do with turning the mass of air then the bottom does.

Edit: Ignore that last bit Cloudbasher, I see you've clarified your position.

[Andy_RR]

Cloud Basher, I think you'll find that the downwash/displacement theory doesn't fully explain all of the lift a wing generates - it is too simplistic. For example, how does this quasi-Newtonian theory explain a positive lift coefficient at zero AoA? Or, a zero CL at a negative AoA? Both of these cases are possible with cambered aerofoils

Getting back to the split flap question, I think deploying a split flap increases both AoA and camber if you consider the cord line to be between the leading edge the average of the (now) two trailing edges. In practice, the flow will be more complex than this, but close enough for descriptive purposes.

[Gargleblaster]

Seemingly, some are saying that it's not the faster airflow over the upper side of the wing (Bernoulli) that's creating the lift, but rather the lower surface pushing the air down.

If so, wings wouldn't need to be shaped as they are, they could simply be constructed as a flat plate !

Non believers of Bernoulli, do this: Go outside into a 15 - 20kt wind with an umbrella. You'd think it'd be pushed down, but feel how much lift it generates !

[Walrus 7]

Here's my one cent worth (adjusted for the global economic crisis).

I find it interesting that people are mentioning the water-over-the-spoon demonstration, but no-one has yet mentioned the name Coanda. The spoon demo has nothing to do with Bernoulli, it demonstrates the theories of Henri Coanda. Remember, Bernoulli was not an aerodynamicist and never claimed his theories had anything to do with flight; other people decided that they did. Coanda stated that a stream of fluid will cling to the contour of any shaped surface over which it runs (the water & spoon bit), thereby deflecting the fluid downward. This is where Newton comes into it. The reaction to the deflection is what we call lift. If you do the demo at home (don't waste the water, though) you will feel the spoon being drawn into the water stream if you hold it gently between two fingers.

Bernoulli's theories are, I believe, negligible, because he formulated them in a closed system (between two curved surfaces). You cannot get a more open system than the rest of the sky. I'm not saying that there are not differentials in pressure over the upper and lower surfaces, but just that they aren't powerful enough to generate the lift needed to fly.

And there you have what I reckon!

Walrus

[AerocatS2A]

Bernoulli's theories are, I believe, negligible, because he formulated them in a closed system (between two curved surfaces). You cannot get a more open system than the rest of the sky. I'm not saying that there are not differentials in pressure over the upper and lower surfaces, but just that they aren't powerful enough to generate the lift needed to fly.

And there you have what I reckon!
Well, you reckon wrong then. Read the NASA link I provided further up on this page. NASA have directly addressed the different popular theories of lift in that link. Bernoulli's theories are NOT negligible, they will account fully for the lift on a wing but they don't tell the whole story. Bernoulli wasn't just talking about a closed system either. Bernoulli's theory describes the change in pressure in a fluid as velocity changes. The velocity changes around a wing are sufficient to produce the pressure changes that account for lift. What the popular equal transit time theory gets wrong is that the transit times are not equal, the acceleration over the top of the wing is greater than that proposed by the "equal transit time" theory. But that's fine, Bernoulli is not the equal transit time part of the theory, Bernoulli is predicting the pressure changes due to velocity changes and it is accurate.

Once again, read the link (or at the very least read the section I've quoted.) If you think it's wrong, then I'd like you to explain how it is that you are right and the collective aeronautical minds at NASA are wrong.

Bernoulli and Newton (http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html)

There's the link again so you don't have to search for it in this thread.

[The Green Goblin]

There is a particular formula which explains anything to do with these particular issues. I use it quite often.

[insert statement] = P.F.M :ok:

(or Pure Farken Magic)

[Walrus 7]

Definitely hard to go past NASA, but the "PFM theory" is starting to emerge as a real possibility. If we consider that the Dept. of Physics at Frankfurt Uni in Germany has some credibility, then we find this statement.

Conclusion

The deliberation of Bernoulli's law in schools and textbooks has serious drawbacks. Unfortunately many applications are erranous and misleading. One source of confusion is the derivation of Bernoulli's law based on the theorem of energy conservation. Bernoulli's law should be derived from the tangential acceleration as a consequence of declining pressure. Another source of difficulties is the fact that many physics textbooks do not mention normal acceleration of flow and the resulting pressure gradients perpendicular to the flow.

Both, Bernoulli's law and the generation of pressure gradients perpendicular to the flow are consequences of Newton’s laws. None of them contradicts those.

Bernoulli's law is insufficient to explain the generation of low pressure. A faster streaming velocity never produces or causes lower pressure. The physical cause of low or high pressure is the forced normal acceleration of streaming air caused by obstacles or curved planes in combination with the Coanda-effect. Pressure gradients generated by the deflection of streaming air can be clearly demonstrated by simple experiments which would substantially improve the discussion of fluid mechanics in schools and textbooks.

You can locate the paper here: Misinterpretations of Bernoulli's Equation (http://user.uni-frankfurt.de/~weltner/Mis6/mis6.html).

I love this argument! Lets have a look for what else we can find.

Walrus

[jimshutt]

Oh yes Andy, but what is the definition of CL and what about the flat plate and symetrical aerofoil flight?

[jimshutt]

The only similarity between a wing moving through the air and a stationary wing with air moving over it is that they both create downwash.

[AvEnthusiast]

Dear posters,
I think this discussion is now getting more to "lift" issue than the "split flap". Let's leave the lift issue whatever it is, because we all know lift by one of these means, and my concern was only with "split flap" and if you still insist that it's the issue; then; those who are claiming that the lift is byproduct of differential pressure tell me how doest split flap can create extra differential of pressure on upper and lower surface of the wing when deflected? and those who are claiming that the lift is byproduct of downwash and split flap increase downwash, tell me how can an aircraft then fly better in "ground effect" where downwas is disrupted by ground interference and the downwas is smoothened or reduced?

[currawong]

Downwash is proportional to the weight of the aircraft.

Not equal to it.

Therefore Newtons Third is not satisfied by downwash alone.

[currawong]

To answer your question AvEnthusiast,

the split flap changes the mean chord of the wing (somewhat inefficiently) as some have already pointed out.

Downwash/wing tip vorticies are disrupted in ground effect, leading to a reduction in induced drag.

Try it - a 100 kt aircraft seems to gain 5 - 10 kts with a smaller angle of attack in ground effect.

[Wally Mk2]

One has to love this sort of conversation:). At the end of the day does it really matter how lift is generated? I didn't invent it, you lot out there didn't invent it. Why get all overheated about something that just 'happens'. Bothers me none how as long as it keeps doing it:)

Flaps, LE devices anything that hangs off a wing is designed for one thing, to improve the lift, increase drag etc of a wing at lower fwd speeds to assist in T/Off's & Ldg's. But just to add more to it here 'cause I'm as human as the rest of us ( I think:E) here split flaps are a basic design, need little in the way of mechanisms to operate therefore less weight & they crudely do the job. There also known as the simple flap, I wonder why!
Hands up ALL the pilots in here who answered what needed to be answered to pass the CASA exams about lift etc whether they knew how it really worked or not & then do a memory dump? I'd say we all did bar a few 'geeks':}

[superdimona]

Downwash is proportional to the weight of the aircraft.

Not equal to it.

Therefore Newtons Third is not satisfied by downwash alone.
I'm not sure I follow you. My understanding is, in level flight, the aircraft is being subjected to 1G = 9.81 M/S/S of acceleration. For a 1000kg plane, thats F = MA = a force of 9810N.

Since the aircraft is NOT accelerating downwards, the lift force must be equal but in an opposite direction. Therefore some force of 9810 Newtons must be generated to counter gravity.

The downwash theory says that as the wing moves through air, it pushes air downwards. Since the air was stationary, and is now moving, there was some acceleration. Since air has mass, then good old F=MA applies. This is the force holding the wing up, countering gravity.

Our F must be 9810 newtons - BUT we can get this either by moving MORE air, or accelerating less air, faster!

So, the weight of the air moved doesn't have to equal the weight of the aircraft.

[Cloud Basher]

I love it! I have to say, it doesn't concern me at all as to what causes lift at the level we are talking about here. The only reason I jumped in was because there were some blatant untruths being spread and I dislike people getting incorrect info, especially when it comes to a subject that I know a little of:E.

Anyway perhaps people should go back and reread the link aerocat provided.

After that reread all my posts, people are going around the tree and to now answer and address a couple of other points that have been brought up.

Andy RR, to answer your question, yes the upper surface of a wing does indeed add to the downwash of the airflow, the air is accelerated downwards (due to it "sticking" to the wind due to the coanda effect. However breaking the coanda effect down, it also come right on back to Newtons three laws. If we treat the atmosphere is a closed system (and it is) energy is never destroyed, it is only changed from one state to another. (we can actually decrease the "system to that around the vicinity of the wing or aircraft if you like, it makes no different to our discussion here) What happens is in the closed system around an aircraft travelling through the air as air accelerates pressure has to reduce so that the total energy of the system is constant. Therefore the air above the air with the lower pressure actually forces the air flowing over the wing, down and onto the wing, thus it follows its surface. Thus it is deflected down aft of the maximum camber by the pressure of the air around it which is now greater than the air passing over the wing. Everything that has mass has momentum, thus this accelerated air is now deflected downwards and the thing it is "sticking to" (the wing) has to rise to equalkise this pressure. nature abhors a vacuum. What does all this crap explanation mean? Well it means that for every action there is an equal and opposite reaction. Newtons third Law!

Now to actually answer your question, Angle of attack is an arbitrary figure. A wing with a positive camber will produce lift at zero AoA by definition. However we have arbitrarily defined AoA as the angle of the relative airflow to the chord line It is just a measurement so we have a baseline, it really means nothing. it is just a simplified way to compare wings with one another and allow pilots to argue over why an aircraft stalls (that will get some people going!!) For a wing with camber the pressure above the wing is less than the pressure below because the air is accelerated and even at zero angle of attack, because of the camber of the wing and the coanda effect, the air is still accelerated downwards and so the mass of air displaced is the same as the weight of the aircraft. I am sorry for the explanation, I am trying to keep it as simple as possible. Everything I have said can be proven with mathematics. Note that the pressure differentials we are talking about here can be very very small. For a 747 in cruise the pressure differential is less than 0.1 Psi between the top and bottom of the wing. So you don't need to accelerate the air much to cause this pressure differential. But remember the pressure differential is simply a function of an action being applied to the air and an equal reaction occuring.

You are correct in that some explanations are too simple. The downwash description is fully correct however we have simply come up with other equations credited to other people who have derived equations that explain the results of Newtons third law in various situations. Take Boyle's law and Charles law as two examples, they both say exactly the same thing, but from different perspectives and we use their equations in two different situations, but it still comes back to pressure and more importantly that the total ENERGY in a system is constant. This is the fundamental tenet of our existence. You can't increase or decrease energy only convert it from one form to another. Sorry for the digression.

Well if you can follow that you are doing well!

Currawong, absolutely postively correct. It is proportional to downwash, but you are completely wrong that is it not explained by Newton. The force of the mass of the air displaced (weight) exactly equals the weight of the aircraft at any one time. This is exactly what Bernoulli is saying and measuring. L = W = 1/2(rho)V^2SCl. Another way, 1/2 x density x V^2 is the kinetic energy of the air, (there is that energy again), S takes into account how much air is affected (actually area of wing reduced to a flat plate equivalent...) and Cl is essentially a measure of the direction of the accelration of the air, ie what portion of that air is accelerated downwards. The entire equation equals Weight which is measured in Newtons and it is a FORCE! Note I deliberately said weight, as this takes into account acceleration of the aircraft in a taking into a turn, a loop or whatever and it take sinto account the mass and direction of acceleration of that air So far from not accounting for it, it actually proves Newtons third Law! However in another sense you are absolutely correct in that it is proportional to downwash as the total mass of air "downwashed" is proportional the weight of the aircraft as in the above equations we are only talking about the vertical component of the downwash and we know that downwashed air is only deflected a few degrees so much much more mass has to be displaced for the vertical component to equal the actual weight of the aircraft, this is simple basic trigonometry. And Superdimona has stated this perfectly and chimed in at the exact right time!

Superdimona, What you are saying is absolutely correct as well, and I am glad you brought it up. Have a close look at Bernoulli's equation and you will see that bernoulli's equation is simply a derivation of Newtons F=MA, it is Netowns second law. It is simply the Mass of air accelerated in a certain direction (downwards) to produce a Force and every force has an equal and opposite reaction, in this case Lift!

If we actually went into the derivations and the maths we could show that force produced by displacing the air downwards is the lift force. Therefore hopefully everyone now can see that Bernoulli is saying exactly the same thing as Newton, just from a different perspective. As I said in my first post it doesn't matter whether you believe Bernoulli or Newton as they are both correwct because they both say EXACTLY THE SAME THING!!!!!!!

So to finally get to the point about split flaps, lets put it this way. The spit flap causes more airflow to be directed down than thus the opposite reaction is to FOPRCE the slpit flap and whatever else it is jopined to UP. As the equal and opposite reaction. Using Bernoulli and pressure, there is an increased pressure below, thus with an increased pressure (ie force per unit area) it means you are producing a greater force upwards. As more air is being deflected down it creates a lower pressure on top of the wing and as nature abhors a vacuum the higher pressure tries to move to the lower pressure so a force is generated and Lift (a force) is produced. So (hopefully) you can see they are the same thing!!!!!!!

Hopefully this long winded explanation has bored most and we can now get back to more exciting topics like first job propects for newbies and what the conditions are like at Rex.

Have fun
CB

[AerocatS2A]

You can locate the paper here: Misinterpretations of Bernoulli's Equation.

I love this argument! Lets have a look for what else we can find.

Walrus
Walrus, your cite doesn't disagree with the NASA website. It takes issue with the "equal transit time" explanation, as does NASA, and it takes issue with the common explanation that the increase in velocity causes the decrease in pressure. The NASA article doesn't address this. What they both agree on is that Bernoulli's equations do accurately predict the pressure differential around the wing and that you need to use the Euler equations to get a more complete picture of what's happening.

Just to make my position clear. The common simplified explanations of lift using Bernoulli coupled with the "equal transit time", and Newton coupled with the flat plate, are both wrong in one way or another. For Bernoulli it's the equal transit time part that is wrong, but that's fine because the equal transit time has nothing to do with Bernoulli it's just a poor explanation of why the upper flow is accelerated. Likewise, the flat plate explanation ignores the very important role the top of the wing plays in turning (or accelerating) the air mass. Once again, the Newton part is right, it's just that the rest of the explanation is incomplete.

[currawong]

Cloud Basher - FYI

Downwash decreases as speed increases.

Downwash increases as speed decreases.

Weight of aircraft remains the same excepting fuel burn etc etc.

Newtons Third can be applied to what is happening on the top of the wing...

Bernoulli tells us what is happening, Newton explains the resultant forces.

Point I am making - lift increases with speed. Downwash does the opposite.

[toolowtoofast]

i still don't get flat plate/newtonian theory at 'negative' angle of attack.

or (using the theories above) why a wing of say 150sq ft with a 'high lift' camber can fly much slower than a 150sq ft 'skinny' wing.......

here's bernoulli at work. along with newton, and coander effect.. :)

YouTube - Fuel leak (http://www.youtube.com/watch?v=DQhe-Ds7ghQ)

[AerocatS2A]

i still don't get flat plate/newtonian theory at 'negative' angle of attack.

or (using the theories above) why a wing of say 150sq ft with a 'high lift' camber can fly much slower than a 150sq ft 'skinny' wing.......
Because the "high lift" camber is much better at accelerating a mass of air downward, therefore it can accelerate enough air at a low airspeed to achieve the required lift. When you think about Newtonian lift, forget about the "flat plate", the entire shape of the wing is very important in turning the air mass. The shape of the top of the wing plays a large role in how much air can be turned.

At negative angles of attack, a cambered wing works against the angle of attack, so you may need a much bigger negative angle in inverted flight to get enough lift. That's why some aerobatic aircraft such as the Pitts Special have a symmetrical airfoil, it makes the wing just as (in)efficient either way up. The downside of the symmetrical airfoil is that it's not all that efficient to start with and you end up with a relatively fast stall speed for a biplane.

[toolowtoofast]

i thought the pitts lower wing was symmetrical to reduce the pressure distribution inteference with the upper wing.

and also so it can fly with not too high aoa when inverted.

[AerocatS2A]

i thought the pitts lower wing was symmetrical to reduce the pressure distribution inteference with the upper wing.That may be the case. I thought the upper wing was symmetrical too, but I may be misremembering.

and also so it can fly with not too high aoa when inverted.
That's what I was saying. The camber on a wing works with a positive angle of attack, and against a negative angle of attack. So a wing with camber will fly inverted it just needs a higher angle of attack as it's working against the camber.

[Cloud Basher]

Currawong,
Absolutely correct! (and I have no idea where you think I ever disagreed with what you are saying).

However you seem to be implying that downwash decreases with speed simply because speed increases (if you aren't then I apologise)

Downwash decreases with speed because we as pilots reduce the angle of attack to remain straight and level. A reduced angle of attack reduces the downwash, so you are correct, but it isn't a function of speed, it is a function of the pilot controlling the flight path. As speed increases if we kept the AoA the same we would prodce more lift because the mass of air being deflected in greater per period of time, therefore the equal and opposite reaction is greater, thus greater lift, ie we would climb! (as we have now converted more energy from the more air deflected and as according to Newton energy cannot be destroyed merely changed from one state to another in this case it gets "stored" as potential energy ie an increase in altitude!) Newton was one smart cookie!

Lift increases with speed as we are pushing more air downwards iaw Newtons third law and explained by Bernoulli's equation as V^2. Therefore we reduce the angle of attack to reduce the downwash which keeps the Lift constant. And bernoulli explained this as Cl, ie reduce the AoA reduces Cl so L remains constant. What do you know, Bernoulli and Newton agree once again!!!!

So your last point about lift increasing with speed is absolutely true, but downwash does not decrease with speed UNLESS Cl (ie AoA) is reduced. And we all put this into practice when we fly the aircraft.

As I said Cl is an arbitrary ratio that tells us how much "downwash" there is. Well actually that isn't absolutely correct, what it is is actually a contstant (at a given AoA) that factors the vertical component of the downwash into Bernoullie equation.

This is easily proven by the following: If you hold a constant AoA and change the speed Cl does not change. The ONLY thing that affects Cl is AoA (which is true by definition, Cl being coefficient of lift!) So if you hold Cl constant and change speed then the amount of downwash is EXACTLY THE SAME! However as you say lift will increase, however this is due to the increase amount of air deflected downwards due to the increased velocity.

Hope this wasn't too confusing.

CB

[Cloud Basher]

People seem to be getting wrapped around the axles (or aerofoils!) about the flat plate theory. Please don't worry about it. The reason for the flat plate was to reduce all wings to one common denominator so they could be compared.

I.e. say we have two wings, wing X which has a 5% camber and wing Y which has a 10% camber.

We know that a flat plate of a certain size gives a certain amount of lift, simply measured in a wind tunnel (also gives drag but lets ignore that). So we know that wing X produces B amount of lift at a given AoA and wing Y produces C amount of lift at a given AoA, so we can compare exactly what the wings do we can say Wing X has an equivalent flat plate area of D and wing Y has an equivalent flat plate area of E, thus the two wings can be directly compared using a baseline of a flat plate area. The beauty of this is that it is a very accurate way of baselining as most (and I stress most, not all!) wings have a linear Cl gradient up to the point of stall. So it is really easy to compare wings planforms directly with one another.

Flate Plate area could have been called Cauliflower equivalent or Dangleberry equivalent, the name is irrelevant. In all science we must reduce all variables to a common denominator so that they can be directly compared. In this case just happens to be a "flat plate". So please don't worry about the flat plate points, it is just a way of comparing apples with apples.

Another example of baselining. We shoot rounds into calibrated 10% ballistic gelatine so we may compare one rounds terminal performace with anothers. It means we can say that all other things being equal round X has a penetration of A and a wound channel of B etc etc and round Y has a penetration of C and a wound channel of D etc etc. However when we chuck it into real world we have peoples clothing to contend with, body armour, chest harnesses with chocolate bars to get through etc. Again it is just a way to reduce variables so things can be directly compared. That is all flat plate theory is.

Again, I hope I wasn't too abstract!

Cheers
CB

[djpil]

Suggest that you agree on the measurement of downwash - talking Cls then angle of downwash is appropriate however talking basic physics then vertical component of velocity makes more sense to me (both of course vary with distance from the wing).

Yep, the Pitts has symmetrical aerofoils top & bottom - see Curtis' original patent here (http://musclebiplane.org/htmlfile/pittspat.html).