How does split flap increase lift?
 

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?

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

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.

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:

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

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.

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!!

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.

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.

As Lodown stated, the split flap turns the air so just like everything else, it develops lift by pushing the air down - Newton's law.

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?

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...

:ok::ok:

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

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

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 .

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

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

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

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?

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