How does split flap increase lift?

[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) 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.

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]

Originally Posted by Walrus7

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

[AerocatS2A]

Originally Posted by 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.......

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]

point on flat plate

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.