Aircraft lift - Angle of Attack?

Question

Hello guys and girls,

I wanna know whether an aircraft's lift acting on the wings depends on the angle of attack of an aircraft?

From my understanding, lift is generated on the wing by deflecting air downwards.. and it seems to me like increasing the angle of attack doesn't really increase the lift acting on the WING, since the airflow over the wing is not parallel to the ground when an aircraft pitches up.. Instead, airflow is almost always parallel with the wings of an aircraft..

So, am i correct to say that lift on the wings of an aircraft doesn't rely much on the angle of attack, but instead, extending the flaps would increase the lift on the wing??

I stand corrected..many thanks.

Answer 1

There is a fundemental problem with the question - while flying straight ahead the amount of lift created by the wings is a constant.   The wing of a 3000 pound airplane creates 3000 lb of lift in level flight. It creates 3000 lbs of lift when climbing, and 3000 lbs of lift when decending.   Changing the angle of attack only changes the amount of power  and airspeed required to create that lift.  An airplane does not climb because you increase the amount of lift - it climbs because the engines are supplying more power than needed to over come drag at the current airspeed and angle of attack.

                 The only time you actually increase the amount of lift created by the wing is while in a bank, to make up for the amount lift that is acting off of the vertical axis.  You do that by increasing the angle of attack, applying back pressure on the controls.

                 When you learn to fly, one of the the first things you are taught is that "pitch +power =performance".   In most aircraft the angle of attack is controlled by adjusting the attitude ot the entire aircraft in pitch, and measured indirectly by the pilot by watching the airspeed.   As you increase the angle of attack, you decrease the amount of airspeed required to supply the required lift, but you increase the amount of induced drag caused by creating that lift, so the plane slows down.

             This holds true until you reach the angle of attack that causes the wing to stall- the point that the air flow stops being directed  downward, but instead becomes a turbulent flow in the wings wake.

           One of the requirements before you can solo is flight at minumum controllable airspeed ( both with and without flaps extended) - a condition that requires a pretty extreme angle of attack and and about as much power as the plane can muster.

     "High lift devices" work in a number of different ways: flaps increase the wings camber, allowing the wing to produce the required lift at a lower angle of attack, but other "high lift" devices like slots, slats, and vortex generators work by allowing the wing to achieve a higher angle of attack before the stall occurs.

  

    You've also made a second faulty assumption: "Instead, airflow is almost always parallel with the wings of an aircraft." Airflow is parallel to the forward motion of the aircraft.  The wing of an aircraft is mounted at a positive angle of attack, so that at normal cruise the plane is level but the wing is close to the most efficent angle of attack for the anticipated airspeed.

Answer 2

>angle of attack doesn't really increase the lift acting on the WING, since the airflow over the wing is not parallel to the ground when an aircraft pitches up<

the angle of attack depends on the vector of air mass relative velocity and longitudinal axis of the aerofoil /the chord/. Thus if the plane pitches up while maintaining the same altitude, its angle of attack increases and positively afects the lift.

thats why you perform a backwards control input before landing - you increase the lift through increased AoA at the account of increased drag /which actually helps you to slow down the plane and is beneficial this time/

when the plane takes off, /speaking of stabilised climb, the air flow is obviously not horizontal relative to the earth but is horizontal relative to the vector of speed of the plane / the speed is a 3D value, having a value horizontal to earth, climb fraction,perpendicular to earth, and sidewards fraction/ the plane may increase the AoA by further pulling the controls. Accidentally the only way to maintain the stabilised climb while AoA is increased would be to deploy the air brakes /speaking of airliner, not agile fighter jet/.

You are possibly confusing the AoA with longitudinal axis tilt value. Once again AoA is angle between airflow and chord of airfoil, while the longitudinal axis tilt is where the nose of the plane points.

to add another example - imagine flat spin. one wing (the forward moving one) would have at about 45 degrees /depending on the ratio of vertical speed and rotary speed of the spin/ AoA while the other has in fact 135 degrees  since it is being passing the air backwards.

edit> deploying flaps does not help to influence the AoA anyhow, because the AoA is undisturbed airflow   IN FRONT of the wing. --well in fact the flaps WILL induce the change in AoA but through their nose-heavy momentum when deployed - deploying flaps should generally decrease the AoA unless proper elevator input is done by pilot.--

simulate the situation this way /dont take it personally/ place a model plane at your desktop. now move it forwards. this is plane flying straight and level. the surface of the table simulates the air flow. check recent AoA angle of the chord and table surface. now push the tail of the plane until it hits the desktop. now again roll the plane forwards.  DO NOT LIFT IT. now the plane is flying high AoA.

Notice nothing said about the position relative to earth. the same can be done at ANY surface - thus simulating climb / descent / bank.

the only two values taken into consideration are the surface(=airflow) and the chord. do not think about what the actual airfoil is and whether is it able to lift the plane. IT implicitly IS, period. even when flying upside down - if your experiment is setup so that the plane flies stable.

edit2> very well :) like john R summarized, the airflow is not parallel with the earth but with the aircraft motion vector/ velocity vector

besides note that the two values go against each other when producing lift - similar plane, ismilary lodaed when flying fast, needs lower AoA because the lift is produced "by sufficient speed" the faster it flies the lower the AoA. once the plane needs to slow down and maintain the same lift, it has to increase the Cy which is basically done through increasing the AoA - by nosing slightly upwards this increases the lift coeficient to supplement the drooping airspeed. at certain AoA the Cy starts to drop as well, because of the critical AoA reached. to prevent this the flaps and other lift increase gadgets are deployed.

To be fair, the SUkhois and F18 flying low pass lowspeed displays actually fly behind their envelope, and majority of forces holding them in flight are provided by their engines, but still the planes are controllable by the pilot, that means they still have certain lift left at their wings to stabilize the flight without vectoring the thrust.

Answer 3

Lift is majorly affected by angle of attack. Imagine holding your hand out flat in front of your mouth and blowing at it edge on. It's not going to push very much air down. Now rotate your hand to a 45-degree angle and blow on it. It will deflect nearly all the air you blow into it downwards.

Of course, that's a tremendous oversimplification for a lot of reasons. But the principle is basically correct, the angle of attack will directly affect the amount of lift. In fact, angling the wing downwards (a negative angle of attack) will result in downward "lift".

Unfortunately, a people are erroneously taught that the lift results from the shape of the wing. If that were true, planes would not be able to fly upside down. (Of course, some planes can't, but it's for other reasons, like the fact that the oil wouldn't flow through the engine or the fuel wouldn't be at the bottom of the tanks where the pipes are.)

It is true that the lift of some wings is enhanced by a shape that's longer on top than on the bottom. But that's generally not efficient for airplanes because they spend most of their time going at high speeds where the amount of lift they need is very low compared to the amount of air going over their wings. Of course, on take-off they are going much slower and need more lift (to climb, rather than just stay level), so they need much more lift. Angle of attack gives it to them.

It is true that with some planes flaps are used to increase lift for things like takeoffs. However, the drag the flaps cause is, on most planes, bad enough that zero flaps is the preferred takeoff setting unless you are very heavy, have an unusually short runway, or are taking off into obstacles.

Flaps are most useful when drag is good, such as during landing. In that case, flaps allow you to maintain a low forward speed without descending too rapidly.

Interestingly, angle of attack is pretty much the only parameter that affects stalling. You stall if the angle of attack is too high, period. (Although configuration changes like flaps will change the critical angle.)

Also, the angle of attack is defined as the angle between the wing chord and the relative wind. So if you're "flying" straight down and pointing straight down, the angle of attack is zero just the same as if you're flying level horizontally and pointing straight horizontally.

Answer 4

Angle of attack is the angle formed between the airfoil's chord line and the relative wind.  --As JoelKatz stated.

Lift can be changed by changing the angle of attack, or by changing the camber of the wing by flap extension or retraction, or by doing both.

Flap extension can greatly change the camber of the wing without changing the angle of attack.

Flying a model plane or an aircraft would clear up much of this for you in about a minute or two.