Aerodynamics ( flap question)

[parasite tango]

Hello everybody I have a question on flap deflection


What I understand is when we lower the flaps ,It changes the camber of the aerofoil and this cause an increases both lift and drag.
Now more lift means more induced drag , With more induced drag wingtip vortices formation will be more. And also downwash also increases when we deflect flaps downwards.

Now what I know is if we have more wingtip vortices and downwash on the wing , it reduces effective angle of attack and to generate the same amount of lift you need to increase angle of attack , the same case happens when we take off and comes out of ground effect.

sir now the question is that with flaps lowered your Clmax of the aerofoil increases but your stalling angle decreases.

If the effective angle of attack of the wing decreases don't we get more range of angle of attack , as the particular aerofoil will always stall on the critical angle of attack?

Please somebody explain me where I am going wrong in understanding the concept

Thanks

Parasite

[Mr.Vortex]

Hi there,

As we extended the Flap, more wing area were make, and more angle of attack
too [Don't forget that now the chord line have change from LE wing -> TE wing
to LE wing -> TE flaps] Because of that the boundary layer might not have enough energy to flow along the wing at higher AoA and hence stall.

Best Regards

[27/09]

with flaps lowered your Clmax of the aerofoil increases but your stalling angle decreases.

Really!

Sure CL increases but I think if you do a bit more research you will find that tha stalling angle remains pretty much the same. As Mr Vortex mentions the chord line has changed

The deck angle decreases but the stalling angle isn't changed.

If the effective angle of attack of the wing decreases don't we get more range of angle of attack , as the particular aerofoil will always stall on the critical angle of attack?


The Cd increases to a much greater extent than CL so so far as the wings efficiency is concerned flaps don't help. You need more power to overcome the increased drag.

Flaps have two purposes:

1. Increase the lift thereby reducing stalling speed to give slower takeoff and landing speeds allowing shorter runways to be used

2. Increase drag to help slow the aircraft down

[Microburst2002]

Here whe have to define Angle of Atack precisely.

I think that the typical graphs showing how stalling AoA decreases when lowering trailing edge flaps are referred to the "airplane AoA", that is, to the angle of the wind relative to the airplane's longitudinal axis.

Strictly speaking, an airfoil's AoA is the angle of the wind relative to the it's chord. When lowering trailing edge flaps you actually change the airfoil. And these airfoils have different dimensions, characteristics and performance.

Their chords are different, so: How to compare stalling AoAs?

In the graph we see flaps down curve parallel and above clean curve. But why the stall occurs at lower (airplane's) AoA?

And another question (the more I think about it, the more questions, Argh!):
Why is the flaps curve parallel??


The reason for the "anticipated" stall when flaps are down is, I think, the increased airflow circulation due to the increased camber. The negative pressure gradient after the maximum camber is more intense, so separation occurs earlier. I suppose the answer to the 1st question is this fact plus the fact that flap airfoil's chord angle of incidence is increased with respect to clean airfoil, so everything has to occur earlier with respect to the airplane's logitudinal axis.

On the other hand, flaps down zero Lift occurs at an earlier AoA, too. According to the typical graphs, the flaps down range of possible positive Lift AoAs is much greater than the decrease in stall AoA. I suppose this can be read as "camber increases lifting capability of an airfoil". But still I don't understand why the slope of both curves is exactly the same. Or are they not?

[decurion]

This website High Lift Systems: Introduction (http://adg.stanford.edu/aa241/highlift/highliftintro.html) could give some basic understand of high lift devices (slats, and flaps). Slats increase the maximum lift coefficient and the Angle-of-Attack where this maximum lift coefficient occurs, flaps increase the lift coefficient at zero Angle-of-Attack.

I can also recommend the NASA study "High-Lift Systems on Commercial Subsonic Airliners" (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960052267_1996080955.pdf)

[Escape Path]

Why is everyone talking about changes in the stall AoA when flaps are up or when flaps are down when I've always been taught that the wing will stall at the same AoA? :confused:

[Pugilistic Animus]

this is truly a fearsome topic:\:\:\...the way you all are going...I'll have a little more tomorrow perhaps...I'm not sure where to start, also this subject is very experimental and empirical...and actaully involves as does everything else...lots of compromises:ugh::sad::uhoh: ...also check out some of Mad(flt)Scientist's:8 posts...he really knows the down and dirty of getting planes to fly---for money;):ok:

[Volume]

with flaps lowered your Clmax of the aerofoil increases but your stalling angle decreases.Correct. Using only trailing edge devices (flaps) the AoA corresponding to CL max is decreasing. Looking at the old propeller transport aircraft (DC-6, Lockheed Constellation) during approach you can notice the nose down attitude of the aircraft. With zero AoA you are already very close to stall with full flaps. Today Aircraft like the BAe 146 are a good example of aircraft that approach with nose-down attitude due to trailing edge devices only.

Figure 1.33 (page 41) in Decurion´s NASA link illustrates this behavior.

With leading edge devices (Slats, Droop nose, Krüger flaps...) you increase the AoA corresponding to CL max, hence today you see significant nose-up attitudes of most modern Aircraft when landing.

[27/09]

Volume

AoA in my book relates to the angle between chord line of the the wing and the relative airflow (wind).

When you say Correct. Using only trailing edge devices (flaps) the AoA corresponding to CL max is decreasing. Looking at the old propeller transport aircraft (DC-6, Lockheed Constellation) during approach you can notice the nose down attitude of the aircraft. With zero AoA you are already very close to stall with full flaps. it appears to me you are talking about deck angle.

Show me a reputable text that goes remotely close to stating With zero AoA you are already very close to stall with full flaps

When flaps are lowered, the chord line changes, it is lowered at the trailing edge, so even though the deck angle is reduced the AoA hasn't decreased. If I was any good with diagrams I would draw some and post them here to show what I mean.

I would recommend "Mechanics of Flight" by A C Kermode as an easy to read and understand text which covers a wide range of topics like this one.

[Pugilistic Animus]

nothing can be said absolutely without speaking in specifics...but in general simple flaps increase wing loading, decrease stall speed, decrease limit loads...however the basic airfoil lift characteristics are not changed so the Clmax point of the wing [not the flap section] changes..you have more lift at lower speeds,..hence changes in the loading with effective camber increase is independent of aoa...I'm not sure what this conversation is truly about..:confused:

when going on about slots and it's effect on boundary layer wrt to laminar and turbulent separation...that's a whole PhD thesis:\


this is on of the most experimental topic ...the theoretical models all are very limited...though useful for define basic changes in load distribution and with deflaction and change in pitching moment and center of pressure...and the whole thing is sensitive to minute changes and the increase in weight, drag and complexity must also be accounted for...this particular topic is a huge pain in the ass for designers- in real life and only the most general things can be said...even in the textbook...:)

[bfisk]

Why is everyone talking about changes in the stall AoA when flaps are up or when flaps are down when I've always been taught that the wing will stall at the same AoA? http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/confused.gif

There's your problem.

You need to be a bit more precise - a wing will always stall at the same AoA as long as it's in the same configuration. Changing the configuration will change the properties of the airfoil and alter the lift and drag coefficients and - curves.

Rules of thumb; trailing edge devices (flaps) lower the Critical angle of attack, leading edge devices (slats) increase the Critical AoA, both increase CLmax and both decrease the L/D ratio.

[Pugilistic Animus]

You need to be a bit more precise - a wing will always stall at the same AoA as long as it's in the same configuration. Changing the configuration will change the properties of the airfoil and alter the lift and drag coefficients and - curves.



but in general simple flaps increase wing loading, decrease stall speed, decrease limit loads...however the basic airfoil lift characteristics are not changed so the Clmax point of the wing [not the flap section] changes..you have more lift at lower speeds,..hence changes in the loading with effective camber increase is independent of aoa...I'm not sure what this conversation is truly about..http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/confused.gif...the above is true look it uphttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/smile.gif

edit:

fmVhlPviOxY&feature=related

it's important for the new generation to see the possibilites...although nothing to do with flaps--just an excuse to put GK in..:ouch::}:}:}

[Escape Path]

@bfisk

Thanks for the clarification. I forgot that tiny bit :\

[Mad (Flt) Scientist]

There's your problem.

You need to be a bit more precise - a wing will always stall at the same AoA as long as it's in the same configuration. Changing the configuration will change the properties of the airfoil and alter the lift and drag coefficients and - curves.

Stall AoA will also change as a function of ... Mach number, Reynolds number, load (twist and bending of wing), sideslip if present, proximity to the ground (lower stalling angle in ground effect), general condition of wing (steps, gaps, damage, etc.), roll/yaw/pitch rates, and I'm sure I've missed some.

Furthermore, the simplified explanations given by various people above shouldn't say "the wing stalls ..." but rather "the aerofoil stalls ..." - notions of local AoA relative to changing camber lines are reasonable approximations when talking of a 2-dimensional aerofoil section but become pretty much meaningless when considering a real 3-dimensional wing. For example, the flap extends (usually) only over part of the span. So, when I deploy the flaps, do I consider the camber to change or not - the OB wing hasn't had any geometrical change, and if the stall is actually still outboard then the effect of the flap movement may not fit the simplified explanation at all.

Because of that, AoA for an aircraft is pretty much always defined in terms of the fuselage reference relative to the airflow, and is not shifted around in definition by wing configuration changes. Otherwise the standard relationship:
pitch attitude=AoA plus gamma
would not hold.

[Microburst2002]

Hi Mad S.

Regarding the shape of the CL vs AoA curves for clean and flaps down:

They are always parallel to each other. Is this a simplification or that reflects the wind tunnel tests?
If so, What is the meaning of that? Why do these curves not have different slopes?

thank you

[cwatters]

The thickness of 2/3rds of the wing remains unchanged as flaps are lowered so should you really expect major changes to the shaps of the plot?

[Mad (Flt) Scientist]

Regarding the shape of the CL vs AoA curves for clean and flaps down:

They are always parallel to each other. Is this a simplification or that reflects the wind tunnel tests?
If so, What is the meaning of that? Why do these curves not have different slopes?


Theoretically, any "thin" aerofoil has a lift curve slope - rate of change of CL with AoA - of 2 * pi per radian. Which works out to 0.1097 CL per degree. So any two dimensional, infinitely long wing would have the same lift curve slope, and that would apply even if the aerofoil was changed - such as by putting down the flaps. So, in an ideal or perfect sense, the lift curve slopes should be perfectly parallel, and of the value mentioned.

In practice, even a 2D section doesn't quite achieve that perfect 2*pi value, but numbers in the range of 0.105 are typical. Once we start to have three-dimensional effects, the slope changes again, but importantly for this discussion the 3-D effects are dominated by the effect of the finite span, which is pretty much the same effect for a given wing whether flaps are extended or retracted.

As a result of all this, the slopes are slightly different for different flap configurations, but perhaps not more than 5% variable between the different configurations for a given aircraft. As a result its a reasonable simplification to say that the slopes are parallel. The same kind of simplification we make when saying that slats extend the slope. The following two pictures compare the idealised and typical situation. (With even the "typical" one having simplifications)

http://img84.imageshack.us/img84/5280/clalphaideal.jpg

http://img818.imageshack.us/img818/1929/clalphatypical.jpg

Unless you are really getting into the weeds, that first picture is good enough to understand the basic effects, and it's a great deal easier to sketch too!!

[Pugilistic Animus]

Unless you are really getting into the weeds---and smoking it...:}

:ouch:

edit I'm afraid to discuss 'normal incremental loading with deflection';)

[BobM2]

Don't forget the dramatic increase in wing area. As I recall from 727 initial many, many years ago, increase in wing area is some 40% from flap up to flap down. This very complex flap system produces a totally different airfoil with flaps down, very high lift & very high drag.