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Spanwise Lift and Stall Sequence

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Spanwise Lift and Stall Sequence

Old 02-22-2010, 11:02 AM
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tomfiorentino
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Default Spanwise Lift and Stall Sequence

Hi everyone,

I have a book on aerodynamics that includes a section on the above as it relates to rectangular, tapered and elliptical semispans. I continue to be confused by one concept presented that is dogging my understanding . Any thoughts would be appreciated as would any references for additionla reading (I didn't see anything come up in my RCUsearch).

In particular, the book references:

" Because the lift is proportional to chord and angle of attack, the effective angle of attack at each spanwise section is also different. Notice that for the rectangular wing the section with the highest angle of attack is at the root. The tapered wing has the highest angle of attack at about 2/3rds of the semispan. The elliptical wing has a constant angle of attack across the span."

Footnote: "The Ilustrated Guide to Aerodynamics" 2nd Edition by H.C. "Skip" Smith pages 40 - 41.

The section then goes on to describe the first region of stall to be at the root trailing edge for both the rectangular and elliptical wings and 2/3rds on the tapered wing. The progression of the stall is shown as being different for each as well. There is no mention of relative wind and/or dihedral effects so I am assuming everything is straight on.

So, few things:

1. How is the angle of attack across the semispan different (particularly hard to understand on the rectangular wing)?
2. If the rectangular wing has the highest AOA at the root and the elliptical AOA is constant across the whole span, then why is the first region of stall for both the same....at the trailing edge root?
3. What consideration shoud be given to the wing tip vortices? I guess I don't understand why the rectangular wing, for example, stalls last at the tip given the vortices are lift killers at any angle of attack.

What say ye RCU!

Tom
Old 02-22-2010, 12:58 PM
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Tall Paul
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Default RE: Spanwise Lift and Stall Sequence

The wing at the root is influenced by the fuselage, which acts as a winglet, altering the flow around it.
Old 02-22-2010, 01:42 PM
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Red B.
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Default RE: Spanwise Lift and Stall Sequence

Here is a link with some interesting reading about spanwise lift distributions for a WWII Swedish fighter aircraft:

[link=http://www.hobbybokhandeln.se/j22/aero.htm]FVVS J-22[/link]
Old 02-22-2010, 04:22 PM
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Default RE: Spanwise Lift and Stall Sequence

I was in tune with this until he said that the retangular wing stalled at the root because the angle of attack there was the highest. Then I went into cerebral meltdown for a while.

There must be something else that also relates to the portion that you've quoted. And hopefully quoted in full and completely. As it shows in your post I can't help think that there's a reference to spanwise flow and vortex flow around the wing panels that is missing. I know that the vortices that are centered around the tips extend their effect well in on the wing. This is why sailplanes use very high aspect ratios be reduce the vortices and their effect on the airflow passing over the wing. It's not just a tip effect. So all I can think of is that there was some talk of vortex formation on the wing tips and how it affects he local angle of attack along the wing panel. And it was assumed in the portion you provided that you already had that in mind and that it was just a given that you'd understand about the rectangular wing having a spanwise reduction in EFFECTIVE angle of attack.

Do I win a prize?
Old 02-22-2010, 09:01 PM
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Default RE: Spanwise Lift and Stall Sequence

even though the wing may be rectangular, the lift distribution is elliptical and goes to zero at the tip. The wingtip vortex ensures this. The only way for that to happen is if AoA is reduced along the span. The wingtip stalls last because in theory, it never stalls. The AoA is always 0 right at the tip.

To better understand it, you need to look at the spanwise Cl distribution, where Cl x chord = lift

The smaller the chord, the greater the local Cl for a given lift
The greater the local Cl, the greater the AoA at that chord.
Old 02-23-2010, 09:14 AM
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rmh
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Default RE: Spanwise Lift and Stall Sequence

If one were able to get a birds eye view of a wing - and see the the actual paths taken by he air as the wing upsets it's steady state, they woul note that the air ALWAYS takes the path(s) of least resistance.
Depending on speed and effective resistance at any given point - the air goes chordwise or bunches up and 9against the fuselage or moves spanwise around the tip.
The ideal textbook wing has no root and no tip so flow is always ideal!
Which is BS because the flow direction is constantly changing
Setting at 35000 ft on a perfectly trimmed , constant speed passenger plane - I often watched the tiny eddies along the wing making minute changes.
Which proved to me that no matter how one tries - the flow over a wingis never a 100% constant codition- so the equasions about what is happening are only true for a fixed (imaginary) condition.
But you gotta start somewhere -
Old 02-23-2010, 11:39 AM
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Default RE: Spanwise Lift and Stall Sequence

If you want to see what goes on during the stall of a wing just go to youtube and search for "RV-12 stall tests. There you can see a wing that has been tufted for the test and the reverse flow that shows where and when the airfoil of the wing is stalling.

On a rectangular wing, the stall is a gradual thing, starting at the trailing edge at the root and working forward and outward as the stall is deepen.
Old 02-23-2010, 01:56 PM
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tomfiorentino
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Default RE: Spanwise Lift and Stall Sequence

Thank you for the replies.

To Matthews:  Then they must mean the angle of attack is highest at the root of the rectangular wing because of the vortex progression and downwash etc.  It is discussed in the book which is why I referenced the vortexes actually...I'm not that smart on my own.  Induced AOA is discussed also.  So to your point...they are referring to effective angle of attack.  I can understand that.  But I still can't get past the stated stall progression.  Why would a hersey wing stall last at the tips?  Seems to me the vortex would mess up flow and lift there first.  Also can't get past the reasons for the effective angle of attack differences between elliptical and tapered planforms.

I get the stall (at least I think I do), I get the flow separation and the discussion on pressure gradient.   I understand the visual that the tufting gives etc.  The book lays out lift and lift production from a pressure differential point of view and right or wrong leans toward the top of the wing
doing most of the work at normal angles of attack.  And from what I can see, there are differing opinions on some of that stuff.

But I can't seem to grasp the why behind the differences in the first stall region and the sequence of progression from there.  There is a blind spot somewhere in my thinking on this.  I'll go back and re-read some of this again.

To RMH: I hear what you are saying.  The book references the theoretical wing span of infinite length.  No problems.  All bets are off once you add the fuse and the tip.  But all things remaining equal..I am just trying to get the science behind the reality as we can't deny that airplanes with different wings exhibit different characteristics as a result.

To highplains:  I'll check it out.

Thanks



Old 02-23-2010, 02:27 PM
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Tall Paul
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Default RE: Spanwise Lift and Stall Sequence

Dupe..
Old 02-23-2010, 02:30 PM
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Tall Paul
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Default RE: Spanwise Lift and Stall Sequence

Tufting for airflow studies on the L-1011.
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Old 02-23-2010, 11:04 PM
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Default RE: Spanwise Lift and Stall Sequence

Tom,

This is how I understand the phenomenon:

ORIGINAL: tomfiorentino

1. How is the angle of attack across the semispan different (particularly hard to understand on the rectangular wing)?
The wing forces its path through the air, creating a disturbance with its shape.
That disturbance is no accident or chaos, it is meant to create an acceleration and deceleration in the flow of air, which produces pressure imbalances, as we all know.
That pressure imbalance sucks the air that is some distance in front of the leading edge upwards (upwash), and also pushes the air that is left behind the trailing edge downwards (downwash).

The stronger the disturbance, the higher the pressure differential, the lower level respect to the LE and TE the upwash and downwash reach.
Upwash makes the wing hit the air that is moving upwards; hence, the real AOA (or induced AOA) is higher than if it is measured respect to the geometrical direction in which the wing is moving.

The high pressure under the bottom is pushing the air into the lower pressure above the top of the wing.
The easiest path is around the wing tip, and the second easiest path is around the LE.
However, the stream of the downwash makes that path around the TE difficult for a vigorous stream, and no so difficult for a weak stream.

Now, let’s take the top surface of a right half wing for example.
Let’s divide the semi-span in several narrow strips running between LE and TE.
Along the chord of each strip the air is accelerated and its pressure is reduced as it should.

The left side of each strip sees lower backpressure from the strip on the left than from the strip on the right.
This happens because there are infiltrations of air coming from the bottom of the wing and around the wing right tip.

This infiltration kills the “vacuum” or low pressure above the top surface as we move toward the wing tip, and it does it in a non-linear pattern.
In terms of lift, the strip that is closer to the tip is unable of developing as low pressure (or “vacuum”) as the strip closer to the center of the wing (I don’t say fuselage because this also happens for flying wings).

As explained above, the upwash for each strip is a function of the low pressure above the top surface; hence, the strip closer to the center produces an upwash more pronounced than the strip closer to the wing tip.
Then, the strip closer to the center sees an AOA that is higher than the AOA that strip closer to the wing tip sees.

For a rectangular wing shape, what strip sees the critical AOA, the one that leads to the stall, first?
The strip that is closer to the center of the wing.

ORIGINAL: tomfiorentino

2. If the rectangular wing has the highest AOA at the root and the elliptical AOA is constant across the whole span, then why is the first region of stall for both the same....at the trailing edge root?
What happens to the air stream above the top surface just before the stall happens?
The air stream becomes slower and slower, using its dynamic energy to create more and more “vacuum” and adhesion to the top surface.
Because of this reduction, air from the bottom infiltrates around the TE, bursting the low pressure bubble that was sustaining lift.
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Old 02-23-2010, 11:23 PM
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Default RE: Spanwise Lift and Stall Sequence

Or, another way of looking (simplistically) at it, as the tip is in the vortex, and producing no lift, you can't actually stall it. Therefore the rest of the wing will stall, and still the tip won't. Again, if you look at the lift distribution over the wing, it is maximum at the centre, and minimum at the tip. As the centre bit is 'working' the hardest, it will feel the effect of increasing AOA the most, and so on out to the tip. The explanation won't be liked by the experts, but it does illustrate why the stall is progressive from centre to tip.
Evan, WB #12.
Old 02-24-2010, 09:46 AM
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Default RE: Spanwise Lift and Stall Sequence

Hahaha....that definitely won't be liked by the experts, but there are a lot of statements in here that wouldn't be either. Regardless, let me take a whack at this:

Definitions/Misconceptions:
Wingtip vortices - the phenomenon, caused by ANY finite wing that effects the entire wing (for the VAST majority of cases), of the high pressure air under the wing bleeding around the tip and onto the top.
Airfoil - an infinitely long shape with an AR of inifinity and ZERO wingtip effects or spanwise flow. Purely theoretical. Simply not achievable.
Wing - a structure of finite length
Stall - eduction in the lift coefficient generated by an airfoil as angle of attack increases

Wing tips DO stall. What doesn't stall is the plane of the wingtip, as it isn't actually a physical structure or any 3D entity. The tip that produces no lift is the 2D plane on which the wingtip sits. The wingtips DO produce lift, and CAN stall (even though they're NOT related).
Old 02-25-2010, 09:46 AM
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tomfiorentino
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Default RE: Spanwise Lift and Stall Sequence



OK...thank you for taking the time to write that up.

So, not too oversimplify, but is it safe to assume then that the differences in stall sequence relating to rectangular, tapered and elliptical wings is in turn related to the differences that exist in the way each of those wings generate vortices (and other disturbances)?

Old 02-25-2010, 09:52 AM
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Default RE: Spanwise Lift and Stall Sequence


ORIGINAL: tomfiorentino

OK...thank you for taking the time to write that up.

So, not too oversimplify, but is it safe to assume then that the differences in stall sequence relating to rectangular, tapered and elliptical wings is in turn related to the differences that exist in the way each of those wings generate vortices (and other disturbances)?

It's because of many things. For example, since tapered wings experience lower Reynolds numbers as the chord decreases... etc.

But if you're looking for a generalization, it's because of the shape of the wing.
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Old 02-25-2010, 11:20 AM
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Default RE: Spanwise Lift and Stall Sequence

they all stall - sooner or later
the real problem is which one becomes uncontrollable first?
and the answer to that is not found on the wing
IFthe stall is "matched" from tip to tip the stall will just cause a downpitch - (it happens duringn landing quite oftenand is sometimes desirable
But if the plane skids -then the the entire lift distribution along both left n right sides can be nasty.
a veryshort couple design (say-a flying wing - like a Dyke delta ) if skidded at critically lowspeed - will take forever to get realigned and back into control -ditto for a similar model or one with long spna and short front to rear arrangement with small tail group
Jou can practice stalls andfeed in a tiny bit of rudder to see how this all wrks - .
edit this as reqf-I cant see or spell anymore.
Old 02-25-2010, 03:16 PM
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Default RE: Spanwise Lift and Stall Sequence

Wow, that's really obnoxious....I typed WAY more than that! However, I feel like I can add to what others have said since I thought I posted. The wingtip vortices reduce the effective AOA, creating an artificial twist. What it does is it increases the pressure over the top of the wing, moreso at the tip than at the root. You don't have to consider it or calculate it....all you have to do is run the equations that already take those effects into account. It's just a concept you need to understand.

The reason that taper, twist, and aspect ratio are important IS because of wingtip vortices. Twist is partially used to reduce spanwise flow. Taper and aspect ratio is to minimize the area that the vortices effect the most. Vortices effect approximately the same AMOUNT of span where you have to account for it, and you can ignore the rest (for a simple model). Say your plane needs 100 squares per wing-half. Vortices effect (for example) 4 inches of span. If you have a wing that's 10in semispan, 10in chord...the vortices effect 40% of the wing (aspect ratio of 1, no twist). If your wing is 25in semispan and 4 in chord, then the vortices only effect 16% of the wing (4in semispan*4in chord). Taper also reduces the area effected, by making the area at the wingtip less for the same span length. I hope that makes sense. If it doesn't...I'll draw something up.

Mechanical twist is LITERALLY twisting the wing to LITERALLY vary the angle of attack of the wing along the semispan.

I don't think that elliptical wings stall at the TE of the root first, and the picture showing stall tendencies shows that.
Old 02-25-2010, 04:49 PM
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Default RE: Spanwise Lift and Stall Sequence


ORIGINAL: victorzamora

Mechanical twist is LITERALLY twisting the wing to LITERALLY vary the angle of attack of the wing along the semispan.

I don't think that elliptical wings stall at the TE of the root first, and the picture showing stall tendencies shows that.

I'm betting you'd rather say that "Mechanical twist actually twists the wing from root to tip to actually vary the angle of attack of the wing along that semispan."

Also, if you look closely at the elliptical wing and compare it to the rectangular one, you'll see that the rectangular one definitely shows the stall originating at the TE at the root of each halfspan. Compare the arrows that show how the stall migrates out from one specific location, the TE @ the root, to the similar function arrows showing the migration of the stall on the elliptical planform. That elliptical stall's arrows come from along a significantly wide span of the wing's TE. You are correct, elliptical wings do not stall at the root TE, however the picture doesn't show that happening.
Old 02-25-2010, 06:18 PM
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Default RE: Spanwise Lift and Stall Sequence

You are correct, elliptical wings do not stall at the root TE, however the picture doesn't show that happening
No...but the book says that elliptical wings stall at the root TE.
Old 02-25-2010, 07:13 PM
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Default RE: Spanwise Lift and Stall Sequence

ORIGINAL: tomfiorentino



OK...thank you for taking the time to write that up.

So, not too oversimplify, but is it safe to assume then that the differences in stall sequence relating to rectangular, tapered and elliptical wings is in turn related to the differences that exist in the way each of those wings generate vortices (and other disturbances)?

You are welcome, Tom.

I have just added two schematics to Post #10 above for clarity.

I believe that the differences in stall sequence is a simple problem of area and pressure gradient above that area.
In other words, those differences relate to where the cross section (referred as strip in my previous post) that is creating the lowest pressure (on the top surface) is located along the wingspan.

As your book explains (I have one), if you consider each of those strips as a wing with no tip (there is backpressure in both ends or tips), then the area that contributes to lift is directly proportional to the chord (since each strip has the same width or “stripspan’).

From experimentation, we know that the lift force produced by the half-wing decreases exponentially from the root to the tip.

We also know that the lift force produced by each of those strips is proportional to the area of the strip and to the square of the speed at which the wing is moving in the air.

Since all the strips are moving thru the air at the same speed, there are only two things to change in order to obtain different lift from each strip: area and coefficient of lift.

The efficiency of the elliptical wing comes from the fact that it has the surface necessary to create the minimum lift, no one inch less or more.

In order to achieve that ideal, the coefficient of lift of each strip has to have the same value (minimum L/D point).

Then, is the area the only remaining thing to play with.
Because of that, moving from root to tip, the area of each strip is reduced in the same proportion in which the lift reduces itself naturally (due to the infiltrations of air from the bottom of the wing and around the wing tip).
This is achieved only by reducing the chord of each strip, because each of them keeps the original width.
By doing that, and then putting together all those strips, the resulting shape of the half-wing is half an ellipse.

According to the book and other references, an elliptical wing stalls over the whole span at the same time.
The detachment bubble moves from trailing edge to leading edge.

http://www.faatest.com/books/FLT/Cha...ngPlanform.htm
Old 09-19-2010, 08:14 PM
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Default RE: Spanwise Lift and Stall Sequence

Keep in mind that these are very crude generalizations about stall progression on wings. Two different elliptical wings will stall differently due to differences in the airfoil, and protrusions on the wings such as pitot tubes, landing lights, etc..

The reason that straight, untapered wings stall at the root first is because of the tip vortices and fuselage interference, as mentioned before. The air circulation around the tip decreases the angle-of-attack at the tip of the wing, which actually influences the flow around the rest of the wing.

The reason the stall will tend to start at the tip on highly tapered wings has to do with relative Reynolds Number between the root and the tip. The shorter the chord, the lower the Reynolds Number. Most airfoils will stall at a lower angle-of-attack at lower Reynolds Numbers.

The reason the stall will tend to start at the tip on highly swept wings is because of the spanwise flow on the wing. Aft wing sweep induces outward spanwise flow (toward the tip). The opposite is true on forward swept wings.

Also, there are ways to fix stall progressions that begin at the tip by varying the airfoil shape along the span, adding wing twist, leading-edge slats, fences, etc..
Old 09-19-2010, 09:40 PM
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Default RE: Spanwise Lift and Stall Sequence

Fascinating, I really enjoy trying to understand and visualise whats going on aerodynamically. All I will say though is that no matter what theory you subscribe to it HAS to match reality.  Now, i don't get to see many full size go ,into stall, but plenty of our models do quite often. So, firstly when you talk about 'mechanical twist',  i guess you mean what we call 'Washout'. secondly, ask any spitfire pilot, their eliptical wings most certainly do not stall evenly across the span - in fact the spit is notorious for bad tip stalling and most models of it incorporate generous washout to alleviate the worst of it, and it affects big and small models alike, Reynolds numbers or not!

Still, very interesting, maybe i'll learn enough to design better planes eventually.
Old 09-20-2010, 12:48 AM
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Default RE: Spanwise Lift and Stall Sequence

Actually Rick, Spitfire pilots will tell you that the real thing has an innocuous stall, and the full size has 2.5 deg washout too. The FW190 has three deg washout, a higher loading and a harsh stall, enough to limit manoeuver margins. Which is why the Spit can out turn a 190, the pilot can fly a Spit into the stall with confidence, a 190 will stall so deep and quick it will flick into a spin the other way, and end up being the target. Against that, of course, the 190 had magnificent ailerons and could change direction fast enough to pull the wings off a Spit trying to follow. But in general, the narrower the tip, or steeper the taper, the more likely you will have 'Tip stall', and the easiest way to reduce the effects is a bit of washout. Straight, non tapered wings of moderate A.R. don't need washout, as the reasons given, ie stall from the root to the tip. So for most sport and trainer type R/C models you can just build them flat and not worry about the stall propagation at all.
Evan, WB #12.
Old 09-20-2010, 04:10 AM
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da Rock
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Default RE: Spanwise Lift and Stall Sequence


ORIGINAL: Rick.

So, firstly when you talk about 'mechanical twist', i guess you mean what we call 'Washout'.

most models of it incorporate generous washout to alleviate the worst of it, and it affects big and small models alike, Reynolds numbers or not

You might hear someone mention washout, mechanical twist and aerodynamic twist, and it might happen all in the same paragraph. You pegged the fact that those people mean washout when they talk about mechanical twist. The aerodynamic twist they might mention is not a twist at all, but differing airfoils from root to tip. It is just the use of a different airfoil at the tip that stalls later than the airfoil of the wing toward the root. Needless to say, that wing will have a progression of airfoils along it's span.

Keep in mind that the Reynolds number situation that's being talked about is based on the different RN a tip with shorter chord will have versus the RN of the rest of the wing. That creates a disadvantage for tips and helps cause the "tapered wings stall from the tips" result.

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