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Old Feb 09, 2011, 03:39 PM
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Swept wing- leading edge angle vs 25% chord angle

http://en.wikipedia.org/wiki/Swept_wing
Quote:
The angle of sweep which characterizes a swept wing is conventionally measured along the 25% chord line. If the 25% chord line varies in sweep angle, the leading edge is used; if that varies, the sweep is expressed in sections (e.g., 25 degrees from 0 to 50% span, 15 degrees from 50% to wingtip).
??? Is it just me, or does that excerpt not make sense. It seems to imply that the chord line can vary while the leading edge angle does not.

I also need confirmation or additional explanation for these conclusions which I have arrived at:

Definition of sweep vs the effects of sweep. Sweep angle is measured using 25% chord line it seems, however all of the effects of swept wings seem to be a result of leading edge angle, except for increased torsion from an angled 25% chord line and spar.

It makes no difference for a constant chord wing since leading edge angle and 25% chord line angles since they are always the same. But for tapered wings things change

1. A tapered wing with zero 25% chord sweep inherently resulting in a swept leading edge
It will have a swept back leading edge and therefore experience the span wise flows that increase vulnerability to tip stalls or help equalize lift distributions between the overworked root and underworked tips depending on severity of taper which affects severity of sweep. But it will have none of the spar complications or torsion associated wings that have more sweep since the 25% chord line is still

2. A tapered wing swept forward just enough to have an unswept leading edge
It will have the spar torsion complications of swept wings since the 25% chord line is now swept but experience greater structural efficiency than constant chord wings. It will otherwise experience the same aerodynamic effects of a straight constant chord wing (ie. tip stall immunity, lack of increased spanwise flows associated with swept wings, same effective AOA from root to tips except for effects of spanwise flow of regular straight wings) since the leading edge is unswept.

Wings, tapered or untapered, that are swept to the extent that both leading edge and 25% chord line are swept
Experiences the spar torsion complications as well as the effects of span-wise flows associated with swept wings, etc, etc.

Is this right? Namely 1 and 2. 3 is pretty obvious since thats the clear definition.
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Old Feb 10, 2011, 03:12 AM
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Quote:
Originally Posted by DKNguyen View Post
??? Is it just me, or does that excerpt not make sense. It seems to imply that the chord line can vary while the leading edge angle does not.
It seems to make perfect sense to me. If the trailing edge is not a straight line the sweep of the 25% chord line WILL vary. Where's the problem ?

OTOH I have no idea what "spar torsion complications" may mean so I'll leave the rest strictly alone.

Steve
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Old Feb 10, 2011, 04:07 AM
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So what is the sweep angle ?

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Old Feb 10, 2011, 04:18 AM
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Originally Posted by slipstick View Post
It seems to make perfect sense to me. If the trailing edge is not a straight line the sweep of the 25% chord line WILL vary. Where's the problem ?
Steve
Well if the sweep angle varies then nothing is a straight line. Im reading it like this:

A. The angle of sweep which characterizes a swept wing is conventionally measured along the 25% chord line.
B. but If the 25% chord line varies in sweep angle, the leading edge [angle] is used
C. if that varies, the sweep is expressed in sections (e.g., 25 degrees from 0 to 50% span, 15 degrees from 50% to wingtip).

How is B different from A? Because if the angle of the chord line varies, then so will the leading edge angle. So you'd just jump straight to C.

Quote:
Originally Posted by slipstick View Post
OTOH I have no idea what "spar torsion complications" may mean so I'll leave the rest strictly alone.

Steve
I mean that the spar has a twisting force at the root on swept wings.
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Old Feb 10, 2011, 05:13 AM
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You have to be very clear about the geometrical sweep in a wing (single or compound) and the aerodynamic implications of the sweep(s).
I would agree that the definition quoted is not very clear. When you do aerodynamic studies (academic) it is the convention to use the 25% chord line as a reference, even in a multi panel wing. But this is not cast in concrete and when doing design for a wing it is actually up to the designer to take whatever reference he is happy with as long as it is clearly defined. The choices might be dictated by ease of defining the reference or to simplify some calculations/simulations. The softwares I use give me the liberty to choose in most cases.
Now the aerodynamic implications of the sweep (or sweeps) in a wing, beside the obvious lift and drag modifications, are in the torsional moments induced by the sweep(s). The resulting loads in the spar are directly linked to this AND the relative position of the spar in the wing.
So, the torsional load at wing root in the spar (I suspect you refer to it as the wing joiner as well) is going not only to be influenced by the sweep but as well by the position of the spar regarding the wing geometry AND the wing section(s) used. Normally the wing section moment coefficients are given for the 25% chord point. Hence the (relative) simplification of the problem by using a 25% chord sweep. The positioning of the spar is a bit more complex as it is not only linked to the Cm0 but as well by the position of the max thickness, its value v.s. the A/R and any other constrain attached to any specific design.
I hope it is clear enough without going into too complex and long explanations.
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Old Feb 10, 2011, 01:45 PM
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Or the the oddball attached below.

It's probably fair to say that the description that uses the leading edge angle only on a compound shaped wing is something you'd find in the sales and marketing blurb sheets while the segmented 25% chord description would be what the aerodynamics department used when determining the wing forces and other somewhat more important aspects that keep the plane in the air.


If the spars are place in the right spot of the wings they will not see a torsional load related to merely holding up the model other than at the center line where they join. At that point because the wing's lift on a highly swept wing is located well behind the center joint the wings will have a goodly amount of torsion being induced into the connection to the fuselage since the balance point of the loaded fuselage will obviously need to be located back near the wing's center of lift. So obviously that needs to be addressed. But the wings themselves will not see this torque load other than at their roots where the forces all connect to each other in complex ways depending on the design.

For small sweep angles such as a tapered wing where the trailing edge is straight across and the leading edge tapered back the sweep angles and effect on the wing is so small in comparison to other things when it comes to model related structures that you can totaly ignore any torsional effects at the fuselage and in the wing. Only if you're doing something such as a scale airliner model or a B52 model would you want to take such forces into account.
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Old Feb 10, 2011, 05:06 PM
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Definitions and spar issues aside,

the aerodynamic implicatons (such as the increasing effective AOA for rearward swept wing segments due to leading edge span wise flows) are dependent on the angle of the leading edge, correct? And not the angle of the 25% chord line?

Beecause from my tapered wing example you can see that you can have no leading edge sweep while still having a swept 25% chord line. You can also have a swept leading edge with an unswept 25% chord line.
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Old Feb 12, 2011, 01:04 AM
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Quote:
Originally Posted by DKNguyen View Post
Definitions and spar issues aside, the aerodynamic implicatons (such as the increasing effective AOA for rearward swept wing segments due to leading edge span wise flows) are dependent on the angle of the leading edge, correct? And not the angle of the 25% chord line?

Beecause from my tapered wing example you can see that you can have no leading edge sweep while still having a swept 25% chord line. You can also have a swept leading edge with an unswept 25% chord line.
Correct, the 25% line is simply a datum.
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Old Feb 12, 2011, 11:41 PM
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The 25% chord line is the one that counts. That line is far more than just a datum line for reference. That's because the 25% chord point is where the lift is centered under the new(er) format where the center of lift is located at the 25% point and some airfoils have positive or negative pitching moments added to that.

If the wing is tapered such that the leading edge is swept back and the trailing edge is swept forward such that the 25% chord line is straight then the wing has no sweep angle at all.

If the leading edge is straight but the 25% line is swept that implies that the wing is tapered with a straight leading edge and a swept forward trailing edge which generates a swept forward 25% chord line. To have a swept BACK 25% line with a straight leading edge would require an inverse taper with the tips wider than the root.
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Old Feb 13, 2011, 04:58 PM
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I am on vacation but the article and accompanying file called Sailplane Calc will shed alittle more light on this subject.

curtis
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Old Feb 14, 2011, 01:18 PM
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Quote:
Originally Posted by BMatthews View Post
The 25% chord line is the one that counts. That line is far more than just a datum line for reference. That's because the 25% chord point is where the lift is centered under the new(er) format where the center of lift is located at the 25% point and some airfoils have positive or negative pitching moments added to that.

If the wing is tapered such that the leading edge is swept back and the trailing edge is swept forward such that the 25% chord line is straight then the wing has no sweep angle at all.

If the leading edge is straight but the 25% line is swept that implies that the wing is tapered with a straight leading edge and a swept forward trailing edge which generates a swept forward 25% chord line. To have a swept BACK 25% line with a straight leading edge would require an inverse taper with the tips wider than the root.
I understand if wings are considered to be unswept if the chord line is unswept, regardless of the leading or trailing edge sweep (due to taper). But what about the effect of increased AOA towards the rear of the wing (due to air following the leading edge causing the stagnation point to move further underneath the wing). That seems to only be dependent on the sweep angle of the leading edge itself.
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Old Feb 14, 2011, 03:03 PM
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The stagnation point moves back based on the angle of attack but that has nothing to do with leading edge taper or "sweep" if you want to call it that. The stagnation point alters based on the local conditions of the wing. And on regular wings of finite span the tip vortex effect forces a change in the angle off attack over the span of the wing. This effect is shown in the span wise lift distribution curve. So while there may be some effect of this sort it's due more to how the wing's taper affects the lift distribution curve. The link between the distribution curve effect on the angle of attack is real but I'd say that the effect it has on the stagnation point of the airfoil is more circumstantial than it is something of concern or significance. Now if the wing truly is swept as measured at the quarter chord line then you introduce spanwise airflow issues when flown at higher lift coefficients. That affects how the air sees the airfoils and the local angle of attack at any given position. So again the stagnation point is related to what the local angle of attack is for any given configuration flown at a given overall wing lift coefficient. So while there may be a connection again it's a circumstantial one.
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Old Feb 14, 2011, 04:30 PM
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Quote:
Originally Posted by DKNguyen View Post
http://en.wikipedia.org/wiki/Swept_wing
??? Is it just me, or does that excerpt not make sense. It seems to imply that the chord line can vary while the leading edge angle does not.
Most of that Wikipedia article on wing sweep is a disaster. There are many oversimplified, misleading, or just plain wrong statements. I'd go look for info elsewhere.
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Old Feb 14, 2011, 05:44 PM
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I see...well let me ask another question then. I think I attributed the tip stall vulnerability of swept wings because of bad memory. I was supposed to have said upwash.

I read that here:
http://www.rcsoaringdigest.com/OTW/o...4/162-HCP2.pdf

Quote:
"The air flow around a straight wing with an elliptical lift distribution is such that the location of
the stagnation point remains consistent across the semi-span. On a swept back wing, we Ūnd any
segment of the wing has an effect on the upwash of the section immediately downstream and
hence outboard from it. The stagnation point thus moves rearward along the bottom of the lower
surface, indicating an increasing angle of attack toward the wing tip. Figure 8 provides an
exaggerated illustration of this behavior on an untwisted wing. Because of wing sweep, the
effective angle of attack at the wing tip is greater than the effective angle of attack at the wing
root. Its little wonder the wing tips are proportionally overloaded and subject to stalling."
Let me get two basic things straight if they are not correct. With regards to swept wings having increased AOA for rearward segments, for constatn chord wings:

-no sweep will underwork the tips (spanwise flow reduces AOA at the tips...or was that the tip vortex?)
-too much sweep will overwork the tips (upwash from the forward wing segment increases AOA)
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