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Old Feb 29, 2004, 02:52 AM
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Airfoil section for swept wings

I am thinking of building a swept back flying wing sailplane. There are many designs on the web which give you a planform and then specify the wing section, dihedral and washout. Is the airfoil section specified in the direction of the airflow or is it perpendicular to the wing's quarter chord line? How do the wings perform compared to the conventional sailplanes?

Thanks

Shane
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Old Feb 29, 2004, 04:53 AM
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See:
http://perso.wanadoo.fr/scherrer/mat...h/fwairfe.html
http://www.mh-aerotools.de/airfoils/index.htm
http://www.mh-aerotools.de/airfoils/index.htm
http://www.mh-aerotools.de/airfoils/index.htm
http://www.b2streamlines.com/Culver.html
http://www.aerodesign.de/english/profile/profile_s.htm
http://www.b2streamlines.com/Panknin.html
http://www.b2streamlines.com/winglinks.html

The pitching moment requirements of airfoils or combinations of airfoils for efficient flying wings do not have high maximum lift coefficients. Thus, flying wings require lower wing loadings for low speeds and thermalling applications. The flying wing configuration does well for slope soaring applications where most of the flying is done at lower coefficients of lift. There are two main advantages of flying wings. They have low parasitic drag and they have low wing bending if the mass is distributed similar to the lift distribution.

Swept back flying wings have much larger torsional loads than unswept wings. Because of differences in bending loads and torsional loads, swept back flying wing structures are different from unswept wings and the same structure will not work as well in both.

The design of swept back flying wings is more complex than for tailed configurations because every design decision has more interaction with other design decisions than for tailed configurations. In other words each one of the following affects all the others: CG, trim, airfoil, sweepback angle, twist, taper, lift distribution, and aspect ratio.

Bending along the length of a swept back wing panel results in a change in the aerodynamic twist as well. In swept back wings stiffness in both torsion and bending rather than strength are usually the controlling structural considerations.
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Old Feb 29, 2004, 07:21 AM
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Thanks Ollie, you are a mine of information. Of course, you realise that I want to try a little of everything in our aeronautical buffet so I will press on with it nevertheless. Back to my embarassingly elementary question, is the airfoil section to be taken as perpendicular to quarter chord line or is it the section in line with the airflow?

Thanks

Shane
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Old Feb 29, 2004, 09:06 AM
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The vector of the air velocity can be resolved into a component at right angles to the 25% chord line and a spanwise vector. Neglecting frictional drag, the lift and drag are produced by the velocity perpendicular to the 25% chord line and the airfoil should be oriented accordingly. The spanwise vector does nothing but produce some small frictional drag. Since the effective velocity vectors are not parallel to each other, where the 25% chord lines meet at the center of the wing, there may be a middle effect which decreases lift and increases drag in that region. The Horton brothers designs added a bat tail at the wing center in an attempt to curve the 25% chord line from one panel to the other across the centerline in an attempt to reduce the middle effect . The Swiss SB-13 curved the leading and trailing edges in planform to attempt the same reduction in middle effect.
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Old Feb 29, 2004, 10:25 PM
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Thanks Ollie, so the selected airfoil section is applied to the cross section of the wing perpendicular to the quarter chord line. Intuitively, I thought it would be the airflow direction but I remembered a thread sometime ago and Dr Drela answered as you did. It was a case of Intuition vs Memory ....

I like the bat tail ...it puts some volume where equipment needs to be housed. I'll probably build a plank first. After the Allegro Lite I need an easy build to get the momentum going again...

Thanks

Shane
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Old Feb 29, 2004, 11:31 PM
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I heard it first from Dr. Drela, too.

The Panknin twist formula is the easiest to build and results ina wing with built in trim and stability but with a small adverse yaw problen to be solved with a drag producing scheme.

The Culver lift distribution results in a wing with minimum induced drag but serious adverse yaw and even more drag to overcome it.

The Horton lift distribution results in no adverse yaw and the poorest induced drag but some induced lift to partly overcome the higher induced drag.

The plank configuration requires a large vertical tail volume coefficienjt that results in considerasble parasitic drag unless a long tail boom is used.

All require a lower wing loading than a tailed configuration to achieve the same sinking speed.
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Old Mar 02, 2004, 12:09 AM
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Ollie,

I've read (and re-read) your answers here with great interest. But there's something I'm not quite getting.

Are you saying the airflow over a swept back flying wing is not even close to longtitudinal (free stream direction)? You say the airfoil should be oriented perpendicular to the 25% chord line, but does this imply the airflow is close to perpendicular to this line, ie. in towards the centre on the wing?

I understand that the velocity vector can be resolved into the two components you mention, but surely the resultant vectors over most of the wing are going to be close to the free stream direction. If anything, the flows (in plan view) over and under most of the wing should spill out towards the tips except perhaps on the top near the tips where wing tip vortex formation may direct the flow inwards and back.

Therefore, for design purposes shouldn't the airfoil be taken longtitudinally?

If you think I've misunderstood something please say so, otherwise can you please elaborate further so my understanding can catch up.

Graham.

BTW, on your recomendation, I recently purchased (through Amazon) Martin Simons Model Aircraft Aerodynamics, it's a great book and I'm wading my way through it and learing heaps.
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Old Mar 02, 2004, 01:27 AM
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Here is a thought experiment.

Take a wing and put it in a wind tunnel and orient the wing so the 25% chord line is aligned to the free stream direction of the air flow. The wind tunnel velocity is set at 7.07 meters per second. How much lift is produced?

Now reorient the wing so that its 25% chord line is perpendicular to the free stream direction and the angle of attack is 10 degrees above the zero lift angle of attack. The wind tunnel velocity is still 7.07 meters per second. The wing spans the width of the tunnel. How much lift is produced?

Lastly, increase the air velocity to 10 meters per second and orient the 25% chord line to 45 degrees relative to the free stream velocity and the angle of attack of the wing at ten degrees relative to the component of the free stream velocity perpendicular to the 25% chord line. The wing spans a narrowed tunnel such that the area of the wing is the same as in the second case above. How much lift is produced?

If a vector, V, is analyzed into two components, v1 and v2, and the effect of v1 is zero, then the effect of V and v2 are equal.

BTW, in a windtunnel where a wing spans from one side wall to the other, the vortices are cut at the walls and the flow over the wing is very close to two dimensional and, in the direction of the free stream when viewed in plan. It is just that one component of the flow is effective and another component has virtually zero effect.

In this description I am ignoring the very, very small decrease in velocity in the 25% chord line direction due to friction.
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Old Mar 02, 2004, 07:30 AM
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Here are some additional thoughts.

As indicated in the previous post, the component of flow in the spanwise direction produces no lift. The flow in the free stream direction produces the same lift that its component in a direction perpendicular to the spanwise direction produces.

If you want to use wind tunnel airfoil data that was taken with the wing spanwise direction perpendicular to the free stream, then you should use the component of the free stream direction perpendicular to the spanwise direction.

If you orient the airfoil to the free stream direction for a swept wing it has the effect of increasing the percentage thickness and percentage camber of the airfoil by the ratio of the chord in the free stream direction to the chord perpendicular to the 25% chord line.
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Old Mar 02, 2004, 08:22 AM
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I guess the confusion arises because model airplane plans give the rib sections in line with the airflow and then go on to identify the section as XXXX0009. We then adopt the assumption that the rib section is analogous with the airfoil section. As most designs have the quarter chord line perpendicular with the airflow it makes no difference most of the time.

Ollie, you said,

"If you orient the airfoil to the free stream direction for a swept wing it has the effect of increasing the percentage thickness and percentage camber of the airfoil by the ratio of the chord in the free stream direction to the chord perpendicular to the 25% chord line"

Can I say that the Cl of a section is proportional to its thickness within a small range? ie XXXX thinned to 90% has a CL approximately 90% of the original section?

Shane
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Old Mar 02, 2004, 10:36 AM
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Generally speaking, above some critical reynolds number, thickening an airfoil about its mean camber line increases drag over the airfoil's useful range of angles of attack and also increases the range of useful coefficients of lift. I doubt that the relationship between thickness and coefficient of lift range is proportional in general but may be so in a particular case at a particular reynolds number. As the reynolds number decrease beyond some value, increasing the thickness decreases the useful range of coefficients of lift.

Generally speaking, increasing the camber of the mean line makes the zero lift angle of attack more negative and increases the maximum lift coefficient.

The best way of dealing with these questions is to use X-Foil to give quantitative results instead of using my qualitative rules of thumb.
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Old Mar 03, 2004, 06:35 PM
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Quote:
Originally posted by Ollie
The vector of the air velocity can be resolved into a component at right angles to the 25% chord line and a spanwise vector. Neglecting frictional drag, the lift and drag are produced by the velocity perpendicular to the 25% chord line and the airfoil should be oriented accordingly. The spanwise vector does nothing but produce some small frictional drag. Since the effective velocity vectors are not parallel to each other, where the 25% chord lines meet at the center of the wing, there may be a middle effect which decreases lift and increases drag in that region. The Horton brothers designs added a bat tail at the wing center in an attempt to curve the 25% chord line from one panel to the other across the centerline in an attempt to reduce the middle effect . The Swiss SB-13 curved the leading and trailing edges in planform to attempt the same reduction in middle effect.
AS Ollie suggests, the square of the velocity component perpendicular to the quarter cord should be used to calculate the lift, but that is only part of the impact of sweeping the wing. There is a slightly offsetting effect of an increase in the effective camber. For instance, with a 30% sweep measured from the quarter cord, the velocity component perpendicular to the quarter cord will be 86.6% of the free stream velocity (cosine(30)). But the cord perpendicular to the quarter cord will also be reduced to 86.6% of the cord parallel to the free stream. As a result, the wing appears to be 1/.866 thicker (e.g., a 10% foil will behave like a 15.5% foil, with proportional increase in camber an angle of attack.) The ultimate impact is that the lift drops off as the cosine of the sweep angle, rather than as the much higher cosine squared rate implied by lift being proportional to velocity squared.
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Old Mar 03, 2004, 07:28 PM
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Another feature of wing sweep is the section is thicker as mentioned not only aerodynamcially, but structurally.
Thin, say 10% when looking from the front, the way the air comes in, but thicker when looked at perpendicular to the leading edge, in terms of chord to thickness.
One of my bosses used to say the reason for wing sweep was to increase the internal volume of the wing, without making it thicker.
A thicker wing is also stiffer.
.
And it not precisely correct to say spanwise flow produces no lift..
When I was testing verticals on a finless tailless sloper, I found it was quite happy.. for a short time... sliding sideways in the span direction!
The controls made no difference in what happened. being , in that mode, deflecting along the flow direction, instead of perpendicular to it.
With any sort of a vertical, this "quality" went away.. thank goodness!
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Old Mar 03, 2004, 08:56 PM
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Hmmm....so spanwise flow has lift but the section is a flat plate. So if we take the airflow as having 2 components - flow perpendicular to the quarter chord and spanwise flow, and apply the airfoil section characteristics to the former and flat characteristics to the latter we have a reasonable approximation of how the wing will perform. ( Its an approximation because we are leaving out the interactions and other inconeniences)

Close enough?

Shane

PS Thanks to you guys my understanding has improved by leaps and bounds from the time I designed airfoils for control line combat and stunt planes using and ellipse template and french curves. BTW is there a name for an airfoil with an elliptical front and a sections of a circle for the rear?
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Old Mar 03, 2004, 09:06 PM
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Yes, my thinking got sloppy when I started using the term "spanwise direction" when I should have said "25% chord line direction." Its hard to be precise and unambiguous without a diagram to explain the words.
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