Swept wing cutting & aerodynamics

[Fireun]

Hello,

I am trying to cut a flying wing of a swept wing style.
I have read through all the posts I could pertaining to this but
haven't found the answer to this question. It may not be that
important.

My question is:
If you cut a tapered wing that is unswept. Then you cut the root
of the wing at an angle to achieve the sweep. Will this affect the
efficiency or change the airfoil?

Would it be better to cut the block of foam with the sweep angle
first, and then put on the templates and cut the wing?

Any help is much appreciated.

Thanks!

[BMatthews]

Cutting the angles after cutting the airfoil will result in the airfoil being stretched. How much depends on the angle. If it was up to 10 degrees then the amount of stretch is negligable. The formula is L/cos a. For 10 degrees the stretch factor is only 1.5% but at 45 degrees it would be 1.5 TIMES and totally change how the airfoil works. At 1.5% we are talking about the same tolerance that we get with a few sanding strokes. Not a biggie.

Use the scientific functions in your Start/accessories/calculator to check the numbers.

[markdrela]

Airfoils are designed for airflow along their shape (chord line).

Not true. The relevant airfoil shape of a swept wing is perpendicular to the leading edge, and so that's how Boeing stacks up their wing airfoils along the span. The airfoil shape of a slice along the freestream direction is not aerodynamically significant. So the same airfoil templates should be used for any sweep.

[Fireun]

Thanks for the help!

Now there is nothing left but to get to work.

Thanks again

[BMatthews]

Originally posted by markdrela
Not true. The relevant airfoil shape of a swept wing is perpendicular to the leading edge, and so that's how Boeing stacks up their wing airfoils along the span. The airfoil shape of a slice along the freestream direction is not aerodynamically significant. So the same airfoil templates should be used for any sweep.

Oh, I guess we all learned something today. I hadn't realized that although I'd seen cutaway drawings showing the ribs placed that way. I just thought that they compensated for the change in shape.

But Mark, perhaps you could elaborate on this, I'm sure the air doesn't suddenly turn inwards by the sweep angle so wouldn't the air be seeing a longer profile?

[Sparky Paul]

All that occurs when cutting the wedge from the center to get sweep is the airfoil gets thinner, percentage wise.
I did the reverse with a NINJA core, cut it so the leading edge only sweep was changed to both leading edge swept back, trailing edge swept forward, along the 1/4 chord line.
The wing looks more like a u/c stunter now.
What this does to the Eppler 374 of the stock Ninja.. haven't the foggiest, nor is it important.
The plane flies well, both as a sloper and powered as shown.

[rakm]

All that occurs when cutting the wedge from the center to get sweep is the airfoil gets thinner, percentage wise.

Is their a way to tell how much thinning (%) occurs depending on the angle that's cut off or is the amount insignificant?

[Tom Hunt]

Originally posted by markdrela
Not true. The relevant airfoil shape of a swept wing is perpendicular to the leading edge, and so that's how Boeing stacks up their wing airfoils along the span. The airfoil shape of a slice along the freestream direction is not aerodynamically significant. So the same airfoil templates should be used for any sweep.

markdrela....

what aero-university did you go to?

Airfoils are always defined logitudinally on the aircraft, regardless of sweep.

Boieng (and many other aircraft manufacturers) position the ribs as required (sometimes normal to the LE) for structural reasons and ease of manufacturing. But they do not "define" the airfoil section that way. Computational and testing data is derived from the theoretical airfoil along the longitudinal direction and in many cases normal to the horizontal reference plane (irrespective of dihedral or negative dihedral)

A 10% airfoil on a 45% sweept double taper wing could be 11,12, 13% or more when developed normal to the LE. (sorry for the edit... I was thinking backwards for a minute.)



Tom Hunt
Vehicle Design
Northrop Grumman

[Sparky Paul]

Originally posted by rakm
Is their a way to tell how much thinning (%) occurs depending on the angle that's cut off or is the amount insignificant?
.
Measure the new chord. Compare that to the previous value.
A profile that's 1 inch deep on a 10 inch chord is still 1 inch deep on a 15 inch chord, but is now 3.3 % thinner.
One of the benefits of sweepback is it permits use of thick wing structure.. for both strength and fuel capacity (full-scale) and it thins the shape aerodynamically.