Swept wing dynamics

[Antonsoarer]

Bit of an interesting debate over in the slope section centered on this statement by Mark Drela:

From Drela post: http://www.rcgroups.com/forums/showthread.php?t=83663

Quote:
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.

End of Drela post.

If this is true then how does sweep provide the well understood dihedral-like effects during a side slip or yaw? :confused:

This statement also implies (for swept back wings) that root foils impinging upon a fuz will be aerodynamically terminated part way along the foil shape!! :confused:

[Sparky Paul]

I thnk the full-scales use profiles perpendicular to the (usually) front spar to have nice 90 degree joints between spar and ribs, making the structural analysis simpler, as the load bearing stuff is the simple ladder form, not the skewed parallellagram placing the ribs in the chord wise direction with a swept spar.
On the Tristar, the parts are layed out this way:

[Antonsoarer]

Thanks Sparky Paul, sort of confirms my thoughts, constructional profiles not aerodynamic profiles used to make such a wing, the aerodynamic profile is the one that results due to the sweep.

Tony.

[Ollie]

The air flow vector is resolved into two vectors. One air flow vector is along the cord at 90 degrees to the leading edge and the other air flow vector along the leading edge. The air flow (2D) vector at 90 degrees to the leading edge results in lift, drag and pitch moment due to the designed airfoil or the airfoil measured in the windtunnel. The other airflow vector (3rd D) along the leading edge has nothing to do with the airfoil.

The intersection between the wing and fuselage is accounted by the interference drag and interference lift.

See:
http://aero.stanford.edu/WingCalc.html
The AR, sweep angle, AOA, taper and twist of the wing are not referenced to a special airfoil.

See:
http://www.glide.dyndns.org/on-the-wing3/index.html
Four part article to download.

[Sparky Paul]

The airflow doesn't know about the theory... :)
It goes chordwise...

[Antonsoarer]

The airflow doesn't know about the theory... :)
It goes chordwise...


or to be more precise.... resultant-aerodynamic-chordwise :) :confused: :)

[Antonsoarer]

The air flow vector is resolved into two vectors. One air flow vector is along the cord at 90 degrees to the leading edge and the other air flow vector along the leading edge. The air flow (2D) vector at 90 degrees to the leading edge results in lift, drag and pitch moment due to the designed airfoil or the airfoil measured in the windtunnel. The other airflow vector (3rd D) along the leading edge has nothing to do with the airfoil.

The intersection between the wing and fuselage is accounted by the interference drag and interference lift.

See:
http://aero.stanford.edu/WingCalc.html
The AR, sweep angle, AOA, taper and twist of the wing are not referenced to a special airfoil.

See:
http://www.glide.dyndns.org/on-the-wing3/index.html
Four part article to download.

Thanks Ollie, plenty to read, good to see you are still helping our understanding of all things aerodynamic :)

[aussie]

Being someone who likes to visualise things, I wonder whether any of these diagrams can help shed some light on what the heck's going on...

Kye.

[Antonsoarer]

I understand the small degree of inflow on the top and the outflow on the bottom which supports the formation of the tip vortex at higher positive alphas :)

[aussie]

Well, this discussion seems to be spread over 3 threads (2 in slope forum) so aren't exactly sure where I should be posting, but this is probably the most relevent spot I guess...

I've come to grips with the vector explanation. It makes perfect sense to me as long as you accept that it's only the airflow component at right angles to the sweep of the wing that is aerodynamically significant rather than the freestream (direction of flight).

The clincher for me was that it explains why swept wings require a higher angle of attack for a given CL compared to an unswept wing. While the angle of attack is conventionally measured chord-wise (direction of flight), the effective angle of attack for a swept wing would actually be measured along the chord at right angles to the sweep (less) and probably then very close to the AoA of an unswept wing for the same CL.

It would also seem to follow that the effective "thrust" (forward component) of the lift generated by the airfoil is directed at right angles to the sweep instead of the direction of flight (the useful direction).

Perhaps Mark can chip in with confirmation.

Also, I'd be more comfortable if the effective chord of a swept wing was at right angles to the AC line (approx. 1/4 chord line) rather than the wing's leading edge... Is there a reason it should in fact be referenced to the LE Mark or were you just simplifying?

And on the side topic of how sweep provides the same effect as dihedral, I think it is best summed up in the following "On the Wing" article by Bill and Bunny Kuhlman: http://www.glide.dyndns.org/on-the-wing3/135-138_EffectiveDihedral.pdf

PS. Tony, I believe the thread you've quoted Mark Drela from is actually http://www.rcgroups.com/forums/showthread.php?t=83663

Kye.

[aussie]

Here's a helpful vector diagram courtesy of "nauga"...

[Antonsoarer]

Kye, thanks for the check on the url, corrected. I too am reasonably comfortable with most of the theories covered as they don’t all conflict with experiences to date. Your point about AOA and lift is interesting, I wonder if that corresponds to the foil thinned to the resultant section as aligned with the freeflow, :rolleyes: :) I will check in profili.

Tony.