Airfoils

[cft21]

I have a question about the nose down pitching moment. When talking about cambered vs. a symmetrical airfoil, do they both produce a nose down pitching moment or just the cambered airfoil?

Thanks

 

[rb1957]

airfoils, by convention, have their lift vector at the 1/4 chord point, which is close the the CofP of early NACA profiles.

moment is then created by having the CofP away from the 1/4 chord position, so any airfoil "can" have aero. moment.

 

[btrueblood]

Any airfoil producing lift produces circulation, which produces a moment. A flat plate at an angle of attack produces a moment.

 

[bf109g]

Good point btrueblood. It's common to put a symmetric airfoil on a trimmable horizontal stabilizer.

Trimming the stab (ie changing the AOA) will produce a pitching moment.

 

[cft21]

What if both airfoils are at their respective zero angle of attack?

 

[rb1957]

a symmetric airfoil at zero AoA has no lift, no moment (and i guess, theoretically, no drag ... but that looks odd !?)

a cambered airfoil would produce lift at zero incidence and quite likely moment as well.

 

[Timelord]

No drag? Tell us about your new theory, then go read up on aerodynamic drag.

Timelord

 

[rb1957]

yuk yuk ...

i was mentally following on from no lift, no moment (ie no anything). in any case, with no lift (no Cl) so no induced drag, but i guess you are left with a little profile drag.

 

[FeX32]

What if there is a little bit of turbulence in the flow. I would expect the instantaneous pressure differences btw top and bottom to produce some kind of aerodynamic lift/moment.

At what main frequency would you expect the airfoil to vibrate under these conditions?

Fe

 

[rb1957]

that would depend on a host (sory, make that a Host) of factors ... the structure of the wing, the frequency and magnitude of the forcing load, ...

i was answering the OP from a steady-state viewpoint. maybe also a 2D viewpoint

 

[KENAT]

http://www.auf.asn.au/groundschool/umodule4.html

Not sure quite how good it is as it's description of fundamentally why an aerofoil generates lift isn't the one my prof taught at uni. However, it has a section on pitching moment.

For some reason I was thinkin the wing usually generated a nose up moment, I clearly am forgetting all I learnt in aero and need to re-read my old mechanics of flight book.


KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[Kwan]

For a symetrical infinite airfoil:
moment coefficent is essentially zero for any angle of attack.

Chambered produces negative moment coefficient. Essentially constant for any angle of attack.

Now, for a total system such as an aircraft, the center of lift relative to the center of gravity does produce a moment. This is often nose down which is balanced by the horizontal stabilizer.

I suggest looking up a typical NACA airfoil lift curve.