On Sun, 2002-04-14 at 20:05, Simon Fowler wrote: > On Sun, Apr 14, 2002 at 12:14:54PM -0700, Andy Ross wrote: > > David Megginson wrote: > > > According to one of my flight-school texts, high-wing aircraft don't > > > usually need much dihedral on the wings because of the keel effect -- > > > most of the weight of the plane is below the wings and thus the plane > > > will naturally tend towards level. > > > > Actually, this is a myth. :) > > > > The wrong assumption that causes it is that the "lift" force always > > points up, away from the ground. It doesn't, it always points "up" > > from the wings, through the c.g. of the aircraft. By definition, it > > can't possibly produce a torque. > > > > Here's a gedanken experiment to prove it wrong: draw an aircraft from > > the front, in level flight. Now draw the wing lift vector; it goes > > straight up through the c.g. Now draw a banked aircraft. Draw the > > aerodynamic force vector that causes the "righting moment". If it > > doesn't pass through the c.g., you've drawn an air flow that somehow > > "knows" whether it is flowing around an aircraft in a bank or not. > > Air isn't that smart. :) > > > > I'm actually fuzzy on the real reason that high-wing aircraft have a > > built-in dihedral-like effect. It has something to do with the fact > > that while in yawed flight, high wing aircraft have a low-pressure > > region on the lower surface of their wings, producing a rolling moment > > "away" from the yaw (and thus away from the direction of an > > uncoordinated turn). Low-wing aircraft have the opposite effect. > > Like I said, though, I'm fuzzy on the details; so YASim models neither > > effect. Dihedral is much more susceptible to quantitative treatment. > > > Take a look at http://www.monmouth.com/~jsd/how/htm/roll.html . . . > > And I have one question: isn't the point where the lift vector acts > defined by the external shape of the aircraft, Yes. For low speed wings its usually around the quarter chord. Adding in the lift produced by the fuselage and tail changes it a bit.
> whereas the center of > mass is defined by the mass distribution? In which case, what's to > stop the two from being in different places? They usually are. > And if they are, and > you roll the aircraft around it's c.g., wouldn't you get the > components of the lift vector generating torque around the c.g.? A pitching moment is the usual result. A rolling moment can be generated by airplanes with wing dihedral, but this doesn't really have anything to do with the cg vs aerodynamic center location. > In > a high wing aircraft where the lift vector acts at a point above the > c.g., the vertical component of the lift would tend to counter the > bank, though I'm not sure if it'd be balanced by the equivalent > effect from the horizontal component . . . > And I imagine any forces > due to this would be vastly overshadowed by the aerodynamic forces. > > Am I missing something here? > > Simon > > -- > PGP public key Id 0x144A991C, or ftp://bg77.anu.edu.au/pub/himi/himi.asc > (crappy) Homepage: http://bg77.anu.edu.au > doe #237 (see http://www.lemuria.org/DeCSS) > My DeCSS mirror: ftp://bg77.anu.edu.au/pub/mirrors/css/ -- Tony Peden [EMAIL PROTECTED] We all know Linux is great ... it does infinite loops in 5 seconds. -- attributed to Linus Torvalds _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
