Conference vmszoo::rc

    
    That _is_ the best way to minimize weight and get the best flying
    characteristics.  I've been too lazy in the past to do it on any
    of my powered planes, but I've heard it's the right thing to do.
    
    I'd guess that just matching the weight/shape of the ribs and sheeting
    in each wing would net as much benefit as one would be able to notice,
    without any computer program.
    
    Bill

    Its been 10 years since I did any mechanics - and even then not
    too well, but here's a reason for equalising the rolling moment of the
    wings, in addition to their weights.
    
    Say you have one wing heavier than the other. To trim the aircraft,
    you stick on a lead lump on one wing such that the 'sum of the moments'
    of the two wings are equal; to you and me: the aircraft sits on
    an even keel; its balanced.
    
    Intuition says, the best place to add the smallest amount of weight
    to achieve this even keel is on a wing tip; and this is more or
    less correct. Until you come to fly the glider. And thats where
    the problems begin.
    
    Gliders have high aspect ratio wings (long, slender, graceful -
    that's why I like them) which are bendy. Put the glider into a roll
    or bank, and pull out. That lead weight, all the way on the end
    of the wing tip will result in unequal rolling moments for that
    wing. If the weight is heavy enough, and the rate for roll great
    enough, it is possible for this to break or damage the wing.
    
    To reduce the effects of this force, you could make the weight heavier
    and move it closer to the fuselage. But adding unnecessary weight to a 
    glider requires commital to an appropriate institution.
    
    But there is worse! An evenly distibuted load (i.e. weight) behaves
    in a fairly sane manner. Concentrated masses (i.e. lead weights)
    can cause rather nasty and unpredictable modes of vibration. And
    guess where is, practically, the worst place to stick a weight?
    You got it, on the end of a slender, bendy, wing. You can try this
    out for yourself. Take a long thin piece of balsa or spruce (say
    1/8" x 1/4" x 2'). Hold one end and flap it up and down. Now perform
    the same thing with a lump of plasticine or lead weight on one end. Now
    hold the strip in the middle and make like you are quickly pulling
    out of a roll. The end with the weight on it wants to keep on going,
    when th non weighted end has stopped.
    
    There are other second order affects of moments, that may not manifest
    themselves too much in gliders i.e. although the moments and weights
    are identical; the distribution isn't. As I can't even remember
    the name of these affects, I don't think you need lose sleep over
    them.
    
    Angus
                                         

    Angus is right; the rolling forces go up linearly with radius, but, if
    I remember correctly, the moment of inertia goes up by the square of
    the component radius.  I hadn't thought of the effect in flight. 
    
    The reason for asking about a program is this:  given enough ribs
    and given enough sets of identical ribs of differing weights, there
    are a zillion ways to get a net-weight balance --- with a potentially
    different rolling moment associated with each of the zillion ways.
    The extraordinarily heavy ribs can't always be put near the root.
    I _could_ do the calculations by hand, but I probably won't.  And
    this wing will be built before I get around to writing a program.
    
    There is another alternative.  Given that each rib has been marked 
    with its current weight, I could now go back and cut lightening holes 
    in the very heavy ones.  The net-weight exercise already ensures
    that I won't have two such ribs in succession.     Comment?

    Thats its! Thats it! Re .3 Moments and 2nd moments of inertia. What
    'orrible memories it brings back.

    Geezzzz, remember guys these are toy airplanes....NOT rocket
    science!!!!
    
    I ended up with a nickle on one end of the 10' wing in my pantera.  I
    don't notice a thing.

    Don't go overboard with this!  If you sort your heavy and light ribs
    and use them equally on both wings, you will only have to add a very
    slight amount of weight to balance your wing.  I add it at the tip. 
    The worst I ever had to do was add two medium sized washers at the tip
    on a 40 size power plane.  Wait til you're at the highest level of
    competition where every little bit counts before you spend an evening
    with your wood sorting.  Then you would throw the heavy ones away and
    make new ones instead of sorting them.

    Well, I had all the data, so I did calculate the rib contributions to
    the left and right roll moments.  I had already assigned the ribs so
    as to equalize the net weights to better than one percent (by chance).
    
    Using this simple assignment without any changes, I calculate that [all
    else being equal] the wing could be balanced with one eighth of a dime
    at one tip.  (That's about 11 grams, not a penny. :-) )
        
    I won't be cutting any weight reduction holes or writing any program. 

How much Titebond per gram??