Conference vmszoo::rc

136.0. "Need Help on Dihedral" by LEDS::LEWIS () Tue Apr 21 1987 10:46

    Has anyone out there built the .40 size CAP-21 from the Great Planes kit?
    I joined the wing panels last night and found that the dihedral
    you get using the tapered rib is a little over an inch, while the building
    instructions call for 3/8" of dihedral.  The full-size plans also
    seem to disagree with the building instructions.  The tapered rib
    shown in the full-size plans matches the actual tapered rib I got,
    and if you calculate how much dihedral you should get using this
    rib it comes out to 1.15" (assuming outer rib of each wing half
    is perpendicular to the wing centerline).
    	The way I measure dihedral is by putting the wing upside-down
    on a flat table, measure from the table to the center of the
    wing at the middle, and subtract half the thickness of the wing at the
    tips (where they are contacting the table).  I haven't had a problem
    with this measurement technique on other planes, and took a lot of
    care making the outer ribs perpendicular to the wing centerline.
    	I'm wondering if the .40 size needs more dihedral than the .60
    for more stability, and they just forgot to change the number in
    the building instructions.  I roughly measured the dihedral on a
    picture of one (in that Great Planes poster with the CAP-21 flying
    inverted) and it looked more like the 3/8".  That is probably the
    .60 size model, however. 
    
    The question is whether to believe the building instructions or
    the full-size plans and supplied wood.  I'm inclined to believe
    the latter, but more than an inch of dihedral on a 60" wing seems a bit
    high for an aerobatic plane.  I'm getting ready to try contacting Great
    Planes (anybody got a number for them?) but thought I'd ask around
    first.  Thanks in advance for any help!
    
    						Bill Lewis
    						SHR1-4/B10
    						LEDS::LEWIS
    						DTN 237-3673	
                               

    Great Planes has a number to call for technical help on their
    kits. If you can't find it I will look it up. I called them
    once and they were very helpful.
    

    I talked to a guy at Great Planes, he said that in this case they
    meant that with the wing inverted on a table, you should be able
    to get a 3/8" block under the wing at the center.  They took the
    taper of the wing into consideration to arrive at the 3/8" number,
    which is what screwed me up.
    							Bill