Looking at a simple vertical shear moment diagram for a wing fuselage arrangement like the KR(treating the spar as a beam) with a questionable but perhaps not unrealistic assumption that the vertical load on the two longerons is equal (this assumes the weight of the plane is centered within the fuselage including the pilot or the fuselage structure equalizes the loads) and the wing loading is symetrical, the vertical shear force on the spar inside the fuselage is zero and the bending moment is constant and at its maximum across the entire width of the fuselage.
The vertical shear force is zero because of the assumption that each wing provides half the lift which is half the weight of the plane X G factor and that load is transferred to the fuselage structure at the longeron. ref: section 5-8 Shear and Moment Diagrams, Mechanics of Materials, 3rd Ed. Higdon, Ohlsen, Stiles, Weese and Riley, John Wiley & Sons, NY, copyright 1976 ----- Original Message ----- From: "Stephen Jacobs" <ask...@microlink.zm> To: "'KRnet'" <kr...@mylist.net> Sent: Saturday, July 03, 2004 3:24 PM Subject: RE: KR> Drilling Holes in Spars - stress riser? > I can see where this would be needed in what I would assume would be the > working part of the wing ( mostly outside the WAFs ). > > +++++++++++++++++++++++++ > > In the interest of us non-structural types getting the facts straight > and not breaking our necks: > > I would have thought that the further outboard we go, the lower the > bending loads on the spar - thus the lower all the stresses > (compression, tensile and shear). Is that not why the spar can be > tapered? > > Surely the point of max stress for any beam or truss is point where the > load works through the longest moment arm - in our case, right in the > centre of the airplane (for spars). > > I can see that the fuselage structure may have a minor influence on the > spar stress analysis but I figured the fuselage is not contributing > anything meaningful, it is designed for structural integrity in its own > rights (mainly for/aft axis) with the gussets and doublers in the spar > /fuselage area being there to keep the fuselage attached to the wing. > > If anything, the spar is a significant contributor to the fuselage > strength. > > Sorry to keep tugging at this one, but it is important in my > application. I want to put the elevator push-rod through the rear spar > and the rudder cables down the centre through both spars. That way all > the controls go down the middle, the antenna (RF) leads all go down one > side (over the spars) and the electrical power supply wires have max > separation by going aft way over on the other side. Hopefully that will > keep the "noise" from strobes /beacons /pumps away from the antenna > leads. > > The input from Donald R has me convinced that I will be Ok with a small > hole in the centre of the front spar (Just enough for two rudder > cables); plus a hole through the rear spar (enough for two cables and > the push rod) both holes with tapered (feathered) reinforcing rings both > sides of each spar. > > Take care > Steve J > > > > _______________________________________ > to UNsubscribe from KRnet, send a message to krnet-le...@mylist.net > please see other KRnet info at http://www.krnet.org/info.html > >
