07-02-2013: 1.5 hr
While waiting for the canopy skirt brace, I worked on the engine mount. I first enlarged the hole on the up right corner (looking aft) to 3/8", then insert a bolt, and the engine mount. While making sure the other bolt holes on the engine mount can cover the predrilled holes on the fire wall, I drilled the rest of the engine mount holes on fire wall using engine mount as a guide. All went well except the lower right hole (looking aft). I don't have a drill bush, so I just hand held the 5/32" drill bit to drill the undersize hole, then enlarge it gradually to 3/8". When drilling the lower right hole, I must have tilted the drill bit so the hole is inclined and exited the engine mount weldment on the fuselage slightly above the 3/8" on the engine mount. When drilled to final size, the hole looked slightly enlongated (no all the way though, just slight inclined).To make sure the slight enlongation is ok, I modeled the engine mount using FEM assuming the tube thickness is 0.063" for OD =0.75" and OD =1" (for the bigger cross tube between landing gear sockets). The landing gear socket is modeled assuing OD=1.6", t =0.125" (and various thickness to see its effect on the reactions of fasteneres). in one run, I modeled the lower right fastener with noraml constraints (fixed in all three degrees of freedom), and in the other run I released the constraint in vertical diretion on the lower right fastener, simulating enlongated hole. To my suprise, the slotted hole will help reduce the fastener loads. With regular hole, the top fastener will see about 5000 lbs shear acting upward on the engine mount, under 2g down and 0.5g aft loads (assuming gross weight of 1800 lbs), bottom corner fasteners see about 8000 lbs shear. While with slotted bottom right hole, the shear load on the top fastener on the right side is reduced to about 3000 lbs, acting downward on the engine mount. This is because the upward and afterward forces on the tire are trying to bend and rotate gear socket, but the tube connecting the socket to the up right corner is holding the socket from rotating, adding a lot downward force at the upper right corner fastener, and this force is bananced by the upward reaction force on the engine mount. The vertical bar connecting upper and lower fasteners is not seeing any load because the fasteneres are constrainted in all three degrees of freedom.
If the lower right hole is slotted, thus can't transfer load in vertical direction, all the upward force from tire is directly transfered to upper hole through the vertical bar (about 6600 lbs under 2g down and 0.5g aft load). The combined effects of the vertical tube and the tube connecting upper right hole to gear socket resulted a reduced reaction on the upper right fasteners, though reaction direction changed from upward to downward. However, the slotted hole will put the vertical tube under compression (about 6600 lbs force). Assuming tube OD = 0.75", t = 0.063", and length is 20". The buckling load is caculated to be 23926 lbs, way more than compression load in the tube.
So, I am not worried about the slight enlongation of the hole. But I definitely will keep an eye on it to see any looseness.
A detailed analysis, and the effect of assumptions, can be found here.
