737eng
Aerospace
- Oct 30, 2003
- 89
This post is similar to one that I posted in the Aircraft Engineering forum, however, in this post I am only concerned with the drag issue, not the structural integrity and durability.
I have been working as a Liaison Engineer in the airline world for a good while. I always knew that adding external "scab" patches to the fuselage induced drag (I believe it is called Excrescence Drag) and this drag would effect the overall fuel burn of the aircraft. However, due to time constraints, ease, internal structure, thickness of skin, etc..... we have always opted to install external scab patches. In doing so, I always tried to install a "clean" external patch with aerochamfered edges, flush rivets, aeroseal, etc. However, in today's environment, where the fuel cost is so High, I have been recently questioned about the use of these external repairs, especially external repairs that utilize button head fasteners. I have explained that due to the thinner skin pockets and the use of thinner doubler and triplers, that in some cases, button heads are req'd to prevent knife-edge conditions and to improve Fatigue characteristics, etc... Even the B737-CL SRM fuselage skin repairs install a 0.032" thk doubler with a 0.050" thk tripler with all button heads.
Assuming that an internal repair is physically impossible to install. One idea is to install a stepped doubler/tripler configuration allowing the thin doubler, which ends on the thin skin, to extend one fastener row past the tripler. Install button head rivets in this outer row common to the doubler (must be button heads since doubler is too thin to install countersinks). And then gage the tripler to install countersunk fasteners. Thus, only the outer fastener row would have button heads, the rest of the repair would have flush fasteners installed. (typical install would have a 0.032" doubler with 3/16" dia button head rivets in the outer row, with a 0.063" or 0.072" thk tripler with 3/16" dia flush fasteners installed through the remainder of the repair). Would this benefit the aerodynamics and reduce the drag, or would the button heads in the outer row, already trip the boundary layer and negate the installation of the flush heads in the remainder of the repair?
I have been working as a Liaison Engineer in the airline world for a good while. I always knew that adding external "scab" patches to the fuselage induced drag (I believe it is called Excrescence Drag) and this drag would effect the overall fuel burn of the aircraft. However, due to time constraints, ease, internal structure, thickness of skin, etc..... we have always opted to install external scab patches. In doing so, I always tried to install a "clean" external patch with aerochamfered edges, flush rivets, aeroseal, etc. However, in today's environment, where the fuel cost is so High, I have been recently questioned about the use of these external repairs, especially external repairs that utilize button head fasteners. I have explained that due to the thinner skin pockets and the use of thinner doubler and triplers, that in some cases, button heads are req'd to prevent knife-edge conditions and to improve Fatigue characteristics, etc... Even the B737-CL SRM fuselage skin repairs install a 0.032" thk doubler with a 0.050" thk tripler with all button heads.
Assuming that an internal repair is physically impossible to install. One idea is to install a stepped doubler/tripler configuration allowing the thin doubler, which ends on the thin skin, to extend one fastener row past the tripler. Install button head rivets in this outer row common to the doubler (must be button heads since doubler is too thin to install countersinks). And then gage the tripler to install countersunk fasteners. Thus, only the outer fastener row would have button heads, the rest of the repair would have flush fasteners installed. (typical install would have a 0.032" doubler with 3/16" dia button head rivets in the outer row, with a 0.063" or 0.072" thk tripler with 3/16" dia flush fasteners installed through the remainder of the repair). Would this benefit the aerodynamics and reduce the drag, or would the button heads in the outer row, already trip the boundary layer and negate the installation of the flush heads in the remainder of the repair?
