Fuselage Skin Repairs and Drag

[737eng]

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.

My question are: (only considering external repairs, and assuming a flush repair is impracticle)
1. Would it be better to increase the thickness of the doubler and/or tripler, in order to install flush head fasteners and improve the aerodynamic characteristics of the repair? Or would the affects on fatigue outweigh this performance issue?

2. Assuming that a repair utilizes a stepped doubler/tripler configuration in order to allow a thinner doubler to end on the thin skin pocket. Would it be beneficial to leave the doubler thin and install button head rivets common to the doubler outer row, while increasing the thickness of the tripler in order to install flush head rivets? This would allow for a majority of the repair to be installed with flush rivets, with only the outer row having button heads. Would this benefit the aerodynamics, 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?

 

[jetmaker]

737eng

Some data I have with regards to Excrescence Drag for the 737NG aircraft is as follows.

For an external doubler 0.1" thick, the fuel increase is 156 gal/year/ft of frontal length. If you place a 10:1 ramp, this number drops to 9 gal/year/ft. Therefore, using a series of thinner doublers/tripplers looks like it would reduce the penalty due to the design. I believe, but am not sure as I do not have the reference on hand, that a positive excrescence has a great drag impact than a negative excrescence.

Using the button head fasteners, will add to the fuel costs depending on the head height. The amount proportional to the 0.1" thickness number over the combined head diameters. This will also be affected by the type of head shape (universal vs modified).

Finally, if the repair is towards the rear or around the fairing section of the wings/tail, there will be less of a penalty. The critical region being upto the wing trailing edge.

Finally, look at the fasteners already in use in the repair area. That will give you an idea of how critical the aerodynamics are to drag.

Regards,

jetmaker

 

[Sparweb]

Probably more information than you need to answer your question, but:

http://www.esdu.com/graphics/dataitem/90029a.htm

About external doublers, in general: We suggest to our customers that they should install doublers internally. The interior must be removed more exentsively, but the resulting repair / antenna mount / whatever, is much easier to inspect from outside the aircraft. We consider it's more important that the skin be free of cracks, rather than the doubler, and the inspectability assures this. It doesn't always work: chem milled skins make it hard to drive a flush rivet, or to make an internal doubler that spans bays.

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[CRACKKILZ]

737eng Good to hear from you.

I suggest you post this thread in the aerodynamic engineering forum also.

With regard to the use of button head fasteners, I know that this has become an industry trend especially when repairing thin skins (such as on the 737). The recent SRM changes are at least partially based on fatigue testing of various joint configurations. I believe these new recommendations are here to stay.

Also, I am sure you have also noticed the new note to contact Boeing for an aero evaluation under certain conditions (based on cumulative sq In of all repairs)? Obviously Boeing is concerned about the cumulative effect on drag with multiple repairs (of the new type) and this note is how they address that concern.

Regarding the use of internal doublers - While I agree with SparWeb re. the improved inspectibility of the critical row, I don't believe they are practical if the doubler will cross one frame or stringer bay. To maintain your original contour you would have to do a stringer splice and or a shear tie false flange repair in order to accomodate the doubler. If you use a thin doubler and taper shim the internal structure then you will change the contour - how significant is this change??? I am not qualified to say.