anurag2801... Been thinking about this for awhile...
I learned, the hard way, on many aircraft platforms, that external repairs of aerodynamically critical structures can have more effects than just excrescence-drag.
Bare with me on another ‘WKTaylor’ story.
NOTE. My experience is primarily MILITARY, so different factors are often at play relative to commercial or general aviation.
Most aircraft with heavy skins have structural wing-skin maps for upper and lower surface damage. These ‘maps’ account for stress levels that are critical for normal flight regimes. ‘GENERALLY’ upper surfaces are torsion and compression critical… with an occasional tension spike. ‘GENERALLY’ lower skins are torsion and tension/fatigue critical… with an occasional compression spike. These maps help us determine ‘blend-out limits’ before major repair… and define requirements for area and fatigue restoration during major repairs. NOTE. These maps usually assume NO internal structure damage, IE: intact stiffeners, ribs, spars, doublers, etc and fasteners.
On older aircraft where structure was more ‘robust’ due to more unknowns [poorer defined flight and internal loads, less accurate structural analysis and testing, etc], the margins for damage are ‘higher’ and structural repair blend-outs can be fairly extensive. One subtle aspect to older aircraft is the tendency for many skin sections to be spliced together which complicates analysis/design/service… but ‘simplifies’ skin replacement in a field, or more-likely, Depot environment [jacked/jigged/rigged, etc for zero loads].
On newer aircraft designs, where flight and internal loads are precisely defined and known… and the structural analysis and testing are far more ‘precise’, there are much smaller margins for repair limits [IE: reparability]. Also, due to advancements in manufacturing, newer wings tend to have ‘large/long monolithic skin-planks without multiple span-wise joint… which are virtually impossible to replace without heroic effort. In some cases reparability is almost non-existent without heroic effort.
Damage that requires blend-out-into-contour tend to be aerodynamically benign.
Damage blend-outs [or penetrations] that requires reinforcement can be done in many ways: ‘heavy’ external or internal reinforcement of the skin, only; or combined external and internal skin repairs; or ‘light-to-moderate’ external/internal reinforcement of the skin combined with reinforcement of the attaching/crossing sub-structure [stiffener, ribs, etc directly supporting the skin].
External repairs tend to be slightly thinner than internal skin repairs… but by adding in sub-structure reinforcement to skin reinforcement may be even thinner [in-theory]. Practical aspects can really mess with everything, such as accessibility without major disassembly of surrounding structure; contour, minimum thicknesses for adequate load transfer with flush fasteners; the skills/abilities of the repair-parts fabricators [machinists] and the assembly [sheet-metal] personnel; availability of tools, materials, fasteners, etc, etc…
NOW, here’s where it can get really messy.
External repairs accomplished on aerodynamically exposed surfaces MUST conform to location-step-waviness-contour deviation limits as specified in certain documents or drawings.
IF exterior repairs are located on aerodynamically critical surfaces/areas AND exceed limits for step/waviness/contour deviations, then the aerodynamics weenies/group must examine the repair before it proceeds. The aero guys earn their pay by determining ‘magnitude’ of changes… and how they would affect the aircraft on both micro [local] and macro [overall] scales. Changes to performance [drag/lift] and efficiency, controllability and control surface loading [hinge moments, etc], static trim, flow/turbulence-streamlines, Mach wave patterns [sonic shock], ice accretion, etc have to be estimated based on safety, mission, environment, degradation of other elements and even ‘the unintended consequences’ of multiple effects. NOTE: most aero guys tend to be overly conservative due to all the factors they have to consider. A VERY FEW [great] steely-eyed aero-guys tend to have a ‘real-world view’ that allows for 'slightly broader' opinion based on real-world experience... but not all.
External repairs accomplished on aerodynamically non-critical surfaces usually have MUCH broader step/waviness/contour deviation limits and wider discretion on our [liaison] part. I have seen/done crazy things in this area.
Where this gets even MORE tricky is when a combination of factors comes into play, including [in addition to pure analysis]: operational needs and over-all condition of the aircraft [existing structural repairs, paint quality, engine performance, pre-existing flight control/trim issues, robustness of the design, electrical/electronic grounding or interferences, need for special coatings, prior good/bad experiences with similar repairs, etc.
Occasionally leadership [managers] have to make gut wrenching judgments: fly with minor degradation that is ‘most likely safe/predictable’ but gets the aircraft back into the air… or fly with certain degraded flight parameters/limitations for a period of time; or don’t fly and repair restore to ‘like new’… even with great effort/time and expense.
War stories.
On several occasions I have been called-out to examine newly applied paint, that was too thick and/or too rough here-there. Most guys thought ‘ho-hum’… until I dispositioned a 100% paint strip/re-apply to spec. Too thick paint tends to flake/peel; and too rough a paint surface can have a mild-to-severe effect on drag, lift and trim [due to asymmetry]. Over the years, a smooth/even/thin ‘paint-job’ can be almost unremarkable and forgettable… but a really bad/irregular finish can cause irritating and very hard to diagnose [odd] performance and trim problems… and ‘not hitting performance numbers’ during takeoff, landing, cruise, power and fuel consumption, etc.
One worst case of finish degradation I encountered was a rotating structure with the coating intact on one side; and 50% wrinkled/sloughing on the opposite side. The aircraft was yawing/rolling in-sync with the structure rotation and driving the crew nuts and interfering with the mission completion. The only ‘repair’ solution was replacement of the ‘side’ with the degraded finish with a fully serviceable assy [Depot job].
In the field, I have designed/had-installed very carefully tailored/sculpted external repairs [W/WO internal/substructure and lots of grumbling from the technicians] that have lasted to next depot maintenance… and in a few instances have been found acceptable for continued/indefinite use.
ONE case that was startling to a bunch of us older ‘structure guys’ was a supposedly ‘benign’ wing modification: a compact/robust [heavy/thick] maintenance-fitting permanently installed on/over/attached-to the upper-outer structural skin/spar/end-rib… intended solely for field maintenance purposes! The aero weenies analyzed the fitting in a local 3D flow-field. At high-cruise Mach numbers, the resulting standing shock-wave impinged-on the local thin skins and the even thinner skinned wing-tip. The aero guys described the effect of this shock wave: a very high frequency ‘buzz/screech’ that would probably beat the daylights out of the thin skins and ribs! A lot of additional resizing/sculpting and re-locating and alternate reinforcement was required for that maintenance fitting [don’t get me started…].
One major skin repair [fighter, massive corrosion grind-out depression] was almost impossible due to multiple complicating factors. The only possible repair [depot designed/installed, NOT a 100% skin replacement] included a combination of integral stiffener reinforcement and an external low-height [inverted wedding-cake] boron-epoxy patch… which had to be built-up and pre-cured to that unique contour-depression... then hot-bonded-in-place.
More Fud-4-Thot...
In many cases external repairs require special attention to leading edges and the fay surfaces: the paint and sealant MAY erode/peel due to impingement by rain abrasive/dusty air and effects of structural flexure... which usually leads to bared [corroded] metal and pressurized-moisture wicking into exposed fay surfaces.
External repairs MAY [in rare instances] lead to increased probability of a lightning strike and corona/static build-up… with their associated problems. Certainly low resistance electrical bonding-grounding between the repair and the structure is a serious requirement.
External skin repairs on integral wing tanks MUST be designed for maximum structural efficiency and durability while employing good sealing techniques. A leaking repair is a sure sign of a poorly executed repair.
Regards, Wil Taylor
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o We believe to be true what we prefer to be true. [Unknown]
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