talk directly to someone on the certification side ... either a DER or your company's certification delegates or your local airworthiness certification people.

there are So many issues this isn't the right forum. An increase in drag affects field performance, FAR36, etc. Though I'm surprised that a well executed upper skin repair causes a measurable increase in drag at all.

another day in paradise, or is paradise one day closer ?

Just out of curiosity, what was the reason a flush repair could not be accomplished? I'm assuming since you mention "extensive corrosion" that it was too widespread to consider trimming the skin out. Still, if the corrosion depth was only a slight exceedance of allowable and the problem was the surface area affected, leaving the outer surface with an SRM-type blend profile and installing internal repair parts may have had a lesser affect.

There are no real OEM designs that you can compare to to say this is OK or to use to generate a knockdown since as I mention, usually internal repairs are the standard. But that doesn't mean it's not good. Structurally it won't really make a difference, just as rb1957 says, I would get a DER on the horn.

Specifically an Aerodynamics DER. Search google for "FAA DER Directory" and you can find a list of aero DERs by state. They will know exactly which regulations you need to comply with for post production structure affecting aero.

Sorry, don't want to lead anybody astray. Could've sworn there was a designation specifically for aerodynamics, but maybe what you should look for is a "flight analyst" DER. They can deal with data related to performance.

I would say though that if your team is saying the drag is affected, that indicates to me there are some aerodynamic loads on the doubler structure which should be considered in your static and DT analysis. Probably would be very small though.

Keep em' Flying
//Fight Corrosion!

The ceiling height (cabin altitude) to be reduced by 190ft."
Ceiling height and cabin altitude should be two mostly unrelated things. A reduction in ceiling height, increases cabin altitude.

A couple of things
Max ceiling height isn't always due to lift / drag limitations.

Assuming it is 40,000 ft or so range, 150 ft is 0.4%, i seem to recall a FAR giving tolerances on performance figues (can't remember where). So that leads to another question does the SRM require total area of external doublers to tracted.

rb1957, I guess you are this my not appropriate forum to discuss this topic. But I was wondering how to address these performance limitations. We have already performed static and fatigue analysis, which has resulted in additional inspections program. However we are unable to address the limitations due to performance. Could anyone guide me to right forum where I can seek help?

no, forum-wise this is fine. I think there are so many performance impacts that you need to talk to a DER rather than ask the "internet". I think to get sensible answers the issue needs to be described in much more detail.

Have you flight tested this repair ?

How much confidence do you have in your CFD ? If your seeing a measurable drag from a skin doubler, I'd worry about other effects (vortices ? or is something happening that can impact controlability ?

another day in paradise, or is paradise one day closer ?

Verymadmac: I think the issue is that since this is an external upper wing skin repair, there are no guidelines for total repair area affecting performance. There are SRM guidelines for fuselage skin, sure, but in my experience with several different OEMs, the SRM typically only has internal (flush) repairs for the wing skin. Which leads me back to my original question.

Also, anurag, maybe when you say that you've already done "fatigue" you mean you've already done DTE. But just to be clear, unless you are the OEM, per 25.571 you need to do more than a fatigue analysis. True, we are on the upper skin which sees compression under normal flight loads. But you're requirements are to check damage tolerance under +2.5g and also -1.0g. The wing skin in general is subject to a lot of minor exceedances (ie a fatigue spectrum and resulting equivalent GAG cycle). For this you should be showing a spectrum factor on your 1g stress and running crack growth.

You're inspections must be based on either fatigue life with a scatter factor on, or the crack growth life. For this structure, I would expect the critical scenario to be based on crack growth.

Keep em' Flying
//Fight Corrosion!

Without knowning the exact details, one can only observe many possible ideas most of which will be not very helpful at all but one might just be the best thing since sliced bread.

LiftDivergence: One could horse trade the fuse drag for wing drag & issue a suppilment reducing maximum total fuse repair area, then one just has to address lift, icing and stall effects.

How much life is left in the airframe, there are typically more options that make ecomonic sense with end of life airframes. if the engines are going to out live the airframe, a token increase in mimamum engine power allowable maybe possible resolve the drag issue (unless they do things like play musical chairs with hot watch engines).

increasing engine power ? boy, wouldn't that involve a tonne of work (to restore original performance).

but as you said there are so many questions, and we don't like guessing where the OP wants to go.

I guess i'd be interested in what is causing the drag increment ? the only thing I can think of is the L/E (possibly the T/E also) of the dblr is tripping the boundary layer. if so, wouldn't a bit of care here (chamfering, PRC fillet seal wedge) help with the issue ?

another day in paradise, or is paradise one day closer ?

I find it difficult to take seriously the opinion of an engineer who writes fuse because he is too lazy to write fuselage.

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 [sub] 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

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]