Wing to Fuselage Fairing Composite Panel - Loads

[SAITAETGrad]

So a wing to fuselage fairing has many loads - if it is a high wing, you will have technicians standing on it - there's bird strike and other discrete damage - air loads naturally. What about rapid decompression? What effect does a single bay failure have on these panels? Presumably that blows the doors off so to speak...is this a fair assumption?

 

[SWComposites]

Thse fairings are typically just that - fairings. Secondary structure. No bird strike, discrete source damage, etc. sized for pressure loads, plus step/handling loads if appropriate. Sometimes induced loads - body bending, thermal.

 

[Sparweb]

Since many fairings have access panels of one sort or another, it is safe to assume that a decompression of the cabin, blowing into the fairing, will blow the door off.
This ignores the possibility that the source of the rupture in the cabin skin may also penetrate the fairing, thus providing an orifice for both to allow the air to escape.
I once worked on an aircraft that had a number of decompression load cases specified for the wing-body fairings. The OEM analysis I saw didn't explicitly define the requirements that were driving the analysis, but with some guesswork I believe they were afraid of debris ingestion into the tail-mounted engines. They preferred to have a door panel burst, instead of risk the whole fairing peeling off.

What kind of door are you talking about? Panels with screws every 2" around the perimeter, or a hinged access door with a single recessed latch "to check the oil"?


STF

 

[WKTaylor]

SAITAETGrad...

Bird-strike/etc penetrations to impact-vulnerable components... including wing-root fairings, radomes, windshields/canopies, wing/stab leading-edges, etc... can cause substantial damage... up-and including partial/total loss of the component.... and major damage to the structure behind it [pressure bulkheads, spars, fittings, occupants, etc] and/or controllability and aerodynamic instability.

IF penetration of forward-facing structure can be resisted... so that crushing/distortion/buckling/deflection absorbs/re-directs the impact-energy... and/or minimizes the size of any 'penetration'... then the secondary effects of (b) residual bird-impact energy [velocity/carcass] directed against primary structure and (b) the effect of free-stream air 'pressurization' of the cavity behind the fairing further stresses the impacted fairing and primary structure can be minimized. Witness the number of radomes designed to absorb bird impacts, that are crushed but barely penetrated leaving the radomes/antenna damaged; but the radomes stays attached [instead of blowing-apart] and the pressure bulkhead remains fully intact. In these cases the aircraft remains reasonably controllable and structurally intact.

The situation gets really sporty when massive penetration occurs and primary structure damage occurs and/or the fairing partially or fully separates [bad aerodynamics!] and/or free-stream pressurization/energy is unrestrained.

Good news, sorta, for civil aircraft...

Most wing-root fairings have a low forward cross-section and are sculpted with relatively shallow [very acute] forward surface-angles that typically deflect-off birds... assuming they are robustly-enough built in the critical forward areas.

Also, most bird-strikes occur at low altitude where airspeed restrictions [250-Kt?] are in-place and free-stream pressure loads are ‘relatively minimal’.

NOTE. External baggage pods [usually belly mounted on prop birds] often have a blunt protruding profile. However the heavy internal loads they are designed for and slower airspeeds, usually makes-up for any impact vulnerabilities.

A few references for You to consider...

SAE AIR5661 COMPARTMENT DECOMPRESSION ANALYSIS

JAMS 2013 Impact Damage Formation on Composite Aircraft Structures, Hyonny Kim, UCSD

And maybe…

AGARD-AG-238 Design Manual for Impact Damage Tolerant Aircraft Structure

Unfortunately, for most military acft which I've mostly work-on, low-level very-high-speed bird-strike and battle-damage is a fact of life: so these [and other] details become far more critical for 'vulnerability and survivability'. Bird-strikes at high tactical speeds can be devastating... hundreds of sad stories make this so. And 'battle damage' is a compounding/wild-card factor. In all cases structure damage, controllability and unstable/unpredictable aerodynamics are just a few of the crazy surprises waiting the aircraft crew.

CAUTION: blow-open/blow-off depressurization doors have to be specifically designed to open or ‘fail-open or off’ and reliably so that unwanted internal pressure loading is vented very quickly away-from fragile structure. Also the cross-section must be adequate: too small a door/opening and it will take too long to vent pressure spikes; too large a door/opening will be structurally inefficient, costly and questionably reliable. NOTE: inflowing air must be vented to low pressure/flow, so that the resulting/sudden air-flow thrust forces are minimal.

One of the craziest stories I ever heard was about the F-111: The Aardvark had a specific vulnerability on the wing leading edges! A sudden hole or opening in the LE structure would re-direct a concentrated air-stream through it, result in a major controllability problem [hard roll]... or a catastrophic flutter scenario... bad things at low-altitude high subsonic cruise/maneuvering in combat.

BTW... Just saying... 'cleaning-up' the 'guts/bones/blood/feather debris' after a particularly large bird-strike can be both an onerous and a toxic job.


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]