Rapid repair

Hi VD,

Although I am not an expert (in fact I am a propulsion engineer - turbojet, bypass, turbofan, turboshaft engines, namely gas turbines) and I am not quite sure that I understand your problem correctly, I'll try to give some kind of an answer.

My problems are that your 'application' of the terms "composite" and "sandvich" is confusing me and a trade name "nomex", which I do not know of.

Composite material is in fact mimicking of a wood material (that is my interpretation, because composites without reinforcing, "organized" fibres are not composites to me). There is a matrix (carbon, epoxy, or some other resins, even ceramics in hot sections of my beloved engines) and reinforcing fibres (again carbon, "kevlar" which I do not know what actually is, glass or metal).

Sandvich in aeronautics is almost exclusively two sheets (metal or even composite) with honeycomb structure inbetween
(usualy Aluminum, an aircraft cabin floor may be of a such construction - all made of Al). Therefore I will assume that your structure is a sandwich: carbon sheets and that "nomex" inbetween (no composites).

Usual aircraft repair procedure is to manufacture a flanged oversize bushing (from Al or maybe some hard, wear resistant plastic in your case). Then to machine a matching "nest" in that sandwich (make that hole bigger to mach the bushing O/D and a counterbore for the bushing flange if structurally OK, if not flange will rest on the upper sheet). The next step is "bonding of the bushing (to the honeycomb-"nomex" using some of the structural adhesives or already mentioned epoxy and a final machining of the bushing hole to required location and diameter after adhesive curing. Usually bushing is predrilled but not to the final I/D allowing for final hole location correction but if location is not so important (your civil "tolerances" tend to be plus or minus inch, sorry but I couldn't help teasing), than I/D may be mashined to a required dimension while manufacturing the bushing and it could easily be installed in that sandwich with plus or minus 0,020" accuracy, I believe.

VD..

I was agreeing with radomir, up to the last paragraph... from that point, and-on, I disagree strongly.

Repair of sandwich-structure that has honeycomb core and metal or composite facings can be very complex, for MANY reasons. I suggest You aquire the following "library" if You are serious

[bookstore]
Composite Materials for Aircraft Structures (**)
ISBN: 0-930403-11-8

[bookstore]
Care and Repair of Advanced Composites (**)
ISBN Number: 0-7680-0047-5

SAE AIR4844 (**)
SAE AIR4938
SAE AIR5278
SAE AIR5279

SAE ARP1524
SAE ARP1575
SAE ARP1842
SAE ARP1843
SAE ARP4991 (**)
SAE ARP5089 (**)
SAE ARP5256

MIL-HDBK-17 (**)
MIL-HDBK-23 (**)
MIL-HDBK-337 (**)
MIL-HDBK-349 (**)
MIL-HDBK-691 (**)
MIL-HDBK-754 (**)
MIL-HDBK-803 (**)

ASTM D2093
ASTM D2563
ASTM D2657

USAF T.O. 1-1-690 (**)

(**) = SIGNIFICANT VALUE

Regards, Wil Taylor

I am not sure what VD means by "rapid" repairs. The USAF uses recovery milestones such as 6, 8 or 12 hours to gauge what is repairable and what is not. In fact, this branch of repair is known as ABDR, or Aircraft Battle Damage Repair. The objective is to do the bare minimum so as to get the aircraft flying to drop its payload. Static strength is considered, but not fatigue.

Aircraft composites are usually made of aluminium skins with nomex honeycomb cores. In certain areas, such as flight control surfaces, you may have variations such as carbon fiber multi-plies, but nonetheless still sandwiched by Al2024 or 7075. Other areas may be made from fiberglass (especially those long endurance type aircraft which needs to be very light).

Kevlar is a Dupont tradename. It is neither glass not metal, but actually a mouthful -- poly-para-phenylene terephthalamide (aramid). It has superior strength to weight ratio, which is why armor suits and protection helmets are made from them.

Now to go on to composite repairs. If the metallic skin is damaged but not the core, it is sufficient to insert a metallic filler and a doubler patch to reinforce. If the core is damaged, route out the damaged core, replace the core, and then insert the filler and doubler. If the hole is really small, squeeze putty in through the hole and then sand down the putty protruding from the surface. If it is fibreglass, chopped fibre in resin can do the trick. As WK Taylor said, there are many possible scenarios. In fact, one can even pull blind rivets in such composite structures to hold a repair doubler in place.

I'm sorry, but I could not follow VD's repair procedure. Have not encountered this kind of repair yet.

Cheers

You may be looking for the "Rapid" repair software. It's at:


Normb

Oh, THAT Rapid!
Thank you normb. Downloaded and done.
Call me an up to date A&P.