Spare case study LANCAIR 5

[pedro8611]

Colombia is entering a new challenge in the aviation industry. The Colombian government through the Corporación de la Industria Aeronáutica Colombiana - CIAC (Colombian Corporation for the Aeronautical Industry) has signed a cooperation agreement with an American company – LANCAIR INTERNATIONAL INC – to build the first airplane in Colombia. As the project advance, some issues rise. The airplane is mainly made of composite materials being the spars the element with the most crucial building process. The limitations found due to atmospheric conditions in the Colombian location (Madrid – Colombia) have created the need to investigate different options to build the aircraft parts to meet the designer quality standards.

DESCRIPTION

The Colombian Government signed an agreement with LANCAIR INTERNATIONAL INC to build the “SYNERGY” a modified version of the LANCAIR LEGACY FG. The SYNERGY will be used by the Colombian Air Force as the Primary Trainer Aircraft for its new officers. The aircraft is built in composite materials. The primary structures are built with pre-preg glass and carbon fibers. The pre-preg technique uses during the curing process temperature and vacuum pumps to guarantee proper bonding of the parts. The main concern is the low atmospheric pressure of the Colombian location for the curing oven, when dealing with crucial structural components like spars. LANCAIR INTERNATIONAL INC. has its facilities in Redmond OR and in Cebu, Philippines where the atmospheric pressure is 24 in Hg or higher. This pressure is adequate for the debulking process of the spars. LANCAIR INTERNATIONAL INC has expressed its concern with regards to the lower atmospheric pressure in Madrid – Colombia where the atmospheric pressure reaches between 19 and 20 in Hg depending on weather conditions.

Madrid – Colombia, Atmospheric Conditions:

• ALTITUDE: 2554 mts
• RELATIVE HUMIDITY: 60 +/- 20 %
• AVERAGE TEMPERATURE: 14ºC
• ATMOSPHERIC PRESSURE: 19 – 21 “Hg

SPARS

The SYNERGY Spars are built with Unidirectional Carbon fibers. The Curing process is achieved in an oven that provides a controlled heat ramp and vacuum pumps to guarantee a proper bond between the fibers. LANCAIR INTERNATIONAL INC. has expressed that the approximately 4 in Hg difference in atmospheric pressure will compromise the structural strength of the spars due to the fact that the air bubbles within the layers will not be properly vacuumed out increasing the chance of delamination of the structures. (See annex with Layup Schedule for spars)

CASE STUDY

Knowing that the Colombian government wants the whole aircraft to be built in Colombian territory, we need help to find a solution on how to build the spars. A few options to solve the problem are:

1. Simulate the structural strength loss due to the lack of atmospheric pressure and analyze if adding more material will compensate the loss without significantly compromising the original design of the aircraft in terms of weight, shape, etc.
2. Using a different device for the curing process. (Autoclave)

CIAC needs help in finding the fastest option without compromising the quality of the spars. Production is supposed to star in January and this issue might cause significant delays.




El tamaño del molde de las vigas que debe caber en el autoclave es de
16 ft de largo, 20 in de ancho y 2 ft de alto aproximadamente.

Autoclave Cure Cycle
Apply 24?Hg vacuum (minimum) for a minimum of 1 hour before cure ? Apply 40 ?100 psi autoclave pressure, vent vacuum when pressure reaches 15 ? 20 psi ? Increase from room temperature to 180°F +/? 10° at a rate of 1?6°F/min (maximum) ? Hold product at 180°F for 60 minutes ? Increase hold time to 120 minutes for laminate > 0.125? thick ? Ramp product temp to 270?280°F at 2 ? 3° F/min ? Hold product at 270?280°F for 110?130 minutes ? Cool to 150°F at 3 ? 10°F/min prior to releasing autoclave pressure

 

[Compositepro]

So what is the question?

It seems that the original manufacturing process is oven cure (non-autoclave). The lower atmospheric pressure will not affect the ability of vacuum to remove air bubbles. It will reduce compaction pressure during cure, and that may or may not result is more voids. It certainly makes voids more likely.

An autoclave will make voids less-likely and smaller.

The best source of advice should be the prepreg manufacturer.

 

[SWComposites]

So what is the question? Are you looking to hire someone to help with this issue?

Why not just move the spar fabrication facility to a lower level site?

 

[pedro8611]

CIAC needs help in finding the fastest option without compromising the quality of the spars.

How can we do that process, in order to achive high qulity standars without affect the production process?, is necessary to do a material properties caracterization

 

[SWComposites]

A material property characterization does not fix the quality; it might allow you to accept lower quality parts.

Do you have a structural analysis of the spar? If so, what are the margins, and how much material property reduction can you tolerate and still have the required strength?

Have you fabricated a part at your facility? If so, what is the porosity level (%voids)? Does that level meet the applicable process specification? (is there a spec?)

Do you have an autoclave at your facility? If curing under a vacuum results in unacceptable voids, then try curing a part in the autoclave to see if that improves the void level.

 

[MikeHalloran]

I suppose you could pressurize the building to 4"Hg or so. ... with appropriate reinforcing.

Or recycle an aircraft fuselage that's too old to fly but still in decent shape.



Mike Halloran
Pembroke Pines, FL, USA

 

[Hansmeister]

You may want to think about a tooling change as the fastest approach.

Use the trapped rubber molding process. The rubber expands greatly when heated in the mold, giving pressures even with or greater than an autoclave.

Do a Google on trapped rubber molding.

Make contact with Airtech in Torrance, CA.

 

[MNLiaison]

It sounds like Double Vacuum Bag Debulking might be a good option for you. DVDB can produce laminates with lower porosity levels then traditional vacuum baging but still only uses atmospheric pressure. You would have to perform some optimization studies to get the proper parameters but it should be feasible. Also it might be difficult for large parts.

 

[RPstress]

Making a low pressure 'poor-man's-autoclave' is a possibility. We have looked at this sort of thing to get a bit on top of one atmosphere using a welded steel construction. It's a pressure vessel with a gaseous pressurizing medium, so care is needed even for such low pressures as four or five psi. Using a spare pressurized aircraft fuse as suggested by Mike Halloran is not a bad idea to my mind, but it would probably be physically awkward. If you want to use the autoclave cure cycle that you listed you'll need an actual autoclave; you don't want to mess around with pressures of that level.

However, the first thing I'd do is try out a few flat panels then a few specimens with representative geometry (including thickness) and bagging arrangements. Relatively simple visual examination with a suitable microscope of polished cut-ups will tell you if you are likely to have a problem.

You don't mention the material to be used (the spec may be Lancair's proprietary information). The resin and the prepregging details (partial leaving 'channels' for air/volatile removal or 100%) will have a substantial effect on porosity.

Because the manufacture is different from the original, additional work with coupons for establishing allowables with at least three (and maybe five) batches of prepreg will be needed for certification purposes. Full re-qualification almost certainly won't be needed, more of a set of comparison tests.

If you wanted to go the route you suggest as option 1 (redesign with effectively a new, lower sttrength material) then it would be a major undertaking. You would need to restress the composite parts of the airframe at the least, as well as re-qualification of the material, generating all-new allowables with the new, lower pressure, manufacturing.


MNLaison - may I ask if you have any pointers to more info on DVDB? I haven't come across it myself.

 

[KirbyWan]

One other possible fix is to add debulking steps. If the original design calls for a single debulking step after many layers are applied you may be able to decrease porosity by adding a vacuum debulking step after every two layers are applied. it adds a lot to the fabrication time (and hence cost) but without a capital outlay.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[Compositepro]

The double vacuum bag process is a bit of a misnomer but it works extremely well. It uses a vacuum bag inside of a vacuum chamber. The vacuum chamber can be created by using a vacuum bag over a structural shell which allows air to be evacuated from a laminate without applying compaction pressure as well. This gets around one of the biggest problems with the vacuum-bag process, which is air entrapment.

A vacuum bag is not a vacuum chamber, and a vacuum chamber is not a vacuum bag, hence the misnomer. Thinking that the two are equivalent is one of the biggest problems in solving problems in vacuum bagging.

Vacuum gets drawn on the vacuum bag and the vacuum chamber at the same time. Care must be taken to not inflate the vacuum bag like a balloon, which can break the seals. Any trapped air will inflate the laminate enough to open a path to the vacuum pump and allow free flow of air and removal of volatiles like moisture and residual solvents. A very high vacuum is very desirable (i.e., low absolute pressure. Gage pressure or altitude is not relevant). When air and volatiles have been removed, then the vacuum in the chamber is released to apply compaction pressure to the vacuum bag.

Autoclaves can be modified to allow them to be used as vacuum chambers prior to normal pressure cure. This would eliminate a lot of void problems and greatly increase the quality of parts to essentially zero voids in most cases.

The only significant information I've seen published on the process is some patents on repair methods.

 

[MNLiaison]

"Field Repair Materials for Naval Aircraft" discusses the DVD process on a wetlayup resin system, EA9390, showing reduction in void content and increase in interlaminar shear strength.

The paper was part of AGARD CP-550, paper # 10, found here:

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA293056&Location=U2&doc=GetTRDoc.pdf

Its 22MB so it takes a bit to load up.

Also here is a NASA sponsored paper on DVD of a phenolic composite prepreg.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080014267_2008014216.pdf

Thats the best public information that I had dug up on the DVD process.

Regads

 

[RPstress]

Thanks 'Pro and MN.