Wingtip Repair

[Anticept]

As tom said. The materials and construction of an FD aircraft make them relatively simple to fix. To be flat out honest, they aren't that special in regards to construction at all. However, there are valid concerns that a lot of people in the field still do not know how to do proper composite repairs, leading to the current environment of holding info close to the chest.

[Runtoeat]

Hi Tom.  Wondering if you're referring to baking the repair at a controlled temp for a specified time?  When my local A&P facility repaired my stabilator, they were required to do this.  This same facility was also repairing the main spar on a Cirrus after the aircraft struck a wing with a light pole during taxi.  This looked very involved and required direct communication to Cirrus thru the repair but it was completed successfully.  I'm thinking someone might be very busy if they had a mobile trailer set up for this and could travel.

Corey, you also appear to know a lot about composite repair and your comments hit on a valid point and leads to the reason that many of the forum members are interesting in following the repair being done here.  As other owners, I am the typical CT owner who has no known options for finding a composite repair facility within a reasonable distance from me.   Also, as you point out there seems to be a reluctance to remove the mystery from this process.  I'm hoping that others will start offering to do repairs  throughout the U.S. and make this a much better understood and accessible process.

[Anticept]

Composites are as much an art as a science. I cannot demistify them in a forum post. Instead: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_ch07.pdf

There's a reason why you don't want to build an airplane that requires special facilities too: Where will they get ongoing support? Maintainability is an important design consideration.

FD chose materials commonly available. Reduces manufacturing costs too.

As for the special equipment: autoclaves, hot bonders, double vacuum debulking, that kind of equipment. A thermal lamp, a thermostat, and sometimes a tent is all you need for the majority of repairs on an FDCT.

[Runtoeat]

Thanks for the link.  Lots of good information.

[solarguy54]

On 10/26/2017 at 9:28 AM, Anticept said:

Composites are as much an art as a science. I cannot demistify them in a forum post.

You are correct.  There are some other issues too:

1. CT's are SLSA.  Only an A&P or an LSRM can repair them.  An A&P can supervise the work of others.  An LSRM must do all of the work themselves.  An LSRM must have been trained or watched a repair in order to do one.

2. Besides being trained in composite repair, IT REALLY HELPS to thoroughly understand how the part was constructed originally in order to do a quality repair.

3. CT's use carbon fiber, aramid, and kevlar in their construction.  For example, the full-width trim tab on the stab uses kevlar that's about the thickness of newspaper.  Tough but delicate.

4. Deciding on a repair procedure IS more art than science.  It's not something you can write a procedure for as each damaged area will be different.  Replacing leaking fuel caps CAN have a process for them, but most structural repair is custom.

[solarguy54]

Making progress!

[Runtoeat]

Appreciate the photos.  Really interesting.  I did wonder about the need to know how the factory originally constructed the damaged area in order to use the proper fiber cloth and  strand direction for the repair.  You mention this.  How do you find this out.  Does FD provide support to the repair facility?  Did you learn this from the visiting expert from Germany?

[Anticept]

You can take moulds and forms easily from another airplane.

In many cases in non-high stress areas, you'll just create a part in a mold, then put it in a jig to line everything up and lay fabric to attach it.

Or, lay the inner fabric directly to the repair area using a form to guide the shape, and then add the core and outer fabric on top. Lots of different ways to do it.

You'll be making oval shaped access ports for yourself to be able to reach the inside of the wing too, if you aren't building it up in sections. You then repair your access port like it were a hole. You want oval shape so you can insert a stiffened repair ply through it (not possible with a circular hole), then build core and outer layer over it.

[Runtoeat]

Didn't know there was a method of building from the inside out as you describe but unless you can access the inside of the structure, I guess this is the way it must be done.  Really interesting.  I'm still having a hard time understanding how epoxy has the ability to maintain it's adhesion when applied to an old surface.  I know a little about scarving the edges and building up the new layers on this scarf but still don't understand how epoxy adheres so well to the old non-porous surface to maintain enough adhesion to keep the joint from failing.

[Anticept]

Epoxy is a polymer. It has a number of sites along the molecular chains that are activated by a catalyst and initiates cross linking action. It also has a high affinity for other various surfaces, making it one of the most versatile polymers available, but it is expensive.

Still, surface prep is CRITICAL. You must thoroughly clean the bonding site. I typically clean and then lightly rough it up with 400 grit sandpaper, and wipe one more time to remove any dust. It allows for a very good bond site.

[Runtoeat]

Now understanding better how composites work.  Thanks for taking the time to explain.

 

 

[Doug G.]

I have been trained in and done fiberglass repair. How much different is carbon fiber repair? (Especially for small repairs.) 

FD requires training for LSRM folks, but no longer provides it. Where can I get the training, if I need it?

Thanks

[Tom Baker]

54 minutes ago, Doug G. said:

I have been trained in and done fiberglass repair. How much different is carbon fiber repair? (Especially for small repairs.) 

FD requires training for LSRM folks, but no longer provides it. Where can I get the training, if I need it?

Thanks

Lets hope you don't need it. If you do simply do like the starter of this thread did. Have Frank come over and teach you while doing the repair,

[Fly Boss]

Or how about a number of us chip in and have Frank come over and do a class.

[Runtoeat]

FlyBoss, good idea.  I think if Frank did a training session at a spot in the Mid-Western U.S. and the Mid-Eastern U.S., there would be enough people interested and within travel distance to support this.

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[Tip]

Lockwood has had composite classes in the past with either Daniel or Frank. Both guys are from Germany.

[Runtoeat]

Hi Tip.  Did you go to one of these classes?  Is there anyone in our area that does composite repair?  I'm not ready to go back to Woodstock.  Those Pennsylvania mountains are too much for this flatlander. 

[Tip]

Hi Dick. My IA attended a Lockwood class in 2011. Cory is the closest for you and I that I know of. 

[Anticept]

Composites are honestly easy once you think of things in terms of fabric lines instead of sheets like metal. You have to understand how stresses follow those lines, and what happens when the stress is and is not following the fabric threads.

FD aircraft are made with the fabric at a slight bias to the spar. There's a good reason. Think about how the wing wants to twist. Excessive twisting on a wing is almost always SUPER BAD. If it manages to oscillate, you now have flutter.

If you have worked with wood, you probably understand the importance of wood grain direction. Same concept here.

[Runtoeat]

More pieces to the puzzle.  The analogy using wood grain helps.  Thanks.

[Doug G.]

 

On 10/30/2017 at 4:38 PM, Tip said:

Lockwood has had composite classes in the past with either Daniel or Frank. Both guys are from Germany.

They haven’t offered one for a long time, have they? Doesn’t anyone teach carbon fiber repair except FD?

[Tom Baker]

10 hours ago, Doug G. said:

 

They haven’t offered one for a long time, have they? Doesn’t anyone teach carbon fiber repair except FD?

Composite repair is taught, but most production composite aircraft use a different method to their composites compared to Flight Design.

[WmInce]

2 hours ago, Tom Baker said:

. . . most production composite aircraft use a different method to their composites compared to Flight Design.

Tom,

Please expand upon that. How so?

IYO, is the Flight Design methods better?

[Tom Baker]

Flight Design aircraft are laid up by hand using a method similar to what a individual would do with fiberglass. After the resin sets up it is then removed from the mold for a post cure heat.

Other aircraft use a more automated system where the cloth has the resin added during manufacturer. It is laid into the mold, then heat and pressure are used to activate the resin and cure the finished product.

I wouldn't say Flight design's system is better, just different. It does lend itself to easier field repair.

[Anticept]

Flight Design vacuum bags many of the parts, including each half of the airplane shell, so it's closer to the head and pressure than you think. But, the stuff laid in after the two halves are put together are not bagged.

I am pretty sure the spar is also done this way too. It's built up, bagged and heated, then everything is assembled around it. I do not know if the wings are multiple pieces and assembled.

I have a hot bonder. Right now it's inoperative (uses ~20 year old computer hardware), but one of these days I'll sit down and upgrade the internals so it would be in service again. I just had not decided on the controllers; I'd like to get away from using chip PC control, and instead move to a programmable logic controller with a watchdog chip, that way I don't ever have to worry about "drivers". But the suction and heat blankets appear to work fine. It would make repairs take next to no time at all.