F3F repairs

[Kyri]

14 minutes ago, isoaritfirst said:

Probably doesn't need it but if there areas that look weak, I have in the past dremelled in slots across the break and fed in carbon tows before wrapping.

Tows yes, I have removed quite a bit of thickness, to be able to apply 2 -3 layers of tows all the way around. 

I didn't post a picture of it in process (didn't have enough hands!) but the brass scraper in an earlier post is used as a blade which I push into the balsa, and pull the tows through it to ensure complete wetting with minimal epoxy. 

Then I apply a tape which is relatively thick and stiff with a silicone adhesive, so doesn't leave any residue. It is around 4x thicker than most tapes.

Then, some stretch wrap, applied carefully without stretching for the first wrap, then stretched for the rest - this will try to recover and apply pressure to the area. I made sure the hatch was on to try and kept the tows near the hatch opening, tight whilst maintaining the shape of the hatch opening. That area is quite critical and would be under tension in the event of a hard landing as the hatch is on the bottom on the needle.

Then back onto the oven to cure...

[Kyri]

Tape removed after it has set:

 

 

[Kyri]

A little sanding with some fine paper, and a thin layer of epoxy to fill the steps at the end of the fibres, followed by re-taping, and wrapping. The benefits of using a very stiff tape which is not elastic is that as long as the surface being covered is relatively flat, it creates a nice surface for the repair to mould to.

Upon removal of the tape it looks like this

 

[oipigface]

On 01/02/2019 at 07:08, Kyri said:

However I did debate whether this is a design feature, ie. it is meant to break there, but went ahead with "serious" strengthening anyway.

Most planes these days are built with a kevlar nose built onto a carbon fus, but this feature has nothing to do with anticipating where the break might occur. The reason for it is that carbon blocks radio signals and kevlar doesn’t. In the old days we used to use long aerials that would dangle outside the fuselage, but for reasons that I don’t know 2.4MHz gear comes with aerials that are just a few inches long and which should ideally be fitted so that they are at right angles to each other. This creates some redundancy, so that if one aerial is masked, or end-on to the signal, the other has a good chance of picking it up.  Sometimes things can be arranged so that the aerials are outside. My all-carbon Pace Wing has the two whiskers emerging on opposite sides of the fus, but it is more usual now to install the whiskers internally. Best practice I think is to run them into guide tubes, so they stay at right angles. I usually try to find room for one running along the fus and the other either across it, or up and down.

If, as you have done, you extend the carbon part of the fus, you need to make sure that your aerials are well away from it.

You’ve done a very neat job there. Can you tell us what tape you use? I use Frog Tape which does a pretty good job for me, but has a tendency to crease if you are not careful.

[Brett82]

It looks really good mate. Hopefully it won't be too too long and you will be able to bring it along to a comp.

[isoaritfirst]

I’ve never used tows in this way, typically using cloth , I add a few tows in strategic places or where they fit the shape better  

Tows look like a good solution, like many carbon items these days single strands running in the correct direction is more effective for less bulk and weight. 

So tows may be the way forward. 

Proviso possibly being that like you have done they need to be well wetted out but then have as little remaining epoxy mass in them when used as possible

[Kyri]

8 hours ago, oipigface said:

Most planes these days are built with a kevlar nose built onto a carbon fus, but this feature has nothing to do with anticipating where the break might occur. The reason for it is that carbon blocks radio signals and kevlar doesn’t. In the old days we used to use long aerials that would dangle outside the fuselage, but for reasons that I don’t know 2.4MHz gear comes with aerials that are just a few inches long and which should ideally be fitted so that they are at right angles to each other. This creates some redundancy, so that if one aerial is masked, or end-on to the signal, the other has a good chance of picking it up.  Sometimes things can be arranged so that the aerials are outside. My all-carbon Pace Wing has the two whiskers emerging on opposite sides of the fus, but it is more usual now to install the whiskers internally. Best practice I think is to run them into guide tubes, so they stay at right angles. I usually try to find room for one running along the fus and the other either across it, or up and down.

If, as you have done, you extend the carbon part of the fus, you need to make sure that your aerials are well away from it.

You’ve done a very neat job there. Can you tell us what tape you use? I use Frog Tape which does a pretty good job for me, but has a tendency to crease if you are not careful.

That is very informative, thankyou. I will take care to make sure the increased carbon is not impinging the signal. The rx is most likely going to be in the front due to the servo tray which I have test fitted.

As for the tape, I used 100 micron (4mil) peek but the easier tape to get hold of is kapton of a similar thickness. Both peek polymer and  PI (polyimide, of which kapton is a brand) are high temperature materials and so generally have silicone as the adhesive when made into tape form. They are generally very expensive, but chinese kapton variants are in abundance and can be found at lower cost. I found one called koptan. The are usually 25microns (1 mil) and 1/2 to 1" wide, you really need is 100 micron, 50mm or 2" wide as I have used, not to have a witness mark unless the repair really small. 

[Kyri]

4 hours ago, isoaritfirst said:

I’ve never used tows in this way, typically using cloth , I add a few tows in strategic places or where they fit the shape better  

Tows look like a good solution, like many carbon items these days single strands running in the correct direction is more effective for less bulk and weight. 

So tows may be the way forward. 

Proviso possibly being that like you have done they need to be well wetted out but then have as little remaining epoxy mass in them when used as possible

I have previously used cloth, and on the needle that you saw a few weeks ago, just behind the wing is repaired with some layers of cloth as well as tows. I struggled to put on as much as I would have liked without impinging the flap movement and with hindsight should have done it like this. As we said though, repairing is a journey of learning, and it is great to be able to share techniques. I enjoy that part a lot - then you have a list of options to try when mending stuff. 

As for the wetting out without adding too much epoxy, here is a pic of how I do it. The balsa has a layer of plastic on it, and is a little compressible. The brass scraper has an edge but it is rounded with fine papers to give a nice surface to pull against. A roller would be another option, but this works for me. 

[Kyri]

4 hours ago, Brett82 said:

It looks really good mate. Hopefully it won't be too too long and you will be able to bring it along to a comp.

Thanks Brett. there are some other repairs, flap wipers missing / damaged, a creased wing to inspect and repair (if needed), and I hope to get pilot skills increased with the existing plane first. It would be great to get this one flying though, really great. 

[Kyri]

To repair the nose damage, I made a mould (from the other needle) and a 2nd one slightly smaller, which I forgot to take a photo of. The smaller one went inside, pushed as far forward as possible and glued in place. Then I chopped some carbon up, and mixed it with epoxy. This mix was put in the hole, and after estimating how much was needed, I put the lubricated mould over the top and let it cure. 

[isoaritfirst]

Hi Kyri. You talk about not being able to put down enough material on an earlier repair.  I’ve tried on recent repairs to try to repair with similar properties to the original  

Its aways hard but I think once you have a stabilised join, perhaps repaired/held  on one side it’s worh considering removing most of the damaged area for a few cm’s around the break and knitting in several new pieces of cloth . Flaring them in should in theory at least, give a good as new finish with no additional thickness or change in the materials flex etc. 

Just takes a brave man to sand awayore than may seem necessary. 

[Kyri]

13 hours ago, isoaritfirst said:

Hi Kyri. You talk about not being able to put down enough material on an earlier repair.  I’ve tried on recent repairs to try to repair with similar properties to the original  

Its aways hard but I think once you have a stabilised join, perhaps repaired/held  on one side it’s worh considering removing most of the damaged area for a few cm’s around the break and knitting in several new pieces of cloth . Flaring them in should in theory at least, give a good as new finish with no additional thickness or change in the materials flex etc. 

Just takes a brave man to sand awayore than may seem necessary. 

The previous repair, I could have done better. Since each time is a learning opportunity I am ok with that. Unfortunately I didn't take many pictures but I do have this one which shows a pre shaped kevlar tube, slit on one side so that the balloon can push it against the inside. I would have used carbon and more of it, if I was to do this again. On the outside the wall thickness was reduced - but could have been reduced more as you mention above. Next time I will be braver but hopefully next time is not for a while!

[Kyri]

The nose was almost perfect when taking the moulded piece off the top, but I had not filled it enough for fear of a bulge. The mix would not be able to escape in the inside as I glued the internal bit behind the nose first. So, I applied a touch more epoxy (no chopped tows this time) and refitted the mould which had been lubricated first so was re-useable. This time it came out fine, and once I spray it with primer it may show up any irregularities which can be sanded as it does not feel perfect to the touch all the way round. 

[Kyri]

A sanding all over the nose area, to help the paint grip. Unfortunately, however this turns out it won't be as durable as the original 2 pack paint so I tried to leave as much of the original paint on as possible. 

The primer helped lighten up the repaired area, and certainly helped identify which parts of the very front need tidying up with some papers.

 

 

[oipigface]

You won’t mind if I muscle in on your thread, will you, Kyri? Your idea that

On 02/02/2019 at 14:30, Kyri said:

repairing is a journey of learning

made  me think that my latest repair may be of interest. This is a repair that I have now done three times to the boom of a Pitbull. The breakage first occurred at La Muela last Easter when  things got a bit out of hand in some rotor. Mark and I did a jury-rigged repair with cyano and glass reinforced tape, which was good enough to keep me in the air for a few more rounds. It wasn’t hard to align, because the boom was not completely snapped, so it was just a matter of getting it straight and running a bit of cyano in. The danger was that the cyano might get into the pushrods and stop the whole thing working, so we were very careful.

When I got back home, I had a second go at it, following my usual procedure with these partially snapped booms. This was suggested to me by Tony Fu, but I’ve developed a number of variants of it since. Do a temporary fix with cyano, then use a Dremel to open up a window placed so that you can see the damage. Then you can scrape, grind and fit carbon cloth or kevlar to the break, and then fit the window back. When I have done this before, I’ve always been able to move the pushrods out of the way, but this is not possible with the Pitbull, unless you are prepared to, or have to, fit new pushrods. 

The back end of a Pitbull is tiny, and the pushrods which run close to the bottom of the fus take up most of its internal diameter. My main concern when doing this repair was to avoid getting glue on the pushrods, so I slit some heatshrink lengthwise, and fitted it over the rods, then I wrapped some plastic sheet around them. This way I could get lollystick loads of glue past them in sufficient quantities to fit some carbon cloth over the cracks. I was also able to push some foam wrapped in the same plastic sheet under the rods, where they actually helped to exert pressure on the cloth behind. The big problem was fitting the window back. This is easy if you can get a bicycle inner, or foam wrapped in plastic inside the fus. You tack a piece of carbon cloth a little larger than the window to its underside, slather it with epoxy, then fit this into the hole with bicycle tube in place. It’s a little tricky, but not impossible to do. Then you inflate the tube and the job’s done. At the time, I couldn’t think of any way to get pressure applied other than destroying the pushrods, so I just glued the window back in and hoped for the best. This was not quite as foolish as it might sound because the rear end of the window coincides with a bulkhead carrying the pushrods. This repair lasted about 10 months, until three weeks ago...

.....I was out on a good breezy day at a local slope, misjudged the landing approach and ended up arguing with a tree. Among other smaller issues, the tailboom repair gave way, cracking in exactly the same places as it had before, and delaminating from the cloth I had fitted inside. Once again, realigning the break was simple to do with cyano. In fact, having reopened the window, I ran it into the gap where the previous repair delaminated, and left it at that. Then  I came up with a solution to the problem of applying pressure to the cloth that holds the window in place. I tacked some 1” kevlar tape to a piece of cereal box which I bent to conform with the inside of the boom. I made two holes in the front of this, ran a loop of string through, so that I could apply pressure to the rear end (where the bulkhead is) with tape, and to the front end by pulling on the string and taping it down. Pictures follow.  I put epoxy around the edges of the kevlar, slipped it inside, locate the card against the bulkhead, pulled on the string, taped it all down, and left it overnight. The result was better than I expected, because the epoxy wicked into the kevlar tape creating a very solid feeling new interior to the boom. Then I realised that I had forgotten where I put the window, so I cut four pieces of carbon, and filled the cavity with them. Pressure was easy to apply with tape over the top, and the result is probably better than it would have been if my search for the missing window had been successful

I’ve been painting it this afternoon.

 

 

[Kyri]

4 minutes ago, oipigface said:

You won’t mind if I muscle in on your thread, will you, Kyri?

Not at all! I had hoped to learn more from others by posting this repair, and seeing variations and better ways to do things helps us all. As you just described, the difficulty sometimes is finding the balance between ease of repair, and effectiveness. The really clever repairs are ones which are both easy and effective, for examples Mike's cyano stiffened CF tow tail repair. Looking forward to your pit pics!

[isoaritfirst]

Most effective way that I have found for rail boom repairs is very quick and easy. 

As John did I usually cyano everything back in place. You can tack cyano and pull and twist etc then tack again until everything is lined up nicely. 

For some I have heated the joint area with a hot air gun, then the cloth will soften and strands can be woven back together if needed. 

Then flood with cyano. 

Then I use a *****  file to cut back until the area of the break is opaque and on its very last layer of original cloth. I file each side of the break for probably 25mm or more. 

60 or so in total length and flaring back to original thickness. 

Then with a long length of glass cloth cut about 15mm wide. I wet it out leave for 5 minutes then remove as much epoxy as I can before wrapping the fuselage as you would with tape. It gives a nice flared repair with several layers of cloth of the area where the break was and the cloth becomes naturally biased slightly. 

Very quick and easy and can be done within the normal thickness of the  original fuselage  

Pressure and low heat to dry 

See earlier for bag technique which I typically use as I never have anything other than what’s lying around to repair with. 

Although if your more organised like Kyri then that krypton tape sounds good 

 

i have tepaired through windows in the past and used inner sleeves. But unless you can be sure that you are getting the surfaces stuck together with almost no epoxy and the right pressures etc, I don’t think the repairs are as strong as they may look. They also add bulk and effect flex characteristics. 

Watching slow motion movies of landings, fuselages flex a lot and need to 

[oipigface]

53 minutes ago, isoaritfirst said:

 I use a *****  file 

Language, Mike! What sort of file is that. Does Liz know you've got one?

[Kyri]

Masking then spraying, a number of thin coats, resisting the temptation to spray too thick and get runs. This is when summer weather is more suitable, but by pre-heating the fuz and paint a little, it can be done without moisture trapping even in the cold.

 

[Maria Freeman]

Would that be a “ bar steward “ file ?