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Hasse

Cracks in the window and doors

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Wow, that's a lot of cracking, looks just like fuel induced cracks to me, did he have a fuel tank leak or maybe a spill while refuelling? Always have doors closed when refuelling for that reason.

Yes Rick, I had a pretty heavy spill of fuel during my last flight, siphoning fuel out via the venting tube under the wing. For some stupid reason, don't ask me why, before flying I tested if the wing pumps of the extra tanks could fill up the main tanks. They could and the main tanks got overfilled and spilled it out during flight. It was quite a lot of spill and as matter of fact it seemed as if the left wing was more active and indeed I think I saw some fuel even hit the left wing root close to where there is a crack in the window. It is not unreasonable to think that the fuel fumes also could have hit the left door. Hmm, I think we are coming close to the answer. How fast will a crack appear on lexan after hit by fuel?

 

 

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Holy guacamole batman..thats a lot of cracks mine are nothing like that

The pics attached the smaller cracks are on the pax door and they are still the same and have not grown...the other 2 pics with the bad cracks is after a fuel leak which you can see on the underside of the wing

 

[ATTACH=full]38801[/ATTACH] [ATTACH=full]38802[/ATTACH] [ATTACH=full]38803[/ATTACH] [ATTACH=full]38804[/ATTACH]

 

Holy guacamole batman..thats a lot of cracks mine are nothing like that

The pics attached the smaller cracks are on the pax door and they are still the same and have not grown...the other 2 pics with the bad cracks is after a fuel leak which you can see on the underside of the wing

 

[ATTACH=full]38801[/ATTACH] [ATTACH=full]38802[/ATTACH] [ATTACH=full]38803[/ATTACH] [ATTACH=full]38804[/ATTACH]

Hi Mark,

 

Your cracks are nice but not much to boost about compared to the Swedish Viking Cracks. Nevertheless, and seriously, I now start to Believe that they do originate from fuel spill (even though drilling holes etc may weaken the strength of the lexan). If this is correct then a lot of precautions should be made to prevent fuel coming into Contact with the lexan material. Not only during filling the tanks but also during siphoning because siphoning will be very hard to avoid. The vent tube should either be much further back of the wing away from the lexan or perhaps even better, on top of the wing.

 

btw, who is Holy guacamole batman? Couldn't find him in the Holy Book.

 

 

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As someone who has the whole lexan-installing process to go thru, thanks for all the advice and tips!

 

I probably would have been very blase about the whole painting process and painted with the windshield in; after this I'll definitely be removing before painting. (The 701 specifies screws instead of rivets for the windscreen, through a metal spacer I believe - this may also help prevent cracking as there's no pressure on the lexan at the fixing points.)

 

 

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As someone who has the whole lexan-installing process to go thru, thanks for all the advice and tips!I probably would have been very blase about the whole painting process and painted with the windshield in; after this I'll definitely be removing before painting. (The 701 specifies screws instead of rivets for the windscreen, through a metal spacer I believe - this may also help prevent cracking as there's no pressure on the lexan at the fixing points.)

Marty, I hope you don't have a fuel tank forward of the instrument panel like some 701's have, because you will surely wreck your windscreen if you do.

 

 

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Marty, I hope you don't have a fuel tank forward of the instrument panel like some 701's have, because you will surely wreck your windscreen if you do.

No, I've got a Sav setup. Sav wing tanks (only 2) going into a 7l header tank behind the seats.

 

 

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There hasn't been any fuel spilt on or near them Hasse?

Hi again SDQDI,

 

I may have been too Quick when I turned down your proposal about fuel spill. No spill during refuel but a lot of spill siphoning out overfilled tanks via the vent tube during flight. I wouldn't be surprised if that spill especially from the left tank came in contact with the Lexan on the pilot side where I now have the cracks.

 

 

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I will soon have to replace a windscreen in my Xair Hawk (H or Hanuman) due to a load of small cracks spreading from the tight wing root bend.

 

The usual stuff is polycarbonate (Lexan, Makrolon etc.), but there is a similar mechanical spec. material called PETG which is reputed to be fuel resistant and slightly more flexible - has anyone tried it?

 

 

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My doors got small cracks in them I gather it from some extra stress on the lexan when I built the doors. I replaced the pilot side door but left the pax door still the same the cracks have not gone any further. My main screen developed a crack at the bottom and I stopped it with a 1mm drill hole at the end of the crack. The screen is also under a lot of tension as well. I think it has to do with the rivets and how they fit into the lexan. The hole size maybe too tight so when the rivet is pulled it puts stress on the inside of the hole. The holes in the lexan may need to be slightly larger than supplied.

Mark

If you read the fitting instructions on the protective film of the lexan, they are very particular about clearances around fittings. I fitted mine as supplied and read the writing on the film after they were built. I thought back to the laser cut holes and wonder weather it would be a good idea to run a drill through each hole prior to assembly to de-burr

 

them out to the specified clearance next time I build one. some of them are a bit ragged as supplied and that could lead to tight spots as the rivets are pulled up.

 

 

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This is very true too, I leave the thin plastic protection on while painting plus masking over the top to try and avoid this. Also do not have your windscreen in place while gluing in the carpets- the fumes from that glue will crack the plastic or permanently fog it.

Something doesn't add up here. The factory planes come out with the roof panel partially painted, and I think Dennis painted some of his roof panel too. I am not aware of cracking when this is done.

 

 

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Something doesn't add up here. The factory planes come out with the roof panel partially painted, and I think Dennis painted some of his roof panel too. I am not aware of cracking when this is done.

Yup i painted my roof..

 

20151004_150231.jpg.461bea0a7b2be8087746dff192286661.jpg

 

 

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Hasse:

 

There are some very interesting videos on YouTube about what happens to polycarbonate sheet that is rubbed with alcohol. Apparently this happens especially if the edges are flame polished. I don't have a link to hand, but if you search youtube for alcohol and plexiglas ...

 

 

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I will soon have to replace a windscreen in my Xair Hawk (H or Hanuman) due to a load of small cracks spreading from the tight wing root bend.The usual stuff is polycarbonate (Lexan, Makrolon etc.), but there is a similar mechanical spec. material called PETG which is reputed to be fuel resistant and slightly more flexible - has anyone tried it?

Hi xair,

 

What I've Heard is that PETG isn't very UV resistant and shouldn't be used at temperatures more than 60 degrees centigrade

 

 

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Hasse: There are some very interesting videos on YouTube about what happens to polycarbonate sheet that is rubbed with alcohol. Apparently this happens especially if the edges are flame polished. I don't have a link to hand, but if you search youtube for alcohol and plexiglas ...

 

Something doesn't add up here. The factory planes come out with the roof panel partially painted, and I think Dennis painted some of his roof panel too. I am not aware of cracking when this is done.

Yup, there's a little bit of confusion here and there.

 

First - to get the different materials and brand names right -

 

Polycarbonate sheet is sold under various brand names like Lexan, Makrolon, Hyzod, Cyrolon, Zelux, Paltuf, Tuffak, Ensicar, Unicar, Texin, Apex and many more, and is quite flexible especially in thin sheets. For ultralight aircraft windshields it's usually used in 1-1.6mm thickness and can be curved to shape cold (flat-wrapped) and screwed, bolted or riveted into place. It is very tough and doesn't normally shatter in event of birdstrike. Contrary to popular myth it isn't bullet-proof, rifle bullets make small round holes in it and shotguns blast big jagged holes in it.

 

Also - polycarbonate sheet is impervious to almost all chemicals that we normally come into contact with, it isn't affected by any of the paints we use or the thinners in glues and adhesives except cyanoacrylates (super-glues). Consequently polycarbonate sheeting can be painted with any of the paints we normally have available although some of them will adhere better than others. It can also be glued with contact cement, industrial adhesives like liquid nails, silicones of all types and foaming or non-foaming polyurethanes like gorilla glue and sikaflex.

 

And - polycarbonate sheet is not at all affected by petrol or avgas. Try it for yourself - cut a small piece and leave it in a can of petrol overnight, it won't be softened or cracked or go misty or change in any noticeable way. Neither is it affected by alcohol ...

 

The rapid and totally destructive cracking that we all know about is caused by a temperature differential between the two sides of polycarbonate sheet under stress. If you have a flat polycarbonate windshield on your plane and spill fuel on it, it won't be at all damaged, just wash it and go flying. The problem is with curved windshields, the curve puts the polycarbonate under stress - the outer surface is under tension and the inner surface is under compression. Polycarbonate is a pretty good temperature insulator, so when you spill fuel on it, and the fuel evaporates, the latent heat of vaporisation is taken from the surrounds - in this case from the outer surface of the windshield. Because the polycarbonate is a good heat insulator, the temperature between the inner and outer surfaces doesn't quickly balance up and so the outer surface is forced to contract due to the cold, more than the inner surface. The contraction of the outer surface that is under tension (i.e. is being stretched at all times while instralled) causes stress cracks in line with the axis of curvature, which propagate rapidly. It's not the fuel, but the rapid cooling that causes the cracking. The cracks won't normally get any worse, so depending on how bad they are it may (or may not) be safe to apply some 'hundred mile an hour' cloth tape and be able to fly home.

 

As people have mentioned, polycarbonate sheet windshields require some care when being installed or they will readily crack if stress is caused by the fasteners. The holes in the sheet should always be deburred and oversized so that the fastener never comes into contact with the perimeter of the hole. The pressure applied on the sheet surface can also cause cracking so I like to use elastomeric washers between the sheet and head of the fastener, polyurethane washers are best but hard to source, butyl are a bit easier to get. Low density polyethylene and teflon (PTFE) washers are also better than using hard washers or none. I prefer to use screws or nuts and bolts, rather than pull rivets but rivets are neater. If you use rivets make sure they're aly not steel as the aly ones don'e pull up so tight.

 

Plexiglass is a different thing altogether. Plexiglass, Lucite, Acrylite and Perspex are brand names for acrylic sheet. Acrylic sheet is not usually flat wrapped, it is heated and then drape-moulded or blow-moulded into shape. Acrylic is what is used for the bubble canopies of gliders, for example.

 

Acrylic is quite a different matter as far as chemicals are concerned because it is quite absorbent and even though it is curved, because that curve was created while heated, it is not under stress. Acrylic can be painted with some paints (water-based acrylic paints (latex in USA, emulsion in UK)) are most compatible, some abrading of the surface to be painted is an advantage for adhesion.

 

Fuel spilled on an acrylic bubble canopy won't hurt it if the fuel is washed off quickly but if not the fuel dye will penetrate and be hard to remove.

 

Alcohol and acetone will affect acrylic canopies if left on it for any length of time, turning it misty.

 

Acrylic is very susceptible to the ethers/chloroform etc. Until quite recently they were the only adhesive used for acrylic-to-acrylic bonding as they melt the surface and effectively weld the parts together a bit like using PVC cement. These days there are purpose made acrylics adhesives but if you want a really good and virtually invisible bond get hold of some chloroform.

 

 

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my lexan windows in my tornados have cracked, small cracks which are cosmetic. Where they have cracked it looks like a combination of age, sun/heat cycles and being held in tension. When replacing them I was advised you should also use a dull drill bit when drilling so that the polycarbonate melts as you drill it to minimize stress fractures.

 

 

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my lexan windows in my tornados have cracked, small cracks which are cosmetic. Where they have cracked it looks like a combination of age, sun/heat cycles and being held in tension. When replacing them I was advised you should also use a dull drill bit when drilling so that the polycarbonate melts as you drill it to minimize stress fractures.

That seems a bit counter-intuitive... wouldn't a sharp bit cut through with less pressure required?

 

 

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a sharp drill bit will tend to bite.

 

drill a small pilot hole to locate the hole, spin the drill up with light amount of pressure, so that it starts melting the polycarbonate and ease it through

 

 

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The differences between Polycarbonate and acrylic are considerable. Polycarbonate is the way to go especially if if using around curved surfaces. It is softer, easier to drill and resists cracking when bending.

 

Acrylic on the other hand although offering much better scratch resistance in use, is more brittle and more prone to cracking when bending or drilling.

 

Additionally when drilling any holes for securing always drill those holes bigger than the fastener to avoid cracking. Screws are much better than pop rivets as pops expand when pulled and fill the hole completely. This is why the cracks have appeared in old mates Savvy. Additionally it is always a good idea to attach a strip of adhesive tape or foam along the edge to be secured this gives the windscreen etc a little flexibility as things flex in flight.

 

 

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I wonder how much money the RAAF has spent on dulled drill bits over the last 100 years?

They are much easier to buy since Frost started making them in China. Much harder to get a sharp one with a lasting edge.

 

 

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I am particularly intersted in this thread because have always wondered if it was the fuel alone or the stress that caused the cracks in the polycarbonate windows. The post #38 by HITC got me enthused to experiment with some strips of polycarb and various solvents to see if it true and also to see if I could stop it by destressing with a heat gun. I tried the experiment with methylated spirits (denatured alcohol), petrol, enamel thinners, and lacquer thinners. I tried pouring a small amount of these solvents on pieces of unstressed, stressed and destressd polycarb. To destress I bent a strip of poly over double then lightly heated it with a heat gun. All of these had had the same result to a degree, but the lacquer thinner was the most aggessive, so i repeated the experiment and made a little video to show you (just did it with my phone, pretty crappy but you will see the results). Next time i put in a window i "might" try destressing it a bit, what do you think?

 

[MEDIA=dailymotion]x3aeizg[/MEDIA]

 

 

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My bubble canopy was made by Todd's Canopies USA the material used was High Impact Acrylic.Used a 1mm thick metal cutting disc in an 5" angle grinder to cut excess [sanded edge with 400 wet/dry] and 1/16" pilot drill followed by a step drill bit for finished hole sizes [also drilled from other side to de-burr holes] and no cracking to date. [ the rye bread should have eggs and herring on it] Cheers

 

 

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About using blunt drill bits -

 

That used to be the advice given to 'first-timers' when drilling acrylic particularly, the trick - which works incidentally - is to use an old drill bit of the right size and drill it into some concrete or a cement block for a few seconds before drilling the perspex. There's no reason why it wouldn't work for polycarbonate too.

 

But - if you want to do the job properly what you should be doing is modifying the cutting edge of a sharp drill bit. When drilling different materials you need different shaped cutting edges and the cutting edge on the drill bit is very similar to the cutting edge on a lathe tool, except the drill bit is designed to cut while spinning.

 

There are two angles to consider, one is called the rake (or helix) angle and the other is the lip relief angle. On a drill bit the rake angle is the angle between the flute and the vertical axis (a line passing down through the centre of the drill bit. The drill bits you commonly buy in a hardware store have a rake angle that is suitable for the most commonly used material i.e. steel, the rake angle is around 8-10 degrees. The relief angle is the angle on the end/bottom of the drill compared to the horizontal plane i.e. a line at right angles to the vertical axis. Steel cutting drill bits have a relief angle of about 5-10 degrees.

 

The rake angle on the front of the cutting edge determines how much the drill wants to 'bite' into the metal, and the relief angle behind the cutting edge controls how deep it can bite. Consider - a high rake angle and the drill will want to bite viciously but if the relief angle is zero, the drill cannot enter the material it is cutting into because the flat back/end of the drill doesn't allow the cutting edge to bite. Add a little relief angle and it will cut slowly and produce thin swarf, and the cut will be very controlled. As the relief angle increases the swarf gets thicker and the cut becomes less controlled.

 

Next consider a moderate relief angle, say 5 degrees, and zero rake angle. The cutting edge is scraping away, like dragging a spade vertically across the dirt, and in steel the edge would blunt very quickly. Increase the rake to 5 degrees and it will cut steel nicely but if you want to cut aluminium optimally you need a higher rake angle than that. Unfortunately it isn't easy to increase the rake angle because it is a function of the helix angle (or flute) so you need to buy another drill designed for aly. If you're clever you can actually grind more rake into a standard drill and add a little more relief too, but for the most part a drill designed for steel cuts aly well enough.

 

However, if you want to drill brass nicely you should grind a vertical flat on the front face of the cutting edge (lip) because brass cuts perfectly with a zero rake angle. Anyone ever had problems drilling stainless? Screaming drills, overheating, blunt in a few seconds? For stainless the rake and relief both need reducing, and cut slower, with more pressure and use a coolant and then it will cut beautifully.

 

And finally - for efficiently and safely drilling plastics, including sheet acrylic and sheet polycarbonate, without being a blunt-drill-using-neanderthal ... you need a negative rake angle of about 5 degrees and a small relief angle to control the rate of penetration. In a horribly crude way that's what drilling into concrete or cement blocks does to your drill bit.

 

Happy drilling folks ...

 

 

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