I have just had a couple of skins replaced on the stabilator due to corrosion on a 30 year old Piper. As an aside, Piper supplied one skin primed and the other skin with no internal green primer!
I was wondering why Aircraft skins are not all Anodised? I am told that paint sticks quite well to an anodised surface with the correct primer but that might be wrong.
The rivets would join the skins so the Faraday Cage for lightning protection would be maintained. – Do any Aircraft have anodised parts? Is there a reason why not?
I think the main reason is money. I think you are well off with your thirty year old Piper. Recently I had a chance to see spanking brand new Piper Mirage (actually at its first 50h inspection) and I was able to find quite many aluminium panels in the engine compartment with absolutely no surface protection. No Alodine, no primer, no nothing! And the topcoat is also on the top surfaces only. It is obvious that they paint the completed and assembled aircraft so the engine cowling top coat is not present on the parts covered by propeller cone. The same sloppy work can be seen on the upholstery and so on. For example the carpet in the front baggage compartment is just a crudely shaped cut by scissors, not sewn around its edges and fixed by self gluing velcros that were peeling off at this first 50h inspection.
You can hardly expect something like this nowadays (pictures of 1989 Malibu Mirage converted to JetPROP) :
fine carpets sewn around the edges
new paint with the attention to details
new paint with the attention to details
Recently I had a chance to see spanking brand new Piper Mirage (actually at its first 50h inspection) and I was able to find quite many aluminium panels in the engine compartment with absolutely no surface protection
The PA46 is a Piper… I hear the same story from maintenance people today.
I was wondering why Aircraft skins are not all Anodised? I am told that paint sticks quite well to an anodised surface with the correct primer but that might be wrong.
Maybe the “aviation grade” aluminium doesn’t accept anodising well? I really don’t know. But painting onto anodising is difficult and even the double glazing business has managed to do a bad job, historically, with paint peeling off the window frames in large chunks.
The rivets would join the skins so the Faraday Cage for lightning protection would be maintained. – Do any Aircraft have anodised parts? Is there a reason why not?
That would be conductive anodising. Normal anodising is not conductive. Again, I don’t know about the material compatibility. I am sure somebody here does know…
But maybe the answer is that normal 2-pack polyurethane paint systems are good enough. One coat of the primer should last decades, especially if using ACF50. Or even epoxy, although epoxy paint has zero UV resistance.
Rivets should join unprepared aluminium panels OK – certainly to the extent required i.e. you may get a bit of dirt on one rivet but it won’t matter because the other 50 are OK. This is especially a problem with doubler plates which – if done properly, which is rare because it’s a hassle – have a sealant (PR1422 etc) between the panel and the airframe, so bonding is needed.
Aircraft are manufactured from Alclad, which carries its own corrosion protection in the form of a thin layer of pure aluminum on the surface. Nowadays they are painted externally, but obviously when Alclad aluminum aircraft structures were introduced, paint was considered optional both inside and out. If you want more corrosion protection internally, zinc chromate or epoxy paint is the conventional technique. The Russians anodized their skins, for example on the Yak-52 with unbelievably durable quality. There is really no need for paint on those aircraft.
I have an extra pair of wings that were lightly zinc chromated in 1946, so lightly that the stamped 24ST material spec can be read through the paint, and they have no corrosion. So paint works OK.
kwlf wrote:
Anodising reduces fatigue life.
Yes, and a whole bunch as well. The main wing spars from Vans can be bought anodized. They will never corrode, but they will reduce the life of the spars. I think the un-anodized spars will live “forever” (20k+ hours at least, or too long to matter in a practical sense) while the anodized spars is 10k I think I remember.
Not all aircraft aluminium is alclad either. It’s mostly 2024 sheet metal that is alclad, spars are typically not. Most other alloys has (much) better corrosion resistance, but not as good fatigue resistance.
Are the Vans spars milled from solid, or formed from a sheet?
Got a couple downstairs 
I am pretty sure they are rolled thick plates and cut out from there. They have to be build up using angles and other plates. Riveting all that together is a hell of a job. You can also get pre-built spars, but I think when I ordered they were all anodized, so I didn’t want them.
In other words, built up from milled parts.
Milled and milled… A prefabricated (ligthening holes) angled beam (U shape) is not exactly milled from a solid. It’s still rolled. It could even be extruded or drawn.
