Timber aircraft - service life

[Guest Baphomet]

Given that a particular timber aircraft is well maintained and stored under cover, what is the expected service life? and what is the limiting factor, does the glue fail or weaken over time? lots of variable I know, comments from those of you with timber aircraft i.e. Corby, Mini-Cab, Jodel and Piel etc welcome.

 

Ian

 

 

[geoffreywh]

As far as I can tell the life span has not been determined yet. It must be well over 50 years...............I read about Jodels from the fifties still flying happily around, BUT they MUST be kept dry! Get it wet and it will rot.

 

 

[Guest Graham Lea]

Thoughts of avro ansons, mostiquitos and other wood/glue aircraft come to mind which were banned from flying by casa after it was found that the glue was had deteriorated badly.

 

 

[kevinfrost]

From memory if the timber is kept at around 3 - 5% moisture it has unlimited life, any less or more loss of strength occurs. Casine glue was use a lot in Europe but was found to crystalize in hot dry climates. A lot of wood A/C and gliders became uneconomic to repair. Resorsinol two pack glues fixed that problem. There are a number of two pack glues used today on wooden A/C. It may suprise some to know wood is still one of the strongest per weight material for building A/C.

 

 

[geoffreywh]

Yes I was going to say that!......My Jodel was started 40 years ago, Built using spruce, birch ply and resorcinol glue...Still looks perfect, I have many test pieces to check, not a single sign of aging apart from a darkening of the wood. ...That's 40 years old NOW , I would say it should be good for 80 years at the rate it's deteriorating!... I have model aircraft from the late sixties built using epoxy. The cheap stuff went rubbery but Araldite is still as it was when it set...Bit darker in colour....Wood is really the best aeroplane building material although it's getting scarcer and dearer.. But then so is everything that is good.......

 

 

[Yenn]

Those old Avro Ansons and Mosquitoes would not have had the good glues we have now, plus they were built to fly and not expected to last more than a few years at most. Any military plane would be out of date before it rotted or the glue failed.

 

 

[facthunter]

Glues.

 

Ian ,.the mosquito gave trouble after a very short time in the tropics, (understandably) and the Wacket was current when I was a young pilot and it shed wings. The glue was Casein (derived from milk?) and a lot of the laminated spar equipped wings were pulled out of the air, in the late 50's. Quite a lot of others continued past that date but I don't know what was required in the way of inspections.

 

I think wood still has a future especially for home builders who generally would build to a high standard, and use modern glues. It' sometimes referred to as Nature's Composite. It can rot, sometimes, due to fungus attack, and faults can exist in the timber ( Shakes etc ) Laminating can assist there, (as in Corby spar.).

 

I favour the idea of making the pilot enclosure of steel tube,4130 ( to attach the wheels, seats, wings, engine mounts, seat & seat belt attach points etc) and the rest of wood ,either ply or fabric covered. Just a thought Nev.

 

 

[Guest Baphomet]

Thanks all, for your input.

 

 

[JohnMcK]

Hi All,

 

My 2 cents worth. Before I took early retirement wood, glues, resins, composites etc was my day job.

 

Wood, if looked after and you keep the rot out of it, has an unlimited life (no fatigue). It was only let down in the early days by the available glues and resins. Good quality resorcinol glues (they are not all the same) have an unknown life span at this stage. I would often use resorcinol as the control glue in an accelerated aging test on other glues.

 

The big drawback with resorcinols is they are not gap filling at all, unlike epoxies, so your bonding surfaces must be absolutely "tight". A glue line more than a "poofteenth" thick will crack. Also being water based you need constant pressure on the glue line until full cure. (you need auto clamp pressure, or you need to keep tightening up the clamps as the water is absorbed into the timber or evaporates. Actual glue molecules only make up about half of what you see, the rest is the water carrier.

 

Epoxies, on the other hand, are 100% glue (at least the good ones are) and they are excellent gap fillers. (You only use low clamping pressure with epoxies) Their life is also unknown at this stage but accelerated aging tests show a life well in excess of 50 years, if you keep the UV away from the glue.

 

The downside of epoxies is they don't like UV and will break down if exposed, particularly the Amine types. The other downside is they are thermoset. That is they will soften with heat. Some common imported epoxies from the Northern Hemisphere have a heat distortion temperature as low as 50 deg C. Not something you want to build a plane out of in our climate. But then again there is always Tasmania to go flying. (Once the heat source is removed epoxies will go back to their origional strength.)

 

A good quality epoxy is the best glue readily available to the average home builder. If you are an excellent woodworker, the best glue not affected by heat, is a good resorcinol. But unless you are real perfectionist, you will lose all the advantages of a resorcinol.

 

Polyurethanes are fairly new on the market and currently are showing real promise. As in all things, there are good ones and bad ones.

 

White glues. The 2 part crosslinking ones are real tough glues but these must never be used on aircraft as they suffer from "creep".

 

Fatigue. We all know metal fatigues (wood doesn't) but it is a little known fact that composites fatigue as well. In fact most composites only have half the strength as they did originally after only one million cycles. (one movement per second.) In one hour of flying you have 3600 cycles, or after only about 380 hours your plastic plane is only half as strong as it was when you bought it. Food for thought, isn't it. To stop flexing you need to build a composite strong, but then you add weight. Build a plastic plane real light, and it has a short life span.

 

Wood, on the other hand, will last forever if you look after it.

 

Cheers,

 

John McK

 

 

[Yenn]

Wood is wonderful stuff. We used it for mower pitmans and I expect they still do. That is the arm that works like a con rod, between the drive and a reciprocating blade mower. Steel arms used to break, but not wood.

 

The problem with wood comes when you try to join it to metal. The join is usually by bolting and that leads to high stress locations.