Strathallan Magister auctioned and restored

Ah, the Trigger’s Broom theory!

hopefully the following will be of interest and explain some way the differences between Casein and Aerodux glues.

Casein glue is a type of adhesive made from milk protein. The glue is known to be very strong over a long period of time and is highly resistant to water. It has a drying time that allows enough time for pieces to be positioned accurately. The process of creating this glue is nearly identical to that of making cheese and can easily be performed at home. Although most adhesives are based on synthetic chemicals, casein glue still enjoys use as a natural binder.

There have been many uses for casein glue throughout history. There are records that show it was created and used by the ancient Egyptians. It was employed in the Middle Ages to bind together thin panels into thicker plates on which artists could paint. It also is believed to be used by makers of famous musical instruments that have lasted for a century or more. It was used extensively in woodworking, furniture making and to assemble wooden aircraft as late as the 1940’s

While casein glue can be purchased commercially, it also can be made at home with similar results. The process involves heating milk and then adding an acid such as vinegar to separate the curds from the whey. The curds are taken out of the excreted liquid and mixed with baking soda to neutralize the acid. The remaining liquid from the curds is removed by pressing them under a heavy weight. The final result is a white paste-like substance that will become a powerful adhesive when dry.

If one is making casein glue at home, it should be noted that the fat content of the milk plays a part in the strength of the glue. Milk that has a high fat content, such as whole milk, will produce weaker glue than non-fat and low-fat milks. This is because the fat in the milk prevents the casein from polymerizing.

In addition to being very strong and having a long lifespan, casein glue is also very water resistant. Unlike some adhesives, water will not weaken or dissolve casein. In some cases, however, water that comes into contact with the adhesive can pass on microorganisms that will eat the glue and eventually destroy it, but this is not a function of the water itself. Here is the problem with wooden aircraft from D H, Miles and Percival that have sat outside allowing water ingress into their structures. Glue failures were widespread in the late 1950’s early 1960’s causing many countries aviation authorities to ground the types affected. I know of a Miles Gemini that has never spent a night outside in its life and is still perfectly bonded with the original casein glue.

The drying time of casein glue can be a benefit in many situations. It takes a long time for the glue to properly cure and harden. This can allow the assembly of multiple complex parts that might need to be moved during assembly before securing them to allow the glue to set. This also might be a disadvantage for projects that require something that sets more immediately.

Aerodux of Phenole Rescorscine type, the Aerodux is a glue that certainly has come of age. It belongs to one of the oldest glue (and plastic, in fact) families around. Like all glue of aero quality it should arrive with a data sheet.
The glue is two component, the glue itself a reddish brown liquid that often comes in 1 litre cans. As filler is cheaper than glue it sometimes contains filler in the form of ground nut shell.

Hardener is a whitish powder packed in tin cans or plastic bags.

Phenols are poison to fish, that means if you pour your surplus Aerodux into a pond the fish might die. It contains very little solvents, and in the hardened state is chemically neutral. Apart from the mentioned effect on fish I know of no health hazhard connected to this glue. But I certainly would not drink it.

I used to mix my glue in a paper cup, and use a letter weight to assure mixing ratio. The ratio is 100 to 20 (100g of glue and 20g of hardener). The ratio is not critical. The glue also can be mixed by the volume, ratio here is 100 to 55.

After weighing the glue should be stirred properly to dissolve all hardener. Stirring should be continued till no lumps remain. The mix then should rest for about 10 minutes before being used.

The following times are relevant to Aerodux 185:

Shelf life: Here the literature seems to give different information. Some batches I have used made by Dyno Chemicals, UK, state a shelf life of 1 year. Others say only 1/2 year. This is true for the glue component, shelf life for the hardener is indefinite. It should be noted that once the can is opened, the glue starts deteriorating. It becomes darker in colour and later lumps appear. So when a can is opened the shelf life is reduced. Close the lid as quickly as possible! It is up to your judgement when to throw it away, but as a rule the glue should be liquid. No lumps are allowed. Aerodux should be stored in temperatures between +5C and +20C.

Pot life: The time from mixing till application is done. Varies with temperature of course, but a normal value is 2,5h at 20C. Temperature should not be lower than +15C (by the way this goes for the materials to be glued as well) and higher temperatures than +25C are inconvenient (I understand you aussies). High temperatures make application a real rat race – more on this later. Pot life does not increase very much on lowering temperature.

Open time: Now here is the usual criticism against this stuff coming up. Open time is the time from when spreading the glue is commenced till the two surfaces are put together. The rule here is that on putting together the first glue spread should still be clearly liquid. Open time varies, shortening with increased temperature and increasing slightly wit increased air humidity. A slightly cool (+15C) and moist atmosphere is favorable. Still the open time rarely exceeds 15min, so application of the glue should be swift – certainly an unexpected problem for the epoxy freaks. On suspicion that the open time has been exceeded the parts should be scraped clean and possibly sanded before a new (and swifter) application is done. Friends with brushes is a big asset when big areas are to be glued! To add to the problem it seems that open time is reduced gradually with time after mixing.

Closed time: The time between closing of a joint till glue pressure is applied. Now here things can move more leisurely. Again varying with temperature, we can have more than 1/2h here. But check that no part of the joint remains open in this phase – it can ruin your whole day.

Glue pressure time or clamped time: A normal value is min. 8 hours at +20C. The glue demands medium high glue pressure to produce a sound joint, to attain this often can be difficult. I use to leave the glued parts for about 24H with clamps attached. Aerodux can be express hardened in high temperatures without any side effects. The joint has full strength after about a week.

Brushes, paper cups and other equipment can be rinsed in cold water as long as the glue is not hardened. After hardening a hammer is a possible cleaning tool. Once hardened the glue is not soluble by any solvent, but have some thermoplastic properties. In fact, the ply we use is also Phenole Rescorcine glued. Here glue comes in foil that is put between the thin wood layers, and fused by high temperature and high pressure in big presses.

Using Aerodux takes a little while to get used to, especially the old epoxy users will find the short open time and need for relatively high glue pressure inconvenient. Personally I find that reduced (if any) health risk and unsurpassed aging qualities more than outweigh the disadvantages. In the end the choice will be that of your controller’s as well as your own. You have to comply to his demands.

Likewise I haste to add that in case of conflict between information found in my writing and the data sheet, the last mentioned is always correct!

The airplane business surprisingly (to some) is quite conservative. While this may be frustrating I find comfort in using tested and tried materials and solutions. New materials and methods certainly can be exiting, convenient and have other qualities. But for me there is a big difference between believing and knowing. So I guess I belong to a somewhat conservative fraternity. At least glue-vide. The oldest Aerodux glued aircraft still flying is now over 61 years old. On its last annual the glued structure was found to be perfectly solid with no sign whatsoever of glue failure anywhere. This and other Aerodux glued aircraft will fly on for decades to come.

…and im guessing a Stradivarius that has had the glue and wood replaced is probably not considered a proper Stradivarius any more.

Ah, the Trigger’s Broom theory!

Three points. Firstly, I disagree that the stresses imposed on either old musical instruments or furniture are commensurate with those imposed on the glued joints of a flying wooden aeroplane. Secondly, old musical instruments and furniture do not need to fly and have not flown. Thirdly, neither old musical instruments nor furniture need to support human beings whose safety will be imperilled if their glued joints fail in flight. I think your analogy is a rather like chalk and cheese. A wooden aeroplane with casein glued joints is fine in a museum – like old musical instruments and antique furniture – but not acceptable for flight.

with all due respect in structural engineering all that matters is stress, strain and load….the fact that one object flies and the other doesn’t, does not effect the structural mechanics either way. Unless you have done the stress analysis you cant really tell and i think you may be surprised!.

I don’t know much about the world of Stradivarius violins but from the violins i have examined up close it would be very difficult to pull them apart and reglue them without destroying all the wood…and im guessing a Stradivarius that has had the glue and wood replaced is probably not considered a proper Stradivarius any more. Furthermore any minute changes in the structure of these antique instruments would be audible to those who play them for a living probably rendering them unplayable.

I guess im trying to understand by what mechanism does casein cement break down over time? assuming the wood structure is kept bacteria and fungus free at a relatively constant moisture and temperature? I know different size pieces of the same type of wood will expand and contract at different rates depending on how they are cut with respect to the grain and this can cause working of the joint when you have two dissimilar shaped pieces glued together (eg spar web and cap). wouldn’t this be a problem with aerodux as well? what is it exactly that prevents aerodux from breaking down?

Is this really the case? is there any hard data to back this? there are very valuable musical instruments and antique furniture several hundreds of years old held together with casein cement. The stresses and strains found in parts of these instruments and furniture would be approaching that found in the spars of some of these early light aircraft. There was a problem with the early synthetic glues (urea formaldehyde) which was used in many post war light aircraft such as the miles messenger and percival proctor. i have a keen interest in this topic! truth is there a few engineers around with much knowledge of this stuff nowadays…

Maybe no hard and fast data to prove the above. However look at Jodels as an example. For instance the oldest flying is now over 60 years in age, screwed and glued with Aerodux. We have one at Great Oakley, a 1961 model in superb original order, never re glued or rebuilt, shes now 54. Annual inspections show the glued joints to be perfect and I know the former BA captain that owns her is a stickler for safety. Any hint of delamination and he would not consider flying it. The difference as stated is that with casein your life depends on it retaining its bonding qualities whilst aloft, if the shaft of your casein violin snaps whist playing you live on!!

Touché?

Surely, Laurence, the plural is Stradivari! What’s more, I don’t think that many of their owners will be testing their glued joints under flying conditions by flinging them across the concert hall!

I understood that old violins are regularly taken apart and re-glued. I don’t think there are many Stradivarius with their original glue.

….. there are very valuable musical instruments and antique furniture several hundreds of years old held together with casein cement. The stresses and strains found in parts of these instruments and furniture would be approaching that found in the spars of some of these early light aircraft

Three points. Firstly, I disagree that the stresses imposed on either old musical instruments or furniture are commensurate with those imposed on the glued joints of a flying wooden aeroplane. Secondly, old musical instruments and furniture do not need to fly and have not flown. Thirdly, neither old musical instruments nor furniture need to support human beings whose safety will be imperilled if their glued joints fail in flight. I think your analogy is a rather like chalk and cheese. A wooden aeroplane with casein glued joints is fine in a museum – like old musical instruments and antique furniture – but not acceptable for flight.

I would suggest that Tango Charlie has a keen interest in the subject of the strength of Proctor glue joints too! 😉

Its highly likely the aircraft when she was rebuilt used Aerodux glue throughout. If so and stored as she was in a well aired garage the structure should be fine. Obviously a very thorough check was required and all inspection hatches will have been removed and possibly small areas of fabric to inspect the timbers below. Casein glue from the 30’s whilst fine at the time suffers from the ravages of time, hats why wooden aircraft in the early 60’s Miles, Percival etc were grounded in large numbers. The cost of opening up the spars and a major airframe check or full rebuild was well in excess of the aircraft’s value, the flood of new metal types from the US meant most were broken up or burnt.

Had Aerodux been available in the 30’s there would likely be dozens more Miles and Percival types around today rather then the mere handful left.

Is this really the case? is there any hard data to back this? there are very valuable musical instruments and antique furniture several hundreds of years old held together with casein cement. The stresses and strains found in parts of these instruments and furniture would be approaching that found in the spars of some of these early light aircraft. There was a problem with the early synthetic glues (urea formaldehyde) which was used in many post war light aircraft such as the miles messenger and percival proctor. i have a keen interest in this topic! truth is there a few engineers around with much knowledge of this stuff nowadays…

Currently at Compton Abbas I believe

Private airfield at Rendcomb.

Currently at Compton Abbas I believe

I see its now registered to a Cheltenham address? Any idea where it is based and weather we are likely to see it out and about at any events this year?

Taken from an article in the local paper in 1976 ( with apologies for any eyestrain from reading this but it was the best I could do!)

Does anyone know if it is still in RAF colours?

She wasn’t totally rebuilt in Strathallen hands . The fuselage and wings were stripped of fabric and a few woodwork repairs done but the ply was not removed from the fuselage. Recovering was done in 1976 and she flew again in June of the same year

In that case I have no idea how you check the structure

Moggy

Its highly likely the aircraft when she was rebuilt used Aerodux glue throughout. If so and stored as she was in a well aired garage the structure should be fine. Obviously a very thorough check was required and all inspection hatches will have been removed and possibly small areas of fabric to inspect the timbers below. Casein glue from the 30’s whilst fine at the time suffers from the ravages of time, hats why wooden aircraft in the early 60’s Miles, Percival etc were grounded in large numbers. The cost of opening up the spars and a major airframe check or full rebuild was well in excess of the aircraft’s value, the flood of new metal types from the US meant most were broken up or burnt.

Had Aerodux been available in the 30’s there would likely be dozens more Miles and Percival types around today rather then the mere handful left.

Wish I did!

I’m sure Mr Labouchere knows what he is doing.

In that case I have no idea how you check the structure

Moggy

It has not been refabriced. It has already been annualled, had a fresh Permit issued, and been delivered to its new owner.

You take the fabric off. It almost certainly needs replacing anyway.

Moggy