Shaping & bending perspex sheet.

[wonwinglo]

One of the most useful and attractive features of perspex is the ease that it can be shaped,when heated to 140 degrees C.it becomes quite pliable.

The easiest and most straightforward way of heating is to immerse it in boiling water.

But before you do this dont forget to remove the protective masking first.You can do this by washing the sheet in warm water,this removes all of the gelatine coating a the same time.Now,before boiling the water be sure to get all of your jigs and formers ready.These are the things that shape the perspex and you will probably find a lot of these items around the house,you can put metal spoons to good use for example,strips of perspex can be easily wound around rods,jars etc.

One tip is to never boil the perspex,always boil the water first and add the perspex when the water is just off the boil,the time the perspex is left in the water depends on the thickness of the sheet but generally it is between two and ten minutes,it is ready for use when sufficently pliable to bend by itself when lifted out of the water.

Take care when handling and use tongues or a similar tool,taking care not to mark or dent it while it is still soft.Wipe off all of the moisture and place the plastic in or around the shaping former.

To stick perspex requires special cement called perspex cement,however if you have difficulty in finding any buy a bottle of acetone from the chemists and soak shavings or small broken pieces of perspex of perpex into it for 48 hours,this makes a very acceptable adhesive.

Make sure that the product you are working with is proper perspex and not acrylic sheet an entirely different material.

 

[OOLILISSIMA]

I've just registered, and I'm really pleased to find such a great forum!!

Barry, I'm planning to make a 1/3 scale canopy for an Auster A5. The sheet before moulding will be something like 1100x370mm, and the canopy shape is complex. I'm therefore thinking that the need to keep the material soft for long enough to work it around all of the plug former might make the boiling water technique difficult for me.

I wonder if you have any experience of anything of this sort of size, and whether you think that an electric paint stripper (with controllable temperature, of course) would work.

 

[OOLILISSIMA]

Another quick one if I can, please Barry. You seem to imply that acrylic sheet other than Perspex is not suitable. Could you possibly say in what way you mean, please?

 

[alan2525]

Isn't Perspex a brand name for Arcylic?

You can also get two different grades of Acrylic sheet, drawn or cast Acrylic. Cast sheet is best for forming.

 

[OOLILISSIMA]

Thanks Alan. That's what I had understood, but i'm new to this and I'd like to know the differences before I jump in with both feet.

While writing, I should correct my first post above. I meant that I am planning a model of an Auster Mk 5.

 

[wonwinglo]

Paul first of all forget moulding a canopy with such a deep draw out of sheet employing this method,it simply will not work,what you need to do is to vacform the canopy,best to make up the plug from block balsa or even softwood,take or send it to a specialist who will run you off as many as you wish.

True Perspex is no longer made and my notes are out of date,modern clear commercial sheet is called Acrylic,it has a distinctive sweet smell to it when moulded and used a lot in the double glazing industry,persex was used originally in making full sized aircraft canopies as it was good for getting things optically perfect,its use can be quite critical as regards working temperature.

Also do not forget to slightly undersize your plug to allow for the thickness of the clear sheet.

There are also many other clear sheet materials with obscure names,some have plasticizers added others without,check out craft suppliers such as Opitec who may be able to assist with vac forming supplies,vac forming is much preferred for anything with a deep draw to it.

 

[alan2525]

I've got access to a vac forming machine if you have a former I'd be happy to vac form some.

 

[OOLILISSIMA]

Barry, many thanks for your advice.

Alan, that's very helpful. Could I contact you again when I get a plug made? It might be a month or three, as I am in the early stages of design and building, although I'll give making the plug as much priority as possible in view of your kind offer.

 

[alan2525]

Do you have any images of the Auster A5? I could only find images of an A3 online. Would the canopy be vacuum formed in sections?

 

[OOLILISSIMA]

Alan, it's actually an Auster Mk 5 (or Mk V). This is the military version of the Auster J/1 Autocrat. The only difference in the superstructures of the two is the length of the canopy. The Mk 5 canopy extends back past the trailing edge to facilitate military observation duties. I am not familiar with all of the Auster planes; maybe there is also an A5, although I tend to think it should be Mk 5.

So far, I have only a few images from the net. I haven't spent much time looking for them, so you'll easily overtake my efforts by "Googling" these Auster Mks in some appropriate phrases.

I have been and photographed a real Mk 5, which is the property of the Leicestershire Museum Service. If they have no objection, I'll let you have copies, either through the forum or to you individually. If you're wanting to scratch build a model, their volunteer Auster specialist is very helpful with drawings and advice. He is also very knowledgeable. He can provide extracts of your choice from Auster's factory drawings. They also have very good three-view and other modeller-orientated drawings, prepared a few years ago by an enthusiast. You'll find the necessary contact details on the Leicestershire, Leicester and Rutland County Records office web site.

 

[OOLILISSIMA]

Whoops, sorry Alan. I missed your question about the form of the canopy.

In both the Auster Mk 5 and the Auster J/1 Autocrat, the canopy is in one-piece.

 

[wonwinglo]

Auster Aircraft canopies,and some Auster memories.

All Auster aircraft employed what is known as a blown canopy,these are literally blown into a mould from the inside outwards,this expensive process ensures that the material is optically perfect from inside the cockpit,the canopy screws around the upper longerons with a break for the door panels,clear sheet extends to the rear of the doors and is framed with light alloy.

It is quite common for the canopies to form small vibration cracks,one of my many jobs as an apprentice light aircraft engineer was to stop the cracks from spreading,to do this a small drill hole was placed just at the end of the crack.

G-AGOH Auster 5 J/1 Autocrat is in fact an hybrid machine built at Rearsby in 1945,a lovely aircraft and I was saddened when she was withdrawn from flying a few years back.

As you probably know the design was introduced and built in the UK originally as the Taylorcraft from America,they used to advertise them as 'The all steel aeroplane' by virtue of their welded steel fuselage frames,very tough machines.

What finished the Auster was when severe American competition forced the company to try and produce a more sophisticated design,made under the name of Beagle,however this suffered as the Airedale which lacked adequate range and was very heavy due to the modifications incorporated,the company went under,however the factory at Rearsby,Leicester still exists to this day but not used for aircraft manufacture.

There are many interesting stories about Auster aircraft,but the one that I like best was when one jumped the chocks and took off pilotless one day from Rearsby,after flying around for some two hours it crashed into open land after running out of fuel !

The company test pilot was called Ranald Porteus,he had a really first class party piece when demonstrating these aircraft at Farnborough etc,he would take off and land using only one wheel,he managed to skillfully balance the machine that way.

She really was a rugged aircraft,despite working on them for hours the only chance that I had to fly in one was from the tiny airfield at Skegness,Ingoldmells,we flew out to sea and came in over the beach for the landing,I remember it well especially as the pilot only had one eye !

 

[Kiwi]

One other thing, perspex can be shaped, bent etc when heated to 140°C BUT water boils at 100°C (at least it does down in this part of the world) so, unless Barrie's water is different, you can try either putting salt in the water (which will increase the boiling water) or better still using a deep fryer with either cooking oil or fat in it, these usually have thermostats fitted so you can set the temp. See, they have other uses than just cooking your fish and chips for tea. Just be bl...y careful when taking it out, mind you it isn't really any more dangerous than cooking the aforementioned meal.

BTW I used to own a Mk 5 Auster, ZK-AUH ex G-AJVU

 

[OOLILISSIMA]

I see you know G-AGOH as well, Barry. It so happens that it is the very one that I have photographed.

As you say, G-AGOH is actually somewhere between an Auster Mk 5 and an Auster J/1 Autocrat. However, the superstructure is Mk 5 rather than J/1. It has the Cirrus Minor 2 engine (and corresponding cowl shape) of the J/1 rather than the military version's Lycoming. It is in civilian markings despite essentially being the military type. Also, its internal fittings and upholstery are 1950s rather than the original (1945), owing to a rebuild after a major prang.

G-AGOH was originally used by the Blackburn aero engine company for testing their engines under development.

For the past 11 years, the aircraft has been displayed at the Newark Air Museum (very much worth a visit). Now, however, it is under notice to quit as Newark need the display space for one of their own aircraft. It's a beautiful example (if that's the right expression for a unique aircraft), and it ought to be in the air. I do hope at the very least that it won't be mothballed.

As for making the scale canopy, it looks as if I'm going to have to think hard about this. When I estimated the size before my original post, I hadn't taken into account the way the windscreen incorporated in the canopy turns back on itself along the fuselage, underneath the wing. I'm unsure, but I expect that will also have implications for the viability of vacuum forming.

 

[OOLILISSIMA]

I just noticed Kiwi's note about softening temperatures. I must admit that I'd assumed that Barry (Wonwinglo) meant Degrees Fahrenheit and hadn't given it much more thought. Now you mention it, 140 deg.F does seem a bit low. Could Barry have been thinking of a different type of plastic?

 

[alan2525]

When I made some aircraft canopies in the past by press forming them from thin Acrylic sheet they needed to be heated to about 185 degrees Celsius for the acrylic to soften.

 

[wonwinglo]

Not at this moment in time,all of my data sheets are boxed up ready for my move,and they will not be unpacked for some time.If you want to see the finest collection of Auster aircraft anywhere in the world then contact Mr.G.Baker at Carr Farm,Nottingham who has a whole hangar full of different marks of these aircraft,he holds the remains (rebuildable) of the rare Auster B.4 Ambulance freighter,and the last Auster B.8 Agricola low wing crop duster in ostensibly airworthy condition,the aircraft are worked on as and when time is available.

Wonwinglo wrote:I'm sad to say that I haven't, and am never likely to get, amnywhere near such experience and pleasure... missed my vocation, I think.

Is it possible, Barry, to clear up the apparent misunderstanding over softening temperatures of perspex and other acrylic sheet?

 

[OOLILISSIMA]

CORRECTION

I earlier replied to Alan2525 that the Auster J/1 Autocrat and Mk 5 had single-piece canopies. Having looked through my photographs again, I am now almost certain that they are in two pieces.

I am going to see G-AGOH again next week, so I will be able to make absolutely sure, then post again.

I can't believe that I could have got such a major detail so wrong despite having "crawled all over" this plane no fewer than three times already!!

I thought I had better mention it on the off chance that I misled someone.

 

[OOLILISSIMA]

:damnit:

Further apologies. Even more embarrassingly, having been for a further visit to measure G-AGOH today, I have to report that the canopy is in THREE pieces.

This will be clear from photographs that I shall be posting over the next few days in my gallery.

The good news is, of course, that it makes moulding the canopy much easier.

 

[wonwinglo]

Paul,did you also know that clear canopies can also be made from clear casting resin obtainable from art shops ? this is normally used for encapsulating small objects,firstly you need to make a plug from wood or plaster then stretch cling film tightly over the former to aid release,break the surface tension on the cling film first by applying one thin coat of model aeroplane clear dope,clear casting resin is then mixed up and applied in thin layers with a brush,once cured the resin form is gently eased from the plug and trimmed up.

The secret of using the resin is to measure out the quantities carefully,mix and allow to settle so the air bubbles go away,and always use well in date material and not old stock.