WIX Archives

OK.... How to make cockpit canopies! (long)

Posted by Paul McMillan on Mon Nov 19, 2001 05:33:45 AM

A while back I posted an article on this site asking how to make a new cockpit canopy. This was after I had seen the work being done on the TAF Oscars..

Brandon K. kindly responded (directly) to my question, and after consultation he said he wouldn't mind me posting his reponses to my questions.. I post the whole text of his article here and his answers to my specific questions for the interest of all... I am certainly more educated as a result!

Basically, there are four ways of making a new cockpit canopy.. These are:

1. "Blown Canopy"; The plastic (typically
polycarbobate- Plexiglas or Lexan) is placed in a
specially shaped frame and heated in an oven. Air
pressure is then used to "inflate" the flat sheet into
a bubble. The operator decides how much air to put in,
this is a free form operation with only the shape of
the base where the canopy attaches being controlled by
tooling. The "Malcolm Hood" was done this way I
believe.

PM Question: "I for one never realised that the term 'blown canopy' was really that. I had heard the term, but no one had explained to me how it was done. I suppose no two hoods were exactly alike, except at the base...."

BK Answer: "You are correct, no two are exactly alike unless a mould tool is used."

2. Vacuum Formed Canopy; Just like a vacuum formed
plastic model, the plastic sheet is heated and then
sucked into a female mold or over a male mold. The
vacuum is usually stored in a big tank and then
released more or less suddenly so the operation can be performed before the sheet has a chance to cool significantly. If you just evacuated with a pump, the sheet would start to cool down and might tear. Canopy moulds are typically covered with felt to prevent scratching or mark off.

3. Drawn; The heated plastic sheet is drawn by hand
over a form. This works OK on shapes that are
2-dimensional (flat wrapped or nearly so) in shape
like a P-40 canopy. If truly a flat wrap, you may be
able to just heat the plastic right on the mold and
let it sag to shape. You may need to shape the mould
to account for springback.

4. Casting or injection moulding; I think that modern
canopies (like F-16) may be done this way.

I have seen methods 2 and 3 done using plaster and
wooden moulds. These methods are highly operator
dependant (a "black art"), but have been mastered by
many homebuilders and really don't require much
specialized tooling. Often, an oven can be made from
heat lamps (like those used to keep fish and chips
warm at the take-away place) and bits of aluminum
foil. You have to watch out for hot spots though to
prevent melting or uneven heating/sagging.

Blemishes can be sanded out and then polished with
excellent results. Removing distortions in the plastic
from uneven thinning during forming takes experience.

The plastic is easily cut with hand and pneumatic or
electric tools, but drill bits require a special angle
to be ground on their tips (as well as slow RPM) or
the plastic may chip or crack during drilling.

I used to work at Fighter Rebuilders in Chino, as well
as being involved in some other aircraft relatated
endeavors. While I don't have lots of irect
experience, I have dealt with many transparencies over
the years.

Finally....

Have a look at the following two links:
---------
Excellent for the basics:

http://www.crclarke.demon.co.uk/Pages/English/Thermoform%20intro.pdf
-------
This looks interesting as well:

Eyes of Flight
Rohm and Haas, color, 1957, 33 min. $25
Order Online Now, with Secure Ordering http://www.tinmantech.com/html/vintage_aircraft_videos.html

If you want to learn how to handle, cut, drill, form
and care for Plexiglas, then this is the film! Many
experienced craftsmen have told me how much they
enjoyed this film. Special drills, saw speeds,
solvents and polishing techniques are shown, along
with the causes of "crazing". Includes footage from
Bell showing how canopies are blown. Warbird footage!
Great info!



Brandon
Daytime job: Composite Structures Engineer (design and
stress analysis) for The Boeing Company on the C-17
Globemaster III Transport- Long Beach, California
Weekend job: Restoring my Harvard MkIV at Chino
Airport


Thanks again Brandon.. Most interesting...


Paul Millan

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