PIK 20E canopy misfit

[john firth]

After detaching the canopy from the front end support to investigate the basic fit,

It is slightly proud of the fuselage front and rear.

It seems that the plexi has contracted pulling the frame ends inward.

Maybe relieving the midpoint of the rail would improve it.

Even for Canadians , Spring is nearly here.

John F

[Francis Savage]

Hi John,

I would check your theory at different ambient temperatures, before making any structural changes. On very hot days, >35c, I have trouble closing my canopy latches.

I assume differential thermal expansion rates between the acrylic and the FRP frame. Or the frame and the monocoque.

Frank

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[Michael Davis]

Several brand-new Arcus canopies have been reported to be difficult if not impossible to close. Factory said the gliders are made in a location where the ambient temp/humidity that might be incompatible with the glider's operating environment and that they couldn't (wouldn't?) be able to do anything to help. My friend, whom I fly with and who owns an Arcus, has to wet a towel and drape it over the canopy before moving the glider from his hangar to the flight line.  If he doesn't do this, the canopy won't re-close. 

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From: pi...@googlegroups.com <pi...@googlegroups.com> on behalf of Francis Savage <franka...@internode.on.net>
Sent: Friday, April 2, 2021 3:42 PM
To: john firth <johnf...@gmail.com>
Cc: PIK20 <pi...@googlegroups.com>
Subject: Re: [PIK20] PIK 20E canopy misfit

 

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[xjos...@aol.com]

The only final solution is: rework the frame and the canopy.

Had the same Problem on my 20E since I have it.

Last year I refinished the fuselage and worked the canopy in.

The workshop has to be minimum 25° C.

At the whole time you rework the canopy.

And you have to rework it in a manner that the canopy dies not stuck when you have painted.

[Bret Hess]

Very nice beautiful canopy and fuselage Joshy!

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[Bret Hess]

John, so is the main problem with latching the canopy?  A different fix might be to resize or refit the metal inserts (holes) that the canopy pins fit into.  

[Bret Hess]

Some reference numbers:  The thermal expansion/contraction of acrylic (plexiglass) is 2x that of fiberglass.  Acrylic: about 71 parts/million/deg C.  Fiberglass: 36 parts/million/deg C.    So if you glue your 1.5m long canopy on the fiberglass frame (fitted on the fuselage) at 15 C (60 F) then try to close it at 38 C (100 F), the plexiglass vs fuselage misfit will be (71-36) x 10^-6/C * 23 C * 1.5 meter = 1.2 mm too long.   Or, if the aft canopy end instead just slides up the slanting aft fuselage opening (about 45 deg slope), the hole will be 0.8mm too low for the pins.   

On Sun, Apr 4, 2021 at 10:22 PM Michael Stockhill <sto...@gmail.com> wrote:

I made the mistake of leaving my canopy unlatched one winter. It seemed to take a set and was very hard to latch the starboard side. That issue mostly went away after deciding to always latch it. Different materials coefficient of expansion are likely at play. It reminds me of herding cats.

[Francis Savage]

I ended up decreasing the bevel on the pins to be more shallow angle, and also owned up the entrance to the holes in the fuselage. Once the pins are started, things come together. But keeping the canopy cool with a cover definitely helps.

Frank

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[xjos...@aol.com]

The main problem is indeed the diffrend expansion between canopy and fuselage.

I´m often in enviroment with more than 40°C on the canopy.

When I took off I could only close one latch.

The other after take off when on altitude and the canopy did cool down.

Now, after rework and some more gap it works as it should.

Here a photo of a repair point which I heated to about 60°C for fast drying.

You can see, the canopy became much longer and poppt out of the frame.

The canopy latches were closed.

Naturally its better when the complete aircraft is heated.

Unfortunately, on ground inside cockpit, and with this the canopy, is more heated than the remaining glider.

My gap is now large enough that I can close the canopy latches at 40°C.

End of the year I´m ( hopefully ) in Namibia with the ship.

There I expext up to 60° in the cockpit on ground. Lets see what will happen...

[john firth]

Thanks to Brett for the thermal analysis; however my canopy frame I believe is carbon fibre ( a negative thermal coeff)

However also in the equation  is the elastic modulus of plexi.

My canopy has settled down after being latched overnight at around 5 C.  The front edge was proud  and

I have filed it to match the fuselage; not pretty but heh,  it is the laminar flow  that matters.

I have ground the shape of the latch pins to a wedge so the  the bottom engages first.

JMF

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[Bret Hess]

I'm glad to hear you got a working solution, John.

I've sanded my 1985 PIK down to the composite and I have seen carbon fiber only in the spar caps.  Did some runs use more carbon fiber?

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[avrontal]

If you are unfortunate to breaker the canopy frame, there in the middle wrapped by glass fibers there is strip of carbon.