Cocklpit heaters and canopy cracking

[MacCool]

I've read a fair amount about canopy crackling due to temp variations, and it seems like it would be an inconvenience that I'd like to avoid.

Around here, it gets cold. Today it was 20F (high) but last night it was -30F and this time of year such temp extremes are not rare. My airplane lives in an unheated hangar and its canopy is in great shape, but it was built in Atlanta 11 years ago and lived the major part of its life in St. Louis until I bought it. From a climate standpoint, Minnesota is a whole other ballgame. It's been through one winter here with no issues.

I have a cockpit heater that I've started using (Hornet 45). It's 120 volts and is designed to keep the cockpit at 55F. I can either leave it on 24/7, or I can turn it on remotely at the same time I turn on my Rieff engine heater, usually about 6-12 hours before a flight.

Question: Would using a cockpit heater put my canopy at risk for cracking in this climate? Should I use it all the time, intermittently, or skip it altogether?

[AlexPeterson]

No way to know the answer for a specific plane short of doing it a few seasons. My plane was in a hangar heated to 45F for about 18 years. I'd pre-flight inside the hangar and then drag it out into the cold. When I'd start the engine I'd hear horrifying clicks from the canopy for a few seconds as the cold air would blast across it, but never had a crack, thankfully. It all depends on how the canopy edges and holes were prepared, as well as how it is or isn't constrained, etc. The differential expansion of plexi vs steel is a bit absurd. One would think that a glued one is less problematic than a per-plans one, but I'm not sure the data proves that or not, as there are just too many variables.

Now that I'm in an un-heated hangar, I've wondered it an electric blanket draped over the seats might be worth trying. The heat that is inside the cockpit lasts about 2 seconds once you open it to get in, so the seat becomes the only thing that matters. The coldest I recall flying it from the 45F hangar was about -15F... I'm too old for that now!

[JeremyL]

Quick side question, how do you turn them on remotely? I like that.

[riseric]

I would suggest it depends on how the canopy/windsheild is installed to their frames.
I'm in a hangar that's kept to 5C wintertime.

I recently flew a few times with the outside temps down to -15C.
After pre-flight, the airplane is pushed outside.
After flight, back into the hangar at 5C.

Never noticed clicking or cracking sounds from the acrylics.
I flew at -18C, that was the coldest as to date in this airplane.
My canopy has 1/4" holes and silicone tubing glued in, surrounding the CS-4-4 rivets to the frame.
My windsheild has slightly oversized holes for the screws (with countersunk washers) in the rollbar. Some washers or spacers are between the windsheild and rollbar.
It's a small sample with only 22 hours flying, but all winter time.
No cockpit heaters.
No cracks to date.