Would they have most likely been damaged or chipped beforehand, or might they just spontaneously break for no apparent reason? Do they break at the hub or could they break anywhere on the blade? I've only ever broken a few tipping during takeoff, but I'd prefer not to have one break in flight. I fly the APC slowfly's. But these new APC's seem to have a larger hub, maybe to prevent breakage? http://www.apcprop.com/ProductDetails.asp?ProductCode=LP14055MRP
They shouldn't break mid-flight. I flew mine through a small tree branch that was in my way No damage. Maybe if you used the "sand paper method" balancing the prop vertically, then you would be removing material on one side of the hub. I've been using hot glue on the hub and sand paper on the blades, not sure if this would weaken your prop though.
Have you had one break in flight? I haven't seen that happen personally, although there have been numerous YouTube videos showing what appear to be prop failures, but it always hard to trust what you see on YouTube. I fly the original APC "slow flyers", too. One thing to be aware of is possibly over-tightening the bolt, as that could cause undue stress on the hub, and probably weaken it over time.
Hmm, I have even scrapped the APC props on a hard landing and they haven't cracked. Usually I have found the APC slow flyers very durable.
Tree are stronger than props. AMHIK. However, bear in mind that when you're spinning a prop at several thousand RPM, there is pretty significant lateral force on the prop blades, and each prop (at least on a Cinestar 8) is experiencing 1/8th of the weight of the aircraft when hovering -- and more than that when you are maneuvering. I've not seen or heard of an APC prop blade departing the hub in flight except when subject to impact by things that get in the way. Use a really bright light behind the APC props and you can see if there any hairline cracks. I've checked some of the props that broke and I can see internal cracks in the plastic. Andy.
I have not broken one in flight either, and I want to keep it that way - that's why I asked the question. I just have read often enough on forums where people say they crashed because of prop failures in flight. I thought perhaps it was just a factor of using hobby grade components - that maybe they sometimes just mysteriously fail. I would hope that if you did a good preflight and checked the props before takeoff then it would be unlikely that one would just break out of the blue.
I had one APC prop break in flight and impact the prop next to it and break it as well, it was on my hex and needless to say the death spiral that ensued was a sight to see, luckily I found if you hit the GPS come home the copter does a decent job as trying to stabilize the copter. I also used the same trick when I had one motor go out in flight and it worked even better, it stops the spin and gets the copter close to home where you can level the throttle and let it come down although it starts spinning again, but its more controllable and it allows for a less damaging, UN-scheduled landing.
Usually those APC's can take a lot of abuse. I smacked one on a pole once with my hex and it didn't even crack, needless to say I replaced it after landing just in case. Granted it was only an 8 Inch prop not sure if the outcome would have been same on a 14 incher.