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Great Planes Kunai, fuselage breakage

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Great Planes Kunai, fuselage breakage

Old 07-10-2022, 01:42 PM
  #1  
baerster
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Default Great Planes Kunai, fuselage breakage

Hello,

I bought two used Kunais. One is assembled and had been flown and had some repairs before I got it, while the other is still in the box and has not been assembled. I am learning to fly the assembled one before I put the in-box one together. I am a beginning-intermediate pilot, and I have had two slow-speed crashes with the Kunai that resulted in the fuselage breaking just aft of the wing, because of how the wing tip hit the ground and the twisting force that stressed the fuse. I have repaired it twice with epoxy and fiberglass cloth. My question is about whether it's normal for a fiberglass fuselage to break even if the crash is only about 5 mph. I suspect it is normal, and that I have been spoiled by foam planes that can take more abuse. As I mentioned earlier, I am going to improve my flying skills before I put the other Kunai together and risk breaking that one. I am also going to try using a nylon screw to hold the wing on, so that it might shear off before the fuselage snaps if there is a twisting force on the wing in a crash.
Old 07-10-2022, 07:57 PM
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tedsander
 
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I've had two Kunais - and yes, for that plane pretty normal. It is built to be reasonably light, which means not a lot of material for the fuse. With that long wing hitting the ground, the force at the root attachment point is really strong and concentrated. RCGroups has some very long threads about Kunai's and mods that can be done to them - both to improve on durability and to turn them from mild to wild.
Old 07-10-2022, 08:21 PM
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baerster
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Thank you much! I have been looking for the threads on Kunai mods, but have only found threads on how to make it go faster. By any chance would you happen to know where I could find the thread(s) on mods for better crash resistance? No worries of course if you don't know. I am just glad to know that my fuselage breaks are to be expected given the design of the plane. It seems like a great glider platform, and fortunately I have been able to work with an instructor who has shown me how to fly it. That said, I have still gotten into crashes on my own, and it may be many more hours of stick time for me before I am able to reliably fly it without having to glue it back together.
Old 07-11-2022, 05:47 AM
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speedracerntrixie
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The Kunai fuselage is weak due to the construction method used. First they put in the mold a heavy layer of gel coat followed by a single layer of loose weave fiberglass cloth. It’s also polyester. IMO they should have gone with an epoxy paint with a couple layers of lighter weight fiberglass with the weave in 45 degree and 135 degree orientation. Obviously that would be more expensive. There’s not much you can do to strengthen it without adding weight.

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