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Jun 11, 2019, 02:26 PM
Everything's A Composite
Knoll53's Avatar
This link is fairly heavy plane being bungee launched with minimal force for the maiden flight of new prototype. This design and this plane had never been flown before.
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Jun 11, 2019, 02:57 PM
An unexpected error occurred
Phaedra's Avatar
Thread OP
Looks great.
I do have a bungee, for my lighter models. For my Twin Astir, I would need to find a way to install a launch hook, maybe retractible?
Thanks for the idea, I'll seriously consider it.
Jun 12, 2019, 01:19 PM
Everything's A Composite
Knoll53's Avatar
For planes without a dedicated tow hook, I have fabricated a tape on tow hook that is attached with strapping tape. If it should fail under load, it shoots forward, not at you, so relatively safe. Opposite is true if the bungee anchor fails.

If you have a light bungee then plan on using multiple strands to increase the force. 8 strands may be right for your plane. This will reduce it's length, which is not a problem. You only need to be on tow for about one second, with the right bungee force. I find that a high force launch is the safest because it absolutely gets the plane flying right away. The moment it is off of the bungee, you can hit the power.
Jun 16, 2019, 01:00 PM
An unexpected error occurred
Phaedra's Avatar
Thread OP
Meanwhile, I received a replacement prop. I installed it on the prop adapter and balanced it.
When I tested it, I had quite a bit of vibration, so I feared a bent motor shaft or a shot bearing somewhere. I disassembled everything and inspected, but I couldn't find anything with the motor or the shaft. So it must be something with the motor mount. When I looked at the opening in the nose where the motor shaft comes out, it was cracked and a small piece was broken off. That can only mean that the shaft had been pushed aside enough to cause this damage.
I mounted the motor again, for extra leverage, then put some sideways force on it, and sure enough, it moved.
Looking inside the fuselage, I could clearly see where the motor mount had come loose from the fuselage.
I cleaned up the area around the cracks as good as I possibly could, and epoxied it back in. After the required drying and hardening period, I tested again, and it all runs smoothly again. Happy days.
Jun 16, 2019, 09:57 PM
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rrcdoug's Avatar
Good detective work Phaedra!
Jul 01, 2019, 12:22 PM
An unexpected error occurred
Phaedra's Avatar
Thread OP
Meanwhile, I have taken the Twin Astir to the field last week. When I tried to take off, the motor developed a loud vibration noise again, so I had to abort. It then dawned on me that I never tested at WOT after re-glueing the motor mount.
Luckily, I have a replacement motor now, and so I mounted that one in the fuselage. When I test at WOT now, everything is fine. This can only mean that something is wrong with the first motor.
I disassembled it and checked the motor shaft again. I still can't find any sign of a bent shaft. All three bearings show quite a bit of play when I insert the motor shaft into them, so I don't know what to think of that. One destroyed bearing after such an impact, I can understand, but all three? That just seems unlikely.
One thing left to consider, is bad alignment between the three bearings; the third one is mounted inside the extension which is bolted to the front of the motor. If that extension is only a bit off-center, it could develop these sort of vibrations. But how to properly align and test this....
Good news is that the Twin Astir is ready for flight again.
Jul 18, 2019, 01:55 AM
An unexpected error occurred
Phaedra's Avatar
Thread OP
I thought the vibration problems were over, but the very next flight, takeoff I still had excessive noise for the motor. After about two minutes of motor time, I wanted to start it again to gain some altitude again, but it didn't respond. Luckily, the landing circuit was free and I was able to land safely. Immediately after landing, I tested the motor, and I could see it would start, but it stopped almost immediately.
I let it cool down, the tested again for lateral movement.
At first, nothing seemed wrong, but when I really put some sideforce on the motor, it surely moved. The motor shaft coming out of the nose moved a couple of mm, which is a lot. Very frustrating, but the good news is that it wasn't the motor after all, it has been the motor mount all the time.
When I got home in the evening, I started disassembling the motor, and with a little reasonable violence, I was able to get the motor mount out.
From the forensic evidence, I could see that in fact there was only limited contact surface between motor mount and fuselage that was actually glued. That could explain why it came loose so easily.
I now made sure to have a larger glue contact surface, and glued it back in. I made one big mistake by not mounting the motor while glueing, so I didn't check motor alignment! I realized this way too late, and when I checked I noticed a LOT of side-thrust to the right. So much that the motor shaft touched the fuselage. Since it is a real pain to bolt the motor to the mount (long story), I didn't feel like getting it out again to correct this and I decided to widen the hole in the fuselage to make more room for the motor shaft. I wanted to try with this much side-thrust, since I always had to give a lot of right rudder on takeoff.
Yesterday was the first flight after this fix, and it went very well indeed. It tracked really straight on takeoff, even though I found the side-thrust to be excessive. But what I did notice was more motor noise than before, some sort of resonance. My guess is that the shaft still touches the fuselage at WOT.
When I look at the telemetry, I now have a maximum current close to 80A, which is a lot more than before (68?), so that could also confirm a problem with the shaft touching the fuse.
But the model now flies like a dream, I was able to use light thermals to pick up a little altitude at a time, and in strong thermals I had to chicken out and look for some sink. I only need to improve roll control and turning: some more aileron throw and use more rudder. And I have to do something about the side-thrust and the motor shaft touching the fuse.
Landing are still a breeze, even with the crosswind we had yesterday. Extremely nice model to fly, this one is really my favorite.

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