Today I was finishing off my scratch built pusher Versa wing, with the idea of maidening her tomorrow. I built it a bit different from the FT directions, in that I did not used the FT pusher firewall. Instead, I bent two PVC gift cards (by heating them) in a square shape and glued them together. Drilled four holes and presto... I had a firewall. Or so I thought.
Once everything else was assembled I tested the whole plane set up on the workbench without a propeller: the Turnigy D2822/14 with a 30A HobbyKing ESC.and a 3S 2.2Ah battery ran WOT (wide open throttle) without issues for a couple of minutes, both ESC and motor getting a bit warm to the touch, but nothing out of the expected.
I then fitted in a 7x4.5 prop and repeated the test holding the wing with my hand. After 30 seconds or so, I noticed the motor started to point slightly upwards, and thought that it was just the PVC bending a little under the load of the motor. However, after another 30 seconds, the motor started to rattle noticeably. Then something hit the propeller and I immediately cut throttle. One of the bolts in the firewall had gone loose and aspired by the prop, fortunately not damaging anything or anyone. And I can tell for sure I had tightened those bolts very, very well.
But what was really happening was that the PVC firewall was starting to melt. The motor pointing upward was not due to the motor flexing the plastic, but because the firewall was deforming under the heath, and the bolt loosening was just a side effect of the PVC becoming flexible and allowing the motor to vibrate. In fact, the motor was very, very hot. I touched the motor and the result was a blister on my finger. The ESC however was only mildly hot, about the same temperature as when I ran the setup without a propeller.
So now I have three questions that I hope someone with more experience can help to answer:
1- Was such a good idea to build a firewall out of PVC gift cards? I don't think that motor temperature is normal, but obviously had I used wood or metal (heavier, more time consuming to build, and above all, not available at that moment) they would have not melted, but I seriously doubt the glue would have not melted as well.
2- Does the central hole in the firewall contribute to dissipate heat significantly? I did not bother drilling it, as there seems to be no way for the air to circulate via that route since the shaft is sealed with a bearing in that part, but perhaps I'm missing something.
3- Is such a high motor temperature normal? It does not look like so. The motor has been on a previously failed attempt to fly a Delta, so it has not been run much before, only crashed hard to the ground four times (problems unrelated to the motor, I used too thin wire on the horns and was not responding to input) On the last crash the shaft moved inward a bit and I just pushed it forward again. The ESC and prop are the recommended by HK. I tried again with a 7x4 prop, which gave noticeably less lift and took a bit longer to start melting the plastic, again not heating the ESC noticeably, but eventually the motor started to melt the PVC again.
On the other hand, I'm glad I did this test, otherwise the maiden flight would have turned my wing into a smash of crushed foam board and I'd be puzzled forever as to why. Lesson learned, don't underestimate bench/ground testing.
Thanks in advance for any suggestions.
Once everything else was assembled I tested the whole plane set up on the workbench without a propeller: the Turnigy D2822/14 with a 30A HobbyKing ESC.and a 3S 2.2Ah battery ran WOT (wide open throttle) without issues for a couple of minutes, both ESC and motor getting a bit warm to the touch, but nothing out of the expected.
I then fitted in a 7x4.5 prop and repeated the test holding the wing with my hand. After 30 seconds or so, I noticed the motor started to point slightly upwards, and thought that it was just the PVC bending a little under the load of the motor. However, after another 30 seconds, the motor started to rattle noticeably. Then something hit the propeller and I immediately cut throttle. One of the bolts in the firewall had gone loose and aspired by the prop, fortunately not damaging anything or anyone. And I can tell for sure I had tightened those bolts very, very well.
But what was really happening was that the PVC firewall was starting to melt. The motor pointing upward was not due to the motor flexing the plastic, but because the firewall was deforming under the heath, and the bolt loosening was just a side effect of the PVC becoming flexible and allowing the motor to vibrate. In fact, the motor was very, very hot. I touched the motor and the result was a blister on my finger. The ESC however was only mildly hot, about the same temperature as when I ran the setup without a propeller.
So now I have three questions that I hope someone with more experience can help to answer:
1- Was such a good idea to build a firewall out of PVC gift cards? I don't think that motor temperature is normal, but obviously had I used wood or metal (heavier, more time consuming to build, and above all, not available at that moment) they would have not melted, but I seriously doubt the glue would have not melted as well.
2- Does the central hole in the firewall contribute to dissipate heat significantly? I did not bother drilling it, as there seems to be no way for the air to circulate via that route since the shaft is sealed with a bearing in that part, but perhaps I'm missing something.
3- Is such a high motor temperature normal? It does not look like so. The motor has been on a previously failed attempt to fly a Delta, so it has not been run much before, only crashed hard to the ground four times (problems unrelated to the motor, I used too thin wire on the horns and was not responding to input) On the last crash the shaft moved inward a bit and I just pushed it forward again. The ESC and prop are the recommended by HK. I tried again with a 7x4 prop, which gave noticeably less lift and took a bit longer to start melting the plastic, again not heating the ESC noticeably, but eventually the motor started to melt the PVC again.
On the other hand, I'm glad I did this test, otherwise the maiden flight would have turned my wing into a smash of crushed foam board and I'd be puzzled forever as to why. Lesson learned, don't underestimate bench/ground testing.
Thanks in advance for any suggestions.
)
