70mm EDF Failure

[telnar1236]

A couple weeks ago, my modular trainer jet crashed due to a power system failure. I initially was thinking it was the ESC since I was using an old and somewhat unreliable one, but turned out to be something I have never seen before. The metal support for the motor stator and bearings fatigued through, and broke off, making the motor non-functional and, unfortunately, destroying the ESC. The EDF was a bit out of balance from being crashed a few times in my modular F-104 when I was developing it and I guess the vibrations were high enough and the metal cheap and brittle enough that a crack formed. I still think this is the best 70mm 4s EDF out there, but only if it remains in balance. If there is any significant unbalance, watch out! Like I said, I have never seen this happen before with an EDF unit, and I have flown units that were far worse balanced than this one, but I think I'll be respecting the importance of balancing far more.

The two shiny bits I circled used to be a solid piece.

 

[bisco]

bummer, good luck with the next one!

 

[telnar1236]

bummer, good luck with the next one!

It's such a weird and cool failure, I'm not even upset about it, I just have to laugh

 

[bisco]

i know, never seen that before

 

[leaded50]

thats a QX motor, not expencive ones.

 

[telnar1236]

thats a QX motor, not expencive ones.

Definitely a cheaper EDF, but it gets a ton of thrust and pretty good efficiency. Prior to snapping, which was due to unbalance caused by a crash, I was getting four-and-a-half-minute flight times and 1600g of thrust one a 4s 4000 mAh battery which I have not seen any other EDF replicate. But looking at where it broke, it does look like a case of bad design. There's a cut-out with very sharp corners that as far as I can tell serves no purpose apart from maybe saving 3g of weight or something but definitely results in a much weaker internal structure

 

[LitterBug]

A little glue, and it will be back in the air!! 😁

 

[Mr NCT]

A little glue, and it will be back in the air!! 😁

That should be an automated post!!!

 

[telnar1236]

A little glue, and it will be back in the air!! 😁

Maybe a bit of glue and a welder. The plane it was in is already back up and flying, but that motor is toast. When I was going through and repairing the plane, I discovered that the motor breaking like that killed not just the ESC, but every bit of electronics in the whole airframe except for the receiver and one servo

 

[telnar1236]

Update on this EDF: DO NOT USE IT. I had another failure with pretty much the same results, this time in an EDF that was only a couple of months old and was very well balanced. This time the rear ball bearing essentially exploded in the EDF and the little balls went everywhere locking the unit up. It appears that as soon as that happened, the unit went into over current and one of the bits of copper wire melted and stuck to the wall where it seems to now be permanently fixed. Again, with the result of complete loss of power/control in the airplane (my 3D printed F-104) and resulting in a crash. With the first failure, I was ready to chalk it up to my bad job at rebalancing it after replacing the rotor, but this time it was with a relatively new unit that had not been mistreated in any way. It had never been crashed, nor even really in any hard landings, and the plane it was in could only pull 6g at its top speed if you yanked all the way back on the stick.

While I cannot be certain on the exactly how it happened, it looks like the bearing race was either in two pieces to start off with or cracked into pieces. Either way, the forward piece slid forwards releasing the balls. When I removed the motor from the airplane, the whole shaft and fan had slid forwards about 3mm and the motor was pretty much locked.

The only conclusion I can come to is that either the quality control on these units is extremely lacking or that the design is inherently flawed and I would lean towards the latter. The use of a 3,400 kv motor on 4s in a 70mm unit results in an exceptionally high performance EDF, but I suspect that the materials are just not up to supporting a rotor at 54,000 RPM in a 70mm form factor. It's a real shame since apart from this I actually really liked this EDF unit design to the point I designed my F-104 around it. It is fairly efficient and powerful for 4s and was also pretty cheap. But now I'll be looking for a new EDF for my F-104 since two failures, both of which caused a crash is too much for me.

 

[Piotrsko]

Would you accept that they used cheap low rpm rated bearings? Fast is easy , high load is easy, but both together is expensive and somewhat counter intuitive. Wonder what the loads might be, but do know they get horrendous for a moment every so often

 

[telnar1236]

Would you accept that they used cheap low rpm rated bearings? Fast is easy , high load is easy, but both together is expensive and somewhat counter intuitive. Wonder what the loads might be, but do know they get horrendous for a moment every so often

I'd accept it is part of the problem. The second failure was definitely a bearing going bad. But the first failure it was actually the aluminum supporting the bearing that broke. To your point, fast and cheap don't go together, so I'm leaning towards blaming it on the combination of cheap parts and high levels of vibration from the high RPM.

I'm planning on using the freewing 4s 70mm fan as a replacement. Supposedly it gets 200 grams more thrust and is a bit lighter while only having a 2550 kv motor, but I'll believe that when I see it.

 

[telnar1236]

The mystery continues to deepen. I bought a thrust test stand to try and compare replacement EDF units and decided to test my remaining Powerfun EDF as well. There was good and bad. The positive - it gets 1770 g of thrust instead of the rated 1640 g. The negative - it draws 78 A instead of the rated 62 A. Basically, it seems the EDF unit is performing well above design specifications. My suspicion is that everything in it was designed/chosen for what it is rated to, so it just has a much shorter lifespan than what I might want because of its higher performance. It also explains the persistent overheating problems until I added a NACA duct blowing directly over the ESC. I was pulling almost 80A with a 60A ESC which is less than ideal.

Unfortunately, the replacement EDF (which is not in fact a Freewing unit, but looks almost exactly alike except for the spinner) only gets about 1500 g of thrust at 55 A. With a 20% reduction in thrust, my F-104 would still fly ok, but it wouldn't have the power for aerobatic maneuvering. It's also the only 4s 70mm EDF I'm aware of that came close to claiming the same power as the Powerfun unit. At this point, I'm debating a partial redesign to strengthen the structure enough for 6s so I can use something more reliable while keeping enough power.

So, does anyone have suggestions for a 4s 70mm EDF that generates at least 1800 g of thrust and can run on a 60 A ESC? I cannot fit anything larger. Apparently, it can pull up to 80 A since the Eflite ESC seems to hold up to it fine

 

[Tench745]

Maybe a dumb question, but are you sure that spec sheet is written for the motor you have in your EDF?

 

[telnar1236]

Maybe a dumb question, but are you sure that spec sheet is written for the motor you have in your EDF?

Completely sure, or at least for the motor it claims to have in it. It is the spec sheet on the exact page I ordered it from and my motor is labeled 3300 kv on the barrel, so it should be what it's claiming.

 

[Bricks]

Never checked on bearings for these electric motors , just wondering if an upgraded replacement bearings are out there? At the RPM that ducted run at I wonder if a little lubrication might solve some issues?