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Old 06-08-2017, 01:59 PM   #1 (permalink)
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Default Lost my Tail

The belt came off the pulley. I was inverted, about 50 ft above Texas prairie. Just a sudden fast pirouette. Had a fairly soft inverted crash. Damage:

one frame spacer stripped by skid mounting bolt
one plastic skid mount broken
main blades delaminated
canopy dinged but useable

I was surprised that the skid popped off rather than broke. It's still serviceable. I thought the tail pushrod was broken, but it was just hidden by the belt

Here's the cause:
https://www.helifreak.com/attachment...1&d=1496947379

Here's how loose/tight the tail belt was when I re-strung it on the pulley. It was quite tight with the nut driver shaft under the belt. I didn't think it was too loose, but it will be tighter next time.
https://www.helifreak.com/attachment...1&d=1496947410

It's a pretty quick fix, but when I took off the tail servo to get to the push rod the servo standoff mounting spun. I had to take the pulleys out to re-tighten those bolts with new locktite.

By the way, rescue couldn't keep up with the rapid tail spin so never righted the heli. Since it was spinning farther away slowly, I gave up on it and hit throttle hold for a crappy semi inverted auto.

Dan
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Old 06-08-2017, 04:45 PM   #2 (permalink)
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Sorry this happened dogbreath.

Are the idler pulleys straight and square? Do both of them turn freely?

I run my tail belt quite loose and have not had this on happen.

Typically a belt will try to climb uphill, so I would look for alignment issues of the idlers.
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Old 06-09-2017, 03:09 AM   #3 (permalink)
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Sorry for the crash, at least the damage report is pretty short With the belt stuck and tail rotor off spinning must have been crazy

I can't see how the tail belt could have slipped in normal conditions unless it has gone through a sudden loss of tension. The idler and AR pulley gides are huge. Have you checked the tail boom support clamps? Maybe the boom have moved forward?
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Old 06-09-2017, 05:21 AM   #4 (permalink)
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sorry for the crash. I noticed in the pic showing how tight it was, was this crashed before? I noticed rub marks on the main pulley guides- I run mine without one guide and no chance of it coming off.
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Old 06-09-2017, 08:17 AM   #5 (permalink)
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Just posting to share information. With me, crashes are inevitable but with modern FBLs they're spaced much farther apart. I already ordered a new canopy as a spare, so when this one becomes unserviceable I'll pop the new one on.

No prior crashes, I had a tip over on an auto once. The rub marks were from me troubleshooting the gap between main & tail pulleys when I was building. I had the rubbing that was solved by moving the OWB a bit. I reversed things to troubleshoot and was rubbing the two pulleys together--cosmetic only. I had posted a video on that.

The belt tension test was done without moving the tail boom--that was the position it flew in. The belt has always run on the bottom of the gearing--so I will check out all the alignment things and the guides when it goes back together.

And yes, it was a minor crash--I landed in taller brush/grass. The Spirit says the vibrations were at 467 percent, the moment it hit the dirt (love telemetry!). I'll check it out pretty well before flight, but my old Skookums have survived much worse (I sent one flying 50 feet from a crash once).

Dan
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Old 06-09-2017, 03:56 PM   #6 (permalink)
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I did a teardown, and found out that the rotor hub is bent also. For those who don't know already, you can't check alignment by spinning the grip bolts--there's no spindle shaft to turn! Take the hub off the main shaft and you can feel the misalignment (if it's bent) by spinning the hub while holding the grips. Something new I learned.

Now some kit bashing. I did a lot of checking the tail belt guides, the belt, and the tail pulley placement. I decided that the guides needed to move up 0.5mm so the belt would run in the middle of the tail pulley. The pulley is 8mm wide, and the belt is 7mm wide. 3mm washers are 0.5mm thick, so I ground down the upper (longer) tail belt standoffs 1mm and added a washer to the bottom and top standoffs next to the carbon shelf. My test runs have the belt off the bottom of the pulley now. Plus, I can adjust up another 0.5mm if need be by moving the top washer to the bottom. Fool me once......

Now back to the freeway, which is already in progress.

Dan
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Old 06-10-2017, 06:19 PM   #7 (permalink)
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You can see the washer raising the guide up 1/2 mm. Just waiting on parts, and she'll be back in the air soon. Dan

https://www.helifreak.com/attachment...1&d=1497136636
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Old 06-10-2017, 06:51 PM   #8 (permalink)
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You might want to put tiny washers (82-3502) on both sides of the bearings on all idlers.

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Old 06-10-2017, 08:22 PM   #9 (permalink)
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The idlers run extremely smooth. I added the tiny washers to the spacers inside the bearing as required, so even if I mashed down on the bolts the bearing would probably turn smoothly. But that's great advice and anyone who doesn't have smooth running idlers/guides should consider that.

Dan
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Old 06-24-2017, 07:37 AM   #10 (permalink)
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Well, I lost the tail again. Sixth battery since repairs, doing a piroflip. It slipped under the pulley but this time wrapped so hard around the main shaft it pushed the bottom tray down and cracked it. I had marked the boom, and it had not slipped (trying very hard after, I couldn't make it slip).

Now I'm thinking that the belt jumps off the guide pulleys when it's quickly loaded. So...how tight is too tight? I had about 3mm play when pushing between the guide and the tail pulley.

I'm also wondering if my boom may be angled a bit low, compared to it's "perfect" alignment. I will have to test this. When one tightens up the clamps, the upper and lower trays bend towards each other slightly, although the actual angle of the boom is controlled by the bolts on the side of the clamps. I will check boom angle with a yard stick when it's back together. This time main blades survived, but tail blades didn't. Maybe the new belt will help as well.

Lower sheets & tail rotor hubs are out of stock currently at Experience RC, so it will be down for a bit.

I've had to comb the internet to find new servo arms for the JR servos, so if anyone knows a reliable source I'd like that info.

(Sigh)...Such a good flyer, and the dead tail piros are amazingly fast. Hitting rescue couldn't right the heli with it doing a blur piro, but the neg pitch gave me enough time to decide to hit throttle hold and give some positive pitch to crash (inverted) a bit softer.

Dan Dan the crashing man
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Old 06-24-2017, 07:45 AM   #11 (permalink)
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Quote:
Originally Posted by dogbreath View Post
Well, I lost the tail again. Sixth battery
I'm also wondering if my boom may be angled a bit low, compared to it's "perfect" alignment. I will have to test this. When one tightens up the clamps, the upper and lower trays bend towards each other slightly, although the actual angle of the boom is controlled by the bolts on the side of the clamps. I will check boom angle with a yard stick when it's back together. This time main blades survived, but tail blades didn't. Maybe the new belt will help as well.

Lower sheets & tail rotor hubs are out of stock currently at Experience RC, so it will be down for a bit.
Dan Dan the crashing man
Looking at my EXO, the tailbelt wont to run at the bottom of the drive pulley and i suspect as you the tail alignement against the main mast is slightly off.
I lube my belts and run the tailbelt on the tight side so hope i wont jump over the edge.
When checking the warp it does the same and on that one i have run the belt so loose that the taltpulley have slipped.

When it comes to parts i order from here and they mostlu have parts for my Warp and as far as i can see they are well stocked for the EXO to.
http://www.shop-compassmodel.com/Compass-eXo-500_1
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Old 06-25-2017, 11:16 AM   #12 (permalink)
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This is interesting, and as you have said I would look at boom angle as well.

I run my cogged belts pretty loose (minimum required tension that will hold against moderate hand torque) and have not had any indicators of a slipping belt.

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Old 06-26-2017, 04:05 AM   #13 (permalink)
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I think my Exos belt also rides on the lower side of the drive pulley. Slipping is not an option and I cant imagine how it can jump over the edge but obviously things can change much under tension. Hope we find a clue about what is going on!
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Old 06-26-2017, 09:22 AM   #14 (permalink)
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I had a crash this weekend. While not cause of the crash, the tail belt ran up over the main belt as a result. I don't worry that the belt rides on the upper or lower portion of the pulley. As mentioned part of the reason is the boom angle (not a problem). I run my belts about the same as Saje runs his. I've never had one slip off a pulley.

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