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500 size Apache Gunship Build

Had a close call with the Apache yesterday.. was cruising around and having a great flight after finally getting it dialed in to how I want it to fly. I was about 30 ft in the air going into the wind over the runway, and heard the unmistakable sound of a gear being shredded. Sure enough, I lost most of my power and entered an autorotation, and made a successful run-on landing on the runway. Only damage was a busted tail wheel from a runway pothole.

When I opened up the canopy, about 20% of the main gear was stripped clean. I honestly think this main gear was not designed to take the flight loads of a 4 blade head and tail. For the moment, I think I am going to limit myself to about 25 flights before changing main gears is required. They are cheap enough that it is worth the extra caution. I have almost emailed Roban about the issue, and hopefully they use a tougher gear material in the future.
 

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PsyBorg

Wake up! Time to fly!
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Kinda looks like an alignment grind. There are no wear marks on the pinion gear so that usually means the gear mesh worked loose.Did a motor screw slack and the motor drifted maybe? I learned that lessen quick with my nitro heli. Trying to balance the pinion tension with the tail rotor belt tension and not smoke the clutch was a serious balancing act.

Oh and Glad you brought it down safely... Sorry I get scope locked on something and forget the humanity part of posting.. Don't ever get assimilated its a pain at times.
 
Thanks guys, it failed at the right spot, and right time. If his had happened over the grass, it would not have ended well.
Kinda looks like an alignment grind. There are no wear marks on the pinion gear so that usually means the gear mesh worked loose.Did a motor screw slack and the motor drifted maybe? I learned that lessen quick with my nitro heli. Trying to balance the pinion tension with the tail rotor belt tension and not smoke the clutch was a serious balancing act.

Oh and Glad you brought it down safely... Sorry I get scope locked on something and forget the humanity part of posting.. Don't ever get assimilated its a pain at times.
Thanks for the troubleshooting advice Psy! I did check the alignment with a piece of receipt paper, and it still seems fine for the remaining teeth. No evidence of a sliding motor. I might try to set the alignment a tad closer with this next gear though if at all possible without putting any side load on the motor shaft.
 

PsyBorg

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Ohh just thought of something too. Check the thrust bearing (I know you know this already but its in my head and needs to get out hehe) and make sure there is not a lot of vertical play so it eats the lower part of the gear faster then once it hits a certain point it gobbles the rest.
 
Well, things could have gone better today. I took the Apache to the Texas Scale Championships today. First flight went awesome, the chopper performed perfectly and I completed all of the maneuvers without too much difficulty. The second flight however, did not go well. I was climbing into a wingover turn, and heard the dreading shredding noise again. Power dropped almost immediately, unlike last time where I was level and at altitude, I was pointing downwind, with the nose aimed at the sky at about 50 ft. I was able to get the bird level, but couldn’t recover enough RPM for a successful autorotation. Keep in mind this is a 7.5 lb heli 500 size bird, so she drops like a brick without power. It hit level, and in the tall grass about a foot off of the side of the runway.

Damage was admittedly nearly not as bad as it could have been. Since pretty much all the rotor energy had been expended, the head and blades are undamaged. The worst is a few large rips in the fiberglass, and a broken gear strut. Otherwise, it was just the glued on detail pieces that came off. It will fly again, but the main gear really concerns me. There was no clear point of failure like last time. Pinion was still aligned properly, one way was tight, and it performed the exact same maneuver before without issue. I’m starting to think that looking into a custom CNC gear might be a better way to go. Or, just sticking to level flight and low torque maneuvers.
 

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PsyBorg

Wake up! Time to fly!
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Wow that really sux. The good side is you are a good enough pilot to take it in as controlled as you did. that says a lot about your ability to think and remain calm under serious pressure of all that happening so quickly. Again Well done on the save.

Were you in a hard yaw when this happened? I am wondering if your tail rotor servo is pushing the mechanical limits and causing binding and heavy resistance on the main gear and when that gets released it surges the main gear on the pinion then the pinion catches up and slams it back into a powered state. This would put a lot of pressure and wear on a nylon gear train.

You might want to check without the pinion gear on before you get it back together and feel how free the tail rotor spins when the servo is placed at full deflection in either direction and feel the resistance when spinning it by hand at the rotor head. If that is binding and then releasing constantly that could easily eat thru gears.

The only other thing I can think of is the pinion is not square to the main gear either by the motor mount having a slight bend or the main rotor shaft is not perfectly aligned vertically.
 

Namactual

Well-known member
That sucks man, that was such a beautiful bird.

I am glad it is salvageable though. And while I know "about two things" regarding helis, I would agree with getting a custom alloy gear. I am not sure what that would cost, but I am sure it would be worth it considering what has happened twice now.
 
Thanks Psy, I’m not sure I would call it a save, but it at least turned from a crash into a hard landing. As for staying calm, first priority is fly the aircraft. You haven’t crashed until it’s in the ground!

I have been looking over the airframe, and it doesn’t seem that the tail slider was binding at all. I was not in a hard yaw. I was climbing up for a wingover turn, nose pointing toward the sky, but had not started the turn yet. I also checked the motor mount/mainshaft alignment and it looks good to the best I can tell. The gear really seems to be the culprit. 2 stripped gears back to back on entrys to the same maneuver seem a bit telling to be. I’ve never stripped a gear in flight on any other heli I’ve owned, and yet it’s happened 3 times on the Apache. The gear that the Apache uses, is made of nylon and seems more flexible than your typical align gear. Making it out of a harder plastic would very likely increase part life and damage tolerance.

Namactual, I am considering it for sure. The issue is that metal-to-metal gearboxes start to have heat and wear problems. That’s why full scale aircraft use a wet oil filled gearbox design. A harder plastic or composite would the preferred solution, but I have to see what I can figure out. Will be doing all I can to get it back up in the air, and reliably operating!
 
Finally getting back to this project after a few months. Apparently Roban moved factories, so production of new parts stopped for several months. The new landing gear and main gear are finally on the way here. If all goes well, I’m hoping to have the Apache back in the air by Flite Fest TX.

In the mean time, I wanted to try to tackle some of my issues with intermittent tail rotor authority. The first item on the list was reversing the direction of tail rotor rotation. For the most efficiency, your forward-most tail rotor blade should be rotating up into the rotor wash. The opposite was true on my model due to my angle gearbox. So, I decided to switch up some of the internal gearing, by moving the bevel gear that turns the drive shaft from the top to the bottom side. This involved very carefully pressing off the stock gear, and securing it on the other side with some JB weld and a pin. It’s nice and solid now, and the tail rotor direction is correct!

This is only part 1 of the fixes. I also have a few pieces on the tail rotor itself that I want to change that I’ll get to next..
 

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Finally got around to working on the rest of the tail rotor. I had 2 main issues with the CNC helicopter tail rotor I’ve been using. Problem #1 was vibration. The “barrel” that slipped over the tail shaft was not machined 100% correctly. When rotating that rotor by hand, there was a slight, but noticeable wobble at the end of the barrel. Problem #2 was a lack of full throws. The tail rotor only went to +/- 25 deg pitch or so. Most tail rotors can go +/-45 deg, and this definitely contributed to my tail authority issues.

Old tail rotor (CNC Helicopter)
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The solution was to start from scratch, and build my own “cascade” type tail out of 2 stock Trex 500 tail rotors I pressed the bevel gear off of the stock tail rotor shaft, and pressed it on to a longer 4mm shaft. I also will be drilling a pin through the gear to pin it to the shaft. The two tail rotors are linked together with Trex 450 ball links. The result is balanced tail rotor with +/- 45 deg of pitch. And, it uses stock Trex parts, so it will be easier to repair if anything happens. One step closer to getting back in the air!

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PsyBorg

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Wow that looks impressive!
And heavy.

I thought you had a 4 blade hub assembly on that not two pairs offset like that.

As I said IMPRESSIVE.

Having that added 20 degree travel will be HUGE change in tail authority. You might be into full on 3d range control now hehe.

Would be so cool seeing you out competing with all the saab goblins with a Apache hehe.
 
Wow that looks impressive!
And heavy.

I thought you had a 4 blade hub assembly on that not two pairs offset like that.

As I said IMPRESSIVE.

Having that added 20 degree travel will be HUGE change in tail authority. You might be into full on 3d range control now hehe.

Would be so cool seeing you out competing with all the saab goblins with a Apache hehe.
Thanks Psy! Definitely should be an improvement. And without the long “barrel” that slips over the tail rotor shaft, this is rotor is actually lighter! Don’t think I’ll be competing with Goblins anytime soon, but it would be sweet if I could make it capable of some scale aerobatics like loops, rolls, or wing overs. Limiting factor is still that main gear though. Might end up going with some CNC aluminum work there.

As for the design of the tail rotor, it is certainly unique! The full scale bird is arranged the same way with 2 offset pairs of 2 blade tail rotors:
36953EFB-82DA-48D3-A5D6-BC488ACC1A38.jpeg

This was done to let the blades fly in more clean air, out of the turbulence of the other blades. This, along with the angle offfset, was done to reduce tail rotor noise levels.
 
Sorry, I JUST saw this thread. I don't know the first thing about making an RC chopper but I have around 30 years working on the real thing so if you ever have a question about what goes on what model or such I'll be glad to help.
Thank you for your service! Definitely might have to hit you up on some of the fine details once she is back to flight status. So far, I’ve mostly been working from walk-arounds and other internet photo resources. I am a Flight Test Engineer at Bell Helicopter, which has made it easy to hunt down info for other projects (Huey and Kiowa), but finding info (especially on the A model) for Apaches has been a lot more difficult.
 
After a lot of back and forth, I finally found a place willing to make me a gear. It is a two phase operation. First, the gear has to be machined without the teeth on a machining center. It will come out of that machining looking something like this:
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Then, I take the gear to a gear specialist that will cut in the teeth. Hopefully, it will be flying again soon with a new aluminum main gear installed.