600 size UH-1N Build - 336th Rescue Squadron

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SuperStallion
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600 size UH-1N Build - 336th Rescue Squadron

Post by SuperStallion »

Over the last 6 months, I have been working on the 600 ARF Bundle Package for the Roban UH-1N. The bundle package combines the original UH-1N Scale fuselage, with bundled electronics, and a Chaos 600 airframe in one package. I decided to build and finish the Huey in the colors of the USAF 36th Rescue Squadron based in Spokane, Washington:
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They have operated the UH-1N for almost 50 years, and are currently the only squadron operating it for dedicated SAR missions. To date, the Squadron has completed more than 690 civilian rescues, and the build will be dedicated to their service. For more info on the 36th RQS, see this article: https://www.verticalmag.com/features/no ... pt-secret/.
Everything out of the box, it looked like this:
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And after some time with an airbrush, I finished painting it in USAF camo:
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For the mechanics, I kept things stock except for a few small changes. I switched it over the CCW rotation, and switched to Helitec blades on the mains, and KBDD tail blades.

In a rush to try to get it flying before Dalton, I decided to risk the first flight with the mechanics in fuselage. Very. Bad. Idea.

The aircraft spooled up smooth with no hint of vibration at flat pitch. As soon I got to light on gear heard a loud bang. The H stabs, and 3 of 4 doors simultaneously departed the aircraft. I immediately hit throttle cut. When the blades came to a stop, the windshield was cracked, pieces were laying around in the scale cockpit, and fiberglass around the swashplate was chewed up. The whole center console in the cockpit has broken loose (probably contains a 1/2lb of ballast or so), and the mechanics were loose on their mountings.

I then decided to do the smart thing, and put the mechanics on a test rig. I test flew the mechanics more and ended up switching tail servos to a BK7006HV. This greatly improved the tail hold, and after a few more flights I was satisfied with the tuning of the mechanics. Although I played with gains on the FBL, I found no track or balance issues and very little vibration as-is. I was never able to repeat the large abrupt vibration light-on-gear.

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After repairing the damage to the fuselage, I moved them back over to the fuselage on Friday and did a few test takeoffs and landings, and no sign of vibration issues when light-on-gear. Did a few patterns without issue as well. Just as I was about to call it a success, I heard a loud bang and huge vibrations followed. I cut throttle and auto rotated it to a safe landing. The plywood around where all 4 bolts are for mounting the mechanics was cracked and shattered, and it even sheared one of the 1/8" aluminum mounting brackets on the mechanics:

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At this point, I've only got 2 guesses. The first is that my initial ground resonance issues had damaged the plywood at one of the mounting points, and it failed in flight, and caused the huge vibration. Or, that I have a fuselage resonance harmonic that is lining up with a main rotor vibration frequency. Any thoughts? Has anyone ever had an issue like this?
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trooper7
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Re: 600 size UH-1N Build - 336th Rescue Squadron

Post by trooper7 »

FWIW,

It certainly would appear to be vibration based incidents.

I noticed that the push pull option isn't being used on the lever arms. Was that by choice?
Also appears the link rod on the right side servo may have minimal threads in the ball link?

Wondering if you did mechanics vibration runs - with and/or without blades and heads?
What head speed U B using?

Fuselages can change the dynamic qualities of the mechanics. I had to add additional lateral supports in my B222 and MD500 when installed in spite of being smooth when run on their own.

Food for thought.
Beating the Air into Submission One Flight at a Time.
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SuperStallion
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Re: 600 size UH-1N Build - 336th Rescue Squadron

Post by SuperStallion »

trooper7 wrote:FWIW,

It certainly would appear to be vibration based incidents.

I noticed that the push pull option isn't being used on the lever arms. Was that by choice?
Also appears the link rod on the right side servo may have minimal threads in the ball link?

Wondering if you did mechanics vibration runs - with and/or without blades and heads?
What head speed U B using?

Fuselages can change the dynamic qualities of the mechanics. I had to add additional lateral supports in my B222 and MD500 when installed in spite of being smooth when run on their own.

Food for thought.
I am running 1500 rpm, and I did do vibration runs of the mechanics with and without head and blades. I didn’t use the Push/Pull option due to interference with the aft servo on the left side. The geometry there makes it impossible to use the upper push/pull linkage:

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Since I did it that way on the left, I duplicated the setup on the right and never gave it much thought. If it was the linkage flexing, that would explain the dynamic nature of the issue. I’ll dig through the parts bin and see if I can get another push pull link added on the right side. I’ll also double check the thread engagement as well.
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Re: 600 size UH-1N Build - 336th Rescue Squadron

Post by goodhunting »

Align Trex 600 PRO uses this kind of servo setup for the third servo. But there is a spacer for it, so that it "sits" within the mechanics. This will enable you to use the push/pull rods as intended.

Klem
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SuperStallion
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Re: 600 size UH-1N Build - 336th Rescue Squadron

Post by SuperStallion »

goodhunting wrote:Align Trex 600 PRO uses this kind of servo setup for the third servo. But there is a spacer for it, so that it "sits" within the mechanics. This will enable you to use the push/pull rods as intended.

Klem
I actually bought one of those spacers just for that purpose, but it interferes with the aft swashplate arm:

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If the servo was ~6mm farther aft, it might work, but the Roban frames don’t allow for that placement. Both the manual (which is pretty much useless) and the one build log I could find online both appeared to locate the servo outside the frames and only use one pushrod on the bellcrank. If it comes to it, I might be able to dremel out some space to push the servo farther aft. Then maybe I could get the spacer to fit. At the very least, I’ll add a carbon tube over the linkage of the right side to minimize any flex, and double up the linkages on the other push/pull bellcranks.
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Re: 600 size UH-1N Build - 336th Rescue Squadron

Post by co_rotorhead »

I've built two of these now (because I'm a masochist, apparently). They are both flying great now, but DEAR GOD... it was a long, long road to get them there.

Below is a brief synopsis of my journey, but I have to tell you right now... I never found a single root cause or smoking gun. No quick fix. It was a slog. I hope you fare better than I did.

First one I built, exact same thing happened, but not as bad. Made the mistake of spooling it up for the first time on my concrete driveway (you think I'd know better by now). It was spooling up fine. At first. When the RPMs got close to 1500, which is what I had it governed to, it quickly went nuts. Crazy vibration, dancing on the skids, etc. The dog house popped off and up into the blades. Pretty messy. Dog house was trashed, but repairable, and some damage to the woodwork inside, also repairable. Bent pushrods and links. Frustrating beyond belief.

Went down the same road you're going down now. Pulled the mechanics, mounted them in pod and boom fashion, they flew great. Mounted them back in the fuselage, this time without the fuselage tailboom or doghouse. Also, spooled it up in the grass in my yard, NOT on the concrete. Still terrible vibration just spooling it up, but thanks to the softer ground, no damage.

From there on, I replaced every single part of the main rotor and tail rotor systems -- tried different main blades, main shafts, rotor heads, torque tubes, etc. At some point, I got it to spool up, hover and fly without vibration.

Then I built the second one. You'd think with all of the trial and error, this one would be a cake walk. You'd be wrong. I don't know WHAT it is about this particular model, but both fought me every step of the way. I've built a 600 Little Bird, 2 600 Blackhawks, an 800 Huey and a 700 Blackhawk and NONE of them gave me a tenth of the grief as these 600 UH-1Ns did. Anyway, in the end, I guess I just wanted it more. I refused to shelf them. (but there were times man... there were times)

I'm not trying to discourage you -- I'm just sharing my experience with this particular kit. If you can keep your patience, you CAN get this bird to fly:

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trooper7
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Re: 600 size UH-1N Build - 336th Rescue Squadron

Post by trooper7 »

It's hard to tell by the pictures but...

Currently the elevator servo is mounted to the outside of the frame. Would it work to mount it in the same orientation but from the inside and put the balls on the 'bottom' of the servo horn? And/or mount the other two servos with spacers/blocks to move them out from the frame a bit? This might free up enough room to double up the servo links?

Hope that made sense.
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Re: 600 size UH-1N Build - 336th Rescue Squadron

Post by co_rotorhead »

Would it work to mount it in the same orientation but from the inside and put the balls on the 'bottom' of the servo horn?
I think I tried mounting the elevator servo from the inside on mine at one point, but IIRC, once the servo horn (or wheel) is on, I think the pushrods were parallel to the A-arm assembly (i.e. exactly in the center of the frames, directly behind the A-arm)

I know that Joel (JD) had mentioned he actually used a longer pushrod on the pitch servo and put a small angle bends in it such that it could go over the top of the elevator servo and connect to the bellcrank.
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Re: 600 size UH-1N Build - 336th Rescue Squadron

Post by trooper7 »

trooper7 wrote:It's hard to tell by the pictures but...

Currently the elevator servo is mounted to the outside of the frame. Would it work to mount it in the same orientation but from the inside and put the balls on the 'bottom' of the servo horn? And/or mount the other two servos with spacers/blocks to move them out from the frame a bit? This might free up enough room to double up the servo links?

Hope that made sense.
ImageA7974-D81-3-A2-D-4-FE1-A3-A7-636-DEF9-AEE2-F by Rob McFadyen, on Flickr
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SuperStallion
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Re: 600 size UH-1N Build - 336th Rescue Squadron

Post by SuperStallion »

trooper7 wrote:It's hard to tell by the pictures but...

Currently the elevator servo is mounted to the outside of the frame. Would it work to mount it in the same orientation but from the inside and put the balls on the 'bottom' of the servo horn? And/or mount the other two servos with spacers/blocks to move them out from the frame a bit? This might free up enough room to double up the servo links?

Hope that made sense.
Tried that as well when I was initially installing the servos. Unfortunately, even with the ball links on the bottom of the servo horn, the pushrod was still interfering with the elevator arm. The only way I could get everything to line up and clearance properly was the way you see it now. Also, if you start moving the aileron servos outward the linkages interfere with the engine cowlings. Appreciate the suggestion though, as I said I'm still scratching my head on this one :frust: .
co_rotorhead wrote: I know that Joel (JD) had mentioned he actually used a longer pushrod on the pitch servo and put a small angle bends in it such that it could go over the top of the elevator servo and connect to the bellcrank.
Thanks, I'll give that a try! Dang that description definitely sounds familiar! Glad to hear that you eventually got it sorted, but it scares me that there was never a "smoking gun". Your Huey certainly looks smooth in the air though!
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