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Old 08-09-2015, 01:10 AM   #1 (permalink)
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Question What is that "skidding" noise during a funnel?



No, I cannot fly like that. Let's get that straight first

However, it's an awesome flight, and it reminded me of something I'm curious about with my own helis.

Starting at ~2:28, there is a secondary noise during the funnel maneuvers that sounds a bit like the tail is momentarily skidding/stalling before regaining grip. In this video it's quite subtle, but still audible especially when the heli passes close to the camera. Like a warbling "wah wah wah" with a frequency of 3 or 4 Hz, superimposed on the turbine sound of the Goblin. Seems to happen most often in funnel maneuvers.

What is the source of that secondary noise? Gyro fighting to keep the tail pointing sideways? Belt vibrations? Something else?
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Old 08-09-2015, 04:26 AM   #2 (permalink)
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Tail fighting torque from the mai rotor. Perfectly normal. I used to freak out when my first protos did that, tore it down and swapped tail servo only to find out later that it's nothing to worry about.

It could also be attributed to too high tail gain. Often I find myself having to compromise on gain and take into account the overall performance and not just how it works in one specific manouver.
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Old 08-09-2015, 06:13 AM   #3 (permalink)
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Yup, that's just the tail blades fluttering. The heli is moving along the axis of the tail rotor so there's a lot of load on it. Additionally every time a main blade passes by, there's some turbulence, so there's higher pressure and lower pressure air going over the tail blades. During low pressure the gyro feeds in more tail pitch because the tail starts drifting, then it has to let go again because the high pressure air is coming and the air has much more bite to it.

The shrouded tail boom makes it a bit louder, you got the same effect with a T-Rex 700 E with the "speed fuselage" on.

An easy "fix" for that problem is to have very slight bending pressure on the tail pushrod so it doesn't switch from flapping around in the pushrod support and being compressed under tail load. When the pushrod support is slightly off center, it alway keeps the pushrod bent a little bit. Gotta lube it well though or it'll squeak.
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Old 08-09-2015, 06:39 AM   #4 (permalink)
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It's something I'd personally look into.

I've never heard any helis (or Goblins) sound quite like that during funnels or hard 3D. Tail slider with too much slop, struggling servo?
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Old 08-09-2015, 07:56 AM   #5 (permalink)
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All 3 of my goblins sound like that in funnels, perfectly normal
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Old 08-09-2015, 08:10 AM   #6 (permalink)
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I agree with possible tail flutter. If it just started I would not dismiss it, but I may not be overly concerned either.

I have observed this a number of times this year on several models of Trex almost always during funnels. Almost every time it could be traced to mechanical issues with tail; often, boom supports.

If it were me I would closely check every part of tail including how your boom connections look even tho you don't have boom supports in the traditional sense.

My advice is from limited experience and I dont want to represent myself as one of these very experience HF pilots. But when I heard this during the manuver you were doing, I'm like wow that's exactly what i have run into often this year. And so I've learned more on this item from some good pilots with many years of experience.

Your flying looks very nice btw. Good luck on your diagnosing. And please follow up. I am interested in anything you might find.
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Old 08-09-2015, 08:18 AM   #7 (permalink)
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Quote:
Originally Posted by Badhelipilot View Post
All 3 of my goblins sound like that in funnels, perfectly normal
My post after yours started before your post. I concede any of my comments to experienced Gobbie pilots. If it's perfectly normal for Goblins that's great. Honestly I only know one person with a Goblin at this time. In fact I've even flown it easy a little bit. I don't remember his sounding like that either, but I do value other Goblin owners' opinions above mine.
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Old 08-09-2015, 08:51 AM   #8 (permalink)
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Quote:
Originally Posted by mqowen View Post
I agree with possible tail flutter. If it just started I would not dismiss it, but I may not be overly concerned either.

I have observed this a number of times this year on several models of Trex almost always during funnels. Almost every time it could be traced to mechanical issues with tail; often, boom supports.

If it were me I would closely check every part of tail including how your boom connections look even tho you don't have boom supports in the traditional sense.

My advice is from limited experience and I dont want to represent myself as one of these very experience HF pilots. But when I heard this during the manuver you were doing, I'm like wow that's exactly what i have run into often this year. And so I've learned more on this item from some good pilots with many years of experience.

Your flying looks very nice btw. Good luck on your diagnosing. And please follow up. I am interested in anything you might find.
That's not the OP flying.
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Old 08-09-2015, 08:53 AM   #9 (permalink)
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Sounds like the tail gain is just slightly too high and its on the verge of chattering. Not a big deal.
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Old 08-09-2015, 08:59 AM   #10 (permalink)
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Quote:
Originally Posted by seefest View Post
That's not the OP flying.
Duh...

I thought he meant he can't fly his heli like that with that flutter.
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Old 08-09-2015, 09:05 AM   #11 (permalink)
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Quote:
Originally Posted by mqowen View Post
Duh...

I thought he meant he can't fly his heli like that with that flutter.
For a moment there, it felt like I might be able to fly like that, since someone else clearly thought that was me. Then reality set in and my ego suffered a blowout

I just saw the video on YouTube and thought it was not only great flying, but also interesting because of the "skidding" sound. Thanks everyone for your input.
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Old 08-09-2015, 09:44 AM   #12 (permalink)
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My tail during right rudder funnels sounds like you are running a wood plainer. It actually scares non heli people. Nearly silent for left rudder. Assumed it's a Goblin thing but in this video his doesn't do it. It sounds bad ass and performs well so I haven't messed with tuning it out.

Any other Goblin owners want to chime in?
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Old 08-09-2015, 09:50 AM   #13 (permalink)
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I fly my G700C a lot at 1900 - 2000 and that sounds normal to me.
The Goblin boom definitely amplifies this sound, and I like it
The 3 bladed tail with 24T tail pulley sounds even more intense.

The tail gain sounds close to the edge, but that's where it needs to be to hold solid. Listening to the tail in this video, I'm guessing it may overgain at higher funnel speeds. Perfect as-is if the heli is never asked to do that though. The heli needs to be tuned for how you fly and what you will ask of it.

When tuning the tail gain, I like to do fast funnels and find the setting that just barely overgains (high speed oscillation / tail wag). Then back down the gain a few points at a time until I can't make it overgain at all.
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Old 08-09-2015, 10:59 AM   #14 (permalink)
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Quote:
Originally Posted by BP58 View Post
My tail during right rudder funnels sounds like you are running a wood plainer. It actually scares non heli people. Nearly silent for left rudder. Assumed it's a Goblin thing but in this video his doesn't do it. It sounds bad ass and performs well so I haven't messed with tuning it out.

Any other Goblin owners want to chime in?
During hard nose - right funnels, mine raises hell - I love it , yet is quiet in the other direction. However, mine doesn't oscillate like the one shown in the video... I would agree his tail gain is right on the edge of wag for his setup, and shouldn't be anything to worry about if it's suited to his style of flying.
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Old 08-09-2015, 11:07 AM   #15 (permalink)
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Def normal
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Old 08-09-2015, 11:25 AM   #16 (permalink)
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Thanks Skidpad and Excalibur, nose down clockwise full speed funnels. At least one per flight
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Old 08-09-2015, 02:02 PM   #17 (permalink)
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I don't fly a 700, but both of my helicopter make more tail noise in a right rudder funnel than left, and all other helicopters I have seen do this.
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Old 08-10-2015, 01:14 PM   #18 (permalink)
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It is the helicopter telling you it is not supposed to fly like that. It should be flown skids down in a scale like manner. See "3D is not for everyone" thread over at RCGroups for further information on this phenomena. ��������

ROG

Ps.......if you were to fly in the Norhtern hemisphere you would not here this noise!!����
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Old 08-10-2015, 03:08 PM   #19 (permalink)
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Quote:
Originally Posted by jmackey View Post
When tuning the tail gain, I like to do fast funnels and find the setting that just barely overgains (high speed oscillation / tail wag). Then back down the gain a few points at a time until I can't make it overgain at all.
Thanks, that sounds like a good plan.

Are tight right funnels the most demanding maneuver in terms of tail gain? In other words, if that oscillation is going to be heard, is it more likely during a hard right funnel?

My other pet concern right now is minimising tail blowouts during high-speed reverse, and to that end I believe more gain is a good thing. Does too much gain inevitably result in these funnel oscillation noises?

Does more head speed help to minimise both of these phenomena - oscillations during hard funnels and tail blowouts in reverse?
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Old 08-10-2015, 03:10 PM   #20 (permalink)
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The lower the headspeed the more 'noise' since the tail blades are really pitching strong.
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