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HomeAircraftHelicopterCentury Radikal G20-30 N640 Hawk Predator › Radikal G20 Tail Problems
08-27-2013 12:43 AM  5 years agoPost 1
RocketDog

rrNovice

College Park, MD

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I'm having a problem with my Radikal G20, I hope someone can help because I'm completely out of ideas. Helicopter is flybar'd with a belt driven tail.

I had about 2 gallons through my G20 when the tail hub failed in flight and it threw a tail blade and grip. I auto'd down and had a successful landing. Damage was missing blade grip and blade, broken tail hub (sheared off at the radius change), broken tail shaft, and I was unable to remove the tail pulley from the broken tail shaft.

I didn't like the design of the tail hub (how some of the material is faced off to get adequate pitch travel), so I replaced it with a Trex 600 V2 version along with the 600 V2 tail pitch slider. I also upgraded to the larger aluminum tail pulley and follower. And I obviously replaced the broken tail shaft. All bearings are good, no slop, not notchy. The whole tail system is very tight, slop free, and extremely smooth. I can drive the tail system from the servo or the tail blades effortlessly. All blades are balanced.

After replacing the mentioned parts, my tail now wags inconsistently in hover. If I reduce the gain, the wag amplitude gets larger and more unpredictable, but at a lower frequency. Increasing the gain causes lower amplitude, but higher frequency wagging. If I go to rate mode, there is a little less wagging, but my inputs are muted and very inconsistent. It is extremely difficult to hover in rate or HH mode.

I have since burned almost a gallon of fuel trying to track down the problem, here is what I've tried:

a) 2 different tail servos, the original one and a brand new one. They heat up if I try to fly very long, I assume because they are working so hard with all the wagging.

b) 3 different gyros including the original. All gyros behaved similarly. Current gyro is a JR G7703D.

c) Different belt tensions. I noticed with the new pulley my belt was a little bit tighter than before, I extended the slots at the tail case so that I could try less tension. I have swept the range from loose to tight, no noticeable difference.

d) Gyro Tapes. I have tried every conceivable combination of gyro tapes. Everything from very thin double sided tape, thick foam tape, align gel tape, then I introduced a metal plate in between layers of the different tapes. Sometimes I could see minor improvements, but never significant improvements. Before the incident I only had one layer of Futaba gyro tape. I did have a small tail drift problem though, and was never really happy with how the tail held. It would blow out on fast tail slides.

e) Engine tuning. I read if your engine was not tuned, it could induce vibrations. It shouldn't have changed from before the tail failure, but I was getting desperate, so I played with the needles. No change. I cannot even see any vibration in the vertical fin, the heli looks like it is running smoothly.

I've taken the heli to some local, much more knowledgeable, pilots than me and they could not figure out what was causing the problem. None of them have flown a gasser though. A big thank you if you've managed to read this far, I know it's a lot. I appreciate any and all ideas and help. Heli Specs are below:

Radikal G20
+stock belt driven tail
+stock flybar head
+sprag auto unit
+frame stiffeners
+aluminum tail pulley and follower
+Trex 600 V2 tail hub, grips, and slider
Mavrikk G5 Wide 620mm blades
Edge 95mm tail blades
Stock Zenoah G20 with RCEXL ignition
Century air filter
Futaba GV-1 governer
align 610's on cyclic
JR G7703d gyro
JR8900G tail servo
Reactor X2 voltage regulator
single 2s lipo (Rx and EI on same battery)
IR kill switch for ignition

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08-27-2013 06:09 AM  5 years agoPost 2
RM3

rrElite Veteran

Killeen, Texas - USA

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I have had and flown a G20 for almost three years and have seen two others do the same thing to include mine...they ALL have some sort of tail wag, either a tiny bit, random tiny corrections, or constant annoying ones, then one day they are not there . My G20 is moddified alot...and most of the time the wags (tiny ones) are not there...then one day, there it is...and nothing changed mechanically, nor weather, nor tune.

My guess is its somtimes some sort of harmonic that is not seen with the eye during flight that causes these strange tail issues on the G20 and are being picked up by the gyro sensor despite the tape, foam or gel mount.

you have to take into account that the G20 is one of the lightest gassers out there...and that in itself can be a problem given the fact that it uses a gasoline engine which is known to produce more vibs. Tune, balancing and vibration absorbtion (by the frame) all play a part...but honestly its alot of trial and error to get the wags to go away.

remember you did change the tail mechanics, so now you need to bring everything else to work with that change (sounds strange, I know)...

So since you tried most everything else...try changing things like head speed, main blade brand...

also,

you mentioned that you managed to bring her down ok...but the G20 also has rather soft main shafts and feathering shafts...belive me when I say these are made from rather soft alloy and sometimes bend slightly...even with rough landings, so you may want to check those too. Also check the straitness of the bottom aluminum frame and side frames...you'll be surprized how soft these sometimes are and they too tend to bend a bit with rough landings or in some cases constant tugging of a pull start over time...a bent bottom frame will throw the clutch bell/clutch shoe out of alignment and cause stange vibs as the two are no longer rotating in the same axis.

alot to take in, but I hope it helps some.

showing a preference will only get you into trouble, 90% of everything is crap...

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08-27-2013 11:52 AM  5 years agoPost 3
JuanRodriguez

rrProfessor

The Villages, Florida

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Are your tail rotor blades matched ? (weight/balance)

Did you balance your tail assembly prior to install ?

Been there, done that and old enough to know better.....

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08-27-2013 01:09 PM  5 years agoPost 4
Pistol Pete

rrProfessor

Seffner, FL

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Head speed?

If you are having drifting issues in RATE mode, it will most likely follow HH.

You should balance tail blades with hub assembled as mentioned. You may have one longer bolt by a thread or two that can make your day ugly.

Out off tune engine affects mainly the tail.

If using plastic canopy, remove it for troubleshooting your issue.

There are two different hubs for this bird. Which one came with yours?

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08-27-2013 02:23 PM  5 years agoPost 5
RocketDog

rrNovice

College Park, MD

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Thanks for the ideas everyone, keep them coming.

RM3 - It is not a minor tail wag, it's major. It is extremely difficult to hover. I also don't want to buy more main blades ($$$), I don't think they are a problem. The landing seemed reasonably soft, but I think I'll take your advice and just replace the main and feathering shaft. Anything else I should order while I'm at it? I think my frame is fine, the clutch is still aligned with the clutch bell.

Juan - Tail blades were checked for balance, but not the entire tail assembly. I will work on that.

Pete - Head speeds are (IIRC) 1700, 1800 and 1950. I try to test them all. I have a FG canopy, but most testing is done without it. It is the LT hub with G10 frames.

Someone also mentioned that I should check to make sure the thrust bearings were installed correctly, so I will do that too. Thanks.

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08-27-2013 05:35 PM  5 years agoPost 6
Pistol Pete

rrProfessor

Seffner, FL

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My experience has been that anything below 1800 is too low for this bird on 620s unless flying slow and scale with a well tuned engine. It's doable but you want/need blade speed for higher or emergency autos.

I started with 1800/1840/1880 to find a sweet spot and found it around 1885 with +/- 11 pitch range. For throtle hold I use -2 and while keeping blades level she glides in FF for some sweet autos.

Anything over 1900 and you're looking to possibly bog it if you're quick and heavy on the sticks.

***

Heavy landings may bend the frame and or the feathering spindle.

For spindle, remove one blade and turn bolt and if bent, one or both grips will oscillate. I remove plastic black or white ball that comes with it. Apply loctite to spindle thread vs bolt to avoid getting any in the bearings.

Check frame length againts a straigh edge such as a 6" steel ruler for straighness or remove landing gear and place on a flat surface for reference. The eyeOmeter is usually off by a degree or two.

***

Tail blades tension should be equal. Given align tail, I use same by the way, I also apply loctite to hub vs bolt and I don't go for the extra ummp to overtighten.

Ball to tail servo screw is around 12mm.

***

Belt tension is to where I can almost have them contact each other when pressing it forward of tensioner pulley.

M3x40 cap screw is tighten enought to keep itself gap free between right plate and vertical fin, keeping them parallel to each other.

***

I twisted by hand while forcing down the "fan hub assembly" onto engine shaft without the KEY. The key slot in engine shaft will shave inside fan hub, IF needed, in order for it to sit all the way in or bottom out. I also sanded down the KEYs flat side to ensure above.

***

The engine screws are the last to get tighten while turning the starter shaft by hand to ensure the most true assembly possible.

Hope these help as I had well over 100 flights out of my stock parts before upgrading. Sans one clutch and its one way bearing that I chucked to beginners errors.

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08-28-2013 01:16 PM  5 years agoPost 7
RocketDog

rrNovice

College Park, MD

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So I took the tail apart, checked all the bearings and the balance of the tail system, all was good. Reassembled and decided to do another test flight. The tail was holding. I have no idea what changed, but it is much better now. It was still not perfect, but my engine was 4 stroking some, so I will adjust the needles and maybe play with different gyro tapes again, and hopefully I can get it the rest of the way there. I have no idea what could have changed, any ideas?

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08-28-2013 02:48 PM  5 years agoPost 8
RM3

rrElite Veteran

Killeen, Texas - USA

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I have no idea what changed, but it is much better now....any ideas?
...what did I tell you...
they ALL have some sort of tail wag, either a tiny bit, random tiny corrections, or constant annoying ones, then one day they are not there

ahh viva la Radikal...

showing a preference will only get you into trouble, 90% of everything is crap...

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08-28-2013 03:07 PM  5 years agoPost 9
Pistol Pete

rrProfessor

Seffner, FL

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FWIW...four stroking engine = gyro nemesis.

Oil ratio? It took me several gallons to break-in or fine tune the stock engine. I finally settled at 40 to 1.

You will really know how badly the whole thing vibrates when you convert to FBL.

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08-28-2013 03:32 PM  5 years agoPost 10
RocketDog

rrNovice

College Park, MD

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I have put over 3 gallons through the engine, and I bought it used, so it should be broken in. I just started a new gallon of fuel, I mixed it to 32:1. I don't use a specific brand of oil.

I probably wont get a chance to fly again until this weekend, I'll let you know how it goes after I get the engine tuned. Hopefully the tail behaves.

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08-28-2013 10:14 PM  5 years agoPost 11
Gearhead

rrMaster

Vt

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the thrust bearings may feel smooth in the grip, or in your fingers, but in the air is something different,
most people when building a tail they test thrust bearings by putting the T-bearing assemble between 2 fingers and push together and twist, but they do not put side pressure on them, the washers can get dimples in the sides of the gruves, even waves in them from the balls, or the balls can become oval shaped or even flat on one side, if one or two balls have become oval you may not see this or even feel this, but once in the air when the grips get side pressure on them from the pitch the trouble starts

check the T-beaings well, even try replacing them

Jim
Buzz Buzz Buzz

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08-29-2013 03:47 PM  5 years agoPost 12
RocketDog

rrNovice

College Park, MD

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Gearhead - The tail is brand new, but I did take a close look at the thrust bearings. They look and feel new. When I reassembled them I held both blade grips and pulled radially with a fair amount of force while twisting the grips. They still moved smoothly. Is there a more definitive test?

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08-29-2013 06:42 PM  5 years agoPost 13
RM3

rrElite Veteran

Killeen, Texas - USA

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wonder if your clutch is slipping causing the tail issue...like Pete said HS is important but I wonder why it affects the gyro... so what pinion are you using, 13 or 14? is the clutch gap good? (doesnt take much to eat up the liner either BTW)...a slipping clutch will cause random tail corrections as the torque from the engine is not constant and therefore loading and undloading the rotor cauing the tail to try to compensate.

Radikals seem to have a sweet spot when it comes to HS and engine RPM (when the engine is tuned just right) that the tail will hold steady...and depending on your setup, it can differ from some one elses as well...below that and you get drift, wobbles and tail issues...above that and the drift get bad, tail gets bad and in some cases FBL systems want to throw the heli around.

showing a preference will only get you into trouble, 90% of everything is crap...

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08-29-2013 07:43 PM  5 years agoPost 14
Pistol Pete

rrProfessor

Seffner, FL

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Good call on the clutch liner.

If I recall correctly, Century liner avgs .040 thickness.

I was advised to shoot for a gap of =<.012 and replace liner when at .020 before clutch goes .

I leaned high needle until I got around 240F avg +/-10F measured between second and third fin. That may help take care of your four stroking.

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08-30-2013 03:16 AM  5 years agoPost 15
Gearhead

rrMaster

Vt

My Posts: All  Forum  Topic

Rocket, I don't think I can answer that question, but I just remembered this, as you know Century buy's their bearings, well some time back a while Century did get a few Tail Thrust Bearings whereby some of them had the wrong size hole and this created problems, maybe this is your problem, but then agin yours may be Align bearings

as for the Clutch, before you take it all apart try lowering your Pitch Curve just off Idle as your going into Spooling up, this will let the clutch grab stronger

Jim
Buzz Buzz Buzz

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09-06-2013 01:48 PM  5 years agoPost 16
Pistol Pete

rrProfessor

Seffner, FL

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Any good news?

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09-09-2013 12:34 PM  5 years agoPost 17
RocketDog

rrNovice

College Park, MD

My Posts: All  Forum  Topic

Everything seems to be fine now, I guess something was miss-aligned with the tail, but it's holding good now. I have been tuning my engine, which I'm apparently really bad at. My temp gun is really inconsistent, I'm going to have to get a new one. I'm just leaving it on the rich side until then. Thanks for your help guys.

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