RunRyder RC
WATCH
 1 page 1256 views POST REPLY
HomeAircraftHelicopterCentury Radikal G20-30 N640 Hawk Predator › Tail shakes lead to Head suspicion
04-22-2016 08:28 PM  31 months agoPost 1
RM3

rrElite Veteran

Killeen, Texas - USA

My Posts: All  Forum  Topic

So after rebuilding my modded engine to replace a busted lower crank/conrod roller bearing resulting in machining of a new Titanium connecting rod and hardened bearing race. My modded stretched G20 is flying again...

but during the engine rebuild process I had installed a stock engine...it flew OK...but a tail shake was still present...it always has been to some extent. But this time it was really bad. the tail was shaking up and down about an inch.

So after tearing it completely down, taking alot of gear slop out of the tail gears tail drive, crown gears front and back, and main gear to pinion, balancing the main blades and tail blades to within a 0.1 gram...
double checked the tail shaft, Torque tube, and mainshaft

I still have the F-ing tail shakes!

so I check the main hub to see exactly how well the grips are running perpendicular to the main shaft with a dial indicator...the difference is 0.01...so I adjust the main hub pinch bolts to bring this error down to 0.002 (+-0.001)...even have the blade tracking perfect.

now with the new engine back up and running the tail is not shaking as bad as before...unless I bring the headspeed over 1800...at 1450 to 1600 there are no shakes other than a tiny tail fin vib which is common to all gassers Ive seenand in tune with a rich running engine. but the tail (and to some extent the whole frame) have the shakes in tune with the head rpm at over 1600.

So im really suspecting that the quality of the metal FBL head is the source of the tail shakes as I have covered and checked everything else...I have tried everything but a different brand head for years to get this fixed...

Im going to swap it out with an Align Trex 700 FBL head and see if that fixes anything....

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

PM  EMAIL  Attn:RR  Quote
04-24-2016 02:26 AM  31 months agoPost 2
Gearhead

rrMaster

Vt

My Posts: All  Forum  Topic

check the Balls in your Thrust Bearings (main and tail), some of the balls could come to have flat spots on them

Jim
Buzz Buzz Buzz

PM  EMAIL  GALLERY  Attn:RR  Quote
04-24-2016 07:22 AM  31 months agoPost 3
oldfart

rrProfessor

Vancouver, Canada

My Posts: All  Forum  Topic

You stated it is a "up and down" tail shake, so I take it you mean it is not a high frequency vibration (as one would see as a buzzing vibration of either the horizontal or vertical fin.

If so that is usually the result of the rotor head damping relative to the rotor speed you are set for?

What is you rotor speed and what blades are you using.

Phil

PM  EMAIL  GALLERY  Attn:RR  Quote
04-24-2016 03:43 PM  31 months agoPost 4
Pistol Pete

rrProfessor

Seffner, FL

My Posts: All  Forum  Topic

Which head from Century and dampers?

~~Enjoying the hobby one flight at a time~~

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
04-25-2016 01:09 AM  31 months agoPost 5
RM3

rrElite Veteran

Killeen, Texas - USA

My Posts: All  Forum  Topic

its the 50 sized metal head from Century...

there are no vibes under 1600 and yeah they are not a high freq vib, they are consistant with head speed, almost as if it had a very badly out of balance set of blades...there is side to side wobble under 1400 but thats from the FBL unit...but I get significant vertical tail shaking starting at about 1750 and it just gets worse from there.

it had the hard plastic dampers at first but they quickly wore out, then I replaced them with a hard rubber damper and it was ok for a while...they still feel good...

I do remember when It had the rigid plastic dampers that it worked really well but they didnt last but one summer.

I have nylon and delrin stock i can machine some rigid dampers for it, but I have an Align 700 FBL head i picked up months ago maybe I should try...

so you all say its a damper problem?

desired head speed 1850 - 1900, 696mm Rail Blades, 6.92 gear ratio (puts engine closest to HP peak at 12500)

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

PM  EMAIL  Attn:RR  Quote
04-25-2016 02:17 AM  31 months agoPost 6
Gearhead

rrMaster

Vt

My Posts: All  Forum  Topic

so you all say its a damper problem?
as for me, the only times I have had a up/down tail wobble was on my 50, the faults are what follows

A) tail rotor un-balanced with blades mounted, fault> one tail blade had a chip in it and was way light

B) main head dampeners bad, but the heli wobbled side to side as well as the tail bobbing up/down

note, I have seen flat spots on the Thrust Bearing Balls after a crash, if your thrust bearings have been in a crash of any type, even a hard landing you should check them

or, did you check your Tail Output Shaft

Jim
Buzz Buzz Buzz

PM  EMAIL  GALLERY  Attn:RR  Quote
04-25-2016 04:06 AM  31 months agoPost 7
RM3

rrElite Veteran

Killeen, Texas - USA

My Posts: All  Forum  Topic

all bearings checked, all shafts checked, all blades balanced, drive train aligned, all gears mesh from pinion to main to tail drive to crown front thru torque tube, to tail perfect.....

like I said the tail system was checked with blades on, I even used a dremel tool to drive the front crown gear and held the tail assembly in one had while I drove the tail up to operating speed...super smooth.

Front gear train is smooth, no tight spots...

blade tracking is perfect on both tail and mains

engine is smooth.

going to swap the head out...just cant seem to lock down this vertical tail shake.

will report when done...need to machine a bushing to allow the align head (12mm) to fit the 10mm shaft on the radikal.

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

PM  EMAIL  Attn:RR  Quote
04-26-2016 03:46 PM  31 months agoPost 8
RM3

rrElite Veteran

Killeen, Texas - USA

My Posts: All  Forum  Topic

well replaced the head with a trex700 fbl head...

vertical tail shakes are gone...

must be bad (too hard?) dampers...

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

PM  EMAIL  Attn:RR  Quote
04-27-2016 04:03 AM  31 months agoPost 9
oldfart

rrProfessor

Vancouver, Canada

My Posts: All  Forum  Topic

Yes, I would bet it was the dampers being too hard for the rotor speed you where set to turn at.

Phil

PM  EMAIL  GALLERY  Attn:RR  Quote
04-28-2016 12:57 PM  31 months agoPost 10
Pistol Pete

rrProfessor

Seffner, FL

My Posts: All  Forum  Topic

well replaced the head with a trex700 fbl head...
vertical tail shakes are gone...
must be bad (too hard?) dampers...
Going to test new dampers in old head?

~~Enjoying the hobby one flight at a time~~

PM  EMAIL  HOMEPAGE  GALLERY  Attn:RR  Quote
04-28-2016 05:07 PM  31 months agoPost 11
RM3

rrElite Veteran

Killeen, Texas - USA

My Posts: All  Forum  Topic

well its a century head...and no choice in dampers other than the super hard rubber ones that get harder over time....so have to source some softer rubber dampers for it.

tried shopping for a complete align head in 600 (the 700 head I have is for another project), so shopped around and found a complete trex600FBL at grand RC for $120.

Now both the head and tail will be Align with parts more readily available.

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

PM  EMAIL  Attn:RR  Quote
11-15-2016 04:11 AM  24 months agoPost 12
jjoos99

rrApprentice

Yellow Springs Ohio USA

My Posts: All  Forum  Topic

so what causes a high freq vibration in the tail fin? My gasser is rock solid at say 1600 but when I go into idle up with a 1800 speed I get the tail buzz.
thanks
jeff

PM  EMAIL  Attn:RR  Quote
11-15-2016 04:38 AM  24 months agoPost 13
RM3

rrElite Veteran

Killeen, Texas - USA

My Posts: All  Forum  Topic

I did solve it... at least with my Radikal... I had posted this question to ask and got all sorts of ideas and possible solutions but none panned out, until I went out and began putting my hands on the heli while spooled up without main blades. here is the topic:

hope it helps
https://rc.runryder.com/t812115p1/

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

PM  EMAIL  Attn:RR  Quote
WATCH
 1 page 1256 views POST REPLY
HomeAircraftHelicopterCentury Radikal G20-30 N640 Hawk Predator › Tail shakes lead to Head suspicion
 Print TOPIC  Make Suggestion 

 3  Topic Subscribe

Friday, November 16 - 9:42 am - Copyright © 2000-2018 RunRyder   EMAILEnable Cookies

The RC discussion world needs to consolidate. RR is now one choice for that. Its software is cutting edge. It hosts on-topic advertising. Help RR increase traffic buy making suggestions, posting in RR's new areas (sites) and by spreading the word.

The RunRyder Difference

• Category system to allow Rep/Vendor postings.
• Classifieds with sold (hidden) category.
• Classifieds with separate view new.
• Answer PMs offsite via email reply.
• Member gallery photos with advanced scripting.
• Gallery photo viewer integrated into postings.
• Highly refined search with advanced back end.
• Hosts its own high end fast response servers.
• Hosts thousands of HD event coverage videos.
• Rewrote entire code base with latest technology.
• No off-topic (annoying) click bait advertising.
Login Here
 New Subscriptions 
 Buddies Online