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03-08-2012 10:53 PM  6 years agoPost 1
Peter Wales

rrElite Veteran

Orlando Fl

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It wont stop wagging its tail, but only in HH mode.

The subject is a Trex 500 with one of them flybar doohickeys. The TR servo is a DS520 and the headspeed is 1750. Its aimed at a scale fuse so I dont want to go faster but going faster makes it worse.

I have dropped the gain down to 10% and up to 100% and every where in between. The linkage runs smooth, nothing is binding and in normal mode at 60% gain the heli is dead smooth, switch into HH mode and it wags cycling about once a second at low gain. At high gain, it does what you would expect wagging pretty fast.

I have tried a Logictech 6100 gyro and a GY701. Both wag the same. The sensor position seems not to make any difference and I have moved it from the bottom gyro mount to the one on top of the tail boom.

The tail belt is reasonably tight, not drum tight and not loose enough to skip a tooth.

This one has me stumped, any ideas would be appreciated

Peter Wales
http://scalehelicopters.org

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03-08-2012 11:00 PM  6 years agoPost 2
deanguy

rrVeteran

TX

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Curtis Youngblood told me one time the stock tail rotor blades are too short on that helicopter.. That he had to put a "special" T-Rexx 500 setting in the design of the Solid G

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03-08-2012 11:01 PM  6 years agoPost 3
GREYEAGLE

rrElite Veteran

Flat Land's

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Add a spare ball to the servo wheel - move it it a few mill's and flip the wheel to reduce the " Mechanical Gain " - leave the other ball in place in case you go back to it.

greyeagle

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03-08-2012 11:09 PM  6 years agoPost 4
Funknrock

rrApprentice

San Marcos, CA

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make sure the belt is tight or the TT is assembled correctly (pushed all the way together but not binding)

all your links need to be smooth, no binding. your tail (pitch slider, grips, blades) also need to be tight but not binding. your tail blades should be fairly well balanced also.

your ball link needs to be in the hole on the servo arm closest to the servo (10-12mm) and the arm should be 90deg to the boom.

i have not used either of those gyros so i don't know what gain setting they work best at.

oh, and make sure your transmitter and receiver are correct. gain wire from the gyro in the correct position and the radio looking at that position for gain settings. that one KILLED me for a week (when i was a newbie) til someone asked if my Tx was looking at the right channel!

Hope this helps...

My crashes are cooler than your flying! ;^)

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03-08-2012 11:29 PM  6 years agoPost 5
MarshallB

rrVeteran

Middle Arkansas

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One item to check on that particular heli is the tail grips and the thrust bearings if it has any. I know some folks put two radial bearings on there as a mod. I believe what we saw before was the tail bearings may be locking up at times. The tail is not the best.


CurtisYoungblood.com
Next-D
Spyder Batteries

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03-08-2012 11:32 PM  6 years agoPost 6
Peter Wales

rrElite Veteran

Orlando Fl

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Mechanically all is correct, no binding and the belt tension is right. I used one of those screw in servo balls so I could change its position on the servo arm easily. I tried it in all positions on the Logictech gyro but right now it is in the middle hole at 13.5mm. I'll try moving it in to the closest hole tomorrow.

The 701 has a readout which tells me the actual gain of the gyro and I can see I have it set at 50% in normal mode. The wag rate does change when I increase the gain.

I am confused as to why its ok in normal but not HH mode.

I posted that while you were posting. I have another 500 which flies fine. If moving the servo ball doesn't do anything, I'll pull the whole tail assembly off the other one and switch it out. Thanks for the ideas guys

Peter Wales
http://scalehelicopters.org

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03-09-2012 01:08 AM  6 years agoPost 7
jgunpilot

rrKey Veteran

Pollock, LA

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Take the two tail blades and pull out on them and twist to simulate flight loads. If there is any notchiness, there is your problem. The fix is new bearings or a new tail assembly.

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03-09-2012 02:36 AM  6 years agoPost 8
Kaborkian

rrNovice

Lafayette, LA USA

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Slow wag on a gyro when in hh and not in normal is caused by integral term response of the PID control. While in normal, the control loop is setup as P only, when in hh mode, its setup as a cascaded PI and sometimes D loop. Classic cyclical response of I term is usually somewhere around 1/10 the speed of response of the P term gain cyclical response, hence the slow wag symptom.

In English, it means you most likely have hysteresis (slop) in the linkage or something binding.

As has been mentioned, check thrust and radial bearings. Also could try lighter weight tail blades.

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03-09-2012 03:19 AM  6 years agoPost 9
sdlloyd

rrApprentice

Delaware, Ohio

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Try turning up the head speed a little. I have one that did that at a lower head speed

Shannon Lloyd
IRCHA Secretary

President
www.corflyers.com

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03-09-2012 03:55 AM  6 years agoPost 10
3D-ALEX

rrApprentice

Daytona Beach, FL

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could be the servo. 520's are junk to start with, check the gear train for slop.

Alex Morley
Logo 6003D

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03-09-2012 04:35 AM  6 years agoPost 11
Dr.Ben

rrMaster

Richmond, VA, USA

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Especially since it acted up with the 701(which is IMMENSELY resistant to wag even at extremely high gain settings) I would indict either the tail servo or the tail rotor hub +/- linkage and slider as the culprit. It is not uncommon for the problem to occur only in HH because the the difference in the HH versus rate control algorithms.

Ben Minor

Peak Aircraft/Team Minicopter Team Futaba Team Kontronik USA

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03-09-2012 02:33 PM  6 years agoPost 12
Shawn Behrens

rrApprentice

DEEP IN THE BOG

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pm sent

shawn

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03-09-2012 02:45 PM  6 years agoPost 13
Peter Wales

rrElite Veteran

Orlando Fl

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I switched out the complete TR assembly from the other machine. The other one (old one) is metal, the faulty one (new one) is plastic. With the TR from the old one, the heli is smooth with no wags. So it's something in the tail rotor. Now, I will take it apart and start checking bearings etc, but first, I'll switch tail blades over and see if that works.

I'll report back as soon as I find out whats going on.

Peter Wales
http://scalehelicopters.org

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03-09-2012 03:15 PM  6 years agoPost 14
Richardmid1

rrProfessor

Leeds, England

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The problem with the Trex 500 is the small diameter boom, plastic boom clamp (if you have the plastic one), skinny boom supports and those stock plastic tail blades (they are too sensitive to slight pitch changes around 0 pitch), the carbon tail blades are much better and they certainly solved my wag. I also used a quickuk metal 3D boom clamp and quickuk boom supports for a 600 cut shorter.

The GP780 gyro does not help things neither does the DS520 on the 500, it is much happier with a full size tail servo like a DS650, 8900G, 9254 etc.

60% of the time, it works every time!

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03-09-2012 04:12 PM  6 years agoPost 15
Peter Wales

rrElite Veteran

Orlando Fl

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I have tried switching to the CF blades from the other tail, no change. So I stripped the hub down and one side looked ok, but the thrust bearing on the other side looked far too big. The inner had a significant gap around it and the outer was a sloppy fit on the shaft.

I decided to replace the whole mess with the metal parts from the pro 500. They will be here tomorrow. If that doesn't fix it, I'll be back.

Thanks for all your help guys, I would never have thought of some of the things you have suggested and certainly not the tail rotor itself.

I still cant understand why it works in normal but not HH, but I guess thats model helis for you

Peter Wales
http://scalehelicopters.org

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03-09-2012 05:29 PM  6 years agoPost 16
Kaborkian

rrNovice

Lafayette, LA USA

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Peter,

Go read about PID control algorithms when you have some time.

As I mentioned above, non hh gyro mode is Proportional mode only and hh is Proportional plus Integral.

Proportional simply looks at how fast the heli is rotating, the outputs what it thinks is the right amount of servo command to make it stop.

Integral integrates the error (rotation rate compared to how fast you are asking to rotate with the stick command) over time. In other words, if the heli starts to rotate from wind, the integral term will move the servo a little bit, then recalculate. If the heli is still rotating, it moves more. If the heli is stopped, it stops moving the servo. If it moved the servo too far, it moves it back the other way. All of this happens quick, but still much much slower than the proportional term.

So imagine if your tail has some sticky it. Heli gets disturbed by wind. Integral moves servo tiny amount. Nothing happens because linkage is stuck. Integral says "oh ****, move more". Still nothing happens. Integral says "oh ****, move even more". Now linkage breaks free,but it overshoots and heli goes the other way. Integral says "crap, too far" and does the same process in the opposite direction. Hence the wag.

Now consider non hh proportional only. Heli gets moved by wind. Proportional says "move servo", which it does. As before, linkage is stuck, so heli is very slowly rotating. However, since proportional doesn't care about time, its done its job and stays put. Now since the heli is slightly rotating, you the pilot breath on the left stick without even really realizing it, or you add a click of trim, and its done.

Clear as mud?

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03-09-2012 05:52 PM  6 years agoPost 17
Andy from Sandy

rrElite Veteran

Bedfordshire, UK

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Nicely described Kaborkian

As soon as I read the post I would of said the bearings are possibly binding in the tail hubs.
So I stripped the hub down and one side looked ok, but the thrust bearing on the other side looked far too big. The inner had a significant gap around it and the outer was a sloppy fit on the shaft.
Have you inadvertently put both inners in one hub and both outers in the other. I am sure you are aware that a thrust bearing has an inner and an outer with different diameter holes.

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03-09-2012 08:20 PM  6 years agoPost 18
Peter Wales

rrElite Veteran

Orlando Fl

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No, I found on side was as I expected, a loose inner and a snug outer. The other had about 1mm gap around the inner and an outer about as sloppy as the other sides inner.

Kaborkian, I was writing PID loops 40 years ago so I understand them, I simply hadn't connected the cause and effect. What you have explained makes perfect sense. I'll add another one to my limited knowledge base, thank you.

Peter Wales
http://scalehelicopters.org

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03-09-2012 08:43 PM  6 years agoPost 19
Shawn Behrens

rrApprentice

DEEP IN THE BOG

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If memory serves me the newer (silver grip) trex 500 pro tail assembly also has had its share of issues. The early versions lacked a shim/washer between the radial and thrust bearing (the v2 silver grips in 600& 700) had the same design issue which has been recently corrected. Welcome to the align test pilot club

shawn

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03-09-2012 11:22 PM  6 years agoPost 20
jgunpilot

rrKey Veteran

Pollock, LA

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It looks like my work here is done. On to help the next humble citizen...

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