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HomeAircraftHelicopterAerial Photography and Video › Near fatal tail wag ... CSM 560 SL Micro and JR DS811
02-08-2008 02:54 PM  10 years agoPost 1
Hogster

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Surrey, UK

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Hi all,

I'm using a CSM 560 SL Micro gyro and JR DS811 tail servo on my stretched Logo-10 AP helicopter and have had no real gripes with its performance since I started flying with it in June 2006.

As I'm only using it for AP, obviously I'm not testing the system to its limits. I've noticed a small amount of wag after relatively quick tail movements, but nothing major.

However, I was experimenting in the garden today and I found something rather shocking! If I give a sudden jab of yaw to the right, the tail will oscillate more and more violently until literally the heli is rotating 90°+ either way! This situation doesn't improve and so I've had to hit throttle hold to bring it in to land ... and it's nerve-wracking to say the least!

I've been in touch with CSM and they were very helpful in trying to solve the problem and suggested, amongst other things: 1) to move the tail pushrod in one hole on the tail servo arm (it's about 15mm out at the moment, and they said moving it in would increase the torque available for the tail, albeit at the cost of slowing down the response); and 2) fitting the tail braces again (have been flying with them off for many months with no problem).

Other than that his only other suggestion was to buy a faster servo. I'm not sure why this heli was supplied with such a slow servo (I bought the heli second hand and it came with this gyro and servo) It's rated at 0.18s/60°! Specs here.

Would I see a noticable improvement if I upgraded to something like the 9254? Although I've never had to use extreme amounts of yaw when flying, it's alarming that I've only just discovered the fatal effects of giving sharp jabs of yaw with this gyro/servo combination!

If not the 9254, are there any other good tail servos you could recommend for me to investigate?

Many thanks for your help,

David

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02-08-2008 05:33 PM  10 years agoPost 2
AceBird

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Utica, NY USA

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It's rated at 0.18s/60°!
I am sure it is not speed of the servo. I would suspect that your endpoints are too large and a sudden snap of the rudder forces the pitch to go full tilt. This may cause an over locking bind of the linkages or just too much resistance for the servo to overcome on the way back. The lag snaps the blades in the opposite direction full tilt and the problem repeats itself.

You should be able to pull it out of this situation by holding the stick in one direction until the tail goes in that direction and then slowly bring the stick back to center.

Shortening the servo arm will do both increase the available torque and decrease the throw. I think you could have just decreased the throw and not suffered any response. But why do you need to pirouette so fast for AP in the first place?

Ace
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02-08-2008 07:16 PM  10 years agoPost 3
Hogster

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Surrey, UK

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Hmmm I'm not so sure. I'm not talking a *huge* stab of yaw here, maybe 50% right stick for 1/4 second or something .... It feels very sloppy when trying to correct it, and then it will suddenly get into this vast over-compensation, even with the gain as low as 20%.

Naturally I have no need to do these types of manoeuvres on a regular basis, but I want to know that if I need to manouevre very quickly or harshly, that the helicopter won't set itself up into the sort of uncontrollable oscillation I'm seeing now.

I will be picking up a second hand Futaba 9254 tomorrow which should make a big difference.

David

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02-08-2008 08:02 PM  10 years agoPost 4
BigguyOz

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Forster, New South Wales, Australia

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Dave, disconnect the servo and move the tail pitch through its travel slowly by hand. I suspect you will find a spot where there is a little bit of resistance. This could cause a problem like this, which could be fixed by gently smoothing the shaft/some graphite powder.

Tony Stott

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02-08-2008 08:04 PM  10 years agoPost 5
Hogster

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Surrey, UK

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Hey Tony,

With the servo disconnected the pitch slider is very very free No hint of sticking/resistance

David

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02-08-2008 09:32 PM  10 years agoPost 6
AceBird

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Utica, NY USA

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The resistance comes when the links are jamed to there extremes. In a servo mechanism if the device lags behind the command to move it builds up a huge error that results in a slam in the other direction. And then history repeats itself. At least this is what I am thinking.

Ace
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02-08-2008 09:41 PM  10 years agoPost 7
Hogster

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Surrey, UK

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Hmmm although on this gyro you set the end-points electronically, so no matter how much the gyro thinks the tail pitch should change, it'll never go beyond those points.

Do you think reducing those end-points would help this problem? Also, in a reply to the same topic on another forum, a pilot said he was experiencing a similar problem until he moved the pushrod in a hole on the tail servo arm. I can't see why that would help, but I'm willing to try! I've already moved it in one hole, perhaps moving it another hole would help ...? Moving it in one hole helped a little ...

I'm not picking up the new tail servo tomorrow as I've discovered I need a new cyclic servo first ...

Thanks for the suggestions guys Please keep them coming!

David

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02-08-2008 09:41 PM  10 years agoPost 8
shuttlepilot

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Mullins, South Carolina

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I would try a faster servo. I used a Hitec 525 on the tail of one of my 46's and it did OK with a 401. The specs on it gave a .13 speed at 6v. I still had to add in some delay at the gyro to keep it from doing exactly what you have described. I know that you are using a CSM gyro, and the settings will not be the same, but I would say that you will see an improvement with a faster servo, and may even see the problem dissappear with a fast digital servo.

Gas is Great
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QWW Helis

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02-08-2008 09:45 PM  10 years agoPost 9
Hogster

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Surrey, UK

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Hmm that does seem to be a definite solution. Maybe I should collect that 9254 tomorrow!

I must say the current servo does feel pretty slow ....

David

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02-08-2008 11:08 PM  10 years agoPost 10
rroback

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Irvine (UCI), Ca

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get a 9254, but it's important that the linkage be smooth. I'd use boom supports.

Rhett..... I can't fly, but the Profi sure can.

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02-09-2008 12:33 AM  10 years agoPost 11
AceBird

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Utica, NY USA

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so no matter how much the gyro thinks the tail pitch should change, it'll never go beyond those points.
These are still end points controlling throw. If you move to another hole in the servo arm it will change the throw even though the end points are controlled by the gyro. It also changes the torque advantage. Servo speed is not the problem it works fine if you don't jab it right?

I am just thinking now, rate can be a problem. If you set the ATV of the servo to high the gyro might get ahead of the servos torque capability. Where do you have the ATV set at?

Ace
What could be more fun?

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02-09-2008 12:37 AM  10 years agoPost 12
bill the greek

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Greece

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important
As I'm only using it for AP, obviously I'm not testing the system to its limits.
take a look on pc interface at "Acceleration Gain" and "Conventional Gain" here is the manual: rcmodels.org/csm/interface%20manual.PDF
Maybe the "curves" between HH and normal mode its too different.
Bill

Vasilis

Nobody is Perfect but who want to be Nobody...

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02-09-2008 12:41 AM  10 years agoPost 13
AceBird

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Utica, NY USA

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take a look on pc interface at "Acceleration Gain"
There is an Acceleration Gain on that gyro? Don't get that one too high.

Ace
What could be more fun?

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02-09-2008 01:45 AM  10 years agoPost 14
FCM

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Surrey, England

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Junk the servo - the JRDS811 is junk when it's new
Also be careful about using a second hand 9254 as although this is a good quick servo, it does not like to be abused by stalling out etc. If in any doubt - buy new.

Leave all of the PC menu parameters as factory default to start with anyway. For AP flying, you will really only have to do some fine tuning such as servo speed, stop gain adjustments (depending on direction of rotation) not fiddle with things like accel gain.

Hope this helps.

Paul.

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02-09-2008 03:01 AM  10 years agoPost 15
Vortex Aerial

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Riverside Ca

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and 2) fitting the tail braces again (have been flying with them off for many months with no problem).
Are you talking about the tail boom supports or the rudder pushrod supports?

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02-09-2008 03:34 AM  10 years agoPost 16
lejon

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Las Vegas, Nevada

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One thing that happened to me awhile back was that the end points were set such that the servo arm would travel to the point where it was almost inline with the push rod and was binding a little at each end which resulted in the gyro and the servo playing cat and mouse. Might be worth your while to make sure that at full left or right the pushrod does not line up so as to bind.

Lejon

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02-15-2008 01:27 PM  10 years agoPost 17
Hogster

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Surrey, UK

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I've just tested the 9254 servo and it's a million times better than that piece-of-junk DS811! Lightning fast reaction and NO wagging whatsoever, even after heavy jabs of yaw.

I got it for a total of £46 (£40 + £6 for a new case as the mounting lugs on the old one were damaged). Not bad considering they retail at £70

Cheers!

David

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02-15-2008 02:39 PM  10 years agoPost 18
shuttlepilot

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Mullins, South Carolina

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Glad to hear it Hogster. Nothing beats a fast digital on the tail!!

Gas is Great
Camper Fuel is Better!!
QWW Helis

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