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why set a gyro to "mechanical neutral" in normal?

Tday

Key Veteran

Needham, MA

Hi guys,

We had a debate at the field without resolution---and opinions seemed to be pretty strong, so I thought I'd pose the question here.

I only run my 611/601 in HH---never in normal. My set-up has the limit settings equal both sides and around but no more than 130 when running the tail from full left to full right. In normal mode that tail would drift, but I'm not sure why I would care. Those of us that do this reason that the servo is "optimized" in both directions---not over torqued and the same in both directions. And the gyro is determining whether the heli rotates or not at all times, the tail can go to both limits, so who cares what it does in normal?

Those that argued it was not properly set-up couldn't tell me why the gyro wouldn't be happy with my set-up, but their experience is that it makes a difference with csm gyros, for example, and of course the 401.

Anyone explain why that isn't the best set-up on a 611? Sure seems to work fine, and I really stress the tail, but perhaps there's more to it.

Thanks,
Tom

10-30-2006 Over year old.
Colin Mill

Veteran

England

Hi Tom

The answer to this one lies in the dynamic performance of the gyro - especially in stop situations. If you don't set the linkage up so that the servo neutral provides something close to the hover pitch then at the end of a rapid piro stop the gyro will centre the servo to a point that is not quite the 'in-trim' position. The tail will then move slightly as the heading lock term makes the necessary correction. This causes the two stops to be slightly unequal (although high gyro gain helps mask the problem.

I also like to accurately set up the pushrod length etc so that I know that the linkage geometry is right. There are a whole bunch of minor ill-effects you can get from the tail linkage so if you are after the absolute maximum performance from the tail there is no substitute for real attention to detail in this department.

Best regards

Colin (CSM)

10-30-2006 Over year old.
Tday

Key Veteran

Needham, MA

Ahhhhh...I see what what's going on! Very nice. Thanks, Tom

10-30-2006 Over year old.
z11355

rrMaster

New England

Colin,

Is there a 'feed-forward' aspect to the control loop design that
makes the gyro want to seek 'center' (even if center is wrong) ?

10-30-2006 Over year old.
Tday

Key Veteran

Needham, MA

Good question Z---I was wondering why there would be any drift despite being in HH for any gyro.

Tom

10-30-2006 Over year old.
BarracudaHockey

rrMaster

Jacksonville FL

In addition to what Colin said, a round servo wheel naturally has positive expo (futaba speak), that is is gets more movement at the control surface around center than it does at the extremes of throw, thus the gyro has less work to do in a hover if the linkage is setup correctly vice having hover offset the center by say 10 degrees and letting heading hold take care of keeping the nose straight.

Andy
AMA 77227
http://www.jaxrc.com

10-30-2006 Over year old.
z11355

rrMaster

New England

I'd counter by saying that the shape of wheel is totally
irrelevent.

Also, having the hover point be at 90 may introduce a bit more
negative expo since every move is amplified vs. the slight offset.

But then there are the mechanical/dynamic aspects of driving the
tail vs. just relaxing the natural tendency

10-30-2006 Over year old.
Dr Lodge

Elite Veteran

Guildford, Surrey - UK

There's also the fact that the tail tends to be more effective in moving "right" (clockwise) from the hover position, than "left". Hence you may want to offset the tail servo horn so its slightly to the "right" position (eg forwards) when in the hover. But with todays gyros, I doubt you'll really notice the difference, it'll just help to give you a more constant rate of rotation.

Vibe 90, Vibe 50, Vigor CS x2,
Dyna-x, Knight 503D, Logo 10,
TRex 500, Furion, Gaui EP200

10-30-2006 Over year old.
HOMEPAGE  
BarracudaHockey

rrMaster

Jacksonville FL

I'd counter by saying that the shape of wheel is totally
irrelevent.
Are you saying there's more than one shape to a wheel than round?

Though I have seen square ones, they are usually attached to cars with grass growing out of where the windows once were.

Andy
AMA 77227
http://www.jaxrc.com

10-30-2006 Over year old.
Colin Mill

Veteran

England

Hi z11355 and Tday

Controlling the behaviour of the HL term during aerobatics is one of the more interesting problems of HL gyro design and one which I have been amusing myself with for a significant part of the last ten years so it's not easy to give you a simple answer. If you have a look at some of the things about PID loop design on the web you will get an outline of the problem - the bits about integral term wind-up are especially relevant. Some gyros resolve the problem by turning off the heading lock term when they are pirouetting at a significant rate and then turn HL back on after the rotation has stopped (with or without a delay). This is a nice easy solution to the problem but does not give you the piro speed control in travelling manoeuvres that a 'full time' HL system. The full-time HL systems have to manage the HL term even during the pirouettes etc. and this can be 'interesting' in travelling situations where the tail load is changing constantly.

If you are going to give the gyro the ability to stop at the maximum rate possible the problem becomes significantly simplified if the algorithm has a 'first estimate' for the servo position at zero rotation. How one arrives at such a value is also interesting. I personally prefer to have this identified at a specific time (during the quick trim time). Running averages etc are attractive but in our experience tend to be fooled from time to time depending on flying conditions and flying styles giving less predictable results.

The alternative is to arrange that the gyro 'engineers' a sub optimal stop with reserves of stop capability in hand during which it can manage the HL term continuously during the stop. However, the loss in stop performance is significant and not something most 3D pilots would enjoy I think.

As with all such things there is no perfect solution but its fun trying!

Best regards

Colin (CSM)

10-30-2006 Over year old.
Tday

Key Veteran

Needham, MA

Now that's fun to think about---or at least know that you're thinking about it. The mechanical zero and square set-up facilitates pirouette and stop management as I read what is being said. And from what you're saying, there is nothing about my original set-up that would baffle a "high end" HL gyro and cause drift in a hover (reasonable weather/wind.)

Thanks again---and what a great time to be thinking about problems like that, given the miniature tools/equipment/chips to work with. Enjoy!

Tom

10-30-2006 Over year old.
Nightstalker

Elite Veteran

UK (BEDFORDSHIRE)

sorry to butt in on this convo but can i ask a question about a slight problem i have with my 401????

THANKS


I have a 401 on my airskipper i have a carbon push rod and no play in my linkages so i know i have no slop there,
now i can pirouette the heli at and speed in either direction and the gyro will stop the tail on a dime and hold spot on, but when im in forward flight i seem to get a drift to the left (nose going right) and if i do a full power out,
now i set it all up in normal mode till it was spot on then went over to HH but i dont get drift in a hover,
my settings are N 65 HH 68

Any ideas what to try???

10-30-2006 Over year old.
Colin Mill

Veteran

England

Hi Tom

Yes, its great fun to work on this sort of problem in aeromodelling. As you say we have almost all the goodies we could wish for these days with most of them getting cheaper and better all the time. I got hooked on model aircraft as a kid and if I had known then that I would one day get to play with models full time you would never have wiped the grin of my face.


Hi Nightstalker

Yes, I think that is almost certainly a small amount of vibrational precession of the gyro sensor due to the cyclic vibration that any two blade head has in forward flight. Changing the gyro mounting slightly can really help with this. I like a location close to the C of G as the cyclic vibration is a nodding motion of the heli so the further away the gyro is from the C of G the more up and down motion it gets (which can disturb the mounting)

Try fitting it on top of a 720 style mount (you can get them as a spare but you could make one out of say 2mm thick stainless or brass sheet). Since the 401 is quite a bit heavier than the 720 you will need about 8 of the 10mm x 10mm square pads to get the right compliance.

I hope that helps

Best regards

Colin (CSM)

10-30-2006 Over year old.
Nightstalker

Elite Veteran

UK (BEDFORDSHIRE)

Hi Colin

as u can see from the pic iv did that but iv only used 7 pads,
3 each side and 1 in the middle and that is the nearest to the cg i can get it

10-30-2006 Over year old.
Colin Mill

Veteran

England

Hi

The only thing that comes to mind looking at the picture is to remove some of the spiral wrap near the gyro to allow a bit more flex in the wires to the gyro. Allow say 30 to 50mm free then make sure the spiral wrap is anchored to the chassis to make sure the mass of the wiring can't shake the gyro about.

Colin

10-30-2006 Over year old.
Tday

Key Veteran

Needham, MA

Nightstalker---to be perfectly honest, this seems off topic(bad form)and given the thread title, will give you fewer answers than you would otherwise get by setting up a new thread. "Drift in FF and full power climb out" or some such thing.

Personally I doubt it's vibration but rather in a full power climb out, heli's can go nose right a bit esp on the initial full power push---the rotor head slows and the instant the tail blades slow---it drifts prior to the gyro correcting. I'm guessing that's what's happening in some fashion in both situations---but then again others would know better than I.

Tom

10-30-2006 Over year old.
Nightstalker

Elite Veteran

UK (BEDFORDSHIRE)

thanks colin il try that,
but why would i get drift in forward flight?? i understand the fullpower out bit but not forward flight, i dont do 3d and stuff like that im just doing gentle 8s

10-30-2006 Over year old.
Tday

Key Veteran

Needham, MA

I honestly don't know enough of what's going on. Perhaps aileron and elevator is not perfectly square so you're doing little (unnoticeable) and slightly out of sync corrections along the way. Perhaps you're "leaning" on the rudder a tad when moving collective during your eights. Not sure. I don't know the 401 gyro that well except to say it is pretty old school and I don't know if they really lock that well. I think it's something other than gyro set-up.

Tom

10-30-2006 Over year old.
Nightstalker

Elite Veteran

UK (BEDFORDSHIRE)

i know what u mean, but i know the 401 is a good holder a friend of mine ian (coolice) he has a 401 in the turbin and it holds spot on,
il try what colin said and see if i can norrow it down, just thought it could be a quick guess,
thanks again

steve

10-30-2006 Over year old.
Tday

Key Veteran

Needham, MA

I hear those guys (401 users) talking all the time. Seems like it's touchy thing and some work better than others. Good luck!

10-30-2006 Over year old.
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why set a gyro to "mechanical neutral" in normal?

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