Thoughts on Linkage Compensation

[siado]

As I am setting here reading Cliff Hiatt and Wayne Mann's excellent article on setup for the 14MZ from littlerotors.com, I started to get confused when I got to the part about linkage compensation. I have played with it and played with it and have just now started to see what could be a shortcoming...scratch that, software improvement for the 14MZ. Now, let me say first that I have had this radio for all of about 10 days now and I haven't even begun to explore it's abilities.

I am using a dial indicator attached to the mainshaft to level the swashplate. I leave the washout hub on the mainshaft so that the inner swash ring is fixed in relation to rotation of the mainshaft. I set the indicator point on one of the inner swashplate balls and rotate the mainshaft/inner swash ring/dial indicator around probably about 340 deg. with my setup. I can't move past the anti-rotation bracket with my setup, so I just have to be careful not to bump into it and mess with the indicator assembly.

- I have finally managed to get my swash +/- .001" all the way around at high, low, and mid collective.

- I have managed to get less than .001 interaction between fore/aft and left/right cyclic at zero collective.

Now comes the tricky part. In order to get rid of the cyclic interactions in the swash at the high and low collective settings, you use the linkage compensation function from the Swash Detail screen. As I understand it this function allows you to make up for the fact, that at high collective, the aileron and pitch bellcranks will have an angle of >90 deg. so that when you put a roll input into the system, the bellcrank which pushes the servo up will be losing its effectiveness at moving the swash as the pushrod angle approaches 180 deg. Now, on the other hand, the bellcrank which is moving the swash down will be getting closer to a 90 deg angle, therefore gaining effectiveness at moving the swashplate. Since one side of the swash will be going up less than the other is going down, this will introduce an aft cyclic effect.

The 14MZ is handy in that you can adjust for this, but it will only let you assign both positive or negative values for the left and right roll inputs. So while I am watching my indicator, I can adjust out all of the aft cyclic effect by increasing the linkage compensation setting for aileron in the right direction by some "positive" percentage. However, when trying to do the same thing for a left roll input, I find that adjusting the value from zero actually makes the interaction worse. I can not get rid of it in one direction because I have to choose the same "polarity" of compensation for the left AND right sides. Seems so easy that Futaba could have made these independent of each other.

So, what I gather is, that since the opposite servo is getting more effective from right to left roll input, the opposite percentage of compensation would need to be applied. Am I off base here? It is fairly late and I may be thinking about this too hard, but this seems to make sense.

Jim

[DavidH]

If you have the swash plate where is stays within .001 level thru out the whole pitch range. I would not worry, .001 could be slop in the system.
Wayne and Cliff are eyeballing the swash plate level. They are really great at what they do. I doubt they have there swashs staying level within .001 thru all the range if they used a dial indicator to measure them. Most pilots I know just eyeball everything when setting up. They may use a swash plate level tool that slips on the main shaft. But that is about it. They will fly the model and work out the rest once they fly the model. Because there is other factors that come into play also like the balance of the model, blades, fly bar paddles.

David

[Gary O]

To add something to what David already said. The chances of the swash staying at .001 after you fly it is very slim. There are so many moving parts that it only takes a slight amount of play somewhere and your .001 is gone. In theory what you're saying is great, but I'm not sure it's practical to chase it.

[siado]

Let me clarify what I was trying to do here. I know that the ship will never maintain those tolerances after flight, crash, hard auto, etc. I am perfectly satisfied with the adjustments I have been able to get so far with the exception of the linkage compensation. In reality, .001" of swashplate travel will certainly never be seen by myself and I imagine many top pilots would have a hard time picking it up. The servo mounting grommets themselves introduce more error than this, so that's not what I am concerned about.

When I am trying to adjust the linkage compensation, I am seeing close to .015" of travel on my indicator, which made me step back and look at the swash itself and you can physically *see* it move. On the Avant, with the elevator bellcrank forward and the anti-rotation bracket in the back, you know the elevator servo shouldn't move and the forward part of the swash won't move either, and they don't. But, as you input left/right roll, the pitch and aileron servos should move the same amount in opposite directions from each other. This way, the swashplate pin which keys into the anti-rotation bracket will not move up or down, and the swash will remain level from forward to aft. This is the adjustment the linkage compensation is trying to account for, but it can only do one side, not both. Now, again .015" is still not a huge amount, but I know if it can be seen by the naked eye, it can be felt in flight.

There is still much to be done for setup once the bird is in the air, but I just think that the more I get done on the ground, the better I will understand the radio/heli combination and it will be easier for me to aplly that knowledge to what "feels" right in the air.

My weather is pissy here in the afternoons when I have the chance to fly, so what else am I going to do? :mrgreen:

[RickyM]

Hi Iwas about to post the same question about my 12z, just got it and tryed to mix all interaction out and it's not possible :( the mix reqired is a 3 way program mix which is not featured as you have said at high swash and low swash the ail- elv values cannot be ajusted at theese points only :shock: me thinks we have been pilaged!!!

[siado]

I was wondering if I was nuts and put the Avant back on the shelf for a while. I know why I prefer mechanical mixing, that's for sure. In reality, I will never be able to fly well enough to see or feel these interactions, but the engineer in me can't stand the idea of it not being right.

It seems like it would be such a simple change if Futaba wanted to. Just a small interface tweak to allow independant variables for the settings.