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Jun 02, 2013, 08:39 PM
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Using a plumb line pitch gauge on a V120D02S


The other day it suddenly occurred to me that, after over a year of ownership, I had not yet gone through and tuned up my little Walkera V120D02S. It had been flying fine for my purposes, but I did notice from day one that while the swash plate looked level, the servo arms weren't very well centered and I'm sure there was some binding at the pitch extremes. Certainly, the swash plate was not remaining level throughout the pitch range.

In a previous write-up, I discussed creating a plumb line pitch gauge, and I touted itís usefulness across a wide variety of helicopter size classes. I thought, what better time is there than now to demonstrate the usefulness of this pitch gauge on a helicopter as small as the V120D02S. Figures 1 and 2 show the plumb line pitch gauge taped onto one of the main blades of the V120D02S. As is standard operating procedure, the receiver was bound to the transmitter with the motor leads disconnected for safety's sake.

The blade pitch-angle is clearly shown in Figure 1 to be 0-degrees with the throttle stick positioned at mid-stick, but things didn't start out this way. On one hand, the blade tracking appeared to be set up very well at the factory, and the swash plate appeared to be vertically centered along the main shaft when the blade pitch-angle was 0-degrees. On the other hand, however, the swash plate obviously had been leveled with the servo arms positioned at different angles. Thus, there was no way the swash plate was actually level except when the throttle was precisely positioned at mid-stick. The first order of business, therefore, was to center all of the servo arms.

Not one of the servos was correctly centered at 50% throttle, and one of them was much farther off than the others. Removing the servo arm from the offending servo and then properly reinstalling it brought that servo into compliance with the others; but still, all three servos remained out of center to slightly varying degrees. All of the servos were too far from center to use Subtrim, but also too close for reinstalling the servo arms to be completely effective. It soon became apparent that no matter what I did, I simply was not going to have these servos correctly centered at 50% throttle. So, I just gave up, frustrated.

Later in the evening, I revisited the V120D02S problem. After some fiddling around, I discovered that the servos all centered fairly well with the pitch curve set to 60% at mid-stick. I already knew that the whole pitch range of this helicopter amounted to roughly 45 percentage points in the transmitter, and thus centering the entire pitch curve at 60% would still allow for the desired blade pitch-angles at low-stick and high-stick. With the mid-stick configuration determined and all the servo arms correctly centered, I then leveled the swash.

One goal I had in mind was to set up this small helicopter just like its larger cousins, using the techniques discussed in Finless Bobís video, "Pitch and Throttle Curves 101." I could then set the Normal mode pitch curve to match the Idle-Up curve above mid-stick, but also set the Normal curve shallower below mid-stick as graphically shown in Figure 3. Setting up my pitch curves similarly to those shown in Figure 3 would enable me to switch from Normal mode to Idle-Up mode before the helicopter even leaves the ground. This called for creating a linear pitch curve in Idle-Up mode.

Since I already had my mid-stick position figured out, all that remained was to set up the low-stick and high-stick pitch-angles and then make sure the pitch curve was a straight line. The plumb line pitch gauge made setting up the pitch curve very easy--I just positioned the throttle stick at, first, the high-stick position and then adjusted the pitch curve in the transmitter until the pitch gauge displayed a desired blade pitch-angle of +12 degrees. I used the same approach to get -12 degrees at the low-stick position, as shown in Figure 2.

It is worthwhile to mention that the +12-degrees pitch-angle required the pitch curve to be set to 84% in the transmitter. This put the high-stick position at 24 percentage points above the mid-stick position of 60% as shown in Figure 4. So, I thought, it stands to reason that if my swash plate was properly centered along the main shaft at mid-stick, then the low-stick position should be 24 percentage points below mid-stick, or at about 36%. Well, this is not quite how things turned out. The low-stick pitch-angle of -12 degrees required the pitch curve to be set to 39%. This indicates that the blade pitch-angle was not truly 0-degrees at the mid-stick position, or that the swash plate was not vertically centered along the main shaft, or perhaps both. At any rate, the difference between the high- and low-stick pitch-angles amounted to a difference of about 1.5% at mid-stick. Figure 5 shows my final pitch curves for Normal mode and Idle Up mode. My Idle Up curve is not quite a straight line; thereís a slight bend in the pitch curve at mid-stick. Considering that I missed the mark by as little as 1.5%, I decided to let this go for the time being.

Once the pitch curve was suitably set up, the only thing remaining to be done was to adjust the cyclic throws. Again, the plumb line pitch gauge made things easy. I simply moved the cyclic stick to each of its maximal positions and observed the blade pitch-angle on the plumb line pitch gauge. I found that my cyclic throws are about 7-degrees in all directions. This has been working well for me, and so I left all the cyclic travel adjustments at 100% in the transmitter. One noteworthy exception is that I did increase the tail travel adjustment to 135% so as to gain more tail pitch range. I had previously set up the tail in Rate mode, and thus the tail needed no further adjustment.

It took me about 10 minutes to perform all of the steps described above; and now, my little V120D02S flies much better and smoother than I had thought possible. The V120D02S also demonstrated that the plumb line pitch gauge can work fabulously for all helicopters, large and small.
Last edited by navigator2011; Jun 03, 2013 at 10:20 AM.
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