[trust]

  I managed to build a new frame from 10mm square cf tubing arms (round hole ID -very strong) and two plates of cf/1/8"balsa/cf laminate, sawed off the previous motor mounts and remounted them to the CF tubing. Found a lock nut for the lost prop & replaced prop. Test flights with default gyro settings work fine.
On the swaying -
This doesn't act like the east/west oscillation. It is more random, and seems to be more related to wind direction and the mass of the aircraft. Cross to the wind, I would see some random swaying. Into the wind - very little swaying - pretty stable. Also the RTB when pointed into the wind was quite smooth. Wind was out of the SE.
 I'll do an autotune and try with its parameters. Then try modifying the VtolPF PIDs.

[trust]

Well, I tried running autotune. The aircraft seemed to shake much more violently than last time I tried this (although it may have started with a twice as high PID settings). The end result settings were nearly unusable to fly with however. Low PID settings - less than half of what I started with (defaults). I reloaded the defaults.
   But now another issue has surfaced. My OPLINK is dropping and reconnecting - connect for a second or two, drop for a second or so, then reconnect. Any ideas how to fix this?

[TheOtherCliff]

The trick with AutoTune is that you must not run it with an unstable set of PIDs.  Stock ones usually work OK, but there are quads where that is too high.  There is a PID issue that I call a D term oscillation that is so fast that you don't actually see it.  Setting D to zero fixes it, but others should then be reduced.  Long story.

Bad OpLinks are a known issue.  Here is a link.  To know if you have the issue, pay attention to the talk about X2 markings.  Basically bad works with bad and good works with good, but bad does not work with good.  It's hard to define which is bad so the assumption is made that original OP Revo and OpLink are good and the new ones that are incompatible are bad.
https://forum.librepilot.org/index.php?topic=4197.msg28502#msg28502

I suspect that at very least, this issue could be coded around.  I don't honestly know if it is a hardware issue or an OP/LP issue.  Knowing some things about the way the GUI says that the code defines channels, it may be that we do something that is undefined "at the edge" and one brand of RFM22B handles it one way and another handles it a different way.

It works fine, but regularly drops the link, like at a specific channel.  I was not able to tweak the settings (starting and ending channel) to avoid the issue, but that is because the code for instance does not let you use just one channel.  If you are running next, you can also see that the frequency is way off (41 kHz).  I haven't played with it in a while.  A project that I never completed is to allow a Revo to be used as an OpLink so if you have two good Revos, at least you can get telemetry working.

[trust]

I checked the AFC correction - it says 10khz - which seems acceptable. These units were working fine. The only difference is it is warmer today by about 10F.
  I tried swapping OPLINK units from another aircraft not in use (but the OPLINK had been working). It had the same problem! At first it seemed fine, then 10s or so it started cycling connect/disconnect. I then noticed if I held the receiver antenna between fingers - it would work!
Probably enough to detune it. I decided to try recalibrating outside. I had to try to hold the antenna while doing this. Which stalled at one point (probably dropped connection) and hung the GCS.
  But after a about 15m, and as the temperature outside was cooler, it started working without having to hold it. I was able to do a mag recalibrate and do a short flite test, but it was getting dark so I quit.
  Something is temp sensitive.
  When I ran autotune before I had higher levels of gyro to start with. I've gone back to these higher levels (P=0.06) on pitch and roll - feels much more connected.

[TheOtherCliff]

You probably already know but there is a connector called RP-SMA (SMA-RP) where they swap the male/female pin parts.  With one SMA and one RP-SMA it's possible to put two female pin parts together and not get a connection.

Also, are you sure you are using two 433MHz antennas?

I would also ohm out the coax cable if you are using one.

I recall that with the "bad OpLink" issue, you set the power to min on both sides, then if the two antennas are close (wild guess 1m) you regularly get a big drop in "Rx Level".  Move them farther apart (5m?) and you regularly get a disconnect.  Flying at higher power you can regularly get a disconnect.

I don't think I recall ever seeing a 10kHz deviation between a good pair.  Channel width can be 40kHz, so I wouldn't consider 10kHz to be good although there is AFC to handle deviation/drift.

You can't tune (next) a 41kHz deviation out.  I actually coded a 41kHz deviation for a test on that set and it didn't help.

[trust]

These units came with antennas, and they are correctly paired connectors.
I assume these are tuned with digital PLLs - can I adjust the PLL manually from the GCS menu or an offset?
Do they frequency hop to find a clear channel? Once acquired do they stay on channel?
It seems odd that  BOTH pairs failed with the same deviation (about 10khz).

[trust]

I decided to try a different tactic to fix the OPLINK rcvr. I rolled some pieces of aluminum foil around the rcvr antenna, and slid it up and down until I got consistent solid signal back at the GCS.  Short pieces of 2" length seemed to work well - 1" was not enough.
I'm also going to try recutting the antenna lengths a bit shorter to detune and see if that will also work. Simple enough to pull off the black plastic covers.
  I also looked at the GPS system display upper right showing the GPS "speed" - proxy for rate of change of GPS position from sat data I presume. When 11 or more sats this stayed well under .1 m/s and was in the .02-.04 range. At 10 or less the numbers would occasionally jump over 0.1 and sometimes .2 or .4.
  I also tried GPS Dynamic Model to 4G instead of the 1G it had been set. 4G seemed worse - more meandering. But then sats was 9 or 10, so hard to tell.
  I did get one good flight in a moderate, gusty north wind (10ish) that held pretty solid with 12 sats.

[TheOtherCliff]

I assume these are tuned with digital PLLs - can I adjust the PLL manually from the GCS menu or an offset?

Tuning (and reading the frequency deviation) can be done in next, not 16.09.  You will have to reflash everything since next is not compatible with 16.09 OpLinks.  When receiving, the RFM2x we use uses AFC to lock on to a slightly different frequency transmitter.  Tuning shouldn't be necessary, but that is with good OpLinks and good antennas.

Do they frequency hop to find a clear channel? Once acquired do they stay on channel?

They do frequency hop, but they do not detect clear channel.  If you know of a particular problem frequency you can adjust the start or end channel number to avoid it.  All OpLinks and Revos must be set the same.

Internally the cheap 433 antennas are a coil of wire like a spring with no wire after the coil.  If it is not a coil of wire it probably isn't a 433 antenna.

A "store bought" 433 antenna really shouldn't need any tuning.

A thread about 433 antennas:
https://forum.librepilot.org/index.php?topic=4199.msg28510;topicseen#msg28510

[trust]

Thanks for the links. I've built dipoles, crossed dipoles, & yagis before.
I took a broken antenna, which was one of the same that was shipped with these units, and added 6.3cm wire to the end of the coax (that's all that would fit into the tube).
The original gave about -40db with 10' between OPLINKS, parallel orientation.
The replacement was virtually identical -41db.
During the tests the freq shift needed was only 1khz.
These must have a coil in the base - that length is not for 1/4 wave 415Mhz is 17cm.

[TheOtherCliff]

Those coil type antennas are about the worst you can get that is actually a correct antenna. 

The only case I know of where adding length to a "store bought" antenna actually helps is the old 72MHz RC receivers where the antenna (30") was shorter than a quarter wave (but no ground plane) and more than doubling it gives much longer range.

My advice is that unless you are building the antenna and using a length formula, leave the length as it comes.  This is especially true of antennas used on transmitters, where you can burn a "final" just because the impedance isn't right.  I wouldn't worry too much about burning a 100mW OpLink/Revo though.  I have found a disconnected Revo antenna after flying it several times that way (@ 50mW IIRC).  None the worse for wear.

My second advice is to always use a half wave dipole antenna.  Quarter wave antennas need a ground plane to work correctly.  That antenna article has some info at the end that you might find useful. 

[trust]

I agree - these are usually pretty bad, and may be why there have been these problems with the OPLINKs. For me I'm mainly using them to set up and calibrate systems, with plans in the future to upload waypoints. SO the short ones are fine. But I think I'll order or make up some dipoles. Its pretty easy to make a dipole from a piece of cable already made up with connector. But it'll end up pretty long - 35cm. Fine for base station but long for the quad.

[TheOtherCliff]

With Retevis 771's at the lowest power setting on both ends, you should expect to easily get a couple hundred meters on the ground.  That is how important a good antenna is. 

Even with the coax sleeve dipole, by the time you have paid for materials, not to speak of tuning equipment, if you consider your time to have value, buying a pair of the best antennas I have found for $13 (shipped) for the pair is the best deal for me.  Else:  What kind of coax?  Is the sleeve diameter large enough?  Should I just fold the braid back or use a sleeve?  Should the wire be longer than the sleeve (look at the insides of any 2.4GHz antenna)?  How to tune?  How to make it stiff enough (both connector and antenna) for vertical use on an airplane?  Of course you may enjoy making antennas and I can't fault that reason. 

I always use these Retevis on the ground side.  I don't consider quads to be long range, so I usually go with the best shorter antenna I can find on the quad.  Airplanes are another matter.  I use the Retevis 771 in the air too when flying airplanes.

https://www.ebay.com/itm/2X-SMA-M-VHF-UHF-Antenna-for-Baofeng-UV3R-Retevis-RT3S-YAESU-VX-3R-Two-Way-Radio/362947038362

These are a bit heavy at a little over an ounce IIRC, so if you want to save some seconds of quad flight time, you may want to get a lighter one (like the Nagoya NA-24 at $16) for the aircraft side.  If you want the best range, then a full length dipole in my opinion is the only way to go.  Truthfully, most of the time we don't need best range and a 771 on the ground at medium-low power with something less in the air at medium-higher power is plenty good enough.

If you simply enjoy making antennas and don't mind buying a pair of Retevis 771's for $13, you might find it interesting to compare range of 2x Retevis vs. Retevis + DIY.

This started out to be a short post... 

[trust]

It is sometimes fun to make antennas. But I take your point and ordered 2 to compare against 2 other types I've also ordered.

BTW:
Reloaded firmware on all devices, configured next rev on my original Hellaplane V1, calibrated, tested in basic, then tested in INS13 mode - a rare no wind day. Hovered nicely. Was getting up to 14 sats today with the DJI GPS/mag.
Then tried VR mode - oddly, it slowly rotated 180 degrees, then hovered reasonably smoothly - no pitch or roll craziness like before - no oscillations. Don't see any issues with the mag - it is showing the correct direction.
Was able to switch back to stab flying mode, transition to horizontal flight, do a short hop and land successfully.
Still working on building V3, but this was a chance to test systems before I transfer them.

One other oddity - I tried to set GPSassist on the VR mode, but it gave me a config error. It did not do that on the SK450 quad.
Any ideas why?

[TheOtherCliff]

Then tried VR mode - oddly, it slowly rotated 180 degrees

That absolutely should not happen unless it is commanded to do it or the yaw stabilization is completely ineffective.  It should be locked in to the compass heading.  Yaw trim, zero PIDs, or no yaw stabilization configured (design / mixer) are the explanations I come up with.

One other oddity - I tried to set GPSassist on the VR mode, but it gave me a config error. It did not do that on the SK450 quad.
Any ideas why?

If you are running the same LP version on both, then I would guess that you have Hella* set up as custom mixer/aircraft.  The default when you do that is to treat it as fixed wing.  I am guessing that fixed wing is not allowed to use GpsAssist since it can't hover / changes heading constantly when loitering.

I ordered another pair of those antennas to test against the ones I have and make sure they are still as good as when I bought them before.  It looks like you ordered 2 pairs. 

[trust]

Yes, because I am using servos to effect yaw on the elevons, I have a custom mixer set. So it apparently switched it to fixedwing.
That's probably why it yawed - it was drifting backwards - so it needed to move to a point behind its position so it yawed the aircraft thinking it needed to point that direction. It didn't really need to but no harm done - in one sense cool as then it always tries to point in whatever direction it's drifting to.
It would be better to have GPSassist on for this VR hover mode. I suppose this takes a separate rebuild of the code to enable.
  There's probably a lot of features that "safety" protocols are disabling the feature, unaware that users may try something unusual.
I'm not a fan of that approach. Warnings are better than preventing features.