[karla]

Interesting.
Now you are doing 10s dives and pull out in good order.

How does that tail rotor looks from the side view?
Is it canted like this
 https://www.google.se/search?ei=ZbQIW6GxD8exsQHu_rLYBg&q=canted+tail+rotor&oq=canted+tail+rotor&gs_l=psy-ab.3..35i39k1j0i22i30k1.19401.21345.0.21760.6.6.0.0.0.0.414.414.4-1.1.0....0...1c.1.64.psy-ab..5.1.414....0.PvzdiYXefxI

[jcg1541]

Sorry I have been distracted by the search for abandoned tall buildings. I am sure I would have dived many decaying skyscrapers of Detroit if I lived there. Here in the north east, I found the rusting old Ramington Arms munition factory with a tall smokestack. See attached photo. I passed by it over the highway just this memorial weekend. Guess I will dive it in the next few weeks.

[karla]

 

[jcg1541]

Just a question: are you using TPS (Stabilization->Advanced->ThrustPIDScaling)?  Mathematically at least, whenever you double the head speed, you should halve the PIDs, etc.  So if you are using a throttle controlled variable head speed, this will keep the PIDs in sync with the head speed.

That was an excellent question. I have tried it today, and the result is a dramatic improvement. Previously either the ascending was very slow or too much vibration to be shown on video. Now the punch up is forceful, yet no oscillation anywhere in flipping or diving. The ascending is 28 seconds, then the 9 second dive follows,

.  And the extreme scaling down is between thrust 75% toward 100% . The flipping and diving thrust is below and right adjacent to 75%.
The curves are attached in pictures.

[jcg1541]

This is a tribute to Boston Marathon spirit. We lost citizens, and we lost limbs. But our spirit is not hurt one bit.
The takeoff weight of this drone is under 250 grams. I have the PIDs for quadcopter-like diving now,
http://nocomputerbutphone.blogspot.com/2018/09/converged-iot-drone-build-notes.html
. FAA rule is that if you are within 400 feet next to a structure, your altitude limit is based off the top tip of the structure. The dive is about 600 feet from the top of the tower to the church spire. The following video will be deleted after being viewed by this forum.

[karla]

Well done

[jcg1541]

I might have run into the Vortex Ring State now. I have been trying to push the limit of my tail motor ESC to the point of failure by forcing full throttle torque at the bottom of my dives.

2 incidences . The first dive starts at 1:00 mark.  In either case, maximum throttle-pitch was applied at least 2 seconds before the touch down crash and the craft crashed with full throttle-pitch still in effect. The pull-up deceleration appeared weaker than with "regular" pull up moderate collective pitch.


The solution is to just go back to the more moderate pull-up that we instinctively develop.

I have a diagram my reasoning of the vortex , which is a stall vortex due to wind direction change with the dive itself, in attached picture.

I believe this happens to quadcopters too, just that no one tries to push the limit of the tail ESC because there is no tail motor there.

I have found my tail ESC that allows the the vortex ring fault to occur before the ESC fails.

John

John, just an idea.

You could consider doing the dive in a different way.
Do it fully upright, horizontal and apply negative collective.
You now have a fully 3D heli setup right.

You will not have any issues with 'retreating blade stall', not even for the tail rotor since it spins typically 4 * faster than main rotor.
However, you will likely run it to another issue, Vortex Ring State.
But, I was just thinking this should just be initially, and then you can push through it, descending much faster.

This style of dive will give same speed and sensation as 40-50 m/s but you can control the descending speed so it keeps under control.
This will allow you to use Attitude stabilization for simplicity but more interestingly, a much longer dive, like any distance you prefer, really.
Move the camera to a downward position, or have two cameras, or maybe a camera that can tilt.

This is the problem of Vortex Ring State and how to recover from it. But you don't want to recover



But very little or no research has been done if you want to push it faster downward.
No one have ever had an interest doing that.

Fascinating John!

[TheOtherCliff]

It could be a simple matter of the main blades stalling out if you pull out too sharply; just like a fixed wing.  High angle of attack associated with backwards flow through the main disk / high G's.

[karla]

I am not sure, but I think this looks like another trap for helis called 'settling with power' (main rotor stalls).
It looks like your heli is slowing down the descend but just do not have time enough to stop before it hits the ground.
That is different from Vortex, since you will just keep falling without slow down.

Anyways, it do seem you really reached the limit now

This guy from Canada is trying to separate the two things.

[jcg1541]

OK. I have been testing a 3-second collective hold-off rule. I keep the collective pitch at zero for the high-G pull-up at the transition out of the dive for 3 seconds, and I have no more crashes.
During the 3 seconds, the craft can do all cyclic banking and piro maneuver without any problem.


I have to admit that, in my previous crash tests, I did feel that the craft was going to keep sinking indefinitely with the hard full-collective pitch, especially in the first crash test. To the point that I doubted that there was some gear-strip, even though the build is gear-less. 

The operating note about this trick and all other tips are at
https://nocomputerbutphone.blogspot.com/2018/09/converted-iot-drone-operation-notes.html
https://nocomputerbutphone.blogspot.com/2018/09/converged-iot-drone-build-notes.html

[TheOtherCliff]

I've wondered about that.  At zero collective, I would guess that the rotor acts more like a wing flying out of the high G maneuver, rather than a prop (with air blowing backwards and probably stalled out) trying to thrust out.

[jcg1541]

latest test with 3150 rpm , 5 degree hovering collective angle , 1 degree during diving, PIDs tuned down 2680/3150


all previous tests before today had rpm 2680 for 6 degree hovering collective angle maximum fuel economy, 0.5 degree during diving.

[TheOtherCliff]

Looks very stable and controlled.

The only thing I would wonder is how well it responds to the 3 stick axes in the dive.

[jcg1541]

I have been trying a new servo that is marketed as "high precision helicopter servo" , but the high integral gain in the servo is interfering with CC3D's PIDs.
In low-speed, line-of-sight flights, it looks nice, but the PIDs gains becomes inadequate at terminal speeds unless I dedicate a CC3D mode with 180% proportional gains for the dives.
The following video is with the 194% "high-gain mode" during the dive. And immediately after diving pulloff, CC3D PIDs are changed back to normal acrobatic mode.

. The servo is E-Flight S60, 6-gram servo. The parts of the video of switching-back-and-forth of the flight modes have been trimmed away.

Prior to this month, I used the Emax 9051 that likely reacted to split-millisecond adjustments from CC3D at terminal speeds so that the low PIDs numbers for cruising/acrobatics was adequate for diving. The emax 9051 servo likely had no integral circuitry. A youtube reviewer compared it with a more expensive servo and showed that the Emax 9051 servo had larger error angles.
Further more, the Emax 9051 servo likely used large proportional gain to compensate the omitted integral circuitry, serendipitously giving it excellent sub-millisecond reaction time.

[TheOtherCliff]

What does it do if you leave the normal PID active in the dive?  Oscillate?  Wobble around as if the servo isn't strong enough when you know it is strong enough?  Come straight down and require more pull out pitch than normal?  Is the new servo rated to have at least as much torque as the old servo (maybe the strength is marginal)?

I wish we had telemetry to look at to see what the servo is being asked to do.  It could be something like a very jittery servo signal that is being smoothed too much by the new servo, or a PID oscillation with one (slower) servo and not the other (faster one) like an oscillation when you run quad ESC protocols that are too slow for your PIDs.

Just an observation: Measuring servo error with a fixed servo signal is not the same as measuring the heli leveling error with a stabilizing FC running.  Any slop in the servo or even the linkage is removed by the FC.  Imagine a roll in manual mode with the stick released when it is level.  Ignore inertia and assume that the only issue is slop.  Imagine that with the slop the swash plate never comes all the way back to neutral and so you get a continuing slow roll.  Now imagine what the servo does with the FC in Rate mode when you release the stick when the copter is level.  It continues to drift past level and the FC sees that the rate is not zero and drives the servo far enough past neutral to both counteract the slop and the inertia.  Servo and linkage slop doesn't really matter because the FC takes the slop out.

I think I saw a slight circular roll/pitch oscillation in forward flight.  I wonder if the FC is not well aligned to the frame or if the heli roll and pitch response is not well aligned to the frame.  I would fly in Manual mode and test to see if pitch and roll flips are well aligned to the frame.  If not, I would adjust whatever is needed to get the manual flips aligned well.

I might also be inclined to measure vibration with GCS and a USB cable attached and try to get a good rotor balance if you think it could use it.

Hope these ramblings help.