[karla]

Cool.
Glad to see you got it sorted out and got it this far.
So your speed at the end of that dive, when you bailed out, should have been like 25 m/s right?
Interesting to see next step!
Best of luck

[jcg1541]

Yes, At the bottom of the dive, the speed was about 25m/s . The top of the dive was about 30 m above ground.

25  * 2.5 / 2 = 31.25 .

The nose pitch up was very sensitive at the pull up. I will gradually reduce the pitch up so that it will be a pull out to a horizontal level flight instead of a stand still hover.

I have ordered a 1080p cam runcam split mini.

[karla]

Interesting, the sensitive pitch up may be an early symptom of a 'retreating blade stall'.
As discussed before, maybe 4 sec vertical dive is the maximum speed and you did already 2.5 sec.
Hopefully when the heli reaches its max speed the nose will pitch up by itself and interrupt dive and slow down in a controlled way.

[TheOtherCliff]

I assume that somehow you are running an "idle up" or similar.  I would guess that you go to neutral collective pitch during the dive.  It is important that you keep your main rotor speed high during the dive, so your neutral collective should still have full RPM.

[jcg1541]

Yes, it is idle up, fully acrobatic mode.
OK. Got a 5 second dive. Dive starts at 0:03 and pulls up at 0:09 . It could have been longer, but I chickened out. The zero pitch was not clean zero pitch because I was frightened and left 3 degrees on the collective during free fall. It was FPV. It felt like I was the one falling from sky.


The craft was only 240 grams because the 10-gram raspberry PI was removed before taking off.

[jcg1541]

OK, Got a slightly over 6 seconds dive. The collective pitch is cut to zero at 0:01 , then the nose is pointed down in the next 1-2 seconds. So, technically the dive starts at 0:01 . The video feed does not look like diving until 0:03 .


My rate mode PI gains are on the high region.
https://drive.google.com/file/d/1sZWrp5uXmfFvvFws00yjMYA2tGCyeg8t

[TheOtherCliff]

So are you using higher head speed during dive that now allows you to dive longer?

[jcg1541]

I have not changed head speed. I am doing the 110 degree dive instead of 90 degree straight down dive. 90 degrees is very unstable.

I am thinking about cheating by tilting the camera lens up 20 degrees, so that when I am doing 110 degrees dive, the video feed will look like a straight 90 degrees down.

[TheOtherCliff]

The apparent singularity when pointing straight up / down very probably only exists in Attitude mode.  You might consider Rate mode test.

Even easier, you might consider trying Rattitude mode on Roll / Pitch.  Holding about half stick should give you about 90 degrees.  Beware that the stick gets very sensitive when away from center stick, 79% of stick is 180 bank (upside down), 81%+ of stick is continuous flips.  It basically feels like Attitude with so much expo that full stick does flips.

I would also guess that the issue doesn't exist in Revo INS13 even in Attitude mode.

[jcg1541]

I have been in pure rate mode for the last 10 dives.

Attitude mode could not have a single second of stable straight down dive. Attitude mode does not allow any "110" degree dive because the singularity problem yanks the heli out of any dive when it approaches 90 degree.

Rate mode does have stable zero collective pitch straight down dives for the first 2 seconds until the velocity is high.

The rate mode does not have singularity problem. The rate mode problem is the pitch up/down sensitivities with single rotor at high velocity.

At high enough velocity, like 50m/s when air moves parallel to the rotor disk plane , 0.01 degree positive collective pitch will generate the pitch up force equivalent of full elevator stick down; 0.01 degree negative collective pitch will generate the pitch down force equivalent of full elevator stick up.

It is worse than "complete stalled out" of the retreating blade at 50m/s. The air blows from the rear edge  of the retreating blade to the front edge of the retreating blade. The retreating blade's collective pitch has a reversed lift force.

[jcg1541]

Karla, yes, the 'retreating blade stall' gets worse and becomes a lift reversal when air moves much faster than the retreating speed, when I go beyond 5 seconds.

Interesting, the sensitive pitch up may be an early symptom of a 'retreating blade stall'.
As discussed before, maybe 4 sec vertical dive is the maximum speed and you did already 2.5 sec.
Hopefully when the heli reaches its max speed the nose will pitch up by itself and interrupt dive and slow down in a controlled way.

[karla]

Wow John, I see you are really pushing the limits here
I had a look at your PIDs. The only thing that sticks out is your I terms, never seen so high in 250 or 450 size heli. But I would not worry, if its working.
B t w your outer loop PIDs are very high but you only do rate mode so it is not used.
Totally see your point of 'falling out of the sky' when dive using fpv 
When you did the 6 sec dive, heli reached 60 m/s over 210 km/h? Or even faster since you do 110 degree dive and not just falling, but pushing the dive.
So how does it behave when approaching stall, nose pitches upward? any roll?

[karla]

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!

[jcg1541]

The nose pitch is either up or down. There is no involuntary roll. Roll is very stable other than a twitch when elevator stick is applied suddenly to start the dive. My attitude mode gains are high, and it is very stable and precise maneuver when I fly it in the office aisles. Thanks for looking at my UAV file.

I have not used the heli's own thrust to dive faster than free fall. It is all free fall. Yes, would be close to 120 mph or 200 km per hour if air had no resistance to slow it.

My zero collective pitch is not 0.00 degree. It is maybe 0.3 degrees positive. My camera lens is mounted horizontally. In the last dive video, when it was in 150 degrees dive, it was stable. When it was 90 degrees straight down, it had pitch up. But, I can only imagine that if my zero collective pitch was 0.3 degrees minus, the pitch up sensitivity problem would reverse and became a pitch down.

Wish I can dive next to a building to get a visual of the speed.

When you did the 6 sec dive, heli reached 60 m/s over 210 km/h? Or even faster since you do 110 degree dive and not just falling, but pushing the dive.
So how does it behave when approaching stall, nose pitches upward? any roll?

[karla]

Got it.
Maybe good to wait diving close to building until very familiar with heli behavior?