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Users => Vehicles - Helicopters => Topic started by: jcg1541 on October 27, 2017, 10:38:27 pm

Title: Cliff Diving PID Setting
Post by: jcg1541 on October 27, 2017, 10:38:27 pm
Has any one done the urban proximity diving with a traditional heli, like youtube RotorRiot team?
I think most helis with fpv video onboard are too expensive to do those dare devil acts where only
50% chance a craft can get out alive. But, how about purely direct drive helis, where the main
shaft is the motor shaft itself? And whole craft is as cheap as quads.

And it is carbon fiber shaft, so no bending, either crack and die or live. And the entire craft has
only the same number of moving parts as a quad with folding props.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on February 28, 2018, 01:16:25 am
OK, I got the pitch and throttle curve for the gearless heli. Video of the onboard FPV camera DVR is here
Liftoff happens at 0:25 , and the 1 second dive happens at 0:56
https://www.youtube.com/watch?v=1ss5iPBlxnk
. And the ground camera video is here
https://www.youtube.com/watch?v=jDdS0xVSPgA
I was afraid that my high P gain will put the airfoil in the stall region at the bottom of the dive when
it needs no stalls. But it looks OK, and the audio at the pulloff does not sound like stall turbulence
happened.  The CC3D config and building discussion is 
https://nocomputerbutphone.blogspot.com/2017/12/converged-drone-developers-platform.html
. I am still trying to dive it at straight 90 degrees. Right now I fly this in attitude mode, and the dive is
80 degrees downward when throttle is lowest and pitch is at extreme forward. I don't know if I want to
fly it in rate mode. 
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on February 28, 2018, 02:01:55 am
Be careful when heli forward speed becomes comparable to blade tip speed.  Even 50% of blade tip speed.

Of course the trailing blade will loose lift (worse effect in high G maneuvers), but the leading blade will have more effect and you may see oscillations.  This happens in fixed wing aircraft.  The faster you fly the faster the roll (/pitch/yaw) rate and so you must reduce the PID at high speed.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on February 28, 2018, 02:42:49 am
Interesting. I have not thought about the forward speed problem.
Title: Re: Cliff Diving PID Setting
Post by: karla on February 28, 2018, 09:01:47 am
In Heli mumbo this is called "Retreating Blade Stall".
Its related to the inevitable effect from the unbalance of lift with a leading and trailing rotor blade.
When forward speed increases it will put a theoretical limit to how fast forward a Heli can go (not very fast).
The retreating blade will be unable to create any lift since forward speed is equal or higher, and nose of Heli will be forced upward.
For the purpose of your project (as I have understood it) it will not be a concern.
Let me get some docs for you to indicate the speed at which your heli will experience the retreating blade stall.
...
Dont let it stop you :)
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on February 28, 2018, 07:52:23 pm
Ok, my throttle curve and pitch cure here in the pictures.
During slow descent, at the 1:13 and at 1:30 - 1:40 mark of the youtube video, the cyclic
compensation has oscillations that didn't occur during ascending because the collective lowers
down while throttle is still high at 55% when collective stick is 70% position and blade collective
pitch is 80%. I though about lowering  the 55% to  50% , but to keep the high rpm during diving,
the 55% seems unavoidable. So, I am planning to overhaul the entire curve scheme for such a
converged drone.

I want to eliminate any negative collective pitch all together. Normal people shouldn't do any
inverted flying. Dedicate the entire 5 tuning points for the pitch curve should give me much
finer tuning. And the entire craft will fly just like a quad, except that the zero collective pitch
point will be a true zero lift-drift. Most stunt quads have a non-zero "idle up" lift-drift to maintain
the rotor's spinning momentum.
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on February 28, 2018, 08:31:34 pm
Some guesses.  :)

Making a guess for 250mm diameter rotor turning 4000RPM (I think that is probably very fast even for 250mm).

Guessing that the blade stalls completely out if 75% of the way out is stalled (farther out is not stalled, but farther in actually has negative lift) and assuming that you start to have problems at half that speed (trailing blade has only half the lift)...

250mm diameter x .75 x PI = about .6 meters of 75% tip distance per revolution
4000RPM is 4000/60=67RPS
67 x 0.6 = 40 meters per second is completely stalled out

For 4000RPM I would guess that, unstabilized, you would have to hold a lot of cyclic at 20m/s to fly level.  And don't try to pull a lot of G's.
For 3000RPM head speed that happens at 15m/s.

Diving straight down with no vertical thrust (since blade is vertical) and the acceleration due to gravity is 10m/s/s you could dive for no more than 2 seconds with no initial velocity (2 seconds at 10m/s/s is 20m/s).  1.5 seconds would get you 15m/s.  1.0 seconds would get you 10m/s.  But I bet you are already flying forward fairly fast when you start the dive.

Larger heli is better, larger diameter means higher tip speed for a given RPM
Higher RPM is better, more tip speed

No negative collective?  I guess you don't want to fly any autorotations.  OK with electric, but for nitro, I would want to allow for motor failure that requires an autorotation.  :)
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on March 01, 2018, 04:11:13 am
250mm is indeed the radius of the rotor. The blades are oxy2 210mm, and the feather shaft and grip add another 40mm. Diameter is 500mm.
4000 rpm is close to it. The motor is 400kv, and voltage is 12 volt.

Not bad guesses.

The speed will tip the nose up to help pulloff the dive, so, the P gain should be small. That makes sense.

The youtube "Trump tower building dive" free falls for 6 seconds.  Will be interesting to see how it goes.

So it will be 60 m/s at the high balcony palm tree tips. So, the blue trump tower is about 6s * 60m / 3 = 120m from the tree tips to penthouse , about 400 feet. 

But I can't find the tower on google. The las vagas one is golden yellow. The new york one is white.
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on March 01, 2018, 04:40:23 am
Twice the diameter that I described, so twice the speed before you have problems and you can dive twice as long time before you have problems.

80m/s is completely stalled out

40m/s is half stalled out and might be OK for max speed (certainly slower would be safer :) ), but that is very fast (144kph / 89mph) and is probably only possible in a dive (4 seconds when starting from 0 speed).
Title: Re: Cliff Diving PID Setting
Post by: karla on March 01, 2018, 05:13:55 am
Wow, now I really do think retreating blade stall is something to worry about here.

At least to be aware of and take pilot measures at early symptoms,
or it could potentially get permanently out of control.

Since the high speed is the problem, recovery can be difficult when the heli is in a vertical dive. Thats exactly what you want to do. Its said that in horisontal flight, the symptom of a retreating blade stall is a pitch up of the nose and it may roll as well, mostly to the right for CW rotor. The recommended recovery is not to move cyclic forward or aft (pitch) since that will only worsen the situation, but to first lower the collective and then use aft cyclic to slow down the speed (pitch nose up). But you will have no collective angle to reduce when in the vertical dive.

Have a second look at the video we discussed before.
I noticed how quickly he pulls out of the dives, even though he could go much further.
It may have to do with this risk.

https://www.youtube.com/watch?v=N-AqVbbfIoI
Title: Re: Cliff Diving PID Setting
Post by: karla on March 01, 2018, 08:26:21 am
I want to eliminate any negative collective pitch all together. Normal people shouldn't do any
inverted flying.

No need to do inverted flying just because having some degrees of negative pitch :)
Just thinking it may be easier to control it and have more options to handle any drift.
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on March 01, 2018, 08:27:26 am
For me, I think it makes more sense to use a powerful fixed wing with a thick airfoil.

Powerful enough to easily fly straight up.  Thick enough wing to slow it down enough so that zero throttle terminal velocity straight down is reasonable.

Then you can fly straight up and straight down for as long as you want.  You could even have an FC to assist in hovering.

There are many 3D airplanes that can do this from small sized to large sized.

I always think about this when I see a video of somebody doing mountain / cliff diving with a quad where the mountain is very very steep
https://www.google.com/search?q=drone+mountain+diving
Title: Re: Cliff Diving PID Setting
Post by: karla on March 01, 2018, 08:51:31 am
The youtube "Trump tower building dive" free falls for 6 seconds.  Will be interesting to see how it goes.

That was done with a quad right?
I am not sure about this but, I think a quad will also experience trailing blade stalls but have a better chance than a heli to get away with it since they have 2 CW and 2 CCW propellers?
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on March 01, 2018, 09:49:41 am
There is certainly a terminal velocity issue too.  The rough calculations I did to calculate number of seconds to reach a certain velocity assume there is no drag.  It looks to me that the quads doing long cliff dives are hitting terminal velocity and don't get any faster.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on March 02, 2018, 07:36:29 pm
All the quads crash 10+ times for each youtube take. We can't be too concerned about the physics.
My servos are 3 dollars each, and it takes at least 20 crashes to destroy it. They want us to crash, and I will crash it for them. See the video how resilient the servos are in simulated crashes,
https://www.youtube.com/watch?v=nKVo3AxR_8I
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on March 04, 2018, 08:52:46 am
I am having doubt about using attitude mode to do straight vertical diving. In my testing
when I hold the heli with my hand, straight downward position causes roll oscillation vibration, in
this video, at 0:37 mark and again at 0:52 mark,
<video removed due to private information involved>
.

It appears that attitude code loop is trying to find the gravity acceleration on the bottom (belly)
side of the heli , but the ground is pointed to by the nose when in straight downward dive. 
The attitude mode diving is only stable when I dive at 80 degrees.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on March 04, 2018, 09:28:49 am
OK, I just tried rate mode with the same exercise. Rate mode has not oscillation when nose points straight down.
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on March 04, 2018, 07:20:10 pm
I can imagine that.  That doesn't help you much.

I can imagine it does that because with nose pointed straight down, then any direction of roll is as level as any other.  So it does not know where to put the roll axis.

From that I would think that if you tried to do more than 90 degrees of pitch, where the roll is actually upside down that it would try to roll out.  Imagine holding 180 degrees of pitch so it was perfectly upside down in pitch.  Then the roll would also think it was upside down even though the roll stick was neutral.  It would (I guess) try to roll back to upright.

I don't think this is related to gimbal lock, but it's possible.  It makes me wonder what INS13 would do in this case on Revo.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on March 05, 2018, 12:58:28 am
Thanks for confirmation.

I had a bliss with my first ever FPV flight today with this gearless setup with low P gain (36) and in
attitude mode.I didn't turn on the onboard DVR recording. So, no video recorded to share.  It was
so intense leading up to today's flight that I forgot to attach the FPV goggle's antenna and still
managed to fly it. The low P gain makes it very stable in open field, but the drift makes it unsuitable
for testing hover in office cubicles.

I figured out why FPV videos on youtube are always action packed. Today in my first FPV flight, I
instinctively had to do non-stop forward flight because the FPV video feed gave no perspective of
depth of the scene. The FPV pilot needs to continuously turning and moving to get the visual ques
to know where the craft is. It is the opposite of line-of-sight flying, where we stop all actions and
try to hover still when we lose track of the heli's movements.

I am still bewildered by what happened today. The feel of insecurity about what is on my sides
and what is behind the craft made me want to land the heli on a landing strip while in forward
flight to avoid "danger". I used to land the craft next to my parking spot in between the fence
and the trees, but now I landed it on the open grass. It was not planed. I didn't plan a landing
approach. But it was a safe touch down without a scratch.

Title: Re: Cliff Diving PID Setting
Post by: karla on March 05, 2018, 01:30:31 am
Yes  :) I can certainly relate to your fpv description.
I flew my first fpv with a heli that did not even have a flight controller, just a tail gyro.
It was exhilarating. So different from los flying.
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on March 05, 2018, 04:12:05 am
The first one is much harder than the second or third.  :)  You quickly get used to seeing it from the air.  I hope your camera does well when looking into the sun, else it is all silhouette and you don't know how close the tree is.

My first FPV was fixed wing, but you would probably say that I cheated.  I used a laptop display and took off LOS.  It was still hard to do.  Fixed wing it is flying faster than a quad and will fly away sooner.  If you get lost FPV you will have a hard time looking up to find it.  ;)  After about a half a flight I got used to seeing things from the air.  Second flight I even landed FPV.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on March 07, 2018, 02:14:02 am
OK, Did my first rate flight today with a hard landing because I didn't realized that I could/should have switched to attitude mode for the landing approach.

Next is to book a flight to Las Vegas.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on March 25, 2018, 03:08:27 am
I have been struggling with rate mode tuning for 2 weeks. Whenever I fly rate mode toward some direction for
more than 2 seconds, there will always be a "rebound" to stop my action on track. CC3D seems to take on
a life of its own.

I still have not managed to dive straight down for more than 2 seconds.
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on March 25, 2018, 07:56:51 am
Just some thoughts:

You might try increasing pitch/roll PID "I" term.  Quads "blow out" (unstable pitch) at high speed if the "I" term is too low.

You could try speeding up / slowing down the main rotor to see if the problem happens later / sooner.  If so, it may be you are hitting the copter top speed.

Have you tried from a dead stop so that you know you are not flying faster than 20m/s after diving for 2 seconds?  Free fall is about 10m/s ... for 2 seconds is 20m/s.  Is it powerful enough (and you brave enough :) ) to fly horizontally at faster than 20m/s?  I bet it has the same problem when flying fast horizontally.

Rate mode is similar to unstabilized.  Can it be flown unstabilized to see if it is a copter issue or a FC issue?  If it always fails in pitch direction, maybe just set pitch to manual for a test??
Title: Re: Cliff Diving PID Setting
Post by: karla on March 25, 2018, 09:44:47 am
... Whenever I fly rate mode toward some direction for more than 2 seconds, there will always be a "rebound" to stop my action on track. CC3D seems to take on a life of its own.

John, can you please elaborate a bit what you mean here? What are you doing and what is happening with heli?
Rate is hardest to fly but still the most predictable stab mode, in my experience.
I think it will not be difficult to fly just with yaw set to axis lock and all others manual. To test what Cliff suggests.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on March 25, 2018, 10:12:38 am
Good suggestion. I will try the axis lock of the tail and manual cyclic with my gearless direct drive.

If anything goes wrong, I will just switch back to attitude mode to regain control.

I was too cocky today and didn't turn on the fpv recording for tracking flight path in case of lost direction. The geared setup flew too far. My brain froze and didn't switch back to attitude mode to regain control before I lost sight of it. It flew away. Lost somewhere in the snow. 250 dollars.

I will have a good story to tell the Dunkin Donut girl tomorrow. She is curious about my heli on my dash board when I drive throu for lunch every day. I put both the geared and gearless builds on the dash board and tape them down.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on March 30, 2018, 04:25:07 am
I tried both manual cyclic and high gain rate cyclic today. Both with tail axis lock.

Both gave me toilet bowl effect. But I don't know if it was pilot induced oscillation of if the gyro physics makes the wobble. Manual cyclic is unflyable . High gain rate cyclic is barely flyable when I try to counter the flushing action.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on March 30, 2018, 06:37:32 am
It should have been pilot induced oscillation. I had the roll stick with zero exponent but pitch stick with very large 75% exponent.

I was not aware of the situation until just now. No wonder it behaves so out of wack. When I try to correct roll, it is very sensitive, so when I try to correct the pitch it is unresponsive, so I force it hard, but then when I try to correct roll , I over correct it. Then I soften my finger, and the pitch becomes unresponsive, and go back to over correct roll, and then it just spirals. Roll is over corrected and pitch is under corrected. I have been flying like drunk for 2 weeks.
Title: Re: Cliff Diving PID Setting
Post by: karla on March 30, 2018, 05:16:56 pm
You are talking about stick exponential settings of pitch and roll in the transmitter right?
If so, yes then I can imagine its difficult to fly, regardless of rate, attitude or PID settings of the FC.
Sorry about your fly-away. I have just gotten a pair of these TrackR bluetooth units I am trying out.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 01, 2018, 01:08:19 am
OK, Got my first rate mode "110" degree diving . Apologies for the protection film smudge.
The dive lasted 2.5 seconds. I was so tunnel-visioned on this dive, I flew it with the smudge on FPV,
totally unaware of the blurred feed,
https://www.youtube.com/watch?v=UuLjEBTcxl4 .

I flew another round laid-back attitude mode after taking out the smudged film. And it was a beautiful day,
https://www.youtube.com/watch?v=MYZc57_7Byw .
Title: Re: Cliff Diving PID Setting
Post by: karla on April 01, 2018, 08:16:54 am
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
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 01, 2018, 10:41:02 am
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.
Title: Re: Cliff Diving PID Setting
Post by: karla on April 02, 2018, 03:01:49 am
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.
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on April 02, 2018, 04:16:06 pm
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.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 10, 2018, 02:30:44 am
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.
https://www.youtube.com/watch?v=wVyx11qVg-Y

The craft was only 240 grams because the 10-gram raspberry PI was removed before taking off.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 11, 2018, 01:09:07 am
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 .
https://www.youtube.com/watch?v=XZJF7gACwoc

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

Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on April 11, 2018, 01:17:07 am
So are you using higher head speed during dive that now allows you to dive longer?
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 11, 2018, 03:13:44 am
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.
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on April 11, 2018, 05:50:59 am
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.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 11, 2018, 07:43:16 am
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.

Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 11, 2018, 08:01:28 am
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.
Title: Re: Cliff Diving PID Setting
Post by: karla on April 12, 2018, 04:25:29 am
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  :o
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?
Title: Re: Cliff Diving PID Setting
Post by: karla on April 12, 2018, 09:36:08 am
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 :)

https://www.youtube.com/watch?v=HjeRSDsy-nE&t=95s (https://www.youtube.com/watch?v=HjeRSDsy-nE&t=95s)

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!

Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 12, 2018, 12:43:20 pm
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?
Title: Re: Cliff Diving PID Setting
Post by: karla on April 13, 2018, 04:51:43 am
Got it.
Maybe good to wait diving close to building until very familiar with heli behavior?
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 13, 2018, 04:51:30 pm
Yes, good idea.

Looking at the picture, the vortex ring seems to be some kind of inverted ground effect that comes out of nowhere.
Title: Re: Cliff Diving PID Setting
Post by: karla on April 14, 2018, 07:31:21 am
Haha yes that's a way to put it I guess.
Its the number one dangerous flight conditionfor a full size heli and a common cause of crashes. It may happen when descending vertically (90, tops 70 degrees) and rate of descent is relatively high for a normal heli, 2-4 m/s. I don't know how this translates to our smaller helis, the tip of a rotor moves typically at 200 m/s, we have higher rpms but much shorter rotors. I just have a gut feeling our helis will need to have a higher rate of descent to get in to it. Anyway, the heli basically gets trapped in its own main rotor downwash. Descending faster or slower will move it out of the predicament.
I think you could expect to move through this state as you dive.
Other than that it could be useful to your ends.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 14, 2018, 11:52:55 pm
Karla, My setup is closer to 200 size than 250. The rotor head is from the Blade 180 , and blades are Oxy2 210 blades that are barely over 200mm from bolt screw to the beginning of the sloped cut.
The rotor diameter is close to 490 mm . The weight is less than 250 grams including Raspberry Pi.
I do use the attitude mode to launch/land from/on hand .  The outer loop high gain is very helpful for catching.
https://www.youtube.com/watch?v=TrHm1BYM3fo

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.
Title: Re: Cliff Diving PID Setting
Post by: karla on April 16, 2018, 07:14:00 am
Got it John,
Will try later to calculate at what decline rate such heli would expect a vortex ring state.
Meanwhile, I think you can really just experiment and try it out.
Now you know what to look for and how to recover from it.
symptoms
. heli vibration
. random yawing, rolling and pitching
. increasing rate of descend
. less cyclic stick respons
recover by rolling

What do you think?
 
Title: Re: Cliff Diving PID Setting
Post by: karla on April 27, 2018, 05:59:43 pm
I have been asking around a bit but seems no readily available method to do this calculation.
Answers mostly like this:
 - VRS is a very turbulent state and therefor stochastic and hard to predict/calculate.
It happens, roughly, when rate of descend of downwash is equal to heli descend. But how to figure out difference between full scale heli compared to rc size heli - not clear.
So no real help for you.
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on April 27, 2018, 07:55:55 pm
My experience with this is with quads.  I never had much issue with my unstabilized .30 to .50 sized nitro helis.

Falling straight down is part of the issue.  Descending at a 45 degree angle does not have the problem in my experience.

Also if you are falling quickly (straight down) there is no problem.

At least with a quad, well tuned PIDs (such as from AutoTune) change the problem almost into a non-problem.

Best to experiment in a controlled way to know where the issue is, to avoid it when needed.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on April 28, 2018, 02:55:10 am
Karla, I have cut 40% of the gains with much larger swash travel range to make the heli agile enough to flip.
Previously I tried to optimize the collective for long duration by stipulating the collective range in Librepilot PWM output tab.
Now the swash travel range is almost doubled, and the gains are all different now. I am still yet to cut the gains further down, maybe half of my previous.
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.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on May 06, 2018, 03:54:19 am
Just some thoughts:

You might try increasing pitch/roll PID "I" term.  Quads "blow out" (unstable pitch) at high speed if the "I" term is too low.
That was a valid idea. I had a Proprotional 35, high Integral 150 dive today. And it felt more controlled and not on the whim of heli physics pitch-up-down.
The dive is at the end of the video, 1:06 to 1:15 , sorry it took a minute to climb to the diving height,
https://www.youtube.com/watch?v=2InEBgHKfI0
I was able to do 120 degrees from 1:06 to 1:08 , then to aim for 90 degrees dive at 1:09 - 1:11 , then again to go at 120 degrees from 1:12 to 1:15 . I was in control.
Tank you my friend.
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on May 06, 2018, 05:55:35 pm
 :)
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on May 14, 2018, 04:37:31 pm
OK, I have the leisure to do a aileron flip while diving straight down now with a even higher Integral .
The first 15 seconds is climbing, then the diving immediately follows at 0:16  ,
https://www.youtube.com/watch?v=R1DbTP3CIHc
. I still have the bad habit of elevator pitch up trying to pull up the imaginary airplane at the end of
a dive that looks stupid. In helicopter, elevator pitch up doesn't really help any thing, and elevator
pitch up should be substituted with elevator pitch down for forward pull out.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on May 16, 2018, 12:22:29 am
I think it will not be difficult to fly just with yaw set to axis lock and all others manual. To test what Cliff suggests.
Karla, I have tested manual mode with this 210 sized heli now. The result is that it tilts and twists all directions.
(the heli is too small to be seen most of the time, and police cars zipping by not noticing what goes on there)
https://www.youtube.com/watch?v=CsPT7BiAaVE
The rotor is too small to have stabilization by itself without flybar. This is a FBL setup. This is a FPV session.
0:01 tilt right and backward
0:02 attitude mode to rescue
0:03 tilt right and forward
0:04 to attitude mode
0:05 tilt left straight
0:06 attitude mode
0:08 tilt backward
0:09 attitude mode
0:10 tilt forward
0:11 attitude mode and stay in attitude mode until safe landing
0:25 landing approach too high and close to trees
0:35 backing off far and fast from the botched landing
0:40 good landing approach
0:53 touch down
Every second is an adventure of twists and turns. It needs a computer to stabilize it every second of the way.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on May 18, 2018, 02:14:42 pm
Smoothest and stable dive so far,
https://www.youtube.com/watch?v=C6aAeSFlfw4
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on May 18, 2018, 08:37:40 pm
That is looking acceptable!  What, about 4-5 seconds?

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.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on May 19, 2018, 03:14:58 pm
Interesting. I am not using the thrustpidscaling. My thrust ranges from 78% to 87%. And P is 15 now. The scaling will reduce 1.5 of P.
I will have my pid around this number for a while until my proximity dance skill leap to the next level.
https://m.youtube.com/watch?v=3imtW_ho2zU

The initial half flip overshot made it dive on the side, still straight down dive though. And then elevator erroneously used to correct it. The elevator pitch up was intended to turn the nose down after a half flip. But now the elevator only made it swing on the horizontal plane. And it ended up with piro correction. All this need to be fixed on the pilot's brain. And work on the brain starts now.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on May 25, 2018, 04:02:07 am
Got leveled pull off at the bottom of the dive now thanks to Sikorsky's research for turbulence solution with a canted tail rotor,
https://www.youtube.com/watch?v=64EaENqfc2s
. The heli now has the same 20 degree slanted tail as the Blackhawk to fix previous ugly high-G-pullback at the closeup of the dive videos.

The swinging during diving is due to the high wind today at 15 MPH on the ground and even higher in the air at 120m altitude.  You can see how the wind blows at the tree at the end of the second video,. It is not PIDs problem, this is the view from the FPV goggles, transmitter is a generic tiny whoop 200mw VTX,
https://www.youtube.com/watch?v=gAIVv7u5EAA
. No changes to PIDs since last 2 posts.
Title: Re: Cliff Diving PID Setting
Post by: karla on May 26, 2018, 03:16:31 am
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
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on June 01, 2018, 05:29:17 am
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.
Title: Re: Cliff Diving PID Setting
Post by: karla on June 01, 2018, 10:14:33 am
 :P
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on July 01, 2018, 05:25:12 pm
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,
https://www.youtube.com/watch?v=ZRezTcZxFwM
.  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.
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on September 16, 2018, 01:47:28 pm
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.
https://www.youtube.com/watch?v=SjaLnku8ueQ
Title: Re: Cliff Diving PID Setting
Post by: karla on September 17, 2018, 11:30:54 am
Well done
Title: Re: Cliff Diving PID Setting
Post by: jcg1541 on November 24, 2018, 01:10:03 am
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.
https://www.youtube.com/watch?v=AP6Rui1JB-k

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 :)

https://www.youtube.com/watch?v=HjeRSDsy-nE&t=95s (https://www.youtube.com/watch?v=HjeRSDsy-nE&t=95s)

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!
Title: Re: Cliff Diving PID Setting
Post by: TheOtherCliff on November 24, 2018, 01:27:52 am
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.
Title: Re: Cliff Diving PID Setting
Post by: karla on November 26, 2018, 01:32:33 am
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.

https://www.youtube.com/watch?v=ehV9vLnBICE