jcg1541

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Cliff Diving PID Setting
« 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.
« Last Edit: February 28, 2018, 02:27:51 am by jcg1541 »

jcg1541

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Re: Cliff Diving PID Setting
« Reply #1 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

. And the ground camera video is here

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. 
« Last Edit: February 28, 2018, 01:27:18 am by jcg1541 »

Re: Cliff Diving PID Setting
« Reply #2 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.

jcg1541

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Re: Cliff Diving PID Setting
« Reply #3 on: February 28, 2018, 02:42:49 am »
Interesting. I have not thought about the forward speed problem.

karla

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Re: Cliff Diving PID Setting
« Reply #4 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 :)

jcg1541

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Re: Cliff Diving PID Setting
« Reply #5 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.
« Last Edit: February 28, 2018, 08:10:49 pm by jcg1541 »

Re: Cliff Diving PID Setting
« Reply #6 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.  :)
« Last Edit: February 28, 2018, 08:57:43 pm by TheOtherCliff »

jcg1541

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Re: Cliff Diving PID Setting
« Reply #7 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.
« Last Edit: March 01, 2018, 04:58:53 am by jcg1541 »

Re: Cliff Diving PID Setting
« Reply #8 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).

karla

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Re: Cliff Diving PID Setting
« Reply #9 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.

« Last Edit: March 01, 2018, 11:23:02 am by karla »

karla

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Re: Cliff Diving PID Setting
« Reply #10 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.

Re: Cliff Diving PID Setting
« Reply #11 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

karla

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Re: Cliff Diving PID Setting
« Reply #12 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?

Re: Cliff Diving PID Setting
« Reply #13 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.

jcg1541

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Re: Cliff Diving PID Setting
« Reply #14 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,
« Last Edit: March 02, 2018, 09:57:43 pm by jcg1541 »