Tiger-Motors U8 + 28x9,2 props

Old Man

Old Man

Active Member
The U8-170 and 28" prop combination will pull in the area of 28 amps at ~2500 full throttle Rpm using 6s. A Wookong FC will not provide stability n any one of wind. The A2 is worse than the Wookong. The system requires more voltage than a 6s system provides to work correctly. 8s will be a minimum if you want the FC to stand a chance in providing any stability with low kV motors and large props. Few have had any success n this area. I have tried it in a quad config and determined that only controllers with wide user input parameters stand a chance of generating stable flight in winds over 5 mph. There have been a couple of private aerospace firm tests that have demonstrated the X8 config to be slightly more efficient than a flat 8, regardless of prop sizing. 5% to 6%.


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Q

Quinton

Active Member
Old Man said:
The U8-170 and 28" prop combination will pull in the area of 28 amps at ~2500 full throttle Rpm using 6s. A Wookong FC will not provide stability n any one of wind. The A2 is worse than the Wookong. The system requires more voltage than a 6s system provides to work correctly. 8s will be a minimum if you want the FC to stand a chance in providing any stability with low kV motors and large props. Few have had any success n this area. I have tried it in a quad config and determined that only controllers with wide user input parameters stand a chance of generating stable flight in winds over 5 mph. There have been a couple of private aerospace firm tests that have demonstrated the X8 config to be slightly more efficient than a flat 8, regardless of prop sizing. 5% to 6%.


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Click to expand...

edit *Actually forget what I said below, as I just found out from another site that the 2X5S are run in series, so its really 10S*
Still 28" prop is massive is it not for 44.4V on a 100Kv motor?

Now that is an interesting statement.
If you look at the setup that steadidrone uses, they are actually using 5S lipos along with their 100Kv motors and 28" props using the A2 flight controller.
5S isn't even on the motor details chart, you can see a video of it flying here...

http://droneflight.co.uk/collections/qu4d/products/2014-qu4d-x


 
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A

ACP

Member
Quinton, I suspect they are using two packs in series giving 10 cells.
To test the thrust I used a weighing scale, the type you hang weights from. This was anchored to the ground and then attached to the Heli. When the Heli is powered it it is pulling on the scales. This is a little hairy as it is restrained as if it is anchored to the ground by a rope at 6 m relative altitude. The motors are 400 kv running 15x5 props. Yes the thrust is 18% less @ 50% and 9% @ 100% but the amps is lower which has to be taken into the equation since we are looking a efficiency here.
 

A

ACP

Member
Old Man,
when you say a Wookong FC will not provide stability. is this because it is unable to change the RPM quick enough to stabilize due to low RPM and rotational mass?

Looking at this in terms of varying RPM to thrust , as this is actual what is happening. Since the props are large, relatively small change is RPM alters the thrust in a bigger way in compassion to higher RPM smaller prop configuration.
This means the FC has to have tighter control of each motor's RPM. this would require a higher resolution requiring higher processing speed.
Is this the problem?
Perhaps this is the time for a change in technology these systems. I feel a new standard of FC and ESC development coming on in my head.
 

Old Man

Old Man

Active Member
ACP

You are correct. FC's rely on generating extremely fast reaction from motor/prop combinations to provide stability correction inputs. I don't know the correct terminology but I'll throw out there the large diameter props, although generating more thrust, do not provide reactive forces fast enough, especially when pushed with "slow" motors. Fromm perspective it's like the relationship between torque and horse power. Torque is how much work something can do where horsepower is how fast it can do it. The big prop/slow motor combination is like having lots of torque but the slow motors don't have the horsepower (speed) to do the work unless voltage is elevated. After that one can factor in the controller, which is limited to much less voltage than what is required for the motor/ prop combination to be effective.

Things work ok, with long flight times and great lift capability when in dead calm conditions at 22 to 25 volts but introduce a destabilizing breeze or wind and watch out. It won't take long for the system to get far beyond the reaction curve and hit the ground. 8s and up will make it work but your FC better be very, very good. You will also require the ability to custom tailor every PID used in the FC.
 

FerdinandK

FerdinandK

Member
[MENTION=12570]Old Man[/MENTION],

That you did not succeed does not imply that success is impossible.

If you have a look at the wind-mark in this
https://vimeo.com/87954117
and that video
https://vimeo.com/101842668
I would not call this "dead calm" conditions.

If only I would get a 1$ for every "this is impossible" ...

If only I could show the videos I have ... (hopefully that damn copter will be released soon ...).

best regards
Ferdinand
-everything is possible, the limit is in your mind only-
 


Old Man

Old Man

Active Member
Where did I say it was impossible? I said few have been successful. Right now Foctech is marketing a unit that will be rife with destruction because few will understand the problems they will be faced with. Those that do understand, and are not afraid o delve deep into their electrical and control systems can do well, but even with those their will be expensive failures.

I will suggest those that are willing to try use ballast weight, not gimbals and payloads, to learn their aircraft well before committing to "missions". Btw, mine flew, and will fly, a 1260mm Vulcan with 4, U8-170 and 28" T-Motor props with an empty weight of 28 lbs at 50% throttle, but never again using 6s and a Wookong or A2.
 

Old Man

Old Man

Active Member
The video notes 8s, which supports my experiences. 8s is the minimum voltage one should attempt this with.
 


Old Man

Old Man

Active Member
You can fly 6s and large props if flying very light but that bypasses the purpose of the larger props. Thrust and efficiency is the intended goal and as soon as props go smaller efficiency is sacrificed. One might say it's about "bragging rights" in marketing. The advertiser doesn't care if many of those units crash, they care only about selling them. There's probably no need to mention the deplorable state of customer support with Multirotor marketing. They also know most believe anything a manufacturer tells them if the price is low enough.

If you use 28" props on 6s using a U8-170 you'll obtain somewhere around 4200 to 4400 grams of thrust per motor in bench testing from a full charged battery. You'll still be generating over 1500 grams of thrust a half to 3/4 hour later if you maintained 50% throttle for the duration. Unfortunately those power numbers do not factor in the other areas required for safe flight. This is where one gets bit on the butt. Great power, long duration, but some important FC stability factors are missing from the equation when using the lower voltage of 6s. Twitchy if light and unstable in wind if heavy. Sort of like flying a very tail heavy airplane, each control input is larger than the one previous until control is lost. My caution is in voltage selection. Don't go with 22.5 nominal.
 

A

ACP

Member
I understand your point that 6s is not going to produce reaction speed to keep the system stable.
Running 12s with a 22x6 prop may produce better results.
 

FerdinandK

FerdinandK

Member
6s:
https://vimeo.com/101842668

6s: with payload
https://vimeo.com/86122621

6s: without payload
https://vimeo.com/85514725

All with WKM or Naza

all "tail-heavy" copter ....

All the same arguments came 2 years ago when I started to use 14" props, people started with the myth of the "low disk loading" which makes copter unstable when using bigger than 14" props. Now 14" is even small, and 18" is normal and 28" with "low voltage" is impossible ... The same story all the time ...

best regards
Ferdinand
 
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Q

Quinton

Active Member
Old Man said:
My caution is in voltage selection. Don't go with 22.5 nominal.
Click to expand...

Aren't these things supposed to be used with 6S, it is the recommended lipo size on tigers page?

Also Ferdinand is talking about the 100kv motor, not the 170Kv one, there is quite a difference 3.44kg at 50% thrust compared to 7.280kg for the 170Kv with 4 motors..
Noticed in one of those videos that it was mentioned that it was not possible to use a 8S with AUW of 7kg as there was no room for descent.
Just wondering if there is a bare minimum weight for take off for each lipo size, and what sort of throttle % are these things even hovering at?

GPS must be completely out the window, as these things are hovering at almost stall speed :)
The numbers involved with these motors are certainly fascinating, but there is a lot more than thrust and weight you have to think about.

Love the videos [MENTION=3871]FerdinandK[/MENTION] I am actually starting to believe you are a bit crazy, especially with the video where you have an x8 taking off from a flying x16.
If anyone has not seen it check it out, it is something you won't have seen anyone else do I'm sure.

https://vimeo.com/56430449
 
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FerdinandK

FerdinandK

Member
Voltage is a parameter you have to (you can) adjust for your setup. It depends on your motors/propellers and AUW. Or course you can also see it the other way round and fix the voltage (because you have already packs of a given voltage) then you have to adjust (the KV) of the motors to fit your propellers and AUW.

As an example, if you fly with 100kv and 8S, you can replace that by 135kv and 6S
Or if you fly 135kv at 8s, you can replace this by 170kv and 6S (approximately)
...

What do you mean by "GPS is completely out of the window"? Of course all is flown in GPS-mode in the videos above.

best regards
Ferdinand

P.S. Next week I start training to "dock" back on the carrier while in the air.
 

Q

Quinton

Active Member
FerdinandK said:
...

What do you mean by "GPS is completely out of the window"? Of course all is flown in GPS-mode in the videos above.
Click to expand...

I was thinking that GPS mode would not be able to tell if it can control the motors as the throttle would be so low when hovering it may shut itself off (I don't know as I can't imagine how these things would fly being so light)
Like a DJI FC would not take off in GPS unless you are over 50%, I would imagine with these and the throttle was >50% it would shoot off.

Look forward to next weeks "dock" video, no idea how you manage to fly 2 machines and video at the same time ;)
 

R

R_Lefebvre

Arducopter Developer
Yes, but you have to remember that on most DJI flight modes, the throttle stick position has absolutely no connection to the actual motor throttle output. I think too many people actually don't know this, and I don't understand why. This is why we have so many people who come try Arducopter, and then find out that their flying throttle is over 80%, which we consider unacceptable as it leaves little margin for flight control.

When in DJI GPS mode, you simply give it 50% stick, and it will fly. It will output 10%, or 90% throttle, whatever it takes to fly.
 

R

R_Lefebvre

Arducopter Developer
ACP said:
Old Man,
when you say a Wookong FC will not provide stability. is this because it is unable to change the RPM quick enough to stabilize due to low RPM and rotational mass?

Looking at this in terms of varying RPM to thrust , as this is actual what is happening. Since the props are large, relatively small change is RPM alters the thrust in a bigger way in compassion to higher RPM smaller prop configuration.
This means the FC has to have tighter control of each motor's RPM. this would require a higher resolution requiring higher processing speed.
Is this the problem?
Perhaps this is the time for a change in technology these systems. I feel a new standard of FC and ESC development coming on in my head.
Click to expand...

No. The flight controller does not control RPM directly. It doesn't even know what the RPM is. It controls throttle output, which controls thrust. PWM generally allows for about 800 steps of resolution here, regardless of motor size, voltage, or anything else.

Now, what does matter, is the rotational inertia of the props vs. the torque of the motor. This gives a maximum response rate to the power system, or "control bandwidth". There's a widely held theory that after a certain size, we can't control a copter with fix pitch anymore, as the control bandwidth is too low. But I have yet to see that demonstrated rigorously.
 

R

R_Lefebvre

Arducopter Developer
FerdinandK said:
@Old Man,

That you did not succeed does not imply that success is impossible.

If you have a look at the wind-mark in this
https://vimeo.com/87954117
and that video
https://vimeo.com/101842668
I would not call this "dead calm" conditions.

If only I would get a 1$ for every "this is impossible" ...

If only I could show the videos I have ... (hopefully that damn copter will be released soon ...).

best regards
Ferdinand
-everything is possible, the limit is in your mind only-
Click to expand...

Well, maybe not dead calm. There's a light breeze. I would like to see it flying in 30-40km/h winds. I'm not saying it can't. I'd honestly like to see it, to see how well it performs because I don't know.
 

FerdinandK

FerdinandK

Member
If you have a look in this video:
https://vimeo.com/87954117
at 0:57 this is what it was about in that flight.

If you fly for almost an hour conditions can chance. In this video at starting it was calm, but as the time passed (and I sit and wait in front of the copter) wind speed raised up.

The point is you can lean back and say, "this is impossible", "DJI-crap", "... if forums they said, that this is not useable ...", or you can get (*** ) up an make it work. (this you can read as sarcasm)

I always add:
If it works for me, it needs not to work for you. What makes me happy can drive you crazy.

... and finally, the less people know, the better my advantage, it is not that I am not willing to tell about it, it is just people will not believe what they hear.
Also, speaking about my videos, I have a lot of cpus and cores available, so basically I can render everything. (this was ironic)

best regards
Ferdinand
 

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