Hex very unstable in wind and ground effect

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schebb
Posts: 7
Joined: Tue Jul 24, 2012 8:26 pm

Hex very unstable in wind and ground effect

Post by schebb »

Hello,

New to forum, first problem with multirotors. Built two quads in past, one with Aeroquad hardware and one with MultiWii Warthox board, both on Multiwii software and they fly great in all conditions. Just finished a hex rotor with Paris Aerial Photo Board 4.0, original MP and NK. Using spectrum 6 channel receiver and DIYDrones ppm mix board. Balanced all props. It flies horribly. In windy conditions such as was our back yard today and in our closed garage in ground effect. Hex is constantly and quickly rocking in all directions and requires a seriously skilled pilot ( my son ) to manage it. As soon as you raise the Hex about 5 feet high in a wind free environment like the garage it flies and feels rock solid. I'm at a complete loss, any ideas please. Thanks.

Steve

TS00
Posts: 64
Joined: Sun Jun 17, 2012 7:35 pm

Re: Hex very unstable in wind and ground effect

Post by TS00 »

Sounds like it needs PID tuning. Have you tweaked the values?

What kind of instability are you seeing? Wobble?

schebb
Posts: 7
Joined: Tue Jul 24, 2012 8:26 pm

Re: Hex very unstable in wind and ground effect

Post by schebb »

Tried P from 3.4 to 4.6 in increments of 3 and had no effect on this instability. Not a wobble, an uncontrollability, like a fly barred heli without the flybar and no electronic compensation? Requires really good heli pilot to manage till the hex is away from wind or ground effect, then stable?

Steve

TS00
Posts: 64
Joined: Sun Jun 17, 2012 7:35 pm

Re: Hex very unstable in wind and ground effect

Post by TS00 »

That's quite a narrow range, I think you need to spend some time tuning it.

I have two identical quads, except they have different motors. One has 1000kv motors, the other has 1450kv motors. The PID difference between them is huge in order to achieve stable flight; one has a P of 6, the other has a P of 1.8.

Follow the PID tuning procedure. Keep increasing P until it starts to wobble, then back it off a little. Then set I (increase if you've had to increase P) until it starts to drift, then back off a little. Now increase D until it starts to wobble when recovering from fast changes, then back off a little.

schebb
Posts: 7
Joined: Tue Jul 24, 2012 8:26 pm

Re: Hex very unstable in wind and ground effect

Post by schebb »

Spent the remainder of the day tuning PIDs and no joy. Have two quads which fly perfectly with a minimum of tuning. This hex was tuned the same way and hovers fair in dead calm but is completely squirrelly in windy conditions or in ground effect. Will post a link to a video shortly. Aside from PID tuning, what other possible causes could there be for this phenomenon? The hex is 630mm on motors centers with 9x4.3 props leaving 3.25 inches between prop tips. Should I go to 8x3.8 props to increase motor speed and increase distance between props? My son is an exceptional 3D heli pilot and extremely capable with a quad and this hex is more than a handful for him. thanks.

Steve

tovrin
Posts: 705
Joined: Tue Sep 20, 2011 4:08 pm

Re: Hex very unstable in wind and ground effect

Post by tovrin »

a couple things i am curious about, how much do you trust that your min/max throttle on all 6 ESCs are identical? this would mean so much more when compensating for adverse atmosphere (would guess that if you gave it 100% throttle it gets a bit squirrelly too) I use a 4to1 cable and connect my TX throttle directly to my ESCs and power up, this makes sure they are 100% the same. you have 2 more motors then i.

trying to think outside the box i am curious if you have a motor accidentally flip flopped with an adjacent one on the FC. during normal lift the change could prove inconsequential, but during adverse conditions when its constantly adjusting all motors it would mean everything.

chris ables
Posts: 317
Joined: Wed Feb 08, 2012 8:42 pm
Location: United states

Re: Hex very unstable in wind and ground effect

Post by chris ables »

That sounds right to me too tovrin ! Esc's out of sync ! Thats why i use the servo y connectors to calibrate all at once on rx throttle channel after i set endpoints in gui ! One esc at a time and sometimes they are not perfectly in sync !

aidensere07
Posts: 12
Joined: Fri Mar 16, 2012 3:47 pm

Re: Hex very unstable in wind and ground effect

Post by aidensere07 »

Just a thought, I'm sure there was a problem with the hex mixes in one of the versions, are you using the latest firmware?
Aiden

schebb
Posts: 7
Joined: Tue Jul 24, 2012 8:26 pm

Re: Hex very unstable in wind and ground effect

Post by schebb »

Thanks all,

Am using the latest firmware but your other points are valid. I will make certain that I do not have an ESC crossed up. On calibrating the ESC"s, I did make a 6 way cable but was concerned about 6 BEC"s connected together! Since all but one now has the positive lead pulled from the connector I will now callibrate them together. Originally did one at a time as I have done with all my multirotors with no adverse effects but I certainly understand the inherent problems. Will look into possible ESC mixup and recalibrate all together. Below is a link to the video of the hex getting squirrely on landing. Thanks.

Steve

http://www.youtube.com/watch?v=dJmDkK6vXaA&feature=plcp

TS00
Posts: 64
Joined: Sun Jun 17, 2012 7:35 pm

Re: Hex very unstable in wind and ground effect

Post by TS00 »

schebb wrote:Thanks all,

Am using the latest firmware but your other points are valid. I will make certain that I do not have an ESC crossed up. On calibrating the ESC"s, I did make a 6 way cable but was concerned about 6 BEC"s connected together! Since all but one now has the positive lead pulled from the connector I will now callibrate them together. Originally did one at a time as I have done with all my multirotors with no adverse effects but I certainly understand the inherent problems. Will look into possible ESC mixup and recalibrate all together. Below is a link to the video of the hex getting squirrely on landing. Thanks.

Steve

http://www.youtube.com/watch?v=dJmDkK6vXaA&feature=plcp


You can connect as many ESCs together as you want as long as they have linear BECs. I can't think of any popular make ESCs used for mutis which don't have linear BECs.

From the video, I'd say that your hex is fine, I'd be surprised if it could fly that well with two ESCs swapped (but that was a perfectly reasonable suggestion of course).

I think you need to just up P and D a little, try a P of ~8 and D of around 40 on the roll and pitch.

chris ables
Posts: 317
Joined: Wed Feb 08, 2012 8:42 pm
Location: United states

Re: Hex very unstable in wind and ground effect

Post by chris ables »

If your son is a heli pilot im sure you know what gyro gain is ! The P value works just like the gyro gain ! And if you think it's a handfull try lowering the rc rates in the GUI . :D

schebb
Posts: 7
Joined: Tue Jul 24, 2012 8:26 pm

Re: Hex very unstable in wind and ground effect

Post by schebb »

Originally stepped P up to 6 but hex wobbled horribly. Down to 3.7 was the value where hex had no wobble. Will try the higher D values today. We understand "gain" or P, its the other two values which we need a better handle on. The rates are not the problem as my son likes high rates and quick response. What he has trouble with is a flying machine be it a heli, a plane, or a multirotor which does not feel connected to the sticks! This hex seems totally disconnected. The smallest gust will pitch or roll it excessively and you correct with the stick and nothing happens so you correct more. All of a sudden it sees that overcorrection and reacts. Your constantly chasing the sticks, worst kind of machine to fly. Our quads fly like they are locked to the sticks and gusts and turbulence do not change their attitude violently, they bounce and get pushed but they stay relatively level and feel locked to the sticks! Will keep you informed, other ideas appreciated. Thanks.

Steve

TS00
Posts: 64
Joined: Sun Jun 17, 2012 7:35 pm

Re: Hex very unstable in wind and ground effect

Post by TS00 »

This explains the effects of PID:

http://www.multiwii.com/wiki/index.php?title=PID

Sounds like you've found your max P, if you reached oscillations then backed off.

Decrease I a little, and increase D to 25 or 30. How do these changes affect the behaviour?

What is the motor - motor distance, and what are the motor specs? What batteries are you using?

schebb
Posts: 7
Joined: Tue Jul 24, 2012 8:26 pm

Re: Hex very unstable in wind and ground effect

Post by schebb »

Thanks all,

Hex is flying great. All in the PID, just too afraid to try things out of my comfort zone. Your suggestions solved my problem. Raised D to 30 and flew OK. A few more flights and final settings: P-5.5, I-0.032; and D-42. Raising the D allowed a higher P. Thanks for all your help and suggestions. Have a video and trying to publish it now. Will post link when done. Thanks again.

Steve

Got a link. Hope it works!

http://www.youtube.com/watch?v=p1wLrL73 ... el&list=UL

TS00
Posts: 64
Joined: Sun Jun 17, 2012 7:35 pm

Re: Hex very unstable in wind and ground effect

Post by TS00 »

Hooray for PID tuning. You are not the first, and won't be the last to be caught out by this.

The default values work great from medium sized quads with low-medium speed motors and fairly low weight (e.g. 450mm, 1000kv, 1kg). But as soon as you go outside this, you need to tune. Faster motors => lower P & D. Bigger frame => higher P & D.

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