Crunch! Crashes are all a part of the fun :-)

My quad (recently renamed to the "flying duckling") had a bit of a mishap today

the angle on that left motor and the shape of the prop isn't just an optical illusion unfortunately! Luckily I had already ordered some spares from jdrones, so I should be flying again tomorrow.

The crash happened when I was successfully flying in LOITER mode for the first time. The quad was holding nicely at about 20m, with just a little bit of wind drift, when one motor suddenly stopped. Unfortunately I didn't have my ground station running at the time, so I have no clue as to why it stopped. I certainly can confirm that a quad flies very badly on 3 motors - it tumbled to the ground and hit pretty hard. I think I was lucky it was on grass, and I only lost one arm and a prop.

It has been a week of unfortunately accidents for the CanberraUAV team. Last Sunday we were flying our UAV plane on autopilot for the first time, when we had a bad crash:

That bit of grey stuff on the right is the edge of the engine sticking out of the ground. It buried itself quite deep when it hit at nearly 30 m/s. Luckily we did have mavlink logging on for this one, and after looking at it for a bit we were able to determine that we'd screwed up the dip switch settings. We had the plane in elevon mode, which doesn't work well if you don't in fact have elevons!

We're trying again tomorrow with a cheaper foam plane, just to get some more experience with APM on a disposable plane. We'll move up to the more expensive planes again once we really know what we're doing.

Meanwhile, I'll put another arm on my copter, and make sure I turn on logging before my next flight, so I have some hope of working out why an engine stops (if it happens again). My best guess is that one of the connectors to an ESC may have become disconnected due to vibration, but thats just a guess. I'll also try running it on my bench for a while, in the hope that the problem may happen when its tied down.

More toys

Meanwhile, a new toy arrived from ebay. I bought a laser tachometer for the princely sum of $6.99 (plus $6.95 in postage). I used it to test the speed of the props on my quadcopter. Very handy! I just put little reflective stickers (which come with the tachometer) on the props and the tachometer quite happily locked onto them. You can see them in the above photo.

 

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Developer
Comment by Jason Short on March 26, 2011 at 11:20am

Hey Tridge, 

I do know that there has been problems with the stock bullet connectors. I've seen Mike's copter do that more than a few times because of a resetting ESC.

But, maybe you had a low voltage ESC cutoff? That's how my original AC kit died. You may want to set your ESCs to soft cutoff instead of a hard one. This change has saved many quad models for me.

I also bought one of those battery alarms from rctimer that plugs into the balancing plug. Another good investment.

 

 

 

 

Comment by Greg Fletcher on March 26, 2011 at 12:09pm

Ok Canberra team, two things could have prevented the last crash.

1) Preflight testing of the control response in stabilize mode.

2) Having sufficient altitude to recover manually when auto doesn't work.

Develop a checklist to prevent this in the future and you will go through less airframes.


Developer
Comment by Andrew Tridgell on March 26, 2011 at 1:38pm

Hi Greg,

We did actually do preflight testing in stabilize mode, as per the APM manual. The problem is that we only tested "do the control surfaces move in the right direction when we move the plane in stabilize mode". What we didn't test was if the mixing of transmitter input with the auto-stabilization worked correctly. The combination of dip-switches we had led to it flying quite well in stabilize mode, until we started to add some elevator override. The mixing of the tx controls went in the wrong direction.

You are right that more height would have helped, and in fact the first time we switched to stabilize mode we were higher. Jack brought the plane lower for the 2nd try as it was hard to see exactly what was happening when the plane was high up. The 2nd try was at 50m, and unfortunately at 30m/s that doesn't give much time to recover when it starts to nose dive.

We will certainly be doing things a lot more carefully with the new plane today though!

 


Developer
Comment by Andrew Tridgell on March 26, 2011 at 2:01pm

Hi Jason,

yes, in looking for a cause and reading comments from other people, it seems the bullet connectors are the most likely cause. So I chopped them out last night and soldered the cables directly.Unfortunately that makes HIL testing a bit trickier as it means I can't disconnect the motors and still have my receiver powered. I'll need to sort out a separate receiver power source for HIL.

I don't think it was the battery dying, as it was quite early in the flight, but I've ordered a battery alarm anyway. I haven't looked into the soft/hard ESC cutoff. I'll read up on that.

 


JDrones
Comment by Jani Hirvinen on March 28, 2011 at 10:15am

All new motors those bullets are removed and they are now in accessory bag. Connector should be tight to work properly and some of those seems not to be tight enough. Squeezing with a pliers helped a lot but as said, in future motors they are removed.

 

Battery alarm is also a good idea, if your battery is starting to run low = immediate crash :)

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