How much does rebar in concrete affect takeoff?

[Skyryder]

I live in a concrete jungle. Almost every place I can find away from crowds are walkways or bike way, concrete decks with grass on top etc. So I read this is a problem because of rebar in sidewalks may interfere with the compass or IMU. My question is there a way to know if this is true or going to happen? If I start up the propellers and don't get an error message does it mean I am in the clear or is it possible I could have an instant fly away? Urban legend or real thing, has anyone actually experienced this first hand?

 

[Faster]

Generally you won't have compass problems taking off from concrete. For checking your compass before take off you need to corroborate that the drone arrow in your map is pointing in the same direction as your drone. If your drone is pointing South, the arrow should be pointing South, if you move your drone pointing to the North the arrow should move pointing North too. This procedure should be added to your take off check list.

 

[Meta4]

Urban legend or real thing, has anyone actually experienced this first hand?

It's not an urban legend at all and every week a couple of drones are reported lost or crashed because of it.
Search for yaw error and you'll find detailed , some with plenty of explanation.

If I start up the propellers and don't get an error message does it mean I am in the clear or is it possible I could have an instant fly away?

Not at all.
If the magnetic field is significantly greater than the earth's normal magnetic field, you will get a compass error warning.
But if the field is of similar strength, but in a different direction, you get no warning.
The compass is deflected .. the IMU gyro sensor picks up an incorrect heading from the compass and the trouble can start soon after takeoff.

The first post in this thread will give more details:

A short explanation of compass function, calibration and errors.

BACKGROUND The aircraft compass is often misunderstood, both in terms of function and purpose, but it is important to understand what the compass measures, why, and what happens to that information. At the most basic level the purpose of the compass is to measure the earth’s magnetic field in...

mavicpilots.com

Search for yaw error and I won't have to type out a lot of details all over again.

 

[Phantomrain.org]

The Drone is pretty good about letting you know if there is a compass error or Magnetic Interference .
The Problem comes when it flashes on the Screen and you ignore it because it goes away in a flash.

What happens to most pilots is they miss the warning or dismiss it because it went away so quickly.
So what you need to do is realize that your drone just got Compromised and you need to shut down the drone and Restart from another area .

Once that Compass gets Hammered , there is no going back until it can reset itself.
Find some grass , dirt or parking lot and try again from scratch.

Phantomorain.org
Gear to fly in the Rain. Float on the Water, Capture the storm

 

[Meta4]

If you get a compass error, it doesn't just flash on screen and go away.
If you get a compass error, simply switch off, move away and start again.
The problems happen when there is no warning at all as explained in post #3.

 

[DougMcC]

I always simply activate my M2P and before activating the props simply twist the drone 180 degrees while looking at the map. If they correlate I’m good to go. Launch.
If you are at a new launch site many miles from home base one can get a request for a compass calibration - do it and then check with “the Twist”

 

[Dave Maine]

Hand launch will remove the problem, as well as launching from the top of a wood or plastic crate.

 

[Wild Drone Pilot]

I live in a concrete jungle. Almost every place I can find away from crowds are walkways or bike way, concrete decks with grass on top etc. So I read this is a problem because of rebar in sidewalks may interfere with the compass or IMU. My question is there a way to know if this is true or going to happen? If I start up the propellers and don't get an error message does it mean I am in the clear or is it possible I could have an instant fly away? Urban legend or real thing, has anyone actually experienced this first hand?

GPS and Magnetic Interference
I have seen it 1st hand

I did a ton of research to find out the -[Toilet Bowl Effect]- is NOT uncommon when you takeoff around metal
DJI users guide should include this very real problem!

My Done had this happen twice that I can recall
One of the those times it hit my new car and scratched the paint!
The other time it almost hit cars and people nearby as it drifted out of control

Now I take off from the top of a cardboard box do distance the drone from rebar or other hidden metal
HTH

 

[Meta4]

If you are at a new launch site many miles from home base one can get a request for a compass calibration - do it and then check with “the Twist”

Travelling any distance, even thousands of miles has no effect on your compass calibration.

 

[EsaT]

So I read this is a problem because of rebar in sidewalks may interfere with the compass or IMU.

It doesn't affect IMU directly, but in far more insidious indirect way:

In short magnetic compass is used to calibrate inertial unit for heading during power on.
And if there's magnetic interference making compass point to wrong direction, IMU heading calibration will become erroneous.
Which actually isn't problem for as long magnetic interference continues and compass and IMU agree about heading.

But once drone moves away from magnetic interference compass turns to follow actual geomagnetic field by itself, while IMU is affected only by drone's actual heading change... Or really lack of true heading change.
With compass and IMU now pointing to different directions drone's flight computer doesn't anymore know what's the proper action.
And hopefully goes to ATTI mode fast enough to avoid drone "liberating itself from your control".

 

[KB9Radio]

I live in a concrete jungle. Almost every place I can find away from crowds are walkways or bike way, concrete decks with grass on top etc. So I read this is a problem because of rebar in sidewalks may interfere with the compass or IMU. My question is there a way to know if this is true or going to happen? If I start up the propellers and don't get an error message does it mean I am in the clear or is it possible I could have an instant fly away? Urban legend or real thing, has anyone actually experienced this first hand?

I used to get the compass calibration thing alot, and I was talking off from concrete. I picked up a landing pad, just one of those metal hoops with the orange nylon material about 42" in diameter when deployed. It folds quickly into a nice zipper case about 15" in diameter when not needed. I just look for a grassy spot about 8 or 10 feet from any concrete, and I'm good to go. Now I can drive from Wisconsin to California and it doesn't ask me for a compass calibration ever.

 

[snowghost]

I almost had a crash from launching on concrete. My early flying days. Luckily I wrestled it to the ground as it was erratically flying around the roof. I use a little plastic stool to launch if needed or its Case Club case.

 

[EricJT]

Hand launch will remove the problem, as well as launching from the top of a wood or plastic crate.

Only if you are good and practiced with that method. At my age I don't trust myself enough and want all my digits intact.

 

[offtheback]

I live in a concrete jungle. Almost every place I can find away from crowds are walkways or bike way, concrete decks with grass on top etc. So I read this is a problem because of rebar in sidewalks may interfere with the compass or IMU. My question is there a way to know if this is true or going to happen? If I start up the propellers and don't get an error message does it mean I am in the clear or is it possible I could have an instant fly away? Urban legend or real thing, has anyone actually experienced this first hand?

Perhaps foolhardy,but I have often taken off with the compass error showing as long as the direction is correct.Was I just lucky 100 times?Never had a problem.

 

[Skyryder]

Generally you won't have compass problems taking off from concrete. For checking your compass before take off you need to corroborate that the drone arrow in your map is pointing in the same direction as your drone. If your drone is pointing South, the arrow should be pointing South, if you move your drone pointing to the North the arrow should move pointing North too. This procedure should be added to your take off check list.

ARRG, I never thought of that, so simple and yet makes so much sense. I usually just check to see if it's in the right place on the map, but duh, why didnt I look at it's postion. Thanks, I will do that from now on.

 

[Skyryder]

I would be cool if there was a landing pad you could put on the concrete to mask out the interference like how Faraday bags work for electronics, but I don't think it's even possible. Short of elevating it off the surface that is.

Maybe that's what I need, an elevated launch pad a few inches off the ground. I thought I saw one once when doing a search but cant find it now.

 

[Meta4]

I would be cool if there was a landing pad you could put on the concrete to mask out the interference like how Faraday bags work for electronics, but I don't think it's even possible. Short of elevating it off the surface that is.

Maybe that's what I need, an elevated launch pad a few inches off the ground. I thought I saw one once when doing a search but cant find it now.

The simple remedy is to avoid reinforced concrete surfaces or launch from something that lifts teh drone a foot or so above the concrete surface.

 

[umanbean]

I take off from atop a plastic 5 gallon bucket with an orange lid... replaced the metal handle with a small rope. The bucket doubles as storage / transport for a fluorescent green safety vest, a DIY 12" tall cardboard hood for my tablet (non-folding), bifocal safety glasses, sometimes spare batteries, and anything else I need to throw in to take with me, etc.

I know that solution won't be for everyone, but it gets my drone well off the ground no matter where I am, and it works for me

 

[Wilson Photography]

I keep a shallow plastic storage bin in my vehicle and launch from that when necessary. It is wide enough to hold my propeller guards, hoodman shade for ipad, etc. and gets the drone off the ground a few inches or so. It is also large enough to land on if I'm in an area with really loose dirt, dust such as camping in southern Utah.

 

[Robert Prior]

Travelling any distance, even thousands of miles has no effect on your compass calibration.

My Mavic 2 Pro asks for a compass calibration when I'm more than a few dozen km away from the last place I flew.