Concrete Slabs

[Skywalker]

Don’t take off from them...got it! BUT, it dawned on me that I’ve been launching off my pressure treated lumber deck, 10’ above a reinforced concrete slab, surrounded by a flagstone patio set on, you guess d it, reinforced concrete.

Is the rule to not launch directly off a slab, is there a safe elevated distance, or is the issue with the AC being between the metal and satellites?

I’ve been told there is never a dumb question...someone else may be afraid to ask. If this breaks the dumb question rule, be gentle.

 

[Starz]

It’s not the concrete itself, but the metal possibly inside.
Where I am, they don’t use reinforcing in sidewalks and most times seldom in the floors of buildings. The perimeter footings typically are loaded with rebar for structural support.
In other parts of the country, that may be a different story.
The key is to be aware of what your compass can have an issue with.
You can check the status in the app.
Any form of metal in your surroundings is always something to watch for.
I’ve seen a video of a guy flying from a car that I thought was truly a joke.
By the end of the video, it’s obvious that the gray matter is somewhat lacking.
Do some research just to be safe.

 

[Meta4]

Is the rule to not launch directly off a slab, is there a safe elevated distance, or is the issue with the AC being between the metal and satellites?

It has nothing to do with satellites and it's steel, not any other metal.
There's no simple answer, it's a combination of how much steel and how close to your compass.
Also be aware of steel bolts or brackets in your timber deck.
If you can't avoid the reinforced concrete surface, a two feet separation would probably be all you need for safety.
Here are some X-ray views of a concrete slab and a sidewalk to show you what they look like inside.

 

[Skywalker]

Ok thank you, that is helpful. I’ve been using a cushioned wicker stool for launch, so it sounds like I’m fine.

I laid the rebar for the slab under the deck and watched the patio go in. It dawned on me reading a recent post that I’m setting on a massive grid.

Thanks again!

 

[spamgnome]

I operate from my Patio all the time with no issues.

 

[Neil Reid]

I'm not so sure about the reality of rebar in concrete affecting unlicensed radios.

When I began my career as an RF professional many years ago, I used to believe rebar could affect radios, but other than the odd but of unexplained behavior which most radios exhibit at some point, I'm not so sure it's a thing. The Russians have some excellent papers on this confirming the general overestimation of magnetic effect by rebar in concrete. Lots of mythology about rebar and radios; in general there's a lot of dearly held mythology about radios. Compasses in the same category.

Not looking to be contentious; just hoping to add a perspective from someone who's been there and done that on this issue for a couple decades. Fly safe, fly smart.

 

[DanMan32]

It's not a radio problem but rather a compass problem. Ferrous materials distort compasses and DJI flight control relies heavily on its accuracy.

Just try the compass on your phone and see how messed up it can become near concrete with rebar.

 

[Flycaster]

Check your compass readings before you place your AC on the take off location you have been flying from. ALL low green readings? Then place your AC in position. Check your compass readings, Still ALL low green readings, same? Then proceed to the next step. Just take off (after GPS lock) and hover while checking your controls, and other sensors. Compass still reading the same? IMU reading happy (all green).
Go fly, you're more than likely safe from that particular point.

I always use this process when flying from a new location.

 

[Meta4]

The Russians have some excellent papers on this confirming the general overestimation of magnetic effect by rebar in concrete. Lots of mythology about rebar and radios; in general there's a lot of dearly held mythology about radios. Compasses in the same category.

Not looking to be contentious; just hoping to add a perspective from someone who's been there and done that on this issue for a couple decades.

I've never seen any suggestion that it affected radios, but each week I look at incident reports where launching from reinforced concretr surfaces causes yaw errors that often have unfortunate results.
Launching from reinforced concrete surfaces may be placing your compass inches from enough steel to cause compess problems and is just asking for trouble.

Search for yaw error to see examples of the issue.

 

[Flycaster]

Once the compass is "confused as to orientation", either one of them, and once GPS lock kicks in, the AC is confused with conflicting information and does not know where/orientation is, and hence the uncontrolled flight/crashes.

That's why I always check my compass/IMU sensor states before take off, and during control check before heading off....

 

[bobscheel53]

Once the compass is "confused as to orientation", either one of them, and once GPS lock kicks in, the AC is confused with conflicting information and does not know where/orientation is, and hence the uncontrolled flight/crashes.

That's why I always check my compass/IMU sensor states before take off, and during control check before heading off....

This. Always check the direction the drone is pointing in the app before take off and verify that it actually is pointing in that direction.

 

[Skywalker]

Good reading. Thank you all for your input. I’ll be doing some testing this evening.

 

[msinger]

is there a safe elevated distance

Shoulder height is safe in many cases. However, it might be easier to take off from a location where it's safe to place the aircraft on the ground.

Before taking off, you should always do the following:

1) Make sure your drone is never near magnetic metal objects after it has been powered on. It’s best to power on your drone in the exact location where you plan to take off.

2) Check the status message at the top of DJI GO and make sure it’s not reporting a magnetic interference error (like in the screenshot below). If you see this error, that’s a sign that your drone is near some type of magnetic metal object. Move your drone to a different location.



3) When your drone is on the ground at the takeoff spot, look at the map in DJI GO and verify that the red aircraft symbol is pointing in the same direction as the drone. If the drone and arrow are not both pointing in the same direction, that’s a sign that your drone is near some type of magnetic metal object.

4) Make sure the red aircraft symbol in DJI GO is not slowly rotating as your drone is sitting on the ground. If the red aircraft symbol is rotating, that’s a sign that your drone is near some type of magnetic metal object.