Rngfnd gndclear min setting is 5?

I have lidar on a very small heli mounted under the skids. Its less than 4cm but the range allowed for this fiend is 5-127. Will setting it to 4 work? Does this mean the FC is going to be confused trying to land with this setting at 5? Why is 5 the min when my lidar works down to 1?

Thanks any help or guidance about how I should configure it will be appreciated.

You shouldn’t have any problems. I’ve mounted a small lidar on a quad and it’s positioned facing down just a few millimeters off the ground and I’ve had no problems. In any case you can exclude it with RC option 10 and activate it only in flight

Can you tell more about your heli? Do you have any photos or videos? I would like to build a small heli and am looking for experiences

Id rather have it for landing. Are you saying I should leave it set at the minimum of 5 or that I can set it down to the actual distance of 4 and not have any problems. The min reliable setting for the rangefinder has no lower limit, its just the expected ground distance that only has a range starting at 5cm and going up.

This heli isn’t finished yet but its a T-Rex 300x with a Matek H743-Mini. Iv designed and 3d printed mounts for the FC, GPS, Flow/Lidar, Camera that sort of stuff. I found some 3000mah 3s that I am hope will give me more time than most are flying their 300x. I also have it on 450 sized skids instead of standard to make more space for things under the belly. Id be happy to start another thread in the trad heli section and upload pics if there is somrthing specific you wanted to see on it.


On my quad it defaults to the minimum distance and I haven’t found any problems even if it’s a few millimeters away from the ground (only the thickness of the frame) and I’ve also done automatic lands. The real problem in my case is the lidar EMI noise on the GPS.

You could open a post in the blog section and upload images and videos of your heli 300, it would be very interesting. Thank you

Setting 4 should work just fine.

I argue that it’s a moot point. If you lose LIDAR data 2.5 inches off the ground, fallback fused sensor data should still be reliable enough to make a successful landing.

I’m not talking about the rangefinder min setting. The range finder reads so far very reliably at much lower range. Though I have not tested yet with the motor running, however there is a lot of separation between the two. The ground clear settings is a separate setting from the range finder min and max settings and in the docs it says you can set it between 5 and 127. I don’t understand why this setting would have limits when the min and max settings do not have limits.

Using the lidar down to the last 1cm then having it call back to the barro could create a lot of unexpected movement right at the last critical bit if you want soft landings. This is not a long range range finder. It only works out to 2m so its not really good for terrain following or anything, basically the whole point of it is for smooth accurate and soft landings.

The ground clear setting supposedly tells the autopilot what the range finder reading should be when the aircraft is on the ground. I’m guessing leaving it set 1cm high wont matter as it will pass through that distance and be landing pretty fast no matter what but wanted to check and see what issues I would have with leaving it set too high. Will the auto pilot have a panic when it isn’t landed at 5cm? Will it just kill the motor early and drop the last 1cm? If I set it to 4cm its outside the range for this setting but what will it do? Error and not launch? Ignore the setting completely?

" RNGFND1_GNDCLEAR: Distance (in cm) from the range finder to the ground

This parameter sets the expected range measurement(in cm) that the range finder should return when the vehicle is on the ground.

Increment Range Units
1 5 to 127 centimeters


This differs from
" RNGFND1_MIN_CM: Rangefinder minimum distance

Minimum distance in centimeters that rangefinder can reliably read

Increment Units
1 centimeters

You didn’t read what I wrote, or you chose to ignore it.

Also, a cursory glance through the source code shows that it does not appear to constrain this parameter. You could set 1cm and simply acknowledge Mission Planner’s warning (many of those min/max values are guidelines, not hard limits).

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I didn’t ignore your response. I just wasn’t confident I had communicated my question clear enough that the response was the info I was looking for. I appreciate your help and coming back to clarify.

The copter won’t leap off the ground if rangefinder data is discontinued. The EKF prevents that situation via sensor fusion.