ianwood said:
The Phantom will experience very minor improved lifting efficiency when a few inches above the ground. The reason is two fold: 1. reduced velocity of downwash air generates more lift, 2. rotor tip vortices are disrupted by the ground making the rotor more efficient.
This results in a lower power level needed to hover just above the ground. That lower power level may result in a dip should one or both of the GE efficiencies be reduced or countered even just briefly.
Also, your vertical height is not exactly precise. You should expect at any time the Phantom will move up or down 1 or 2 feet simply due to variability of the barometric readings even if it normally does maintain a pretty stable height normally.
This is by far the best response to the initial concern, instability during landing...
My day job makes me pretty familiar with rotary winged flight (US Army Apache Pilot) so I am always happy to share the knowledge with those that like to listen / learn.
In flight school I remember my instructor (an old crusty OH-58 pilot from Vietnam with about 11,000 hours in the OH-58 / Bell Jetranger variant) telling me "DON'T HUNT FOR THE GROUND!"... He said this because students have a tendency to get right above their landing point, suddenly become rather tense and then do a little **** dance right above the ground (going up and down by just a few feet). In the helicopter world, THIS is one of the most dangerous parts of flying. We are told that if we are going to terminate the approach to the ground then we commit to it before entering ground effect (roughly the 2/3rds the width of the rotor diameter - turned vertically), we know to expect slightly more power if we are high and slow (steep approach angle) and that that power requirement will drop off by about 10-15% once we are about 5 - 25ft off the ground (in a Bell Jetranger that is)...
Anyway, I said all that to say this... If you are going to land, COMMIT TO IT. Keep taking out power until you touch down. Don't do a "**** dance" right above the ground because you run the risk of tipping the craft over...
As far as
Vortex Ring State (VRS) - I did a little write up on that in a post (linked the reference in my post signature)... Not really a factor while hovering. DEFINITELY a factor while descending vertically and even more so when we are heavily loaded.
Ground Effect of multi-rotor Drones - I think it would be safe to say that the Phantom is in "some form of Ground Effect" at roughly 2/3rds it's total width... Technically speaking we don't know exactly because unlike a conventional single rotor helicopter, the Phantom has 4 props. And the Vortices from those props interact with eachother... So who knows, maybe I'll get ahold of a smoke grenade someday and will be able to do some fun experiments aimed at the air flow around a multi-rotor during flight... Then again, there is probably already a good deal of research online about them... Long story short, Ground Effect is not something that we really need to worry about... Just remember that the last foot or so of decent to the ground will require continued reduction of power.
Ground Resonance (traditionally speaking regarding flight dynamics, not RF interference as it sounded like someone was suggesting) is not a factor for us. Reason being, we do not have a fully articulated rotor system. Hell we have possibly the worst thing you could use for rotary-wing flight - a Fixed Pitch propeller... But THAT is another story entirely...