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Flying in Ground effect??

Discussion in 'General Discussion' started by usaken, May 30, 2014.

  1. usaken

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    I am trying to be very precise about my quad landings. I am a regular pilot and just think it's good practice. I always land into the wind. I have noticed a couple if times that the bird starts a rhythmic wobble a foot or so above the ground. Thinking that anything rhythmic is probably not good I abort the landing and go around for another approach. You have to pass through some GE to get the bird back on the ground, but what are the hazards? I am aware of ring vortex but that seems like a descending problem, not hovering. Thanks
     
  2. WillemvdK

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    I think it is some sort of ground effect indeed. There is probably a good technical term for it. I have experienced it too. If you try to hover close to the ground (say like 20 - 50cm off the ground) then after a few seconds the craft starts to wobble up and down. Sometimes very close to the ground. Freaks me out. I must have something to do with bouncing updraft off the ground. Its worst on tarmac or anything hard, and the effect is less on something like grass. So it must be the downward air bouncing back up into the craft which makes it wobble like that.
     
  3. FLGulf

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    I look for patterns. Have noticed in cases where bird was hovering over water resulted in bird taking a swim or flying away. My theory was reflections of the signals affecting communication. Another person's theory was hovering for a long time results in VRS. In one case, the bird was flying into a strong head wind over water and the bird dropped. Perhaps the bird was flying so slowly against the wind that it had the same effect as hovering.

    Wondered if hovering over land would also have effects.

    Here is an example of propagation effects of reflection:
    "The studies concluded that physical propagation effects such as dispersion and diffraction of electromagnetic signals propagating through wind turbine farms produce low-level, long-delay, multipath distortion on wireless telecommunications equipment. As a result, microwave network planners have considered windmill farms to be “exclusion zones,” and have built links around or over them rather than through them. " ... "New digital microwave radios with Forward Error Correction (FEC), Adaptive Coding and Modulation (ACM) and high dispersive fade margin are better equipped to deal with interference and multipath distortion produced by wind turbine blades." ... "Besides the improvements in digital microwave technology, wind turbines have also changed. In the 1980s they were smaller (compared to contemporary units) and mostly made of metal; today wind turbines are bigger, and the blades are mainly made of reinforced fiberglass, which is transparent to microwaves."
    http://www.wirelessweek.com/articles/2013/02/wind-farms-and-microwave-links-–-are-they-incompatible

    You said the bird wobbled when hovering close to land. Apparently hovering can result in VRS.
     
  4. ianwood

    ianwood Taco Wrangler
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    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.
     
  5. ElGuano

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    That's what I've noticed as well. I spend a good bit of time (for what reason, I can't even say), just hovering and skitting across the ground a few inches up in the air, and it's far from unstable/uncontrollable. The FC is very good at maintaining attitude in GE, it's essentially the same as how it handles getting hit by shifting winds and thermals.
     
  6. ianwood

    ianwood Taco Wrangler
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    Ha. Almost totally changed my response as you were replying but yes, the FC does a very good job of maintaining it's height. I would attribute that mostly to good algorithms that take full advantage of the IMU.
     
  7. Michigan_PI

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    I have seen resonance issues frequently with conventional r/c helicopters landing on pavement or cement.
    Maybe bring out a "Welcome" mat to take off and land on?
     
  8. usaken

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    I use a piece of foam core as a landing pad in grass. Keeps the bird from tipping on landing
     
  9. GreenMarine

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    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 dick 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 "dick 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...