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High altitude decent

Discussion in 'Phantom 2 Discussion' started by tupes, Feb 8, 2014.

  1. tupes

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    When coming straight down from a high altitude, is it possible to defend TO fast? I'm asking because today when I was coming down for the first time I went high, at around 20' from the ground it started to wobble very bad and crashed. It may be a stupid question since it did happen to me but wanted to ask anyway. Also is it possible to shut the motors off while in flight? If so that may what happened. I wasn't thinking about holding the elevator control all the way down until after the crash.
     
  2. tupes

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    Sorry for the misunderstanding. It started to do the "wobble of death" at about 20'. At max height I was probably around 100' when I started my decent. This is a screen shot of my height. Sorry in advance for the poor quality picture. [​IMG]
     
  3. Gizmo3000

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    it's ideal to descend at a slight angle so that you don't descend into the prop-wash (which can always produce instability)
     
  4. macheung

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    100ft wont generally be considered high altitude. I thought you were coming down from like 400ft plus...
    Anyhow, rotor craft including heli and phantoms get into trouble if they descend too quickly into the air disturbed by their own propellers. It is called "settling with power", to avoid or escape it, mix horizontal movement with and rapid descends (do not try to power out of it without going forward or sideways first). I typically come down in sport of a wide spiral without issues.
     
  5. tupes

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    Thanks to the both of yall. I consider 100' high just because it was my first time going straight up like that. I have now learned from my errors and Hopefully it don't happen again.
     
  6. Damienlovegrove

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    It sounds like you used the left joystick at the bottom of it's travel for 5 seconds or more. At 5 seconds of bottom position the motors are switched off.
     
  7. goldfishrock

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    I think it's more likely to be something called Vortex Ring State aka Settling With Power.

    This is something that full size helis suffer from too. Simply put you are descending so quickly you actually descend into your own prop-wash and lose most of your lift. I found this out after experiencing exactly the same thing a few weeks ago. :eek:

    The quick and simple way to avoid this is to put in a little forward movement when you descend. This way the prop-wash gets pushed behind and leaves clean air under the blades.

    You can read more here
    http://en.wikipedia.org/wiki/Vortex_ring_state
     
  8. jwuman

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    is there a more favorable condition that can tent to lead to prop wash issues, ie: stright down rapid decent or any contributing weather factors that will induce or help create the prop wash issue?, i ask becuase i have had 30+ flights and have not once experienced as severe prop wash issue with my vision that would cause a crash,,,,i do decend usually with forward stick but have tested straight down decents controlled and slow but never a loss of full control, i did notice some wobble during decent on a few occasions i asssume this was from a bit of prop wash.
     
  9. goldfishrock

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    Dropping straight down rapidly will result in this effect. A slow controlled descent of < 2 m/s should mean you don't see it however if you descend quicker than about 3m/s you'll find very quickly you'll lose stability and then control. If you just release the controls it'll recover, however you need a good 7-10m of altitude for it to regain stability. Anything lower and you risk ploughing it into the deck.
     
  10. havasuphoto

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    Simple answer is yes-there are conditions that make "settling with power" more likely. 1. if the aircraft is heavy, it's producing more downwash for any given flight state.
    2. If you're descending with a tail wind(very common), you are most likely to enter this state, and that's what bites a lot of people.
    3. you have to be using some power......this is usually when things get really ugly. You see the wobble, and add throttle, and the aircraft plants itself firmly into the ground. The correct course of action is to reduce power slightly(don't go below 10%).

    Most people like to take-off and fly into the wind. However, little thought is given to the descent upon returning from the take-off point. And, it's at this point, as the aircraft is fast approaching your view, you realize it's still too high, so you lower the throttle even more...and you enter the "death wobble".

    You can have a 10mph tail wind, and appear to be moving forward at 10mph on your descent, and still get into VRS(settling with power). And, I believe that's where people get confused-because it happens so fast.

    Vision props will make this situation much worse-so use extreme caution when flying with the bigger blades, and plan your descent accordingly.

    What I do, is descend in steps. For example; if I'm 100 meters up, I will descend to 75 meters(I have FPV and iOSD Mini), and stop the descent/level off.....then another 25 meters, and stop. once I get down to the final 25 meters, I fly a long, crosswind approach to my landing pad. Always know which direction the wind is coming from.
    When I'm at about 10 feet, I add power, while still going side-ways(cross wind), and bring the aircraft to a hover.

    Always leave yourself some altitude and time to recover. If you see the aircraft start to wobble-immediately apply forward stick....if you know you have a tail-wind, then apply right/left stick as well as forward, and after about a second or two, add some power.
    These things recover very quickly from this situation, if you recover quickly. But, when the aircraft is really descending fast in a well developed VRS situation, the controls will be very slow to respond-so full stick movements are necessary.
    Never lower your throttle below 10%, to descend. And, always descend at least 1/2 as fast as you went up.

    The closer you get to the ground, on your descent, the more you want to know you have control of the aircraft-that's why I recommend you stop the descent high enough to realize you have control-then just slide down a bit more.
     
  11. fxmodels

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    [edited this to reflect a single missing word which gave the impression we have done this already... sorry... IT is an experimental drop system we are working on...]

    One thing we are experimenting with is ballistic recovery systems. For example, when you zip up to say 800 feet and want to get down FAST, cut the throttle entirely. Let it fall... It will probably tumble if you dont mod it as we WILL do. By simply adding the equivalent of a weathervane to the top which only deploys with negative Gs and cannot reach the props, the vehicle rights itself and remains in a flat spin so to speak. Then you are using very little battery as it falls to the ground, when it gets 30 feet up, kick in throttle and do a powered landing, or, shut off the radio or tell it go home and let it do all the work from there.
     
  12. ianwood

    ianwood Taco Wrangler
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    Good point. This didn't occur to me. It's the same principle in general aviation. Your aircraft and others generate wake turbulence (in case of the P2, it's all from the props) and that wake turbulence sinks at a certain rate and moves with the wind. When you get into it, you can quickly lose much if not all lift.

    Or you could do it like this guy and sink like a stone twice to get down in a hurry!

    [youtube]http://www.youtube.com/watch?v=83fraTicuUk[/youtube]
     
  13. jwuman

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    thanks for all the great tips, went out today and noticed some wobble on decent a few times,,nothing serious but for sure will use some of the tips here.,,,
     
  14. Gonzoshots

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    Here is my recent hard lesson learned about descending too fast (although I didn't mean to), I'm sure there must have been some wind sheer and possibly a backwind. Lesson learned, always descend at an angle, if death wobble occurs DO NOT increase power, reduce and use forward stick. Anyway I lucked out, the craft hit in a pile of snow, 12" to the North and it would have hit on concrete and been destroyed. Here is the video from this nightmare event.

    http://youtu.be/yQvyqSl2Mo0

    The picture below was taken a few seconds before the death spiral downward.
     

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  15. Meluk

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    Only if you have your naza motor settings set to immediate. I think the default is intelligent and the motors will not turn off until it's landed. No matter how long u hold the throttle at zero.


    Sent from my iPhone using Tapatalk
     
  16. Meluk

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    Only if you have your naza motor settings set to immediate. I think the default is intelligent and the motors will not turn off until it's landed. No matter how long u hold the throttle at zero.



    I have a P1 with original props and I have never experienced this wobbling/loosing control on descent after descending from dozens and dozens of flights. Many from v high altitudes as well.

    Just descended 20mins ago from around 300ft at zero throttle with no issues at all. Slightly wobbly but that's to be expected as the craft isn't producing half as much lift.

    I think this only affects the phantoms with the bigger more aggressive props?



    Sent from my iPhone using Tapatalk
     
  17. ianwood

    ianwood Taco Wrangler
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    A better way to put it is the P2 is much more vulnerable to this than the P1.
     
  18. havasuphoto

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    It effects all Phantoms. You may have been descending with virtually no power. The condition we refer too is "settling with power"...so, if your power is near zero, you will descend past your own downwash. This will only be a problem when you add throttle for the recovery. Also, if you fly into the wind-and say the wind is 10mph or more, you can descend very quickly, and not get into your own downwash, because it's always going to be behind the aircraft.
    There are many factors at play here. But, the biggest factors are weight, and the amount of downwash(Vision props).
     
  19. Meluk

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    I agree I think having a heavy phantom and the new props is a big cause for it. My phantom have never been over a 1000g.


    Sent from my iPhone using Tapatalk
     
  20. havasuphoto

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    ...and, you were descending without power. Basically, you need to be using around 15% of available power, and descending at least 300 feet per minute, or more, with near zero airspeed, or going backwards into a tailwind.
    The safest way to descend, always, is to fly a "traffic pattern", like a box, as you descend. Also, descend slowly.

    I have a heavy Phantom 1.0 with Vision Props-so I know I can get into this condition very easily. Sometimes it's hard to avoid. It's what you do when your in it that matters. Just remember, "fly out of it". your controls will not be very responsive...so, plan ahead.