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Thread: Measuring take off Distance

  1. #1
    jimboflying's Avatar
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    Measuring take off Distance

    Anyone have a good way to measure takeoff distances while on floats?

  2. #2
    this would be a title NimpoCub's Avatar
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    Take off, turn around & look @ your tracks.
    Nimpo Lake Logan... boonie SuperCubber
    200mi (300km) from nearest stoplight... just right! - "Que hesitatus fornicatus est"
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    If you use ForeFlight with a Stratus, the "track" feature can help... You can see where the climb began (assuming that's how you define "airborne"), and where the acceleration run began, both via GPS coordinates. From there, it's just math to find the distance.
    Jim Parker
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    ?? Bearhawk Patrol - Building

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    SJ's Avatar
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    I thought it was always measured with seconds... (time)
    "Often Mistaken, but Never in Doubt"
    ------------------------------------------

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    mvivion's Avatar
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    Quote Originally Posted by NimpoCub View Post
    Take off, turn around & look @ your tracks.
    What he said.

    We don’t measure the length of lakes in “seconds”. One of the more useless metrics I’ve ever encountered.

    MTV
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    The only other way is to use the shoreline or anchor some buoys. It is hard enough on wheels, with good marks and a qualified observer.

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    jimboflying's Avatar
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    What I was looking for was a way to see how the present flap setup works and compare it to the same plane with double slotted flaps. I would want multiple records taking into account density altitude for each measurement.
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    mvivion's Avatar
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    Jim,

    Probably your best bet would be to anchor some small buoys, such as balloons, with a small weight that will rest on the bottom. Put those at some known distance apart and have someone in a boat or on shore note takeoff distances.

    Might not be real precise, but if there is a significant difference, it’ll be obvious.

    How about the water ski ski pond west of town along the interstate? Ready made buoys.....

    MTV

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    Recognizable landmarks and Google Earth or a similar mapping app that has a measurement tool.

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    BC12D-4-85's Avatar
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    Some young gadget pilot could find a way to mark a takeoff track via GPS like noted above. Start the run as a route to a destination then stop the breadcrumb track at takeoff (or whatever). Maybe do a quick turn prior to takeoff then after departure and note the straight track length on the moving map overlay. My GPS experience is limited.

    In the past looking back at the takeoff run on the water vs landmarks on the shore...trees or beaver lodges for example, was how I did it.

    I also developed a habit of mentally timing a takeoff (one one thousand, two one thousand, etc.) to guess if it was going well but also used visible go/no go marks to shut it down and reevaluate. Knowing how long it takes to stop a loaded floatplane is another valuable tool, especially going downstream on a river with a turn coming up.

    Gary

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    spinner2's Avatar
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    Quote Originally Posted by jimboflying View Post
    What I was looking for was a way to see how the present flap setup works and compare it to the same plane with double slotted flaps. I would want multiple records taking into account density altitude for each measurement.
    I think counting off seconds is a good method. I don’t have much seaplane time but I do that on wheels. There’s a big difference in a 5 second takeoff compared to a 15 second takeoff. I do the same for landings. Time isn’t measuring distance but it is a good measure of performance.
    "Fast is fine, but accuracy is everything." Wyatt Earp
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    hotrod180's Avatar
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    Timing a takeoff might be great for comparing performance of airplane A vs airplane B,
    but IMHO it doesn't do much to tell you whether a pond is long enough to safely take off from.
    "Let's see, I take off in 13 seconds and that lake is 23 seconds long" ......
    Cessna Skywagon-- accept no substitute!

  13. #13
    mvivion's Avatar
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    Again, on floats, find a lake where you can drop a few balloons with anchors (old A/N hardware), spaced a known distance apart. Use one as your starting point. Line them along shore so someone on shore can see the balloons. We did something like this when we evaluated new models of floats and different airplanes. Make sure everything else is equal...load, fuel, etc.

    Its pretty easy really.

    MTV
    Last edited by mvivion; 06-04-2018 at 03:43 PM.

  14. #14
    BC12D-4-85's Avatar
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    Lots of variables like debris on float bottoms, daily density altitude, load distribution in the plane, water conditions, wind, technique, plane configuration and trim, engine and prop health, and so on. With GPS the available takeoff distance can be estimated from a detailed map. Add the variable factors plus obstacle clearance requirements makes it an always moving target. Manufacturers publish tables but the rules they used rarely remain the same.

    The counting can help evaluate expected performance to various stages of the takeoff profile; estimating distance isn't guaranteed.

    It's a learned art with a little science mixed in which is why float flying can be fun.

    Gary
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  15. #15
    BC12D-4-85's Avatar
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    Once we estimate the takeoff distance required the dance begins.

    A week ago I watched a C-185 (loaded?) try to get on step in a ~5400' water lane. Temps in the '60's but didn't look up density altitude. Not much wind on what was likely a Saturday's planned adventure to the bush. Tried twice - about 30 seconds into each experiment they failed to get on step and shut down and reevaluated the conditions and maybe load. Made it off on the third try. It looked to me like they used the same takeoff technique and might have used weight reduction or redistribution to finally make it work.

    If the plane doesn't get on step soon there's a reason to expect it may never in time for a safe departure.

    Gary

  16. #16
    Alex Clark's Avatar
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    DISTANCE: A: I use my laser range finder. From my shooting / hunting gear. I keep it in my flying gear for measuring lakes and no-go markers.
    .................B. If that does not work I use google earth measuring gizmo.

    MARKERS:
    A: Land marks along a shore, big rock, old house, driveway, dock or whatever.
    B. Floats....Regular balloons are rather small. I found some kids punching balloons at the party store. They are more heavy duty, larger and they come with a big rubber band so kids can bounce them off their fists. https://www.amazon.com/50-Pack-Jumbo...SIN=B074JGRS84

    I just took some kite string and a few old nuts and washers to use as anchors. Either retrieve them after they have been used. or shot them with a pellet gun.

    I occasionally make a box of floats ( balloon buoys ) that is only a couple feet wider than the floats. Then I have the clients practice touching down inside the box

    Attached is a photo of a lake occasionally use and places where various planes can break free of the water. I hit the marker on my hand-held gps.
    Attached Images Attached Images
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  17. #17
    Cub Special Ed's Avatar
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    Im not sure, but if u had handheld gps, you could mark the start of your run, then mark the break water point, measure distance between to points on gps when you land (you can do this on the gps itself). Prob easier to have a passenger do this for you when you say mark.

    As far as landing, i use the 69mph measure. Fly over intended landing run at groundspeed of 69mph and that is for evey one-1000 count=100ft. Sometimes 69 is too fast but thats ok. Slower than 69 mph is more than 100ft.
    "There are 3 kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves." Will Rogers
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