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Thread: Measuring take off Distance

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    jimboflying's Avatar
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    Measuring take off Distance

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

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    NimpoCub's Avatar
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    Take off, turn around & look @ your tracks.
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    JimParker256's Avatar
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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
    2007 Rans S-6ES

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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.
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    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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    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

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    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 Thumbnails Attached Thumbnails Click image for larger version. 

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    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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    While taxing, I punch a punch a GPS waypoint at the spot I want to start the takeoff , then hit the GOTO button for that waypoint. I do a 360 degree turn and navigate back to that waypoint. When back at the exact spot, the distance to WPT is 0, I start the takeoff run, and read the distance to WPT, when in the air. Work great and very precise. Got and average 600 feets in standard conditions.

    I also mentally count the time, to check the takeoff sequence, if I'm not on the step after 5 seconds, something is wrong (water rudder down, carb heat on ...). I'm usually in the air at 8 seconds, If pass 12 seconds, I will abort as something is not right, (hole in float ..) Also establish an absolute no go point, at which I will abort.
    Last edited by Olivier; 03-03-2021 at 01:33 AM.
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    TurboBeaver's Avatar
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    Old pro (Elmer Wilson)that had flown Maine/ Labrador with 40k of bush flying, were talking Cubs on floats, I had a PA 11/90 on 1320's at the time, I asked Elmer how can you tell if a pond is just too small to land in it??
    He laughed and said " if your circling it not sure, and it just seams to short..............it probably is.��

  20. #20
    Aeronut's Avatar
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    You can use a bit of easy math.
    1. Calculate your acceleration using the equation: Velocity at rotation divided by time in seconds for takeoff run. (v/t = a)
    2. Next you can calculate the distance using the equation: distance = .5*a*t^2 where time is squared (t^2)

    This equation should allow you to compare configurations. All you need is the time and velocity in ft/s at rotation. You will likely need to convert the latter from knots.
    Last edited by Aeronut; 03-03-2021 at 05:16 PM.

  21. #21
    cubdriver2's Avatar
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    Yeah, what he said

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
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  22. #22
    aktango58's Avatar
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    Comparing two airplanes by time may not work so well, as some planes accelerate quicker, and others lift off slower... the important figure is length of run, and distance to climb. Everything else is just parking in the bushes at the end.
    I don't know where you've been me lad, but I see you won first Prize!

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    RVBottomly's Avatar
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    Quote Originally Posted by Aeronut View Post
    You can use a bit of easy math.
    1. Calculate your acceleration using the equation: Velocity at rotation divided by time in seconds for takeoff run. (v/t = a)
    2. Next you can calculate the distance using the equation: distance = .5*a*t^2 where time is squared (t^2)

    This equation should allow you to compare configurations. All you need is the time and velocity in ft/s at rotation. You will likely need to convert the latter from knots.
    Assuming constant acceleration, of course. That works pretty well on wheels, but for floats, the part before you get on step is a lower rate of acceleration than once on step. At least it is for boats unless you have a lot of power to jump out of the hole quickly.
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    BC12D-4-85's Avatar
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    ^yes^ plus floats can develop a hull speed below flying speed when finally on the step beyond which further acceleration requires a reduction in water drag, added lift, or lots more power. W/O some help that's where the bushes deal happens. Not all are willing to just cut power and regroup.

    Gary

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    Gordon Misch's Avatar
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    Right. Valid for constant acceleration only. Not even exact on wheels because drag increases with speed.
    Gordon

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    This app MIGHT work. I haven’t tried it yet:

    GPS Measure

    https://apps.apple.com/us/app/my-gps...e/id1335951628

  27. #27
    BC12D-4-85's Avatar
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    Since Post#10 I found my G-660 had a quick way to add waypoints and measure distance - once on and GPS constellations located tap power on/off button then push Add Waypoint on upper right touchscreen (locks 1st position) then tap Yes to accept (no rush). Repeat power button tap and hit Add Waypoint at 2nd waypoint (takeoff spot) then Yes. Later, use the measure distance function from Map>Menu>Measure Distance option while zoomed in to the two waypoints. How accurate? Depends on GPS quality when measured.

    Gary

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    skywagon8a'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.
    jimbo,
    Since your old thread has been brought to the light again and you have posted pictures elsewhere of your plane with the new flaps, what did you find for a change with the new flaps?. Was the take off distance reduced? Was the time to get on the step reduced? Was the landing distance reduced?
    N1PA

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    Aeronut's Avatar
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    Quote Originally Posted by RVBottomly View Post
    Assuming constant acceleration, of course. That works pretty well on wheels, but for floats, the part before you get on step is a lower rate of acceleration than once on step. At least it is for boats unless you have a lot of power to jump out of the hole quickly.
    Good point and I agree with the other comment that even on wheels acceleration wouldn't be constant as drag increases. I'm not sure how far off the constant acceleration assumption would be on wheels but it would be neat experiment using known distances.
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  30. #30
    skywagon8a's Avatar
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    Quote Originally Posted by Aeronut View Post
    Good point and I agree with the other comment that even on wheels acceleration wouldn't be constant as drag increases. I'm not sure how far off the constant acceleration assumption would be on wheels but it would be neat experiment using known distances.
    Something to consider is the angle of attack of the wings during acceleration. There are many shown in videos who appear to like to carry the tail in a very high attitude during the take off run. When in this attitude the drag of the wheels on the ground increases as the speed increases due to the increased downward pressure from the wings. If the tail was carried with the tail wheel just clear of the ground, the drag of the main wheels would be decreasing as the speed increased due to the wings taking more of the weight of the plane. There is less drag in the air than there is when rolling contact with the ground is happening.
    N1PA

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    Aeronut's Avatar
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    There's a lot of variables going on no doubt, Skywagon8a. This is where I wonder if some testing wouldn't show an approximation that is good enough. One day, when up and flying again, I may take up something like this.

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    RVBottomly's Avatar
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    Quote Originally Posted by Aeronut View Post
    There's a lot of variables going on no doubt, Skywagon8a. This is where I wonder if some testing wouldn't show an approximation that is good enough. One day, when up and flying again, I may take up something like this.
    I guess I'll show what a nerd I am. I did this very thing using a few STOL videos. In those cases, the distance is known and you can get the time from the video.

    The only thing I didn't really have was wind velocity and airspeed at takeoff. Just guestimates there. But comparing various aircraft and using what I could observe from airplanes I flew, I figured that acceleration on wheels is roughly constant until around rotation. I think best I could guess is 10% variation through takeoff roll--which is helpful for back of napkin estimates but could be disastrous in tight places.
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  33. #33
    jimboflying's Avatar
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    Skywagon,
    I have continued to use the HP double slotted flaps. The landing speed is slower. The takeoff distance is shorter. If you pull full flaps on to pop off the water with conventional flaps the plane will fly at slower speeds. If you try that with the double slot flaps the nose pitches down enough to go off the sweet spot. The actual times and distances vary with the conditions of the day.
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  34. #34
    skywagon8a's Avatar
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    Quote Originally Posted by jimboflying View Post
    If you pull full flaps on to pop off the water with conventional flaps the plane will fly at slower speeds. If you try that with the double slot flaps the nose pitches down enough to go off the sweet spot.
    That alone is proof the double slotted flaps have more lift.
    If you like to pull the flaps to pop off the water, coordinate simultaneously pulling back on the stick to maintain the sweet spot.
    N1PA
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  35. #35

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    Google maps will let you click and 'measure'. After that.. it's picking some point and seeing how it works before and after. Ordered my PSTOL flaps too. We'll see how they do.

    JP

  36. #36
    labrador_cub's Avatar
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    if you have a garmin gps you can do a few things. turn before power in and turn off the water, plug gps in computer and on garmin base camp you'll see the turns and the program has a measure feature. that would be most accurate. you can also click on parts of your track and see the speeds and altitudes at set intervals, this gets you pretty close as well and is probably close enough for floats because it don't take much to add length to a takeoff. for testing mods first one is probably best but if you want a measurement for checking out ponds on a map you can go to, second one will give you some margin for safety.

  37. #37
    labrador_cub's Avatar
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    seeing I'm here i figured I'd check out my own and get some pictures.

    here you can see I started my takeoff at point 1450, next point i was doing 20mph so I didn't start there.

    Click image for larger version. 

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    where I got off is a bit harder to tell, i went with elevation at point 1455 but looking at the speed I was probably off at 1454. either way at 1455 I was flying in .4 mile/ 2112 feet. this was just a random pond it my flight track with only 1 in and out so it was easy to follow, wasn't trying to get off quick by any means but you get the concept I hope.

    Click image for larger version. 

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    if I went with point 1454 off the water then my takeoff was .3 mile or 1584 feet.

    Edit: should probably go with speed rather then altitude, my gps at least is very inaccurate apparently now that I'm looking at the trip log, apparently a few time I was 700 feet under water as I was climbing out leaving my base lake.... I don't remember THAT much water coming across the windscreen!
    Last edited by labrador_cub; 03-09-2021 at 11:50 PM.

  38. #38

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    Quote Originally Posted by RVBottomly View Post
    Assuming constant acceleration, of course. That works pretty well on wheels, but for floats, the part before you get on step is a lower rate of acceleration than once on step. At least it is for boats unless you have a lot of power to jump out of the hole quickly.

    the acceleration on wheels is relatively constant. Not so on floats. The variation in acceleration rates is minimal enough for me to discount in a cub that flys at about 40 MPH.

    to put this in numbers and I hope more simply and practically:

    On wheels a cub will fly at about 40 MPH.
    it starts at 0. Since acceleration is relatively constant the average speed of the ground run is 20 MPH 40/2=20

    We know that 60 MPH is 88 feet per second. If we don’t know it we should. Flying over a landing area in both directions at 60 MPH and timing it will give you the approximate length. If we round the 88 feet per second to 90 feet per second it is easier to do the math mentally. If it takes us 10 seconds to fly over a potential landing area at 90 feet per second it is approximately 900 feet long. More time in one direction than the other gives us an indication of wind speed, and direction, the average will give us a better idea of length.

    since we’re going 90 feet per second at 60 MPH at 1/3 the speed we’re going 30 miles an hour.

    if we take the 1/3 of our velocity and apply it to our take off speed average of 20 MPH we are taking off at, the distance we travel on a takeoff roll is approximately 30 feet per second.

    a ten second takeoff is approximately a 300 foot take off. A three second takeoff is approximately a 90 foot take off.

    it is my understanding and outside my experience that big aircraft like jets don’t have constant rates of acceleration.

    this is an approximation only. It’s pretty accurate but you can’t beat good judgement.

    with floats I would estimated my distance along the shore then come back over and time the distance at 60 MPH which gives me an estimate. After I have that down for a particular aircraft I can then fly over a landing area at 60 MPH and get an estimate of It’s length.
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    RVBottomly's Avatar
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    reliableflyer, that's the kind of stuff I do in my head often. Sometimes it ends up being useful!

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