Measuring take off Distance

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.

Aeronut said:

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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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.

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.

jimboflying said:

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.

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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.

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

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.

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.



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.



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!

RVBottomly said:

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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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.

reliableflyer, that's the kind of stuff I do in my head often. Sometimes it ends up being useful!