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Using GPS to measure takeoff roll

How feasible is this? Should be do-able with SkyDemon / ForeFlight, etc?

I should like to gradually acquire a dataset of how many metres it takes me to get off the ground in various conditions. Would make me much more comfortable with shorter strips.

I envisage a software button you hit marked ‘start takeoff roll here’ and it locks that position as the starting point. It then decides you are off when it sees a rate of change of altitude greater than whatever. A significant runway slope might fool it, but it could cross-check with speed to avoid logging the lift-off too early.

EGLM & EGTN

I used ForeFlight for this purpose quite a bit when I first started flying my current plane. It was useful in figuring out the real world performance, in my hands, as you suggest. You just record the flight and view the file afterward. Recording frequency is fast enough so you can tell the liftoff point without any additional pilot input. I can’t remember now whether I viewed the data in native format or saved it first in something else.

I’ve used a Garmin Etrex to check my height, with the actual AUW, at the distance of powerlines from a planned Fly-in runway. We decided not to go.

Maoraigh
EGPE, United Kingdom

Did this when I evaluated my new plane. It worked perfectly well with the Garmin 496, but the data export is so early ‘90s. It is way more elegantly done with a tablet or smartphone, but measurement frequency of their built-in GPS is 1Hz instead of 5 Hz and the altitude with my iOS devices sometimes deviates by 100 to 200 ft from the Garmin GPS and the barometric altitude. Thus you may need to look at the raw data and use the change in altitude as a parameter instead of some comfortable graphic representation. You don’t need to put a marker as the sudden change in altitude is easy to detect. The horizontal position is sufficiently reliable. I later bought a 5Hz GPS logger (30€ IIRC) for this purpose from China which works very well, however I now don’t need it anymore.

Last Edited by a_kraut at 21 Jan 23:43
Bremen (EDWQ), Germany

There is an App / website called Could Ahoy (must have some Czech backers / software guys…) that translates the data coming off ForeFlight into a comprehensive flight debrief. It’s great for IR training (I discovered it a bit late, alas). It can also read your GPS data directly, IOW you can log your flights directly in the App, although I’ve never tried that. It’s a subscription based thing, about $ 60 p.a. and a free one month (?) trial.

Any GPS app which can record data at time (not distance) intervals can do this. Set it to say 1 second intervals. Then the data (GPX, KML, etc) can be plotted and the distance measured. Not exactly convenient though.

For landing distances, it is not easy to do this, because small geoid errors will prevent the software detecting the touchdown point accurately. This is the problem I always get on my flying videos (which usually have a GPS position+altitude) subtitle). Correcting the geoid error at the start of the flight is easy but the error can be 10-20ft different after a long flight.

Administrator
Shoreham EGKA, United Kingdom

Using gps, I wouldn’t have expected the accuracy of the aircraft on the vertical plane would be anywhere near good enough, considering the tolerances required. IIRC, the accuracy is ‘only’ 5m on the horizontal, so due to the location of the satellites in orbit wouldn’t expecting anything approaching this on the vertical plane be optimistic?

a_kraut wrote:

It worked perfectly well with the Garmin 496

A while back I walked up a couple of flight of stairs with an old 496 but didn’t see any change in altitude?

You could write a script to do it from the logs after the fact.

Better still would be to use the logging feature of something like a Garmin G5 since that also includes values seen from the pitot/static system as well as GPS.

Andreas IOM

I thought about using a skiing app for this but you’d need your passenger to hit the stop/start at the right points.

Forever learning
EGTB

DavidJ wrote:

Using gps, I wouldn’t have expected the accuracy of the aircraft on the vertical plane would be anywhere near good enough

Using Foreflight Track Log generated data worked fine for me. You can see a number of data points at the same elevation, then one that’s higher, followed by more that are higher still. Lift off occurs between the two obvious elevation change points and the take off run could be estimated between them. IIRC I exported the data into Google Earth, with graphics like that shown below, and measured point to point distance on the screen in the usual way. It was not rocket science.

Last Edited by Silvaire at 22 Jan 18:44
14 Posts
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