If we know our barrier speed, typically Vx, we can work out the distance to the later of achieving barrier speed, or achieving barrier height. Alternatively if you have a typical lift off speed, which avoids you from not accelerating out of ground effect you could also calculate the distance of the ground run to lift off.
Assume Puddle-jumper Sport takes ten seconds to accelerate to 60 mph IAS (we assume the IAS is close to CAS). To work out acceleration v is final velocity, u is initial velocity (ie zero) and acceleration is a. Formula would be v= u + at. v is approximately 27 m/s. Solving for a we get 2.7 m/s2
Now how far did we travel in reaching v. The formula would be ut + 1/2at2. In this case 0 + 1/2 × 2.7 × 10 squared resulting in around 135m.
Now presumably this is all based on a level, dry tarmac runway at SL with no wind etc
You would then need to apply rough safety factors, for example as in:
The time & distance it takes to accelerate to a given ground or indicated speed (combined with wind & weather/runway data) are way more relevant than distance to liftoff
One can liftoff at early speeds aircraft will be stuck in ground effect if acceleration is not enough, same one can delay liftoff speed…
I found that using a CSV file from ForeFlight and Google Earth (?) one could interpolate between points pretty easily to determine the ground roll. I haven’t done it in a few years but it served my purpose at the time.
My initial thought is that it might be more accurate to use an accelerometer/gyroscope (to feel for rotation and detect an upwards acceleration) or microphone (to listen for rumbling wheels) than GPS to detect when the wheels have left the ground, but that GPS will probably give a good value for the 50ft obstacle clearance. With accurate timings, one should be able to interpolate the GPS ground track well enough to get an accurate take-off distance.
If I wanted an accurate value for my own aircraft I would probably rig a £10 cycle computer sensor to one of the main wheels, and use it to make ‘clicks’ to the microphone input of a phone or ipad. Usually brakes are set very lightly ‘on’ to stop wheels spinning in the air, so they should slow noticeably at the point of take-off. Rules and regulations may apply.
This software was pretty useful when I was doing my flight testing. https://www.cloudahoy.com/
Exactly but that’s a LOT of data – already at 1Hz the csv files easily reach 5Mb.
Yes if your GPS chip and breadcrumbs software was sampling 10 times a second that would probably be sufficient for good info.
You’d certainly then be able to check altitude against time for a rate of increase that is too great to be an upslope or undulation and must mean lift off. Probably wouldn’t work for departures where you hold it in ground effect to build speed.
I’ve built a tool to scorecard all my flights, and finding the point of take off, to measure take off roll and initial climb performance, is surprisingly difficult because… runways are not flat. So I focused on distance to 50 feet instead, which is a bit easier, and afaik is part of every poh so an easy comparison point.
LeSving wrote:
The GPS chip itself is slow. I think max 1 Hz. It used to be 0.2 Hz for older ones. O
Lots of modern chipsets do 10 Hz updates.
LeSving wrote:
Several glass panels have logging capabilities and include AHRS and air data.
Yep. Even my “low cost basic” dual G5 panel allowed me to perform, record, and plot the flight test data that was required for a MTOM increase on my steed 2 years ago. Loads of extractable data, such as accelerations, pitch/bank/yaw, speeds, etc.
Having seen aviation pass from steam to digital has been a mesmerising experience 
