Featherweight Datalog Velocity Units

[jdoose]

I recently flew a Featherweight GPS tracker with my university team up to ~7,000 feet and while reviewing the logs, noticed an interesting quirk in the flight data. In both the ground station and the tracker logs, the vertical velocity measurement seems abnormally high. Our estimated maximum velocity for the flight was 770 ft/s, and the maximum recorded was 2,450 ft/s. Similarly, our estimated mains velocity was -25ft/s and the log indicates -90 ft/s.

Post-processing the data some, calculating the vertical velocity based on altitude and time data series, results in data matching what we expected. It appears like the logged velocity is always about 3.3x higher than the actual velocity. Has anyone noticed this? I'm wondering if it could just be some user error on my part, or if there's an erroneous conversion from m/s to ft/s with data that's already ft/s. Data & graphs attached.

@Adrian A curious if you've seen this as well!

 

[Buckeye]

You are correct. Something is messed up in the velocity log. Altitude is in feet, so it stands to reason that velocity should be ft/s. A factor of 3.3 sure looks like a feet to meters issue.

Looking at the slope of the altitude curve, your drogue rate is -59 ft/s and your main rate is -26 ft/s.

 

[Buckeye]

I looked at my FW GPS flight from last summer, and the velocity values in the log files are correct. Also, my output does not have these columns:

Altitude AGL

Launch detection

Apogee detection

Landing detection

Distance (feet)

So, something has changed. What model tracker and ground station software are you using? I am still using the original, iOS-only, Test Flight app.

 

[jdoose]

I looked at my FW GPS flight from last summer, and the velocity values in the log files are correct. Also, my output does not have these columns:

Altitude AGL

Launch detection

Apogee detection

Landing detection

Distance (feet)

So, something has changed. What model tracker and ground station software are you using? I am still using the original, iOS-only, Test Flight app.

The tracker is dated April 2019, and I've been using the new Featherweight UI app for Android. Maybe it exports differently, I'll try on the iOS app as well later today.

 

[Adrian A]

I recently flew a Featherweight GPS tracker with my university team up to ~7,000 feet and while reviewing the logs, noticed an interesting quirk in the flight data. In both the ground station and the tracker logs, the vertical velocity measurement seems abnormally high. Our estimated maximum velocity for the flight was 770 ft/s, and the maximum recorded was 2,450 ft/s. Similarly, our estimated mains velocity was -25ft/s and the log indicates -90 ft/s.

Post-processing the data some, calculating the vertical velocity based on altitude and time data series, results in data matching what we expected. It appears like the logged velocity is always about 3.3x higher than the actual velocity. Has anyone noticed this? I'm wondering if it could just be some user error on my part, or if there's an erroneous conversion from m/s to ft/s with data that's already ft/s. Data & graphs attached.

@Adrian A curious if you've seen this as well!

Thanks for letting me know about this. It appears to be a bug, I agree with your theory that it might be applying the conversion from m/sec to ft/sec twice somewhere in the chain. I'll put his on my developer's to-do list.