NZFF

Driving past Whenuapai I noticed a Herkiebird doing circuits.
I stopped and shot a short video as he passed overhead.
It was a bit lower than I anticipated so I framed mostly shaky bits of wing and wheel.
I lifted this one from the clip - I guess if you knew the engine RPM you could work out how fast he was going, from the contrail spacing!

According to Google for nearly all flight regimes the prop speed is 1020 RPM.
Looking at your pic it would seem that a quarter of a turn is about 1m (each prop blade makes its own contrail so 4 contrails for one rotation). That makes it 4m for a full turn.
1020 rpm x 4m = 4080m/min x 60 = 244800m/hr
1 nautical mile = 1852 m therefore,
244800/1852 = 132nm/hr or 132 kts.
Probably not too far out.

According to Google for nearly all flight regimes the prop speed is 1020 RPM. The prop diameter is approx 4.11m, so using that as a "yardstick" (ok "metrestick") it seems that the contrails are spaced about 1 to 1.2m apart.
Looking at your pic it would seem that a quarter of a turn is about 1m (each prop blade makes its own contrail so 4 contrails for one rotation). That makes it 4m for a full turn.
1020 rpm x 4m = 4080m/min x 60 = 244800m/hr
1 nautical mile = 1852 m therefore,
244800/1852 = 132nm/hr or 132 kts.
Probably not too far out depending how high it was.

At 1.2m spacing it would be 158 kts.

Via Google search:



and, reverse engineering the Wisdom of Cowpatz: using V=137kts (obstacle clearance) gives a contrail spacing of 1.04m!

Given a prop diameter of 4.1m to scale the picture, looks about right

Fascinating stuff ....I love this place ! I've just learnt so much from a close up shot of prop contrails