C130 Props

[Charl]

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!

[cowpatz]

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.

[cowpatz]

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.

[emfrat]

[Charl]

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

[Splitpin]

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