First, this was done with Minecraft 1.14.4, on the public server Minepurist, using the F3 debug key data, and the utility SCROT referred to in this posting:

but with a new feature programmed into scrot so that it records the fractional seconds in the filenames.

Pointing Angle: 0.7

Horizontal Rate: 29.81 b/s

Sink Rate: -2.95 b/s

Glide Ratio: 10.10

Pointing Angle: 1.2

Horizontal Rate: 30.28 b/s

Sink Rate: -2.99 b/s

Glide Ratio: 10.10

Pointing Angle: 1.5

Horizontal Rate: 30.14 b/s

Sink Rate: -3.00 b/s

Glide Ratio: 10.05

Pointing Angle: 1.6

Horizontal Rate: 30.29 b/s

Sink Rate: -3.00 b/s

Glide Ratio: 10.10

Pointing Angle: 1.9

Horizontal Rate: 30.28 b/s

Sink Rate: -3.00 b/s

Glide Ratio: 10.09

Pointing Angle: 2.2

Horizontal Rate: 30.34 b/s

Sink Rate: -3.01 b/s

Glide Ratio: 10.09

Pointing Angle: 2.5

Horizontal Rate: 30.36 b/s

Sink Rate: -3.01 b/s

Glide Ratio: 10.08

Pointing Angle: 2.7

Horizontal Rate: 30.40 b/s

Sink Rate: -3.01 b/s

Glide Ratio: 10.09

Pointing Angle: 3.3

Horizontal Rate: 30.46 b/s

Sink Rate: -3.02 b/s

Glide Ratio: 10.07

Pointing Angle: 3.7

Horizontal Rate: 29.47 b/s

Sink Rate: -3.06 b/s

Glide Ratio: 9.61

Pointing Angle: 4.0

Horizontal Rate: 30.72 b/s

Sink Rate: -3.06 b/s

Glide Ratio: 10.05

Pointing Angle: 4.5

Horizontal Rate: 30.44 b/s

Sink Rate: -3.06 b/s

Glide Ratio: 9.96

Pointing Angle: 5.2

Horizontal Rate: 30.99 b/s

Sink Rate: -3.10 b/s

Glide Ratio: 10.01

Pointing Angle: 5.4

Horizontal Rate: 30.70 b/s

Sink Rate: -3.09 b/s

Glide Ratio: 9.93

Pointing Angle: 5.7

Horizontal Rate: 30.89 b/s

Sink Rate: -3.09 b/s

Glide Ratio: 9.99

Pointing Angle: 5.8

Horizontal Rate: 30.75 b/s

Sink Rate: -3.11 b/s

Glide Ratio: 9.88

Pointing Angle: 6.6

Horizontal Rate: 31.29 b/s

Sink Rate: -3.14 b/s

Glide Ratio: 9.95

I was measuring over periods of 5 seconds, and changing angle every 20 seconds or so, allowing 15 seconds for the speeds to stabilize in most cases. This I think might be a source of error -- not waiting long enough for the speeds to stabilize. Also, some were measured when stepping up in angle (accelerating), some when stepping down in angle (slowing).

In general, it seems there is no significant optimum angle between 1.2 and 5.8 down angle. As far as getting as far as one can on one pair of elytra (maximum speed over ground), it does seem down angles of more than 4.0 degrees give marginally faster speed over ground.

Your mileage may vary, on different versions, on different servers, but it seems there is no magic number for down-angle for elytra flying that is substantially better than other numbers. Do not have a potion of slow-fall activated, because that really does slow down the flyer and greatly reduces total range on one pair of elytra.

The maximum range on one pair of elytra with unbreaking III seems to be 28 minutes and 44 seconds times 31.28 blocks / second = 53,927 blocks.

First, this was done with Minecraft 1.14.4, on the public server Minepurist, using the F3 debug key data, and the utility SCROT referred to in this posting:

https://www.maketecheasier.com/take-screenshots-in-ubuntu-at-regular-interval/

but with a new feature programmed into scrot so that it records the fractional seconds in the filenames.

Pointing Angle: 0.7

Horizontal Rate: 29.81 b/s

Sink Rate: -2.95 b/s

Glide Ratio: 10.10

Pointing Angle: 1.2

Horizontal Rate: 30.28 b/s

Sink Rate: -2.99 b/s

Glide Ratio: 10.10

Pointing Angle: 1.5

Horizontal Rate: 30.14 b/s

Sink Rate: -3.00 b/s

Glide Ratio: 10.05

Pointing Angle: 1.6

Horizontal Rate: 30.29 b/s

Sink Rate: -3.00 b/s

Glide Ratio: 10.10

Pointing Angle: 1.9

Horizontal Rate: 30.28 b/s

Sink Rate: -3.00 b/s

Glide Ratio: 10.09

Pointing Angle: 2.2

Horizontal Rate: 30.34 b/s

Sink Rate: -3.01 b/s

Glide Ratio: 10.09

Pointing Angle: 2.5

Horizontal Rate: 30.36 b/s

Sink Rate: -3.01 b/s

Glide Ratio: 10.08

Pointing Angle: 2.7

Horizontal Rate: 30.40 b/s

Sink Rate: -3.01 b/s

Glide Ratio: 10.09

Pointing Angle: 3.3

Horizontal Rate: 30.46 b/s

Sink Rate: -3.02 b/s

Glide Ratio: 10.07

Pointing Angle: 3.7

Horizontal Rate: 29.47 b/s

Sink Rate: -3.06 b/s

Glide Ratio: 9.61

Pointing Angle: 4.0

Horizontal Rate: 30.72 b/s

Sink Rate: -3.06 b/s

Glide Ratio: 10.05

Pointing Angle: 4.5

Horizontal Rate: 30.44 b/s

Sink Rate: -3.06 b/s

Glide Ratio: 9.96

Pointing Angle: 5.2

Horizontal Rate: 30.99 b/s

Sink Rate: -3.10 b/s

Glide Ratio: 10.01

Pointing Angle: 5.4

Horizontal Rate: 30.70 b/s

Sink Rate: -3.09 b/s

Glide Ratio: 9.93

Pointing Angle: 5.7

Horizontal Rate: 30.89 b/s

Sink Rate: -3.09 b/s

Glide Ratio: 9.99

Pointing Angle: 5.8

Horizontal Rate: 30.75 b/s

Sink Rate: -3.11 b/s

Glide Ratio: 9.88

Pointing Angle: 6.6

Horizontal Rate: 31.29 b/s

Sink Rate: -3.14 b/s

Glide Ratio: 9.95

I was measuring over periods of 5 seconds, and changing angle every 20 seconds or so, allowing 15 seconds for the speeds to stabilize in most cases. This I think might be a source of error -- not waiting long enough for the speeds to stabilize. Also, some were measured when stepping up in angle (accelerating), some when stepping down in angle (slowing).

In general, it seems there is no significant optimum angle between 1.2 and 5.8 down angle. As far as getting as far as one can on one pair of elytra (maximum speed over ground), it does seem down angles of more than 4.0 degrees give marginally faster speed over ground.

Your mileage may vary, on different versions, on different servers, but it seems there is no magic number for down-angle for elytra flying that is substantially better than other numbers. Do not have a potion of slow-fall activated, because that really does slow down the flyer and greatly reduces total range on one pair of elytra.

The maximum range on one pair of elytra with unbreaking III seems to be 28 minutes and 44 seconds times 31.28 blocks / second = 53,927 blocks.