[jthomaslm]

The 24 hour test was with a sub-scale prototype at low altitudes. It turns out that testing a small aircraft in the thicker air at these altitudes matches some important physics of the larger aircraft (Reynolds number).

Perhaps unexpectedly, climbing to 20km altitude isn’t too large a problem from an energy perspective. In a typical energy cycle, the aircraft has low battery in the morning, so if we launch with a full battery charge we have plenty of extra energy to climb up to altitude.

Prime Air - definitely a serious project, but I’m sure Amazon didn’t hesitate in milking the PR! Making a reliable drone delivery service at Amazon scales certainly isn’t easy. That team is still going strong and I’m sure we’ll see more from them as they ramp up commercial deliveries.

[hermitcrab]

>The 24 hour test was with a sub-scale prototype at low altitudes.

That's fine, but I think you need to make that clear or some people might think you aren't being straight.

[frontiersummit]

Just over 20 years ago, a small model aircraft (https://en.wikipedia.org/wiki/The_Spirit_of_Butts%27_Farm) crossed the Atlantic in 39 hours using about a gallon of Naptha as fuel. The same year, the Design-Build-Fly club at my university built an electric aircraft which had some serious endurance and payload capabilities, though I don't remember the details. Keeping a model electric plane aloft for a day is impressive, but how many hobbyists have built something similar by now without any expectation of VC money?

[hermitcrab]

Impressive. Did it rotate the propellor the whole 38.9 hours on 1 gallon of fuel?