Hacker News Reader

Comment by quercusa | Hacker News Reader

>All the while, the bird was zipping along at 90-170 km/h (56-106 mph). Given that these birds generally cruise at 10-60 km/h (6-37 mph), at his top speed

I assume the higher speed is "across the ground" but it's possible he flew his usual airspeed the whole time.

View on HN

What’s the airspeed velocity of an unladen swallow in a hurricane?

Japanese one?

Aaaand hop, you go into the canyon! ;-)

Unless you can tell me your favorite color!

The thing about storms is that the wind changes abruptly all the time. Maybe the bird knows how to navigate it, but it's not a safe environment for balloons like people used as example downthread.

Where the bird was flying the wind did not change abruptly all the time, that only happens in the turbulent lower atmosphere. Higher up the wind is more constant, in the case of a hurricane/typhoon in a constant vortex. The bird was caught in something resembling a local jet stream, circling around the storm's centre while being carried along.

Yeah. I imagine it is like a ballon that blows away with the wind. To it, there is no wind. Just the ground is moving away below it.

but wouldn't the wind exert some pressure on the back of the bird? wouldn't it not generate any lift otherwise and just drop back to the ground? like sails on a sailboat, for the sailors it feels like there is no wind, but the sails are carrying massive pressure

If you’re in a boat in a river, your “natural” speed is the speed of the river. Same with air.

You point about a sailboat and wind is confused; the pressure comes from the fact that the boat is traveling at one speed through the water and at a different speed through the air.

ok, if the bird's natural speed is that of the ambient air, then how does it stay up? the bird is not lighter than air, so where does the upwards pressure on the bird come from? either the bird must flap its wings to stay up or there must be speed difference between the bird and the ambient air to generate lift.

There is a speed difference between birds and the ambient air. Birds don't just drift aimlessly with the wind, they flap their wings and gain airspeed. But from their perspective, the "speed" at which they're flying is entirely relative to the mass of air, and not the ground.

This isn't some mystery, it's the same way boats and planes work. Consider a plane flying at 100 knots of airspeed. If the mass of air they're in is moving perpendicular to the plane at 50 knots, the plane will track diagonally across the ground even though it's pointed forward. The plane won't experience side loads because it's tracking 50 units sideways (with respect to the ground) for every 100 units it moves forwards, the exact same as the "wind". If the plane is instead in a 100 knot headwind, it will be stationary with respect to the ground. It won't drop out of the air, but it also won't make headway to its destination either.

From a mechanics perspective, neither the plane nor the bird care about what the ground is doing once they're airborne. The only thing they care about is the mass of air they're aloft in.

If the bird’s natural speed is 7mph, and the tail wind is 7mph, the bird is still flying at 7mph airspeed, its ground speed just increases to 14mph.

No different than a jet flying with or against the jet stream.

https://science.howstuffworks.com/transport/flight/modern/ai...

that is my point - in order to stay up, the bird would need to either flap its wings for 700 miles non-stop (unlikely) in order to maintain velocity difference between it and the ambient air OR glide which basically means he was not traveling at the speed of ambient air, otherwise there would be no lift pressure generated on his wings and he would drop to the ground

Ignore the storm for a moment, and ask whether a bird can fly for 7h+, either from flapping, gliding, or navigating updrafts. The answer is yes, that's trivial for many many bird species, especially those with a propensity for ocean travel. The storm then just changes the baseline ground speed.

There's stuff like dynamic soaring that Albatrosses use to travel thousands of miles over the ocean using little energy.

Some enthusiasts use this to make RC gliders go really fast, and the record is over 500 mph.

https://en.wikipedia.org/wiki/Dynamic_soaring

The comparison with the balloon is perhaps not entirely accurate since birds like you say, fly using lift from the wings. So they need to have some relative speed vs the air.

The point is that as long as the entire body of air the bird is flying through is moving uniformly and without acceleration, it's perceptually indistinguishable from calm air (except visually, and even that only when flying pretty low).

For rotating and turbulent air, which would both not be totally unheard of in a hurricane, this probably doesn't apply though.

This is probably like when you swim out at the beach, and back and find you are 20m away from where you started due to currents. But you didn’t feel it.

With dead reckoning you could probably figure out.

You can’t feel linear/unaccelerated motion, and biological organisms aren’t great at indirectly deriving it from acceleration and rotation over time the way inertial navigation systems do.

Hell, even those have a hard time doing it without experiencing drift! So they periodically re-establish a baseline using something like GPS.

It doesn't really matter in this case, but hummingbirds would like to have a word with you.

A sailboat uses the speed difference between the water (basically stationary) and the wind. A bird (or sailplane) just moves along with the wind. Birds and sailplanes can however hang around areas of rising air to overcome their natural sink rate.

When you're flying, wind is no longer the air moving over the ground, it's the ground moving under the air. It doesn't produce acceleration except for a brief moment when you leave the ground.

>for the sailors it feels like there is no wind, but the sails are carrying massive pressure

The only way this is possible is if the sails are moving at a different airspeed than the sailors, which is only possible if 1) the sailor is running up and down the deck or 2) there is an windspeed gradient with altitude, which the sails penetrate by virtue of being tall.

If your stall speed is 6mph and the winds are 100mph, you can fly into the wind at just above stall speed and still be doing 94mph tailfeathers-first.

It would be yes because ground speed is what a GPS measures

Reply on news.ycombinator.com