Hacker News Reader

Comment by noonespecial | Hacker News Reader

I'd really like to see a "you can fly anything commercially that weighs less than 5 pounds and has a terminal velocity below 30mph in freefall" rule. Bigger loads and speeds would be subject to some regulation and licensing.

This would cause a Cambrian explosion in development. It would be wild.

View on HN

Imagine one of these payloads landing on someone's head. Wolfram Alpha offers up "2.4x impulse force acting on a baseball being hit by a bat" as a comparison when you plug in "5 lbs * 30 mph".

I like the idea, but there probably needs to be more development in safety systems before anyone could reasonably consider allowing that.

> Imagine one of these payloads landing on someone's head. Wolfram Alpha offers up "2.4x impulse force acting on a baseball being hit by a bat" as a comparison when you plug in "5 lbs * 30 mph".

I'm not sure what value this comparison has. Impulse force doesn't tell you much of anything about how much damage would be done to a persons head.

You are working with the wrong concept, m * v is momentum, not impulse. Impulse would be equivalent to the change in momentum over time [1]. Actually 5 lbs falling at 30 mph would probably crush a human skull or at least crack it open. It is easy to see why from the equations:

F * delta t = m * delta V, therefore F = m * delta V / delta t

This means that the force generated in the impact is inversely proportional to the time of the impact. Since the skull is pretty hard, we can say that the impact time will be very short, probably in the order of 1e-2 seconds. Such a sudden change in velocity, from 30mph to 0mph in 1e-2 seconds, generates a big force, and therefore a big impulse.

[1] http://en.wikipedia.org/wiki/Impulse_%28physics%29

Indeed, however to say anything about damage you need to know the area where the force is applied and more about the materials. If the bottom of the copter is padded the duration of the impact coud be much longer and spread over a larger area.

Also, F = m * delta V / delta t is only valid if force is constant in this context. I would expect acceleration to vary during the course of the impact so this relation would likely not hold [EDIT] in terms of determining damage I mean.

Yes, in this case the equation refers to the average force over an interval of time: if at time instant A we had a speed v1 and at a later instant B we have some speed v2 then a force with a magnitude of at least m * (v2 - v1) / (tB - tA) must have acted, therefore the analysis should hold.

As you point out the area is very important in the analysis, since Pressure = F / A. For large areas the force "spreads out" and the damage is less.

The fatality of the accident can vary due to many circumstances however I think we can safely assume that for the purposes of our initial analysis, 5 lbs traveling at 30mph and crashing into my head is not something I am looking forward to ;)

Umm, the point was more about the rule, not the specific numbers. Most of the little quad's like the parrot(1) can free-fall unpowered right onto your head and cause little injury, but you are absolutely not allowed to use the pictures your parrot sends back for any commercial purpose whatsoever. Even as a hobbyist, you are not allowed to fly your parrot outside of direct line of sight, even though its technology clearly begs to be used this way.

Also bear in mind that a 5 pound sponge that hits you at 30 might be a bit more pleasant than a 5 pound chunk of concrete. Perhaps I should have worked out a formula for average density per unit of speed modulated by compressibility, but again, that's not the point. First, acknowledge we need common sense rules that allow a great deal of freedom at the bottom end. Worry about the specifics later.

The worst thing that can happen is that all unapproved drone flights remain off limits "for our safety" and exceptions are made only for those rich and patient enough to clear a bunch of bureaucratic hurdles (which will become higher each year as bigcorps dominate the space and lobby to "increase our safety" and keep the upstarts out).

(1) I am well aware that a parrot weighs less than 5 lbs.

Is it possible to guarantee a maximum free-fall velocity? Couldn't structural failure change it?

Mind you, I think it's an exciting idea in some respects.

Reply on news.ycombinator.com