[Robotbeat]

Ground effect in this case would be virtually non-existent because of the extremely small disk loading in the case of those miniature jet engines. The ratio of the jet diameter to each jet's height off the ground shows that the ground effect is pretty small in this case. Probably less than 1 percent thrust increase in the case where the jets are attached to his arms and back according to this graph: http://www.copters.com/aero/pictures/Fig_2-39.gif (This graph proves that he's getting essentially zero ground effect.)

The actual exhaust velocity of those jets is extremely high, and for a lot of the video he is holding the jets at a significant angle with respect to vertical, giving cosine losses. With a thrust to weight ratio of over 1, I have no doubt he could achieve extremely high speeds (yeah, 200mph or over) given even a slight amount of lift from his body in horizontal(ish) flight.

[SideburnsOfDoom]

> Ground effect in this case would be virtually non-existent

Then it should be easy for him to demonstrate a sustained hover at 5 or more meters up. Or if it's not a lack of power, does he not have the necessary fine control do do that?

[Robotbeat]

How would you like to fall from 5 meters height while carrying 6 lit jet engines and a bunch of flammable liquid on your back?

The answer is pretty obviously safety. That, and I doubt flying is terribly reliable either given he is doing it with muscles and his muscle memory (super impressive, by the way), so if he did it over water just to prove a point, there's a good chance he'd ruin tens of thousands of dollars worth of equipment.

[rm445]

He could fly to five metres attached to a safety line on a ten-metre tower, or a crane. A simple auto-descender would do fine.

My impression watching this guy's videos when they have hit the news over the past few months has been that, for whatever reason, it skims the ground and can't ascend. If he wants to disprove that, it should be possible to rig up a safe test.

[Robotbeat]

As far as I can tell, they're working on a garage budget. A ten meter tower and crane is non-trivial to rent and setup.

No reason to disprove something that is disprovable with a basic understanding of basic physics and aerodynamics.

I expect the big challenge with a device like this is NOT ascending too high where you'd be at risk of breaking a leg or thousands of dollars worth of red-hot jet turbine. This isn't a drone, it's a flying person.

[SideburnsOfDoom]

That fits the "lack of fine control", doesn't it?

So he's not going to fly much then, is he?

[Robotbeat]

I think this takes some time. It takes 2 years of constant practice to learn how to walk well, and we evolved that capability. Learning to hover and fly might similarly take years. He most certainly could do it, but at significant risk of breaking bones and wrecking his equipment and/or starting on fire.

[SideburnsOfDoom]

> Learning to hover

But if there's no ground effect in those videos, then he can already hover. So if there's plenty of footage of stable hovering at 0.5 meters "in mid air", why is there none at all of hovering at 3.0 or 5.0 meters? It doesn't entirely convince me that it can be done.

[Robotbeat]

For the same reason that just because you can balance on a balance beam 30cm from the ground, you'd probably be hesitant balancing on one 5 meters off the ground.

EDIT: If you want to see him testing the concept in a bunch of iterations, you can check out the vimeo page here: https://vimeo.com/gravityindustries/videos/

Arguably, the leg-mounted jets COULD be close enough for ground effect, but they're apparently destabilizing so they switched to two backpack-mounted engines instead.