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Comment by larrydag | Hacker News Reader

I believe the key is a strong headwind. I would like to see if this could be accomplished without the headwind. I believe short takeoff is possible but not quite that short.

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The steady headwind was everything. The power and STOL mods just made it somewhat safer to do, although, had that headwind suddenly died, I am fairly sure we would have landed very, very hard.

We were young and stupid.

With the plane adjusted for hovering (or nearly so), a sudden drop in wind speed would cause you to quickly lose altitude, but you would also be accelerating forward relative to the ground to get back up to the former airspeed that the throttle is still set to produce. And the pilot would have had plenty of headroom to increase power, and that would have been an immediate response to an unwanted rate of descent. So a landing would not have been guaranteed, let alone a very hard one–but if you were too close to the ground for the pilot to react in time I could still see the landing being uncomfortable.

We were about 20ft off the ground. Tom said he was trying to stay in ground effect although I am not really sure that was a factor.

Intuitively the effect should be the same regardless, it's still the air being forced down by your wings forming a cushion increasing lift. There's probably some differences but the bulk of the effect should be the same.

Hmm

Once you're airborne you're part of the airmass and moving with the airmass. Your ground speed becomes irrelevant for purpose of lift, as you've amply demonstrated.

"Wind died" implies that wind speed relative to the ground would change.

But whatever caused the massive airmass to alter velocity would impact you as well, right?

So my intuitive assumption is that while sudden air motions might disrupt you and flail you around, I'm not sure you would actually stall and fall down from loss of lift strictly because "Wind died down". (though you might be slammed into ground).

Any thought from anybody with more flight hours and aerodynamics theory than me? :->

It'd be like a microburst. Not so much the way the airmass moving fast is somehow different than the plane moving fast in a still airmass… more that it's a sudden change IN the state of the plane in the airmass. If the air suddenly stopped moving past the plane, that doesn't mean the plane suddenly starts moving past the airmass (ignoring its own inertia relative to its position in the world) to make the same airspeed.

Instead, if the airmass suddenly changes velocity, the plane stays in position, stalls, and drops like a rock. It's the rate of change that'd get ya: if it changed velocity very very slowly, you'd end up just zipping forward and not stalling at all. But the airmass suddenly reversing, that'd stall you immediately.

Not a pilot, but trained in physics. To a certain extent, what you say is true. However, the aircraft also has inertia, so if the wind suddenly dies, the aircraft doesn't simply surge forward to maintain its airspeed. It is inclined to stay where it is and the amount of air flowing over the wings will consequently drop. Constant wind is easy and can be treated more or less as you describe, but gusty variable wind can and will cause problems.

Without headwind you cannot get negative groundspeed with any conventional airplane design.

Yes, of course airflow over the wings is required for a plane to take off, and therefore it must move forward.

Unless it has a large (>1) power to weight ratio.

You still need airflow over the wings to turn it up to allow the propeller to produce lift. Unless you are flying a V-22 Osprey

Or you could have a tail-sitter... but I thought none of these were necessary points to mention.

You mean thrust to weight ratio?

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