9
$\begingroup$

Isnt resultant force allways pointing slighlty backward at every wing,so why hang glider is not push downwind by the wind? From theory that cant be possible,so does wind somehow coming at him at angle from below? ground seems flat,maybe slightly angled..

(Is it easy what his doing or that can do only very very skilled pilots?)

Can he penetrate continusly forward into wind direction in zag-zag motion,first straigt up and then down-forward or maybe with dynamic soaring like birds do?

Improve this question
$\endgroup$
8
  • $\begingroup$ Important to note that the hang glider isn't actually hovering. It's flying like it always does, just not fast enough to move forward. A hover means no forward airspeed. $\endgroup$ – John K Aug 31 '20 at 14:59
  • 7
    $\begingroup$ Does this answer your question? Is it possible to fly backward if you have really strong headwind? $\endgroup$ – Dave Gremlin Aug 31 '20 at 17:47
  • 2
    $\begingroup$ He's flying into a headwind that is the same velocity as his forward airspeed. Therefore, ground speed = 0. Yes he can move forward by increasing his airspeed by lowering the nose of the glider to descend more steeply and faster. But in that particlular upslope wind angle and velocity, this may increase his descent rate which results in closing with the surface, so he can only do it if he has clearance below to allow for the steeper descent, or if the upslope wind velocity increases. $\endgroup$ – John K Aug 31 '20 at 17:52
  • 2
    $\begingroup$ @JohnK: You're absolutely right about almost everything you said. However, a hover does not mean no forward airspeed. A hover means means holding a fixed position relative to a stationary spot on the ground. When I hold hover in a helicopter with a headwind, it is with a certain degree of forward cyclic - the forward airspeed is not zero. Unless it is a windless day, a hover will always involve horizontal airspeed in some direction. $\endgroup$ – Aaron Holmes Aug 31 '20 at 19:26
  • 2
    $\begingroup$ "Wind" means something completely different to aircraft than what it means to those of us who are standing on the ground. Once you're in the air, "wind" is just the difference between your airspeed and your speed over the ground. I had some flight training in a little Cessna (never went for the license) and I remember one day, while practicing slow flight, I noticed that I was stationary over one spot on the ground. But that wasn't "hovering." I was flying at 40 knots through an air mass that coincidentally happened to be moving at 40 knots in the opposite direction relative to the ground. $\endgroup$ – Solomon Slow Sep 1 '20 at 14:46
21
$\begingroup$

The ground is not flat, and indeed slightly angled. This is enough to "hover".

Specifically, the ground slope should be equal or more than the max L/D ratio of the hang glider at the current wind speed. The wind should go 'up' as much as the glider goes 'down' per unit of distance. The ground seems to go up about 2m over a distance of maybe 10m, which is a slope of one in five. An L/D of 5 seems perfectly reasonable for a hang glider.

Improve this answer
$\endgroup$
8
$\begingroup$

Yes, the force from the wing is indeed always pulling slightly backwards. Upwards and slightly backwards relative to the wind that is. Since the wind here is coming in slightly upwards along the slope of the beach instead of horizontally, the force from the wing is turned to clean upwards instead of slightly backwards.

Here is the link to a force diagram showing a gliding plane in another thread. Just change the glide velocity vector "V" in the picture with the opposite of the wind vector on the beach in the film.

Edit: Presumably, this exercise is difficult and dangerous as it takes place very close to the ground. However, the so called "ground effect" might add a bit of stability. The closer to the ground, the less induced drag a wing experiences; hence there is relatively speaking more lift the lower you go, or relatively less force to sweep you downwind.

Edit 2, on risks: After learning from another answer that the practice of hovering close to the beach is called "dune gooning", i googled it and it really seems like even practitioners deem the activity as "higher risk".

Improve this answer
$\endgroup$
3
  • $\begingroup$ @MastLind Is this easy move to learn? youtube.com/watch?v=gTA12kyB4IE $\endgroup$ – member2017 Aug 31 '20 at 14:54
  • $\begingroup$ From what I've now read about "dune gooning" and assuming it's the same flyers in the two movies; I guess this is so hard to learn that those who really master it make it look simple. $\endgroup$ – Mats Lind Sep 1 '20 at 13:58
  • $\begingroup$ It would seem that there’s a smaller margin for errors, and I could see it being quite tricky, especially if it’s gusty at all... $\endgroup$ – Michael Hall Sep 1 '20 at 15:01
4
$\begingroup$

As should be obvious this was shot in Sydney, Australia. This type of flying is often referred to as Dune Gooning and it is great fun and not particularly dangerous (pilots rarely carry parachutes when engaged in this type of flying as they are too low for the chutes to do them any good if the glider breaks apart).

Getting close to the ground is, of course, the point. Skimming along an inch or two of it. Even a slight rise in the shore line allows for a upward wind component.

Improve this answer
$\endgroup$
7
  • 3
    $\begingroup$ This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review $\endgroup$ – Manu H Sep 1 '20 at 7:52
  • 3
    $\begingroup$ @ManuH One of the questions asked was "is it dangerous" and this definitely answers that. Maybe try and be a bit less enthusiastic with the "not an answer"-button? $\endgroup$ – Sanchises Sep 1 '20 at 8:40
  • 1
    $\begingroup$ @Sanchises I'm not the only one to have hit the "not an answer" button, only the first one. The review process exist so that such action are reviewed by at least 5 people. Moreover, the last revision when I was reviewing did not mention anything about danger. $\endgroup$ – Manu H Sep 1 '20 at 18:05
  • $\begingroup$ @ManuH Are you sure? There's no edit history on this answer and your review comment was posted three hours after this answer was posted so well outside the quick edit grace period $\endgroup$ – Sanchises Sep 1 '20 at 18:58
  • 1
    $\begingroup$ @ManuH Regarding "reputation", you may not be aware that Davis Straub is a world champion hang glider pilot. Imagine that someone had asked a question about motorsport, Alain Prost happened to chip in, and someone downvoted him for not having enough reputation on the site? You've just done that. $\endgroup$ – Graham Sep 1 '20 at 20:51
3
$\begingroup$

It's possible, yes. I often see paragliders hovering in the air, thanks to the so-called 'ridge lift'. And yes, in that case, the wind, forced by the terrain, comes from below, at an angle...

Improve this answer
$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.