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Let's assume that a "perfect" humanoid robot flying a ram air parafoil (paraglider or canopy) performs some maneuver for landing exactly the same way multiple times.

The only difference between the trials is the wind velocity relative to the ground (downwind/ crosswind/ upwind/ no wind/ etc...).

We collect statistics for a bunch of sensors (airspeed, vertical speed, etc...) for each trial until just before contact with the ground.

Question: What are all the things that we know (via theory or repeatable experiments) that would cause detectable difference in the statistics due to the wind velocity OTHER than ground speed or direct interaction with the ground?

What I've already come up with

(Feel free to correct me on any of these)

  • A wind shadow or wind gradient would cause the canopy to pitch forward and lose some effective airspeed / lift/ (energy?) momentarily while flying upwind. The opposite would happen flying downwind.

  • I've heard that gusts are sharper on the upwind side due to how pressure waves work. In this case, the "average" effect of each gust would be the same whether you are flying upwind or downwind, but you might detect a difference in the average shape of oscillations if you always fly through the sharp side first (upwind) or the gradual side (downwind).

And here is one I'm still thinking through:

Assumptions:

  • Let's assume an infinite number of trials, so random events that don't follow a pattern (such as individual thermals or gusts) can be ignored. However, things that follow patterns (such as wind gradient generally decreasing towards the ground) should be considered.

  • Let's assume a flat, circular landing field that is longer/ wider than the maneuver requires. It is symmetrical in all directions. It may or may not have buildings and trees surrounding it (also symmetrical).

  • Let's assume the parafoil has stabilized in whatever air mass it's in before starting the maneuver.

  • The maneuver might be anything from multiple 360 turns to build speed for a freestyle CP competition, to a simple flare from half brakes. Anything that a human might regularly perform.

  • The canopy might be anything from a lightly loaded large paraglider to a heavily loaded, high performance competition canopy. Anything that an average size human might regularly fly.

A note about parafoils:

Parafoils are much lighter and have much greater separation between CG and CL/ CD than any other aircraft. They have a uniquely soft, flexible wing that may be full of air.

As a result, effects that are thought to be insignificant in other aircraft types can be very significant in parafoils.

Examples:

So I'm particularly curious if any of these relate to wind-related effects that wouldn't be a factor in other types of aircraft.

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    $\begingroup$ Re "I've heard that gusts are sharper on the upwind side due to how pressure waves work. In this case, the "average" effect of each gust would be the same whether you are flying upwind or downwind, but you might detect a difference in the average shape of oscillations if you always fly through the sharp side first (upwind) or the gradual side (downwind)." -- this alone would be worthy of it's own question, or maybe two (one about the asymmetric shape of the gusts, and one about the resulting effect on flight dynamics.) $\endgroup$ Commented Apr 24 at 19:51
  • $\begingroup$ @quietflyer Thanks. I actually did expect more questions to come out of this question! Your comment confirmed that it's a thing, so I'll move it out of the uncertain category and that's good enough for this question. :) I'll create those two questions when I get a chance (unless you'd like to beat me to it). $\endgroup$ Commented Apr 24 at 21:17

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Here’s one, based on my own experience: wind shear (or wind gradient) does appear to affect lift. What I’ve observed when soaring a hill is that I can fly into wind for some distance (hundreds of metres) without losing any height but then I turn downwind and sink steadily until I return to the hill. My rationalisation is that even at a few hundred feet there’s still a shear as the air speeds up to go over the hill. My wing generates lift by having the air over the upper surface going faster than the air underneath, so when heading into wind the shear assists this and when flying downwind the shear works against the airflow differential that the wing generates. Since this effect is apparent at altitude it’s very likely to be significant when close to the ground. Hence when landing into wind (as is normal) you will experience something similar to ground effect. As regards to ground effect itself (as would be experienced in still air) the effect seems to be very small but probably not immeasurable. TL;DR landing upwind in a breeze has a lower rate of descent than in still air.

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  • $\begingroup$ Interesting! I've never heard of this. It seems the opposite of the wind gradient effect (where landing into the wind might reduce lift, if the wind speed is decreasing towards the ground). Do you have any links or references for this wind shear effect? $\endgroup$ Commented Apr 25 at 0:09
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    $\begingroup$ I don’t have track log data with accurate height, my observations were based on a variometer which would indicate minimal change in pressure altitude while flying around 500m into wind but then lose around 200ft while flying downwind. The sink rate was steady in straight flight and couldn’t be attributed to losses while turning. It’s quite repeatable, most easily done in evening conditions where the wind is steady and minimal thermic activity. $\endgroup$
    – Frog
    Commented Apr 25 at 1:42
  • $\begingroup$ Regarding wind shear near the ground, I don’t have any references other than my own experience. Because if the nature of paragliding it’s difficult to repeat observations under identical conditions, and pilots are generally more interested in horizontal rather than vertical speed when landing, so gathering objective data would be challenging. $\endgroup$
    – Frog
    Commented Apr 25 at 1:48
  • $\begingroup$ what about a reverse wind gradient? If I remember correctly, the wind near a rounded hill is strongest at the level of the peak. So if you were returning above the hill, you'd have been going into faster and faster wind. The effect would have reversed below the top of the hill, possibly without you noticing since by that point you'd have already returned to the ridge lift. $\endgroup$ Commented Apr 25 at 1:48
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    $\begingroup$ I couldn’t rule that out $\endgroup$
    – Frog
    Commented Apr 25 at 4:36

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