Wind and Canopies

[airdolphin 0]

Hi,

I'd like to begin a discussion on how winds affect canopies. Based on my basic knowledge of aerodynamics, the winds should not affect canopies at all (only their ground speed and direction). Unless the winds cause turbulence, the parachute will fly exactly the same whether or not there is wind and whether the parachute flies crosswind, upwind or downwind.

Frequently I hear a common misconception that landing upwind will produce better flares than downwind, since the speed of the canopy in the latter case is lower. Landing downwind indeed will produce a lower ground speed, but the speed of the airflow over the canopy's surface will remain the same! Thus the flare characteristics of the canopy would be the same (that is why some people can easily land their parachutes on a windy day and have trouble on no-wind day: they get scared by the ground rush).

So the question I am posing is this:
if we were to ignore the effects of turbulence caused by wind, how does flying a parachute (including making carving riser approaches and flaring) change due to windy conditions (speed/direction)?

Thanks.
Pavel [email protected]

[parachutist 1]

Well I thought I had an answer about why there is actually more lift produced on windy days near the ground that on no-wind days... but then I did some research & found out that "ground effect" is not really the correct explanation. I believe that air pressure is greater near the ground on windy days because the air is flowing across a solid wall surface (the ground). Can anyone elaborate on this? I'd like to know a clear definition here too =]

- Chris

[ernokaikkonen 0]

Oh dear. Not again.

The canopy does not understand whether it's going upwind or downwind. The reason why people tend to think "upwind landings have more lift than nowind/downwind landings" is this (IMO, phycisists please chime in):

Regardless of the direction of the wind, the airspeed of the canopy is always the same. The difference is only in the groundspeed.

Landing upwind, the canopy has no lift left(little airspeed) when the groundspeed is near zero(groundspeed=canopy airspeed-wind velocity).

Landing downwind, the canopy has no lift left at the same airspeed, but the groundspeed is much faster(groundspeed=canopy airspeed+wind velocity) than that of an upwind landing. So you think "Man, the canopy had no lift left, I had to put my feet down and I was going forward really fast!"

A FAQ about "canopies and wind" wouldn't be a bad idea...

Erno

[Staso 0]

Quote

the winds should not affect canopies at all (only their ground speed and direction). Unless the winds cause turbulence, the parachute will fly exactly the same whether or not there is wind and whether the parachute flies crosswind, upwind or downwind.

that's correct.

stan.

--
it's not about defying gravity; it's how hard you can abuse it. speed skydiving it is ...
Speed Skydiving Forum

[diverds 0]

This topic has come up before in this forum with many people dishing out a lot of misinformation. You are right in your original thought that a canopy is not affected by the wind unless turbulent conditions exist. It does not know the difference. Only ground speed changes.

For a good laugh, read through this thread. It still amazes me that so many people have such a hard time grasping a concept that should have been taught to them in their first jump course.

http://dropzone.com/cgi-bin/forum/gforum.cgi?post=118233;sb=post_latest_reply;so=ASC;forum_view=forum_view_collapsed;;page=unread#unread

Skydive Radio

[Jimbo 0]

For the lazy:

http://dropzone.com/cgi-bin/forum/gforum.cgi?post=118233;sb=post_latest_reply;so=ASC;forum_view=forum_view_collapsed;;page=unread#unread

I'm surprised Quade and a few others haven't jumped all over this one yet. Then again, maybe not...

-
Jim

"Like" - The modern day comma
Good bye, my friends. You are missed.

[diverds 0]

Jimbo,
How do you do that. I tried but did not get the hyperlink.

Skydive Radio

[quade 3]

I've been sort of busy the last few days . . . what can I say?

Anyway, for a normal, simple straight ahead landing (no super swoops and carving turns), not only does the canopy react exactly the same for an upwind, downwind and nowind landing, but if you give approximately the same toggle inputs, you'll get approximately the same results right up until the time your feet contact the ground.

Your perception of things may change, but the physics of a wing moving through the air will not.

The reason you get softer landings into the wind as opposed to no wind is that your forward movement over the ground can be less and therefore the energy required to be absorbed by your legs is less.

BTW, speaking of ground effect . . . I'm not sure that most of our canopies actually get close enough to the ground for ground effect to affect them. My guess is not. Generally speaking you'd take ground effect into consideration at a distance of about 1/2 a wing span or less to the ground. Even then, ground effect does NOT create more lift, it only reduces drag. Your swoop might be longer, but your landing wouldn't be any softer.

quade -
The World's Most Boring Skydiver

[billvon 2,446]

>Even then, ground effect does NOT create more lift, it only reduces
> drag.

?? Not sure that's true; I know aircraft can rotate at lower speeds due to ground effect (i.e. a low wing can get off the ground at a slightly lower airspeed than the equivalent high wing.) This would indicate that lift is increased at a given speed.

[kallend 1,654]

Quote

Replying to:
Re: [quade] Wind and Canopies by billvon
Post:
>Even then, ground effect does NOT create more lift, it only reduces
> drag.

?? Not sure that's true; I know aircraft can rotate at lower speeds due to
ground effect (i.e. a low wing can get off the ground at a slightly lower
airspeed than the equivalent high wing.) This would indicate that lift is
increased at a given speed.

It's true. The ground reduces downwash, which is responsible for induced drag. It's also true that you need to be within about 1/2 wingspan from the ground to get the effect.

My Mooney has long wings and short landing gear. The drag reduction is very pronounced in the flare.

[quade 3]

Bill --

Sorry for the delay in answering this. I just didn't see it until this morning.

Anyway, I think you're making a very common mistake, confusing lift with thrust required. This is another one of the common mistakes that gets folks into trouble up at Big Bear during the summer. They assume that since they rotated and are skimming down the runway in ground effect, they have enough horse power to actually climb out of the airport. Well, most do, but some don't. The pilots of those aircraft end up swimmin'!

You can look at a lift equation all day long and you're just not going to see any reference to ground effect, however, take a quick look at climb equations and you'll see a pretty big reference to drag.

The reason the airplane gets off the ground in ground effect is that the thrust required for the 1st 1/2 wingspan portion of the climb has been greatly reduced by the reduction in drag. If the aircraft doesn't have the thrust (horse power) to climb out of ground effect, then that's where it's going to stay.

quade -
The World's Most Boring Skydiver