Wing Load and Canopy Flight Speed

[PrairieDoug 0]

In searching the forums for info. on canopy flight, I found a post which stated that "with the same canopy and jumper, the ratio of freestream flight velocity will be proportional to the square root of the ratio of wing loadings. Compared to a wingloading of 1.0, a canopy loaded at 1.3 will be (1.3/1.0)^0.5 = 1.14 times faster."

I have a question and an observation.

Question: Is this formula correct?

Observation: The impact of WL on flight speed is much less than I would have expected, based on my own (limited) experience. My exit weight is 190, so if I had a flight speed of 30 mph on a Sabre2 190 (WL=1.0), then my flight speed on a Sabre2 150 (WL= 1.26) would be 33.8 mph. I would never consider a 135 at my experience level, but the comparable speed would be only 35.6 mph. Again, that's not as much of an increase as I would have expected.

Knowing how much attention is given to downsizing gradually, does this mean that most of the concern about downsizing is due to factors other than the higher flight speed? I know that the smaller canopy will be more responsive apart from the increase in WL. Is that really the concern with downsizing, rather than speed per se?

[ianmdrennan 2]

I don't believe it's so much the forward speed in regular flight, but rather the length of the recovery arc and the amount of speed that can be generated in that arc that creates the downsizing concern, not to mention other factors like sensitivity to body symmetry and inputs.

Basically, the bigger the wing and lighter the wingloading, the more the pilot can "get away with" close to the ground.

Edit: As for the formula. I'm sure Kallend or one of the other physics buffs can shed some light on that.

Blue ones,
Ian

Performance Designs Factory Team

[skynole 0]

I have no idea if that formula is accurate or not, but from my experience, that seems about right....in full flight mode. I was very surprised when I first got my Velo (WL ~2.0-2.1) that it was not as fast as I thought it would be. In full flight, it really wasn't all that much faster than under my Stiletto (WL ~1.5ish).

As Ian stated though, once you start talking about speed building maunevers, it's a whole different story...

Edited to answer your post more fully: There are many other factors to consider when downsizing besides just the forward speed of a canopy due to increased WL, namely planform design, line length, sq footage, etc... I can expand on these if you would like but it's been covered extensively in here.

[quade 3]

You should note that the formula refers to wingloading on the same canopy. By changing canopies, you're changing a number of factors that do not scale in a linear fashion. As a rough approximation it's fairly close, but the reality is it's not exactly the same.

quade -
The World's Most Boring Skydiver

[crazy 0]

Quote

Compared to a wingloading of 1.0, a canopy loaded at 1.3 will be (1.3/1.0)^0.5 = 1.14 times faster."

Even though it's just an approximation, in my opinion and experience it is very accurate. Even though it's easy to compute it more accurately, it's not worth it.

Quote

Knowing how much attention is given to downsizing gradually, does this mean that most of the concern about downsizing is due to factors other than the higher flight speed? I know that the smaller canopy will be more responsive apart from the increase in WL. Is that really the concern with downsizing, rather than speed per se?

The concerns about wing loading go a bit further than this. The energy is proportional to the square of the speed, hence it is proportional to the wing loading. An impact at a WL of 1.3 has 30% more energy that at a WL of 1.0.

In addition, there are concerns about the size of the canopy as well: the same pilot under a smaller canopy has more input (the length of the arms is the same but the length of the lines decreases proportionally with the square root of the size). This means potentially steeper turns. If the turn is 14% steeper, this means 30% more energy.

Altogether, a more cautious way to see it is that at a WL of 1.3, compared to 1.0, you have ~14% less time to make the right decision. This might increase significantly the risk of a panic turn. After a panic turn, you might hit the ground with up to 70% more energy.

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Skydive Asia