CA (dry) vs. FL (humid) Turn Initiation

[jacketsdb23 49]

So its no secret that some of us had to drop our turn initiation point in Florida during the recent FLCPA/Nationals competition. For me it was significant, about 100' lower than I usually turn.

My home DZ and Zhills are both roughly 80' - 90' MSL. Water vapor was much higher in Florida, which would suggest less dense air. Temperature was fairly high in FL, but we get 90ºF - 100ºF days frequently here as well.

Something else was definitely a factor here and I'm trying to wrap my head around it. There is usually a lot of talk about density altitude tied to temperature and LZ elevation, but I'm wondering what other factors might be in play.

Losers make excuses, Winners make it happen
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[wildcard451 0]

jacketsdb23

So its no secret that some of us had to drop our turn initiation point in Florida during the recent FLCPA/Nationals competition. For me it was significant, about 100' lower than I usually turn.

My home DZ and Zhills are both roughly 80' - 90' MSL. Water vapor was much higher in Florida, which would suggest less dense air. Temperature was fairly high in FL, but we get 90ºF - 100ºF days frequently here as well.

Something else was definitely a factor here and I'm trying to wrap my head around it. There is usually a lot of talk about density altitude tied to temperature and LZ elevation, but I'm wondering what other factors might be in play.

I was seeing density altitudes of -200 to -250ft consistently during FLCPA and Nationals. I was also turning comfortably 100' lower than normal (VA/NC).

Also, we were routinely diving through thermal areas that are unfortunately due to the placement of the pond relative to the hangars and trailers at Zhills.

[diablopilot 2]

Humidity usually does not affect DA as much as people tend to think. Pressure and temperature are far bigger influences. Try this on for size....

http://www.boldmethod.com/learn-to-fly/performance/density-altitude/

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You're not as good as you think you are. Seriously.

[Divalent 137]

diablopilot

Humidity usually does not affect DA as much as people tend to think. Pressure and temperature are far bigger influences. Try this on for size....

http://www.boldmethod.com/learn-to-fly/performance/density-altitude/

The point that Humidity is only a minor contributor is correct, but that source is clearly not a good place to go to learn about things. Here's a quote from the article:

Quote

In the atmosphere, Nitrogen usually exists as a N2 molecule - which means two Nitrogen atoms are bound together. Since Nitrogen's atomic mass is 7, one Nitrogen molecule weighs 14 units.

Oxygen in the atmosphere generally exists as a O2 molecule, which means two Oxygen atoms are bound together. Oxygen's atomic mass is 8, so one O2 molecule weighs 16 units.

Water molecules are made up of two Hydrogen atoms, each of which weighs one unit, and one Oxygen atom, which weighs 8 units - so a molecule of water vapor weighs 10 units.

That is just incorrect. The author is confused about the difference between atomic number and atomic mass.

[vision 0]

I can concur that one must turn lower in humid air than in dry air...

Not a lot for me (30 feet or so), but definitely enough for me to drop the initiation or vert the entry.

Why? I am not sure.

But that is not what is important, what is important is scoring the gates!

[Polorutz 0]

vision

what is important is scoring the gates!

And trusting your rears!

[Pendragon 1]

Discussed this with Scott Roberts years ago. What we came up with was:

You do get a slight increase (as pointed out) in density air altitude as a consequence of moist air being less dense than dry air. This is because the same number of molecules occupy a given volume at a given temperature, and that water molecules (H2O = 18 g/mole) weigh significantly less than the main constituent of dry air (nitrogen, N2 = 28 g/mole).

However, this effect would make the turn higher and not lower, but as pointed out, the effect is small.

What is overriding is the change in air viscosity. Moist air is more viscous (like treacle) than dry air. As a consequence, the drag on the canopy is increased. Compared to the canopy, the suspended weight (the canopy pilot) has little surface, and so is little impacted by this change when compared to the canopy. After the turn has been initiated, the canopy will decelerate faster in humid conditions, causing a the pilot to travel a sharper arc under the canopy, thereby allowing the wing to recover to level flight faster.

I've had similar experiences myself with the difference in turn initiation height this makes; back when I was jumping a Velo 90 or 96, I was doing the same 270 degree turn roughly 80-100' lower in Florida than back in the UK.

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