Temperature of air

[Metekanya 0]

Do you change the altitude of start of the turn from winter to summer?

Talking about how the plane has a harder time to get to altitude in summer due to air density, I thought that one surely dives longer on a hot summer day compared to a cold winter day. I was wondering how much difference there could really be since it is very noticeable for the plane. Searching in internet I have learned that the air density changes around 3% for every 3º Celsius. Where I mostly jump now, the change in temperature from winter to summer can be up to 30º (more than 50º F) meaning a difference in air density of around 10%. This can lead to a difference in the altitude of start of my dive of 25 metres or 75 ft which really is considerable, specially now that most of us are using digital devices.

I have never noticed because I used to jump at a place in summer which is 1.500 ft higher than were I jumped in winter and had to adapt every time.

Any thoughts?

[billvon 2,400]

Density altitude definitely changes significantly. From a cold winter morning at Perris to a hot summer afternoon, you could go from an altitude of 1900 feet MSL to 6500 feet MSL (assuming your altimeter reads 1000 ft AGL.) That's a huge change in terms of how your canopy will respond.

[AggieDave 6]

I typically jump really close to sea level (300msl or less), with the changes in density altitude I don't vary my turn more then about 20ft typically between the winter and the summer. The rate of turn changes a little bit, which is something I've been trying to train out of me.

Now, going from here to Eloy, 1500msl, (before swooping was made illegal to "save us") I bumped the turn about 70ft and from here to Longmont I bumped my turn up about 125ft. In fact if I had done my normal turn at the normal height in Colorado I would have hit the ground or the pond incredibly hard. Most likely hard enough to seriously injure myself.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[morris 0]

I now (finally) got all my "numbers" together for different wingloadings, fieldelevations, temperatures,...
It´s just almost impossible to give any numbers that are valid for everyone or for you.
For this reason I don´t wanna post numbers here.
I could give you an estimation (without any guarantee!) if I´d know more about your WL, type of turn, initiation altitude,... PM welcome...

[Pendragon 1]

There's other effects to be considered here, not just air density, but humidity as well.

Moist air is less dense than dry air (Molecular weight of H2O=18; N2=28), so on that basis you would expect places like southern Florida to behave like they were at higher altitude. Wrong!

Humidity also changes the viscosity of air, increasing induced drag and shortening the recovery arc. So actually, humid conditions will lower, rather than raise, the initiation altitude.

--
BASE #1182
Muff #3573
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[Steel 0]

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So actually, humid conditions will lower, rather than raise, the initiation altitude.

More humidity means less lift or a higher density altitude. Here is a calculator you can use to plug in number and see how it changes http://wahiduddin.net/calc/calc_da.htm

You won't see an entry specifically for humidity but you see one for dew point which pretty much comes back to humidity. If the difference between the dew point and ground air temperature divided by 3.5 (talking fahrenheight ofcourse) is a good prediction for where visible moisture (clouds) will form.

All that being said temperature has a far greater effect on density altitude than humidity. Also with regards to concerns for density altitude, its true that you should take it into account, for where you start your turn. However, if anybody thinks that they always start their turn at 710 and now they are consciously going to start it at 730 feet instead because density altitude is higher, then they are kidding themselves, because nobody is that accurate, with the point at which they start their turn.

You could be that accurate as you approach the gates but that is with the minor modifications you make along the way. You are too high turn slower, you are (a little bit) too low turn faster, you came ouf of your turn too high hand on double fronts a little be longer, you are short of the entrance gates, pull a little be less on the double fronts. All these different inputs will make a much bigger difference than 20 vertical feet.

In the end if you are making you approach with a safety margin, your biggest concern with a higher density altitude is that your canopy will have less lift. It will tend to have a higher air speed which will give you more lift, but your speed to lift ratio will be smaller. This essentially means that if you feel comfortable with the lift you get at standard temperature (59 F/ 29.92/0 MSL) with a 100 squarefoot canopy, in a (59F/29.92/5000 MSL), you probably will feel about the same level of comfort on touch down with a 110-120 squarefoot canopy. The 10-20% loss of live for 5K is my best guess from my experience. If you feel compelled to get and exact ratio, I am sure you can google it.

If I could make a wish, I think I'd pass.
Can't think of anything I need
No cigarettes, no sleep, no light, no sound.
Nothing to eat, no books to read.

[matt6242 0]

I have never jumped a very high altitude DZ.

I would know to expect more speed, but most simply put are you saying your flare power is lessend?

[Steel 0]

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I have never jumped a very high altitude DZ.

I would know to expect more speed, but most simply put are you saying your flare power is lessend?

You wouldn't have to jump at a high altitude dz. Perris Valley is at about 1400 MSL. It often gets over 100F there. Look at the link I provided earlier and see what happens to the Density altitude there if you have a day with the temperature at 104 F, barometric pressure of 29.82 and dew point of 75. It already has you at a density altitude of over 5000.

Yes, I am saying that if you take it easy and start your turn on the high side on your first couple of jumps in the day, to get used to your altitude lost in your dive, the only problem you will be left to deal with it the lessened flare power/lift.

Some people think that by coming in really fast you can resolve this issue, because more speed will equal more lift. But that is only part of the story. Yes more speed, with Bernoulis principle (you can google that if you want) will give you more lift. However you can't walk or run away from your landing at 50-75 MPH. No person can run that fast. So after doing that nice pretty high speed approach your canopy will need to start slowing down, until it gets to a speed where you can realistically touch down. If you can run really fast, you might be able to get away with touching down at 15-17 MPH and running like bloody hell to avoid falling foward. If you have mastered the skill of sliding in, you can touch down faster and just not start running until you have slowed down enough that you will be able to keep up with it.

Regardless, I don't believe anybody is touching down while they are still moving over 45MPH and starting to slide it in from that point.

Now lets picture two different senarios. You start to dive you canopy and get it going somewhere between 80 and 105 MPH. You start your swoop somewhere between 70 and 85. As you go through your swoop you loose your speed and when you've slowed down to maybe 35, you touch down and started to slide it in. As you're sliding in, you are slowing down more, your canopy is loosing its flare power/lift. Your goal is to be comfortable on your own by the time you canopy lost all its lift.

Now lets think about it differently, you fy that canopy straight in, the foward speed ofcourse is going to vary on your wingloading. But lets say for this example you are at 2.5. Your foward speed with no turns might be 45, you pull double fronts and get your foward speed up to 60. By the time you let up your front risers, level out and start your swoop, you're going 50. You still can't touch down or you will face plant. You have to slow down until you find yourself in the same situation as you were in the previous example, with the same lift/flare power to work with.

If I could make a wish, I think I'd pass.
Can't think of anything I need
No cigarettes, no sleep, no light, no sound.
Nothing to eat, no books to read.

[AggieDave 6]

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As you're sliding in, you are slowing down more, your canopy is loosing its flare power/lift.

As you're sliding and slowing down, your canopy is holding up less and less of your weight, so you're also lowering the stall speed of the canopy. This is giving you "more canopy" to work with.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[Steel 0]

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As you're sliding in, you are slowing down more, your canopy is loosing its flare power/lift.

As you're sliding and slowing down, your canopy is holding up less and less of your weight, so you're also lowering the stall speed of the canopy. This is giving you "more canopy" to work with.

Absoloutely.
However, my point was that you still had to touch down at the same speed.

If I could make a wish, I think I'd pass.
Can't think of anything I need
No cigarettes, no sleep, no light, no sound.
Nothing to eat, no books to read.

[matt6242 0]

I am pretty moderate, Katana @ 1.6.

If i was at perris on a hot day should I expect then to have a reinforced ass in my suit or would pulling toggles all the way down to the bottom of the flare arc ( which kills a swoop ) result in a soft touch?

Thanks this is insightful as I have never left a Southeast DZ.

[Steel 0]

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I am pretty moderate, Katana @ 1.6.

If i was at perris on a hot day should I expect then to have a reinforced ass in my suit or would pulling toggles all the way down to the bottom of the flare arc ( which kills a swoop ) result in a soft touch?

Thanks this is insightful as I have never left a Southeast DZ.

I am glad to hear that my post may be helpful.

I have never flown a Katana. If I did I would most likely lengthen my brake lines, as I have done with just about every other canopy I have flown (Thats just my preference). In any case, for that reason, it would not be right for me to give you specifics as to how low you should pull your toggles down during your flare, the recovery arc or any specific part of the landing.

I will however say that I am certain that with the proper technique, (notice I said with the proper technique) a Katana loaded at 1.6 can be landed at any density altitude with little or no run out steps at all, while still maximizing your swoop distance.

If I could make a wish, I think I'd pass.
Can't think of anything I need
No cigarettes, no sleep, no light, no sound.
Nothing to eat, no books to read.

[matt6242 0]

Thanks,

My best friend moved to So Cal and Perris is on my hit list for a near future visit.

I dont ever land to try to impress anybody, but I also dont want to dork out with a world record, full landing area, up through manifest, across the landing area, barely miss the parked DC-9, ass slide.