Jan 16, 2003, 3:21 PM
Post #1 of 10
can someone help me on this:
I'm wondering how to quantify variation in altitude, temperature and humidity on density altitude? what's the actual effect on the canopy?
how do you calculate?
the bottom line is that I'm jumping on various DZ with various climate, altitude... And I'd like to be able to make an accurate idea of how my canopy is going to behave before getting up there. 'tnink it could help getting a better undertanding of what's happening and therefore avoinding bad surprises...
ok, Paul: you give us some good stuff here, thanks , it's easy and as you've stated in some other thread, it's accurate enought regarding skidiving.
how ever, I have two questions: Paul, could you tell me where I can find more details about this (just curiuous really), like some pilote instruction source ?
all: I understand that lower density altitude makes the canopy fly faster and stall easier, but how much does it affect the recovery arc? any other interesting aspect of canopy flight influenced by this?
quade (D 22635)
Jan 16, 2003, 4:14 PM
Post #4 of 10
First lets clear one thing. In the end the only thing that matters is density altitude. However Density altitude is determined by temperature, humidity and elevation. Whether you are aware of this or not if you do a hop and pop from 13,500 (full altitude) and fly your canopy around to get used to your flight characteristics you are actually not experienceing the same flight characteristics of your canopy all the way down. As a matter of fact even if you were on a day at standard temperature (59 F or 15 C) and with standard barometric pressure (29.92 inches of mercury or 1013 milibars), if you did a hop and pop from altitude the difference in the flight characteristics from opening to landing would be far more than the possible difference of density altitude could ever be on the ground. Now if your looking at a manual for a specific aircraft it will tell you how much runway is needed for landing in a day with standard temperature, plus 10 or 20 degrees, on and airport at sea level or with some elevation and so on. You can look at this and make determinations for what is required for your flight plan. Such data is not available for skydivers for various reasons. For one thing most skydivers don't even understand density altitude. But even if they did there are no real standard aproaches for experienced skydivers, specially swoopers. Some do 180 degree turns to final, some 270, some even 360 and ofcourse different wingloading and different airfoils will be affected differently. Even the speed at which these turns are made are different, that is not everybody banks at the same angle on there approach and therefore not everybody looses the same altitude on a turn even if the both turn the same amount of degrees. So now that I have said all the reasons for which you can not find this answer I will say the one answer that you can have. That answer is you can know what the density altitude was where you jumped previously and you can know what the density altitude is where you intend to jump. There are sites on the interenet (made for pilots) where you can plug in temperature, elevation, and humidity and it will return density altitude. If the density altitude is higher then you can be sure that your parachute will have less lift on flare, will have a faster foward speed and will loose more altitude with the same turn on approach to final. Once you know this you can expect that you need to be more conservative on you first couple of jumps at a new dropzone until you adjust your swoop so that you know at which altitude to start your final turn. Blue Skies,