Terminal velocity descents.

So you get a call from the repair station.  Your envelope passed but you need a new parachute.

How can this be?  “you ask”.

The balloon has less than 200 hours on it!  And I never fly it over loaded.  I am a little aggressive when I fly competition but that can’t hurt it can it?

Sure it can!

Here is the situation.   To get over to the target (or landing area) you had pop up to 2,000 feet AGL.  Then at the last moment you had to give the balloon a huge vent and drop like a rock down to the surface.  Then about 600 feet off the ground you lay on that burner with a death grip to stop the descent.  It takes forever to pull out of that descent but you do it about 75 feet off the ground!

Then you land, or toss your marker right on the center of the X.  FANTASTIC job you say, as you give yourself a big “ATA Boy”, till you shell out a couple of grand for a new parachute.

So how did this happen?  What can you do about it?

Let’s say we are flying a 77.  We have the PIC and your crew chief on board.  It is a warm day but not too hot.  You are flying in level flight and the envelope temperature is 220 degrees Fahrenheit.   So now with just those two pieces of information let’s go and make the same maneuver. At 2,000 feet AGL we give the balloon a huge vent.  The envelope temp drops to 200 degrees as we begin to fall.  Watching the VSI we notice we are up to 600 fpm and the envelope temp is now down to 190 degrees.  A couple of moments later we see 800 fpm and the envelope temp is now 185 degrees.  We wait a moment more and our envelope temp is now down to 175 degrees but the rate of descent has not increased.  So we wait a little longer and we are now down to 160 degrees but our rate of descent is still 800 fpm.  Your balloon has reached its terminal velocity but the envelope temperature continues to fall.   By the time you get to 600 feet the envelop temperature is down to 155 degrees.  You lay on that burner and give the balloon a good 5 second blast and nothing seems to happen!  So you lay on the burner again for another 7 seconds and the VSI begins to move.  Again on the burner for 9 seconds and you are starting to pull out of the descent.

You have just scorched your parachute.  You may not see it now but your repair station will see it!

Pass your hand over a candle and you won’t be burned but hold your hand over that candle and you get a bad burn in a short time.  Long and repeated blasts on the burner will do the same to your parachute.

The trick is to hold that envelop at the 185 degree (or whatever temp allows for maximum descent).  Letting the balloon cool any more will not increase the rate of descent but will require great amounts of heat to get the balloon back up to the point where you are still falling at the terminal descent rate.  If you add some heat on the way down you won’t slow the balloon at all but when it is time to pull out of that descent it “WILL NOT” take the huge long burns to pull out.  If the balloon temperature is held at the point where you achieved your terminal descent rate, when you do begin to add heat, you will immediately begin to slow you down.  It will take much less heat to stop that decent and cause the parachute to be exposed to much less intense heat.

Another benefit is you keep your balloon stiff.  As your envelope cools it becomes soft and can begin to cave in.  As one side of the balloon begins to cave in your trajectory will change.  The best example is dropping a quarter into a bucket of water.  It will zig and zag its way down to the bottom with no way to tell where it will land.  Drop a marble in that same bucket and it will fall fairly straight.  So when you let the balloon begin to get soft it can actually make you slide left or right of where you want to go. So keep your balloon warm even when you are trying to make an aggressive descent to a target or landing area.  Letting it get too cool can cause more troubles than you imagine.

Post your replies or questions below.

Fly Well,  Have FUN & BE SAFE

Jeff A Thompson

Leave a Reply