terminal velocity descent
Robert Wood
deminstrated for the commercial balloon certification. How do you get to
terminal velocity and how is it recovered?
phantom
2. Heat it back up before impact.
phantom
I went to 10200 agl. stopped heating....... started descent. didn't
burn..........
descent built up eventually to ~1300 fpm
fired it back up at 2500 feet agl
leveled out at ~ 1500
and thats the truth.
William W. Plummer
eat? --Bill
phantom wrote in message <35DE1C4F...@worldnet.att.net>...
Craig Wall
phantom wrote:
> Better description:
>
> I went to 10200 agl. stopped heating....... started descent. didn't
> burn..........
> descent built up eventually to ~1300 fpm
Interesting- that's a quite survivable impact velocity (my old sport
parachute, a converted military emergency 'chute lowered me at about 22 fps,
which is very close to 1300 fpm).
What size balloon was that, and is that a typical terminal descent speed for
balloons? Was the envelope collapsed? (Of course not....but was it starting
to? Would it? Was that *really* terminal velocity? I'm not challanging you,
but I'm really curious....)
Craig Wall
phantom
That is typical. It also depends on how much weight is being carried. If I had
been alone, it probably have been around 1000 fpm. The envelope doesn't collapse,
more air is going in the mouth than escapes. And, the vertical "wind" pushes in on
the sides, and it starts getting shaped like a mushroom. Yes that's really
terminal. Terminal means as fast as it will go under the conditions, mostly
determined by weight.
Pete
wrote:
>It was a Raven AX7.
>
>That is typical. It also depends on how much weight is being carried. If I had
>been alone, it probably have been around 1000 fpm. The envelope doesn't collapse,
>more air is going in the mouth than escapes. And, the vertical "wind" pushes in on
>the sides, and it starts getting shaped like a mushroom. Yes that's really
>terminal. Terminal means as fast as it will go under the conditions, mostly
>determined by weight.
>
>Craig Wall wrote:
>
>> Interesting- that's a quite survivable impact velocity (my old sport
>> parachute, a converted military emergency 'chute lowered me at about 22 fps,
>> which is very close to 1300 fpm).
Yup, it's typical and survivable. I've terminaled AX-6 (56,000 cu ft)
to AX-8 (105,000 cu ft) balloons of three different manufacturers and
gotten similar results. Depending on the load, 1100 to 1300 ft per
min. As long as the envelope stays intact, you have a very good
chance of surviving, even walking away.
PLEASE NOTE! - I do NOT recommend flying into the ground at these
speeds, just point out that in a bad situation, you still have a
chance.
BTW, Fossett reported descent rates of at least 2500 fpm shortly
before contact with the ocean. (Damaged envelope and thunderstorm)
Alan Sanderson
descent rates. A sphere on a cone shape such as a Thunder&Colt will
develop a much faster terminal descent rate than a Cameron Viva (8
bulbous gore) or a Firefly.
My Firefly 7 (older Flexnet design) with a normal load would get up to
800 to 900 fpm terminal descent. My Viva 77 did about the same. A T&C
12-gore 7 would develop closer to 1200 fpm. The newer shape Firefly
envelopes with the higher load capacity will have slightly more drag.
Richard D. Piccard
.
.
.
> >
> >Craig Wall wrote:
> >
> >> Interesting- that's a quite survivable impact velocity (my old sport
> >> parachute, a converted military emergency 'chute lowered me at about 22 fps,
> >> which is very close to 1300 fpm).
>
> Yup, it's typical and survivable. I've terminaled AX-6 (56,000 cu ft)
> to AX-8 (105,000 cu ft) balloons of three different manufacturers and
> gotten similar results. Depending on the load, 1100 to 1300 ft per
> min. As long as the envelope stays intact, you have a very good
> chance of surviving, even walking away.
The envelope staying intact is presumably one of the reasons why the
certification requirements include terminal velocity testing, but I
wonder how much safety factor there really is. Does the testing have
to be done with extra weight in the basket (to increase terminal
velocity above that expected in the worst-case scenario)?
With conventional gas balloons, a failed envelope may collapse up into
the netting, forming a small parachute. The only case I know of where
this happened resulted in multiple broken bones (ankle or leg, I don't
recall) for some but not all of the occupants, but no fatalities. I
suspect that the most likely envelope failure mode of a modern hot air
balloon at terminal velocity would be to split part or all of a gore
near the equator, which would permit the balloon to deflate, at which
point the terminal velocity would increase, probably beyond any chance
of survival.
> PLEASE NOTE! - I do NOT recommend flying into the ground at these
> speeds, just point out that in a bad situation, you still have a
> chance.
Survivability would presumably also be enhanced by a resilient basket
structure base (woven wicker rather than welded aluminum), to permit
the occupants to slow down less suddenly.
Your chances might be much reduced if the burner assembly comes down
and whacks you on the noggin! Do certification requirements include
drop testing the basket/burner assembly? From what height (with what
impact velocity)? Is there a maximum permitted deformation of the
burner supports? I seem to recall designs in which the burner is
part of the load ring from which the basket hangs by flexible
cables; in such a design the burner would be pressed down onto the
occupants of the basket first with its own inertia and then with some
portion of the inertia of the tons of air in the envelope.
RDP
--
==================================================================
Dick Piccard Academic Technology Manager
pic...@ohiou.edu Computer Services
http://oak.cats.ohiou.edu/~piccard/ Ohio University
Richard D. Piccard
.
. [helpful answers to my questions removed to save bandwidth]
.
>
> Are you related to Don Piccard?
> He has probably forgotten more about gas and hot air balloon design
> than most people ever knew.
Yes, he is my uncle (my dad's kid brother). He was the pilot on the
only balloon ride I have taken, nearly 20 years ago.
He and I do correspond from time to time by E-mail.
James W. Moore
Richard D. Piccard wrote in message <35E475...@ohiou.edu>...
>
>The envelope staying intact is presumably one of the reasons why the
>certification requirements include terminal velocity testing, but I
>wonder how much safety factor there really is. Does the testing have
>to be done with extra weight in the basket (to increase terminal
>velocity above that expected in the worst-case scenario)?
>
The only example that I have personal knowledge of was a STC basket to be
flown under a type certified envelope. In this case, and I would think it
would be true in any case, the terminal velocity testing was with full
design load. There was, of course, no actual terminal descent, the test was
done by dropping the fully loaded basket unto a concrete floor, repeatedly.
The test also dropped the basket on a corner, rather than on the bottom,
which would tend to magnify the effect. Pass/Fail was that the basket be
structurally intact, one would like to believe this means flyable. I believe
the basket was flyable though it was never subsequently flown. Some
deformation occurred in the uprights. There is no real concern with the
envelope, we are
talking here about 15 miles an hour; any envelope with current airworthiness
will have no problem at that speed. Note the 1300 fpm is about the same as a
drop from 15 feet, the impact is considerably less due to the prep time one
has in a balloon as opposed to falling off a building. My own balloons have
had
a terminal velocity of considerably less then 1300 fpm. This may be due to
the fact that I never fly at sea level, I seldom launch below 5800 msl, thus
cannot carry max design load, and when I have tested the descent rate it has
usually been with at most one other person on board. It usually maxes out at
about 800 fpm. The certified max descent rate is 1300 fpm at 2000 lb. load.
This translates to about 7 people on board, no way.
>With conventional gas balloons, a failed envelope may collapse up into
>the netting, forming a small parachute. The only case I know of where
>this happened resulted in multiple broken bones (ankle or leg, I don't
>recall) for some but not all of the occupants, but no fatalities. I
>suspect that the most likely envelope failure mode of a modern hot air
>balloon at terminal velocity would be to split part or all of a gore
>near the equator, which would permit the balloon to deflate, at which
>point the terminal velocity would increase, probably beyond any chance
>of survival.
>
Envelope failure, as noted above, is not a problem. For a non-airworthy
balloon, who cares, dead is good. Only pity the unsuspecting passengers,
never fly with a stranger without checking their qualifications and
reputation. (Check your friends, too.)
While illegal, it is possible to fly a hot air balloon with substantial
damage. I know of a case in which 16 panels were ripped out in the vicinity
of the equator due to contact with a rock cliff at modest speed. The balloon
was subsequently flown over the cliff and landed without (further) incident.
A very experienced balloonist I know, while flying a Raven balloon with the
old Velcro sealed top, wondered why he was burning so much. Upon glancing up
he discovered he had forgotten to seal the Velcro and had a substantial hole
in the top of the balloon. Other than increased fuel consumption he
experienced no problem. Provided your burners are still working it is
possible to decrease descent rates substantially.
It is possible to exceed the design terminal velocity in a variety of ways.
Partial deliberate deflation; sometimes used in competitive events to get
down to the target in a hurry (usually frowned upon). Down drafts; I have
some experience here, unfortunately. I have hit the ground at about 1200 fpm
with no damage or ill effects on more than one occasion.
>Survivability would presumably also be enhanced by a resilient basket
>structure base (woven wicker rather than welded aluminum), to permit
>the occupants to slow down less suddenly.
The occupants are usually standing on the floor of the basket, not much room
for compression. There is a theory that in a hard landing you should be on
the side of the basket to take advantage of the resiliency, or hang onto the
superstructure with your arms. I know of no one who has actually tried. For
one thing you are much more likely to be thrown out and dragged, much better
to hide in the bottom.
>
>Your chances might be much reduced if the burner assembly comes down
>and whacks you on the noggin! Do certification requirements include
>drop testing the basket/burner assembly?
The burner is indeed included in the drop tests.
> From what height (with what
>impact velocity)? Is there a maximum permitted deformation of the
>burner supports? I seem to recall designs in which the burner is
>part of the load ring from which the basket hangs by flexible
>cables; in such a design the burner would be pressed down onto the
>occupants of the basket first with its own inertia and then with some
>portion of the inertia of the tons of air in the envelope.
>
You might well recall such designs, (your cousin Don?) Piccard balloons are
the only balloons all of whose occupants are required to wear helmets. Many
balloons are required to have helmets on board. All
of this discussion has sort of tacitly assumed that the descent is vertical,
with no horizontal component. In over 20 years I don't recall having such a
landing. During a hard landing the envelope continues to descend around the
basket until it hits the ground, it then rebounds rather well and the basket
is rather abruptly in the air again. In the balloons I fly (rigid
superstructure) the suspension cables, normally rising from the
superstructure to the envelope, are now descending from the superstructure
to the envelope. On rebound these (on my current balloon there are 16) can
become caught on any projection on the basket (handles, knobs, burner
attachments, arms, or heads). Should they become caught the basket may no
longer be suspended vertically under the envelope, or the cables may be
damaged severely. One of the (dangerous) things that one attempts to do is
to prevent that from happening. Note that the problem is exacerbated by the
forward motion; the envelope continues to move forward, while the basket
remains stationary. When the basket leaves the ground again the cables may
be well to one side of the basket. In Piccard balloons the forward motion
acts in favor of the occupants as the burner plate will tend to follow the
envelope and is more likely to land on the side of the basket than descend
into the basket. I find it difficult to believe that Piccard could be
certified today due to the lack of burner support, indeed Don's later
designs were more protective, although I don't believe any were actually
sold.
One final note, after a hard landing in my brand balloon - to maintain
airworthiness an extensive inspection of the balloon must be conducted and
any damaged parts replaced. This inspection must be done by a certified
balloon repair station. It ain't free! The requirements are effectively that
for an annual inspection.