Balloons respond to various air currents to a greater degree than other
aircraft. Except for momentary delays wherein inertia of the balloon mass
resists the energy of a new air current, the flight path of a balloon in
equilibrium exactly mirrors the direction and velocity of the air current
in which it is operating. This simple fact of balloon operation holds true
for horizontal (the kind we like to fly in), vertical, and rotary air currents
(when Mother Nature flies). This article is about flying with Mother Nature
and includes a few tips on what to do when Mother Nature's flying becomes
too exciting.
FALSE LIFT
False lift is an aerodynamic phenomena which occurs during the initial
acceleration of the balloon. A balloon standing in the wind acts as an
obstacle to normal air flow causing the wind speed to increase on the surface
of the balloon. Changing the wind speed causes a pressure conversion to
occur (static pressure decreases and velocity pressure increases) which
results in false lift. During launch false lift couples with the balloon's normal lifting forces
to cause the balloon to take off. The problem with having false lift is
that it quits when the balloon reaches wind speed. Ready or not, false
lift dissipates rapidly after takeoff causing the pilot to apply heat constantly
in order to gain real lift before acceleration is complete.
In a practical sense, false lift does not pose any operating problems
until the wind speed exceeds approximately 10 mph. False lift is unavoidable
in a fast wind making takeoffs a challenge for the balloon pilot. Simultaneously,
the pilot must watch burner operation, fabric, control the crew, assess
the balloon's physical readiness for flight and judge the lift. The confusion
of burner noise, passenger and crew demands, and the physical jolting caused
by the gondola dragging across the ground can quickly overload an unprepared
pilot. The pilot should carefully prepare, inspect and rig the balloon
prior to inflation to minimize the tasks required during inflation and
prior to takeoff.
This procedure is necessary to insure that the balloon is safe in the
event takeoff occurs unexpectedly. The pilot must always assume false lift
is present during takeoff, that the envelope lift is inadequate, and continue
heating until balloon acceleration is complete and a positive climb rate
is established. During takeoff and acceleration it is not possible to differentiate
between real lift and false lift. The best policy is to heat past equilibrium
temperature and then vent as necessary to maintain a comfortable rate of
climb. False lift is easier to overcome when burner output is high, therefore
fuel pressure is the best information available to judge the balloon's
ability to overcome false lift. Small burners on big envelopes are obviously
a poor choice of equipment for fast wind operation. Obviously the takeoff
field is important in fast wind operations. Obstructions downwind require
additional clearance depending on wind speed due to the reduced initial
climb rate of the balloon after losing the false lift.
WIND SHEAR
Wind shear is a phenomena that resembles false lift in many ways. The
principle difference is that the balloon is airborne and may be accelerating
or decelerating. In a practical sense, there is no difference between acceleration
or deceleration since the balloon is symmetrical and responds uniformly
to air flow in any direction. The real difference from a pilot's view point
is the location of the occurrence. False lift occurs from a known position
selected by the pilot. Wind shear locations are usually unknown to the
pilot which can increase the hazard. I recall a windy landing which was
unfortunately expedited by a strong decelerating wind shear. At that time
my balloon was not fitted with a skirt. The wind had increased unexpectedly
to a force of about 25 to 30 mph. On a previous approach I accidently struck
a bank and tree causing the deflation valve to open over 3 panels. Frankly,
I was scared, nervous and excited and probably not thinking too clearly.
I was flying down a river valley and the landing sites were typically small
and surrounded by trees. I decided to descend to tree top elevation so
that I would be ready when a suitable landing site appeared. I wasn't experienced
enough to realize that strong shears are typically present just above tree
lines. The shear destroyed the balloon's lift and I descended through the
trees and into a small clearing. We hit hard but there was no damage to
occupants or balloon. I learned 3 big lessons that day. First, that velcro
tops are not rated for the life of the envelope. Second, wind shears can
deflate the balloon sufficiently to lose control. Third, that skirts are
a necessity because they allow you to heat during a shear.
Let's examine what is happening dynamically and thermodynamically to
the envelope in a wind shear. The initial reaction of the envelope to a
wind shear is to distort and gain false lift. False lift occurs instantaneously
and is momentarily beneficial. Distortion occurs a little slower and is
proportional to the gross weight of the balloon, envelope volume, and the
strength of the shear. There is no evidence to suggest that one envelope
design is more resistant to wind shear forces than another. The effect
of distortion is to reduce the envelope volume causing exhalation through
the throat which is followed by cold air inhalation. The distortion or
breathing of the envelope may, and typically does, cycle several times.
Each distortion causes the average envelope temperature to decrease because
of the inhalation of cold air. The heat transfer rate of the balloon envelope
is also greatly increased. The wind shear wipes away the dead air layer
which is laying on the surface of the envelope. This dead air layer, or
boundary layer, represents significant insulation to the envelope. When
the boundary layer is wiped away by a gust the envelope heat loss nearly
doubles. To view this in perspective the balloon will act like you doubled
the load insofar as the operation and feeling of the balloon is concerned.
The net result is a significant decrease in lift.
I mentioned that the reaction of the envelope to shear is also dependent
upon, the internal pressure, the volume of the envelope and its gross weight.
The internal pressure of a balloon envelope of a given volume is directly
proportional to its lift. The limiting pressure is a function of envelope
height. Given an identical load, a small envelope will have a higher internal
pressure than a larger envelope. A large envelope, loaded to its rated
gross weight, will have a higher internal pressure than a small envelope
loaded to its rated gross weight because of its greater height. What does
all this mean to a pilot? Simply that a lightly loaded balloon will distort
more than a heavily loaded balloon when it encounters a shear or gust because
the internal pressure offers the only resistance to those wind forces.
There is a serious hazard to flight associated with wind shear which
could be easily overlooked. A heavy shear gust can displace the vertical
axis of the balloon. This displacement from vertical can be extreme, perhaps
as much as 30 to 40 degrees in severe turbulence. If the burner is operating
at the moment the envelope displaces, severe structural damage to the throat
can occur. Structural damage is not loss of fabric; fabric is not significant
in the throat area because there is no internal pressure. Structural damage
is loss of load tapes or ropes. The gravity of this problem is not easily
visualized. The scenario involves the loss of lift and thus pressure in
the envelope and the loss of two or more load tapes or ropes. When the
load elements are burned free the fabric of that portion of the envelope
is no longer supported. If the throat pressure is low or negative the fabric
repositions itself into the balloon throat thus blocking the burner from
the envelope. As the balloon begins to descend the velocity pressure created
by the downward flight of the balloon presses on this unsupported fabric
causing it to close the throat further. At some point, probably over 1500
feet/min., the force on fabric becomes so great that the fabric tears.
This enormous internal flap of fabric effectively splits the balloon from
the inside. This is a known failure which has contributed to at least one
fatal balloon accident. Advice? Don't use console mounted burner controls
in turbulent air or high winds because they have too much afterburn. In
turbulence and shears be ready to shut off the burner immediately. When
at altitude it is seldom urgent to maintain equilibrium thus there is no
particular reason to operate the burner when heavy envelope distortion
is present. Be aware that high burner pressures can cause the flame to
reach to distorted fabric on the side of envelope and result in major damage.
ROTOR
Rotor winds are associated with mountainous terrain and strong winds
aloft. Mountain flights should anticipate rotor winds, and in particular,
when wind aloft forecasts exceed 15 knots at mountain peak altitude. Prediction
of rotor winds is not a well developed science. Typically a rotor wind
will have forces which exceed the performance of the balloon. Downward
airflow will therefore carry the balloon to the ground even with the envelope
at red line temperature. Further, it is difficult to escape from a rotor.
The only apparent escape is climbing out through the top. The vertical
segment of the rotor enhances balloon performance. A good rate of climb
on the upside of the rotor will force the balloon out the top by inertia.
Once the rotor area is defined, the area can be avoided by clearing it
at high altitude. An approximate safe altitude is 3 to 4 thousand feet
above highest terrain. Surprisingly, rotor winds can be completely free
of turbulence; however, this is not always true. Fortunately balloon flight
over mountains is not the typical flight. Those who choose to fly in mountains
should realize that there are serious hazards which can supersede the scenery;
rotors are just one of those hazards.
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