LEVEL FLIGHT
Level flight is achieved when lift equals gravity. For all
practical purposes, gravity is constant as far as the
balloonist is concerned. The heated air trapped in the
balloon envelope overcoming gravity creates
buoyancy.
Equilibrium is achieved when lift exactly matches
gravity and the balloon neither ascends nor descends,
but remains at one altitude. Theoretically, equilibrium
aloft is level flight, because the wind may raise or lower
the balloon and the ambient air temperature is
constantly changing.
Since it is the difference between ambient and
envelope temperature that gives the balloon lift, the
difference is really what a balloon pilot is interested in
knowing, but few balloons have a temperature
differential gauge.
In addition to fuel gauges, regulations require all hot
air balloons to have an envelope temperature indicator,
a rate-of-climb or vertical speed indicator (VSI), and
an altimeter. The envelope temperature gauge helps
prevent overheating and damage to the fabric. All
balloon manufacturers have established maximum
envelope temperatures that should not be exceeded.
The VSI is a trend indicator showing the balloon's
tendency to ascend or descend. A mechanical VSI
will lag behind while an electronic VSI is more
sensitive. An altimeter is similar. A mechanical altimeter
lags behind and a digital altimeter is very sensitive.
Since the air around the balloon is constantly moving
and changing temperature, instruments lag. The human
eye is the best gauge for determining if a balloon at
low altitude is ascending, flying level, or descending.
Theoretically, if a pilot were to hold a hot air balloon
at a constant temperature, the balloon would float at
a constant altitude. However, there is no practical way
to hold the envelope air temperature constant. When
the first modern hot air balloons were flown, pilots
used a metering valve to adjust precisely a constant
flame, hoping to hold the balloon in level flight. Anyone
who has tried to fly that way has found the technique
is noisy, fuel inefficient, damaging to the burners, and
nearly impossible. Most modern hot air balloons are
flown with a blast valve using unregulated tank
pressure.
For every altitude there is an equilibrium temperature.
If a pilot is flying at 500 feet mean sea level (MSL)
and wants to climb to 1,000 feet MSL, the balloon
temperature must be increased. This is not only to
attain equilibrium at the new (higher) altitude, but some
excess temperature must also be created to overcome
inertia and get the balloon moving. Newton's law
states, "A body at rest tends to stay at rest, and a
body in motion tends to stay in motion."
A pilot flies level with a series of standard burns.
Ideally, if the burns were of identical length and
perfectly spaced, the balloon would hold a nearly
constant altitude. The word nearly is used because
the ambient air temperature is always changing. Each
time a pilot burns, the balloon will tend to climb, but
the air in the envelope is always trying to cool and the
balloon will tend to descend. If the subsequent burns
are perfectly timed, the balloon will fly in a series of
very shallow sine waves.
An interesting experiment is to use the second hand
of a wristwatch to establish a usable level flight pattern.
Establish a standard burn and count how many burns
are necessary to hold the balloon relatively level for 5
minutes. Assume you make five burns in 5 minutes
and the balloon stays pretty much at 1,500 feet AGL. That means if you make a standard burn every 60
seconds, the balloon will fly level. Now you have a
basis for controlling the balloon.
Of course, any variable will change the balloon flight.
A heavier basket load, higher ambient temperature,
or cloudy day, will all require more fuel (by shortening
the interval between burns) to maintain level flight.
By experimenting, you can establish standards that
can be used as a basis for all flights. On a hotter day,
or with a heavier load, you could make the interval
55 seconds. A standard baseline to work from will
then have been established.
With practice and using the second hand of a
wristwatch, a new pilot can fly almost level. The
exercise of trying to learn the pattern of burns (each
day and hour is different) is an interesting training
exercise, but not a practical real-life technique. The
ability to hold a hot air balloon at a given altitude for
any length of time is a skill that comes only with serious
practice.
Level flight is probably the most important of all flight
maneuvers. Nearly all other flight maneuvers are based
on level flight. Level flight is very important to the
student pilot aspiring to pass the practical test.
Maintaining level flight is one of the few balloon
maneuvers for which pass/fail tolerances have been
established.
Unfortunately, most pilots do not spend enough time
practicing level flight. If, during the practical test, you
have a choice of altitudes, pick a lower, rather than
higher altitude, to perform level flight. There are more
visual references to assist in maintaining level flight
when you are closer to the ground. Of course, you
do not want to fly below minimum safe altitudes, and
you should always be alert for powerlines along your
flightpath.
Level flight is the basis of most other flight maneuvers.
Evenly spaced, standard burns can produce smooth,
relaxing level flight. The ability to fly straight and level
comes with experience and practice.
|
