roswell

ture difference between the lifting gas and free air. Evidently
the.effect of ventilation as the balloon moves through the air
oauses the lifting gas to remain at a temperature approximating
that of the air, and the increase of lift due to temperature
variation is small in magnitude.
Since changes in the value of free lift appear incapable or. causing
any appreciable increase in rate of rise, other possible variations
such as a ohange of the drag, or fluid friction, effect must be
considered.
The equation of Korff is based upon the assumption that the effect
of the change in Reynolds number and the change in size are of equal
magnitude, but in opposite directions. Therefore, these variables
are eliminated to obtain the stmple engineering formula of Korff.
With a non-extensible balloon, however, the change of drag effect
is probably less than the effect of change of Reynolds number. There-
fore, it is likely that the rate of rise would increase l'r:i,.th alti-
tude. The change in drag effect may be realized by a decrease of
relative size of the flabby, unfilled portion of the balloon. Thus
there will be a decrease of the drag caused by flow of air past this
flabby portion as the shape of the balloon changes; the result will
be an increase in the rate of rise of the system.
c. Superheat and Its Effects
The effect of the heatin~ of lifting gas by the sun's rays has long
been of interest to those using balloons for atmospheric investigation.
In cosmic-ray studies using freely extensible balloons, this heating
effect was used to advantage in extending the length of flights.
These flights were often released at night using the heat added at
sunrise to replenish lift lost during the night by diffusion and
leakage.
In constant-level balloon work, using non-extensible balloons, the
effect of superheat of the lifting gas is more often a disadvantage
than an advantage. The disturbance of the flight is not great when
the gas acquires this superheat but may be disastrous when the super-
heat is lost. It is at this time that a lerge amount of ballast is
required to keep the balloon system afloat.
Let us investigate the effects of gain and loss of superheat on a
full, non-extensible balloon. We shall try to explain these
effects in terms of percentage loss or gain of lift of the balloon
system by use of simplified engineering formulas. First, the
genera 1 formulas:
(1) Lift: , where
vb = balloon volume
d0 , dg = density of air and liftirg gas, respectively
-34-