roswell

of' the su,rrounding, atmosphere. Upon reaching the altitude at
which it is full it will still have en unbalance in the
direction of ircrease of altitude due to the excess buoyancy
causing ascent. This unbalance is gradually decreased as tho
balloon rises (with a fixed vobwe) into less dense e.ir. Mean-
while excess gas pressure is relieved by velving gas through
the appendix until the balloon system is in a condition of
equilibriun. The porticn of the ascent after the balloon has
become full is known as the 11 leveling-off" period.
The lifting gas which is valved out through the appendix will
cause a "back pressure" inside of the balloon which rrust be
transferred to the balloon fabric or film. In other words,
there must be a pressure difference across the appendix opening
during this period to force the excess lifting gas out of the
balloon. Let us analyze this back pressure by the method used
by Picard. Using the rules of subsonic aerodynamics, Picard
suggests that air at sea level escaping at 1333 f~sec. pro-
duces a beck pressure of 1 atmosphere and that back pressure
induced is proportional to the square of escape velocity of
the gas and inversely proportional to the density of the gas
escaping. Volume of gas lost in ascE:nt through 1 foot is
within a reason&ble degree of accuracy: '
(e) AV :Y~. T+AT
lli. P di! T
~~ = volume lost per foot of ascent (ft.3/ft.)
v = balloon volume (ft.~)
p = pressure of free air (psi)
dp = pressure
di! ehange with increase of Z (psi/ft)
T = tempereture of air ( 0c abs.)
AT : change in air temperature durinb rise ( 0 c)
For ascent in the troposphere this relationship will reduce to:
(7) v
27,800 .u'FT
The velocity of escape of e;a_s, then:
(8) V • dJ! V I
· di. 27,800 · Aa
V = velocity of escape of lifting gas (ft./sec.)
-52-