$$
F = \left( \frac{V}{412} \right)^2 \times \left( G \right)^{\frac{2}{3}} \quad \text{(Approximate)}
$$
where $ G = $ gross load
A chart, Figure 24, has been drawn up, based on this equation, expressing free lift as a percentage of gross load, allowing the rate of rise to be approximately predetermined.
E. Ballast Requirements
The amount of ballast which must be dropped through the manual ballast valve to keep the balloon at its ceiling, can be approximately determined by the following measurements: a balloon of similar size and construction is inflated and its loss of lift with time is measured with correction for variation of temperature. This inflation is not complete, but is of the same magnitude as that of a balloon ready for release, approximately 14% of full inflation in the case of a General Mills balloon. The loss of lift per hour, multiplied by a factor representing the increase of the surface which results from total inflation, is thus obtained. This factor is the reciprocal of the fraction of inflation raised to the two-thirds power for a spherical balloon, and is approximately the same for the tear-drop shaped General Mills balloons.
Field experience has shown that ballast leak pre-set to slightly exceed the computed loss of lift is insufficient. A ballast leak of double the computed loss of lift has usually been adequate. It is believed that increased liquid viscosity and valve closure caused by the colder temperatures of the high atmosphere are responsible for the need for this higher ballast setting. An investigation into temperature effects on the ballast release systems has been started.
The amount of ballast which must be released at sunset to compensate for the loss of superheat, may be computed as follows:
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