Basic Rules for Aerostatics
These Aerostatic Rules are taken from an Australian Government Document for getting ones "Airship Endorsement" on their pilot's license. Published in 2004 much of the document concerns the specific information an Airship pilot needs that the pilot of a heavier than air craft doesn't. One interesting point is that the rules are written for both hydrogen and helium airships!
Keep your sightglass full, your firebox trimmed, and your water iced.
Recall the following basic rules for aerostatics and how to use them to calculate airship performance:
(a) Rule 1
Lift of an airship varies with the volume if all other conditions affecting lift remain constant.
(b) Rule 2
Lift of a given volume of gas increases if barometric pressure increases and lift decreases if pressure decreases.
(c) Rule 3
Lift of a given volume of gas will decrease if atmospheric temperature increases and will increase if temperature decreases.
(d) Rule 4
The higher the atmospheric humidity the less the lift.
(e) Rule 5
There is no change in equilibrium due to a change in barometric pressure when the gas is free to expand.
(f) Rule 6
Where air and gas temperature change an equal amount there is no change in equilibrium if the gas is free to expand.
(g) Rule 7
An airship in equilibrium at any altitude will be in equilibrium at sea level, providing no weight is lost or gained and the superheat value is not changed in descending.
(h) Rule 8
An airship rising from the ground in equilibrium will be in equilibrium at any altitude below pressure height if no weight is lost or gained and the superheat value does not change.
(i) Rule 9
Barometric pressure will decrease approximately 1 inch Hg for every 1 000 feet of ascent in the lower atmosphere.
(j) Rule 10
Atmospheric temperature will decrease approximately 2°C for every 1 000 feet ascent.
(k) Rule 11
Gas volume is changed 1% for every 2.75°C change in gas temperature.
(l) Rule 12
Gas density is changed 1% for every 2.75°C change in gas temperature.
(m) Rule 13
1% change in gas density or specific gravity for helium changes the lift 0.2% when at pressure height.
(n) Rule 14
Lift is changed 1% for every 2.75°C change in superheat in flight as the gas is free to expand.
(o) Rule 15
At pressure height (where gas is not free to expand) the lift will change only 1% for 11°C superheat with helium and 1% for every 25°C superheat with hydrogen. This shows the danger in going from maximum daytime superheat conditions to zero superheat value at night if not properly understood and counteracted.
(p) Rule 16
(i) 2.75°C superheat will lower the pressure height 360 feet at altitudes below 7 000 feet.
(ii) 2.75°C superheat will lower the pressure height 400 feet at altitudes above 7 000 feet.
(q) Rule 17
In ascending under average atmospheric conditions the volume will increase 1% for every 360 feet of ascent in rising to 7 000 feet and increase 1% for every 400 feet above 7 000 feet.
(r) Rule 18
In going above pressure height, lift is reduced 1% for every 360 feet below 7 000 feet and 1% for every 400 feet when above 7000 feet.
(s) Rule 19
1% of the original mass of gas is lost if going 360 feet over pressure height when below 7 000 feet and 1% is lost for every 400 feet ascent above 7 000 feet altitude.
(t) Rule 20
If, when full of gas, a weight equivalent to 1% of the lift is thrown over as ballast, equilibrium will be reached when 1% of the gas has been ‘valved’.