When I was in the Navy, the officer in charge of our Navy SH-2F Seasprite helicopter crew, Lt. Cmdr. Chuck Taylor, briefed us before we flew off the back of frigates and destroyers, and the subject of air density was often discussed. Here’s why:
Helicopters fly by chopping the air; the denser the sky, the more efficient the rotors become and the less power they need from their jet engines.
If we ever had an unexpected loss of energy from even one of the two engines during periods of low air density, the consequences would have been tragic. You see, the highest amount of energy required from the helicopter’s engines occur at takeoff, landing and especially during a hover.
During landings, particularly at night, pilots often struggle to put the helicopter in a stable hover over the flight deck as it pitched and rolled with the ever-changing seas. At times like that, we would move left and right, forward and back, in an attempt to center the helicopter’s main landing gear over a frightfully small flight deck safety circle that was painted white on a deck coated with thick and crusty black, nonskid material.
If the helicopter’s landing gear was too far forward of the white circle, you ran the risk of the main rotors striking the ship’s hangar. If you were too far back from the oval, the rear landing wheel on the helicopter’s tail pylon could end up in the flight deck safety net hanging over the water. At times like this, we all thought about the loss of an engine.
Even if you did an emergency jettison of the helicopter’s sonobuoys (they would shoot out the side of the aircraft like a Gatling gun), external fuel tanks, torpedoes and other equipment that would literally shed hundreds of pounds of weight from the helicopter in a split second, you might still not have enough power to safely land on board the ship when the air was less dense.
These thoughts of doubt would circle back to basic physics and chemistry that we learned in school. The four factors that most affect air density are altitude, atmospheric pressure, air temperature and humidity. Generally, the higher your altitude, the less dense the air will become. Because of changing weather conditions, atmospheric pressure fluctuates continuously. Increasing air pressure raises air density.
On the other hand, increasing the air temperature will decrease its density. Think about a hot air balloon. Heat the air inside by a propane torch and it becomes less dense. Because it’s less dense, the hot air balloon rises through the colder and denser atmosphere that surrounds it.
It may seem counter-intuitive, but humid air is lighter, or less dense, than dry air. Most of the water that is in the atmosphere is in the form of a gas or water vapor. Water vapor weighs less than nitrogen or oxygen.
If you remember the periodic table from school, it was arranged by atomic number. The weight of an individual atom is represented by its atomic weight; the chemical element with the lowest density is Hydrogen. The atomic weight of hydrogen (H) is 1; oxygen (O) is 16. Consequently, a water molecule (H2O) has a molecular weight of 18 (1 + 1 + 16). Free Nitrogen (N2) has a molecular weight of 28, and an oxygen molecule (O2) has an atomic weight of 32. Therefore, a water molecule is lighter than either a nitrogen or an oxygen molecule, according to a U.S. Navy training manual (Aerographer’s Mate).
Avogadro’s law states that “equal volumes of all gases, at the same temperature and pressure, have the same number of molecules.”
This means as heavier molecules of N2 and 02 are displaced by lighter H2O molecules, the air becomes less dense, and helicopters and fixed-wing aircraft will require more power to stay aloft. By the way, nitrogen and oxygen, on average make up about 99 percent of the air we breathe. Water vapor can range anywhere from less than 1 percent to as much as 5 percent of the atmosphere near the Earth’s surface or troposphere.
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John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at firstname.lastname@example.org or follow him on Twitter: @PGE_John.