Atmospheric Pressure in Aviation
July,14,2022
Air has weight and thickness. Right at 14.7 psi. Just like everything else, what goes up must come down. This is all dependent on temperature and altitude. Temperature falls at 3.5 degrees Fahrenheit for every 1,000 feet in altitude until 36,000 feet. Similarly air density will decrease by 1Hg for every 1,000 feet until 10,000 feet. (PHAK chapter 11-2). This seems like a simple concept at first, but as we dive into it the more complex it becomes. The temperature and density of air is totally different on the surface than FL300. For us this may cause ear popping but what does it mean for our aircraft? There are three main effects on an aircraft as it ascends in altitude. All three issues are a result of the air thinning as you fly higher. The performance an aircraft has on the ground will be drastically different than high above ground level. Your power, thrust, and lift will all suffer due to thin air in high altitudes (PHAK 11-2). The power plant needs oxygen to perform effectively. Your propeller will lose the amount of air it is pushing thus making it more difficult to maintain speed aka thrust. Lastly without as much power from the engine and thrust from the propeller added with less air pressure under the wings the end result will be a lack of lift (PHAK 11-2). One mitigation technique we use for our military aircraft is the implementation of a turbo on the engine causing more air to enter the engine at high altitudes to compensate for the loss of air density. This allows the engine to perform just as well no matter our reasonable altitude. Technology has come a long way through the years. We now have a way to combat the loss of thrust when it comes to the propellers. Many aircrafts are now equipped with Variable pitch propellers (Variable Pitch Propeller for UAV-Experimental Tests,Podsedkowski,2020). As the aircraft climbs higher the angle of attack(AOA) can adjust to make up for the loss of thrust (PHAK 3-7,3-8). With these mitigations added together with modern designs of aircraft the fight against low atmospheric pressure is easily won.
References
Pilots Handbook of Aeronautical Knowledge chapter 11 Aircraft Performance, chapter 3 Aircraft construction
Variable Pitch Propeller for UAV-Experimental Tests, Podsedkowski,2020
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