(By Bert the Chicken, an expert on high altitude flying - An explanation your kids will understand). I wrote computer programs for a Jet Engine Test Stand company, for testing Jet Engines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .
Jets operate more efficiently way high up where the air gets colder and colder (cold air is more dense) but they also operate more efficiently close to the earth where there is greater air pressure (more dense). Jet engines depend on "heavy" (dense) air for their efficiency. So for maximum fuel efficiency, they have to pick an altitude where the inverse gradient curves of temperature and pressure meet, for maximum density. Moisture content and Jet stream are also a factors.
Jet engines use up 80% of their power to produce the pressure needed inside the engine to produce continuous combustion and thrust. They gain momentum and speed by "throwing" the (dense) air backwards. Similar to standing on a skate board and throwing a bowling ball backward, you'd move forward.
Air temperature decreases at a rate of 3.57 F degrees for every 1000 feet from the Earths surface. So planes climb to an altitude where the air is coldest for better efficiency, but as it climbs the pressure is decreasing at a rate of .934 (inches of Mercury) per 1000 feet thereby reducing efficiency. Therefore, at some point as it climbs, it ceases to become more efficient.
A strange thing happens at 36,089 feet. The temperature STOPS decreasing for the next 28,911 feet and remains at a constant minus 69.7 f degrees, but then beyond that, at 65,000 feet, the temperature continues to decrease again. Therefore 36,089 feet is in theory the altitude of maximum fuel efficiency because it ceases to get colder at that altitude.
The noise a jet engine makes comes from the difference in speed of the very fast air in the exhaust making contact with the relatively slow air moving past the outside of the engine.
Trick question: If a jet is flying at Mach 3 (2000mph+) how fast must be the air passing thru the inside of the engine??? Answer: Actually the air is SLOWED down to below the speed of sound (769 mph~) by increasing the pressure, until it exits the engine. The reason is, that air exceeding the speed of sound causes turbulence, cavitation, and vibration within the engine, therefore the air is compressed to slow it down (giving it more density).
A Hi-bypass engine (Large diameter) gives more thrust but is less efficient at high speeds, and therefore is used for shorter distances. Lo-bypass or no-bypass (Smaller diameter) engines produces less thrust but are more efficient at high speeds and at high altitudes and therefore are used for longer distances.
The actual altitude at which a plane flies is determined by many other factors also. Other factors are: other air traffic, weather, distance to travel, and the Jet Stream.
Most commercial jets fly around 30,000 to 33,000 feet and about 480-520 mph. Recently I noticed my flight was at 38,680 feet and traveling at 628 mph (My GPS told me), very close to the speed of sound at that altitude. We flew that high in order to get into the Jet stream. We were 40 minutes early on a 3 hour flight.
The above information is from Berts most favorite book in the whole world "the Aircraft GAS TURBINE ENGINE and its operation", by Pratt & whitney Aircraft Group. It not JUST about jet engines, its about physics too. Knowing information from this book also teaches one about thousands of other everyday physical phenomenon.
(Bert used to write computer operating systems and test programs for a Jet Engine Test Stand manufacturing company, analyzing data for peak efficiency)
