How is density related to an aircraft engine being able to produce more or less thrust during a hot/cold day?

Question

hey peeps this question has always gotten me confused due to air being less or more dense based on humidity for a hot or cold day, especially when it is related on a molocular level ......so for any of you aircraft enthusiast ...pls HEEELLP!!

Answer 1

Folks! It is the atmospheric pressure we are talking about for low or high engine thrust....

The equation for calculating the thrust of an engine with the same pressure both inside and outside of the engine is F = (me/te x Ve) – (mi/ti x Vi), where me is an amount (mass) of the gas exiting the engine and te is the amount of time it takes for all of this mass of gas to pass through a given area. Ve is the velocity of this exiting gas, mi is an amount (mass) of the air just before it enters the front of the engine, and ti is the time it takes for this mass to pass through a given area. Vi is the velocity of the entering air. To make the terms more understandable, the terms with the subscript “e” refer to events and amounts relating to the exit of gas. The terms with the subscript “i” refer to events and amounts relating to the intake of gas.

In other words, the total thrust created is the force of the gas being ejected out of the back of the engine minus the force of the gas entering at the front.

Agreed that dense or cold air is thick but we should not forget that hot air expands rapidly. So, a dense air volume entering the engine will effect an efficient combustion. And that's why, not only the aircraft engines, in automobiles also it is advisable to run it idle for a couple of minutes before accelerating so that the better combustion takes place later.

Further, one of my relative who flies a Gulfstream V often says that during a rough weather encounter or in a bad weather where the outside temperature is too low and humidity is higher his engine sound changes and power drops to a level that he can sense it (i.e. without looking at the instruments)!

Answer 2

When the air is more dense (at lower altitudes) the engine is more efficient. This means the engine is slightly less efficient when the air is less dense.

Answer 3

Basically, air is less dense on a hot day compared to a cold one. This air density effects an engine in two ways. The first is overall power. This is especially important with piston engines. Air that is hotter or less dense doesn't combust as well when mixed with fuel. This limits the amount of power the engine can produce.  To counteract this, a piston engine can be fitted with a turbocharger. This effect isn't as evident with jet engines because a jet compresses air so much as part of it's operation, but it's still a factor.

The other major effect is in the actual drive system. A propeller is basically a wing. In thinner air, the propeller has less to exert force against and thus generates less thrust. This also occurs with a jet engine. Thicker air gives a more solid mass to react against. These differences are most evident at high altitude. An interesting note is that airliners are equipped with turbofans (turbojet engines driving large fans) to to exert force over a larger area behind the engine for more efficient operation at high altitudes. For an example of this effect think of your hand striking water. Striking with the same force, your fist will penetrate farther than your open palm.

Answer 4

guy no.1 answer is OK but just add a few nice to know which I happen to know during my B737 re-currency course.

A good example of thrust is your back to a wall and you kick the wall to propel yourself forward that is thrust and the wall behind you is the air now think of it being dense or thin the thrust will be lessen isn't it?

Answer 5

Ok, no one answered the question of density.

Humidity is water vapor that is suspended in an airmass.

Water Vapor molecules are less dense than air molecules, meaning that the more water vapor (humidity) air has the less dense it is. Density is mass per volume, so the more mass it has the more dense, meaning the drier the air has more higher the density. Temperture is related to volume and pressure, so the higher the temperture, the more volume, and less pressure. so if mass is constant, and volume is increased (higher temp), the less dense it is. This is why cold, dry days have the highest density, and usually pressure will be much higher than hot days.

In physics, Force = Mass x Acceleration, so the more mass a volume of air has the more force is can produce when accelertion is constant. This is how an engine can produce more power when flying in cold dry air, as the air has more mass, and the acceleration (combustion) of the engine is constant, the more power it can create.