Date: 19 Feb 98 01:34:26 From: Chris Pitzel <firstname.lastname@example.org> Organization: PSINet References: 1 Followups: 1 2 3 4
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Nathan Pusey wrote: > Can somebody please explain to me why planes (jet aircraft) fly > at very high altitudes on long range flights, and how does this conserve > fuel and get maximum distance ? How does this conserve fuel and save > time if you can only go half the speed you normally can go at low > altitude and you have to use twice the amount of power to get too half > the power you get at lower altitudes ? There are two main physical reasons this is true. There may be others which others can elaborate on as well (I'm not too familiar with some of the operating characteristics modern gas turbines, so I couldn't really comment on efficiency issues). a) The earth's gravitational attraction decreases as you get further away from the earth. When you're at sea level, the earth's acceleration due to gravity is approximately 9.81 metres/second (or 32.2ft/sec for the non-metric types). As you go higher, the attractive forces between the earth and a particle decrease (the particle being the aircraft). This translates into less energy being needed to overcome this gravitational attraction in order to keep an aircraft airborne. If you were to measure your weight on an aircraft at FL400 and your weight at sea level, you would find that you would weigh less at altitude. For example, I weigh roughly 130 pounds at sea level, yet I would only weigh perhaps 110 pounds at 40k feet. There are formulas to determine this which can be derived from the relationship: F = G*m1*m2/r^2 Where F is the force between the two bodies, G is the gravitational constant, m1 is the mass of one of the particles, m2 is the mass of the other particle, and r is the distance between the centres of the two particles. Anyone who has taken an introductory physics course of any kind will have seen these formulas. b) The density of air (and therefore pressure) decreases with altitude. Air is much more dense at lower altitudes than it is at high altitudes. If you want proof of this, just try climbing Mount Everest without oxygen ;-). Temperature also decreases with an increase in altitude until you reach a certain altitude, then it levels off and (I believe) even starts to increase. Since it is much easier to move an object through a very low-density fluid than it is to move it through a higher density fluid, this means that an aircraft will require less power in order for it to overcome frictional forces due to wind resistance (ie: drag). This is pretty self-evident; take a tub of syrup and try moving your finger through it. Then take a tub of water and move your finger through it. Observe which is easier to move through. You'll find that the water exerts much less drag than does the water. The resistance is proportional to the viscosity of the fluid. Overcoming this resistance is very costly in terms of fuel burn, and pilots often will fly hundreds of miles out of their way just to fly with the wind (in the jetstream) instead of against the wind to reduce this drag and make the flight faster. To calculate wind resistance involves a bunch of logarithmic relationships, but in general, to make an aircraft fly twice as fast requires 8 times as much power (velocity cubed relationship). In terms of fuel burn, gas turbines have certain operational power settings at which they are most efficient. It may be more efficient for an operator to run an engine at 66% of full power over a period of 15 hours than it would be for an operator to run an engine at 100% of full power for 10 hours just to achieve the same flight distance (these numbers, are by no means, accurate or anything). However, there are economic reasons for not always selecting the most efficient operating power setting on an aircraft. For example, if you end up having to pay for an extra crew to fly the plane because the first crew can't legally fly a flight that long, then that adds up to some really large costs (fuel is only ~20% of the total operating cost of a typical flight). > Thanks in advance. I hope I've given you some answers to your questions; if I didn't, I at least got the discussion started.