Outcast_Searcher wrote:Tanada wrote:Best solution, require short distance flights to remain below 25,000 feet and require long distance flights to fly at or above 40,000 feet. Contrails almost disappear, no more contrail pollution or global dimming caused.
Interesting. Any idea of the general fuel/economic impact of doing this? If the extra fuel required is minor (percentage wise) then this sounds like a good idea, if you can get the international community to agree on this.
http://science-edu.larc.nasa.gov/contrail-edu/faq.phpQ: In which layer of the atmosphere do we normally see contrails?
A: Contrails usually form in the upper portion of the troposphere and in the lower stratosphere where jet aircraft normally fly, generally between about 8 and 12 km altitude (~26,000 to 39,000 feet). They can also form closer to the ground when the air is very cold and has enough moisture.
For most modern gas turbine powered aircraft you have a balanced set of competing properties to get the perceived best fuel efficiency. I say perceived because for a traditional jet engine passenger aircraft you have a low to mid range bypass ratio ranging from 2:1 to about 5:1, the early generation turbofan engines. The latest and best ultra high bypass ratio turbofans like the ones on the Boeing 777 are rated at 9:1 while the Airbus A380 has a bypass ratio of 8.7:1 and the 2015 version of the Airbus A320 has a super efficient 12.5:1 bypass ratio. In contrast a turboprop engine driving a free spinning unducted propeller has a nominal bypass ratio of 50:1 because very little of the engine thrust comes from the jet exhaust, almost all thrust is from the spinning propeller. The difference is propellers are most efficient below 450 knots air speed and 25,000 feet altitude while the low bypass ratio turbofans are more efficient at higher altitude in thinner air. Contrary to what most people think the high bypass ratio turbofans are an attempt to get the most thrust possible at lower altitude with the lowest noise output possible to let very heavy aircraft take off safely.
Passenger aircraft in particular do not normally climb to high altitude immediately after taking off. Instead they preform a series of step altitude increases especially on long distance flights. They will fly at say 27,000 feet for the first hour or more, then as they burn off fuel weight they will ask permission to ascend to a higher altitude of say 33,000 feet where they will cruise for another hour or two while burning off more fuel weight. Finally when they are light enough they will climb again to their flight plan cruise altitude of 37, 000-39,000 for the remainder of their travel time to the destination where they will descend and enter the holding pattern at much lower altitude before finally landing.
Because much of the thrust on even a high bypass turbofan actually comes directly from the jet exhaust a high altitude cruise is more fuel efficient at typical speeds of 550-650 mph, however no long distance passenger jet climbs directly to that altitude because for long distance flights they are very heavily loaded with fuel and they basically can't until they burn off a lot of weight. To follow the no contrail plan these long distance jets would skip the middle steps in the climb, they would fly at or around 25,000 feet until they burned off enough fuel to climb to their long distance cruise altitude when they had burned off enough weight. While this would be somewhat less fuel efficient than the step climb system used today it would greatly reduce the contrail formation by only spending a limited period in the 26,000-39,000 foot altitude band where most contrails form.
Ideally for short distance flying you should not use engines designed for very high altitude, it is a mismatch between engine capability and fuel efficiency. Climbing to a high altitude takes a lot of energy IOW fuel consumption, and for a short flight it doesn't pay off. For long and very long flights flying high once you have a low enough weight saves fuel because once you climb you stay at the high altitude for 3 or more hours. If you fly from Boston to Seattle you are in the air 6 hours or more so high altitude saves fuel. If on the other hand you are going from Boston to Detroit the whole trip is a little over two hours. For such a short flight the only way to make climbing to high altitude economical is to put a light fuel load on the aircraft so it can climb directly to high altitude without the cruise phase. Meanwhile you could put the same passengers in turboprop engine aircraft flying at 23,000 feet and get there in 2 hours 45 minutes instead of 2 hours 15 minutes and save a lot of fuel in the process. IOW if we only flew aircraft that were most suited depending on the route we could nearly eliminate contrails which are just a side effect of long cruises at optimum contrail altitudes.