fuelburn
hello
Well to answer your question first consider the stalling speed in clean config, or the en-route configuration, which for a jet are pretty high, as you would know.
The first speed we encounter is the minimum drag speed; this is typically about 1.5 times the stalling speed. And as the terminology implies - it is also the speed at which the fuel flow required or the fuel consumption is the least. At this speed the summation of induced drag and parasite drag is minimum, and hence the total drag is minimum. Minimum drag means the least thrust required, as drag equals thrust in un-accelerated flight, and therefore corresponds to a thrust setting to the minimum fuel flow required. Now any faster - the parasite drag will incr and will require more thrust and therefore incur a greater fuel burn. Any slower and u will slip up on the backside, incur an increase in induced drag, and again will require more thrust to hold speed, and so greater fuel consumption. This speed is therefore the one that gives you minimum fuel burn, that is maximum no of air hours per unit of fuel, and is therefore the holding speed of a jet, and flown when endurance is of primary importance. U will stay up in the air for maximum time if u follow this speed schedule.The next speed we encounter, and that is more appropriate for your question is the Max range speed. This is typically about 1.7 times the stalling speed, definitely higher than max endurance (minimum drag) speed mentioned above. This speed gives us the maximum TAS/drag ratio. That is a given IAS for which drag is minimum. Agreed the drag remaining same at a given IAS, as we go higher, the differential between IAS and TAS becomes greater, while the thrust, therefore the fuel flow required to push the plane remains substantially constant at the same IAS at the same weight. Again without going into the graph, this speed is not equal to min drag speed but somewhat higher which actually offsets the benefit of minimum drag, but more than makes by being a higher IAS correspondingly a higher TAS. When u fly this speed u get maximum no of air miles from a pound of fuel - and that is range. Though u will not stay in the air longer as compared to the speed in the previous paragraph. In conclusion if u want to fly farther, fly higher, that is what the combination of the swept back wing and the jet engine prefers.
So do we fly this speed in airliners - actually no. If u maintain this speed, if the airplane wud slow down, it will continually slip back the curve and require more thrust and so greater fuel flow. What the manufacturers have done is that the manuals are provided with what is called a LRC - long range cruize chart. This speed is slightly higher than the max range speed - uses slightly more fuel, but the range penalty is just about 1%. That is to say - for a slightly higher speed, assuring u speed stability u will still achieve 99% of max range. And this is the speed u wud like to fly to get maximum mileage out of your tanks.
Again so the airlines use it, depends on company to company. One thing for sure no one flies slower that this. But many use higher speeds to get shorter block times, and on very long haul legs it may shave off 30 to 45 minutes of flying time, therefore the requirement of an additional crew is eliminated due flight duty time limitations
Again to make it sound simpler..Consider the following examples at atypical weight and crz level
Stall speed- 160 knots.
Max endurance (minimum drag or best L/D speed or normally hold speed) 240 knots.
Max range speed 280 knots
LRC speed 285 knots ( to assure speed stability)
Airline may use 295 or 300 or whatever suits their requirement.
Just a note here. Except the hold speed, all others are normally dealt with in Mach no. figures.
In the end - if u r flying in headwind increase mach no ,and in a tail wind decrease mach no. to achieve max mileage from ur tanks.
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kunal