Originally Posted by extreme P
From the gouge...
What is the difference between Max Range Cruise [MRC] and Long Range Cruise [LRC]?
MRC:
The speed at which, for a given weight and altitude, the maximum fuel mileage is obtained.
It is difficult to establish and maintain stable cruise conditions at max range speeds.
1.32 Vimd constant speed with variable AoA [dependent on weight].
LRC:
Speed slightly faster than MRC at a constant AoA [slightly faster than Vimd]
As weight decreases, speed needs to decrease to maintain AoA
Reducing speed necessitates reducing thrust, though because best SFC for a given engine occurs at a particular design RPM, you must climb
I disagree with the 3rd point on MRC.
You can't have a constant 1.32 Vimd "SPEED" and variable AoA. If you were to slow down and increase your AoA to Vimd (best endurance) for example, you take longer to get there but burn more fuel. If you were to firewall the throttes you'd save heaps of time, but the parasite drag is so high and the power required is so high, you'd dry your tanks before arriving!
1.32 Vimd (best TAS/DRAG ratio) occurs at a CONSTANT AoA and the speed at which that AoA occurs is dependent on weight. SPEED is the variable, proportional to weight, which decreases during the flight, as noted on your LRC notes.
LRC is referred to as 99% MRC. Same principles, but you fly about 5% faster with only a 1% decrease of the best SAR. ie: a good compromise.