pstaney,
It's all about incremental cruise. Looking at the first two cases (Best Gradient Climb and Best Rate Climb), the aircraft at Best Gradient speed will get to cruise level in a shorter distance (but in a longer time) than the aircraft at Best Rate Speed. Even if the time to climb and fuel to climb were the same (they're NOT, the aircraft at best rate will get there first and for less fuel), the aircraft at best Gradient Climb now has to cover incremental cruise distance to the point where the aircraft at best rate commenced cruise. This must be added to the climb fuel. Strike out the aircraft at best Gradient for good fuel economics.
To take it one step further, if best rate climb is examined, the rate is attributable to maximum excess Power (whereas the aircraft at best gradient is flying at maximum excess Thrust).
Power = Force (Thrust) X Velocity (TAS). If we examine Power Required (
Pr) graphs for a
JET aircraft Vs Power Available (Pa), they lie at quite high speed in a region where the
Pr is increasing in a gentle curve upwards with increasing speed, but Pa is also in a gentle curve upwards and almost parallel with each other. Small (20 to 25 knots) variation is speed either side of best rate result in only slight change in Pa Vs
Pr, i.e. excess Power, with very small impact upon Rate of Climb. The aircraft about 20 knots above best rate speed will suffer only a small penalty in fuel burn during climb, but cover a considerably greater distance for that very slight fuel investment.
If I look at typical B777 figures at high weights, Best Rate comes out at about 300 KIAS (Mean TAS = 425), and Minimum Fuel at about 320 KIAS (Mean TAS = 451). For a 30 minute climb (30.5 minutes at best fuel speed), I have covered an additional 13 miles for an investment of .5 minute of climb fuel. The aircraft at the lower speed (Best Rate) must now cruise for 13 miles (about 1.5 minutes) to catch up to the same point.
Yes, the speed is very similar to, or
FASTER than typical Maximum Range Cruise speeds.
Regards,
Old Smokey