Slowing down for fuel efficiency?
Join Date: Jun 2004
Location: Australia
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Hi Hawk37,
Note that I did say, "at a constant level". A series of constant levels is typical of an aircraft conducting periodic step climbs to be at the level closest to optimum, thus, initially the weight is above optimum, decreasing to optimum weight for the level, and then further decreasing to a weight less than optimum for the level until burned down to the weight for the next step climb.
At the initial high weights (when the weight is above optimum), Angle of Attack is high (due to the high weight) resulting in greater than desired acceleration of air over the upper wing, resulting in slightly slower Mcrit. As a result, because MRC is closely related to Mcrit (always being ABOVE it!), flight is at a slower 'than might be expected' Mach Number. As weight decreases, required AoA decreases, acceleration of air over the wing is less, thus the aircraft may be flown faster (increasing Mach Number) for the same wave drag, and MRC increases slightly as the weight decreases. After this very small increase with reducing weight, MRC Mach Number then steadily decreases with reducing weight, as might be expected.
As LRC and all Cost Index speeds between MRC and LRC are inexoribly linked to MRC, the same trend is noticeable at these speeds also.
Actually, it's quite noticeable in the cruise control tables from many AFMs, the initial small increase is usually only of the order of M0.02 or so.
Regards,
Old Smokey
Note that I did say, "at a constant level". A series of constant levels is typical of an aircraft conducting periodic step climbs to be at the level closest to optimum, thus, initially the weight is above optimum, decreasing to optimum weight for the level, and then further decreasing to a weight less than optimum for the level until burned down to the weight for the next step climb.
At the initial high weights (when the weight is above optimum), Angle of Attack is high (due to the high weight) resulting in greater than desired acceleration of air over the upper wing, resulting in slightly slower Mcrit. As a result, because MRC is closely related to Mcrit (always being ABOVE it!), flight is at a slower 'than might be expected' Mach Number. As weight decreases, required AoA decreases, acceleration of air over the wing is less, thus the aircraft may be flown faster (increasing Mach Number) for the same wave drag, and MRC increases slightly as the weight decreases. After this very small increase with reducing weight, MRC Mach Number then steadily decreases with reducing weight, as might be expected.
As LRC and all Cost Index speeds between MRC and LRC are inexoribly linked to MRC, the same trend is noticeable at these speeds also.
Actually, it's quite noticeable in the cruise control tables from many AFMs, the initial small increase is usually only of the order of M0.02 or so.
Regards,
Old Smokey
Join Date: Jun 2004
Location: Australia
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Hi hawk37,
Arrrgh!
I'm guilty of typing too carelessly, all of the text stands uncorrected, I meant to type a difference of M0.002 or so, not M0.02! That's quite a difference.
Most noticeable on high flying aircraft like the Learjet, which have a fairly 'fat' wing, and suffer high low speed and high speed drag polars in the mid thirty thousands, quite low low speed polars above 40,000 or so, but the relatively fat (slatless) wing accelerates air to Mcrit rather quickly at higher angles of attack, a good performance in this respect at more optimal angles of attack.
Mea culpa, mea culpa!!!
Regards,
Old Smokey
Arrrgh!
I'm guilty of typing too carelessly, all of the text stands uncorrected, I meant to type a difference of M0.002 or so, not M0.02! That's quite a difference.Most noticeable on high flying aircraft like the Learjet, which have a fairly 'fat' wing, and suffer high low speed and high speed drag polars in the mid thirty thousands, quite low low speed polars above 40,000 or so, but the relatively fat (slatless) wing accelerates air to Mcrit rather quickly at higher angles of attack, a good performance in this respect at more optimal angles of attack.
Mea culpa, mea culpa!!!
Regards,
Old Smokey
