How can more THRUST mean less POWER and therefore less fuel?
To examine this let’s go back to basics.
Hopefully we can all agree that:
Power required Drag x TAS
And
For straight and level constant speed flight Thrust = Drag
This means that we can use the Drag / TAS curve to test how power required varies with drag.
Now let’s look at the curves in post number 7.
To keep the number small We will ignore the numbers of the scales and use the numbers of squares to measure values of drag, TAS and power.
At 2 squares along the TAS axis we have 17 squares up the drag axis. So the power required at this speed is 2 x 17 = 34.
At 4 squares along the TAS axis we have 5 squares up the drag axis. So the power required at this speed is 4 x 5 = 20.
Continuing this process w get the following results:
TAS------- Drag --------Power Required
2---------- 17 ---------- 34
4----------- 5 ---------- 20
4.5 --------- 4 --------- 18.5
5 ----------- 3.7 ------- 18.5
7 ------------3.3 23.1
10 ---------- 4.3 ------- 43
15 ---------- 7.3 ------- 109.5
Note that the position of the power required curve in post 7 does not reflect these values. This is because it has been positioned to suit the power required scale at the left edge of the diagram.
Between 2 and 4 drag units the drag and power required both decreases. This is because the rate at which the drag is decreasing is greater than the rate at which the TAS is increasing. So the product of TAS x Drag is decreasing.
Between 4.5 and 5 drag units the drag is decreasing but the power required is constant. This is because the rate at which the drag is decreasing is equal to the rate at which the TAS is increasing. So the product of TAS x Drag is constant.
Between 10 and 15 drag units the drag and power required are both increasing. So the product of TAS x Drag is increasing.
Now to answer the specific question:
How can more THRUST mean less POWER?
Let's look at the effect of a deceleration from 7 squares on the TAS scale to 4 squares on the TAS scale.
At 7 TAS we have 3.3 drag so power required = 7 x 3.3 = 23.1
At 4 TAS we have 5 drag so power required = 4 x 5 = 20.
So although the drag (and hence thrust required) went up from 3.3 to 5, the power required went down from 23.1 to 20.
The key factor in determining whether power required increases, remains constant or decreases, is the relative magnitudes of the rates of change of TAS and Drag.