From and related to: What is the relation between drag and weight?
- Friction drag is not affected by the angle of attack change.
- The [induced] drag will increase with the square of the mass increase.
I have no doubt about that, in fact I was going to post the same answer, the reason I waited is because I could not explain the A320 performance figures using this method.
The drag breakdown for an A320 at FL 370 and M0.78 is:
7,900 lbf of drag is composed of 4,700 lbf of parasitic drag (...) and 3,200 lbf of induced drag.
Let's consider a 32% weight increase -- from 50 to 66 tonnes. Applying 1.32^2 to the induced drag raises the total drag by 30%.
If we used the FF (fuel flow) as a thrust (also drag) measure -- since at a given FL / IAS / TAS the TSFC shouldn't vary that much when the N1 is ± a few percent --
... we find the ΔFF is 7% at FL 290 and 20% at FL 370. 20% is closer to the 30% thrust/drag estimate. Why is the thrust/drag increase much lower at a lower FL with weight the only variable?