I'm doing Ex A4 and confused by question 7 on page 7.10:

Refer to the Total Drag/Speed graph [with speed 'S' marked at the minimum point on the curve]. To use the least amount of fuel in a given distance for nil wind conditions, the aircraft must be flown
a) at speed S for any weight
b) faster than speed S if weight is increased
c) slower than speed S if weight is increased
d) faster than speed S if weight is reduced

The correct answer is given as a).

I understand that if we consider "S" to be a variable speed that always resides at the bottom of the Total Drag/Speed curve, then we should fly at variable speed "S" for any given weight. But I also think that b) is the answer that shows a greater understanding of the question. The Total Drag/Speed graph is only valid for that aircraft at one gross weight. If we increase the weight of the aircraft, won't the entire graph move to right (compared to the original graph) and thus, won't the point "S" actually be a higher speed?

Hi Ray,

The wording is perhaps a little ambiguous but answer (a) is the one Bob is looking for. Any aircraft of any weight will have an airspeed which corresponds to minimum total drag and on their own total drag vs speed curve, that speed will be "S". Aircraft of different weights will simply have a different value for S.

Having said that, you have obviously understood the concepts behind the question so don't mark yourself wrong for that one.

Cheers,

Rich