There is very good book by Donald Crawford called A Practical Guide to Airplane 
Performance and Design.  ACS sells it.

This is my all time favorite airplane drag chart. It says it all.
the base line airplane is 150 HP and 1500 pounds.

Another interesting thing is the the decreased performance due to weight
increase down in the cruise speed range. Somewhat unexpected as the top 
speed is hardly affected at all.

Note the drag of most normal airplanes is a quite linear
function of speed squared  in the upper 33% of the speed range. 

What this chart is saying is the simple equivalent flat plate area
number can be used to interpolate performance points in this range. 
It is not necessary to know details that affect the induced drag in this
region of the performance curve.

Paul Lamar
 
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