[Claudius]

It's a shame because I'd love to know in real percentage terms just how superior a modern brushless power-train is, given a direct comparison in a single airframe.

I can answer that for you...

I don't know if you saw it in its first powered configuration, but that 2x6 glider of mine with a Speed 400 + bolt-on speed controller could barely get out of its own way. With a brushless in-runner + ESC on 3S, it practically goes vertical at WOT.

Better yet, the prop/brushless/ESC/Lipo/modern receiver and servo combo is less weight than the original nicad pack/receiver/servos... meaning in its current powered configuration it has a lower all-up weight than the original unpowered glider had!

Consequently, I too have a Speed 400 + ESC to donate to the cause if Jason would like to do a direct comparison.

The thing to bear in mind is that in order to double the speed you need to more than square the power, so you would expect that if 100w = 125 kph then 200w would equal about 150kph and 400W would get you somewhere in the rehlm of 190kph

It goes up by the cube of velocity, not the square. (although perhaps this is what you meant by "more than square the power", since your examples closely represent a cubic relationship)

The Drag FORCE goes up by the square; consequently, the POWER required to overcome it goes up by the cube.

http://en.wikipedia.org/wiki/Drag_%28physics%29#Power

... Because weight also increases drag ...

I'm with you on all the bits of your post - except this bit.

Certainly there's a non-linear relationship between the power transferred to whatever is providing the thrust (propeller, impeller, turbine, whatever ...) and the velocity of the airframe - but in level flight the drag and weight vectors are orthogonal and therefore independent?...

They are related, essentially by the lift to drag ratio. Every extra unit of weight requires more lift to overcome it, and every extra unit of lift induces an extra amount of drag.

If your airframe has a lift-to-drag ratio of 10, and you add 10g of weight, well, you're going to need another 1g of thrust force to overcome it at comparative speeds.