Originally Posted by Mad (Flt) Scientist
So, the question is, what is the L/D for the approach case.
If (L/D) sin3 = cos 3 then we have the diciding case
i.e. L/D = 19 (approx) is the dividing case.
So, if your L/D is less than 19 (which seems a reasonable assumption) you should have increasing thrust with increasing weight for a 3 deg glideslope (for the other minor assumptions). But you will need less thrust than in level flight for both cases (of course)
Nice analysis Mad Scientist. Interestingly for the 747...
"The maximum lift-drag ratio of the aircraft, (L/D)max, is estimated to be about 18, as compared with a value somewhat over 19 for the 707,
..but of course that's in a clean config. On approach you need to fly slower than optimium and that means all the twiddly bits hanging out in the breeze. The LD will be a lot less than 18. I couldn't immediatly find that figure on the web.