ronmarks

I think your premise of fuel flow increasing with altitude may be in error. That would be odd behaviour under any condition I can think of. Fuel flow will always DECREASE with power as altitude increases, jets and props both. This holds true even in the case of a simple carburetor piston engine which gets richer with altitude, if that is what you are basing your assumption on. In this case, the fuel flow does not DECREASE ENOUGH with increasing altitude to keep the proper fuel/air ratio as the air thins with altitude. But again, flow decreases.In the case of a supercharged piston engine flat rated to some (critical) altitude, the flow will be nearly constant to that critical altitude. Above that altitude, the fuel flow will drop along with the power just as a non-supercharged engine would do from sea level on up.OK, that is engines. Now airplanes. If you fly an aircraft level at the same indicated airspeed, the power REQUIRED increases with altitude, because the aircraft is going faster. ( power= force X distance/time). Maybe this is what you have in mind. And finally, re-reading your post, optimum altitude is not some magic altitude of minimum fuel flow. Maximum fuel flow always decreases with altitude. It means that above opt alt you burn more fuel climbing that you gain in descent and/or you L/D, slower TAS or whatever will cause more fuel to be burned for the trip at that higher cruise altitude.

This issue was challenging, so I followed my advice from my previous post and did a bunch of max GW take-off rolls and eng-off rollouts with the PMDG 737-800 sim. I did most of the runs at 1/2 speed to make the timing more accurate. and used the stopwatch function of the panel clock(upper left) for timing the acceleration and decelerations. This is what I got:The average TO roll drag is about 10,800 lbf. This seems to be excessive, but this is what the tests produced. The drag at 150 kt just prior to rotation was 13,000 lbf.The average net thrust during the TO roll was 33,000 lbf total (both engines)Adding the drag and net thrust gave 43,800 lbf total, or about 21,900 for each engine. This is a significant thrust deficit.So, Dr Serin's suspicions are confirmed. The PMDG 737-800 badly needs new engines and some air in the tires. :Worried:I also confirmed that, single engine at gross, the -800 is impossibly marginal. It will only maintain any climb at all if the speed is above V2, so yanking it off earlier will only get you to the first obstruction or an "urban renewal" crash landing.

I agree with the OP's unstated point - that the 737 NG physics might be off a tad. I think so. My impression is that the pmdg 737s ( and original MS 737) are a bit sluggish on the TO roll. After rotation, it seems about right.There is a very large increase in drag upon rotation. With low flap setting, the wheel and aero drag prior to rotation will only be in the high 100's or low 1000's lbf , but will increase more than 10,000 lbf with rotation. So the TO roll net thrust to accelerate the aircraft is only a little less than the engine thrust. It should not be too hard to do an estimate of the effective TO roll thrust, just time the TO roll from brake release to V1 and see what you get... f = ma and all that.