B738 Unstick Speed and VR Prediction Mismatch

[Guest mruane]

HiSorry for starting a new thread, but I realised what may be a -800 problem whilst trying to get the Stab Trim figured out in the B737-800 in another thread.When I start FS9 and load the -800 it has (nearly) full fuel tanks. If I accept that fuel load and program a flight, the FMC Take Off page predicts a VR of 154 Knots. The actual unstick occurs at 155 knots. Close enough.Now, if I start FS9 and select the -800 and then manually reduce the fuel load by emptying the centre tank, after planning the same flight the predicted VR becomes 140knots. The Stab Trim is 4.9. However when I execute the takeoff, the actual unstick speed remains at 155 knots in spite of the predicted VR of 140 knots. In other words, the unstick speed seems to be based on the actual weight at the time the aircraft is loaded into FS9 and not on what it actually is at the point of rotation. Has anyone else noticed this or are your observations different?

[Guest mruane]

Bump - Hasn't anyone else at all noticed this behaviour at takeoff?

[CF104 0]

Are you letting the trim rotate the aircraft or are you manually rotating at VR?Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

[Guest mruane]

Thanks for the interest John, I was beginning to wonder if I was bleating in the dark about this. The 737-800 is such a joy to operate but this little issue is irritating - you know how it is.I am rotating the aircraft manually at the VR speed notified on the Take Off page in the FMC. In the case of the 737-800 loaded with about 17,000lbs of fuel and ZFW=130.0 that comes out at 140 knots (in my examples). However the aircraft will not unstick before 154 knots which happens to be the predicted speed when all three fuel tanks are full.My aircraft thinks that it is heavier than it actually is when I manually unload fuel...Any thoughts?