The effect of CG position on airplane drag in MSFS appears to be the opposite
of what it should be. For a given airplane net lift coefficient, the drag
coefficient should be higher for a more forward CG. So for a given flight
condition, there will be more drag at forward CG than at aft CG. However, in
MSFS, airplane drag is higher at aft CG than at forward CG. See the
screenshots below for the default A320. The first is with a forward CG; the
second is with the CG near the aft limit. Airplane drag is higher for the case
with the CG near the aft limit.
Interestingly, even with the CG
near the forward limit, the lift force on the tail is never negative. At least
the change in lift for the tail went in the correct direction (less positive
lift for FWD CG, but it should have been negative). CL for the wing had to go
up for the FWD CG case, but it did not change the CD value nearly as much as
the decrease in CL for the horizontal tail lowered the CD value for the
horizontal tail. This characteristic is also true in the default 747 as well
as a couple of the best known airliners available from 3rd party developers.
Is this endemic to the MSFS flight model, or is it due to incorrect parameter
values in the flight_model.cfg file?
Alright, after some more investigation, I have at least a partial answer. I’m
working on an airplane for which I have numerous iterations of the flight
model. In looking at a previous iteration, I found that although the lift on
the horizontal stabilizer was still not negative even at the forward cg limit,
at least the variation of drag with cg position was correct. That is, drag
decreased as the cg position moved aft. One of the changes between the flight
models was to reduce the elevator_lift_coef parameter, described as the
slope of the elevator lift curve. Changing this parameter back to the default
(by removing it) resulted in the correct variation of drag with cg position. I
have had other issues with this parameter and the elevator/horizontal as
described in these questions: https://devsupport.flightsimulator.com/t/4671
and https://devsupport.flightsimulator.com/t/4447. (Note: Although the
default Asobo A320neo flight model cfg file does not include the
elevator_lift_coef parameter, it has the elevator effectiveness parameter
set to 0.75, which I believe does effectively the same thing as reducing the
elevator lift curve slope.) I am finding it difficult to find the right
combinations of parameters to reduce the pitch sensitivity (which seems to be
a common issue in MSFS) without messing up something else, and the SDK does
not seem very helpful to me in that regard.
