Maybe something a bit less important at this point than other flight model
issues, but how is one-engine-inoperative drag for turbojet airplanes
currently being determined, and what parameters can we use to influence this?
An inoperative engine typically creates a drag increment composed of
windmilling drag (energy used to rotate the fan by the air inflow rather than
combustion in the engine) and spillage drag (caused by the air that cannot
pass through the engine spilling out of the inlet). There will also be
additional control drag in trying to hold constant heading with wings level
due to rudder and aileron input. I flew a test condition with our airplane
(A320NEO) with all-engines-operating and with one-engine-inoperative to see if
I could observe any drag difference. There did not appear to be any. Are there
current parameters that affect this, or are there any plans to incorporate
windmill/spillage or control drag parameters that would assist devs in trying
to match one-engine-inoperative
performance?
Hello @donstim You probably want to have a look at ram drag: Flight Model
Physics
(flightsimulator.com)
The ram drag value is visible in the engine debug view and leads to negative
thrust when turbines are inoperative. Regards, Sylvain
Hey, thanks for responding. But this doesn’t answer my question. Ram drag is
part of the overall thrust equation for determining the net thrust produced
for a given flight condition. It is caused by bringing the freestream velocity
of the air almost to rest as it comes into contact with the fan and other
engine components. Although it is a “drag” term, it is bookkept as thrust. At
low thrust levels, like idle thrust, ram drag can cause the net thrust to be
negative as you note. The windmilling drag of an inoperative turbojet engine
is an additional drag force that is normally bookkept as a drag force on the
airplane. It is in addition to the ram drag and is due to energy being
extracted from the air to turn the fan (“windmilling”) rather than it being
turned by energy coming from the burning of fuel. I took a look at the engine
debug view as you suggested, which leads to more questions. See the attached
screenshot, which uses the Asobo A320NEO. In the screenshot, engine #1 is at
idle, and engine #2 is inoperative… Given enough time, the inoperative
engine’s RPM reduces to zero, so it is not actually even windmilling like it
should be. (Maybe that’s the issue?) The ram drag of the idling engine is
higher than that of the inoperative engine leading to a lower net thrust (more
negative) for the idling engine than for the inoperative engine. As a result,
not only does the inoperative engine not lead to a drag increase, it results
in a thrust increase in this case.
Hello @donstim My apologies for the misunderstanding. I reviewed the subject
with our core simulation programmers and they confirmed this is not simulated
at the moment. They are aware of the problem but I can’t give any ETA on when
will implement a solution for this at the moment. Regards, Sylvain