Question about engine start.
-
Hello,
The Starship checklist specifies engaging the engine start switch, motoring to at least 12 percent N1 RPM, then moving the condition level to start and disengaging the starter. While this is likely correct for the type of engines the Starship has (?), most turbine engines require the starter to remain engaged to around 60 percent N1 RPM to avoid a hot start condition during ignition.
Also, disengaging or turning off the starter cuts off the igniter (I think), and at 12 percent N1 RPM that seems like it might not be perfect for good light off. Just wondering.
I will add that starting the engines exactly as specified in the Starship checklist works great every time. Just trying to wrap my head around a few detail I don't correctly understand.
Thanks!
-
12% is the minimum RPM for stable combustion to occur, this means that the flame front will progress at the same velocity as the incoming air in the burner can and that the flame front wont contact the burner can. the starter is to remain engaged until 50%N1. The most dangerous part of an engine start is during the initial acceleration phase where there is very little air flowing through the engine. The current engine model over temps if you introduce fuel at 12%, ive seen temps as high as 1100.
the basic process is as follows:
Starter engage
12%N1 fuel cutoff lever to start
monitor ITT for max 1000 degrees, cutoff fuel if over temp seems likely
at 50% N1 disengage starter. keeping the starter engaged above 50% can lead to damage from reverse current as the starter starts to be driven by the engine rather than the opposite. Additionally the starter and generator are the same unit, but the starter turns the generator into a motor. -
12% is the minimum RPM for stable combustion to occur, this means that the flame front will progress at the same velocity as the incoming air in the burner can and that the flame front wont contact the burner can. the starter is to remain engaged until 50%N1. The most dangerous part of an engine start is during the initial acceleration phase where there is very little air flowing through the engine. The current engine model over temps if you introduce fuel at 12%, ive seen temps as high as 1100.
the basic process is as follows:
Starter engage
12%N1 fuel cutoff lever to start
monitor ITT for max 1000 degrees, cutoff fuel if over temp seems likely
at 50% N1 disengage starter. keeping the starter engaged above 50% can lead to damage from reverse current as the starter starts to be driven by the engine rather than the opposite. Additionally the starter and generator are the same unit, but the starter turns the generator into a motor.I always wait for max motoring before introducing fuel. It costs nothing but a few extra seconds and keeps the temps cooler.
@ProfCrash said in Question about engine start.:
The most dangerous part of an engine start is during the initial acceleration phase where there is very little air flowing through the engine. The current engine model over temps if you introduce fuel at 12%, ive seen temps as high as 1100.
Indeed. I once had a disagreement between my hardware levers and the virtual cockpit levers and I had an 1100 degree C or higher hot start in the left engine. I had the tablet up and got to watch the health bar shoot downwards and completely destroyed the engine before I could even really think about it.
Is there some sort of recovery process (aside from setting the condition lever to cutoff), or if that ever happens you've basically destroyed the engine regardless?
-
12% is the minimum RPM for stable combustion to occur, this means that the flame front will progress at the same velocity as the incoming air in the burner can and that the flame front wont contact the burner can. the starter is to remain engaged until 50%N1. The most dangerous part of an engine start is during the initial acceleration phase where there is very little air flowing through the engine. The current engine model over temps if you introduce fuel at 12%, ive seen temps as high as 1100.
the basic process is as follows:
Starter engage
12%N1 fuel cutoff lever to start
monitor ITT for max 1000 degrees, cutoff fuel if over temp seems likely
at 50% N1 disengage starter. keeping the starter engaged above 50% can lead to damage from reverse current as the starter starts to be driven by the engine rather than the opposite. Additionally the starter and generator are the same unit, but the starter turns the generator into a motor.Disengaging the starting at 50 percent N1 seems a lot more reasonable. Perhaps the checklist in the Blacksquare manual needs to be updated?
-
It depends, if you over temp an engine start it depends on how long that over temp lasted based on the highest temp. at 1000 degrees you may have 5 seconds, at 1100 you may only have 1 second before you need to conduct a hot section borescope or tear down. If you are under the limit then mx will do a visual inspection for discoloration and damage and if none is noted then run the engine and then return to service.
-
Disengaging the starting at 50 percent N1 seems a lot more reasonable. Perhaps the checklist in the Blacksquare manual needs to be updated?
@Aaron-Anderson said in Question about engine start.:
Disengaging the starting at 50 percent N1 seems a lot more reasonable. Perhaps the checklist in the Blacksquare manual needs to be updated?
The checklist is based on the Beechcraft language. it seems like the intent was to keep pilots from attempting the get the switch into the off position by 50%. its better to have pilots wait until 50% then get the switch off
-
@Aaron-Anderson said in Question about engine start.:
Disengaging the starting at 50 percent N1 seems a lot more reasonable. Perhaps the checklist in the Blacksquare manual needs to be updated?
The checklist is based on the Beechcraft language. it seems like the intent was to keep pilots from attempting the get the switch into the off position by 50%. its better to have pilots wait until 50% then get the switch off
That makes perfect sense. Thank you!