Hi,
Thank you for your posts.
Leftright, I sent you a private message. it will be nice if you can post info from the manuals.
Originally posted by leftright:
A windmilling engine still produces enough rpm to produce sufficient oil pressure to supply oil to the breaings, and herein lies the problem, ALL 747/340 engines (infact most jet engines) use Labryinth seals at the bearing compartments, these seals utilise Internal Engine Bleed to pressurise the labryinth seals, this helps seal these bearing compartments tightly, thereby decreasing oil consumption radically.
A Jet Engine at windmill speeds cannot produce bleed pressure, this leads to the Labryinth seals being unpressurised, the oil consumption more than doubles because of this, the oil gets dumped overboard via the respective bearing compartment vents, (check the training or ground maintenance manual for precise figures), hence leaving an engine at idle power is far more sensible (if possible), than doing a precautionary shutdown. Doing a precautionary shutdown and then restarting in a few hours time might present you with a mandatory shutdown due to low oil quantity and Low oil pressure light illuminating.
This perhaps explains why there is a windmilling limitation on the 340, small oil tanks/massive endurance, on the classics with either the Pratt & whitney or GE motors there is no windmilling limitation.
If you dont find any info in your ground maintenance manuals let me know and I'll try and paste the Boeing info from their training manual in this thread
I am not postulating, just a guess: a windmilling engine will provide very low oil pressure. Oil leakage via bearings will be not important (through labyrinth or carbon seals, I guess carbon seals are also used) since the oil pressure is very low and the compressor (windmilling engine) is still providing air (even tough not important) that may buffer the bearing seals.
I get a PM that states (during windmill):
Oil pressure almost zero, depends upon N2 ;and the N2 also depends upon airspeed. It will decay to zero below 250kts.
After I received this PM, my opinion is: N2 is turning initially at very low speed and will be driving the gearbox with all its accessories (more load for the N2). The oil pump will provide very low pressure if it is not nil. IF N2 is high the oil pressure will increase but also the N2 or HP compressor will increase the air pressure that will buffer the bearing seals (there is kind of certain proportion and balance between oil pressure and air in the bearing compartments). The oil leakage through bearing seals is existant but not very important: it may depend on the size of the oil tank (as you mentioned), the number of bearings, and the site of the bearings (on N1 or N2). The oil pump will provide the same pressure for all bearings (N1 & N2) but the bearing seals are not buffered by the same air pressure because the air pressure is not the same along N1 or N2: N2 bearing seals may be tightly sealed and N1 bearing seals may leak little bit (assuming N2 is higher than N1) and vice versa if N1 is higher than N2.
This is just assumption, I hope to get answers to be corrected.
In another thread I get this statement:
"FL150, 150kts which gives about N1=10% and N2=0% and FL240, 260kts which gives N1=18% and N2=15%, although these would be much lower if allowed to decay down to the steady state".
My question is: IN GENERAL after drift-down (twin, tri, or quad after engine shutdown) and in steady state and in stabilized flight, is N1 always > N2 ???(let's say especially in medium or high bypass engines, or even low bypass
if it is possible).
Feedback very appreciated. Thank you.
Best regards.