Gulfstream From United States of America, joined Feb 2004, 61 posts, RR: 0 Posted (10 years 5 months 6 days ago) and read 9370 times:
I an aware that there are different ways of calculating engine power that go beyond my capabilities but I have often wondered what percentage of available power is used at cruise assuming takeoff is 100%.
I frequently fly Aer Lingus between Boston and Shannon and can't help but wonder how much is being asked of those two GE engines all night.
Thanks for any feedback.
FredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 4, posted (10 years 5 months 5 days 3 hours ago) and read 9188 times:
In cruise, you are ideally using 100% power - if you use RPM as your power indication.
This does not mean that you are using 100% of the power available at sea level though. You simply climb until 100% RPM will just maintain the ideal cruising speed for your aircraft.
A jet engine is happiest when run like this. Up until you reach the tropopause you gain even more by the reducing temperature.
Of course, the 100% RPM at the top levels will not have nearly the same fuel flows, pressures or temperatures as the lower RPMs down low...
If you want the actual thrust at altitude, the ratio between sea level thrust and cruise level thrust will be largely proportional to the ratio between the air density in cruise and the sea level air density. The power is the thrust times the true air speed... which will be higher at altitude due to the lower air density.
Nothing is so simple it can't get complex when you start looking at it, right?
I thought I was doing good trying to avoid those airport hotels... and look at me now.
OldAeroGuy From United States of America, joined Dec 2004, 3705 posts, RR: 67
Reply 6, posted (10 years 5 months 3 days 6 hours ago) and read 8991 times:
During cruise, the best power settings are those that equate to operating near the best Thrust Specific Fuel Consumption (TSFC) point. This is true regardless of the number of engines on the airplane, so the engines on a Twin are working no harder than the engines on a Quad.
Evidence of this is that all types have approximately the same engine/hr shut down rate during cruise. However, a Quad is twice as likely to have an engine shutdown since it has twice as many engines.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 66
Reply 7, posted (10 years 5 months 3 days 4 hours ago) and read 8960 times:
There are a couple of things that could be discussed here.
1.) The rated thrust of a jet engine is at sea level, standard day. (I won't get into "flat rated") This is based on the density of the air under those conditions. As the density of the air at FL180 is about half that of sea level and it diminishes by half again at about FL360 then a jet engine rated at 36000 lbs of thrust at 100% would produce 18000 at FL180 and only 9000 at FL360. These, of course are offset by similarly reduced drag and greatly reduced fuel consumption.
2.) The thrust of a jet engine increases in non-linear fashion with increasing turbine speed. So you may get more increase in thrust between 92% and 97% than you get in going from 65% to 85%. The real power comes way up there at the top corner of the gauge. That is true until you begin to encounter various limits. You are not going to go faster, necessarily.
For that reason 85% N1 for example, is not 85% of rated thrust.
All of my numbers are figurative and not actual. They do give you the general idea though.
edit: I guess I could also mention that the wear and tear on a jet engine also increases dramatically up there at the top end of the tach. At a place where the highest temperatures are being developed, rotational speed and therefore centrifugal force are also curving up off our chart. Turbine blades can actually stretch and rub the outer case. Not a good thing.
[Edited 2005-02-04 18:33:57]
Happiness is not seeing another trite Ste. Maarten photo all week long.
Mandala499 From Indonesia, joined Aug 2001, 7314 posts, RR: 78
Reply 8, posted (10 years 5 months 2 days 4 hours ago) and read 8852 times:
The thrust of a jet engine increases in non-linear fashion with increasing turbine speed.
PW and RR engines use EPR... engine pressure ratio... airpressure coming in vs air pressure going out of the engines... it's more of a linear thrust relationship... unlike N1 which is more quadratic....
For 732, Take off EPR U can go for 2.14 EPR (-17A), but mostly done with 2.10 to 1.98...
For quick thumb rules... since many have no autothrottle/LVL CH, taught by one 732 driver who is a member of this forum...
Climb thrust = 1.8 EPR + (25C-TAT) for the -15 engines, 1.9 EPR + (25C - TAT) for the -17s...
On a 280KIAS climb, leave the throttles your initial Climb thrust setting, and the EPR should change in a similar rate to the TAT, requiring little adjustment.
For cruise EPR, it's 3000kg/h/engine for 300kts@40Tons gross weight...
Cruise EPR = 1 + (CRZ FL (thousands) x3) for 40T gross weight for M0.76
For every M0.02, reduce EPR by 0.05... For 50T gross weight, add 0.1 EPR to that figure... let it settle and then adjust
So for a 50T 732 to cruise at FL300 at M0.76, 1.1 + (30*3) = 1.1 + 90 = 2.00
For 0.74 = 2.00 - 0.05 = 1.95 on the EPR... and 1.90 for M0.72 cruise
Gotta love the way they figured these out for these old babies
When losing situational awareness, pray Cumulus Granitus isn't nearby !