Posts: 55
Joined: Mon Feb 16, 2004 8:53 pm

Engine Power Setting At Cruise

Wed Feb 02, 2005 6:54 am

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.
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RE: Engine Power Setting At Cruise

Wed Feb 02, 2005 9:04 am

You can't always compare the cruise power to takeoff power, since takeoffs are commonly performed with reduced power. To answer your question, I have no idea, I'm not a pilot.  Big grin

Why grab a Heine when you can grab a Busch?
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RE: Engine Power Setting At Cruise

Wed Feb 02, 2005 9:13 am

Takeoff is usually not done at 100% power, closer to 95% in the ERJ. Max Speed Cruise usually requires something in the neighborhood of 86-88% N1.
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RE: Engine Power Setting At Cruise

Wed Feb 02, 2005 10:53 am

You can't compare the two. Unless you cruise at field elevation.

Turbine engines burn less fuel at higher altitudes and produce higher true airpseeds.

In cruise, approximately 60% of the thrust comes from the fan in a high-bypass (>4:1) engine. At takeoff, the fan is closer to 75% of the total thrust.

Lots of apples and oranges!
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RE: Engine Power Setting At Cruise

Thu Feb 03, 2005 4:15 am

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?  Big grin

I thought I was doing good trying to avoid those airport hotels... and look at me now.
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RE: Engine Power Setting At Cruise

Fri Feb 04, 2005 8:40 am

Here are some real life numbers taken today from a WN737-700(AC# 481)...The flight was from SAN to SJC(flt. 2958)...Conditions:

Gross Weight - 112,600 pounds
Pressure Altitude - 40,000 feet
T.A.T. - (-36) degrees
Mach - 0.80
I.A.S. - 242 knots
S.A.T. - (-61) degrees

Engine data on number 1 and 2 engines as follows:
1 N1(84.4), EGT(603), N2(89.6), F/F(2040lbs/hr), Oil Temp.(100), Oil Pres.(40)
2 N1(84.5), EGT(606), N2(89.6), F/F(2040lbs/hr), Oil Temp.(100), Oil Pres.(40)


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RE: Engine Power Setting At Cruise

Sat Feb 05, 2005 12:33 am

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
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RE: Engine Power Setting At Cruise

Sat Feb 05, 2005 2:31 am

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.
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RE: Engine Power Setting At Cruise

Sun Feb 06, 2005 3:11 am

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 Big grin

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 Big grin

Gotta love the way they figured these out for these old babies Big grin

When losing situational awareness, pray Cumulus Granitus isn't nearby !
Posts: 284
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RE: Engine Power Setting At Cruise

Sun Feb 06, 2005 1:09 pm

The neat thing with modern jet powered aircraft is that the limiting factor in sub-sonic performance is the airframe. It's been this way for decades.

Engines are more and more capable of hurtling us through M1.0 for supersonic cruise. But we are limited by our airframes and current experience.
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RE: Engine Power Setting At Cruise

Sun Feb 06, 2005 6:43 pm

65% for il-76 at cruise
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RE: Engine Power Setting At Cruise

Fri Feb 11, 2005 3:29 am

I want to thank everyone for taking the time to respond. Very helpful.

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