747 EPR Question

[PointyEngine]

Someone here might know the answer to this - its got us prop guys baffled.

On the flight profiles below, it has "Level" EPR of 1.16, and "App" at 1.18. Should the "Level" EPR be significantly higher than "App"? Or is there some relationship between airspeed and EPR that has gone over our heads?

[attachment=0]Screenshot_2015-11-09-13-40-04.jpg[/attachment

[Ki-ll]

Someone here might know the answer to this - its got us prop guys baffled.

On the flight profiles below, it has "Level" EPR of 1.16, and "App" at 1.18. Should the "Level" EPR be significantly higher than "App"? Or is there some relationship between airspeed and EPR that has gone over our heads?

[attachment=0]Screenshot_2015-11-09-13-40-04.jpg[/attachment

I am not too strong on my jet engine theory, but I think the amount of drag on approach is higher due to flaps and gear hanging down hence the higher EPR.

[pelmet]

Someone here might know the answer to this - its got us prop guys baffled.

On the flight profiles below, it has "Level" EPR of 1.16, and "App" at 1.18. Should the "Level" EPR be significantly higher than "App"?

Perhaps it would help if you explained why you feel that EPR in level flight should be more or less than at approach. It is difficult to compare "apples to oranges" so to speak and you are talking about two very different scenarios. A full flap and gear down situation versus a clean aircraft with much less drag. It appears that almost as much thrust is required for the aircraft on approach as the 280 knot level flight scenario with gear and flaps up. Seems believable.

[Boreas]

It's a pretty dirty aircraft in landing configuration - lots of gears sticking down into the flow.

Also, its a pretty fast jet - compared to other heavies - so approaching at 155kts might put it that much further on its power curve.

[complexintentions]

Of course the EPR is higher on approach, tons of drag and the engines are spooled up. Don't know if the 744 has "High Idle" but it's pretty common to have a higher thrust setting (whether EPR or N1) on approach. It's why a low-drag approach is much preferred, saves a significant amount of fuel.

Also, its a pretty fast jet - compared to other heavies - so approaching at 155kts might put it that much further on its power curve.

155kts at 270 tonnes seems about right, B777 at max landing of about 250 tonnes is 149kts which is the same extrapolated....?

[RG500]

.

[JigglyBus]

I'm not a heavy jet guy, but you heavy jet guys have seriously confused me.

You (jet guys) seem to be answering this OP by saying that "the thrust on approach would be just as high as in cruise because of all the drag".

Surely this isn't correct.

The actual thrust (and fuel burn) would be waaaay lower on approach, would it not?

My understanding is that the EPR in cruise would show 'similar' numbers to on approach, due to the ram air effect. EPR measures pressure coming in the front vs pressure going out the back. In cruise, the intake pressure it quite high (cause you're doing 280). The 'output pressure' is 16% (1.16 EPR) greater than the intake pressure. 16% greater is a lot of thrust, when compared to 280kts of pressure. B744 cruise fuel burn is around 24,000 pph, no?

On approach at 150, the intake pressure is significantly less. Therefore the 1.18 EPR is 18% more than the intake pressure, but the actual thrust produce is waaaay less than the previous 1.16 setting. Evidenced by the fuel burn being around 8000 pph, no?

Anyways, that's my understanding of it. Could be completely wrong.

[PointyEngine]

Someone here might know the answer to this - its got us prop guys baffled.

On the flight profiles below, it has "Level" EPR of 1.16, and "App" at 1.18. Should the "Level" EPR be significantly higher than "App"?

Perhaps it would help if you explained why you feel that EPR in level flight should be more or less than at approach. It is difficult to compare "apples to oranges" so to speak and you are talking about two very different scenarios. A full flap and gear down situation versus a clean aircraft with much less drag. It appears that almost as much thrust is required for the aircraft on approach as the 280 knot level flight scenario with gear and flaps up. Seems believable.

I can't see how required thrust in "App" could be higher than in Level flight, unless at 155kts and full dirty it is right down the back of the drag curve? May well be, but I can't see that being normal ops to configure a machine intentionally like this?

"Comparing apples to oranges", I generally find that in the ballpark of a 30% power requirement for types I've flown will work pretty well for an approach power setting. Never anything close to "Level" power as outlined here.

These profiles are what is recommended to be flown in the 747-400 Simulator. Configuration for approach begins at 15DME, with flaps 1, speed limited to 280kts, then flaps 5 at 260kts, then flaps 30 at 180kts. The only thing that makes some sense is that this amount of flap has a huge amount of drag, and that with gear down, descending on the ILS, a power increase is required to even maintain 155kts??!!

Doesn't seem right, but we have no experience here...

[aycarumba]

if this is for cathay sim eval

Don't worry too much; they will give you another set of EPR for you to memorize when you get there

just remember the AOA and you will be golden (expect to fly as a single pilot)

[PointyEngine]

if this is for cathay sim eval

Don't worry too much; they will give you another set of EPR for you to memorize when you get there

just remember the AOA and you will be golden (expect to fly as a single pilot)

Yeah its notes from a Cathay Sim Eval

[xsbank]

You will not be single pilot, you will have a copilot. But you need to command him as he is voice-actuated and will not prompt you unless you instruct him to. They are evaluating your command ability amongst other things.

By the way, the '47 needs lots of thrust on the remaining three engines to maintain. Ask me how I know...

It would behoove you to know what EPR is, even if you just follow the chart and set it on a gauge. You will only need it for the takeoff...

[Bede]

JigglyBus,

EPR measures static pressure, not total (or dynamic) pressure so airspeed has no appreciable effect. Theoretically an increased speed could actually result in a lower pressure at the inlet pressure sensor.

[AuxBatOn]

Actually, EPR, by definition, is the ratio of total pressures...

[Wacko]

The 1.18 IEPR will be your target with flaps 30 on the ILS. If I were you, I wouldn't worry too much about the logic and more about being able to pull the attitude/IEPR out of your ass under pressure. I've seen guys drenched in sweat after the 30 min "ride".

It sounds simple but the 747 is a slippery aircraft, guys with actual hours on the plane hate flying it by hand.. which you will.

Good luck on your Cx journey.

[squash junky]

It sounds simple but the 747 is a slippery aircraft, guys with actual hours on the plane hate flying it by hand.. which you will.

My experience is totally opposite to that Loved hand flying it. I don't have much comparison as I have only flown Boeing, but comparing it to the 737, 767, and the 777, I liked the -400 the most. Gentle and easy.

I never had any experience with epr, just N1, but read some great explanations above that made sense.
N1 cruise 90% or so and for approach 56-58% and that goes for all the B's I've flown.
I think.
Retirement and wine is clouding my memory.

[cgzro]

Got to try the BA 747-400 simulator at LHR a while back and thought it was pretty nice to hand fly. Pretty fracking weird taxing around 20 feet in front of the nose wheel though Of course we were just horsing around at 3:00am.

[Meatservo]

guys with actual hours on the plane hate flying it by hand.. which you will.

Pretty much the exact opposite of everything every single person who has ever flown one has ever said, ever.

[RG500]

guys with actual hours on the plane hate flying it by hand.. which you will.

Pretty much the exact opposite of everything every single person who has ever flown one has ever said, ever.

Couldn't agree more, the B747-400 was a dream to handfly. Responsive and agile for its mass.
If you get the job on a Boeing, disengage the autopilot whenever opportune.

[Eric Janson]

Someone here might know the answer to this - its got us prop guys baffled.

On the flight profiles below, it has "Level" EPR of 1.16, and "App" at 1.18. Should the "Level" EPR be significantly higher than "App"? Or is there some relationship between airspeed and EPR that has gone over our heads?

I've never flown the 747 but I do fly large jets for a living.

Level flight will be in a "clean" configuration. 280 knots is probably close to the best lift/drag ratio speed.

Approach configuration is probably with landing gear down and full flaps. As other posters have said you are on the back side of drag curve. In this configuration you will need a lot of thrust to maintain speed.

Have a look on YouTube to see how much the wing shape changes from the clean configuration to the landing configuration.

Don't overthink this - just set the appropriate thrust for the conditions.

When flying manually (decelerating) add the thrust you need 10knots before you reach your target speed as you have a large amount of momentum to overcome. I normally fly target speed +5 knots when flying manually to avoid having to continually change thrust.

[Wacko]

Well PointyEngine, when is your sim eval? I think we'll all be interested in hearing about your experience!

[pelmet]

I'm not a heavy jet guy, but you heavy jet guys have seriously confused me.

You (jet guys) seem to be answering this OP by saying that "the thrust on approach would be just as high as in cruise because of all the drag".

Thrust wouldn't be as high on approach as cruise. The OP said 280 knots which is not a cruising speed. Like the most other jet airlines 280 is fairy slow. But when the gear is out(especially 5 gear) and flaps are at full, a lot of thrust is required.

[Kitzbuhel]

280 IAS at FL390 must be close to cruise no? About .8 .9 Mach

[all_ramped_up]

280 IAS at FL390 must be close to cruise no? About .8 .9 Mach

Mach 1 is ~570 knots at >FL390 so 280 is M0.49 actually.

[AuxBatOn]

280 KIAS at 390 is 514 KTAS. Speed of sound at 390 (ISA) is 573 KTAS. It means the IMN is 0.88.

Just curious if you fly commercially?

Edit: I see you are an AME with a PPL! You are forgiven

[all_ramped_up]

280 KIAS at 390 is 514 KTAS. Speed of sound at 390 (ISA) is 573 KTAS. It means the IMN is 0.88.

Just curious if you fly commercially?

Edit: I see you are an AME with a PPL! You are forgiven

Yessir. Make the fast ones go and fly the low 'n slow ones. heh

My use of TAS is very sporadic at best.