747 Takeoff and cruising power comparison 1

[TPL]

Can anyone give me some pointers as to the power required by a 747 powered by RB211 engines for takeoff, compared to the power required for the same aircraft to maintain a typical cruising altitude/speed.

I know there are lots of varaibles but I am looking for ball park figures

thanks in advance

 

[cedarbluffranch]

TPL,

Take a look at this link:

http://www.airweb.faa.gov/Regulator...97593c2f5b57b86256b35005d599a/$FILE/a23we.pdf

The RB211-22C engine will generate 41,030 lb of thrust on takeoff (5 minute maximum) and 36,900 lb of thrust on continous duty.

I can't say how much the 747 airplane actually requires but that is how much the engines can generate in flight. Since weight is a huge issue on airplanes, you can bet the engines aren't sized any larger than necessary, so they probably can use the full take off and crusing thrust if flying under worst case scenarios.

 

[RPstress]

On a non-RB211 jet engine (about 23,000 lb thrust) I've been looking at recently the cruise/max-TO is about 0.65 in terms of thrust, which may not quite equate to power.

NB: cruise isn't necessarily "max continuous".

 

[swearingen]

I spoke to my uncle last night (who happened to be sitting in the left seat of a 737-800 on a ground delay at the time) and he said his settings for the flight he was on were as follows:

94% takeoff power
96% climbout power
87% cruise power

He said the power settings do vary depending on flight load, but he said he wouldn't see below about 80% power at cruise.

Naturally, for noise abatement over certain cities, the climbout power will be less until they get over 10,000ft.

I stand corrected on my first post, but the point is that aircraft jet engines do not get throttled to a much lower position while at cruise.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[Surestick]

How is that power measured?

Correct me if I'm wrong but I'd guess 100% of max power at ground level is going to be more that 100% max at 30000ft due to air density.
80% max at cruise altitude might be significantly lower than 80% max at ground level.

 

[GregLocock]

Various ways you could measure it

80% max EGT

80% max fuel flow

80% max rpm


I'm sure there are others.


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TheFeds]

Consider asking at airliners.net, in the forum here:

http://www.airliners.net/aviation-forums/tech_ops/

There are plenty of aircraft pilots and mechanics on that forum who could discuss operation and maintenance.

 

[j79guy]

TPL: I feel for you. I worked 18.5 years for a compression utility, running GE and RR aeroderivitive units, before starting my own turbine overhaul business. I too have encountered the same consultants, and yes, you do have a fight on your hands. However, it comes down to a few key points, most of which have been presented by the previous contributors here on this thread. Industrial RB211 units typically operate steady state at approximately 95% of aircraft "maximum". Contrary to popular belief, and as you know, industrial engines have to built to a higher acceptance standard than aircraft engines, typically with vibration and oil consumption limits 60% that of aircraft acceptability limits. (I'll stand beside an industrial turbine any day. Some flight engines flat out scare me.) To get seven years of reliable operation from an industrial RB211 would entail reducing the firing temperature, to allow the hot section to live. (There goes thermal efficiency.) If you continued to operate at full rated power, you will eventually liberate an HP or LP turbine blade, due to tip shroud abutment face wear. Blade pairs will "lock" together at the tips, dropping the flexural frequency down into the operating excitation range of the engine, tearing the pair out. This is especially pronounced with the can-annular type engines, such as Avon and Spey, but present to a degree in the RB units as well. As you know, getting 30,000 fired hours out of your units is actually pretty good. More than a few operators wish they could get that many hours out of theirs. The Trents certainly won't do it. (Nor LM6000, or LM2500+, or........) Make your case, get backing from Rolls, and make sure you politely let your managers know that THEY have to assume responsibility down the road, if thet adopt a seven year overhaul plan. Also, I bet your insurance underwriter would have a field day with this one, as they generally hold the trump card in the industrial turbine business. (Your risk assessment team is sitting mute on this one?)
Good luck.
j79 guy

 

[dicer]

So could someone explain why there would be a desire to use turbines for any application, when a CI recip engine will beat it in fuel conservation, and in inexpensive as compared to jet/turbine overhauls as well as initial cost?

 

[rb1957]

i guess there are times when costs are 2ndry ... the M1 tank is powered by turbines, many small navy ships (destroyers) are powered by turbines. i suspect the power density of the engine may be relevant in these cases.

for industrial apps, i don't know ...

 

[j79guy]

dicer; Recips are indeed the engine(s) of choice in industrial applications, up to about 2000hp. Above that, recips just get too darn big. This thread is specifically for natural gas compression turbines, in the 32,000 hp class. Gas flows vary greatly hour by hour, and often the engines have to start/stop on minimal notification. heavy frame type engines are not condusive to this operation, and like to run steady-state. Another consideration is in NG compression, you have practically free fuel...........
In NG compression, any pipeline 20" or bigger, or requiring 2000+ Hp, a turbine will be usually be spec'ed. When varying gas flows are the norm, aeroderivitive turbines will be used.
If an engine fails, an aeroderivitive turbine can be replaced with a fresh unit within 8 hours on-site, give or take.
A 32,000 Hp. recip would take how long to repair/replace?
rb1957 is bang on. Power density.

j79 guy

 

[kontiki99]

You might try looking in the Aircraft Flight Manual for an aircraft equipped with those engines. It will contain all sorts of performance data.

Any more most airlines have the crews collect a few minutes of engine performance data every flight. If you have a connection, maybe somebody will share the continuous health data on a couple engines.

 

[mohr]

Hi TPL :

To speech about power under operational point of view is not
practical. We use EPR a relation between exhaust pressure vs.inlet pressure ( pt2/pt1). This relation will give you an idea of "power" and was used by RR, P&w, GE.

Suppose you are taking off with MTOW and the real value of EPR for the engines will be 1.66 for RR 211-524-D4X. As OP Manual indicate,the needed EPR at the optimum initial cruise level,will be 1.64. The value is very similar to take off EPR.

Suppose you are taking off with 330 tons, your TO EPR will be aprox 1.58, the needed EPR at the optimum initial cruise level will be 1.68. This value is nearly the same to all maximum cruise EPR for the optimum initial cruise level.

Conclusion: You can expect nearly the same EPR at the opti-
mum initial cruse EPR ( at M=.84 and LRC )for all T.O. weights ( 1.66 to 1.68 ).

The comparison of T.O, EPR and Cruise EPR and is from 0%
( 370 tons ) to 10% ( 280 tons, reduced T.O. EPR ).

Those values are for B-747-287 series, with Rolls Royce
R 211-524-D4X.( the best airplane ever made !! )

Cheers

 

[DavidLeyman]

rb1957 and j79guy are right. Someone has decided that you are the pigeon on the branch ready to be shot down if it all goes wrong. Get the risk assessment team and the insurance people involved. Get everybody you can find involved.
On the face of it, any engine that has to be run at steady state (here they mean at a steady load configuration) will last longer than an engine that is cycled hot/cold, high/low load.
BUT
The usage of your engines is completely different. Your situation is different. A lot of engines on rigs are sucking in seawater as spray - you get very little sea spray at 36,000'! Trying to make comparisons is really futile.
Speak to RR at East Kilbride. They know; they are experts. They make them, service them, research them.
Bean counters are dumb. Hit thm with every fact you can find - they once told us that, at 16 students per class, we only needed three instructors! For several disciplines of avionics, airframe specialists, propulsion specialists, maths and physics.... This is not to mention that some people were doing Aeroplanes 1 and others were doing Aeroplanes 2.
Go for safety in numbers - flock together like small birds confusing an Eagle. Then dump everything on them and say "when this all comes to a complete clusterf*** it will be you fault. Put it in writing.

 

[KENAT]

Skimmed the thread sorry but something else to consider that may be simple enough for your PHBs.

Although it's been a while since I did A/C ops I (think I) remember that a long hauler such as 747 is only flying for about 13 hours a day on average. Obviously the engines runs a little on the ground but simplistically the engines are running maybe 60% of that 7 years.

.6*24*365*7=36792

Then so about 20% longer than your service interval.

I believe operating hours are more significant in your operation than calander days.

So now the question of why not at least extend out your servicing 20% more.

This is where your differeng service conditions etc come in.

At the end of the day what others say about following RR guidelines etc. is the real answer but I appreciate that the combined intelligence of management & consultants might struggle with this.

Something you may be able to strong arm management, does exceeding the servicing intervals invalidate any warranties or safety regs etc?

I thought this might be useful but on closer examination I'm not sure

http://www.tpub.com/content/aviationandaccessories/TM-1-1510-223-10/css/TM-1-1510-223-10_298.htm

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?