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Flat rating and de rating

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Flat rating and de rating

Old 22nd May 2017, 05:55
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Flat rating and de rating

There are several turboprop and turbofan engines on current transports that
are flat rated or de rated.

For example the CF6-80 engine on the C5M is de rated to 50,000 pounds of thrust.

Back tracking a little, I understand that flat rating limits power at sea level but allows this same power to be maintained to a much higher altitude.

Two part question, does the de rate on the C5M allow, like flat rating for sea level thrust to be held to a higher altitude and how is this actually achieved as the aircraft climbs ?
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Old 22nd May 2017, 21:13
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I usually see "flat rating" associated with turboprops.

Turbofan engines are often derated to extend engine life and increase allowable tolerances before hot section overhaul is required. Our 744s have B1F and B5F versions. Most of the B5Fs have been derated to B1F to decrease maintenance costs.

The CF6 engine will maintain rated or selected thrust after TO as long as possible (or until a CLIMB thrust is selected). The EEC does it by adjusting N1. Since a lower CLIMB thrust is usually used, "flat rating" doesn't have a lot of practical application in airline practice. The Air Force may treat the engines differently...
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Old 23rd May 2017, 03:09
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A flat-rated engine achieves rated thrust at sea-level well below the maximum internal temperatures (ITT). Therefore, that same thrust can be achieved to a high altitude before reaching the limiting ITT ("temping out"). Achieving the same thrust output for a good portion of the aircraft's altitude capability is where the 'flat' term comes from from: the thrust output is 'flat' until a certain density altitude, at which point the thrust curve starts to fall away.

A de-rated engine simply lowers the thrust rating achieved at sea-level.

These two terms are not mutually exclusive.
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Old 23rd May 2017, 06:04
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Interesting, the King Air 350 uses a PT6 that is rated at 1200SHP but flat rated
to 1050SHP as a structural limit.


So the full 1050SHP can be maintained to higher altitude, how is this achieved
with the turboprop engine ?
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Old 23rd May 2017, 06:18
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If it's anything like the older King Airs that I'm familiar with, each power lever simply controls a N2 governor. So the pilot sets power for max torque, (which takes, say, 75% forward on the power lever, and some intermediate N2 and ITT values) and as he climbs, it takes more and more forward power lever (along with associated higher N2 and ITT) to maintain that torque. At some altitude, he hits the ITT limit and from there on starts losing power.

Last edited by Vessbot; 23rd May 2017 at 06:30.
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Old 23rd May 2017, 19:15
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It's been awhile since I was in the C-5 program, but my recollection is that the engine was a 58,000-60,000 pound thrust flat rates to 50,000. The reason for the flat rating was to maintain a relatively low Vmcg and 50,000 pounds was sufficient to meet the required take and cruise performance. One parameter, OBE'd, was gross weight take-off at LETO at 35c. Increased climb made much of the fuel savings.

GF
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Old 23rd May 2017, 19:34
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The B1F version in the 744 is 56,500 lb thrust; the B5F is 62,100. I don't know if there are any versions more powerful than the B5F.

Even the B5F version seldom reaches anywhere near max EGT on takeoff...
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Old 23rd May 2017, 19:54
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Originally Posted by Intruder View Post
Even the B5F version seldom reaches anywhere near max EGT on takeoff...
Exactly... that's the 'flat-rating', even though nobody really calls it that. That same thrust is attainable at higher density-altitudes because there's still 'room' left before reaching max EGT.

Also, the 767 uses B6F and B7F versions. Not sure if there are any higher than that.
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Old 23rd May 2017, 22:44
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The -80C2 gives plenty of takeoff thrust - perhaps even approaching a C-5 airframe structural limit. The bigger benefit I'm told is in ROC and the ability to climb directly to an efficient cruise altitude. Well worthwhile on that count.

Reduced noise is also appreciated, helping avoid curfews at civil or mixed-use airfields.
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Old 24th May 2017, 00:28
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Barit1,

It has new pylons has it was not possible to modify the old due to structure. There was some structure strengthening in the crown area above the troop and rudder/fin.

Correct on the climb performance being the primary component of fuel savings with improved fuels specifics being a minority share. The wing loading was notably light compared to the B747 and it was always thrust limited in cruise. Now it can cruise easily in the mid-upper thirties and F410-430 at the end of cruise. A plane that could struggle to fly ETAR to the East Coast with a load now does it easily and can do Incirlik to Dover with that load.

Interesting Vmcg story. L-M discovered that it could not have the same Vmcg at the higher thrust rating, so they proposed assymetry protection with cost and delays; a 5% reduction on 1 & 4 which could have met the performance and Vmcg numbers or a sympathetic reduction when the opposing engine lost a certain N1 reading. I think they went with the last answer as the AF generals were opposed to not getting the contract thrust even if it did provide the performance in the contract. Interesting times.

Last edited by galaxy flyer; 24th May 2017 at 16:41.
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Old 24th May 2017, 10:13
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Originally Posted by TangoAlphad View Post
I'm going to go for a horrible generalisation here...
So could we say that a type which may be considered under powered or at least, not the pokiest machine, be flat rated to allow you to achieve the most out of it within the reasonable temp range and types with a more comfortable excess (i.e modern transport jets etc) de-rate?
Not really.

Like everything engineering related, this is about compromises, and not paying for benefits that you don't really need.

Flat rating boils down to this:

You engine is limited by (a) thermodynamics (Basically, when are you going to melt the turbine) and (b) mechanics (when is something going to break).

Thermodynamically you can keep on getting more thrust if the OAT is lower. But in order to utilise it you have to beef up everything structurally. This means you are going to be carrying extra mass. Always. Including when it is hot outside.

So, with flat rating you might loose the opportunity for extra thrust, but it is on cold days when everything else perf wise is working in your favour anyway.

Powerplants aren't my area of expertise but pretty much everything modern is flat rated (I'm sure someone will correct me). There is simply no engineering reason not to (unless it is some upgraded legacy design).

Simply say "we need this much thrust at this temperature. Do the mechanics for that much thrust and flat rate below that temp" Then if your jet can take off from main base at Max Mass at that temp, it'll be able to do it at every lower temp as well.

Whereas your generalisation is more about de-rating rather than flat rating. If your jet is 'underpowered' shall we then you won't get to do a reduced thrust take off as much or as often.

I thrust everyone else can correct me if this is all a load of bollocks.
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Old 24th May 2017, 18:11
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Interesting Vmcg story. L-M discovered that it could not have the same Vmcg at the higher thrust rating...
Not the first time. I recall this being an issue on DC-8 re-engining with CFM56. Don't recall the solution specifics, but with over 100 aircraft eventually converted, it seems to have been successful!

Last edited by barit1; 26th May 2017 at 00:09.
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