Flat rated engine

[B738]

Can someone explain the principle of an flat rated engine?
A screening-question says: the thrust of an flat rated engine is
a) increasing or b) decreasing with altitude?

Thanks for the help!

[Old Smokey]

Jet engines essentially have 3 limits - (1) An internal pressure limit, (2) A turbine temperature (typically EGT) limit, and (3) An engine speed limit. The maximum thrust which may be used at Takeoff (and MCL and MCT in some circumstances) will be when the first of the 3 limits are reached.

Consider an aircraft at Sea Level on a very cold day. As the thrust levers are advanced, the maximum internal pressure would typically be first reached as the air is very cold and dense. The engine is well below the EGT and N1 limit. As ambient temperature increases, the air is less dense, and increasing EGT and N1 is required to achieve the pressure limit, although still below their limits. This is a situation where the engine is "Flat Rated", a rise or fall in ambient temperature will have negligible effect upon thrust, it is "Flat", with the thrust available being quite constant and governed by the internal pressure limit.

Ultimately, with EGT and N1 increasing to achieve the required thrust, the EGT limit will be reached, and as temperature rises further, Fuel Flow and thrust must be decreased. The engine is now in a declining thrust situation with thrust being governed by the EGT limit. This is known as "Full Rating".

Engine speed limiting Takeoff thrust is rare, it is usually a high altitude phenomenom. With decreasing density with increasing altitude, engine speed must be advanced to achieve the required thruct, MCL, MCT etc. When the engine speed is at a maximum, the engine thrust is governed by the engine speed limit.

Fairly typical, but by no means absolute, temperature limit to flat rated thrust is around 30°C. Below that temperature the engine is Flat Rated, above that temperature, it is Full Rated. If you happen to be operating the engine right at the ambient temperature limit for the flat rating, stress is at it's maximum, as the engine is simultaneously at it's pressure and temperature limit.

Regards,

Old Smokey

[Alty]

The question is ambiguous, but the answer is probably supposed to be "decreasing".

The explanation provided is sort of misleading if taken to the word. The flat-rated portion of the takeoff rating curve must also account for the thrust lapsing of the clamped-throttle engine as the airplane climbs out on takeoff. The engine control controls the fixed-throttle lapse as altitude increases - the N1 or EPR decrease with increasing airspeed as the airplane goes down the runway, then will increase as the airplanes climbs out. In the old days, the engine controls typically maintained a fixed N2 with fixed throttle, but design of lapse rates has varied over the years. The speed or temperature limits can be reached at any point in the takeoff up to some assumed point above the airfield - typically 4,000 ft. but sometimes less. The takeoff corner-point temperature is typically ISA+15C (i.e. 30C at sea-level) but can vary.

I would disagree that the flat-rated portion of the curve is up against a pressure limit, even when considering the lapsing. The flat is usually chosen to provide the full-rated thrust desired by the airframer, if possible. There is a big set of trades that come into play - and engine life/durability is one of the keys. Only when you get to the higher takeoff altitudes, with lapsing considered, do you really start to encounter limits such as rotor speeds.

[barit1]

I've seen several high-bypass engines where within the flat-rating range (say, ISA+15C and below), the TO thrust was held constant for increasing altitude. In other words, N1/EPR/temps etc. increased with altitude. This is a tradeoff by the manufacturer to meet certain customer requirements e.g. MEX-to-Europe.

On the other hand, if N1 etc. is not "bumped" at altitude, then thrust must decay as air density decreases.

So - to answer the quiz, you have to get inside the head of the guy who wrote the question, and assess how much you think he knew about the subject. My bet - the answer he expects is b).

[cwatters]

Does the thrust of any engine type increase with altitude?

[Intruder]

None that I know of...

The OP's original question is indeed ambiguous, though. A "flat rated" engine is mechanically or electronically limited at sea level and supposed to be [automatically] controlled so that at "full power" it maintains a constant thrust/torque/power output through the climb, UNTIL it reaches an altitude where it can no longer provide that output. Then the output will decrease with altitude.

[hawk37]

Alty, I'm a bit confused. If an engine is well within its flat rating, does thrust remain constant as it accelerates for take off? I'm thinking the anwer is no, that it decreases. But if Smokey is right, and the engine is neither at its limiting rpm, EGT, nor internal pressure limit, there seems no reason to program the engine to decrease it's thrust output.

Hawk

[mustafagander]

there seems no reason to program the engine to decrease it's thrust output.
Hawk

If engine thrust were not flat rated, imagine the chaos on a cold clear day with OAT, say 0*C and the a/c certification done at, say 25*C. The engine at full throttle would probably deliver considerably more thrust, so where is the relevance of certification trials and, more to the point, how are you, captain, going to control the beast when an engine fails at V1 +1kt. It may well be worth watching from a safe distance, though!!

IMHO, flat rating is about the engine manufacturers supplying the airframers with a known level of thrust and maintained to as high an OAT as possible so that they can do all their sums about control moments etc., etc. There's the rub, though, the engine gives more thrust than you really need at low density alts when it's not too critical and gives less at high DAs just when you need it most. Ain't life a pi55er sometimes?

[ElNino]

Fairly typical, but by no means absolute, temperature limit to flat rated thrust is around 30°C.

In the 146, it's only flat rated to a measly 15°C.
At 15°C at MSL, full thrust is 93.5%, any hotter or colder and the full thrust N1 decreases, for the reasons well put by Old Smokey. For example, at 0°C, full thrust is 91.3% and at 30°C it's 91.4%.

[barit1]

That's typical to have highest N1 at the "break point"; Below that OAT, the air is more dense, and the fan turns slower to produce the same physical thrust. As mustafagander points out, this helps control the assymetric yaw for OEI conditions.

Above the break point, N1 is rolled back along a line of approximately constant EGT for hot section protection.

[chornedsnorkack]

If engine thrust were not flat rated, imagine the chaos on a cold clear day with OAT, say 0*C and the a/c certification done at, say 25*C. The engine at full throttle would probably deliver considerably more thrust, so where is the relevance of certification trials and, more to the point, how are you, captain, going to control the beast when an engine fails at V1 +1kt.

Look at it this way: if the plane is at a low density altitude, then the thrust of a Full Rated engine increases with air density. But so do aerodynamic forces on fins and rudders at a given airspeed!

Thus, the more asymmetric yaw an engine out produces, the more compensating authority is available for a given airspeed.

However, it seems that the mechanical strength of the engine, engine mountings, wings, rudder and fin is not improved by low temperature or low pressure altitude...

[mustafagander]

I think you might find that thrust increases disproportionately.

[bobmij]

Gentlemen,
To check my understanding of the flat rating concept,
If we say my engine is flat rated to 30 degrees, then effectively I can acheive rated thrust up to and at this OAT. The engine will absorb more fuel as I chuck it in to overcome the power loss through decreseasing density until, at 30 degrees, the extra fuel and heat gained adiabaticaly through compression, threaten to melt the turbine, cans or other critical part. At this point we see the graph start to drop off in thrust with increasing temperature as the FCU and FADEC do their best to keep the engine within design parameters by reducing fuel flow.
I assume then that flat ratings are also specified with barometric altitude in mind (probably ISA 1013.25Hpa) .
Have I got this right?:confused:

[barit1]

At least two different ways altitude can be handled:

1) Let thrust fall off in direct proportion to reduced air density - i.e. maintain the SL N1 or EPR values. Thus EGT(ITT) is little or no worse on a high altitude TO than at SL.

OR:

2) Boost N1 or EPR to maintain thrust up to some target altitude. EGT(ITT) will increase accordingly.

Most of the engines I have dealt with use the latter method. It is a negotiated power management between the manufacturer and the customer to keep from losing too much TOGW, balanced against increased engine wear & tear. :8