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hu
18th Feb 2002, 08:13
I have read some engine related papers, there is a term-"Flat rated thrust" coming up so often. They do have some explanations about it but too technical, anyone can help me out with some simple way or using plain language!

john_tullamarine
18th Feb 2002, 10:31
Some of these terms are used a bit loosely and it would be helpful if you either cited, or quoted from, your reference so that the usage is seen in context.

There are at least three applications of the term which you might come across ..

(a) A simple mechanical limitation restricting thrust output so as to achieve a maximum power rating as might be seen in a turboprop installation

(b) A manufacturer election to downgrade the maximum thrust output of the engine, ie use a bigger engine where a smaller one would do the job .. often to improve the higher OAT performance

(c) If you look at a typical thrust carpet in a Flight Manual performance chart, the basic appearance of an individual altitude line is of two straight-ish lines which meet at a point somewhere near the higher OAT end. (The line might well be a little more complex than this .. but this is the simplest presentation.)

(i) the more steeply sloping line to the right (higher OAT) of the intersection usually represents the thrust (fuel) reduction necessary to remain within engine temperature limits.

(ii) the intersection usually is called something like the "thrust break temperature"

(iii) the shallower sloped line to the left (lower OAT) of the intersection usually represents a mechanical limit within the engine which precludes operation under temperature limited conditions. This section of the operational envelope often is referred to by the term "flat-rated" which is a reference to the comparatively small variation in thrust within this OAT range .. the line itself is not all that far from being horizontal.

[ 18 February 2002: Message edited by: john_tullamarine ]</p>

barnaby
19th Feb 2002, 00:04
Basically and in a nutshell. A flat rated engine is one that is restricted to a maximum thrust even though it is capable of producing more due to lower ambient temperatures than ISA, higher density, etc. Therefore, at temperatures below the temperature at which the engine is flat rated to, the thrust is not affected by temperature. A flat rating is normally ISA +15 degrees celcius.

In other words, from low temperature, all the way up to 30 degrees celcius you get the same amount of thrust.

Hope that helps.

hu
19th Feb 2002, 05:04
Hi John,Barnaby:. .Thanks for you guys explanations, that was a big help!

[ 19 February 2002: Message edited by: hu ]</p>

Alex Whittingham
19th Feb 2002, 12:34
If you are talking about the JAA exams Barnaby's explanation should suffice. Does anyone know why the engine is flat rated at temperatures below ISA+14 or ISA+15? I have never been able to find a complete explanation. The best I have so far is this:

Engines can be limited by their internal temperatures, usually TGT, or by an RPM limit. If the outside air temperature is high, say 45º, then the first limit you hit as you advance the throttles is the TGT limit. If the OAT is lower, perhaps 35º, you can wind more RPM on before you hit the TGT limit. In this range of temperatures thrust increases as temperature reduces.

There comes a point, though, as temperature continues to reduce and RPM is increased where you hit the RPM limit before the TGT limit. From this point on the maximum thrust you can wind on stays constant and RPM limited.

Harrier pilots have an engine limit they call NF root theta. As explained to me, NF is the speed of the fan and 'root theta' is the square root of the temperature in degrees absolute, a Mach number limit. This limit kicks in at low temperatures when the speed of the fan is so high it creates shock waves in the engine. Could this be what causes the RPM limit in other high bypass ratio fans?

Keith.Williams.
19th Feb 2002, 23:16
Hu,

I suspect you are referring to statements found in engine and aircraft manufacturers' brochures, such as "The ……. engine will produce 80000 lbf static thrust, at ISA msl, flat rated to ISA + 30 degrees C." To understand what this means we must first consider the factors that affect the thrust output of an engine.

Thrust is proportional to the mass flow of air passing through an engine. For any given engine this depends upon RPM and air density. Air density is related to static pressure and temperature, such that increasing altitudes and temperatures, decrease both density and thrust output. An engine in as-new condition, running at a given RPM, might produce 80000 lbf of static thrust at sea level in ISA conditions. But if it is used at greater altitudes or temperatures, it will produce less thrust at that RPM. This reduction in thrust will reduce the maximum mass at which the aircraft can take-off.

The obvious solution is to open up the throttles a little bit further so that the engine runs at a higher RPM, restoring its original mass flow and thrust. This option poses two problems: Firstly it means that a different power lever position is required for every possible combination of airfield altitude and temperature. The probability of every pilot getting this right for every take-off is (as they say in all the best mathematics books) vanishingly small. Some will take-off with too much thrust, while some (will fail to take-off) with too little.

The second problem is that opening the throttle a little bit more will also increase the turbine inlet temperature. While the life of a modern engine might be 20 or 30 thousand hours at normal temperatures, increasing turbine inlet temperature sufficiently will reduce this to matter of seconds! The effects of lesser levels of abuse will be less dramatic, but will still increase the probability of catastrophic failures. What is required is a system whereby the pilot puts the power levers in the same position for every take-off and the engine produces the advertised level of thrust without over-cooking the turbines.

The first part of the solution is (as JT said in his earlier post) to fit engines that are more powerful than is really required. If for example we want 80000 lbf of thrust in msl ISA conditions, we might use an engine that can actually produce 90000 lbf. We can then add a control system that senses the ambient pressure and temperature and calculates the EPR (or N1) required to give 80000 lbf thrust, and the turbine temperature that this will produce. When the power lever is set in the take-off position, the system opens the throttle while monitoring EPR (or N1) and turbine temperature. If the required EPR is achieved before the temperature limit is reached, the engine will produce the required 80000 lbf thrust. But if conditions are such that the turbine temperature limit is reached first, then acceleration will stop at a lower thrust. In the case of the brochure referred to above, the engine will give 80000 lbf of thrust at msl in all temperatures up to 45 degrees C. The extra thrust capability also offers benefits in terms of airfield altitude. With 10000 lbf of thrust is hand the engine described above could clearly still produce the required 80000 lbf at some altitude above msl, provided the temperature isn't 45 degrees C.

The engine manufacturers produce a performance chart indicating the relationship between altitude, ambient temperature and thrust. This typically has altitude on the vertical scale and temperature on the horizontal scale. In the example above it might include a box going from msl up to perhaps 8000 ft, then horizontally to 20 degrees C, then down to 45 degrees C at msl. In ambient conditions within this box, setting the power levers to take-off power will produce the same (flat rated) 80000 lbf of thrust. Outside the box (higher, hotter or both) the thrust will be reduced to avoid exceeding the limiting turbine temperature. So thrust is flat rated within the box and dependent upon ambient conditions outside the box. This aproach is equally applicable at other power settings.

This system provides the following advantages:

a. The aircraft can take-off at maximum mass in a wide range of temperature and altitude conditions.. .b. The pilot always sets the power levers to the same position for take-off.. .c. Engine life is increased because under most conditions the engines operate well below their limiting turbine temperature.

john_tullamarine
20th Feb 2002, 04:38
Alex Whittingham . . . .The normal RPM term (N) in an engineering analysis of thrust output involves sqrt(temperature). To make the numbers a little more sensible, it is usual to reduce the value to a reference sea level condition by introducing standard sea level temperature into the equation. This changes the presentation by using the temperature ratio, normally designated theta. I am not sure that mach number is relevant generally .. I suspect that the RPM limit more usually is a blade centrifugal loading/tip clearance consideration.

As to where the thrust break temperature might be for the advertised flat rating, this will be solely a matter of commercial and technological compromise both in the flexibility offered by the engine manufacturer and desired by the airframe manufacturer. There is no significance, per se, in any particular deviation temperature.

. .Keith presumably has a RR background .... ? The alternative throttle setting procedure seen often with US engines is to predetermine the relevant limits and then set the throttle (power lever or whatever term you might prefer) to achieve the relevant limiting condition.

Killer Shark
20th Feb 2002, 07:16
For a constant engine temperature, you will get more and more thrust as ambient temperature reduces.

On a cold day, engines generally produce a whole lot more thrust than the (commercial) airframer really needs.

There at least two good reasons why the airframer might want to limit his max thrust by "flat rating":

1. Minimise minimum control speeds (Vmc), i.e. help the rudder in its fight against asymmetric thrust in an engine-out case, by defining an artificial max "flat rated" thrust that is not exceeded.

2. Minimise structural loads on the pylon/thrust attachments, i.e. why have massive thrust on a cold day, which you'll probably use very rarely, when you're going to need big, heavy, over-engineered lumps of metal to make sure the attachment is strong enough?

I'm sure there are other reasons, but generally why burn up the engine on a cold day when the extra thrust gives you more problems than benefits.

The temperature below which thrust is flat rated varies significantly from aircraft to aircraft, but is generally between ISA and ISA+20C.

Alex Whittingham
20th Feb 2002, 18:47
Thanks JT, that seems a better explanation for 'root theta'.

Keith.Williams.
20th Feb 2002, 22:59
Alex,

I think the Pegasus problem you refered to is one of high altitude HP compressor surge. In the case of the Pegasus it is (or was some years ago) referred to as NF Root Theta surge. As altitude increases at constant NF, the air mass flow decreases while the HP spools RPM increases. This increases HP compressor ratio to the point where it surges. The old Mk 103s and 104s used a hydro-mechanical pressure ratio limiter in the fuel system, to limit maximum HP pressure ratio a above 10000 ft. The test was done at 0.8M at 40000 ft.

JT,

I must confess I was thinking only of the types where FADEC moves the throttle vavle without moving the power levers in the cokpit. You are of course correct that in some systems the autotthrotle (or pilot) moves the levers. But whichever method is used, it seems to me the overall effect is the same. There are two limits, thrust and turbine temperature, which must not be exceeded. Whatever control method is used, the engine is throttled up until it hits the lower of these two limits. By having a surplus of thrust capacity, it is the thrust limit that is reached within the range of ambient conditions for which the engine has been flat rated.

[ 20 February 2002: Message edited by: Keith Williams. ]</p>

GotTheTshirt
21st Feb 2002, 03:15
Basically the jet engine is a heat engine - the hotter it gets the more thrust it produces. The trade off is that the hotter it gets the shorter its life.The more you open the trottle, the more thrust you get, but this is balance against useful life. The RR Spey in the BAC 1-11 had a much better life than the Spey installed in the Phantom! . .The aircraft manufacturer is looking for a thrust to meet his certification requirements.. .As much of the marketing is aimed at operations above ISA the perfomance is generally rated to +15C although early in the engine developement this can be lower.. .The certified thrust of a particular mark of engine is found in the Type Data Sheet with the maximum ISA temperature at which that thrust can be maintained ( flat rating), above that temperature the thrust falls off. As an example the Type Data sheet for the Rolls Royce RB211-22B-02 shows the engine as producing 41,030 thrust pounds up to ISA+13.9o. This is the flat rating for the engine. Anything from 0 to ISA + 13.9 produces 41,030 lbs and above that the thrust falls off. The -524D4 engine produces 51,980 lbs up to ISA +15o. .Originally, different thrusts meant a different model of engine but it is now common for the same model and variant of an engine to have different thrust ratings even for engines with no physical changes. These are refered to as -A thrust or -B thrust etc. The significant difference is that engines with the same internal parts (e.g. turbine disc's) have a different life on these parts, depending on the thrust level that the engine is operated to.. . <img src="wink.gif" border="0"> <img src="wink.gif" border="0">