GB2193535A

GB2193535A - Gas turbine engine fuel control

Info

Publication number: GB2193535A
Application number: GB08718303A
Authority: GB
Inventors: Robert Wayne Rice; Nicholas Dean Lappos
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 1986-08-04
Filing date: 1987-08-03
Publication date: 1988-02-10
Also published as: DE3725687A1; IT1228547B; GB8718303D0; FR2602270A1; JPS6345429A; IT8721567A0; BR8701672A

Description

SPECIFICATION

Technical Field of the Invention Blow-away limits for turbine engines The invention relates to helicopters having a turbine engine(s) and electronic fuel controls for the engines.

Background of the Invention Electronic fuel controls for governing the operating conditions of a turbine engine are well known. Typically, their object is to maintain a preselected free turbine speed (output shaft speed) under a wide range of sensed operating conditions, both of the engine itself and the associated aircraft. However, in doing so certain constraints (limits) are imposed on various operating conditions of the engine itself, such as its inner stage temperature (T5), its

torque (Q). As a result of these limits (hereinafter referred to as "normal engine operating limits"), the ability of the engine control to maintain the selected free turbine speed is limited.In the context of a helicopter whose rotor is driven directly from the free turbine of the engine(s), the rotor loads imposed under certain flight regimes may overwhelm the ability of the engine to maintain the preselected free turbine speed (i.e., the preselected rotor speed) and, as a result, the rotor speed may droop (decrease below its reference speed). The ability of a helicopter to maneuver is limited in any case by its rotor speed (among other factors), and this limited ability is diminished by a droop in rotor speed. U.S. Patent No. 4,500,966 (Zagranski, et al, 1985), entitled SUPER CONTINGENCY AIRCRAFT ENGINE CONTROL, discloses increasing engine operating limits to supercontingency levels in an electronic fuel control in response to predetermined operating conditions, wherein risk of stressing an engine to its failure point is undertaken in favor of acquiring enough power to avoid a certain crash (see Abstract therein). The operating conditions discussed therein relate generally to the failure of one engine in a multi-engine aircraft and, more specifically, to operation of the aircraft within the avoid regions of the flight regimes (See Fig. 2 therein which shows "dead man" curves based on altitude and airspeed.). The Zagranski patent also discloses increasing the engine operating limits to a supercontingency level if the rotor speed has drooped below 80%, whether or not an engine has failed. Such a circumstance may exist when extremely severe maneuvering has been undertaken in order to avoid an obstacle or the like. In such a case, engine and rotor speed may droop so badly that continuance of the maneuver is either impossible or inadequate. In such a case, the fuel controls for each engine will provide supercontingency engine operation. As noted therein, the supercontingency limits should only last on the order of a few seconds (see column 8, line 60 through column 9, line 7), but even that is enough to require an engine overhaul at the end of the mission. The inventors herein have recognized a need to implement increased engine operating limits (hereinafter referred to as "blow-away limits") which are above normal engine operating limits but below supercontingency limits in response to a predetermined family of rotor speed conditions which are less severe than Zagranski's 80% . Disclosure of the Invention Thus, it is an object of this invention to implement blow-away limits in an electronic fuel control in response to certain rotor droop conditions so as to provide increased capacity for the fuel control to maintain a preselected rotor speed under certain well-defined abnormal rotor speed conditions. According to the invention, blow-away limits for the inner stage temperature (T5), the gas generator speed (N 1 ), and the output torque (Q) are automatically implemented in an electronic fuel control for a turbine engine if certain rotor speed (NR) conditions are satisfied. The rotor speed conditions, any one of which will cause the automatic implementation of the blow-away limits are: - if the rotor speed (NR) droops to a specific value; or - if the rotor speed (NR) droops a specific amount below a selected reference speed (NRREF); or - if the rotor speed is decaying (NRDOT) at an excessive rate for a specified time and the rotor speed (NR) equals the free turbine speed (NF) of the engine. Other objects, features and advantages of the invention will become apparent in light of the following description thereof. Brief Description of the Drawings Fig. 1 is a block diagram of the invention. Best Mode for Carrying Out the Invention Fig. 1 shows a simplified electronic fuel control 10 supplying fuel on a line 12 to a turbine engine 14. The turbine engine 14 has a gas generator 16 turning on a common shaft 18 with a power turbine 20, and a free turbine 22 connected via an output shaft 24 to an input module 26 containing a clutch which, in turn, is connected to a gear box 28. In the context of a multi-engine arrangement, the gear box 26 would also receive as an input the output shaft 24' of another engine 14' (via another input module 26') which is under the control of an electronic fuel control 10'. The gear box 28 drives the main rotor 30 of a helicopter. Appropriate sensors 32,34,36 within the engine(s) 14 (14') provide indications of the following engine operating parameters to the fuel control 10 (10'): - the sensor 32 provides a signal indicative of inner stage temperature (T5); - the sensor 34 provides a signal indicative of gas generator speed (N 1 ); - the sensor 36 provides a signal indicative of output torque (Q); and - the sensor 38 provides a signal indicative of free turbine speed (NF). The object of an electronic fuel control is to maintain the free turbine speed (NF) constant in relationship (determined by the transmission) to a preset, reference rotor speed (NRREF). To this end, an appropriate selection device, such as a rheostat 39 provides a signal indicative of the reference speed (NRREF) to the fuel control, and a sensor 40 provides a signal indicative of rotor speed (NR) to the fuel control. Any deviation therebetween (NRREF NR 0) is translated into an appropriate increase or decrease in the amount of fuel supplied to the engine in an effort to maintain the rotor speed (NR) at the reference speed (NRREF)' However, operating limits are imposed on the engine(s) to assure its (their) longevity.Typical limits imposed on a Pratt & Whitney PT6B-36 engine, two of which are employed on the Sikorsky S-76B aircraft (PT6B and S76 are registered trademarks owned by United Technologies Corporation), are: - for inner stage temperature (T5), 816[deg]C;

design speed (point) of the compressor; and - for output torque (Q), 100% of the design shaft horsepower at 100% rotor speed. These limits are referred to herein as "normal threshold limits". There are times, however, when it is desirable to operate an engine, briefly, beyond its normal threshold limits. For instance, during a single engine failure in a twin engine helicopter, it may be appropriate to operate the remaining engine at "supercontingency" limits. These supercontingency limits are well above the normal threshold limits and allow the remaining engine to deliver additional power so as to prevent a crash, albeit at the expense of engine overhaul. Under less severe circumstances, it is known to operate the engine(s) at limits only slightly higher than the normal threshold limits or a moderate period of time. For instance, it is permissible to operate the PT6B-36 for 2 1/2 minutes at the following limits without damaging the engine(s) or requiring an overhaul: - for inner stage temperature (T5), 850[deg]C; - for gas generator speed (N1), 101.6%; and - for output torque (Q), 136% . These limits, sometimes referred to in the context of a "2 1/2 minute rating", are referred to herein as "blow-away limits", and it is known to equip an aircraft with a pilotoperable switch 42 for selecting the blowaway limits (rather than the normal threshold limits) for such flight regimes as heavy (gross aircraft weight) or high (Barometric Altitude) landings or approaches to hover. Blocks 44,46, and 48 within the fuel control show the normal threshold, blow-away, and supercontingency limits in a hierarchical manner. The switch 42 maually selects the blow-away limits. The invention basically resides in automatically selecting the blow-away limits for flight regimes wherein the pilot may not otherwise have selected these limits and is readily implemented by one skilled in the art in a typical, microprocessor-controlled electronic fuel control. The selection of the blow-away limits and the reversion to the normal threshold limits occurs as follows for an S-76B aircraft. (As background to the example, whereas the S76B aircraft is always operated at 107% NR, the S-76A aircraft has a beeper to allow operation at rotor speed settings between 95% and 107%. Certainly, the S-76B aircraft could be similarly equipped with a rotor speed beeper.In the following example, it is as-

has been set to 107%.) If the rotor speed (NR) droops to 98%, then the blow-away limits are imposed (automatically selected) so as to make more power available for recovery from whatever flight regime caused the excessive (9%) droop in rotor speed. Reversion to the normal threshold limits occur when the rotor speed recovers to 102%, and rotor speed recovery (to NRREF) continues under the constraints of the normal threshold limits from 102% to 107% (NRREF)' If the rotor speed droops to 9% below NRREF (in this case, if it droops to 98%), the blow-away limits are imposed for the reasons given above.(9% is selected as being 2% more than the worst case droop of 7% expected for recovery from autorotation.) Reversion to the normal threshold limits occurs when the rotor speed increases to within 5% of NRREF, from which point on recovery continues from 5% below NRREF to NRREF under the normal threshold limits. (5% is selected as being 2% less than the worst case droop of 7%.) If the rotor speed is decaying at a rate (NRDOT) in excess of 10% per second for 0.2 seconds (typically, two 0.1 second microprocessor duty cycles) and the free turbine speed (NF) equals the rotor speed (NR), the blowaway limits are imposed until there is no more decay in rotor speed (NRDOT = 0), at which time the reversion to the normal threshold limits occurs. By examining the decay rate of rotor speed only if NR = NF, autorotative decays are eliminated as triggers for implementing the blow-away limits. The three scenarios described in the preceding three paragraphs all address events (flight regimes) which cause a defined droop in rotor speed under normal operating conditions. These scenarios are readily implemented in a software module (set of software instructions) 50 that is responsive to the rotor speed sensor 40, and which functions basically as a switch in parallel with the switch 42 (e.g., IF any of the three rotor speed conditions are present, THEN implement. the blow-away limits). It will be noted that the first two scenarios seem to be simply redundant, and for the example given they are. In a situation wherein with different thresholds selected, the first event (NR < 98%) could occur independently of and without the other event (NR < 9% below NRREF) occurring. The events contemplated by these scenarios are of a much less severe nature than an engine out (in a twin engine aircraft), or a rotor speed droop to 80% (see Zagranski's patent). Nevertheless, it is desirable to provide a little extra safety margin at these times without sacrificing engine longevity. It has been deemed unnecessary to implement a 2 1/2 minute limit upon the automatic implementation of the blow-away limits by the software module 50, since it is a virtual certainty that the pilot will have recovered from the rotor speed conditions that caused implementation of the blow-away limits well within the 2 1/2 minutes for which the engine can safely be operated at the blow-away (2 1/2 minute) limits.

Claims

1. An electronic fuel control (10) exercising control over the inner stage temperature (T5),

torque (Q) of each turbine engine in a helicopter, the rotor (30) of which is driven at a speed (NR) directly related to the free turbine speed (NF) of the engine(s), the fuel control striving to maintain the rotor speed (NR) equal to a reference rotor speed (NRREF) which is between a minimum rotor speed (NRMIN) and a maximum rotor speed (NRMAX), comprising: means (44) for limiting the inner stage temperature (T5), the gas generator speed (N 1 ), and the output torque (Q) of the engine(s) below normal threshold limits (A, B, and C, respectively) related to sustained engine operation; means (40) for sensing rotor speed (NF); and means (46,50) responsive to the rotor speed means (40) for allowing the inner stage temperature (T5), the gas generator speed (N1), and the output torque (Q) of the engine(s) to increase in excess of their normal threshold limits by replacing the normal threshold limits with blow-away limits (A', B', and C', respectively) in response to any one of the following conditions: a. if the rotor speed (NR) droops to a first value (NR1) between the minimum rotor speed

or b. if the rotor speed (NR) droops to or below a second value (NR2) which is below the reference rotor speed (NRREF); or c. if the rotor speed (NR) decays at a rate (NRDOT) in excess of a predetermined rate (R) and the rotor (NR) equals the free turbine speed (NF).

2. Apparatus according to claim 1 wherein: the range of normal rotor speeds is between a minimum rotor speed (NRMIN) of 95% and a maximum rotor speed (NRMAx) of 107%; the first value (NR1) is 98%; the second value (NR2) is 9% below the reference rotor speed (NRREF); and the predetermined rate (R) of rotor speed decay is 10% per second for 0.2 seconds.

3. Apparatus according to claim 1, wherein: the minimum rotor speed (NRM,N) is 95%; the maximum rotor speed (NRMAX) is 107%; the first value (NR1) is 98%; the second value (NR2) is 9%; the normal threshold limit (A) for inner stage temperature (T5) is 816[deg] Celsius; the blow-away limit (A[deg]) for inner stage temperature (T5) is 850[deg] Celsius; the normal threshold limit (B) for gas generator speed (N1) is 100%; the blow-away limit (B') for gas generator speed (N1) is 101.6%; the normal threshold limit (C) for output torque (Q) is 100%; and the blow-away limit (C') for output torque (Q) is 136%.

4. A method of controlling engine operating parameters in a turbine-engine helicopter, comprising: establishing a normal threshold limit (A) for the inner stage temperature (T5) of the engine(s); establishing a blow-away limit (A') for the inner stage temperature (T5) of the engine(s) in excess of the normal threshold limit (A) for the inner stage temperature (T5) of the engine(s); establishing a normal threshold limit (B) for the gas generator speed (N1) of the engine(s); establishing a blow-away limit (B') for the gas generator speed (N1) of the engine(s) in excess of the normal threshold limit (B) for the gas generator speed (N1) of the engine(s); establishing a normal threshold limit (C) for the output torque (Q) of the engine(s); establishing a blow-away limit (C') for the output torque (Q) of the engine(s) in excess of the normal threshold limit (C) for the output torque of the engine(s); operating the engine(s) within the normal threshold limits (A,B,C) under normal circumstances for sustained engine operation and a normal rotor speed, unless the any one of following conditions occurs: a. allowing the engine(s) to operate up to'

speed (NR) droops to a first value (NR1) between the minimum rotor speed (NRM,N) and the maximum rotor speed (NRMAX), but resuming engine operation within the normal threshold limits (A,B,C) when the rotor speed (NR) increases to a third value (NR3) between the first value (NR1) and the maximum rotor speed (NRMAx); or b. allowing the engine(s) to operate up to the blow-away limits (A',B',C') if the rotor speed (NR) droops to or below a second value (NR2) (D) which is below the reference rotor speed (NRREF)' but resuming engine operation within the normal threshold limits (A B,C) when the rotor speed (NR) increases to a fourth value (NR4) below the reference rotor speed (NRREF); or c. allowing the engine(s) to operate up to the blow-away limits (A',B',C') if the rotor speed (NR) decays at a rate (NRDOT) in ex- , cess of a predetermined rate (R) and the rotor speed (NR) equals the free turbine speed (NF) but resuming engine operation within the normal threshold limits (A,B,C) when the rotor speed is no longer decaying (R = 0).

5. A method according to claim 4, wherein: the minimum rotor speed (NRM,N) is 95%;

the first value (NR1) is 98%; the second value (NRZ) is 9%; the third value (NR3)' is 102%; the fourth value (NR4) is 5%; the normal threshold limit (A) for inner stage temperature (T5) is 816[deg] Celsius; the blow-away limit (A[deg]) for inner stage temperature (T5) is 850[deg] Celsius; the normal threshold limit (B) for gas generator speed (N1) is 100%; the blow-away limit (B') for gas generator speed (N1) is 101.6%; the normal threshold limit (C) for output torque (Q) is 100%; and the blow-away limit (C') for output torque (Q) is 136%.