60-GTHYD-7 STALL CONTROL S. Drabek Compressor stall has had an increasing effect through stall. The first trace shows how normal op eration may be the years upon gas turbine controls. The general problem interrupted by a surge or st al l. The frequency for surge was reasonably well known in the first decade of jet en­ may fall below S cycles per second for very large engines gine history after the "Whittle Engine". The scheduling and above 20 cycles per second for very small engines. appro ach to the control of compressor stall established This is faster than many control systems in use today. Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1960/79955/V001T03A006/2389346/v001t03a006-60-gthyd-7.pdf by guest on 02 October 2021 during this time has become rooted throughout the industry. Stall frequencies are about an order of magnitude faster. On the other hand, an idealized approach based on sens­ In general, this is beyond the speed of practical aircraft ing incipient stall remains an intriguing challenge. engine type sensors alone. During the early 1940's, the engine opuator was pro­ If stalls and surges are to be avoided some advance vided with a direct manual control of fuel flow. If he ex­ signal must be found in the compressor as indicated on ercised his demands slow enough, he could always avoid the second trace of Fig. 1. Using a very fast control on stall. This simple so lution to stall quickly evaporated. this signal should make possible recovery acticn before The pilots demanded fast thrust re sponse to make the jet serious aerodynamic and thermodynamic degeneration react more like the old propeller driven fighter airplane; occurs. If such a signal is not available, all that is increasing range of flight conditions and maneuvers plus possible is a recovery from stall or surge in a reason­ addition of afterburner transients further aggravated the ably short time and in an automatic manner. The third compressor's sensitivity to stall; and the customer de­ trace shows the nature of recovery that is possible cided that engines should be accelerated much faster in after surge has started. Using arbitrary time values, the keeping with effective military operations. These re­ signal develops an adequate magnitude to be sensed in quirements developed rapidly to push American controls about 10 milliseconds fo llowed by fuel control action of designers away from the automatic controlling of the about 30 milliseconds and a fuel transportation and com­ maximum rate of throttle motion which has been favored bustion lag of about 40 milliseconds. If the surge cycle by the British and towards what was hoped to be much time is more than the total of these three values then a better control performance. second surge cycle may be avoided. No known control Two generally ac cepted practices developed fro m has succeeded in reducing surge to less th an one cycle. these requirements. One is that the engine and its com­ If surge is fundamentally irreversible, then a practical pressor must be so designed that it could operate with limit for speed of response can be established for each acceptable performance under al l req uired conditions engine system. without producing any stalls which would adversely af­ Several requirements must be met to completely avoid fect the performance of the aircraft. The compressor and stall and surge. Over the past decade, one or more of cycle designer carry the primary burden for this req uire­ these requirements has proven to be the stumbling block ment. They must convince the mechanical designers to the successful completion of several proposals, (with customer approval) to incorporate all features re­ studies and actual laboratory investigations. The techni­ quired for stall-free operation. It has probably happened cal requirements are listed on Fig. 2. First - there must more than once that an airplane has been considered exist a reasonably small number of signals covering all "unacceptable" bec ause the engine experienced com­ performance deteriorating stalls for any given compressor pressor stall under some flight conditions even though design. l\lany of one type of signal or several of various the control was holding the engine at es sentially steady types could introduce a complexity which would result in state conditions. The second practice is that controls unfavorable cost, weight and reliability. Next - these applied to these engines would so schedule its vari­ signals must be clearly discernible and distinctive. The able parameters that this performance could be achieved sensors must be able to readily distinguish these signals without experiencing stall. The controls designer is from stray signals and background noise in the compres­ thus primarily concerned with preventing the compressor sor and in the control system. Also , the signals must not from operating near tho se regions where there is be­ be duplicated under completely acceptable operation lieved to be a sensitivity to stall. He concerns himself, where there is no sensitivity to stall unless such opera­ as doe s thi s paper, with stalls that have significant ef­ tion is readily di stinguishable with simple hardware. In fect on engine operation and performance and treats addition - these signals must be sensed enough in ad­ them in a "practical", generalized manner. vance of stall to permit (a) the signal to be transmitted It has been a fond hope of many engineers that some­ by the sensor to the control, (b ) and there converted into how we wo uld obtain a very thorough understanding of an output signal and transmitted by the control to one or stall phenomena and devise a control that will sense an more actuators, (c) which in tum must move to correct the anticipation of stall but prevent its occurrence. These engine configuration (adequate change of fuel valve posi­ proponents hoped for gains of shorter development time tion or compressor bleed valve position, etc.) and (d) the and a maximum capability to experience transient con­ engine to change its thermodynamic cycle conditions ditions without stall. It has been referred to as an (following fuel lags, combustion lags, etc.) to maintain optimum control method because each compressor could stall-free operation. be operated up to it s individual, inherent limits presum­ The limiting item in the response of a control system ably under any and all conditions. is usually the output actuator. The inertia of both the Figure 1 shows three generalized signal traces of a actuator and the mechanical valve, such as a jet nozzle, point in the compressor that may be useful in controlling are fr equently very high. In addition, effective action 23 Copyright © 1960 by ASME - -- �--- - --- ------- •CENERA LIZED PR £SE/./TA T!ON OF .TECH!VICA L R£QUIREMENT.5FO R S!ICC£ .J5 - e CENERA LIZED PR ESENTA TI ON OF STALL -SURGE CONTROL SIGNALS F!IL STALL ANTICIPA TION CtJIVTROL ST'ALL MA RGIN I. Sral/ anlir�af/on s/9aals /77t/.5 I'be: o. S,rnal/ la /llll?l.b er ,6. 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