9

Is synchronization of rpm to avoid beat frequencies also a concern on jet engine aircraft like it is on propeller driven airplanes?

Context:

This question on SE-Aviation discusses prop sync for multi engine prop aircraft to eliminate beat frequencies that can be annoying. But it says nothing about any multi-engined jet aircraft.

On the other hand, this other question on SE-Aviation refers to why it is preferable to run all engines at the same RPM (and hence differential thrust is NOT used for directional control) and I presume the answer applies to both jet and prop aircraft.

Which begs the question above:

Is synchronization of rpm to avoid beat frequencies also a concern on jet engine aircraft like it is on propeller driven airplanes?

If it is, how is this done in typical modern jet aircraft? Is there an automatic system in the FADEC etc?

Bianfable
  • 55,697
  • 8
  • 195
  • 257
curious_cat
  • 8,436
  • 7
  • 45
  • 94

2 Answers2

8

Some aircraft with turbofans have an engine synchronization system for exactly that reason: reducing the beat resulting from different, but similar, frequencies for the two engines.

  • MD-80: The McDonnell Douglas MD-80 can synchronize its engines by EPR, N1 or N2 when the autothrottle is engaged:

    Engine Synchronizer System

    The engine synchronizer system automatically matches the N1 or N2 speed or EPR of left engine to match right engine. The system includes a three position (N1, OFF, N2) ENG SYNC selector, ENG SYNC ON annunciation, and control system with actuator to trim left engine fuel control. The pilot can override the system at any time with throttles.

    When the ENG SYNC selector is placed in N1 or N2, signals from the tachometer generators causes engine synchronizer system to trim left engine fuel control until left engine RPM (N1 or N2) matches right engine.

    (MD-80 FCOM Sec. 18 Power Plant - General)

  • CRJ: The Bombardier CRJ-700/-900 can also synchronize by either N1 or N2 via the FADEC:

    N1 and N2 Synchronization

    In the cruise range, thrust is set to a specific value by the FADEC based on throttle position. Synchronization allows the FADEC to match the fan (N1) or the core (N2) speed of the two engines for noise reduction. Synchronization is selected by using the SYNCH switch on the engine control panel . The left engine is designated as the master engine and the right engine will be slaved to it.

    For N1 synchronization to be enabled, the right engine N1 must be within 1.5% of the left engine N1.

    For N2 synchronization to be enabled, the right engine N2 must be within 7.5% of the left engine N1.

    (CRJ FCOMv1 chapter 20 Power Plant - Thrust Control)

As far as I know, no Boeing or Airbus aircraft has an engine synchronization system. Presumably, the beat is less of a problem for wing mounted engines since the passenger cabin is shielded from direct sound by the wings. For tail mounted engines, engine noise is more of a problem in the cabin.

Bianfable
  • 55,697
  • 8
  • 195
  • 257
  • That really isn't a synchrophase system though. It's jut the FADEC computers tweaking N1s. There is no adjustment for phase issues. – John K Nov 17 '19 at 18:35
  • 1
    It's just synchronizing the frequencies, yes. I don't think phase is an issue with turbofans... – Bianfable Nov 17 '19 at 18:43
  • 3
    @JohnK with a 25 or 30 blade fan, the phase issues are fairly negligible compared with a 4 bladed prop. – alephzero Nov 18 '19 at 03:24
  • 1
    I know of at least one large turbofan engine which was a bit close to the margin for fan flutter at some power settings, and the simplest cure was reprogramming the FADEC to keep the fan speed varying continuously within a small range (a small fraction of 1%) to avoid sitting on the exact speed that allowed flutter to slowly build in amplitude over a time period of about a minute. Obviously that would have prevented any exact engine synchronization even if it was desirable for noise reasons (and AFAIK synchronization has no effect on noise, for wing mounted engines). – alephzero Nov 18 '19 at 03:30
  • @alephzero I would think so also but I'm not sure what motivated GE to develop a fan synchrophaser, at least to the point of patenting it. – John K Nov 18 '19 at 03:33
  • Since you mention CRJ, the 200 doesn't have FADEC, and if the pilots let the N1 get out of sync by more than about 0.4% or so, everyone on board will be able to tell. – nexus_2006 Nov 18 '19 at 16:53
6

GE patented a synchrophaser system for turbofans in 1991 but I've never heard of the system being implemented. My experiences are with the CF34 family (both flying and on the technical side), both FADEC and non FADEC, and none of them had a dedicated synchrophasing system (except for N1 synching by the FADEc computers) and I can't find anything about syncrhophasing on their larger engines like the GE90 either, and PW's geared turbofans also don't appear to use synchrophasing systems (which is a surprise since with the very large gear reduced fans they are a lot closer to being a turboprop).

In any case, the slow woo woo woo of a slightly out of sync pair of turbofans is a lot less objectionable that what you get from propellers, and the crew up front may not even be aware of it, especially if the engines are back at the tail. To the extent that they are aware, or if a passenger complains (more likely), then they would just tweak the N1s a little bit. It's less of an issue on FADEC engines since the N1 is maintained more precisely and to the extent there is a "woo woo" going on, its oscillation frequency is so low (Woooooo.........wooooooo) that most people aren't bothered by it.

John K
  • 130,987
  • 11
  • 286
  • 467