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In some models of multi-engine airplanes there is a switch labelled something similar to "prop sync" or "engine sync".

My questions are:

  1. What is engine synchronization?
  2. How is it achieved?
  3. Why is it important to use?
  4. What happens if you don't use it?
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  • $\begingroup$ Note that this is not limited to propeller aircraft or aircraft with only two engines. Many multi-engine jets have "engine sync" as well. (Updated question to reflect this.) $\endgroup$
    – Lnafziger
    Apr 20, 2015 at 21:17
  • $\begingroup$ Another concern is setting up resonant frequency problem on some airframes. $\endgroup$
    – Mike Geleide
    Apr 21, 2015 at 14:44
  • $\begingroup$ This question has now received an (at least partial) answer at the new (duplicate?) question aviation.stackexchange.com/q/77429/34686 $\endgroup$
    – quiet flyer
    Apr 22, 2020 at 20:48

6 Answers 6

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I don't know how it is achieved, but here is the reason:

If you have two sources of the same noise, with nearly the same frequency, the sum of both noises will be a noise of slowly increasing and decreasing volume. It's called beat and can become very annoying.

The math says

$$\sin(2\pi f_1t)+\sin(2\pi f_2t)=2\cdot\sin\left(2\pi \frac{f_1+f_2}{2}t\right)\cdot\cos\left(2\pi \frac{f_1-f_2}{2}t\right)$$

To demonstrate this, there you see two sinus-tones of almost same frequency, and what happens if you mix them:

enter image description here

The blue curve is of frequency $(f_1-f_2)/2$.

As example, one machine running at 3000rpm and one at 3030rpm results in a noise which increases for one seconds before it decreases within one second again.

As said, I don't know how it is done, but synchronization must be done very precisely to avoid this beat.

Edit:

Here is what happens if the two noises do not have the same volume. One of the curves has three times the amplitude of the other. The envelope is not a pure sin function, but the blue function fits it quite well. enter image description here

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  • $\begingroup$ I think you're missing a $2$ on the RHS of your formula. $\endgroup$
    – yo'
    Apr 20, 2015 at 13:51
  • $\begingroup$ That's right, thanks $\endgroup$
    – sweber
    Apr 20, 2015 at 14:08
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    $\begingroup$ I have noticed this very thing and thought it was driving me nuts! Glad it wasn't me just going crazy. $\endgroup$
    – Thebluefish
    Apr 20, 2015 at 15:00
  • $\begingroup$ One of the curves has three times the amplitude of the other it all depends on the two "volumes", it can be 3 times, it can be 10, it can be anything $\endgroup$
    – Federico
    Apr 21, 2015 at 14:52
  • $\begingroup$ This factor 3 was just an example. However, the larger the difference, the smaller the beat effect. The range of the volume is already smaller compared to the max. volume here. $\endgroup$
    – sweber
    Apr 21, 2015 at 19:20
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On the Junkers Ju-52 the pilot had two little discs which rotated with the beat frequency between two engines. The upper left one would show the beat frequency between the left and the center engine, and the lower right one that between the center and right engines. The pilot would try to stop them rotating by advancing or retarding the throttles, equalizing engine speed in the process.

Instrument panel of the Lufthansa Ju-52 D-AQUI

I highlighted the synchronization indicator in the picture above (source).

Running all engines at the same speed helps to fly straight and reduces vibrations. Not only is one distinctive engine frequency removed by equalizing it with the other, also the beat frequency is deleted.

Sorry, I cannot answer this for the Beech Baron. I've flown it only once and don't remember any instruments (other than my ears) which would help with synchronization.

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    $\begingroup$ my friend had a twin-screw boat that had a pair of lights side-by-side, each of which was connected to cyl 1 of each distributor. when the lights flashed together, the screws were in sync. $\endgroup$
    – rbp
    Apr 20, 2015 at 15:49
  • $\begingroup$ Looks like some of those instruments have had a little update since the WW2 era... $\endgroup$
    – quiet flyer
    Apr 22, 2020 at 13:56
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According to Wikipedidea

In aviation, propeller synchronization is a mechanism that automatically synchronizes all propellers of a multi-engine, propeller-driven aircraft so that they rotate at the same speed.

As for why its used the article states

Propeller synchronization serves mainly to increase the comfort of crew and passengers, since its primary purpose is to reduce the “beats” generated by propellers that are turning at slightly different speeds.

As for what happens if you don't use it, according to the article

Synchronization is not normally necessary for proper operation of the aircraft.

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    $\begingroup$ just because the props are turning at different speeds, that doesn't mean they're not producing equal thrust. but in reality, no two (or more) engines produce equal thrust. they just need to be close enough not to generate undue yaw. (note that this has nothing to do with beats) $\endgroup$
    – rbp
    Apr 20, 2015 at 14:32
  • $\begingroup$ I have edited my answer to reflect this. $\endgroup$
    – Dave
    Apr 20, 2015 at 14:38
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The engines using prop sync will be constant speed props. Constant rpm is maintained with a governor of some kind that adjusts the pitch.

This governor can be tuned by the prop sync to speed up or slow down one prop by adjusting it.

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It's just an automatic system to do prop speed and/or blade location synchronization. On a multi without it, one does it manually by adjusting the prop pitch controls. If the automatic system is not used, it can still be done manually. Prop sync is typically required to be turned off during critical phases of flight (takeoff/landing).

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4 engine prop planes turn at exactly the same(RPM) or percent depending on the type of engine. Number 3 is usually the boss and all the others slave to it. Every blade has to cycle between the other blades on the same wing or it causes a harmonic vibration so fine if cannot be felt by the crew but can shake nut or other tightened safety devices loose. If 4 blades crossed when they are at their closest proximity, you'd have blades which are near the speed of sound crossing about 4000 times a minute, so they must take turns crossing the center line. On 2 engined aircraft it is not a critical.

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