Synchronised engines

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M

marlin

Airman
14
0
Jan 5, 2007
During WW2 Luftwaffe aircraft were, at night, easily indentified by the sound of their unsynchronised engines, whereas those of the RAF were synchronised.
Does anyone know exactly how engines are either desynchronised or synchronised ?
 
This is a guess on my part, but the only thing I could think of is that the
left and right engines were not turning at the same RPM. Multi-engined
aircraft have tachometers for each engine, and syncing them should be
sort of easy.... Maybe the German's didn't do that ???

Charles
 
marlin said:
During WW2 Luftwaffe aircraft were, at night, easily indentified by the sound of their unsynchronised engines, whereas those of the RAF were synchronised.
Does anyone know exactly how engines are either desynchronised or synchronised ?
Click to expand...

That's a good question, it's probably got a simple answer which, no doubt, somebody here can answer easily.
For me, it's a good question because I grew up in the London during the 2nd WW. Young as I was, by late '42, when there were still a lot of German planes about over London, usually at night. I had no problem telling Luftwaffe from RAF types by sound alone.
The weird thing is it's never occured to me to ask what unsynchronised meant. I know what unsynchronised sounds like, but not why.
They never get it right in films and seldom in documentries. Recently I watched a documentary in which a flight of Lancasters approached the French coast. I could have sworn they were He 111's...........
 
Hi Marlin,

>During WW2 Luftwaffe aircraft were, at night, easily indentified by the sound of their unsynchronised engines, whereas those of the RAF were synchronised.

>Does anyone know exactly how engines are either desynchronised or synchronised ?

For a twin-engine aircraft, it's usually done by listening to the sound while doing small adjustments to the (rpm) settings of one engine. Unsynchronised engines give a throbbing "beat" due to two very similar frequencies interfering with each other: Beat (acoustics - Wikipedia, the free encyclopedia) This also results in a lot of vibration in the aircraft, which is why engines are usually synchronised in aircraft.

The Luftwaffe deliberately did not synchronise their engines when flying over England because they thought (and might well have been right in that) that it made it harder to locate them using sound location equipment. The crews did not like the vibration, but it came with the job.

For four-engine aircraft, synchronising all of the engines was somewhat more complex. The two engines on each side could be synchronised visually by observing the picture of visually overlapping propeller disks, and then the two sides had to be synchronized by sound, which was a tedious business because adjusting one engine on that side required re-synchronizing with the other engine before the sound could be compared again.

Later on, electronic synchronizing aids were developed, and I believe I read that they were sometimes installed only for the two inboard engine in order to eliminate the most difficult task in synchronizing a multi-engine aircraft.

Regards,

Henning (HoHun)
 
Hi Charles,

>Multi-engined aircraft have tachometers for each engine, and syncing them should be sort of easy....

I believe the problem was that only very small differences in speed are required to generate the beat (in fact, big differences don't give any beat at all), and the required accuracy for proper synchronization exceeded that of the tachometer reading.

Regards,

Henning (HoHun)
 
This engine sync thing reminded me of an ex-RCAF Lanc pilot I met in Edmonton AB. They lived under the glide path into the old downtown airport and there was a late night TCA [now Air Canada] North Star [a DC-4 with 4 Merlins] which he heard in his sleep. He drove his wife nut when he shouted in his sleep, 'get those effing engines in sync'!
 
Hi Thorlifter,

>I should have known that HoHun would know the answer to this. Thanks for the input.

You're welcome! The amazing thing is that it was Len Deighton's book on the Battle of Britain that made me aware of this synchronization issue so that I was able to put all the more detailed information into context when I came across it much later.

Deighton's book is sometimes considered to be sort of "light-weight" history, but I think he actually did a great job if you consider that he was writing (and illustrating) a popular book for a non-expert audience.

Regards,

Henning (HoHun)
 
You can easily feel and hear the vibrations that you experienced when the proprotors are not sync'd. You can manually set the prop rpm levers and feel when the props are close to matched in a twin. I don't know how you could effectively sync props visually like hohun said on the 4 eng.

When I flew C-12's, they had a prop synchrophaser, which actually matches not only the RPM of the props, but also the phase so the blades pass the fuselage at the same time accomplished via magnetic pickups.

The prop synchronizer probably set a master and slave engine, and matched prop rpm by matching electric currents generated by the each.
 
The JU52 probably had the niftiest prop synchronizer ever made. It used curved mirrors on the nacelles. Using the center engine as the reference, one only had to get the reflections/shadows to match to get both sides synched to the center engine.

I've been flying R/C models for about 40 years. When engines are synched, the sound is unmistakable. To get an idea of what synching sounds like, try humming to your vacuum. When you hit the same frequency as the vac, you'll know!

CD
 
This unsynch method was employed by the Japanese for their nightly nuisance raids commonly referred as "Washing Machine Charlie"
 
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