View Full Version : Unsynchronised beat of Luftwaffe engines

24th Jun 2004, 14:28
During the war I was conscious of the noise of massed German bombers passing over my village in Kent towards London during the blitz.

Their engines gave a monotonous and frightening uneven beat. As a pilot now, I realise that was the sound of unsynchronised engines. My question is this:

Was the unsyncronised engine noise quite a normal expectation given that there were so many aircraft?

Or was it deliberately done (by setting unequal rpm) to frighten the populace? I feel that the former was the most likely explanation.

PPRuNe Pop
24th Jun 2004, 15:49
I remember that! Blimey! I never thought anyone would raise that as a subject. But it was very distinct and very disquieting. I used to know the definitive answer - the reason which involved knowing the engines - but I do re-call that it emanated mostly from the He111.

I expect someone else will have a point or two to make.

24th Jun 2004, 16:03
Not old enough to have heard except on old films, but a reason I was given is that it was a Luftwaffe SOP to confuse the sound-location equipment that some nations used.

vintage ATCO
24th Jun 2004, 18:20
I'm reading Ken Rees' book 'Lie in the Dark and Listen' and he was on Wellingtons and says they use to do the same 'over there'.

24th Jun 2004, 22:52
Being a recently-qualified single engined PPL, with a keen interest in history but not much technical knowledge (especially of twins), I really do have to ask...

How on earth does one go about un-synchronising engines? What does it mean, is it simply a case of ensuring the rpms on engine one are different to the rpms on engine two?

Apologies if this is a bonehead question....

25th Jun 2004, 01:49
Engine Synch

It's the prop noise that you synchronise by careful adjustment of engine RPM. Most multi props these days have automatic synch. Unsynchronised props can drive the crew and pax to distraction

What you would have been hearing on the ground would have been the uneven beat of multiple aircraft, each being a bit out of synch with each other. Each aircraft would have had its own props in synch.

henry crun
25th Jun 2004, 04:49
This thread brings back memories of many hours sitting in an air raid shelter listening to that throbbing droning noise passing overhead.

I'm not sure you are right about your last sentence Milt.

I have witnessed a solitary He111 with unsynchronised engines, it was the noise that attracted my attention to it just before a stream of bullets from it came my way and made me leap into the nearest hedge.

Dr Illitout
25th Jun 2004, 09:48
I was also under the impression that it was to "fool" any sound locator system used against them. What they should have done is bomb all the Carrot fields instead!.
To change the subject slightly, I flew on a Belfast years ago and that had a manual prop sync system. There was a gauge on the flight engineers panel with three prop symbols on it. If the "prop" turned one way, it was turning faster than the "master" prop and if it turned the other way is was turning slower. The poor old flight engineer spent all of the flight moving the throttles backwards and forwards trying to stop the thing shaking it's self to bits!.
Also moving a little further away from the original subject, I seem to remember hearing that the 777's have or had a problem with fan synchronising.

25th Jun 2004, 14:00
But why would sound locators be affected by the throbbing noise of unsychron'd Daimler Benz or Jumo engines versus sync'd engines.

I think Milt is right, however. Just listen to two or more single engine light aircraft in formation and you can pick varying engine noises. With large formations of German raiders there would have been an awful lot of throttle jockeying from aircraft attempting to hold formation and that would be transmitted to the populace below as uneven engines.

henry crun
25th Jun 2004, 21:31
Hudson, I have heard many formations of single engined aircraft and I can assure you that the unsynchronised beat from German bombers was very different.

On the subject of throttle jockeying to maintain formation, by day yes, but not by night when they flew in stream the same as the RAF.

PPRuNe Pop
26th Jun 2004, 06:10
I heard both daylight and night engine noise and the two do not compare. Massive formations during August and September were mainly during the day and it was just noise! Plenty of it too, bombers and fighters together make quite din.

At night the beat would not come from massive formations, they didn't operate them at night, they came from waves of smaller numbers of bombers and the synch would not be from all of them otherwise it would be hard to discern. It was from individual aircraft, and it wasn't always a guaranteed occurrence.

As I said earlier it turned out to be mainly from the He111 which had two types of engines - Daimler Benz and Jumos - it was the former (I think) that were difficult to synch.

In later life I found when flying twin pistons of varying weights that synch was difficult until the engineers mastered the art of auto synch. It wasn't always perfect even then! ;)

26th Jun 2004, 13:39
Don't ask me where, but I'm sure I remember seeing a Luftwaffe pilot interviewed and he said engines were deliberately desynchronised (on nearing the target?) in an effort to add a further psychological dimension to bombing raids - similar to the thinking behind the Stuka siren.
It certainly had an effect on my father who grew up a city which was a major target during the Blitz; "A horrible laughter" he once described it as. It made it very hard (to a teenager's ears) to tell how many aircraft there were, and until the sound had totally gone, whether there were more approaching.

Edited at add, there is no reason why you wouldn't get a so-called 'beat frequency' from two aircraft, each with their own (twin?) props perfectly synchronised, but with both aircraft at slightly different RPM, (the frequency of the 'beat' being exactly equal to the difference in RPM.)

QNH 1013
26th Jun 2004, 19:17
My local pub-expert (so it must be true!) tells me that the unsynched engines didn't have any effect on the sound locators, but the rumour that it did was started so that the german crews would fly with unsynched engines, therefore making their lives more uncomfortable. Sounds a reasonable explanation (like the carrot rumour) but difficult to know if its true or not.

satis 5
27th Jun 2004, 01:05
synchronising and synchrophasing still in use on modern digital turbo props.
a web search should return more info,
reduces cabin noise and vibration when operating correctly.;)

with the psychological effects, british whistles on bombs had to be a pretty effective one.

27th Jun 2004, 21:03
I can't remember when or where I heard it, but I understand that WW2 Luftwaffe crews did not synchronise their engines, for whatever reason.

29th Jun 2004, 01:44
Chap called W.E.Johns (you may have heard of him? Write some books about a chap called 'Biggles' ;)) wrote in said books about the 'Pour-vus Pour-vs' noise made by German engines - but he was talking about the FIRST world war, and not necassarilly multi-engined a/c!

Unfortunately I'm seperated from my Biggles collection by the Atlanic at the moment, so no checking.

Before anyone pours scorn on these works of fiction, Johns knew what he wrote about in the W.W.I Biggles stories, as he was a 2nd Lt. pilot in 55 Sqn; DH-4 a crack day bomber sqn. His W.W.II stories are technical disaster areas, but 99% of what I've read in the W.W.I books has been substantiated elsewhere.


James K

St Helens
29th Jun 2004, 09:54
Thankfully I never heard the sound of German bombers. My mother always went on about the terrible throbbing they made and how even up to the day she died (1984) she still remembered it as the most evil of all sounds (may have had something to do with being bombed as well) She told me on many occasions that she hadn't slept on a particular night as she could still hear the throb of those aircraft and this was twenty and thirty years after the war. Whatever the reason for flying unsynchronised it certainly had a lasting effect upon my mother and many others I bet.

As an aside my mother and sister lived in Dagenham during the Blitz and were evacuated to Sheffield for safety? The evening they arrived there so did the Luftwaffe - needless to say my mum took it very personally and the following day went back to Dagenham - who on earth thought that Sheffield was going to be a safe place being the centre of Steel production at the time.:mad:

1st Jul 2004, 13:37
If anyone is interested I wrote a story about my own experiences as a small boy living in Kent during the war. A wonderful place to view the Battle of Britain and an aeroplane spotter's paradise - apart from the big bangs. The story was called "A Messerschmitt and a Blackbird on a Tree". The reason for the strange title is explained in the article. I recall it was published in an edited form in Pilot magazine several years ago.

I would be happy to email it if anyone wants to read it. It is around 15 minutes of light reading - only text- no pictures therefore a small Word file. Contact me via Pprune. I hope this is legal!

1st Jul 2004, 16:00
Centaurus kindly sent me his article last year and I heartily recommend it !


1st Jul 2004, 22:26
I am nowhere near old enough to remember anything like the sound of Luftwaffe engines, but I have to say I have the utmost respect for those that had to lay there night after night listening to the sound of enemy bombers, engines sychronised or not, and still managed to come through the experience with their sanity intact.

It must have been truly terrifying.

4th Jul 2004, 18:31

Thanks a lot for the stories you emailed me, very interesting reading and beautifully written.

Would recommend it to anyone. Have you any more stories ?

A book of your short stories would be marvellous.....!

6th Jul 2004, 03:41
centaurus' wife and i have told him for too many years that i can remember to put his stories in book form , id recommend it as great reading for anyone interested in flying.

9th Jul 2004, 21:11
There was something multi engined pootling around the Luton / Stanstead area about 2 weeks ago around 11:00 - 11:30 at night. The engines / props were not synchronised and the resulting discordant throb was bloody annoying! Far more disturbing than the normal sounds of aircraft in the area.

9th Jul 2004, 22:20
Quite agree LowNSlow, a few weeks ago a heavy turbine overflew Purley in the wee hours (obviously not the delightful and eagerly anticipated DC-6s!) which had a very bass note and very out of synch... it was aurally quite painful, and that was through double glazing... Don't think it was Herc, certainly not an Antonov (the An-22 can disturb me any time!).

10th Jul 2004, 00:40
Hercules/C130s have auto synch which works extremely well.

If it was a Herc the auto synch may have been U/S.

In a Lancaster or Lincoln the pilot was able to look through the inner props to the outers. Technique was to juggle RPMs to line up each side then take out the beat between left and right by carefully altering RPM on a pair to synch with the other pair.

Don't remember how I did it in a Beverley.

Say again s l o w l y
10th Jul 2004, 12:52
LnS, it could have been one of our sheds. The auto sync on them very rarely works and some of our pilots seem to have cloth ears!

Like others here, I have heard that the Luftwaffe deliberately de-synched their engines, but from contemporary accounts it does seem as if the HE111 was the worst offender.

2nd Aug 2004, 11:21

You may have read that the Luftwaffe didn't synch their engines in Len Deighton's novel "Bomber". I'm sure it's mentioned somewhere in there.


4th Aug 2004, 05:53
Somewhere in the dark recesses of my mind I seem to think some of these multi engined bombers were diesel powered,can somebody confirm that?