De-synchronised engines

[goudie]

As a boy I remember bombers flying overhead and everyone listening for that distinctive sound, to tell whether they were German bombers or ours. I learned later that the Germans de-synchronised their engines and that's why they sounded different to ours. So my question is, why did they do this and why didn't we?

[Robert Cooper]

IIRC it was to defeat the sound locators on the AA guns, which required constand sound to work properly.

Bob C

[A37575]

Personal viewpoint only, but I would have thought With large formations of aircraft it would be impossible to have all aircraft engines perfectly synchronised. That would have been the unsynchronised beat characteristic of German aircraft over England. Allied bombers operating in similar large formations over Germany would have the same apparent unsynchronised beat as German formations over England. During the period of the war when large numbers of German bombers were flying in formations over England, there were no large formations of British bombers flying over England at the same time.

Thus it would not be possible to listen to both and differentiate between British bombers in formation engine noise and German bombers engine noise. Those that claimed the engine noise was "one of ours" were probably hearing one aircraft - be it a twin engine like a Beaufighter or even a Spitfire. It could also have been one Dornier or one Heinkel.

During the war I saw occasional single German bombers flying at low altitude and do not recall anything unusual about the note of their engines.

[welliewanger]

A37575:
Engine sync is nothing to do with another aircraft it's when the engines of a singe aircraft with multiple engines run at EXACTLY the same rpm. As the engine turns it vibrates the airframe. If two engines are not in sync then for a few seconds the vibrations from either engine cause constructive interference (louder) and for a few seconds they cause destructive interference (quieter) This causes a slow wowowowow sound in stead of the constant hum.

[Rory57]

This was also done by RAF crews when over enemy territory:

"The steady drone of the engines had now become broken, with different revs on each engine: Dick had de-synchronised the engines to make detection more difficult........"

P. 39, Lie in the dark and listen by Ken Rees ISBN 1 904010 77 6

[Load Toad]

..but in the Lancaster book it mentions the engineer being careful to synchronise the engines to avoid vibration and excessive noise / discomfort.

[Fareastdriver]

On the Halifax you synced the inners with the whizzwheel. Then you looked through the inner propellors at the outer props. You then adjusted No1 or 4 engine so that the 'ghost' prop was stationary. Not surpisingly it was called 'ghosting'. Difficult at night unless you turned the landing lights on but I never did it at night.

[pasir]

... To concur with Goudie I too as a boy can still recall the sound of
German bombers overhead at night as we sheltered under the stairs during the London and Croydon Blitz - Having an ominous and distinctive sound
as if vibrating that we were given to understand was due to the Luftwaffe
flying with desyncronised engines - although I dont think many of us were aware at that time that this was in order to confuse the British system of sound locating.

...

[Robert Cooper]

A37575

I don't know where you were during the war, but those of us sitting in air raid shelters east of Sheffield could certainly tell the difference between a Heinkel or Dornier and a spitfire or Defiant.

German bombers did have desynchronized engines and were quite distinct.

There is a world of difference between the sound of a Merlin and a Jumo or Daimler-Benz.

Bob C

[Load Toad]

I think Robert that no one is doubting there was a difference.
The question is did the Germans routinely fly with unsychronized engines and if they did why?
Or was the difference in the noise that you've mentioned due to hearing multiple engines on multiple aircraft - or simply the different sound of German engines compared to Merlins etc.

[Robert Cooper]

Well, as I said before, I believe the AA crews had trouble with azimuth calculations due to the desynchronization, which would lead one to believe that maybe it was a purposeful ploy. Certainly the single Heinkel and Dornier aircraft that I recall hearing at night had desynchronized engines. It was nothing to do with multiple aircraft in formation.

I understand RAF bomber crews did the same thing, but don't have a reference yet.

would be interesting to hear from an AA crew here.

Bob C

[Robert Cooper]

This is the only reference on the subject that i can find right now. The pertinent section is at the bottom.

The following U.S. intelligence report on evasive tactics of Luftwaffe bombers was published in Tactical and Technical Trends, No. 16, Jan. 14, 1943.

[DISCLAIMER: The following text is taken from the U.S. War Department publication Tactical and Technical Trends. As with all wartime intelligence information, data may be incomplete or inaccurate. No attempt has been made to update or correct the text. Any views or opinions expressed do not necessarily represent those of the website.]

EVASIVE TACTICS OF GERMAN LEVEL BOMBERS

Evasive action, as employed by German Air Force level bombers, may be divided into two categories. One includes all maneuvers made as a matter of normal routine; the other applies to action taken when the presence of enemy aircraft is detected.

The first type of action consists of flying a weaving course in azimuth and simultaneously changing altitude by as much as 500 feet above and below the mean. Occasionally, an orbit or complete circle is made as a further safeguard and may be repeated as often as the pilot considers necessary. This circle is often effective in preventing the successful completion of a ground-controlled interception, and has the added advantage of being easy to carry out while making good a track to the target.

The second type of evasive action is of a much more violent nature. On the approach of intercepting aircraft, it is not unusual for a bomber pilot to quarter-roll his plane, applying full bottom rudder. The resulting dive is very steep, especially if the stick is pulled back hard immediately before the quarter roll. The bombers frequently pull out of this maneuver as low as 2,000 feet.

However, it appears to be beyond the capabilities of the majority of German pilots to carry out evasive tactics irregularly and still make good a mean track to the target. The result is that evasive tactics are used in a comparatively regular manner which the ground control stations are able to follow and sometimes even to predict.

Different tactics are employed in avoiding or escaping searchlights, but the most common evasive action consists of gliding through a known searchlight belt at high altitudes with throttles closed, so that the sound locators for the searchlight and antiaircraft batteries are unable to detect the aircraft.
Enemy aircraft, when picked up by searchlights in night combat, have attempted to deceive the ground defenses by turning away from the beam and immediately returning from another quarter with navigation lights on.

The bombers take effective evasive action on their approach to, and get away from, a target by desynchronizing the engines and constantly altering the throttle settings. This action upsets the normal action of the detectors and makes difficult an accurate prediction of the plane's course.

Bob C

[A37575]

This is the only reference on the subject that i can find right now. The pertinent section is at the bottom.

The following U.S. intelligence report on evasive tactics of Luftwaffe bombers was published in Tactical and Technical Trends, No. 16, Jan. 14, 1943.


Thanks Robert. Seems my theory was wrong. Story of my life:ok:

Having said that I will come up with another quick story. And that is the violent manouevre called the "Corkscrew" used by British bombers as a means of evading fighter attacks. I could never quite understand why this was used as for a few moments in the manoeuvre as the bomber reverses its direction of turn it is a sitting duck. We used to practice fighter affiliation between Lincolns and Mustangs and I flew both.

A few years ago at an Aviation Historical Society meeting in Melbourne the guest speaker was Tony Gaze a Spitfire pilot with 14 enemy aircraft to his credit. When asked about the practicality of the Corkscrew manoeuvre used by Bomber Command crews, he said he could never understand why this manoeuvre was used as it was so easy to shoot down the bomber as it reversed its turn. He had told the various RAF staff concerned with tactics that the Corkscrew manoeuvre had probably caused the destruction of many bombers. But apparently nothing was changed. Although it is hard to know what else the bomber crews could do when attacked. De-syncronise their engines, perhaps:ok:

[henry crun]

A37575: I would agree with you if the corkscrew was used during daylight, but apart from the start and the end of WW2, RAF bombers flew at night, and the corkscrew was developed to evade German night fighters.

I have been in the position many times of trying to follow an wildly evading target at night.
It is not an easy exercise even when one has it visual, and doubly so if the nav is trying to follow it on his AI radar.

[A37575]

On the Halifax you synced the inners with the whizzwheel. Then you looked through the inner propellors at the outer props. You then adjusted No1 or 4 engine so that the 'ghost' prop was stationary. Not surpisingly it was called 'ghosting'. Difficult at night unless you turned the landing lights on but I never did it at night

True. We did the same on the Lincoln.

[A37575]

and the corkscrew was developed to evade German night fighters.


And a real bugger if the one and only artificial horizon toppled during the steep angles of bank. Back to the basic primary panel, Bloggs ..don't know which was more dangerous - a night fighter looking for you or a limited panel unusual attitude recovery..

[horsefern]

My goodness, what memories come flooding back when I see the words de-synchronised engines! I was living in New Malden during the Blitz and, yes, it was terrifying for a 3-year-old who had been wakened from a deep sleep and carried down to a damp bunk bed in the Anderson shelter in the back garden. I can replay a tape in my head of the sound of the engines, no problem.
In answer to another posting on this forum about retaining sanity, I think that, if you lived through WW2 as a child, your sanity is guaranteed, as you can probably live through anything! I am 75 and am sound in mind and body (the latter not so hot but still working quite well!)
This is my first exposure to the forum and I will probably become a regular visitor, as the subject matter is very interesting. Thank you for reading my posting. (I am ex-Coastal Command RAF -- Air Sea Warfare Development Unit 1955-1959.)

[Shackman]

I can't speak for wartime practices, but I do know that flying for any length of time with the engines desynched was most uncomfortable, as the whole aircraft would vibrate and the rest of the crew would shout at you (or worse). Although it could be done by looking at the props, at least on Avro's finest there was a 'synchrometer', and all you had to do was fine tune the RPM until the little prop indicators stopped moving.

[Krakatoa]

Don't tell him Pike........

[A37575]

Why is it that when hearing a helicopter nearby it has the usual expected blade noise but when two or more helicopters are in formation, the noise sounds quite unsychronised. Isn't this unsynchronised engine noise exactly why formations of Luftwaffe bombers had a similar engine beat?

[longer ron]

Certainly could be partially that but as Bob C posted...individual a/c used to do glide approaches and also changed throttle settings to evade/confuse sound detection.
Peter Stahl refers to these procedures in 'The Diving Eagle'

[Rory57]

Was not the continued de-synchronising of engines to confound acoustic detection a bit of spin to obscure the discussion of radar detection?

The synchronising of an aircraft's propeller speed is practical and desirable: comfort, vibration, human and equipment fatigue etc but synchronising the props of one 'plane with those of another is not possible. Thus a group of aircraft within the range of a detector would sound the same to that detector whether or not each aircraft had synchronised engines.


Some interesting pictures and some notes about acoustic aircraft detection here
Acoustic Radar. (http://www.aqpl43.dsl.pipex.com/MUSEUM/COMMS/ear/ear.htm)

[goudie]

Horsefern
What a coincidence! I too lived in New Malden, during the Blitz and recall quite clearly poking my head out of the Anderson shelter to listen to the bombers...that is until my mum yanked me back in side and gave me a clip round the ear!
It always mystified me why the sound was different, hence me starting this thread. Many thanks to all those who replied with interesting answers

[ZeBedie]

I wondered if the out of sync noise was intended to terrorise the civilian population? I know it did.

[PPRuNe Pop]

A37575, and I. lived under many a flight path of German Bombers of all kinds in 1940 and 41. I can, as, he does, recall the unsynchronised beat. It would look like a 50 cycle sine wave on an osicillorscope I expect. Anyway, I am not in a position to offer a positive view that it was deliberate. The Jumo engines on the He111 was always defineable.

However, I am not sure I buy a deliberate attempt to unnerve us on the ground. Just HOW would large formations as in August, September and October - and beyond - of all types all de-synchronise to make it worthwhile. It didn't bother me that's for sure.

[longer ron]

The Luftwaffe aircrew had little idea of the capabilities of our 'Radar' system in the early days of the war and so certainly some solo bombers did alter throttle settings in an attempt to confuse detectors...

From Peter Stahl 'The diving Eagle'...
Target Liverpool march 1941 ...Heavily engaged by searchlights...
''I try every trick I know to mislead them,but in vain,every time i change direction they follow me.Even the usually effective method of deceiving the defenses,by changing the revs of the engines to alter their sound,fails this time''.

Who knows...inland it may have been effective because I doubt there was much inland radar coverage early in the war.

[longer ron]

Actually - looking at the date of the quote from Stahl's book - possibly it could have been an early encounter with the new (inc inland) GCI system which was coming into service in the spring of 1941 !