Multi-engine synchronisation

AFAIK the main rotor on most helicopters rotates at about 400-600 rpm and that is basically the permitted RPM range. Anything outside thes limits is cause for aprecationary landing to investigate the problem.

Yes, it’s a classic!

The main rotor RPM is surprisingly stable…..or is that quite normal for a helicopter? I always assumed it would vary quite a lot as the helicopter maneuvered.

Found this amusing video… no it isn’t the new top secret version of the Hind with an anti gravity system built it… it is what happens when the RPM of the main rotor is synchronised with the frame rate of the camera…

http://www.youtube.com/watch?v=Do1VYW6rKFM

A very old topic I know but I asked my dads next door neighbour who was a Lancaster engineer what he remembered of syncing. He said that during the daytime it was achieved by looking through the props and manually adjusting the pitch and engine rpm, during the night it was done by ear alone.
Regards Paul

I was searching for some completely different information when I came accross this web site with information on the equipment used to listen and calculate the position of aircraft by sound. Since one of the questions that was raised earlier in this thread regarding the deliberate de-synchronization of the engines to confuse these devices was raised I thought this information might be relevant to this discussion.

Also has some very good information on the old searchlights!

here is the link

http://www.geocities.com/mepurina/victory2.html

Starts off with searchlight info and then into the sound locators. As expected they were not used much after Radar came along – but it is noted that they were left for the enemy to see as decoys.

Also a bit of information on a heat seaking locator that would pick up the heat signature from the engine(s). Never knew they had that capability that early!

Truc

One of the four engined bombers the Soviets had in service during WWII was later fitted with diesel engines because of the shortage of standard engines.

It is my understanding that diesel engines can be run in either direction, though they must be stopped to change direction AFAIK.

BTW fitting diesel engines greatly improved range and bomb load.

Some older diesels will run in the wrong direction ‘accidentally’ but they will not produce much power and will wreck themselves in the process. The critical thing is the ‘timing’ of the injection of fuel into the cylinders.

Very few diesel engines are designed to run in both directions, the exception being very large marine diesels for super-tankers (that run flat-out at 80 RPM). These are designed to run backwards to give power ‘astern’ without the necessity for any costly gearbox.

Diesels by nature tend to be heavier than a petrol engine developing the same power, but they use less fuel, so the trade-off in aircraft is between the weight of the engine plus the weight of the fuel. Basically the greater the range the better a diesel is for an aircraft.

Nice bald statement Garry, care to elaborate / quantify?

OK, let me put some hair on it…

The Plane in question was the Pe-8 four engined heavy bomber.

To Quote a Bill Gunston publication:

“Maximum bomb load was 4,000 kgs, the range of 2,321 miles being raised to over 3,000 miles by the diesel engines substituted when AM-35 production ceased.”

The engines in particular are given as the AM-35A Vee twelve engine developing 1,300hp each in the first production model. Second production aircraft had four 1,475hp Charomski M-30B vee twelve diesels, and the third production batch had four 1,630hp Shvetsov ASh-82FNV 14 cylinder two row radial engines. Speeds were 444km/h (AM-35A), 438km/h (M-30B), and 451km/h (ASh-82FNV).
Bombload is given as 4,000kg in the first model and about 5,250kg in the last model.

All sleeve-valves…but only the Crecy was a two-stroke.

You’re quite right…my fault for reading the chapter too quickly and then jumping to conclusions.

It’s a pity really…I would have liked to have seen how it turned out.

Started life as a diesel…changed to petrol…but kept the direct injection!

I wonder if any survive? 🙂

Just as a good musician can tune a musical instrument to a high degree of accuracy by ear. Likewise a well experienced engineer can tell if an engine is off colour just by its tone.

I always seem to forget that many times links to external sources get changed and then finding information can get a bit difficult down the road.

Here is the paragraph describing the synchronization of the props in the B-24 from the link.

“We had slipped into our place in the formation, and I was concerned about the number three engine, which was just outside my window. I had synchronized the props, first by tachometers, then finer synchronization by changing prop pitch until you got rid of the changing shadows seen by sighting through the two props on the left, then the two props on the right. Final synchronization was by killing the sound “beat” of the two left props being a little off timing from the right. If you listened, you could hear a varoom-varoom-varoom. I would then speed up or slow down the right pair to sound-synch with the left with gentle nudges on the pitch of the props, resulting in the sweet smooth roar of our well-synchronized engines. I’m pretty hard of hearing now, and I like to attribute it to my glory days as a B-24 Pilot and Co-pilot. Well worth it to have had such a once-in-a-lifetime privilege.”

I don’t think the author would mind me pulling this from a very interesting story.

Truc

Truc.
I think the only question left in my mind is that of there possibly being some variation of Left and Right Synching. I had come across two versions.
A. As above with both wing groups being synched first and then brought into tune together by slowing the beat until it went. ( I subsequently deleted this part from my mail because I also then recalled):
B. In which the outers were firstly very slighly pulled back together and the comparitively louder inners initially synched by sound, with then the outers “brought up and in ” by visually strobing. (This of course could potentially result in a bit less yaw compensation being required from the pilot , and therefore be a smoother overall operation if being performed by, for example, the engineer).
Finally ,I do note the remark in the manual of words to the effect IIRC that with experience one could actually do it all on sound alone.

I always seem to forget that many times links to external sources get changed and then finding information can get a bit difficult down the road.

Here is the paragraph describing the synchronization of the props in the B-24 from the link.

“We had slipped into our place in the formation, and I was concerned about the number three engine, which was just outside my window. I had synchronized the props, first by tachometers, then finer synchronization by changing prop pitch until you got rid of the changing shadows seen by sighting through the two props on the left, then the two props on the right. Final synchronization was by killing the sound “beat” of the two left props being a little off timing from the right. If you listened, you could hear a varoom-varoom-varoom. I would then speed up or slow down the right pair to sound-synch with the left with gentle nudges on the pitch of the props, resulting in the sweet smooth roar of our well-synchronized engines. I’m pretty hard of hearing now, and I like to attribute it to my glory days as a B-24 Pilot and Co-pilot. Well worth it to have had such a once-in-a-lifetime privilege.”

I don’t think the author would mind me pulling this from a very interesting story.

Truc

All sleeve-valves…but only the Crecy was a two-stroke.

You’re quite right…my fault for reading the chapter too quickly and then jumping to conclusions.

Just some more info regarding prop synchronization.

Engine and prop synchronization are the same thing – and it does not matter as to the condition of the propeller – damage or a reduction in diameter as the RPM is all that is important.

Engine and propeller vibration is a whole other matter and in that case the differences in prop physical condition will have an effect. We had a DHC 2 Beaver that had a vibration from the engine/prop only at very high power settings – turns out it had one old blade and one new blade – the prop was balanced but the blade resistance to bending was different. At high power settings, the old blade with many hours of flexing was work hardened – making it stiffer than the new “softer” blade. As a result when we pulled high power the new blade tip was farther forward than the old blade – and like a helicopter rotor was out of track!! Took the prop back to the prop shop and they rebuilt it using 2 blades with about the same hours on them – problem solved.

BomberBoy – the first scan that I posted is from the AAF training manual from the B-17 – if you actually read it – it does say that someone in the nose has to look at the prop overlap from that position!!

Also according to this web article the props on the B-24 were also synchronized in the same manner.

http://461st.org/Liberaider/sevenseven_to_tower_really.htm

I have about 3000 hours of flight time – much of it in DC3/C-47, Curtiss C-46s, DC-4 and DC-6s, PBY-5a and PBY-6a, and a whole host of other aircraft. I can assure you that you can hear everything in the cockpit and an engine out of synchronization is immediately noticed. On one long and very boring flight on a DC3 from somewhere in the north of Canada to Churchill Manitoba – I got the idea to bug the flight crew since they were hogging all of the heat by closing the cockpit door – we only had one Janitrol heater in the aircraft and it dumped only into the cockpit (freighter aircraft – not a normal configuration) – so I slid up the center floorboards, found the propeller pitch control runs and gently tugged on one of them – placed the engines out of Synch – crew corrected – I adjusted the other side – crew corrected – etc. etc. etc – until the crew finally realized that something was wrong and flung open the door to catch me in the act of making another “adjustment”. They were not happy campers!!!!

I would presume that many pilots wouldn’t think about telling anyone about synchronizing the engines and props as it was such a routine thing to do. Most of the aircraft that I flew in had the tachometer synchroscopes and were actually very easy to synchronize. See the attached scans of a King Air 200 – this twin engine turboprop has 4 tachometers as the PT-6 engine is a free turbine. The gas generator rotates at a speed selected by the power lever angle and the power section – ie prop rpm is controlled in flight by the constant speed unit that receives its speed signal from the propeller condition lever. In addition the gas generator turbine and the power turbine turn at different speeds and in opposite directions to cancel some of the torque.

The little prop synchronizer unit is driven by differential frequency from the 3 phase permanent magnet tach generators. They used the permanent magnet technology so that even in the event of an electrical power failure of the aircraft electrical system, the crew would still have rpm indications from the standalone system. If you can remember a bit about 3 phase generators and motors – the speed of a 3 phase motor is directly related to the frequency of the ac power. The frequency of the tach generator output is proportional to the input speed of the rotating magnets. The standard rpm input for piston engines is ½ engine speed and if I remember correctly this is a result of having to mount the tach generator somewhere – the front of the crankshaft had a prop attached so that was out, and in most cases the back end of the crankshaft usually had a starter attached. Since the camshaft was available it was used and of course the camshaft in a 4 stroke engine (Except Radials) turns at ½ the speed of the crankshaft.

The American standard for turbine engines using tach generators is 4200 rpm input will give a 100% reading on the gauge. There are other systems out there so not every engine works with these.

The tachometer type synchronizer uses the 3 phase from both engines as an input – if one engine is turning faster than the other then the motor of the synchronizer unit will rotate – the unit is wired so when that happens it will spin towards the faster engine. Use of the propeller controls to stop the spinning of the indicator will synchronize the engines. Once manually synchronized the switch is positioned to the on position if an electronic synchronizer is installed – this will control the propeller constant speed unit electrically to maintain the synchronization – within limits. There are 2 systems in use – the older type I systems will have a master engine and will synchronize the other engine to the speed of the master engine. Obviously we have to have rpm limits for a disconnect – a failure of the master engine must not drag the other engine with it on a shutdown/failure. The second more modern type – type II system doesn’t have a master engine and simply matches the engines to each other by reducing the speed of the fast one and increasing the speed of the slow one. There is no master engine with this type.

Synchrophasing must be done with an electronic unit. There are usually magnetic targets mounted on the spinner backplate that pass by a pickup probe installed on the engine. The positioning of the targets is important and can vary from one engine to the other in their locations. The electronic box will look at the phase pulses using a very accurate clock and using a computer program will know where the propeller blades are positioned in the circle. To control the blade phasing angle they will speed up or slow down the propeller via the constant speed unit to obtain and maintain the specific phase relationship between the engines. It is possible that at the last stages of the war that the automatic synchronization and synchrophasing systems might have been installed in aircraft like the DC-6 but I am not positive about that. I do know that the 6s that I flew in had the systems – very large electrical control box full of tubes etc.

Found it – as I thought, the Exe, Pennine and Crecy were all sleeve-valve two-strokes. Just when things began to look promising, gas turbines arrived.

All sleeve-valves…but only the Crecy was a two-stroke.

Synchronising engines and synchronising props are two very different things.

…the CSU is constantly making adjustments to the prop pitch in order to maintain engine RPM stability not prop RPM stability.

You’ve lost me there…surely engine RPM and propeller RPM are at a fixed ratio (that of the reduction gears)? :confused:

I’m pretty sure some of the wartime experimental Rolls-Royce engines were two strokes. My usual reference for this is Bill Gunston’s account of RR aero engines, but it’s gone walkabout in the house somewhere – can anyone else confirm or deny this?

Found it – as I thought, the Exe, Pennine and Crecy were all sleeve-valve two-strokes. Just when things began to look promising, gas turbines arrived.

Been away from the computer for a while and came across this thread.

Me “DAD” was a pilot on Halifax IIIs – 51 Sqd and he told me that they syncronized the engines by getting the inboards synchronized by ear and then using the inner prop overlap of the outer prop to synchronize the outers like a strobescope.

Without trying to sound off, but I’d like to know how he did that with all the noise being generated in the cockpit.
My dad was in 53 Sqdn on B-24’s and he never ever talked about engine synchronisation issues or methods.

I had a look through my limited British aircraft library – including the Halifax, Lancastrian and York and found no references to the method used to synchronize the propellers – but I did find some info in my training manuals for the B17 and the C-54 that basically state the same thing – Scans included.

Hmmmm.
This would be very difficult in a B-17 as the only practical place that this can be done……is in the nose, so I do not believe your information to be ‘fully correct’ Sorry.

The DO 328 aircraft has both prop sync and phasing – is a bit different in that the engines are only in sync at “Climb” or “Cruise” settings. In addition the props are initially balanced on the ground and then fine balanced during flight. Prop sync on takeoff is accomplished by the CSU governor adjustments – and on this aircraft the digital tachs are very accurate. As a further refinement to the reduction in noise level for passenger comfort the airframe is “Tuned” by the application of sound deadening material placed over the vibrating bays as well as mass balance weights applied to the fuselage frames. Dornier took noise reduction seriously – and the difference between a Dash 8 and the 328 is most astounding! No such luxury for the war and early cargo machines. 🙁 .

I have had the good fortune to fly in the 328 many times from London City……nice aeroplane with tractor the same on both donkeys.
Much more noisy in a Jetstream 41 with opposite tractors.

The prop synchronization was so important from a stuctural point of view that the manufacturers had to place restricted speed ranges on the engines – operating in the restricted ranges caused harmonics to be created that would destroy important things like wing spars. Most people think that the airflow behind a propeller is a solid pressure disk – but it isn’t. As each prop blade passes the structure – think leading edge of the wing – there is a higher pressure “Pulse” that thumps the structure. If the frequency of the thumps is a match for a harmonic or prime frequency of the structure – it will begin to vibrate heavily – think of striking a tuning fork repeatedly at the correct frequency. All structures have a natural frequency and if that frequency is excited the amplitude will increase. In some cases the restricted speed ranges of engine operation were right where the engine operated best – which made for some awkward power and speed settings..

I believe that there is a certain point on any machine including aircraft where it is a central point of focused frequency and can be in different places on the same model of aeroplane.

The deliberate un-synchronization of the engines may have been a requirement for operations – one that would have surely decreased the useful life of the aircraft – but in a time of war might have been an acceptable technique as the aircraft were not expected to last that long under battle conditions. I would wonder if the same result could be obtained by individual aircraft being in synch – but all aircraft in the group operated at different speeds. Would that have caused the unsyncronized beat frequency???

Hope this helps in understanding the syncronization and phasing requirements.

Truc 🙂

Synchronising engines and synchronising props are two very different things.
One might sync the props using the methods offered here, but what if you’re trying to sync a aircraft that has a new prop and an old prop?
The old prop may be an overhauled unit that has had an inch or two removed from the blade radii.
Now i’m sure you’ll get them to sync, but i’ll almost guarantee you that the engine RPM will be out of sync.
Generally basic engine sync’ing in old aircraft is acheived by looking at the engine RPM gauge and inlet manifold pressure gauge and setting the desired settings that way.
Then the CSU is constantly making adjustments to the prop pitch in order to maintain engine RPM stability not prop RPM stability.
Even a slightly heavy prop as opposed to a lighter prop (all of the same prop model of course), will have differing effects on how the individual set ups work but whatever they do engine RPM is the base on which these old engines are operated.

Bomberboy

Thanks for info, looks like just what we were after, I wonder if thats why many older multi prop aircraft have a fuselage window level with the propellors?

Yes, one reason indeed ; and often adjacent to the flight engineer’s position

That sounds familiar Scouse. I’m pretty sure I read it somewhere else too but can’t remember where. It’ll probably come back to me when I’m doing something else and can’t get to the computer then by the time I can get to one I’ll have forgotten what I remembered.

Sorry to be vague, I really can’t remember where I got that from. 😮

.

I’m pretty sure some of the wartime experimental Rolls-Royce engines were two strokes. My usual reference for this is Bill Gunston’s account of RR aero engines, but it’s gone walkabout in the house somewhere – can anyone else confirm or deny this?