Engines coupled as in DB 606 and DB 610

[Elmas]

I'm a structure man, not an engine man so there's one thing that I don't understand.
AFAIK the coupling of the two engines was trough a mechanical device and my question is: how can two engines coupled by a mechanical device based on gearwheels have a tuning so identical to exactly give the same power at the same RPM? Even a few dozen, minus o plus, RPM of difference between the two engines can put an heavy stress on the couplig device, creating harmonics and consequently vibrations, and overheating. Or there was some particular device to avoid this?

 

[davebender]

ZF was founded in 1915 to make gearboxes for coupled engines and apparently they worked just fine even during WWI era. I have no idea how ZF pulled it off but if you read German there must be a wealth of technical information in ZF historical archives.

 

[Elmas]

Thanks, my knowledge of German is very poor but I'll have a try.

 

[GregP]

Another way to do it, though not usually used in aircraft except for starters is to have a one direction clutch such that you can start and run one engine at a time or run both. If one engine is not running, the clutch is disengaged and spinning with very little load. When both are running, as long as both are turning the same rpm, they can put out different power and do just fine. It is very easy to get them to turn the same rpm ... you just apply enough power to get both engines loaded. Since they are geared together, they MUST have the same rpm. If one is throttled back, it loses rpm and the engine delivering more power takes over while the engine delivering the lesser power coasts at lower rpm.

 

[Elmas]

To synchronise exactly the RPM of two, three or four piston engines (before electronic age, of couse) was not an easy task, so I was told by old Pilots, so to synchronise engines coupled trough a gearbox must have been a nightmare, I suppose.....

All those thing are in contrast with Kelly Johnson's First and most important Rule, that he always reminded to his Engineers:
KISS!

 

[rinkol]

I'm a structure man, not an engine man so there's one thing that I don't understand.
AFAIK the coupling of the two engines was trough a mechanical device and my question is: how can two engines coupled by a mechanical device based on gearwheels have a tuning so identical to exactly give the same power at the same RPM? Even a few dozen, minus o plus, RPM of difference between the two engines can put an heavy stress on the couplig device, creating harmonics and consequently vibrations, and overheating. Or there was some particular device to avoid this?

I haven't seen the specific details of the mechanical layout, but expect that each crankshaft would drive a gear that meshes with a larger gear to drive the propeller at an appropriately reduced rotational speed. In any case the crankshafts would turn at exactly the same rpm with a fixed rotational offset (i.e., no slippage). Any small differences in engine output would not matter. I recall seeing a claim that there were clutches that allowed one of the two coupled engines to be shut down, but am skeptical.

The tandem arrangement used in the Fiat AS 6 record breaking engine used a quite different arrangement with a contra-rotating propeller consisting of two 2 bladed propellers, each of which was driven by its own V12 engine.

Robert