"Oversquared" Powersettings

Hi all

Please take into consideration the torque curve of the engines.  Flying below peak torque with heavy loads can result in detonation.  In older versions of the crankcase, pre 2006, the crankcases on engines running hit loads had a tendency to crack.  A large number of aircraft never made TBO for that reason.  

Think of it this way.  Going up on the torque curve you are building pressure in the cylinder.  Once you hit peak pressure the engine can continue to climb in RPM (more HP) and the throttle response is very quick and the pressure diminishes (less load on the parts).  It turns out that 75% power is peak torque and this area and above give us the best throttle response.  Fuels play a great deal into this as well.  

No doubt many have seen the Service Letter but in case you have not i have attached it.  And yes this is for the type 912/914 but the extrapolation is the same for the type 915 and 912iS. 

Cheers

Resurrecting this thread... Pascal I think the answer to your question is in the Operator's Manual:

https://rotax.my.salesforce-sites.com/DocumentsSearch/sfc/servlet.shepherd/version/download/0685c00000Kj6FaAAJ?asPdf=false

In particular you might want to look at the "Specific Fuel Flow" graphs on pages 5-4 (Figure 2) and 5-6 (Figure 5).

I think what you'll find as you look closely at those graphs are two important facts:

1) The most "efficient" way to run your engine is, for any given power setting, to find an RPM value that is at the bottom of those graphs (there may be other concerns).

2) Note that for any given RPM setting, there is a point in those graphs where adding more power becomes clearly inefficient (For example, look at the 4000 RPM line on Figure 2 on page 5-4 just past the 50 KW power point). Based on this manufacturer's data, it is probably not a good idea to run the engine above the 94% throttle level at 4000 RPM.

I expect you can reach similar conclusions about beyond which point it is unreasonable to run the engine at 4500, 5000, etc RPMs.