P68b

[Dude~]

Does the Partenavia P68B have counta rotating props or is there a critical engine?
(Should I say contra, or is that the same as counta?)

Ta

[stiknruda]

Flew a 68c (Lexan nosebowl) for about 10 hrs about 10 years ago, IIRC they both went the same way and as both donks were Lycoming Os the left one would be the critical engine.

Stik

[Who has control?]

Stik - care to explain why?

[Tinstaafl]

Stick. is referring to clockwise rotating donks so the L. becomes the critical engine.

The critical engine is the one that would have the greatest adverse effect if it fails.

With clockwise rotation (viewed from behind the a/c) the overall thrust line each engine produces is offset slightly to the right. This is due to the same reasons as a single engine a/c's yaw tendency with power application.

For the LH engine, this offset moves its thrust line closer to the longitudinal axis. For the RH engine the thrust line moves further away from the longitudinal axis.

The further the thrust line is from the longitudinal the more inefficient flight will be when asymmetric due to greater slideslip effects & additional control surface deflections causing drag.

If the LH engine fails (or is deliberately stopped :p ) then the a/c is left with the RH engine's relatively greater offset thrust line adverse effects. If the RH engine fails, the a/c is left with the relatively lesser adverse effects of the still running LH engine's offset thrust line.

Since the adverse effects are worse if the LH engine fails (for clockwise props.) the LH engine is the critical engine.

[Dude~]

Thanks guys. I shouldnt admit to this, but I actually flew the P68B after a 1.3 hr checkout and wasnt aware of this. I have just been comparing my photos with a Duchess I also flew. Now that really pisses me off that my instructor never emphatically pointed this out. I have kept the questionaire I did at the time and I see now I wrote down left engine as being the critical but the instructor never checked the thing. Makes me wonder what other things one might miss. At least I know I will never be unsure about critical engines now.

[Skylark4]

You have COUNTER-rotating props on a twin for the reasons discussed above.
CONTRA-rotating props are as per Shackletons or Fairey Gannet, two props on concentric shafts going in opposite directions.

Mike W

[Tinstaafl]

It's not as bad as it first seems. Performance & handling considerations eg Vmc are based on the worst case ie critical engine failure.

You could say that if the RH(non-critical) donk quits you may gain a slight additional handling & performance benefit. The benefit may be more in theory than in practice, considering airframe & engine age and pilot handling effects.

[stiknruda]

Tinstaafl has answered 'who has control's' question well.

All I ever remember is that the down going blade creates more thrust than the up-going blade! Works just the same on single engines as well!

Whilst learning to fly, the hardest part for me to understand was why I needed different feet (well, left or right!) pressing on the rudder at varying airspeeds!

Now that I fly fairly passable standard level aeros, I feel myself thinking less about this and compensating naturally as a matter of course and have even got my head around the effects of gyroscopic precession on the prop when I pull or push to the vertical.

I have had lectures about the 3 gyro forces affecting a spinning aircraft; A, B and C and must confess that at this embryonic stage in my understanding of aeronautics, that it is easier to fly the aircraft than it is to master the math behind the 3 spinning components (prop, fuse and wings!)


Stik