Re: Contra-rotating propellers

Date:         26 Dec 97 03:28:53 
From: (G3AV8TOR)
Organization: AOL
References:   1
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There are many propeller driven aircraft that use counter-rotating props.

In a climb, the descending prop blade will create more thrust tham the
ascending blade. This is called P-factor. The result in any aircraft, single
engine or twin with noncounter-rotating props, will be a yawing turn to the
left in a climb, assuming the blades are turning clockwise as viewed from the
cockpit. In a multi-engine, this is due to the right descending propeller being
further from the longitudinal axis than the left descending blade, ie. more
moment or turning force along the longitudinal axis. To offset this, the pilot
must hold right rudder in the climb.(Or adjust the rudder trim to the right{not
usually done for short durations}).

With counter-rotating props, P-factor is nullified. You may say "So what?
What's the big deal?"

Certification of multi-engine aircraft requires establishing Vmc or Minimal
Controllable Airspeed.. This speed is determined with the aircraft in a
specified configuration.  One of the specifications is that the critical engine
be inoperative and windmilling and the operating engine at maximum rated power.
Which engine is the critical engine?

The critical engine is the engine that if it fails, the remaining operating
engine will produce the most yaw. Should the left engine fail. in a
noncounter-rotating engined aircraft with counter clockwise turning props, the
right descending prop will take more rudder force to counteract because it is
further from the longitudinal axis of the aircraft. Should the right engine
fail, left rudder would still be needed, but the resulting yaw force would be
less because the left descending prop is closer to the longitudinal axis. In
this scenario, the left engine is the critical engine.

In an aircraft with counter-rotating propellers, both descending propellers are
equal distant,  meaning both are closer  to the longitudinal axis, so neither
engine is considered a critical engine.

This would result in a lower certified Vmc than the same aircraft equipped with
noncounter-rotating propellers. This would affect aircraft certification and
performance, ie.take-off speeds, approach speeds, etc.

Yes, two sets of engines.left and right, would need to be maintained.