Engine rotation (NAC)

[gjkscruggs]

Although there is convention for engine rotation direction... probably for
ease in spec'ing gears, starters and such... there is no physical reason for
one direction over the other, only cultural.  Balance or imbalance doesn't
know direction.

They don't all turn the same way.  The first guys that used Corvair engines
with VW transaxles in their dune buggies (remember the Meyers Manx?)
discovered that they had one forward gear and four reverse gears... but
flipping the ring and pinion, as I recall, solved the problem.

Twin engine aircraft and boats use counterrotating engines for the same
reasons, to reduce torque reaction.  Let's pick a worse case... the P-51,
for instance... to see why reduction of torque reaction is desirable.  As
you add power for takeoff your single engined P-51the prop rotates to the
right and as you roll and continue to add power the aircraft begins to drift
to the left.  The torque applied to the prop by the engine must be countered
by the aircraft... engine mounts, aircraft structure, landing gear... and
ultimately the port side tire acting on the runway.  As the tire 'flattens'
against the runway it causes more drag on that side and the aircraft pulls
to the left.  You counter with some right rudder but as you add more power
to reach take off speed more torque is generated and you add more rudder.
Soon you simply run out of rudder travel and your P-51 continues to drift to
the left side of the runway.  This is why full power can't be used on
takeoff on a lot of WWII single engine aircraft.  This effect takes place in
all single engine propeller-driven aircraft.  You have to add right rudder
even in your Cessna 150.  But if you're flying the Su-26... Russian
acrobatic competition aircraft... you have to add right rudder on takeoff as
the Vedeneyev M14PF engine turns to the left.

And if you have the twin engined P-38 which has counter-rotating engines
then you add as much power as you want as the engines torque reaction
cancels and you go straight down the runway... absent crosswind interaction.
Reasons are the same but the effects are not as drastic for boats.
Countering torque reaction is also why boat drivers usually sit on the right
side of the boat.

As has been mentioned the only downside of having counterrotating engines
for boats and aircraft is that your logistics folks have to stock twice as
many components thus driving up the cost.

Regards, Gross Scruggs