Hey HoHun,
A twin engine in cruise flight with both props turning the same direction (Co-Rotating). Is this what were talking about, if it is.
Both engine are producing equal thrust and both wings have equal prop wash induced airflow across them. At level flight with no positive angle of attach there is no P-factor present. P-factor is only present in a nose high or positive angle of attach, which will yaw the airframe to the left. In a single engine situation the when the right is only running P-Factor and engine thrust will yaw to the left the airframe far more because of the leverage it exerts from the length away from the CG. With the left engine only running the P-Factor wants to yaw the airframe to the left but the thrust is greater and will yaw the airframe to the right. We get P-Factor in a single engine situation because of the asymmetric Lift nose high angle of attach.
In straight and level flight with both engines running all forces are balanced. Aileron trim is at 0, rudder trim is slightly to the right due to prop slipstream, elevator trim set to hold a given airspeed.
If you think that there would be an induced roll due to engine torque. Well lets discuss this a little. Torgue effects is at its greatest in power application or power reduction in a single engine airplane. With a strong power application there is a greater torque moment. Most pilots just dont jam the throttle to the engine, to reduce the torque effect they add power gradually and to counter this rudder can be used. But aileron can be used also but when you use aileron, you get a adverse aileron effect and sideslip which yaws the plane into the direction of engine torque moment. Correct? So now you need more rudder and more drag of the airframe, this is less efficient then just using rudder and to reduce sideslip to 0. With a decrease in engine torque the single engine plane will yaw to the right, rudder is used to counter this effect. In cruse flight engine torque is present but is not a great influence on the airframe so a slight right rudder trim is used to counter this at a 0 sideslip.
Multi-engine airplane do not experience this torque roll effect. This is a different airframe all together the engine are mounted on the wings and the engine and props are mounted inline to the nose of the airframe, way forward of the CG. Now if you take and draw a line from the CG straight out to the engine placement and measure the distance we have some kind of leveage on the CG. Now draw a line from the engine to the most forward point were the prop location is, and measure the distance. The leverage arm has increased more, and this leverage is off the center of gravity, and off the longitudnal axis. Buy using the coefficient of yaw formula you can calculate the yaw force induced on the airframe. But nowhere is there a input for torque effect in the yaw coefficient formula.
The placement of the engine from the CG and longitudnal axis, torque forces are deminished due to the placement of the engines, but engine thrust and P-Factor and prop slipstream forces have increased.
Now when you see the picture of a twin provided by Hitech you can see this. Lets take the right wing and engine, if the engine is running the wing would rotate the oposit of the props direction of rotation. If I took the hand of god and grab the left wing tip the wing rotation would stop. Lets call the hand of god the fuselage. With now mounted to the left wing tip and the fuselage that has a horizontal stabilizer and elevators, by using up elevator you can stop this rotation in flight. But what happens know because torque force is deminished by the fuselage and elevator force, the placement of the engine will try to create some force or leverage on the airframe.
Remembur that the fuselage is in the CG and longitudnal axis or the pivot point in the roll axis and yaw axis. The engine is the pivot point or roll axis for torque force in a single engine airplane, but not in a multi-engine when the engine is mounted on the wing away from the fuselage center pivot point. This is where the fuselage has a greater moment then the engine torque moment. All the engine can do know is roll the plane to the left by asymmetric lift and yaw, due to the P-Factor and the engine thrust around the pivot point roll and yaw axis. So by using rudder on the fuselage roll and yaw moment, this can counter this force, which has a further distances from the CG and has a bigger leverage arm then the engine. But when airspeed is decreased you will lose this leverage and the airplane will stall/spin out of controll.
The same goes for the left hand engine and the hand of god thing and all other things relate to the right engine.
Later Straiga
