Interesting Roll ?Issue?

[Tango]

Hi,

 

With the rotor system being co-axial, I'm intrigued as to why I require additional left roll input as the aircraft accelerates?

 

Looking from external view at the angle of the upper set of rotors when flying at 250 km/h, they seem to be tilted quite a long way.

 

Why is this?

 

I can understand this phenomenon for single-rotor due to retreating blade stall and the resulting loss of lift, but given the configuration, the loss of lift should be symmetrical???

 

I've also noticed that I'm unable to yaw without pulling a bit of collective (it is very slow and unresponsive). Pulling a bit of collective seems to suddenly make it more effective. This is whilst in the hover for example, so no airflow over the tail.

 

Best regards,

Tango.

[Yo-Yo]

Coax rotors are not symmetrical due to at least two reasons: the thrust of upper and lower rotors are not equal and the force arms are different.

 

The answer for the second Q is obvious: the yaw produced by a difference of rotor moments so the power they absorb. By the way, that's why during autorotation there is a lack of yaw control.

[ericinexile]

Hi,

 

With the rotor system being co-axial, I'm intrigued as to why I require additional left roll input as the aircraft accelerates?

 

Looking from external view at the angle of the upper set of rotors when flying at 250 km/h, they seem to be tilted quite a long way.

 

Why is this?

 

I can understand this phenomenon for single-rotor due to retreating blade stall and the resulting loss of lift, but given the configuration, the loss of lift should be symmetrical???

 

Coriolis Force. You know the same phenomenon that makes the water in a toilet spin as it's flushed. If you were flying south of the equator you'd require right cyclic.

 

Naw...just kidding. I have to idea.

 

I've also noticed that I'm unable to yaw without pulling a bit of collective (it is very slow and unresponsive). Pulling a bit of collective seems to suddenly make it more effective. This is whilst in the hover for example, so no airflow over the tail.

This one is easy...but it's still a guess. The Hokum relies on differential torque in order to yaw at slow speed. The collective is your primary means to create torque. The more torque supplied by you means more total torque available for the rotor system to generate a yaw moment.

 

Smokin' Hole

[GGTharos]

Hi,

 

With the rotor system being co-axial, I'm intrigued as to why I require additional left roll input as the aircraft accelerates?

 

Because you have a slightly unbalanced torque effect.

 

Looking from external view at the angle of the upper set of rotors when flying at 250 km/h, they seem to be tilted quite a long way.

 

Why is this?

 

I can understand this phenomenon for single-rotor due to retreating blade stall and the resulting loss of lift, but given the configuration, the loss of lift should be symmetrical???

 

It isn't symmetrical. The lower rotor produces less lift. Also, the rotors are counter-rotating, so they will angle in opposite directions on the same side - one rotor produces lift on one side, one on the other ... and in different amounts.

 

I've also noticed that I'm unable to yaw without pulling a bit of collective (it is very slow and unresponsive). Pulling a bit of collective seems to suddenly make it more effective. This is whilst in the hover for example, so no airflow over the tail.

 

Best regards,

Tango.

 

Now that sounds a bit strange. If you are hovering at full power then yes, it'll be a problem because in hover you use a torque effect (getting one rotor to spin slower than the other) to yaw. It shouldn't matter if you are hovering (you'd simply lose a bit of power)

[mckee14]

hold down the trim button or activate the flight director disengages the autopilot, which tries to hold the heading you have (so he works against your yaw)

[quo vadis]

Now that sounds a bit strange. If you are hovering at full power then yes, it'll be a problem because in hover you use a torque effect (getting one rotor to spin slower than the other) to yaw. It shouldn't matter if you are hovering (you'd simply lose a bit of power)

 

Do they really slow down one rotor ? I thought they would change the angle of attack of the rotor blades (reducing on one rotor and increasing on the other to maintain same amount of lift) ?

[Tango]

Hi,

 

Yes, to induce a torque reaction, they reduce the speed of one of the shafts (they don't increase the speed as this could result in an over-torque or over-speed of a shaft/drive). This is why I'm intrigued as to its ineffectiveness in certain, specific, phases of flight.

 

Best regards,

Tango.

[AlphaOneSix]

I've also noticed that I'm unable to yaw without pulling a bit of collective (it is very slow and unresponsive). Pulling a bit of collective seems to suddenly make it more effective. This is whilst in the hover for example, so no airflow over the tail.

 

Best regards,

Tango.

 

As you've mentioned, yaw control is effected by introducing differential torque between the two rotors. When stable, each rotor has roughly the same amount of torque, so they balance each other out. To yaw the aircraft, the rotor system must reduce the torque of one rotor and increase torque on the other. At very low blade pitch angles (low collective settings), there is not enough blade pitch to yaw effectively, since the blades only go down so far (roughly 1 degree of pitch).