'Centering the Ball in forward flight.'

[Fish]

Strange. At what speed? Look at controls indication in AirTito's post here http://forums.eagle.ru/showpost.php?p=592330&postcount=12 Rudder to the right, cyclic to the left.

...On second thought, maybe AP is keeping you straight? Do you have DH switch on?

 

K,T on, H,B off. Speeds say, steady 220kph.(in as much as i can maintain this steady.

[Fish]

Well, when you are accelerating......................

 

Another point though is that if you have any wind...................

 

 

Yes I'm talking of steady forward flight, fairly fixed speed, fixed (cyclic)pitch forward (trimmed), and use collective to maintain speed/alt.

 

Did not consider the wind but would expect it has less impact at bigger speeds. Will try again flying directly into/against wind, where there's not cross-wind component, to see how it flies.

:thumbup:

[Yskonyn]

If the ball is right of centre, give right rudder. If it stays right of centre, give more right rudder.

 

And vice versa.

 

Oh, and when it is in the centre and stationary, trim to maintain that.

 

Exactly, a common aviation term is to 'step on the ball' to center it.

[EtherealN]

That's what my instructor always yelled ay me from the back seat, yup. :)

[Weta43]

The rotors are counter-rotating. The "sum" of their rotation speed is zero, as is the torque unless you have rudder input since the latter increases the angle of attack on one blade disc while the other disc is lowered in alpha, causing differential torques from the discs and then you get rotation.

 

This might have already been said, but this is not true.

Even without rudder, each of the counter rotating blades is generating simillar amounts of lift & torque around their hub, but they are set at different distances from the helicopter's center of gravity and so create a net torque on the aircraft when the aircraft is moving forward (or any other direction) fast. At speed, cyclic and rudder is needed to 'center the ball'.

[Fish]

Solved!!!

 

There was a problem with my cougar profile, in that any time i trimmed, it ignored the rudder trim. I changed this now, so i can trim the cyclic and the rudder together. On my first test of this, i could set the ball in the center, with the cyclic forward and left of center, and the rudder slightly right.

 

Now its off for retraining, to fly with rudder trim on.!!

 

Thanks for all who contributed

[sobek]

Even without rudder, each of the counter rotating blades is generating simillar amounts of lift & torque around their hub, but they are set at different distances from the helicopter's center of gravity and so create a net torque on the aircraft when the aircraft is moving forward (or any other direction) fast.

 

This is not entirely true, AFAIK. The two rotors do produce the same amount of torque on their rotational axis, however they generate dissimilar lift due to the lower rotor being in a climbing state IIRC. Once the helo moves, the lift zone transitions to the sides of the rotors and is no longer evenly spread, therefore the rotor which is further from the CG produces a even stronger torque around the center line and the helo starts to roll (you stated the latter correctly). So there are two effects, the rotors produce dissimilar lift on opposing sides and the one producing more lift is further away from the CG, giving it a longer lever.

Edited April 21, 2009 by sobek

[Weta43]

Got to admit I didn't read your initial post carefully & thought you were implying the rudder inputs needed during forward flight were a result of differential torque through the vertical axis.

Seems you did something similar...

me

each of the counter rotating blades is generating simillar amounts of lift & torque around their hub

you

The two rotors do produce the same amount of torque on their rotational axis

Given the rotor's rotational axis runs through their hub, no argument.

 

me

but they are set at different distances from the helicopter's center of gravity and so create a net torque on the aircraft when the aircraft is moving forward

you

Once the helo moves ... therefore the rotor which is further from the CG produces a even stronger torque around the center line and the helo starts to roll

 

Given the only axis that the rotors are different distances from is the longitudinal axis, therefore the torque I mention produced must be around that axis, and torque around the longitudinal axis produces roll, again - different words to say the same thing :-)

 

So - ignoring rudder inputs during forward flight, any time the helicopter is not turning, the two rotors produce the same amount of torque on their rotational axis...

 

So we agree:

 

While in forward flight however, the 2 rotor disks do not both produce the same amount of torque around the helicopter's longitudinal axis...

Which causes the aircraft to roll...

Which causes it to generate a sideways thrust...

Forward thrust from pitch + sideways thrust from roll = net thrust at an angle to the aircrafts longitudinal axis...

Which means that in order to move in a given direction, (without rudder) the nose has to be pointed slightly off course

Which means whenever the aircraft is moving at speed, and in the absense of corrective trim, the aircraft is yawing...

Edited April 21, 2009 by Weta43

[Yskonyn]

Which is nicely modelled in BS! :)

[sobek]

[...]

 

You seem to miss my point. I just wanted to add that the two rotors NEVER (not even in a hover) produce the same amount of lift, because the lower disk is in the downwash of the upper. They do however produce the same amount of yawing torque (or you would have to apply constant rudder). At least so i was told :) In other words, the lower disk works less efficient than the upper, being supplied with the same torque but producing less lift.

 

 

This dissimilar lift adds to the roll effect caused by the different lever lenghts of the two rotor disks.

I hope this makes sense to you.

[EtherealN]

Even without rudder, each of the counter rotating blades is generating simillar amounts of lift & torque around their hub, but they are set at different distances from the helicopter's center of gravity and so create a net torque on the aircraft when the aircraft is moving forward (or any other direction) fast. At speed, cyclic and rudder is needed to 'center the ball'.

 

Entirely true, yeah. I just simplified it in my post to make it easier to understand. Simplifications are dangerous and I should have left a disclaimer in it to say that it's not at all the full story.