Hi Everyone, long time no post!

I have a question regarding ground adjustable trim tabs for the rudder.

In particular, I am trying to get my head around the statements "tab left-tail left, tab right, tail right" and "bend the rudder tab in the same direction as you want the ball to move", both on page 12.2

I would have thought that if the ground tab on the rudder was adjusted to the left lets say, that the local aerodynamic force would push the rudder and deflect it to the right, much the same as it would on an aileron (tab down, aileron down), and not push it left? What have I missed? Also, the answer to Final Test No1, question 7 with regards to adjustable ground rudder trim tabs is (D), which going by the "tab left, tail left" principle should be (C)?

I guess I am missing something very simple here, so it would be great if someone could point it out to me, its doing my head in!

Thanks!

Hi Basketball,

When you yaw to the left, the nose moves to the left, the tail moves to the right and this is caused by the rudder being deflected left into the relative airflow. If you want a tab to deflect the rudder to the left, the tab needs to be bent to the right.

So, bend the tab to the right, the rudder moves to the left and the tail moves to the right. The tab is bent in the direction you want to yaw, or "tab right = tail right".

Similarly, yawing right means the tail moves to the left because the rudder has been deflected to the right and if you use a tab to deflect the rudder it will need to be bent to the left to create that local aerodynamic force required to do that.

So, bend the tab to the left, the rudder moves to the right and the tail moves to the left. Again, the tab is bent in the direction you want to yaw, or "tab left = tail left".

Cheers.

RIch

Thanks Richard, got it now, i was misreading the text which was confusing me! Thanks for clarifying.

Cheers.

Hi Richard,

Just a quick clarification because I've just come across a practice question like this.
If a plane was continuously yawing right, to yaw left your rudder would deflect left yes? And since a Trim tab is deflected in the opposite direction to create a small force from opposing airflow to hold that position, would you not have a fixed rudder trim tab deflecting right? To help minimise this imbalance that continuously causes the aircraft to yaw right.
Struggling a little to grasp this as I firstly understood you would need the trim tab to deflect left to yaw the aircraft back left until I was told otherwise!

An aircraft that is continuously yawing to the right is asking the pilot to apply left rudder to correct the situation. If you apply left rudder you deflect the rudder to the left. That means that the ground adjustable trim tab would need to be deflected to the right. That would apply a local lift force that acts on the trailing edge of the rudder forcing it to move further left.

Bob

Great thats what i thought thanks Bob!