Aeroplane yaw

[Fergal20]

Hello there,

I'm designing a flight simulator for college, and I'm at the stage where I have to integrate the yaw. From my own experience (I have a PPL) yaw is used when on cross - wind landing, balancing the turn, etc. But, my question is this:
How come the plane wont turn fully when rudder force is inputted and the wings are keep level? It just move the direction of the plane left or right, but not fully around.

Thanks in advance for any answers:)

[Tinstaafl]

You mean yaw through 360 deg? Sometimes called a 'flat' turn.

It will. Eventually. The catch is that the size of the forces that can be generated by rudder is a lot less than can be generated by the wings (small fin/rudder vs. big wings).

The a/c has inertia & directional stability so will tend to return to the same heading if disturbed AND the disturbing force is removed. Keep the force there however, and it will eventually achieve a new heading.

Next time you fly on your own (pax don't usually like playing like this) make sure you're below Va comfortably above Vs. Gradually apply up to full rudder. Hold the rudder for 10 or 15 secs & release (under control). Keep the wings level through out. The a/c will stabilise on a new heading.

Secondary effects mean that you would have 'fight' these wtih crossed controls to maintain a wings level attitude. That leads into potential nasty effects if the a/c stalls during the turn.

The fin/rudder could be increased in size to better allow such turns but that in turn can have unfavourable effects.

Overall rather less efficient than a normal, banked turn.

Also the skidding sensation is not as comfortable as a balanced turn that maintains the force acting normally.

[Genghis the Engineer]

By making a rudder input you are creating sideslip.

The aircraft has two important static stability derivatives which are a function of sideslip. These are lateral stability and directional stability. Expressed mathematically both are the first partial derivative of moment (rolling and yawing respectively) with respect to sideslip.

So, when you make a rudder input the first thing that happens is the aircraft sideslips. Then the stability characteristics cause it to both yaw and roll. The only way a truly flat turn could be achieved would be in an aircraft with no lateral stability (either positive or negative), but retaining directional stability. I can't offhand think of any aircraft that has these characteristics - something like a Pitts must come reasonably close I'd guess.

The book I learned much of this stuff from was Donald McClean's "automatic flight control systems", which if you can find a copy may help a lot in working through the maths - it also has sample sets of stability derivatives for known aeroplanes that might help. ISBN 0-13-054008-0.

G