Rate controls vs rudder

[jpilotj]

Quick question on aerodynamic theory that I was asked today...


Aileron and elevator are both rate control controls. That is to say that if one applies roll, the aircraft will keep rolling. However if one applies rudder input, and holds a yaw will occur with a secondary effect of yaw. Why does our rudder control however not manifest into constant yaw?


Responses are appreciated. Cheers

[rudderrudderrat]

Hi jpilotj,

If you held a constant rudder deflection with wings level, you would constantly yaw - and with enough time - yaw through 360 degs heading change.
The reason you don't - is because you'll put in a roll to counteract the heading change.

[jpilotj]

Thanks,

I thought the answer might be along the more simple lines than what I was thinking.

Cheers

[Microburst2002]

The answer is in stability about the three axis and in the way the flight controls achieve moments about the CG:

Airplanes are stable in both the lateral axis (pitching) and normal axis (yawing). What you do with the elevators and the rudder is to change the trim angle of attack and the trim slip angle, that is, the angle of attack and the slip angle to which the airplane will tend after disturbances, as long as you keep the stick (or rudder) fixed.

When you pull the stick, the airplane will raise the nose and the angle of attack will increase, but it will stop increasing as soon as its stability cancels the pitching up moment created by the elevator with an equal pitching down moment created by the stabilizer. The same occurs with the rudder and the slip angle.

However, roll is achieved by differential lift in the wings. There is not an equivalent to angle of attack or slip angle for the roll axis. As long as the ailerons are not neutral there is a rolling moment about the CG. Airplanes are only slightly stable in roll, when they are stable at all, so if you keep the ailerons deflected, the airplane will try to roll continuously.

I think this is what you mean