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If you a small GA plane, and apply full rudder to one side and then let go, there will be some oscillation from the momentum of the rotation about the yaw axis. Does this same effect happen with the pitch axis?

This can only probably happen when the plane is at a high angle of attack, but does it happen when maneuvering?

(Below is what I believe happens with the yaw axis in a sideslip, correct me if I'm wrong)


So say you have a fuselage without wings (still with tail section), fixed on its CoG. If there is a yaw disturbance, the V-stab will counteract that, sending the tail back to the original position. The thing is, once the tail is back where it was before the disturbance, there will still be the momentum of the tail section traveling in that direction, so it will continue past the original position and go the opposite direction of the way the disturbance made the tail section go.

Another way to think of it : Pretend you're in a sideslip because you're holding the rudder to where your nose is to the right of the direction of flight, in a smaller aircraft (just for simplicity). When you let go, the V-stab will counteract this sideslip, pushing the tail back to the right (viewed from behind the aircraft). When the tail arrives at the position where it is parallel to the freestream, it still has the momentum from getting pushed back from the sideslip.

That would make the tail now continue traveling to the other direction, creating some sort of oscillation as the process repeated itself, I think.


Now I don't see anything preventing this from happening in the pitch direction either. Am I missing something, or does it also happen in the pitch direction?

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Yes it happens in pitch. In fact, you generally learn about the pitch oscillations before the other modes.

There are two pitch oscillations that we generally talk about. One is called the short period, the other is called the Phugoid.

The short period is most like what you're thinking of with yaw. It is a fast oscillation that is quickly damped. When a stable aircraft is perturbed from the trim condition (CM=0), there is a moment to correct. That moment accelerates the aircraft back towards the equilibrium condition. Once it gets moving, the rotational inertia causes the aircraft to 'over-shoot' the equilibrium condition, which causes the corrective moment back the other way, etc. This motion is quickly damped out -- on the order of one second.

If you have a throwing dart, get someone else to throw the dart and stand to the side so you can see the flight path. When they throw the dart straight, you can't see anything special because the perturbation from equilibrium is so small. Instead, have them throw the dart 90 degrees (or even 180) to the normal direction of flight. You will be able to see the short period oscillation.

The other longitudinal oscillation is the Phugoid. It is a very slow motion that we consider to happen at constant angle of attack (CL) and at equilibrium pitching moment. It happens when the aircraft's trim airspeed is not the current airspeed. It occurs as an exchange between potential and kinetic energy. Each oscillation takes 30 seconds or longer and it can take minutes to damp out. In normal circumstances, the pilot will cancel out this oscillation without thinking about it.

The source of damping in the Phugoid is aerodynamic drag, so as aircraft are designed to be very low drag, they will have less and less Phugoid damping.

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  • $\begingroup$ Ah okay that makes sense, +1. So the Phugoid oscillation happens basically because the lift coefficient changes with airspeed, and when the aircraft is going a different speed than trimmed for, it will want to pitch. Once it has pitched, the airspeed will change, again changing the lift coefficient, and changing the flight path. Is this correct? $\endgroup$
    – Wyatt
    Apr 20 at 22:31
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    $\begingroup$ The lift coefficient is constant during the Phugoid. When the airplane is going too fast, it will get too much lift, which will cause it to climb, losing airspeed - until it is too slow, then it will descend, gaining airspeed. This is done at constant control inputs -- fixed trim conditions - but the aircraft is out of trim until the perturbation is damped and it gets back to equilibrium. $\endgroup$ Apr 21 at 5:16

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