Long-period longitudinal oscillation

[tailgator]

My M20K has a long-period longitudinal oscillation (a phugoid) of roughly a period of 10s in cruise.

While it is on autopilot, I check to see the trim is not moving at all during these oscillations, so it's the plane's own aerodynamics (interestingly, the KFC200 knows not to over-correct it and stays put?).

Normally a plane should have damping for this motion and the oscillation should eventually flat out (i.e. self-stabilizing). However, in my flights the phugoid is pretty stable and doesn't reduce over time. It is not a big oscillation, but it is definitely noticeable both physically and on my G5.

While it is not a big deal, it definitely bothers me a bit. Curious if other Mooneys have this as well?

[A64Pilot]

ALL small airplanes have a Phugoid although very few pilots notice it, it’s usually pilot damped. It however is usually slight and is not divergent, if it is divergent well then of course we have a problem. I guess I shouldn’t have said all, that was my mentor as a test pilots quote what he told me when I asked, but I’ve learned to never say all, there is an exception to everything.

However 10 sec is pretty short period, I think mine is about three times that but I have never measured it, I just live with it because short of buying an Autopilot there isn’t anything I can do about it of course.

I first ran into it certifying an aircraft for Canada, they have a requirement for an airplane to maintain level flight hands off for some time, I discovered the Phugoid that way, until then I had never noticed it

For those that may not have heard about it, it’s why you can never really get the trim exactly perfect in dead smooth air, the aircraft will slowly climb and then descend in a slow oscillation of maybe 30 sec or so. It’s not much, most pilots may fly for many years and never really notice it, it may only climb and then descend 50 ft or maybe less or so.

[PT20J]

My M20J has a stick-free phugoid response with a period of about 25 seconds and it damps out after 2 cycles. I would expect the K to be similar. It sounds from your description more like an oscillation in the autopilot.

[A64Pilot]

A Phugoid occurs not necessarily because of any control movement it occurs if there is no control movement to dampen it out, normally a pilot dampens it out without any conscious thought. I think the long term is entirely normal and apparently so does the FAA.

https://en.wikipedia.org/wiki/Aircraft_dynamic_modes#:~:text=Phugoid (longer period) oscillations,-Main article%3A Phugoid&text=The phugoid oscillation is a,which it had been disturbed.

“Aerodynamically efficient aircraft typically have low phugoid damping.^{[3]: 464 “}

https://en.wikipedia.org/wiki/Phugoid#:~:text=In aviation%2C a phugoid or,downhill" and "uphill".

If you ever fly a helicopter, they are negatively stable in all axis, three phugoid cycles in an AH-64 is it, you won’t get four, it’s divergent

Instrument flying in a helicopter is “interesting”, takes constant attention.

[tailgator]

On 1/16/2024 at 5:56 PM, PT20J said:

My M20J has a stick-free phugoid response with a period of about 25 seconds and it damps out after 2 cycles. I would expect the K to be similar. It sounds from your description more like an oscillation in the autopilot.

I thought my KFC200 controls pitch through trim only, and my trim wasn't moving in those situations.

[PT20J]

1 hour ago, tailgator said:

I thought my KFC200 controls pitch through trim only, and my trim wasn't moving in those situations.

The KFC 200 primarily controls pitch with the pitch servo. If the pitch servo is exerting  a constant torque, the pitch trim servo is actuated to eliminate it.

https://www.bendixking.com/content/dam/bendixking/en/documents/document-lists/downloads-and-manuals/006-08262-0000-KAP-KFC-200-Pilots-Guide.pdf

[skykrawler]

52 minutes ago, PT20J said:

The KFC 200 primarily controls pitch with the pitch servo. If the pitch servo is exerting  a constant torque, the pitch trim servo is actuated to eliminate it.

https://www.bendixking.com/content/dam/bendixking/en/documents/document-lists/downloads-and-manuals/006-08262-0000-KAP-KFC-200-Pilots-Guide.pdf

And there is of course some dead-band within which the trim will not be triggered and the pitch servo has the whole responsibility.    As mentioned above, 10 seconds is short for a phugoid.   If you push the CWS button on the wheel and the aircraft tends to change altitude then a fine trim adjustment may help, but I think the pitch servo would be producing a 1Hz or so oscillation.    It could be you attitude indicator is starting to go or something to do with the pressure altitude sensor the AP or the pitch servo itself. 

[Nokomis449]

Check your static drain and visually verify there's no water in the static system.  I can easily see the lowest part of mine (where the water will collect) when I stick my head in the fuselage access door, but I'm not sure how the later Mooney's are plumbed.

[Utah20Gflyer]

Does your plane exhibit this oscillation when the auto pilot is off and the plane is trimmed for level cruise flight? 

[wombat]

I've found that I can 'trim' the plane slightly by moving my body forward or back in the cabin.  Even leaning forward or back can have an effect I can see.    If the autopilot is on and in altitude hold mode it'll correct for it, but movement within the cabin can cause slight pitch and roll of the aircraft.

[tailgator]

Thanks everyone for pitching in on the pitch issues

I agree it's more likely an autopilot issue then. The plane is currently down for AP upgrade and I'll check it again with and w/o the new AP once it's airborne.