Piper Navaho Chieftain Stability Question

[Mechta]

Hi, I'm back at college as a mature student doing an aero eng. degree. We have been set a research task into static and dynamic stability, and it reminded me of when I flew(as a passenger) in the Piper Navaho Chieftain (PA-31).

For a while we used to do a regular flight from Southern England up to Scotland and back each week, and it was noticeable on a flight of this distance that the Chieftain would appear to yaw gently from side to side. It was as if first one engine was pulling more, then the other, or maybe the fin was a tad too small. We had three Chieftains, and as far as I am aware, they all did this.

What I would like to know is if anyone else has experienced this motion in a Chieftain or similar, and if so what was actually happening? Was it yawing, was it some form of damped dutch roll (our lecturer suggested a dutch roll could take the form of the tail moving in a circular motion), and what measure might a designer have taken to prevent the effect?

[hawker750]

Hi
Owned and flew on loads of Chieftains and considered the aircraft to be really stable.
Pretty sure the aircraft had a simple yaw damper system, more likely this yaw to be the yaw damper reacting to slight turbulence. I remember the aircraft would trim out really well if the auto pilot was U/S

[Tinstaafl]

Yaw damper is optional & depends on the autopilot system. It does have an aileron-rudder spring connect so that might affect what you see. It's sometimes done to help meet stability requirements for certification so may be a factor in this type. Also, some autopilots aren't the best and can 'hunt' a bit. Is it possible the behaviour you saw was an AP issue? One PA31-350 I fly has an old Century III A/P. Depending on temp & humidity it can spend some time gently rolling L & R as it brackets its LNAV signal. I think I recall this is an analog AP so changes to resistances with temp may affect things.

A PA31-325 I manage has a KAP200 with YD. Much newer and rock solid with no hunting or bracketing. The YD pretty much eliminates any fishtailing, not that I've noticed anything much anyway.

Both aircraft have Panther mods so they have winglets, VGs, fences & 4-blade props. Any of these could affect stability, not that I've noticed compared to non-Panther or non-VG's versions I've flown. The -325 I look after also has -350 engines so that could affect things too.

[Dr. Bunsen Honeydew]

Flew the PA31-350 with a yaw damper years ago and always enabled it when established in the climb until on short final. It flew like a hot knife through butter and I wouldn't dream of not using it. Later we got another one without YD and it was prone to classic dutch roll especially with an aft C of G (usually large pax in the back row + rear baggage). Any turbulence or slightly unco-ordinated manoeuvres would wag the tail and trigger some creative dabs of rudder to regain composure until the next wiggle developed. I've heard it compared to Marilyn Monroe's walk so it never really bothered me...
I always wondered if the stretch from the original -325 model whilst keeping the same fin was the cause of the stability issue. Perhaps a -325 driver might have some thoughts?

[Mechta]

Thank you for all the replies.

It sounds like the yawing motion could have been a combination of several of the items mentioned. We flew with baggage for a week away, all seats filled on occasions, and some of us weren't stick insects.

A friend I've just spoken to turned out to be involved in servicing the fleet. He confirmed that the autopilots were prone to overcorrecting on heading adjustments, and were a constant maintenance issue.

I've also found one of them on Google, and it was a -350 model, so the undersize, or at least 'not increased', fin size may also have been a contributing factor. I can't say if all the fleet were the same model, but I expect they were.

Thanks again, Mechta

[Tinstaafl]

I figured that the additional FS arm to the rudder on the -350s was sufficient. Don't forget a -350 (ie a Navajo Chieftain) is 2' longer than -300/310/325 (all are Navajos). On the Panther Navajo's with the -350 engine.....damned if I know! I haven't noticed any difference from a -310 I used to fly. Mind you, they both have VGs so that changes things a bit. Anyway, it has a YD so that goes on immediately after gear up and stays on until just before landing.

Wonder what a Navajo without any aerodynamic bolt-ons would be like with -350 engines?

[tommoutrie]

You can make a chieftan go a bit faster if you alternately press each rudder pedal - it causes the rudder to wag from side to side which pushes the plane forward in much the same way as a a fish tail works.

[Chinchilla.612]

Tom,
I've told you before.......that excuse just doesn't work for you not coordinating the controls :p
Although I'm glad you stopped doing it with the elevator while saying it works with flippers.......

[Tinstaafl]

Yawing L & R to increase speed only works if you have all seats facing aft and the rear door removed That way when the pax start hurling their lunch out the door the equal & opposite reaction will be in the direction of flight.