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Bruce
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Plane Swaying Side To Side

Fri Aug 06, 2010 6:48 am

Recently I took a long ride in a Mooney M-20 plane. One thing I noticed is that the plane appeared to be "swaying" from side to side, along the yaw (?) axis. I could feel it, and I even looked out the window at points on the ground and could see the movement. What causes this? Is this a characteristic of Mooney aircraft, or bad piloting? or just the wind? We were holding steady at 6,000 with the autopilot on and holding speed and altitude. I think we had a headwind. But I have had rides in Cessnas and never noticed this.

Bruce
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 6:57 am

sounds a lot like the wind to me, pushing around those tiny airplanes.
 
swiftski
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 7:12 am

Quoting Bruce (Thread starter):
One thing I noticed is that the plane appeared to be "swaying" from side to side, along the yaw (?) axis.

Yaw around the Normal axis.

Quoting Bruce (Thread starter):
or bad piloting?

Well..

Quoting Bruce (Thread starter):
with the autopilot on

Did you have your feet on the pedals?
 
Maverick623
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 7:48 am

Quoting Bruce (Thread starter):
But I have had rides in Cessnas and never noticed this.

It happens. It's a combination of wind and the left-turning tendencies of single-engine props.

Quoting swiftski (Reply 2):

Did you have your feet on the pedals?

I think the Mooney only comes with a 2-axis autopilot. It doesn't control yaw.
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muhamed
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 8:21 am

Could it be a dutch roll? Sometimes if you're playing with your pedals or stick to correct your aircraft after doing a yaw-roll you get this kinda effect. It's what's called a pilot induced error I think. The complete description is in wikipedia.
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soon7x7
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 12:36 pm

Even with the auto pilot on, (that aircraft should, I'm not sure of this) have rudder trim. If the rudder trim was not set before autopilot was engaged, this could cause the oscillation. I was in an old Air Florida 737-200 from ISP to FLL. The whole 21/4 hours, she oscillated back and forth. I was seated in the tail. That got annoying real fast.
 
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Bruce
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 3:40 pm

As far as I know, my feet were not on the pedals but I don't know about the pilot. it seemed to be a very rhythmic oscillation. I dont think the wind could create that type of steady rhythm. As far as wind, would this still happen if we had a headwind, not a crosswind?

bruce
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 5:40 pm

Quoting Bruce (Reply 6):
As far as I know, my feet were not on the pedals but I don't know about the pilot. it seemed to be a very rhythmic oscillation. I dont think the wind could create that type of steady rhythm. As far as wind, would this still happen if we had a headwind, not a crosswind?

I've got several hours in an M20B. They are pretty unstable in the yaw axis, probably due to the relatively short fuselage and small rudder. So they do tend to be pretty prone to dutch roll. I'm certain this is what you experienced.

http://en.wikipedia.org/wiki/Dutch_roll

A way to fix that is to put in some slight rudder trim of to either side, so the plane ends up slipping ever so slightly. The catch is you loose a few knots in airspeed.

And dutch roll will happen even in calm wind, though its not as apparent (really obvious in a PA-44 Seminole however). Any gusts or turbulence will tend to make it worse.

[Edited 2010-08-06 10:43:21]
 
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 6:04 pm

Quoting Fly2HMO (Reply 7):

Does this aircraft have aux tanks in the wings?
 
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 6:10 pm

Quoting soon7x7 (Reply 8):
Does this aircraft have aux tanks in the wings?

Nope. But I see what you're getting at. (increased moments due to far out aux tanks, right?) From the factory they came with 48gal wet wings, IIRC. However they were notorious for leaking and many early M20s have been converted to fuel cell bladders. My buddy's Mooney is probably one of the few out there to still have wet wings, and you can see some slight fuel seeping at a few joints. Heck its nearly 50 years old, hardly surprising.

I don't think aux tanks were an option on the early models, unless as an STC.

[Edited 2010-08-06 11:11:27]

[Edited 2010-08-06 11:11:42]
 
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HAWK21M
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 10:14 pm

Sounds like Dutch roll....Maybe the Yaw Damper was INOP.
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RE: Plane Swaying Side To Side

Fri Aug 06, 2010 10:55 pm

Quoting HAWK21M (Reply 10):
Sounds like Dutch roll....Maybe the Yaw Damper was INOP.

Errr, I don't think I've ever heard of a light plane with a yaw damper. 3 axis autopilots pretty much don't exist for GA piston planes.
 
N243NW
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RE: Plane Swaying Side To Side

Sat Aug 07, 2010 12:05 am

Quoting Fly2HMO (Reply 7):
And dutch roll will happen even in calm wind, though its not as apparent (really obvious in a PA-44 Seminole however).

It's obvious in the Seminole indeed. Even in smooth air I've often had to be my own yaw damper.

My guess is this is what Bruce has felt as well.
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soon7x7
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RE: Plane Swaying Side To Side

Sat Aug 07, 2010 1:17 am

Quoting Fly2HMO (Reply 9):

Yeh, thats what I was getting at...my buddies cherokee 6 has aux tanks out on the wing tips and if you were flying off one tank, one wing would wants to lag as a result, so the yaw thing happened, you had to constantly switch tanks if you cared, otherwise your ankles got a good work out.
 
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RE: Plane Swaying Side To Side

Sat Aug 07, 2010 1:29 am

The worst instance of this I have ever felt was while seated in row 57 of a Singapore 747-400 as we took of from SFO bound for HKG... (SQ flight 1). Being further back towards the tail apparently amplified the effect of the tail wagging.
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RE: Plane Swaying Side To Side

Sat Aug 07, 2010 2:02 am

Quoting KELPkid (Reply 14):
Being further back towards the tail apparently amplified the effect of the tail wagging.

That's why I always tell easily-nauseated people to sit somewhere over the wing area. That's where the plane "moves" the least, so to speak.
 
mrskyguy
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RE: Plane Swaying Side To Side

Sat Aug 07, 2010 4:05 am

Quoting HAWK21M (Reply 10):
Sounds like Dutch roll....Maybe the Yaw Damper was INOP.

That was my first thought as well.. the 732 will exhibit some dutch roll tendencies but won't slip in to a full-blown oscillation like a 707. The Yaw Damper definitely curbs this on the 732, however, for a smoother ride.

Quoting Maverick623 (Reply 3):
It happens. It's a combination of wind and the left-turning tendencies of single-engine props.

Not just those lovely little singles.. twins exhibit this behavior too, especially those with a T-tail configuration. Anyone who's ever flown a UA Express/SkyWest EMB-120 flight in to SGU will know precisely what I'm talking about when I say "she moves on all 3 axis', and rather aggressively to boot."
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soon7x7
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RE: Plane Swaying Side To Side

Sat Aug 07, 2010 5:43 am

Quoting mrskyguy (Reply 16):
Not just those lovely little singles.. twins exhibit this behavior too,

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Bruce
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RE: Plane Swaying Side To Side

Sun Aug 08, 2010 9:15 pm

according the the wikipedia article, Dutch Roll is a combination of movement on two axis - tail-wagging and rocking, but I'm pretty sure I didn't feel any rocking. I guess tail-wagging wwould be sort of a good way to describe how it felt. This plane didn't have aux tanks. Maybe a little rudder trim would have helped but he didn't want to do anything that would give up any airspeed.
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muhamed
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RE: Plane Swaying Side To Side

Mon Aug 09, 2010 8:41 am

Next time you experience that try to stop correcting the stick and pedals so much and slowly reduce your speed. In a Dutch roll it gets worse when you try to counter the problem. If that fixes it then it's surely a Dutch roll.
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Bruce
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RE: Plane Swaying Side To Side

Mon Aug 09, 2010 10:34 pm

I'll try and keep that in mind. I'm not a pilot and wasn't flying.  
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RE: Plane Swaying Side To Side

Tue Aug 10, 2010 1:11 am

Sounds like the pilot was only using the rudder peddles as a foot rest  

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RE: Plane Swaying Side To Side

Tue Aug 10, 2010 7:23 am

Quoting muhamed (Reply 4):
Could it be a dutch roll?

Very unlikely...the primary contributor to Dutch roll is wing sweep (that's what couples yaw to roll for this particular motion). A Mooney M-20 really shouldn't exhibit this behavior in a major way.

Quoting muhamed (Reply 4):
It's what's called a pilot induced error I think.

Dutch roll is inherent in the aircraft dynamics, it's not a pilot induced phenomenon. It's the roll/yaw analog to phugoid motion (different physics, but both inherent to the aircraft dynamics).

Quoting Fly2HMO (Reply 7):
They are pretty unstable in the yaw axis, probably due to the relatively short fuselage and small rudder. So they do tend to be pretty prone to dutch roll.

If you've got poor yaw stability due to small vertical fin/short arm, that won't automatically make you prone to Dutch roll, it'll just give you poor yaw damping and stability and any yaw disturbances (like from a gust) will damp slowly. If it's just yaw with no roll, it's not Dutch roll, it's just poor yaw dynamic stability.

Tom.
 
muhamed
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RE: Plane Swaying Side To Side

Tue Aug 10, 2010 11:33 am

Quoting Fly2HMO (Reply 7):
I've got several hours in an M20B. They are pretty unstable in the yaw axis, probably due to the relatively short fuselage and small rudder. So they do tend to be pretty prone to dutch roll. I'm certain this is what you experienced.
Quoting tdscanuck (Reply 22):
A Mooney M-20 really shouldn't exhibit this behavior in a major way.

Now I'm confused... I've never flown an M-20 so I have no opinion on whether the M-20 is prone to a Dutch roll or not.

Quoting tdscanuck (Reply 22):
Very unlikely...the primary contributor to Dutch roll is wing sweep (that's what couples yaw to roll for this particular motion). A Mooney M-20 really shouldn't exhibit this behavior in a major way.

If it's not a Dutch roll then what could Bruce have been experiencing on the M-20?
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RE: Plane Swaying Side To Side

Tue Aug 10, 2010 2:40 pm

Quoting muhamed (Reply 23):
If it's not a Dutch roll then what could Bruce have been experiencing on the M-20?

Poor yaw damping (not the absence of a yaw damper, but actually having a low damping coefficient on the yaw dynamics) and/or marginal yaw stability.

Dutch roll is specifically a coupling between yaw and roll oscillation. You can have yaw instability without coupling to roll instability, in which case the plane will oscillate in yaw only without rolling. Oscillation in yaw only is, by definition, not Dutch roll.

It basically comes down to the magnitude of C_L_beta (change in roll moment with yaw angle)...this can be fairly large on swept wing aircraft, which is part of why Dutch roll can be a big issue on swept wings. With a straight wing it should be much smaller (though not zero unless you've got a symmetric vertical fin).

I'm not saying it's not Dutch roll, just that from the description of the motion it doesn't sound like Dutch roll and, based on aircraft configuration, it's not likely that Dutch roll is a big factor.

If someone actually has access to all the stability derivatives for the M-20 we could figure it out pretty quickly...I can try to calculate them later if I get some free time.

Tom.
 
tdscanuck
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RE: Plane Swaying Side To Side

Wed Aug 11, 2010 1:04 am

Quoting tdscanuck (Reply 24):
If someone actually has access to all the stability derivatives for the M-20 we could figure it out pretty quickly...I can try to calculate them later if I get some free time.

OK, I dug out Blakelock and he cheats...his forumula for C_l_beta is stolen from another reference, which I don't have...anybody got "Airplane Performance, Stability, and Control" (Perkins & Hage) or "Dynamics of Flight: Stability and Control" (Etkin)?

Tom.
 
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RE: Plane Swaying Side To Side

Wed Aug 11, 2010 1:11 am

Quoting FighterPilot (Reply 21):
Sounds like the pilot was only using the rudder peddles as a foot rest

I've found in certain light singles that applying some light, but equal pressure to the rudder pedals actually seems to dampen the dutch roll a bit.

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RE: Plane Swaying Side To Side

Wed Aug 11, 2010 2:40 am

Quoting tdscanuck (Reply 22):
A Mooney M-20 really shouldn't exhibit this behavior in a major way.

It is very unstable in the yaw (vertical) axis for sure. And there is some roll coupling, though minor. Not a textbook definition of dutch roll but pretty close.

One that I'm 100% certain dutch rolls per textbook definition constantly is the Piper Seminole. It's really obvious and fricking annoying as hell.
 
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RE: Plane Swaying Side To Side

Wed Aug 11, 2010 4:25 am

Quoting Fly2HMO (Reply 11):

Errr, I don't think I've ever heard of a light plane with a yaw damper. 3 axis autopilots pretty much don't exist for GA piston planes.

I flew a Bonanza so equipped. Had an altitude preselect and FD as well. Nice set up, but an exception to most.

Quoting DiamondFlyer (Reply 26):
I've found in certain light singles that applying some light, but equal pressure to the rudder pedals actually seems to dampen the dutch roll a bit.

That's the basics of a yaw damper. You were providing you own. It will work in most any airplane not equipped with one, but it isn't a substitute for an MEL'd yaw damper.

Seems like I always flew the Duchess that way even though it's decently stable.
 
KELPkid
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RE: Plane Swaying Side To Side

Wed Aug 11, 2010 4:44 am

Quoting tdscanuck (Reply 22):
Very unlikely...the primary contributor to Dutch roll is wing sweep (that's what couples yaw to roll for this particular motion). A Mooney M-20 really shouldn't exhibit this behavior in a major way.

As I recall, on the Mooney, the leading edge is straight, and the trailing edge ofthe wings is swept, with the chord increasing from tip to root...sort of a reverse Learjet 23 wing, minus the tip tanks  
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RE: Plane Swaying Side To Side

Wed Aug 11, 2010 4:53 am

Quoting KELPkid (Reply 29):

As I recall, on the Mooney, the leading edge is straight, and the trailing edge ofthe wings is swept, with the chord increasing from tip to root...sort of a reverse Learjet 23 wing, minus the tip tanks

Case in point:

 
bri2k1
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RE: Plane Swaying Side To Side

Wed Aug 11, 2010 1:15 pm

Quoting DiamondFlyer (Reply 26):
I've found in certain light singles that applying some light, but equal pressure to the rudder pedals actually seems to dampen the dutch roll a bit.

Based on your forum name, I'd guess you've found this in the DA40 Star, which I have a fair number of hours in and have always felt to "wag" its way through the sky. It's fine in smooth air, but it seems to respond to chop or bumps in any axis with yaw.
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RE: Plane Swaying Side To Side

Wed Aug 11, 2010 1:39 pm

Quoting Fly2HMO (Reply 30):
Quoting KELPkid (Reply 29):

As I recall, on the Mooney, the leading edge is straight, and the trailing edge ofthe wings is swept, with the chord increasing from tip to root...sort of a reverse Learjet 23 wing, minus the tip tanks

Case in point:

Tapered wing. Used to mitigate CG issues by keeping the CoG somewhere between the front and back passengers, and therefore, keeping the arms to those stations short.

A more extreme example of this is the forward-swept wing, with a forward taper, as seen on many twin gliders such as the Blanik L13/23. In this case, it's to keep the rear passenger very close to, or on, the CoG, which simplifies W&B by only needing a min/max front pilot weight + Rear pax weight.


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N243NW
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RE: Plane Swaying Side To Side

Thu Aug 12, 2010 7:09 am

Quoting tdscanuck (Reply 25):
"Dynamics of Flight: Stability and Control" (Etkin)?

Had it, but sold it about 6 months ago. Darn.

Quoting Fly2HMO (Reply 27):
One that I'm 100% certain dutch rolls per textbook definition constantly is the Piper Seminole. It's really obvious and fricking annoying as hell.

As I said before, I've definitely noticed some pronounced tail-wagging in the PA44...although not a ton, since I spent so much of my time in that plane with one engine at idle or feathered. The only real thing my FAA examiner noted on my checkride was how I had my feet rather stagnant, planted on the pedals when I should have been trying harder to counter it.

Quoting bri2k1 (Reply 31):
I'd guess you've found this in the DA40 Star

Interesting...I have a joyride set up for the near future with a Diamond rep. I'll check it out.

[Edited 2010-08-12 00:11:16]
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