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Plane Swaying Side To Side  
User currently offlineBruce From United States of America, joined May 1999, 5049 posts, RR: 15
Posted (4 years 2 weeks 22 hours ago) and read 5095 times:

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


Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
33 replies: All unread, showing first 25:
 
User currently offline413X3 From United States of America, joined Jul 2008, 1983 posts, RR: 0
Reply 1, posted (4 years 2 weeks 22 hours ago) and read 5092 times:

sounds a lot like the wind to me, pushing around those tiny airplanes.

User currently offlineswiftski From Australia, joined Dec 2006, 2701 posts, RR: 2
Reply 2, posted (4 years 2 weeks 22 hours ago) and read 5077 times:

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?


User currently offlineMaverick623 From United States of America, joined Nov 2006, 5592 posts, RR: 6
Reply 3, posted (4 years 2 weeks 21 hours ago) and read 5063 times:

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.



"PHX is Phoenix, PDX is the other city" -777Way
User currently offlinemuhamed From Malaysia, joined Aug 2010, 32 posts, RR: 0
Reply 4, posted (4 years 2 weeks 21 hours ago) and read 5051 times:

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.


There is nothing so useless as doing efficiently that which should not be done at all.
User currently offlinesoon7x7 From , joined Dec 1969, posts, RR:
Reply 5, posted (4 years 2 weeks 16 hours ago) and read 4942 times:

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.

User currently offlineBruce From United States of America, joined May 1999, 5049 posts, RR: 15
Reply 6, posted (4 years 2 weeks 13 hours ago) and read 4829 times:

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



Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 7, posted (4 years 2 weeks 11 hours ago) and read 4745 times:

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]

User currently offlinesoon7x7 From , joined Dec 1969, posts, RR:
Reply 8, posted (4 years 2 weeks 11 hours ago) and read 4727 times:

Quoting Fly2HMO (Reply 7):

Does this aircraft have aux tanks in the wings?


User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 9, posted (4 years 2 weeks 11 hours ago) and read 4720 times:

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]

User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 10, posted (4 years 2 weeks 7 hours ago) and read 4613 times:

Sounds like Dutch roll....Maybe the Yaw Damper was INOP.
regds
MEL.



Think of the brighter side!
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 11, posted (4 years 2 weeks 6 hours ago) and read 4602 times:

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.


User currently onlineN243NW From United States of America, joined Jul 2003, 1629 posts, RR: 20
Reply 12, posted (4 years 2 weeks 5 hours ago) and read 4569 times:

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.



B-52s don't take off. They scare the ground away.
User currently offlinesoon7x7 From , joined Dec 1969, posts, RR:
Reply 13, posted (4 years 2 weeks 4 hours ago) and read 4546 times:

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.


User currently offlineKELPkid From United States of America, joined Nov 2005, 6369 posts, RR: 3
Reply 14, posted (4 years 2 weeks 4 hours ago) and read 4542 times:

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.


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 15, posted (4 years 2 weeks 3 hours ago) and read 4516 times:

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.


User currently offlinemrskyguy From United States of America, joined Aug 2008, 1214 posts, RR: 3
Reply 16, posted (4 years 2 weeks 1 hour ago) and read 4481 times:

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."



"The strength of the turbulence is directly proportional to the temperature of your coffee." -- Gunter's 2nd Law of Air
User currently offlinesoon7x7 From , joined Dec 1969, posts, RR:
Reply 17, posted (4 years 1 week 6 days 23 hours ago) and read 4461 times:

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

Beech 1900's...drove me crazy!...the worst.?...Jetstreams!


User currently offlineBruce From United States of America, joined May 1999, 5049 posts, RR: 15
Reply 18, posted (4 years 1 week 5 days 8 hours ago) and read 4252 times:

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.


Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
User currently offlinemuhamed From Malaysia, joined Aug 2010, 32 posts, RR: 0
Reply 19, posted (4 years 1 week 4 days 20 hours ago) and read 4146 times:

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.


There is nothing so useless as doing efficiently that which should not be done at all.
User currently offlineBruce From United States of America, joined May 1999, 5049 posts, RR: 15
Reply 20, posted (4 years 1 week 4 days 6 hours ago) and read 4036 times:

I'll try and keep that in mind. I'm not a pilot and wasn't flying.  
bruce



Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
User currently offlineFighterPilot From Canada, joined Jun 2005, 1383 posts, RR: 22
Reply 21, posted (4 years 1 week 4 days 4 hours ago) and read 4005 times:

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

Cal   



*Insert Sound Of GE90 Spooling Up Here*
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 22, posted (4 years 1 week 3 days 22 hours ago) and read 3951 times:

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.


User currently offlinemuhamed From Malaysia, joined Aug 2010, 32 posts, RR: 0
Reply 23, posted (4 years 1 week 3 days 17 hours ago) and read 3929 times:

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?



There is nothing so useless as doing efficiently that which should not be done at all.
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 24, posted (4 years 1 week 3 days 14 hours ago) and read 3901 times:

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.


25 tdscanuck : 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 Perfor
26 DiamondFlyer : 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.
27 Fly2HMO : 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 p
28 DashTrash : I flew a Bonanza so equipped. Had an altitude preselect and FD as well. Nice set up, but an exception to most. That's the basics of a yaw damper. You
29 Post contains images KELPkid : 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...so
30 Post contains images Fly2HMO : Case in point:
31 bri2k1 : 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
32 Post contains links and images Goldenshield : 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
33 N243NW : Had it, but sold it about 6 months ago. Darn. As I said before, I've definitely noticed some pronounced tail-wagging in the PA44...although not a ton
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