The Phugoid (Long Period) Mode on the Boeing 777

[Alpha Floor 237]

Hi all,

 

I recorded this video showing the response of the aircraft to an elevator input. What you get is a longitudinal natural mode called "Phugoid" or "Long Period" mode, it looks like this:

 

 

Here's the video, hope you enjoy it!

 

[rondon9898 84]

That's really interesting Jaime. Which kind of stability does this stable phugoid demonstrate? There are several kinds of stability and I oft confuse them

[keight 26]

It is dynamic longitudinal stability (oscillation about the lateral axis).

[Alpha Floor 237]

It is dynamic longitudinal stability (oscillation about the lateral axis).

Exactly. Thanks!

 

Dynamic vs. Static -> Response to the input over time vs. response at t=0. A plane can be dynamically stable but statically unstable or vice-versa.

 

Longitudinal vs. Lateral-Directional -> Axes about which the oscillation takes place. The longitudinal modes are "easier" to analyze because they remain in the vertical plane. It can be easily shown that Yaw and Roll are coupled, so there's not one without the other and hence Lateral-Directional modes are more complex to analyze (here you have the Dutch Roll for example)

 

Free vs. Fixed (or Locked) Controls -> Locking or leaving the controls of the aircraft free can alter the stability of the plane. An aircraft is usually more stable with locked controls, and it could happen that the aircraft is unstable with free controls but stable with locked controls. 

[overspeed3 410]

I wonder who first discovered this aerodynamics principle - have to Goggle it. I also wonder if there is a practical application for it - i.e. in an emergency situation?   

[JRBarrett 3,034]

[overspeed3 410]

The famous aeronautical engineer Phrank Phugoid!

 

TOUCHE!

[keight 26]

A plane can be dynamically stable but statically unstable or vice-versa.

it will not be dynamically stable if it is statically unstable.

a plane has positive/negative/neutral dynamic stability only in case if it is statically stable.

[DaveCT2003 1,820]

Very nice description and video Jaime.

 

Thank you.

[Alpha Floor 237]

it will not be dynamically stable if it is statically unstable.

a plane has positive/negative/neutral dynamic stability only in case if it is statically stable.

Yes, I wrote one "vice-versa" too many, hehe.

Static stability is required, but not enough, for dynamic stability.

[Mike777 163]

Jaime, thanks for a very interesting video.

 

 

 

I wonder who first discovered this aerodynamics principle - have to Goggle it. I also wonder if there is a practical application for it - i.e. in an emergency situation?

 

In1989 the failure of a United DC-10's hydraulics resulted in complete loss of all control surface function, United 232.  The crew landed the plane using thrust of the two wing engines to control pitch & direction.  They did an amazing job landing the plane.  Somewhere I remember reading that Captain Haynes said they never fully overcame the phugoid cycles while trying to control pitch using increases and decreases in thrust.  I don't know whether a greater pre-incident knowledge of phugoid cycles would have helped them out.

 

https://en.wikipedia.org/wiki/United_Airlines_Flight_232

 

Mike

[kevinh 409]

I wonder who first discovered this aerodynamics principle - have to Goggle it. I also wonder if there is a practical application for it - i.e. in an emergency situation?

F. W. Lanchester named the phugoid and analysed it. There is no practical application, it's just a characteristic motion that pilots should understand.

[jcomm 5,889]

Thx for sharing Jaime, and yes, it is VERY VERY well done in the PMDG 777.

 

The 777 is ( for me ) the Masterpiece of PMDG. Their upcoming 744 might change the score :-)

 

I spent some good time testing this and other features after the latest 777 patch that changed the implementation of the stability augmentation system problems of the initial version, and for me they went as far as it is possible in MSFS - and probably in any PC-based flight simulator, for a product designed to be used by normal users not owning extremelly expensive hardware.

[Alpha Floor 237]

 

The 777 is ( for me ) the Masterpiece of PMDG. Their upcoming 744 might change the score :-)

 

I spent some good time testing this and other features after the latest 777 patch that changed the implementation of the stability augmentation system problems of the initial version, and for me they went as far as it is possible in MSFS - and probably in any PC-based flight simulator, for a product designed to be used by normal users not owning extremelly expensive hardware.

 

Thanks for the comment!

 

You think it will be on par with the 744 by PSX, minding the differences?

 

Do you think X-Plane is inherently better for simulating stability and control? That is, would it have been easier for PMDG to get close to the real aircraft if they used XP instead of MSFS?

[jcomm 5,889]

I gave up on X-Plane for other than using it as a visuals generator for PSX...

 

As far as comparing PSX to PMDG's 747 ( v1 ).... Well, I would rather prefer not to... :-)

 

They're the best, each on it's own... but PSX it's a different league....

 

Regarding the upcoming 744 v2, I am eager to test it and see how it performs. If it's at the same level of detail and sophistication and quality of the 777, then PMDG will have two Masterpieces :-)