# The Phugoid (Long Period) Mode on the Boeing 777

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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!

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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

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

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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.

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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?

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The famous aeronautical engineer Phrank Phugoid!

TOUCHE!

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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.

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Very nice description and video Jaime.

Thank you.

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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.

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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

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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.

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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.

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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?

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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 :-)

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

The Phugoid is basically a natural interchange of potential and kinetic energy. It would be hard not to do it well.

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The Phugoid is basically a natural interchange of potential and kinetic energy. It would be hard not to do it well.

Yet, many do not even get that right ...

And, given the 777 is "added" by it's fancy SAS / FBW, it could turn into a more problematic modelling outcome... But it does it plausibly, IMO.

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In flight test, the aircraft responses are usually described as positive, negavite, neutral, divergent or convergent for dynamic stability testing.

Nice video by the way.

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The Phugoid is basically a natural interchange of potential and kinetic energy. It would be hard not to do it well.

Well, I believe it's a bit more complicated than that. Keeping the mechanical energy is "easy", the difficult part is how the interchange of kinetic and potential energy is done. What's the frequency of the mode? What's the damping coefficient? How are these parameters affected by the flight condition? Etc.

The equations behind the stability of an aircraft are HIGHLY non-trivial. Now, I have NO IDEA how this is implemented into MFS and/or how PMDG have done it. Neither do I know how closely the simulation resembles the real aircraft.

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The equations behind the stability of an aircraft are HIGHLY non-trivial. Now, I have NO IDEA how this is implemented into MFS and/or how PMDG have done it. Neither do I know how closely the simulation resembles the real aircraft.

One of the leads within PMDG has a PhD in a related field, so the equations that are a challenge for an engineer with a BS such as myself are not hard for him to crunch and swallow.

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A friend of mine who is becoming a Flight Test Engineer was kind enough to refer me to the following manual:

US NAVAL TEST PILOT SCHOOL - FLIGHT TEST MANUAL 103 - FIXED WIND STABILITY AND CONTROL

Pages 191 and 192 describe the technique for measuring phugoid characteristics.

The Flight Test Manuals, commonly known as FTMs, are the bible for Naval Flight Test. The FTM 108 is used for fixed wing performance flight testing. Being a Naval Test Pilot school grad myself, these manuals require a lot of study, but are paramount when conducting DT (developmental testing). Not sure how deep the PMDG software was developed in order to recreate the true 777's flying characteristics, but the video was very interesting.

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The Flight Test Manuals, commonly known as FTMs, are the bible for Naval Flight Test. The FTM 108 is used for fixed wing performance flight testing. Being a Naval Test Pilot school grad myself, these manuals require a lot of study, but are paramount when conducting DT (developmental testing). Not sure how deep the PMDG software was developed in order to recreate the true 777's flying characteristics, but the video was very interesting.

Wow! Your studies must be interesting! And yes, these manuals not only require a lot of study but a lot of mathematical/physical background as well. Becoming a test pilot must be difficult, it requires the knowledge of an aeronautical engineer together with the skills of a military pilot! Keep it up!

And here's the test card. This test was performed with a Cessna 172 and the initial altitude was 3500', not 4000' as it was planned. The plane was trimmed for 90kt and the pilot pitched up until reaching 70kt at which point he released the control and the clock started. If there was any roll to be corrected he could only use the rudder. Power and trim must be left untouched throughout the maneuver of course.

And here the plot for airspeed versus time (just a "dirty" draft):

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This was a controls free method, meaning that the controls were free in the longitudinal axis following the excitation of the phugoid (also called Long-Term Response). The roll attitude can be maintained at zero, as long as the longitudinal axis is not disturbed (hard to do). Like you said, clock is then started to record peak altitudes, airpeeds, and attitudes, etc. The graph above shows a positive and convergent response, which is what you want in a fixed wing aircraft. In an aircraft such as the 777, you're really observing the response of the augmentation (FBW, FCC, etc) when trying to excite the phugoid. Sometimes during initial testing, these augmentations are turned off to a degree, to observe the "true response" of the aircraft. In some cases, that is not even possible because loss of control would happen in seconds. Flight test is also done in simulators. When a response of the real aircraft is known, variables in the simulators are adjusted to replicate the real thing (not sure if this was done by PMDG).

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So does that diagram show that the C172 is longitudinally stable, Jaime? Sorry to bring the tone down

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