The Phugoid (Long Period) Mode on the Boeing 777

[kevinh 409]

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.

[jcomm 5,932]

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.

[icaruss 18]

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.

[Alpha Floor 237]

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.

[Alpha Floor 237]

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.

[downscc 1,327]

 

 

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.

[icaruss 18]

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.

[Alpha Floor 237]

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):

[icaruss 18]

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).

[rondon9898 84]

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

[Alpha Floor 237]

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).

 

Good comment! I really have no idea how the phugoid is excited or tested in the real 777. I don't know if they turn the FBW off and revert to DIRECT law, so I just left it all on. Can we say this is an "ideal" phugoid then, artificially stabilized by the FBW?

 

One thing is for sure: In the real test you never get back to the initial condition "exactly". 

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

 

Sure! The Cessna is the most forgiving aircraft ever! It will fly out of a spin by itself

[icaruss 18]

Good comment! I really have no idea how the phugoid is excited or tested in the real 777. I don't know if they turn the FBW off and revert to DIRECT law, so I just left it all on. Can we say this is an "ideal" phugoid then, artificially stabilized by the FBW?

Aircrafs are mainly tested in the same way as they are flown. My assumption is that Boeing did the majority of the tests with all the augmentation on. However, tests are also done with degraded systems to ensure the aircraft can be flown safely back to an airport.

[PMDG777 231]

Very interesting video. Certainly didn't know that this was so well simulated in the 777. You're right in saying that the mathematics behind this is no simple feat as I discovered in my degree!

[kevinh 409]

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.

I've only seen one addon that didn't do it right, and that was because the AIR file had a missing pitching moment coefficient (which was fixed by a patch). Even the default planes will exhibit a phugoid response.

 

The FBW shouldn't affect it much as the manoeuvre is hands off, so there is no pitch input after the initial pulse. The 777 is designed to fly like a non-FBW aircraft.

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.

Yes I know about the equations, I have a degree in Aeronautical Engineering. I also know how it's done in a flight simulaton like FSX. You don't have to solve those equations in real time and then somehow replicate the calculated flight path. Lift, drag and pitching moment coefficients are programmed into the AIR file and the sim responds to the resultant forces and moments according to Newton's Laws of Motion. It's all built into FSX.

[downscc 1,327]

 

 

I have a degree in Aeronautical Engineering

 

Good to know. USAF opened a door to BSEE for me, I would have preferred aeronautical but they paid the bills.