The Phugoid (Long Period) Mode on the Boeing 777

[kevinh 409]

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

I was lucky enough to get my degree back when the UK government paid the fees and gave you a grant to live on (though we had a few RAF sponsored students on our course too). So it didn't cost me a penny. Now students in England have to get enormous loans to get such an education.

[icaruss 18]

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.

 

The FBW does affect the excitation (which is not really a pulse, that's something else). Not all phugoid tests are done hands off.

 

Not sure what you mean by "designed to fly like a non-FBW aircraft."

[kevinh 409]

The FBW does affect the excitation (which is not really a pulse, that's something else). Not all phugoid tests are done hands off.

 

Not sure what you mean by "designed to fly like a non-FBW aircraft."

It's a pulse in terms of being a short input then a release. You don't have to hold the input very long. Not a pulse as used to excite the SPPO of course. Yes the FBW affects the input, but once the column is released than the effect is minimal. A true phugoid is always flown hands off. If you make longitudinal control inputs (or thrust inputs) after the initial input then it's not a true phugoid, as demonstrated by Jaime in his video.

 

What I meant was Boeing designed it so the aircraft response is very natural to a pilot used to non-FBW controls. They use a modified C*U control law so the aircraft retains speed stability for example.

 

You should be signing your posts in this forum, first and last name, or they risk being deleted.

[icaruss 18]

A pulse is a very defined term in flight testing, just wanted to clarify that.

 

A phugoid can be flown controls-free or controls-fixed (following the excitation), as described in the flight test manuals. In fact, when I do flight test, I usually do both methods in case there's a difference in the response.

[Alpha Floor 237]

A pulse is a very defined term in flight testing, just wanted to clarify that.

 

A phugoid can be flown controls-free or controls-fixed (following the excitation), as described in the flight test manuals. In fact, when I do flight test, I usually do both methods in case there's a difference in the response.

 

Speaking of controls-free and controls-fixed,

 

On the C172 test card I showed earlier, the test was performed with controls free (which is "worst case" or more conservative since airplanes are more stable controls-fixed). The controls of the C172 are mechanically linked and if left free will "float" in the stream.

 

Now, if you do this test on an airliner with a hydraulic system like the 737 (but no FBW), even if you don't touch any controls after the first input, they would remain in the neutral position making it essentially a controls-fixed test, right? An airplane like the DC-9 which is also mechanical (works with tabs, very interesting), will fly like the Cessna, right? The tabs will float in the stream.

 

Going back to my video, the 777 is FBW and during the oscillation you can glance down at the Multi Function Display and see that the elevator is actually moving. What's the FBW doing here? Another thing I don't understand is that the stabilizer trim changes as well ranging from 3.50 to 5.25, yet the trim speed on the speedtape remains constant at 290. Could anyone clarify this please?

 

I might do a second video with the FBW turned off and the stab trim cutout to see what happens then.

[icaruss 18]

You'll be surprised how little reversible flight controls (like the one on the cessna) move when doing controls-free testing, remember that all controls are trimmed before the phugoid is excited.

 

In my experience, the results for both controls-free and controls-fixed are very similar, and a lot of it depends on the type of flight controls system. For example, there are flight controls that move to automatically trim for an airspeed. In such systems, If you do controls-fixed while the airspeed changes during the phugoid, you'd be fighting the stick.

 

Irreversible flight controls (such as the one on 777), are a little trickier to test because of all the augmentation. You're really testing the FCC when all the system are working. A perfect example is the stabilalizer trim. Not knowing the 777 flight control laws, my guess is that the stabilizer is triming as the airspeed changes. In that case, you're really looking at the "pseudo" longitudinal dynamic stability of the airplane, which is fine because the airplane was designed to be flown that way.

[jcomm 5,930]

One thing I noticed on your 777 video was that the aircraft returned not only to it's initial trimmed speed / AoA, but also the the original altitude.

 

While the phugoid bringing it to the trimmed state is what I expected to happen, the fact that it returned to the original altitude is somehow unexpected....

[Alpha Floor 237]

One thing I noticed on your 777 video was that the aircraft returned not only to it's initial trimmed speed / AoA, but also the the original altitude.

 

While the phugoid bringing it to the trimmed state is what I expected to happen, the fact that it returned to the original altitude is somehow unexpected....

 

Yes, you're right! I did a lot of attempts and tried to "round it up" with the plane returning to its original altitude. Sometimes it happened and other times it went off by some +/- 100 or 200 feet.

[kevinh 409]

A pulse is a very defined term in flight testing, just wanted to clarify that.

 

A phugoid can be flown controls-free or controls-fixed (following the excitation), as described in the flight test manuals. In fact, when I do flight test, I usually do both methods in case there's a difference in the response.

What control inputs do you make during a phugoid and why? My experience is mostly with commercial airliner flight tests and I've never come across a phugoid test result with a pilot control input after the initial pitch input.

 

The controls do move hands free of course, sometimes even with fully powered flight controls.

 

 

Not knowing the 777 flight control laws, my guess is that the stabilizer is triming as the airspeed changes.

In the 777 the pilot has to adjust trim with airspeed changes, the FBW doesn't auto trim for speed. It auto-trims for configuration changes.

[Alpha Floor 237]

In the 777 the pilot has to adjust trim with airspeed changes, the FBW doesn't auto trim for speed. It auto-trims for configuration changes.

 

So any clue why the trim is moving during the oscillation yet the trim-speed remains constant at 290 on the PFD?

 

I will read the FCOM regardin trimming and FBW cause I don't have a deep understanding of the system yet.

[icaruss 18]

What control inputs do you make during a phugoid and why? My experience is mostly with commercial airliner flight tests and I've never come across a phugoid test result with a pilot control input after the initial pitch input.

 

What control inputs do you make during a phugoid and why? My experience is mostly with commercial airliner flight tests and I've never come across a phugoid test result with a pilot control input after the initial pitch input.

 

After the excitation there is not input made to the flight controls. Controls-fixed just mean that we keep them fixed (without making any inputs). There is some variance in the results, and they are both valid.

[kevinh 409]

So any clue why the trim is moving during the oscillation yet the trim-speed remains constant at 290 on the PFD?

 

I will read the FCOM regardin trimming and FBW cause I don't have a deep understanding of the system yet.

It isn't trimming but it might be affecting the controls. My iniitial reaction was the FBW would not affect the response but having thought more about it I think it must be doing something. It may be that the FBW is trying to maintain flightpath (or keep vertical acceleration to 1g). It will use elevator initially but will also follow up with the stabiliser if the elevator input is sustained. I don't know enough about the system to be sure how it should behave. I tried the phugoid in the 777 both with FBW active and in Direct law but the results were rather inconclusive.

After the excitation there is not input made to the flight controls. Controls-fixed just mean that we keep them fixed (without making any inputs). There is some variance in the results, and they are both valid.

My mistake. Stick free or stick fixed makes perfect sense.

[jcomm 5,930]

I don't think "disconnecting the FBW" really has any effect in the PMDG 777. I could never tell the difference... Don't think "Direct Law" is actually modeled.

[icaruss 18]

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?

 

The only practical application is really the relation between pilot workload and the longitudinal dynamic stability of the aircraft. A good example is airspeed maintenance. If the pilot has to constantly make longitudinal inputs to maintain an airspeed (high workload), then the reason could a problem in the longitudinal dynamic characteristics of the aircraft.

[Jet Tech 31]

Jose,

 

What was it you were expecting to see by turning off the FBW?  Boeing FBW doesn't really change the cruise handling characteristics as its a speed trim system,  it simply adds protection and provides some additional compensation in Turns and Flap use but it doesn't really change the way the aircraft "Flies".  We actually have an extensive explanation of the system in the Introductory section of the manual for the 777.   And just so you know, we did model the FBW Disabled mode as well as the protected modes,  you just have to know what to look for when one mode changes to another to even be able to tell.