Playing around with the pitch phugoid

Re what is the point of this thread?

I just find it fascinating that cloud flying without an artificial horizon IS routinely done in sailplanes by some pilots in some nations (see previous posts withing last 6 months to RAS), yet very little information is out there on how to stop the pitch phugoid.

My experience is that some sailplanes are dynamically unstable in pitch even when flown within the published CG limits. Of course I can't rule out an error in weighing or computations, but that's my experience.

This idea of "if your flying it right, a phugoid will never develop" doesn't hold water with me.

The idea of moving the stick forward when the airspeed is high, seems bizarre at first glance. Yet it works. See earlier posts for more. When the airspeed is high but no longer increasing, the glider is in a nearly level pitch attitude, and the excess airspeed will soon make the flight path curve upward strongly. The pilot needs to prevent that. By moving the stick forward.

After what I've seen firsthand in clear skies, I shudder to imagine any pilot ever venturing into clouds without an intimate knowledge of the dynamics of the pitch phugoid, and how to stop it. So you have an artificial horizon display. Do you want to bet your life that it will not fail? Then you are down to partial panel, and then what?

Besides, what's the point in just droning around the sky without understanding what is going on? Any pilot should be interested in these topics even if he never intends to come within ten miles of a cloud.

Those in the "I manipulate the stick to make the sailplane do what I want it to do so why do I need to understand anything about anything" camp need not engage in this topic...

S

On Thursday, July 30, 2015 at 9:07:20 AM UTC-5, wrote:
Re what is the point of this thread?

I just find it fascinating that cloud flying without an artificial horizon IS routinely done in sailplanes by some pilots in some nations (see previous posts withing last 6 months to RAS), yet very little information is out there on how to stop the pitch phugoid.

My experience is that some sailplanes are dynamically unstable in pitch even when flown within the published CG limits. Of course I can't rule out an error in weighing or computations, but that's my experience.

This idea of "if your flying it right, a phugoid will never develop" doesn't hold water with me.

The idea of moving the stick forward when the airspeed is high, seems bizarre at first glance. Yet it works. See earlier posts for more. When the airspeed is high but no longer increasing, the glider is in a nearly level pitch attitude, and the excess airspeed will soon make the flight path curve upward strongly. The pilot needs to prevent that. By moving the stick forward.

After what I've seen firsthand in clear skies, I shudder to imagine any pilot ever venturing into clouds without an intimate knowledge of the dynamics of the pitch phugoid, and how to stop it. So you have an artificial horizon display. Do you want to bet your life that it will not fail? Then you are down to partial panel, and then what?

Besides, what's the point in just droning around the sky without understanding what is going on? Any pilot should be interested in these topics even if he never intends to come within ten miles of a cloud.

Those in the "I manipulate the stick to make the sailplane do what I want it to do so why do I need to understand anything about anything" camp need not engage in this topic...

S

Platyp, if you have a couple hundred hours in a glider and still need to consciously avoid a phugoid - then something is seriously wrong with you or your glider. Your entire postings are not related to anything I have experienced in 35 years of flying. Let it go, nobody else is interested in your musings, you are just scaring the kids, please stop.

On Thursday, July 30, 2015 at 9:07:20 AM UTC-5, wrote:
Re what is the point of this thread?

I just find it fascinating that cloud flying without an artificial horizon IS routinely done in sailplanes by some pilots in some nations (see previous posts withing last 6 months to RAS), yet very little information is out there on how to stop the pitch phugoid.

My experience is that some sailplanes are dynamically unstable in pitch even when flown within the published CG limits. Of course I can't rule out an error in weighing or computations, but that's my experience.

This idea of "if your flying it right, a phugoid will never develop" doesn't hold water with me.

The idea of moving the stick forward when the airspeed is high, seems bizarre at first glance. Yet it works. See earlier posts for more. When the airspeed is high but no longer increasing, the glider is in a nearly level pitch attitude, and the excess airspeed will soon make the flight path curve upward strongly. The pilot needs to prevent that. By moving the stick forward.

After what I've seen firsthand in clear skies, I shudder to imagine any pilot ever venturing into clouds without an intimate knowledge of the dynamics of the pitch phugoid, and how to stop it. So you have an artificial horizon display. Do you want to bet your life that it will not fail? Then you are down to partial panel, and then what?

Besides, what's the point in just droning around the sky without understanding what is going on? Any pilot should be interested in these topics even if he never intends to come within ten miles of a cloud.

Those in the "I manipulate the stick to make the sailplane do what I want it to do so why do I need to understand anything about anything" camp need not engage in this topic...

S

Funny, people have been flying IMC using only a turn and bank and airspeed for a long time - without worrying about the pitch phugoid. As I said before - if you are actively flying the plane (actively controlling the speed via angle of attack, and the bank angle by controlling the turn rate by reference to the needle (while keeping the ball centered), your pitch phugoid is not a factor.

If you so mishandle the controls that you lose airspeed control (which is basically what you are describing) then you are probably going to end up in a spiral dive - a whole different ball game!

Are you saying you run into problems with a pitch phugoid "in clear skies"? How? You letting go of the controls? WHY?

And sure an AI makes IMC flight easier - and sure it can fail (which is why plane certified for IMC fligh have to have redundant gyros), but again that has little to do with the pitch phugoid! If you lose your AI (or T & B), you will most likely end up in a spiral dive.

Again: If you are flying it right - A PITCH PHUGOID WILL NEVER DEVELOP. If you let go of the controls, it probably will - but then by definition you are no longer flying the plane, are you?

IMHO, you are making a big mountain out of a small molehill...

Just fly the damn plane, for christ's sake!

Kirk

I find the best response to the pitch phugoid to be--

When the airspeed is decreasing, or when the airspeed has plateaued at the peak airspeed and is about to decrease, move the stick forward. In short, move the stick opposite to the direction that the airspeed is changing, or is about to change. Try to freeze the airspeed, and then ease the stick aft a bit to ease the airspeed slowly back toward trim.

When the airspeed is decreasing and has dropped below the trim speed or the midpoint of the oscillation, relax your inputs, in anticipation of giving a new input as soon as the airspeed plateaus at the minimum airspeed.

When the airspeed is increasing, or when the airspeed has plateaued at the minimum airspeed and is about to increase, move the stick aft. Try to reduce the rate of increase in airspeed. In short, move the stick opposite to the direction that the airspeed is changing, or is about to change. Go easy on it so you don't invite a stall-- don't be so aggressive as to try to completely "freeze" the airspeed at this point in the oscillation. But don't be afraid to give a little aft pressure on the stick even when the airspeed is quite low-- the dynamics are such that gentle aft pressure at this point in the oscillation will have a helpful effect, and won't stall the wing.

When the airspeed is increasing and has risen above the trim speed or the midpoint of the oscillation, relax your inputs, in anticipation of giving a new input as soon as the airspeed plateaus at the maximum airspeed.

Recurring theme: "Move the stick opposite to the direction that the airspeed is changing, or is about to change."

Recurring theme: relax your input at the midpoint of the airspeed oscillation, or upon passing through trim speed, in anticipation of the next input.

Recurring theme: the peak airspeed is a good time to break the oscillation, by moving the stick forward as needed to freeze the airspeed-- to prevent a rapid decrease in airspeed-- and then easing the stick slowly aft to ease on down to trim speed.

S

As some posters already stated:
If you fly the sailplane with your hand on the stick, there won't be any significant phugoid (I've flown about 3 dozend different gliders in 35 years, from 10m span to 29m span - never seen a phugoid while flying the glider).

If you take your hand off the stick, most sailplanes will start a phugoid.
Best way to end it: grab the stick and fly the glider.

Are you sure that you have actually flown a glider?

Bert
Ventus cM TW

On Thursday, July 30, 2015 at 6:45:39 PM UTC+3, Tango Whisky wrote:
As some posters already stated:
If you fly the sailplane with your hand on the stick, there won't be any significant phugoid (I've flown about 3 dozend different gliders in 35 years, from 10m span to 29m span - never seen a phugoid while flying the glider).

If you take your hand off the stick, most sailplanes will start a phugoid..
Best way to end it: grab the stick and fly the glider.

Are you sure that you have actually flown a glider?

Does that work if you close your eyes at the same time?

Certainly I've never had a problem with unwanted pitch phugoid, but then I've never flown a glider with absolutely no outside pitch reference, even if only showing me changes in pitch, not absolute pitch. e.g. I'm fine in a mountain valley I have my doubts about deep inside a cloud.

I find this an odd discussion since this is in reference to a dynamic mode that is related to the interaction between speed and pitching moment - it requires you not to make control inputs for many tens of seconds and the pitch, airspeed and altitude excursions that result are considerable. I don't know of any pilot or any airplane where this would happen without a deliberate effort to NOT fly the airplane - or if your elevator became disabled.

No one needs to think about controlling the phugoid - it is subordinated to simple pitch and airspeed control that happen on a far, far shorter timescale. The dominant dynamic mode there is short-period, but even that's not a big factor from a pilot control perspective.

9B

This discussion is such a mish-mash.
The phugoid came up in the discussion of the so-called benign spiral mode of exiting a cloud.
I once owned a flew a Grob 104 Speed Astir. It had a benign spiral configuration for exiting a cloud. It was full flaps, trim full aft, spoilers out, and hands and feet off of the controls. I experienced that situation and can tell you that the aircraft still had a phugoid behavior but it stayed within reasonable limits.
I now have an LS8. I practiced the benign spiral situation a couple of times and the phugoid got worse and worse until it was full stall to Vne. However, I neglected to pull the spoilers.
Last week I tried it again but pulled full spoilers. No phugoid! However, there would still be no way to control roll without proper instruments.
Now I have a full time, solid state, artificial horizon in an LX S80. No worries.

On Saturday, August 8, 2015 at 5:40:44 PM UTC-7, wrote:
This discussion is such a mish-mash.
The phugoid came up in the discussion of the so-called benign spiral mode of exiting a cloud.
I once owned a flew a Grob 104 Speed Astir. It had a benign spiral configuration for exiting a cloud. It was full flaps, trim full aft, spoilers out, and hands and feet off of the controls. I experienced that situation and can tell you that the aircraft still had a phugoid behavior but it stayed within reasonable limits.
I now have an LS8. I practiced the benign spiral situation a couple of times and the phugoid got worse and worse until it was full stall to Vne. However, I neglected to pull the spoilers.
Last week I tried it again but pulled full spoilers. No phugoid! However, there would still be no way to control roll without proper instruments.
Now I have a full time, solid state, artificial horizon in an LX S80. No worries.

Yes - good point. One way to damp the phugoid (If you don't want to touch the elevator - either because it's jammed or because you are trying a benign spiral) is to open the speed brakes at the bottom of the oscillation. I can attest that it works on a G103.

9B

On Saturday, August 8, 2015 at 10:40:35 PM UTC+3, Andy Blackburn wrote:
I find this an odd discussion since this is in reference to a dynamic mode that is related to the interaction between speed and pitching moment - it requires you not to make control inputs for many tens of seconds and the pitch, airspeed and altitude excursions that result are considerable. I don't know of any pilot or any airplane where this would happen without a deliberate effort to NOT fly the airplane - or if your elevator became disabled.

No one needs to think about controlling the phugoid - it is subordinated to simple pitch and airspeed control that happen on a far, far shorter timescale. The dominant dynamic mode there is short-period, but even that's not a big factor from a pilot control perspective.

Yes OF COURSE if you are controlling the pitch and therefore the speed then you will never see a phugoid. OF COURSE. But how do you control the pitch if you can't see the pitch or even changes in the pitch? What control input should you make? Normally you don't even think about this because it's so obvious and subconscious. But it DOES rely on outside visual references, and isn't going to work when you're in a cloud and don't have any.