Playing around with the pitch phugoid

You've blundered into the white room. You have primitive gyro guidance in the form of a turn rate indicator and so you are keeping the wings level, sort of. You are trimmed for 55 and the airspeed has just risen (alarmingly rapidly) to 85 and then temporaily plateaued there, staying constant for a couple of seconds. Answer quick: what do you do with the stick?

S

Trim slightly forward. Nothing to stick.

Push aggressively to hold the airspeed; left to its own devices the airspeed will soon decrease rapidly. The glider is in roughly a normal glide attitude (or a bit more nose-down from that) but way above trim speed, so this is not a steady-state situation in any way, shape, or form. Left alone, the nose will rise dramatically as the flight path curves upward, and the airspeed will bleed away to far below trim speed-- you may even get a stall break.

S

On Tuesday, July 7, 2015 at 3:31:25 PM UTC-5, wrote:
Push aggressively to hold the airspeed

And then slowly relax forward pressure to let the stick come back to trim

S

On Tuesday, July 7, 2015 at 4:31:25 PM UTC-4, wrote:
Push aggressively to hold the airspeed; left to its own devices the airspeed will soon decrease rapidly. The glider is in roughly a normal glide attitude (or a bit more nose-down from that) but way above trim speed, so this is not a steady-state situation in any way, shape, or form. Left alone, the nose will rise dramatically as the flight path curves upward, and the airspeed will bleed away to far below trim speed-- you may even get a stall break.

S

I don't think the question can be answered with any degree of confidence if you don't know how the glider will respond hands off. If the CG is aft, it may well diverge and have a bigger change the next cycle due to less stability. If the CG is forward, it may well be damped and be better off left alone.
A very good reason to know the glider well.
Simply saying push seems to mean the writer is more worried about a stall than over speed.

UH

Last post-- you raise some good points. It is certainly true that the phugoid may either diverge into increasing highs and lows, or damp out into decreasing highs and lows, depending on the glider and the CG position. I'm a light guy and so my CG is usually well aft, making the dynamics on the lively side. Even if the CG is well forward, I think that some mild forward pressure on the stick at this particular point in the oscillation would help the situation.

Thanks for chiming in.

S

I guess my mindset is that if that if you can apply whatever forward stick pressure is needed to freeze the airspeed, and then slowly ease the airspeed back to trim, that is always better then simply riding along with the oscillation, regardless of whether the oscillation is going to get more severe, or damp out. Obviously the latter situation is much more user-friendly than the former...

S

So....you are still in the White Room.
What about bank?

On Sunday, July 5, 2015 at 11:50:39 PM UTC-6, wrote:
You've blundered into the white room. You have primitive gyro guidance in the form of a turn rate indicator and so you are keeping the wings level, sort of. You are trimmed for 55 and the airspeed has just risen (alarmingly rapidly) to 85 and then temporaily plateaued there, staying constant for a couple of seconds. Answer quick: what do you do with the stick?

S

Open the spoilers (add drag) and leave the stick alone. A phugoid is damped by drag.

Don't believe it? Put your glider into a stick-free phugoid pitch/airspeed oscillation (roughly 17 seconds peak-to-peak) and then open the spoilers and watch it go away - or at least be significantly reduced.

On 7/7/2015 1:57 PM, wrote:
On Tuesday, July 7, 2015 at 4:31:25 PM UTC-4, wrote:
Push aggressively to hold the airspeed; left to its own devices the airspeed will soon decrease rapidly. The glider is in roughly a normal glide attitude (or a bit more nose-down from that) but way above trim speed, so this is not a steady-state situation in any way, shape, or form. Left alone, the nose will rise dramatically as the flight path curves upward, and the airspeed will bleed away to far below trim speed-- you may even get a stall break.

S

I don't think the question can be answered with any degree of confidence if you don't know how the glider will respond hands off. If the CG is aft, it may well diverge and have a bigger change the next cycle due to less stability. If the CG is forward, it may well be damped and be better off left alone.
A very good reason to know the glider well.
Simply saying push seems to mean the writer is more worried about a stall than over speed.

UH

The first thing I used to do is contact the manufacturer and some pilots
I knew and respected and ask ask what is the recommended CG position.
When I was racing our Duo Discus, I would be changing the water in the
tail daily. It made flying a delight rather than fighting the stick
every flight.

MG
--
Mike I Green